The impact of disturbance on detrital dy
Pores~~~ology
Management
ELSEVIER
Forest Ecology and Management 88 (19%) 273-282
The impact of disturbance on detrital dynamics and soil microbial
biomass of a Pinus kesiyyaforest in north-east India
A. Arunachalam *, Kusum Maithani, H.N. Pandey, R.S. Tripathi
Deparhnent
of Botany, North-Eastern
Hill University,
Shillong
793 022, India
Accepted 28 March 19%
Abstract
Detrital dynamics and microbial nutrient flux due to disturbances such as treefall and tree cutting were studied in a
subtropical Pinus kesiya Ro$e Ex. Gordon forest in north-east India. Disturbance has substantially altered community
structure, and therefore soil nutrient status. Natural gap formation has not resulted in significant changes in dry matter, C and
N accumulation in litter and fine roots, or in microbial nutrient concentrations. However, there was a significant reduction in
all functional parameters in the selectively logged site and soil heap. Soil microbial C, N and P were maximum in the
understorey and minimum in the heap. Fine roots and microbial biomass contributed more to nutrient recycling in the
ecosystem. N-mineralization
was generally higher in the disturbed sites.
Keyworrls: Disturbance; Fine roots; Litter; Microbial biomass; N-mineralization; Pinus kesiya
1. Introduction
The structure and function of plant communities
in terrestrial ecosystems are largely determined by
disturbances (Armesto and Pickett, 1985). Studies on
forest clearing suggest that net loss of soil organic
matter occurs with additional disturbance to soil or
with long-term removal of forest canopy. Natural
gap formation in the forest canopy due to single or
multiple treefalls also perturbs the productivity and
Corresponding author. Present address: Lecturer in Forestry,
Department of Applied Sciences, North Bastern Regional Institute
of Science and Technology. Nirjuli-791109, Arunachal Pradesh,
INDIA. Tel: (0360) 47434. Fax: (0360) 44302, 44307. E-mail:
forest @ nerist.emet.in.
l
0378- 1127/%/$15.00
P/I SO378-1127(96)03801-7
nutrient cycling patterns of a given forest
(Chandrashekara and Ramakrishnan, 1994). Recovery of the disrupted nutrient cycling in a degraded
ecosystem is closely linked with vegetation regrowth. This enhances input of organic matter and
nutrients to top soil through litter and detrital root
mass, helps nutrient conservation by reducing losses,
and increases nutrient availability by favourably altering the hydrology and physico-chemical and biological properties of the soil. Singh et al. (1989)
reported that soil microorganisms also play an important role in nutrient conservation in terrestrial
ecosystems.
In north-east India, tree cutting and shifting agriculture have resulted in the conversion of primary
broadleaved forest into several seral communities.
Copyright 0 1996 Elsevier Science B.V. All rights reserved.
274
A. Arunachalam
et al./ Forest
Ecolo,qy and Manugement
Pinus kesiyu, a rare species of the native forest,
grows well in degraded sites, resulting in the formation of monospecific secondary forests, especially at
higher altitudes (800-2000 m above sea level (a.s.1.)).
Pine is utilized for timber, and urbanization in the
region has also contributed to forest clearing. The
objectives of this study were to assess the impact of
natural gap formation and tree cutting on the detrital
and microbial nutrient dynamics of a Pinus kesiyu
forest.
2. Study area
The study was conducted in a Pinus kesiyu Royle
Ex. Gordon forest at Shillong (latitude 91”56’E, longitude 25”34’N, altitude 1500 m a.s.l.1, the capital of
Meghalaya, India.
The climate is monsoonal with an average annual
rainfall of 2500 mm, 85% of which occurs during
mid-May to September (rainy season). Autumn is in
October and November, a transitional period between rainy and winter seasons. The winter season
(December to February) is characterised by low temperatures (mean minimum 3°C; mean maximum
16°C) and occasional frost. March to mid-May
(spring season) is warmer, with average maximum
and minimum temperatures of 23°C and 16”C, respectively.
The soil is lateritic (oxisol), sandy loam and
slightly acidic (pH 5.5-6.3).
The forest is 22 years old and covers approximately 50 ha. The only tree species in the forest is
Pinus kesiya. The biomass of P. kesiya is 331.44 Mg
ha- ’ , calculated from allometric relationships obtained by Das and Ramakrishnan (1987). There are
neither shrubs nor tree saplings in the forest. The
dense ground layer is dominated by Eupatorium
adenophorum, Luntana camera and grasses such as
Imperata cylindrica and Arundinella benghalensis.
A large number of pine seedlings were also found.
There is a heavy growth of epiphytic lichens, mosses
and ferns.
An experimental area of about 10 ha was surveyed to locate treefall gaps in the forest. A gap was
considered as an “opening in the forest extending
down through all foliage levels to an average height
of 2 m above ground” (Brokaw, 1982). Three gaps
88 (19961273-282
(average area 263.3 m2) originating from multiple
treefails (not less than three) were identified, and the
investigation was carried out in those gaps only. In
the gaps, seedlings of Pinus kesiya, and grasses such
as lmperata cylindrica and Arundinellu benghalensis dominated the forest floor. However, species such
as Rubus ellipticus, Osbeckia stellata and Lantana
camera were also present.
Nearby a portion of the forest was selectively
logged for building construction during September
1993. As a consequence, patches of uncut pine trees
(biomass 160.3 Mg ha-’ 1 were interspersed in the
area. A few cut-stumps were left behind in between
the uncut trees The ground vegetation was dominated by Imperata
cylindrica,
Eupatorium
adenophorum, Luntana camera and ferns. A dense
growth of tree seedlings was observed. Three such
patches (average area 289 m*) were also selected for
the present study.
The logging also resulted in whole tree harvesting
and total removal of top soil. This soil was dumped
aside in heaps, which occupied about 5-10% of the
total forest area. These heaps are now mostly dominated by pine seedlings. A few young dicotyledonous species such as Eupatorium adenophurum,
Lantana camera etc. were also present.
Three plots (25 m X 25 ml of understorey with a
dense canopy were presumed to be undisturbed (UD),
three gaps formed by treefalls were presumed to be
slightly disturbed (SD), three sites disturbed by selective logging were presumed to be moderately
disturbed (MD), and three soil heaps, representing
extreme stages of soil degradation, were presumed to
be highly disturbed (HD) sites. These gradients were
based on the intensity, size and duration of the
disturbances.
3. Methods
3.1. Vegetation analysis
In each replicated site, the vegetation was analyzed during October 1994 in 5-10 randomly placed
10 m X 10 m quadrats for trees and in 1 m X 1 m
quadrats for herbaceous vegetation. Nomenclature of
plant species follows Hooker (1872- 1897). Density,
A. Arunachnlam
et al./
Forest
Ecology
frequency and basal area were determined according
to Misra (1968).
3.2. Forest floor litter, root and soil sampling and
analysis
and
Management
88 (1996)
275
273-282
cedures and the buried bag technique, as outlined by
Anderson and Ingram (1993). The data presented are
all the means of the three replicated plots in each
site.
3.3. Statistical analysis
Litter samples were collected from each plot during October 1994, from the 1 m X 1 m quadrats used
for the ground vegetation analysis. Soil cores (diameter 6.5 cm) were sampled from all these quadrats at
O-10 cm depth for the determination of physicochemical properties, fine-root biomass and microbial
nutrients. Polythene bags containing soil cores from
0- 10 cm depth were also buried in these quadrats for
in situ N-mineralization.
The litter, root and soil samples were brought to
the laboratory in polythene bags and stored at 4°C.
The soil samples from each site (i.e. three plots)
were bulked, air-dried and used for the determination
of texture, pH, water-holding capacity (WHC), organic-C, total Kjeldahl nitrogen (TKN) and available-P following standard procedures (Allen et al.,
1974). The litter samples were grouped into leaf
(pine needles and leaves of monocots and dicots) and
miscellaneous (woody litter of < 20 mm diameter,
bark and reproductive organs) fractions, oven-dried
at 80°C and weighed. The roots were retrieved from
the soil cores following wet-sieving (Bohm, 1979)
and roots of < 2 mm diameter (fine roots) only were
considered. The fine roots were classified into < 1
mm (finer roots) and 1-2 mm diameter classes. Live
roots (biomass) were distinguished from dead roots
(necromass) by visual and textural characteristics
(Persson, 1983; Arunachalam et al., 1996). The fine
roots were washed with a gentle flow of tap water to
overcome soil contamination, oven-dried at 80°C and
weighed.
Ash content was determined by igniting the
oven-dried material at 550°C for 6 h in a muffle
furnace. Carbon (C) content was calculated as 50%
of the ash-free mass (Allen et al., 1974). Total
Kjeldahl nitrogen (TKN) was estimated in the litter
and root samples using a Kjeltec Auto 1030 Analyser. The carbon and nitrogen contents of litter and
fine roots were calculated by multiplying the dry
mass by their respective concentrations. Microbial
biomass-C, -N and -P and in situ N-mineralization
were estimated following fumigation-extraction
pro-
The data were analyzed using ANOVA (fixed
effects model). Linear regressions were also used
where necessary, according to Zar (1974). Tukey’s
test was used to compare the mean values across the
sites.
4. Results
4.1. Microenvironment
and soil nutrient status
Light intensity was significantly higher in disturbed plots than in the understorey. Air and soil
temperatures were significantly lower in the understorey (UD) and gaps (SD) than in the selectively
logged plots (MD) and soil heaps (I-ID), where they
were almost the same (Table 1).
Bulk density, water-holding capacity (WHC) and
moisture content (SMC) of the soil showed a decreasing trend, and pH showed an increasing trend
with increasing degrees of disturbance. The concentrations of organic-C and TKN in soil were highest
in the understorey and lowest in the soil heap (Table
2). The C/N ratio in soil was highest (11.6) for the
understorey and lowest (6.3) for the selectively
logged plot. There was a significant decline in the
level of soil ammonium-N in the MD and I-ID plots,
Table 1
Microenvironmental
variability
( f SE) in the study sites (n = 9)
Variables
UD
SD
MD
HD
Light intensity
(lux x 100)
Air temperature
Soil temperature
Soil moisture
content (%o)
30af2
236bf10
242bf3
250bk5
(“C)
PC)
23 a f 1
19 a f 1
2ga+3
25’+2
21ak2
27=f2
29b+l
26bjz2
lgb+l
29bkl
26b*2
16b*l
UD, understorey;
SD, gap; MD, cut-tree stand; HD, soil heap.
Within
each row, means with the same superscripts
are not
significantly
different at P < 0.05.
276
A. Arunachalam
et al./ Forest Ecology
Table 2
Physico-chemical properties (k SE) of soil samples from the
undmtorey aud disturbed plots of a Pinus kesiya forest (n = 9)
Properties
UD
SD
MD
HD
SL
SL
SL
SL
Soil textural class
Bulk density (g cm3)
1.12 a 1.01 a 0.99 a 0.92 a
i 0.09 f 0.02 kO.01 + 0.02
WHC (o/o)
55.12 * 55.01 a 35.19 b 30.62 b
i 1.12
5.5 a
kO.2
2.78 a
rtO.09
0.24 a
k2.31
5.8 a
*0.1
1.98 a
kO.23
0.20 =
Nitrate-N (p,g g- ’ )
+0.65
6.45 a
+0.07
+ 1.32
6.65 =
Available-P (p,g g- ’ )
5.95 a
PH
Organic-C (%)
+ 1.11
5.9a
*0.1
0.74 =
50.1 I
0.12 a
+ 4.39
6.3 a
kO.0
1.04a
rto.01
0.10 a
*0.01
i:O.Ol + 0.01 kO.01
C/N
11.58 a 10.10 a 6.27 a 10.83 it
Ammonimum-N (kg g- ’ ) 16.55 a 16.44 a 6.64 b 7.05 b
TKN (%)
kO.13
kO.01
6.12 a
*0.15
& 1.17
5.0 a
I!z0.1
5.37 a
f 0.02
and Management
88 119961273-282
Table 3
Density of trees (ha- ’ ) and ground vegetation &h#sX lou0
ha-’ ) in the understorey CUD), gaps (SD), selectively logged
plots (MD) and soil heaps (HD) in a Pinus kesiya forest ecosystem
Vegetation component
UD
SD
MD
HD
Pinus kesiya
Monocots
Dicots
2233 a
(77.3)
22.4 a
(0.44)
22.6 =
(0.4)
12.0 b
(0.2)
6.7 b
(0.1)
1080”
-
(37.4)
8.9 b
--
(0.2)
4.1 ’
(0.1)
3.2 ’
(0.1)
Values in parentheses are basal area (m* ha- ‘).
Mean values with different superscripts within a row are significantly different at P < 0.05 between sites.
-. no vegetation recorded.
f 0.99
6.05 a
zko.01
4.76 a
+ 0.03
UD, understorey; SD, gap; MD, cut-tree stand; HD, soil heap; SL,
sandy loam.
Mean values with the same superscripts across sites are not
significantly different at P < 0.05.
as compared with the UD and SD plots. Concentrations of nitrate-N and available-P were maximum in
the gaps, but differences between the understorey
and the gaps were not significant.
4.2. Community characteristics
and in the gaps (101320 ha-’ 1. In the understorey
and soil heaps it was 56000 and 64000 ha-‘, respectively.
4.3. Biomass, C and N contents of forest-floor litrer
The mass of forest-floor litter was reduced significantly by disturbances (Fig. 1). Litter mass was
maximum (1.5 Mg ha-’ 1 in the understorey and
minimum in the soil heaps (0.02 Mg ha-’ >. Pine
needles contributed 32-758 of the total litter mass
in all sites. Both monocotyledonous and dicotyledonous species contributed significantly (5-25%) to
The total basal area of P. kesiya (the only tree
species) was 77 m2 ha - ’ in the undisturbed forest
and 37 m* ha-’ in the selectively logged plot. The
total tree density was maximum in the understorey
and minimum in the selectively logged plot; no trees
were observed in the gaps and soil heaps (Table 3).
Monocotyledons such as Imperata cylindrica and
Arundinella benghulensiswere dominant in the gaps
and selectively logged plots. However, in the understorey, dicotyledons such as Eupatorium adenophorum, Lantana camera, Rubus ellipticus, Ambrosia
urtimissifolia and Osbeckia stellata contributed
equally with monocotyledons to the total density of
herbaceous vegetation. In soil heaps, only sparsely
distributed shade-intolerant dicotyledonous species
were present. The density of pine seedlings was
highest in the selectively logged plots (113 530 ha- ’ )
Fig. 1. Forest floor litter mass in understorey KJDJ, gaps (SD),
cut-tree stands (MD) and heaps (HD). Open bIocks, pine needles;
diagonal shading, dicotykdon leaves; solid blocks, monocotyledon leaves; vertical shadhtg, miscellaneous fraction. vertie&baFs
represent the standard error (n = 30).
A. Arunachalam
et al./ Forest Ecology
Table 4
Concentration
(%) of nitrogen in litter samples from
storey and three disturbed plots in the pine forest
Litter fraction
Leaf litter
Pine needles
UD
SD
MD
1.23 a
(14.19
")
0.97 a
(4.77 b,
0.91 a
(6.91b)
0.37 a
(Oxsa)
0.84'
(0.34')
Monocots
Dicots
Miscellaneous
;iY3aa,
1.29 a
0.79
(0.41a)
(0.12
0.70 a
(2.00 ")
(5.28
0.75
a
0.93
“1
a
b,
88 (19%)
273-282
277
the underHD
a
1.20
a
(0.14
-
')
0.91
(0.04 “)
0.90
-
a
(l-2mm)
a
(0.11 “)
(2.66
and Management
UD
MD
HD
Fig. 2. Fine-root
mass ( < 1 mm or l-2 mm in diameter)
in the
understorey
(UD), gaps (SD), cut-tree stands (MD)
and heaps
(HD). Open blocks, biomass; diagonal shading, necromass. Vertical bars represent the standard error (n = 30).
"'1
UD, understorey;
SD, gap; MD, cut-tree stand; HD, soil heap.
Values in parentheses are the nitrogen content (kg ha- ’ ).
Within a row, values with different superscripts are significant
P < 0.05.
-, no litter recorded.
SD
at
total litter accumulation. The miscellaneous fraction
of the litter was 46% of the total litter mass in the
gap, 27% in the selectively logged plot and 19% in
the understorey. The carbon content of the forest
floor litter was highest in the understorey and lowest
in the soil heaps (see Table 7). The contribution of
the litter to the total stock of C in the soil was two to
three times greater in the selectively logged plot
compared with the understorey and the gaps.
N concentration was higher in the leaf litter components of the understorey than those of the other
three disturbed plots (Table 4). N stock was maximum in the understorey (17.33 kg ha-‘) and minimum in the soil heaps (0.18 kg ha-’ ). N content in
the leaf litter was higher than that of the miscellaneous fraction in all the study sites (Table 4). However, the contribution of litter to total N capital in the
soil was insignificant.
to 0.7. The total fine-root mass (5.12-0.71 Mg ha-’ )
also followed the trend of the BM:NM ratio. Finer
roots contributed 43-100% of the total fine root
mass, while the roots in the l-2 mm diameter class
contributed up to 57% only. The fine roots accumulated 21.8% of the total soil C stock in the selectively logged plots, followed by gaps (10.2%), understorey (7.3%) and soil heaps (3.5%).
N concentration in the fine roots from different
sites did not show any definite trend (Table 5), but
was generally greater (0.6-l%) in the necromass
than the biomass (0.4-0.8%).
Table 5
Nitrogen concentration
(%) in fme roots in the understorey
and three disturbed plots (SD, MD, HD) in the pine forest
Fine root
fraction.
UD
< 1 mm diameter
Biomass
0.82
MD
a
")
0.62
(3.6
a
b,
1.06 a
0.62
(9.9
a
b)
0.35
(5.7
a
b)
(8.5
Necromass
SD
(12.1')
0.75
CUD)
HD
0.77
a
(8.9 '7
0.77 a
(10.5 "'1
a
(2.3
0.83
(3.4
bd)
a
d)
0.63
a
-
(1.9 '1
1.04 a
(5.6 "1
-
4.4. Biomass, C and N contents of$ne roots
Fine-root biomass was maximum in the understorey (3.4 Mg ha-‘) and minimum in the soil heaps
(0.3 Mg ha-’ ). Generally, necromass was greater in
the disturbed plots than the forest understorey (Fig.
2). An average of 86% of the necromass was in finer
roots (< 1 mm diameter). The biomass (BM) to
necromass (NM) ratio of fine roots showed a decreasing trend (i.e. UD > SD > MD > HD) from 1.9
l-2 mm diameter
Biomass
0.39
Necromass
a
(9.0 ")
0.64a
(3.9
"1
0.78 a
(5.6 "1
UD, understorey;
SD, gap; MD, cut-tree stand; HD, soil heap.
Values in parentheses am nitrogen content (kg ha- ’ 1.
Values in the same row with different superscripts ate not signiticant at P < 0.05.
-, no fme roots recorded.
A, Arunachalam
278
et al./ Forest
Ecology
Table 6
Microbial C, N and P concentrations (p,g g- ’ ) ( jI SE) in soils at
the study sites (n = 9)
Parameter
UD
SD
MD
HD
MB-C
MB-N
MB-P
294.4 a
+ 9.3
118.3 a
rto.9
19.6 a
+0.2
MB-C/MB-N
MB-C/MB-P
2.5 a
15.0 a
287.9 a
*3.1
104.6 b
f0.3
8.2 b
dco.3
2.8 ’
35.1 b
126.3 b
+ 3.9
72.3 ’
*0.1
5.5b
t- 0.9
1.8 b
23.1 ’
55.7 c
+4.1
35.6 d
50.1
4.9b
+ 0.7
1.6 b
11.4 ad
UD, understorey; SD, gap; MD, cut-tree stand; HD, soil heap.
Mean values in the same row with the same superscripts are not
significant at P < 0.05.
4.5. Microbial C, N and P
Concentrations of microbial C (B,), N (BN 1 and
P (B,) in soil were maximum in the understorey and
minimum in the soil heaps (Table 6). The difference
between B,, B, and UD, SD was not significant,
while the differences between UD and MD, and UD
and HD were significant (P < 0.05). C/N ratio in
Table 7
C and N stocks (kg ha-‘) in soil, litter, fine roots and microbial
biomass in a disturbed Pinus ksiva forest
Nutrient/category
UD
SD
MD
HD
Carbon
Soil
Litter
Roots
31136a
732 a
(2.4)
2268 a
(7.3)
330 a
(1.1)
19998 b
695 b
(3.5)
2039 b
(10.2)
291b
(1.5)
7326 ’
516’
(7.1)
1595 c
(21.8)
125’
(1.7)
9568 d
7d
(0.001)
338 d
(3.5)
d
&05,
and Management
88 (1996)
273-282
the microbial biomass of the understorey and gaps
was significantly higher than that in the selectively
logged plots and soil heaps. C/P ratio did not show
a similar trend: the highest ratio was obtained in the
gaps and the lowest in the soil heaps. B, represented
1.1-I .7% of the total soil organic-C stock in the
understorey, gaps and selectively logged plots. The
contribution of B, in the soil heaps was negligible.
The contribution of B, to total soil N content was
more or less the same in the understorey and gaps,
the highest contribution was in the selectively logged
plots and the lowest in the soil heaps (Table 7).
4.6. In situ N-mineralization
Ammonium-N and nitrate-N in the soil increased
significantly (P < 0.01) after 30 days of field incubation. Ammonification rate ranged from 0.2 to 0.4
pg g-’ day-’ and nitrification rate ranged from 0.1
to 0.2 p,g g- ’ day- ‘. The net N-mineralization
varied significantly between the understorey and the
other three disturbed sites (Table 8). pH did not
show any significant variation, but the soil moisture
content declined significantly (P < 0.01) after 30
days of incubation. However, the reduction was much
more pronounced in soils from the disturbed sites
compared with the forest understorey.
Table 8
pH, SMC (%), concentrations (pg g-‘) of ammonium-N and
nitrate-N after 30 days of field incubation and N-mineraliiation
rate (pg g-’ day- ‘) in soils from the disturbed sites and understorey in the pine forest
Parameter
UD
SD
MD
HD
PH
5.8 a
6.0 a
6.0 a
6.5 a
k0.7
Nitrogen
Soil
Litter
2688 a
zo606)
1188 c
b
b
920 d
c
&06~
td.m,
GoO2~
b
b
Roots
Microbes
2020 b
z,
132 =
(4.9)
(:5.2,
106b
(5.2)
Fi.3,
0.1 a
0.1 a
19.9 a
f 1.2
Ammonium-N
22.4 a
* 1.3
Nitrate-N
10.6 a
Net N-mineralization rate
Ammonification rate
Nitrification rate
0.3 =
(41Ioo6,
UD, tmderstorey; SD, gap; MD, cut-tree stand; HD, soil heap.
Values in parentheses are percentages of total soil C and N stocks.
Different superscripts in the same row indicate that means differ
at P < 0.05.
0.2 a
f 0.0
17.9 a
f 0.9
28.1 b
* 2.2
9.8 a
+0.2
0.5 a
0.4 =
SMC
It 0.6
to.2
10.0 b
5 1.0
18.7 ’
k2.1
9.6 a
i:O.2
0.6 if
0.4 a
0.2 a
f0.1
8.8 b
i0.3
16.2 ’
i3.4
12.0 =
+ 1.3
0.5 a
0.3 =
0.2 *
UD, understorey; SD, gap; MD, cut-tree stand; HD, soil heap.
Values are mean * standard error (n = 9). In each row, differentsuperscripts indicate that the means differ at P < 0.05.
A. Arunachalam
et al./Forest
Ecology
88 (1996)
273-282
279
5.2. Litter dynamics
5. Discussion
5.1. Microenvironmental
and Management
variability,
soil and vegeta-
tion
There were a number of changes in the physicochemical characteristics of the soil subsequent to
different intensities of disturbance, caused by microenvironmental changes in the sites. The reduction
in the soil moisture content in gaps, selectively
logged plots and soil heaps as compared with the
forest understorey (see Table I> could partly be due
to increased light intensity and soil temperature, and
in part to a decline in the bulk density and waterholding capacity of the soil. Soil organic-C and
nutrient concentrations (see Table 2) are less than
those reported for natural humid subtropical
broadleaved forests of the region (Arunachalam et
al., 1994; Maithani et al., 1996), and this is attributable to the species composition. The soil fertility of the pine forest did not decline significantly due
to natural gap formation, but selective logging (stem
harvesting) and soil degradation (whole tree harvesting) have significantly altered the soil nutrient status.
This could be due to excessive erosion/leaching of
top soil as the area receives high rainfall. Similar
findings have also been reported by Chandrashekara
and Ramakrishnan (1994) in a humid tropical forest
in Kerala, India.
Community characteristics such as density and
basal area showed marked differences among the
four sites (see Table 3) due to discernable variation
in soil conditions and biotic influences. The forest
understorey, with its optimum soil nutrient status and
microenvironment, has favoured the growth and regular distribution of both dicots and monocots. Generally, the disturbed sites were dominated by the
monocots, as only they could withstand the prevailing high light intensity and soil temperature. Tree
seedling recruitment was very pronounced in the
gaps and selectively logged plots, and very low in
the understorey. This is attributable to the light
availability and soil moisture regimes, as influenced
by canopy opening. Competition for various resources among seedlings and herbaceous species,
especially for light and space in the relatively spacelimited understorey, might have hampered successful
recruitment of seedlings (Whitmore, 1984) and therefore seedling density.
The standing crop of litter in the undisturbed pine
forest (1.5 Mg ha- ’ ) was well below the reported
range (2.2-22.6 Mg ha-’ ) for various tropical and
subtropical forests (Vogt et al., 19861, possibly because it was sampled just after the rainy season when
the decomposition rate is at its peak (Maithani et al.,
1996). Meentemeyer et al. (1982) reported that leaf
litter accounted 70% of the total litter mass on the
forest floor. In the present study, the leaf litter
averaged ca. 77% of the total litter mass. In the soil
heaps, leaf litter was the only fraction, of which 25%
was contributed by the incited dicotyledonous species
and tree seedlings, and the rest was from the nearby
pine forest as an external input into the system.
Relatively higher miscellaneous litter (46%) in the
gaps (Fig. 1) could be due to the accumulation of
branchfall or treefall residues.
Leaf litter had higher N concentration (see Table
4) than the miscellaneous litter. This is in agreement
with the observations of Gosz et al. (1972) and
Arunachalam et al. (1994) that perennial tissues have
lower concentrations of nutrients, especially N. N
content in litter generally reflected the trends in the
litter mass. The least accumulation of N in soil heaps
through litter was due to a very low plant biomass.
5.3. Fine root dynamics
The mean standing crop of fine roots in the
understorey is very close to the value reported from
a 7-year-old subtropical forest regrowth dominated
by P. kesiya (Arunachalam et al., 19961, but more
than those reported from Picea sitchensis (3.5 Mg
ha- ’ ) and Pinus sylvestris (1.8-4.6 Mg ha- ’ ) forests
in temperate regions (Deans, 1981; Persson, 1983).
Despite these variations, the values obtained from
understorey, gaps and selectively logged plots of the
pine forest (3.4-5.1 Mg ha-’ ) were within the reported range (1.0-17.7 Mg ha-’ ) of fine root mass
for various forest ecosystems of the world (Vogt et
al., 1986). The least standing crop of 0.7 Mg ha-’
obtained from the soil heaps was close to that of a
l-year-old stand of a premontane wet forest (0.9 Mg
ha-‘) in Costa Rica (Berish, 1982). It has been
reported that low availability of water and nutrients
promote high production and accumulation af fine
280
A. Arunachdam
et al./Forest
Ecology
roots (Vogt et al., 1986). However, our results do not
agree with these findings, since root mass decreased
as the WHC, organic-C and TKN decreased in soil,
along a disturbance gradient. We have also reported
similar findings from a disturbed humid subtropical
broadleaved forest of this region (Arunachalam et
al., 1996).
Nambiar (1987) reported that nutrients are not
retranslocated from fine roots before senescence. We
agree with this report because the N concentration in
biomass and necromass of the finer roots (< 1 mm
diameter class) did not vary significantly. However,
this was not the case with roots of l-2 mm diameter,
where the variation was significant (P < 0.05). The
reduction in N concentration with the increase in
root diameter simply reflects the increase of woody
tissues which usually contains fewer nutrients
(Khiewtam and Ramakrishnan, 1993). Decline in
elemental concentrations in fine root biomass could
also be related to soil nutrient status. These results
are in contrast with those of Coults and Philipson
(1976), who found that nutrients could be internally
translocated in roots, resulting in similar concentrations throughout the root system.
5.4. Microbial
nutrient dynamics
Methodologies to quantify Bc in soil are still
controversial. Our results for Bc (56-294 P,g g- ’ ),
obtained following fumigation-extraction
procedures, were lower than the values reported by
Arunachalam et al. (1994) for soils of a disturbed
climax forest (1040-1532 pg g-l), but well within
the lower range of the reported values for various
terrestrial ecosystems (61-1900 pg g- ‘) (Vance et
al., 1987; Srivastava and Singh, 1988). There was a
significant positive relationship between soil organic
matter and Bc (r = 0.903, P < 0.05). A relatively
higher stand density and soil organic matter, and
greater accumulation of litter and fine roots could
have favoured the growth of microbial populations,
and therefore Bc in the understorey.
The dynamics of N in the mineral soil is intimately linked with that of C, because most of the N
exists in organic compounds and heterotrophic microbes, which utilize organic-c for energy. As a
result, B, showed a positive correlation with Bc
(r = 0.958, P < 0.01). Joergensen et al. (1995) re-
and Management
88 (19961273-282
ported that above soil pH 5.0.. microbial C/N ratios
vary within a narrow range. Our results fully corroborate this point. The microbial C/N ratios (see Table
6) were smaller compared with those in most of the
literature. This could be related to low soil organic-C.
Joergensen et al. (1995) also suggested that forest
soils with comparatively low C and N availability
may give microbial C/N ratios which are well below the optimum values. i.e. 5-8. However, soilspecific differences in growth form and survival
strategy are also important in regulating the C/N
ratio in microbial biomass. According to the hypothesis of Bremer and van Kessel (1992), the microbial
biomass of our study sites was dormant because of a
very small microbial C/N ratio.
The values obtained for B, (4.9-19.6 Pg gee! i
were very low as compared with other reported
values for grasslands and woodlands (12.0-67.2 p-g
g- ’ ) (Brookes et al., 1984) but were comparable to
those of arable lands (5.3-27.5 pg gg ‘). In the soils
studied, the contribution of B, to available-P content
in soil varied between 1.4 and 4.7%. Brookes et al.
(1984) measured B, and B, contents in soils of
grasslands, pastures and woodlands and found them
to be positively correlated. Our results in the pine
forest soils are also in accordance with that study.
The microbial C/P ratios (see Table 6) were well
within those reported by Brookes et al. (1984). The
significant increase in microbial C/P ratios in the
gaps and tree-cut plots is due to a relatively sharper
decline in microbial P.
5.5. N-mineralization
pattern
The increase in the concentrations of ammoniumN and nitrate-N in the field-incubated soils reflects
the ammonification and nitrification processes. Ammonification rate was higher than nit&?&on
rate in
all the four sites studied. Similar findings have also
been reported for tropical forests by several other
workers (Schimel and Patton, 1986; Singh et al.,
1991). The net N-mineralization rates (see Table 8)
of the subtropical pine forest soils were within the
range reported by Singh et al. (1991) for soils from
tropical India (0.007-0.767 p,g g- ’ day- ’ ). The low
N-mineralization
rate in soils of the understorey
could be due to rapid immobilization of the mineralized nitrogen by the microbes as well as plant roots.
A. Arunachalam
et al./Forest
Ecology
This could be a N-conserving mechanism by which
the nutrient loss is checked. On the other hand, rapid
mineralization of N in the disturbed plots signals
potential loss of available N from the system, owing
to a little dense vegetation. This might have resulted
in a lower concentration of ammonium-N in soils of
the tree-cut stand and soil heaps.
5.6. Relative importance of litter, fine roots and
microbes in C and N dynamics
Fine roots play more important role in soil C
dynamics than the litter and microbial biomass. On
the other hand, microbial biomass contributes much
to the soil N pool compared with the two detrital
fractions (see Table 7). The insignificant role of
these three biological processes in the soil heap
compared with the understorey indicates the extremity of soil degradation, and the site may not be easily
restored in a short time. An increase in the contribution of litter, fine roots and microbial biomass to C
and N pools in soils of the slightly and mildly
disturbed plots is a measure of nutrient retention on
the forest floor, in spite of reduced soil organic C
and total N stocks, and would thus sustain the future
uptake and productivity of the regrowing forest community.
6. Conclusions
The present study concludes that the community
structure, dynamics of litter, fine roots and microbial
biomass, and N-mineralization
have been substantially altered due to perturbations. Natural gap formation did not alter the soil nutrient status, microbial
nutrients and detrital stock significantly when compared with the understorey. Stem harvesting through
selective logging and soil degradation by whole-tree
harvesting have exposed the soil to direct insolation
and excessive leaching; thereby the soil fertility level
declined. Fine-root mass and microbial C, N and P
declined with the decrease in water-holding capacity,
concentrations of organic-C, total nitrogen and available-P in the soil. It was also concluded that fine
roots play a crucial role in C-cycling, and microbial
biomass in N-cycling, of the disturbed pine forest.
This indicates the dynamic nature of C and N circu-
and Management
88 (1996)
273-282
281
lation on the forest floor, as these two biological
processes have a rapid turnover, and thus the fine
roots and microbes are important for nutrient conservation and maintenance of the disturbed secondary
forest. It is therefore suggested that the dynamics of
fine roots on the forest floor in relation to microbial
biomass and nutrient mineralization should receive
more attention from ecologists and forest managers.
Based on the ecology of secondary forest succession and nutrient cycling processes, the natural and
human-impacted Pinus kesiya forest ecosystem can
be sustained for at least a fairly high level of productivity by adopting appropriate forest management
practices and also through the planting, in gaps and
selectively logged plots, of selected tree species
which would conserve nutrients and have low nutrient-use efficiency. Although difficult in practice,
microbial management can also help in restoring the
soil fertility, especially N-status, through synchronization of nutrient mineralization and its uptake by
plants, resulting in fairly high-level productivity of
the Pinus kesiya forest.
Acknowledgements
Special recognition is given to Dr. A.K. Das and
Professor P.S. Ramakrishnan, TSBF, SARNET, for
providing us with the TSBF Methodology Book. The
authors thank CSIR, Government of India, for financial assistance. The first author is grateful to Professor K.B. Misra, Director, NERIST, for providing the
necessary facilities. The authors thank the two
anonymous referees for their useful comments for
improvement of the paper.
References
Allen, LE., Grimshaw, H.M., Parkinson, J.A. and Quamby, C.,
1974. Chemical Analysis of Ecological Materials. Blackwell
Scientific, Oxford, 565 pp.
Anderson, J.M. aad Ingram, J.S.I., 1993. Tropical Soil Biology
and Fertility: A Handbook of Methods. CAB International,
Wallingford, UK.
Armesto, J.J. and Pickett, S.T.A., 1985. Experiments on disturbance in old-field plant communities: Impact on species richness and abundance. Ecology, 66: 230-240.
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Arunachalam, A., Boral, L. and Maithani, K., 1994. Effects of
ground-fire on nutrient contents in soil and litter in a subtropical forest of Meghalaya. J. Hill Res., 7: 13- 16.
Arunachalam, A., Pandey, H.N., Ttipathi, R.S. and Maithani, K..
1996. Biomass and production of fme and coarse roots during
regrowth of a disturbed subtropical humid forest in north-east
India. Vegetatio, 123: 73-80.
Berish, C.W., 1982. Root biomass and surface area in three
successional tropical forests. Can. J. For. Res., 12: 699-704.
Bohm, W., 1979. Methods of Studying Root System. Springer,
Berlin, Heidelberg, New York.
Btemer, E. and van Kessel, C., 1992. Seasonal and microbial
biomass dynamics after addition of lentil and wheat residues.
Soil. Sci. Sot. Am. J., 56: 1141-1146.
Brokaw, N.V.L., 1982. The defmition of treefall gap and its effect
on measures of forest dynamics. Biotropica, 11: 158- 160.
Brookes, PC., Powlson, D.S. and Jenkinson, D.S., 1984. Phosphorus in the soil microbial biomass. Soil Biol. B&hem., 16:
169-175.
Chandrashekara, U.M. and Ramakrlshnan, P.S., 1994. Successional patterns and gap phase dynamics of a humid tropical
forest of the Western Ghats of Kerala, India: Ground vegetation, biomass, productivity and nutrient cycling. For. Ecol.
Manage., 70: 23-40.
Coults, M.P. and Philipson, J.J., 1976. The influence of mineral
nutrition on the root development of trees, I. The growth of
Sitka spruce with divided root systems. J. Exp. Bot., 27:
1102-1111.
Das, A.K. and Ramakrishnan, P.S., 1987. Above-ground biomass
and nutrient contents in an age series of khasi pine (firms
kesiya). For. Ecol. Manage., 18: 61-72.
Deans, J.D., 1981. Dynamics of coarse root production in a young
plantation of Picea sitchensis. Forestry, 54: 139- 155.
Gosz, J.R., Liens, GE. and Bormann, F.H., 1972. Nutrient
coment of litterfall on the Hubbard Brook Experimental Forest, New Hampshire. Ecology, 53: 679-684.
Hooker, J.D., 1872-1897. The Flora of British India. 7 Vols.
London.
Joergensen, R.G., Anderson, T.H. and Walters, T., 1995. Carbon
and nitrogen relationships in the microbial biomass of soils in
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beech (Fagus syluatica) forests. Biol. Fertil. Soils, 19: 141147.
Khiewtam, R.S. and Ramakrishnan, P.S., 1993. Litter and fine
root dynamics of a relict sacred grove forest at Cherraptmi in
north-eastern India. For. Ecol. Manage., 60: 327-344.
Maithani, K., Arunachalam. A., Pandey, H.N. and Tripathi, R.S.,
1996. Dry matter and nutrient dynamics of litter during forest
regrowth in humid subtropics. Ecologia, 15: in press.
Meentemeyer, V., Box, E.O. and Thompson, R., 1982. World
pattern and amounts of terrestrial plant litter production. Bioscience, 32: l25- 128.
Misra, R., 1968. Ecology Work Book. Oxford and IBH, Calcutta.
Nambiar, E.K.S., 1987. Do nutrients retranslocate from fine rooti?.
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Persson, H., 1983. The distribution and productivity of fine roots
in boreal forests. Plant Soil, 71: 87-101.
Schimel, D.S. and Parton, W.J., 1986. Micrcclimatic controls of
nitrogen mineralization and nitrification in short steppe soils.
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Singh, J.S., Raghuvanshi, A.S., Singh, R.S. and Srivastava, S.C.,
1989. Microbial biomass acts as a source of plant nutrients in
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mineralization in dry tropical savanna: Effects of burning and
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Cliffs. NJ.
Management
ELSEVIER
Forest Ecology and Management 88 (19%) 273-282
The impact of disturbance on detrital dynamics and soil microbial
biomass of a Pinus kesiyyaforest in north-east India
A. Arunachalam *, Kusum Maithani, H.N. Pandey, R.S. Tripathi
Deparhnent
of Botany, North-Eastern
Hill University,
Shillong
793 022, India
Accepted 28 March 19%
Abstract
Detrital dynamics and microbial nutrient flux due to disturbances such as treefall and tree cutting were studied in a
subtropical Pinus kesiya Ro$e Ex. Gordon forest in north-east India. Disturbance has substantially altered community
structure, and therefore soil nutrient status. Natural gap formation has not resulted in significant changes in dry matter, C and
N accumulation in litter and fine roots, or in microbial nutrient concentrations. However, there was a significant reduction in
all functional parameters in the selectively logged site and soil heap. Soil microbial C, N and P were maximum in the
understorey and minimum in the heap. Fine roots and microbial biomass contributed more to nutrient recycling in the
ecosystem. N-mineralization
was generally higher in the disturbed sites.
Keyworrls: Disturbance; Fine roots; Litter; Microbial biomass; N-mineralization; Pinus kesiya
1. Introduction
The structure and function of plant communities
in terrestrial ecosystems are largely determined by
disturbances (Armesto and Pickett, 1985). Studies on
forest clearing suggest that net loss of soil organic
matter occurs with additional disturbance to soil or
with long-term removal of forest canopy. Natural
gap formation in the forest canopy due to single or
multiple treefalls also perturbs the productivity and
Corresponding author. Present address: Lecturer in Forestry,
Department of Applied Sciences, North Bastern Regional Institute
of Science and Technology. Nirjuli-791109, Arunachal Pradesh,
INDIA. Tel: (0360) 47434. Fax: (0360) 44302, 44307. E-mail:
forest @ nerist.emet.in.
l
0378- 1127/%/$15.00
P/I SO378-1127(96)03801-7
nutrient cycling patterns of a given forest
(Chandrashekara and Ramakrishnan, 1994). Recovery of the disrupted nutrient cycling in a degraded
ecosystem is closely linked with vegetation regrowth. This enhances input of organic matter and
nutrients to top soil through litter and detrital root
mass, helps nutrient conservation by reducing losses,
and increases nutrient availability by favourably altering the hydrology and physico-chemical and biological properties of the soil. Singh et al. (1989)
reported that soil microorganisms also play an important role in nutrient conservation in terrestrial
ecosystems.
In north-east India, tree cutting and shifting agriculture have resulted in the conversion of primary
broadleaved forest into several seral communities.
Copyright 0 1996 Elsevier Science B.V. All rights reserved.
274
A. Arunachalam
et al./ Forest
Ecolo,qy and Manugement
Pinus kesiyu, a rare species of the native forest,
grows well in degraded sites, resulting in the formation of monospecific secondary forests, especially at
higher altitudes (800-2000 m above sea level (a.s.1.)).
Pine is utilized for timber, and urbanization in the
region has also contributed to forest clearing. The
objectives of this study were to assess the impact of
natural gap formation and tree cutting on the detrital
and microbial nutrient dynamics of a Pinus kesiyu
forest.
2. Study area
The study was conducted in a Pinus kesiyu Royle
Ex. Gordon forest at Shillong (latitude 91”56’E, longitude 25”34’N, altitude 1500 m a.s.l.1, the capital of
Meghalaya, India.
The climate is monsoonal with an average annual
rainfall of 2500 mm, 85% of which occurs during
mid-May to September (rainy season). Autumn is in
October and November, a transitional period between rainy and winter seasons. The winter season
(December to February) is characterised by low temperatures (mean minimum 3°C; mean maximum
16°C) and occasional frost. March to mid-May
(spring season) is warmer, with average maximum
and minimum temperatures of 23°C and 16”C, respectively.
The soil is lateritic (oxisol), sandy loam and
slightly acidic (pH 5.5-6.3).
The forest is 22 years old and covers approximately 50 ha. The only tree species in the forest is
Pinus kesiya. The biomass of P. kesiya is 331.44 Mg
ha- ’ , calculated from allometric relationships obtained by Das and Ramakrishnan (1987). There are
neither shrubs nor tree saplings in the forest. The
dense ground layer is dominated by Eupatorium
adenophorum, Luntana camera and grasses such as
Imperata cylindrica and Arundinella benghalensis.
A large number of pine seedlings were also found.
There is a heavy growth of epiphytic lichens, mosses
and ferns.
An experimental area of about 10 ha was surveyed to locate treefall gaps in the forest. A gap was
considered as an “opening in the forest extending
down through all foliage levels to an average height
of 2 m above ground” (Brokaw, 1982). Three gaps
88 (19961273-282
(average area 263.3 m2) originating from multiple
treefails (not less than three) were identified, and the
investigation was carried out in those gaps only. In
the gaps, seedlings of Pinus kesiya, and grasses such
as lmperata cylindrica and Arundinellu benghalensis dominated the forest floor. However, species such
as Rubus ellipticus, Osbeckia stellata and Lantana
camera were also present.
Nearby a portion of the forest was selectively
logged for building construction during September
1993. As a consequence, patches of uncut pine trees
(biomass 160.3 Mg ha-’ 1 were interspersed in the
area. A few cut-stumps were left behind in between
the uncut trees The ground vegetation was dominated by Imperata
cylindrica,
Eupatorium
adenophorum, Luntana camera and ferns. A dense
growth of tree seedlings was observed. Three such
patches (average area 289 m*) were also selected for
the present study.
The logging also resulted in whole tree harvesting
and total removal of top soil. This soil was dumped
aside in heaps, which occupied about 5-10% of the
total forest area. These heaps are now mostly dominated by pine seedlings. A few young dicotyledonous species such as Eupatorium adenophurum,
Lantana camera etc. were also present.
Three plots (25 m X 25 ml of understorey with a
dense canopy were presumed to be undisturbed (UD),
three gaps formed by treefalls were presumed to be
slightly disturbed (SD), three sites disturbed by selective logging were presumed to be moderately
disturbed (MD), and three soil heaps, representing
extreme stages of soil degradation, were presumed to
be highly disturbed (HD) sites. These gradients were
based on the intensity, size and duration of the
disturbances.
3. Methods
3.1. Vegetation analysis
In each replicated site, the vegetation was analyzed during October 1994 in 5-10 randomly placed
10 m X 10 m quadrats for trees and in 1 m X 1 m
quadrats for herbaceous vegetation. Nomenclature of
plant species follows Hooker (1872- 1897). Density,
A. Arunachnlam
et al./
Forest
Ecology
frequency and basal area were determined according
to Misra (1968).
3.2. Forest floor litter, root and soil sampling and
analysis
and
Management
88 (1996)
275
273-282
cedures and the buried bag technique, as outlined by
Anderson and Ingram (1993). The data presented are
all the means of the three replicated plots in each
site.
3.3. Statistical analysis
Litter samples were collected from each plot during October 1994, from the 1 m X 1 m quadrats used
for the ground vegetation analysis. Soil cores (diameter 6.5 cm) were sampled from all these quadrats at
O-10 cm depth for the determination of physicochemical properties, fine-root biomass and microbial
nutrients. Polythene bags containing soil cores from
0- 10 cm depth were also buried in these quadrats for
in situ N-mineralization.
The litter, root and soil samples were brought to
the laboratory in polythene bags and stored at 4°C.
The soil samples from each site (i.e. three plots)
were bulked, air-dried and used for the determination
of texture, pH, water-holding capacity (WHC), organic-C, total Kjeldahl nitrogen (TKN) and available-P following standard procedures (Allen et al.,
1974). The litter samples were grouped into leaf
(pine needles and leaves of monocots and dicots) and
miscellaneous (woody litter of < 20 mm diameter,
bark and reproductive organs) fractions, oven-dried
at 80°C and weighed. The roots were retrieved from
the soil cores following wet-sieving (Bohm, 1979)
and roots of < 2 mm diameter (fine roots) only were
considered. The fine roots were classified into < 1
mm (finer roots) and 1-2 mm diameter classes. Live
roots (biomass) were distinguished from dead roots
(necromass) by visual and textural characteristics
(Persson, 1983; Arunachalam et al., 1996). The fine
roots were washed with a gentle flow of tap water to
overcome soil contamination, oven-dried at 80°C and
weighed.
Ash content was determined by igniting the
oven-dried material at 550°C for 6 h in a muffle
furnace. Carbon (C) content was calculated as 50%
of the ash-free mass (Allen et al., 1974). Total
Kjeldahl nitrogen (TKN) was estimated in the litter
and root samples using a Kjeltec Auto 1030 Analyser. The carbon and nitrogen contents of litter and
fine roots were calculated by multiplying the dry
mass by their respective concentrations. Microbial
biomass-C, -N and -P and in situ N-mineralization
were estimated following fumigation-extraction
pro-
The data were analyzed using ANOVA (fixed
effects model). Linear regressions were also used
where necessary, according to Zar (1974). Tukey’s
test was used to compare the mean values across the
sites.
4. Results
4.1. Microenvironment
and soil nutrient status
Light intensity was significantly higher in disturbed plots than in the understorey. Air and soil
temperatures were significantly lower in the understorey (UD) and gaps (SD) than in the selectively
logged plots (MD) and soil heaps (I-ID), where they
were almost the same (Table 1).
Bulk density, water-holding capacity (WHC) and
moisture content (SMC) of the soil showed a decreasing trend, and pH showed an increasing trend
with increasing degrees of disturbance. The concentrations of organic-C and TKN in soil were highest
in the understorey and lowest in the soil heap (Table
2). The C/N ratio in soil was highest (11.6) for the
understorey and lowest (6.3) for the selectively
logged plot. There was a significant decline in the
level of soil ammonium-N in the MD and I-ID plots,
Table 1
Microenvironmental
variability
( f SE) in the study sites (n = 9)
Variables
UD
SD
MD
HD
Light intensity
(lux x 100)
Air temperature
Soil temperature
Soil moisture
content (%o)
30af2
236bf10
242bf3
250bk5
(“C)
PC)
23 a f 1
19 a f 1
2ga+3
25’+2
21ak2
27=f2
29b+l
26bjz2
lgb+l
29bkl
26b*2
16b*l
UD, understorey;
SD, gap; MD, cut-tree stand; HD, soil heap.
Within
each row, means with the same superscripts
are not
significantly
different at P < 0.05.
276
A. Arunachalam
et al./ Forest Ecology
Table 2
Physico-chemical properties (k SE) of soil samples from the
undmtorey aud disturbed plots of a Pinus kesiya forest (n = 9)
Properties
UD
SD
MD
HD
SL
SL
SL
SL
Soil textural class
Bulk density (g cm3)
1.12 a 1.01 a 0.99 a 0.92 a
i 0.09 f 0.02 kO.01 + 0.02
WHC (o/o)
55.12 * 55.01 a 35.19 b 30.62 b
i 1.12
5.5 a
kO.2
2.78 a
rtO.09
0.24 a
k2.31
5.8 a
*0.1
1.98 a
kO.23
0.20 =
Nitrate-N (p,g g- ’ )
+0.65
6.45 a
+0.07
+ 1.32
6.65 =
Available-P (p,g g- ’ )
5.95 a
PH
Organic-C (%)
+ 1.11
5.9a
*0.1
0.74 =
50.1 I
0.12 a
+ 4.39
6.3 a
kO.0
1.04a
rto.01
0.10 a
*0.01
i:O.Ol + 0.01 kO.01
C/N
11.58 a 10.10 a 6.27 a 10.83 it
Ammonimum-N (kg g- ’ ) 16.55 a 16.44 a 6.64 b 7.05 b
TKN (%)
kO.13
kO.01
6.12 a
*0.15
& 1.17
5.0 a
I!z0.1
5.37 a
f 0.02
and Management
88 119961273-282
Table 3
Density of trees (ha- ’ ) and ground vegetation &h#sX lou0
ha-’ ) in the understorey CUD), gaps (SD), selectively logged
plots (MD) and soil heaps (HD) in a Pinus kesiya forest ecosystem
Vegetation component
UD
SD
MD
HD
Pinus kesiya
Monocots
Dicots
2233 a
(77.3)
22.4 a
(0.44)
22.6 =
(0.4)
12.0 b
(0.2)
6.7 b
(0.1)
1080”
-
(37.4)
8.9 b
--
(0.2)
4.1 ’
(0.1)
3.2 ’
(0.1)
Values in parentheses are basal area (m* ha- ‘).
Mean values with different superscripts within a row are significantly different at P < 0.05 between sites.
-. no vegetation recorded.
f 0.99
6.05 a
zko.01
4.76 a
+ 0.03
UD, understorey; SD, gap; MD, cut-tree stand; HD, soil heap; SL,
sandy loam.
Mean values with the same superscripts across sites are not
significantly different at P < 0.05.
as compared with the UD and SD plots. Concentrations of nitrate-N and available-P were maximum in
the gaps, but differences between the understorey
and the gaps were not significant.
4.2. Community characteristics
and in the gaps (101320 ha-’ 1. In the understorey
and soil heaps it was 56000 and 64000 ha-‘, respectively.
4.3. Biomass, C and N contents of forest-floor litrer
The mass of forest-floor litter was reduced significantly by disturbances (Fig. 1). Litter mass was
maximum (1.5 Mg ha-’ 1 in the understorey and
minimum in the soil heaps (0.02 Mg ha-’ >. Pine
needles contributed 32-758 of the total litter mass
in all sites. Both monocotyledonous and dicotyledonous species contributed significantly (5-25%) to
The total basal area of P. kesiya (the only tree
species) was 77 m2 ha - ’ in the undisturbed forest
and 37 m* ha-’ in the selectively logged plot. The
total tree density was maximum in the understorey
and minimum in the selectively logged plot; no trees
were observed in the gaps and soil heaps (Table 3).
Monocotyledons such as Imperata cylindrica and
Arundinella benghulensiswere dominant in the gaps
and selectively logged plots. However, in the understorey, dicotyledons such as Eupatorium adenophorum, Lantana camera, Rubus ellipticus, Ambrosia
urtimissifolia and Osbeckia stellata contributed
equally with monocotyledons to the total density of
herbaceous vegetation. In soil heaps, only sparsely
distributed shade-intolerant dicotyledonous species
were present. The density of pine seedlings was
highest in the selectively logged plots (113 530 ha- ’ )
Fig. 1. Forest floor litter mass in understorey KJDJ, gaps (SD),
cut-tree stands (MD) and heaps (HD). Open bIocks, pine needles;
diagonal shading, dicotykdon leaves; solid blocks, monocotyledon leaves; vertical shadhtg, miscellaneous fraction. vertie&baFs
represent the standard error (n = 30).
A. Arunachalam
et al./ Forest Ecology
Table 4
Concentration
(%) of nitrogen in litter samples from
storey and three disturbed plots in the pine forest
Litter fraction
Leaf litter
Pine needles
UD
SD
MD
1.23 a
(14.19
")
0.97 a
(4.77 b,
0.91 a
(6.91b)
0.37 a
(Oxsa)
0.84'
(0.34')
Monocots
Dicots
Miscellaneous
;iY3aa,
1.29 a
0.79
(0.41a)
(0.12
0.70 a
(2.00 ")
(5.28
0.75
a
0.93
“1
a
b,
88 (19%)
273-282
277
the underHD
a
1.20
a
(0.14
-
')
0.91
(0.04 “)
0.90
-
a
(l-2mm)
a
(0.11 “)
(2.66
and Management
UD
MD
HD
Fig. 2. Fine-root
mass ( < 1 mm or l-2 mm in diameter)
in the
understorey
(UD), gaps (SD), cut-tree stands (MD)
and heaps
(HD). Open blocks, biomass; diagonal shading, necromass. Vertical bars represent the standard error (n = 30).
"'1
UD, understorey;
SD, gap; MD, cut-tree stand; HD, soil heap.
Values in parentheses are the nitrogen content (kg ha- ’ ).
Within a row, values with different superscripts are significant
P < 0.05.
-, no litter recorded.
SD
at
total litter accumulation. The miscellaneous fraction
of the litter was 46% of the total litter mass in the
gap, 27% in the selectively logged plot and 19% in
the understorey. The carbon content of the forest
floor litter was highest in the understorey and lowest
in the soil heaps (see Table 7). The contribution of
the litter to the total stock of C in the soil was two to
three times greater in the selectively logged plot
compared with the understorey and the gaps.
N concentration was higher in the leaf litter components of the understorey than those of the other
three disturbed plots (Table 4). N stock was maximum in the understorey (17.33 kg ha-‘) and minimum in the soil heaps (0.18 kg ha-’ ). N content in
the leaf litter was higher than that of the miscellaneous fraction in all the study sites (Table 4). However, the contribution of litter to total N capital in the
soil was insignificant.
to 0.7. The total fine-root mass (5.12-0.71 Mg ha-’ )
also followed the trend of the BM:NM ratio. Finer
roots contributed 43-100% of the total fine root
mass, while the roots in the l-2 mm diameter class
contributed up to 57% only. The fine roots accumulated 21.8% of the total soil C stock in the selectively logged plots, followed by gaps (10.2%), understorey (7.3%) and soil heaps (3.5%).
N concentration in the fine roots from different
sites did not show any definite trend (Table 5), but
was generally greater (0.6-l%) in the necromass
than the biomass (0.4-0.8%).
Table 5
Nitrogen concentration
(%) in fme roots in the understorey
and three disturbed plots (SD, MD, HD) in the pine forest
Fine root
fraction.
UD
< 1 mm diameter
Biomass
0.82
MD
a
")
0.62
(3.6
a
b,
1.06 a
0.62
(9.9
a
b)
0.35
(5.7
a
b)
(8.5
Necromass
SD
(12.1')
0.75
CUD)
HD
0.77
a
(8.9 '7
0.77 a
(10.5 "'1
a
(2.3
0.83
(3.4
bd)
a
d)
0.63
a
-
(1.9 '1
1.04 a
(5.6 "1
-
4.4. Biomass, C and N contents of$ne roots
Fine-root biomass was maximum in the understorey (3.4 Mg ha-‘) and minimum in the soil heaps
(0.3 Mg ha-’ ). Generally, necromass was greater in
the disturbed plots than the forest understorey (Fig.
2). An average of 86% of the necromass was in finer
roots (< 1 mm diameter). The biomass (BM) to
necromass (NM) ratio of fine roots showed a decreasing trend (i.e. UD > SD > MD > HD) from 1.9
l-2 mm diameter
Biomass
0.39
Necromass
a
(9.0 ")
0.64a
(3.9
"1
0.78 a
(5.6 "1
UD, understorey;
SD, gap; MD, cut-tree stand; HD, soil heap.
Values in parentheses am nitrogen content (kg ha- ’ 1.
Values in the same row with different superscripts ate not signiticant at P < 0.05.
-, no fme roots recorded.
A, Arunachalam
278
et al./ Forest
Ecology
Table 6
Microbial C, N and P concentrations (p,g g- ’ ) ( jI SE) in soils at
the study sites (n = 9)
Parameter
UD
SD
MD
HD
MB-C
MB-N
MB-P
294.4 a
+ 9.3
118.3 a
rto.9
19.6 a
+0.2
MB-C/MB-N
MB-C/MB-P
2.5 a
15.0 a
287.9 a
*3.1
104.6 b
f0.3
8.2 b
dco.3
2.8 ’
35.1 b
126.3 b
+ 3.9
72.3 ’
*0.1
5.5b
t- 0.9
1.8 b
23.1 ’
55.7 c
+4.1
35.6 d
50.1
4.9b
+ 0.7
1.6 b
11.4 ad
UD, understorey; SD, gap; MD, cut-tree stand; HD, soil heap.
Mean values in the same row with the same superscripts are not
significant at P < 0.05.
4.5. Microbial C, N and P
Concentrations of microbial C (B,), N (BN 1 and
P (B,) in soil were maximum in the understorey and
minimum in the soil heaps (Table 6). The difference
between B,, B, and UD, SD was not significant,
while the differences between UD and MD, and UD
and HD were significant (P < 0.05). C/N ratio in
Table 7
C and N stocks (kg ha-‘) in soil, litter, fine roots and microbial
biomass in a disturbed Pinus ksiva forest
Nutrient/category
UD
SD
MD
HD
Carbon
Soil
Litter
Roots
31136a
732 a
(2.4)
2268 a
(7.3)
330 a
(1.1)
19998 b
695 b
(3.5)
2039 b
(10.2)
291b
(1.5)
7326 ’
516’
(7.1)
1595 c
(21.8)
125’
(1.7)
9568 d
7d
(0.001)
338 d
(3.5)
d
&05,
and Management
88 (1996)
273-282
the microbial biomass of the understorey and gaps
was significantly higher than that in the selectively
logged plots and soil heaps. C/P ratio did not show
a similar trend: the highest ratio was obtained in the
gaps and the lowest in the soil heaps. B, represented
1.1-I .7% of the total soil organic-C stock in the
understorey, gaps and selectively logged plots. The
contribution of B, in the soil heaps was negligible.
The contribution of B, to total soil N content was
more or less the same in the understorey and gaps,
the highest contribution was in the selectively logged
plots and the lowest in the soil heaps (Table 7).
4.6. In situ N-mineralization
Ammonium-N and nitrate-N in the soil increased
significantly (P < 0.01) after 30 days of field incubation. Ammonification rate ranged from 0.2 to 0.4
pg g-’ day-’ and nitrification rate ranged from 0.1
to 0.2 p,g g- ’ day- ‘. The net N-mineralization
varied significantly between the understorey and the
other three disturbed sites (Table 8). pH did not
show any significant variation, but the soil moisture
content declined significantly (P < 0.01) after 30
days of incubation. However, the reduction was much
more pronounced in soils from the disturbed sites
compared with the forest understorey.
Table 8
pH, SMC (%), concentrations (pg g-‘) of ammonium-N and
nitrate-N after 30 days of field incubation and N-mineraliiation
rate (pg g-’ day- ‘) in soils from the disturbed sites and understorey in the pine forest
Parameter
UD
SD
MD
HD
PH
5.8 a
6.0 a
6.0 a
6.5 a
k0.7
Nitrogen
Soil
Litter
2688 a
zo606)
1188 c
b
b
920 d
c
&06~
td.m,
GoO2~
b
b
Roots
Microbes
2020 b
z,
132 =
(4.9)
(:5.2,
106b
(5.2)
Fi.3,
0.1 a
0.1 a
19.9 a
f 1.2
Ammonium-N
22.4 a
* 1.3
Nitrate-N
10.6 a
Net N-mineralization rate
Ammonification rate
Nitrification rate
0.3 =
(41Ioo6,
UD, tmderstorey; SD, gap; MD, cut-tree stand; HD, soil heap.
Values in parentheses are percentages of total soil C and N stocks.
Different superscripts in the same row indicate that means differ
at P < 0.05.
0.2 a
f 0.0
17.9 a
f 0.9
28.1 b
* 2.2
9.8 a
+0.2
0.5 a
0.4 =
SMC
It 0.6
to.2
10.0 b
5 1.0
18.7 ’
k2.1
9.6 a
i:O.2
0.6 if
0.4 a
0.2 a
f0.1
8.8 b
i0.3
16.2 ’
i3.4
12.0 =
+ 1.3
0.5 a
0.3 =
0.2 *
UD, understorey; SD, gap; MD, cut-tree stand; HD, soil heap.
Values are mean * standard error (n = 9). In each row, differentsuperscripts indicate that the means differ at P < 0.05.
A. Arunachalam
et al./Forest
Ecology
88 (1996)
273-282
279
5.2. Litter dynamics
5. Discussion
5.1. Microenvironmental
and Management
variability,
soil and vegeta-
tion
There were a number of changes in the physicochemical characteristics of the soil subsequent to
different intensities of disturbance, caused by microenvironmental changes in the sites. The reduction
in the soil moisture content in gaps, selectively
logged plots and soil heaps as compared with the
forest understorey (see Table I> could partly be due
to increased light intensity and soil temperature, and
in part to a decline in the bulk density and waterholding capacity of the soil. Soil organic-C and
nutrient concentrations (see Table 2) are less than
those reported for natural humid subtropical
broadleaved forests of the region (Arunachalam et
al., 1994; Maithani et al., 1996), and this is attributable to the species composition. The soil fertility of the pine forest did not decline significantly due
to natural gap formation, but selective logging (stem
harvesting) and soil degradation (whole tree harvesting) have significantly altered the soil nutrient status.
This could be due to excessive erosion/leaching of
top soil as the area receives high rainfall. Similar
findings have also been reported by Chandrashekara
and Ramakrishnan (1994) in a humid tropical forest
in Kerala, India.
Community characteristics such as density and
basal area showed marked differences among the
four sites (see Table 3) due to discernable variation
in soil conditions and biotic influences. The forest
understorey, with its optimum soil nutrient status and
microenvironment, has favoured the growth and regular distribution of both dicots and monocots. Generally, the disturbed sites were dominated by the
monocots, as only they could withstand the prevailing high light intensity and soil temperature. Tree
seedling recruitment was very pronounced in the
gaps and selectively logged plots, and very low in
the understorey. This is attributable to the light
availability and soil moisture regimes, as influenced
by canopy opening. Competition for various resources among seedlings and herbaceous species,
especially for light and space in the relatively spacelimited understorey, might have hampered successful
recruitment of seedlings (Whitmore, 1984) and therefore seedling density.
The standing crop of litter in the undisturbed pine
forest (1.5 Mg ha- ’ ) was well below the reported
range (2.2-22.6 Mg ha-’ ) for various tropical and
subtropical forests (Vogt et al., 19861, possibly because it was sampled just after the rainy season when
the decomposition rate is at its peak (Maithani et al.,
1996). Meentemeyer et al. (1982) reported that leaf
litter accounted 70% of the total litter mass on the
forest floor. In the present study, the leaf litter
averaged ca. 77% of the total litter mass. In the soil
heaps, leaf litter was the only fraction, of which 25%
was contributed by the incited dicotyledonous species
and tree seedlings, and the rest was from the nearby
pine forest as an external input into the system.
Relatively higher miscellaneous litter (46%) in the
gaps (Fig. 1) could be due to the accumulation of
branchfall or treefall residues.
Leaf litter had higher N concentration (see Table
4) than the miscellaneous litter. This is in agreement
with the observations of Gosz et al. (1972) and
Arunachalam et al. (1994) that perennial tissues have
lower concentrations of nutrients, especially N. N
content in litter generally reflected the trends in the
litter mass. The least accumulation of N in soil heaps
through litter was due to a very low plant biomass.
5.3. Fine root dynamics
The mean standing crop of fine roots in the
understorey is very close to the value reported from
a 7-year-old subtropical forest regrowth dominated
by P. kesiya (Arunachalam et al., 19961, but more
than those reported from Picea sitchensis (3.5 Mg
ha- ’ ) and Pinus sylvestris (1.8-4.6 Mg ha- ’ ) forests
in temperate regions (Deans, 1981; Persson, 1983).
Despite these variations, the values obtained from
understorey, gaps and selectively logged plots of the
pine forest (3.4-5.1 Mg ha-’ ) were within the reported range (1.0-17.7 Mg ha-’ ) of fine root mass
for various forest ecosystems of the world (Vogt et
al., 1986). The least standing crop of 0.7 Mg ha-’
obtained from the soil heaps was close to that of a
l-year-old stand of a premontane wet forest (0.9 Mg
ha-‘) in Costa Rica (Berish, 1982). It has been
reported that low availability of water and nutrients
promote high production and accumulation af fine
280
A. Arunachdam
et al./Forest
Ecology
roots (Vogt et al., 1986). However, our results do not
agree with these findings, since root mass decreased
as the WHC, organic-C and TKN decreased in soil,
along a disturbance gradient. We have also reported
similar findings from a disturbed humid subtropical
broadleaved forest of this region (Arunachalam et
al., 1996).
Nambiar (1987) reported that nutrients are not
retranslocated from fine roots before senescence. We
agree with this report because the N concentration in
biomass and necromass of the finer roots (< 1 mm
diameter class) did not vary significantly. However,
this was not the case with roots of l-2 mm diameter,
where the variation was significant (P < 0.05). The
reduction in N concentration with the increase in
root diameter simply reflects the increase of woody
tissues which usually contains fewer nutrients
(Khiewtam and Ramakrishnan, 1993). Decline in
elemental concentrations in fine root biomass could
also be related to soil nutrient status. These results
are in contrast with those of Coults and Philipson
(1976), who found that nutrients could be internally
translocated in roots, resulting in similar concentrations throughout the root system.
5.4. Microbial
nutrient dynamics
Methodologies to quantify Bc in soil are still
controversial. Our results for Bc (56-294 P,g g- ’ ),
obtained following fumigation-extraction
procedures, were lower than the values reported by
Arunachalam et al. (1994) for soils of a disturbed
climax forest (1040-1532 pg g-l), but well within
the lower range of the reported values for various
terrestrial ecosystems (61-1900 pg g- ‘) (Vance et
al., 1987; Srivastava and Singh, 1988). There was a
significant positive relationship between soil organic
matter and Bc (r = 0.903, P < 0.05). A relatively
higher stand density and soil organic matter, and
greater accumulation of litter and fine roots could
have favoured the growth of microbial populations,
and therefore Bc in the understorey.
The dynamics of N in the mineral soil is intimately linked with that of C, because most of the N
exists in organic compounds and heterotrophic microbes, which utilize organic-c for energy. As a
result, B, showed a positive correlation with Bc
(r = 0.958, P < 0.01). Joergensen et al. (1995) re-
and Management
88 (19961273-282
ported that above soil pH 5.0.. microbial C/N ratios
vary within a narrow range. Our results fully corroborate this point. The microbial C/N ratios (see Table
6) were smaller compared with those in most of the
literature. This could be related to low soil organic-C.
Joergensen et al. (1995) also suggested that forest
soils with comparatively low C and N availability
may give microbial C/N ratios which are well below the optimum values. i.e. 5-8. However, soilspecific differences in growth form and survival
strategy are also important in regulating the C/N
ratio in microbial biomass. According to the hypothesis of Bremer and van Kessel (1992), the microbial
biomass of our study sites was dormant because of a
very small microbial C/N ratio.
The values obtained for B, (4.9-19.6 Pg gee! i
were very low as compared with other reported
values for grasslands and woodlands (12.0-67.2 p-g
g- ’ ) (Brookes et al., 1984) but were comparable to
those of arable lands (5.3-27.5 pg gg ‘). In the soils
studied, the contribution of B, to available-P content
in soil varied between 1.4 and 4.7%. Brookes et al.
(1984) measured B, and B, contents in soils of
grasslands, pastures and woodlands and found them
to be positively correlated. Our results in the pine
forest soils are also in accordance with that study.
The microbial C/P ratios (see Table 6) were well
within those reported by Brookes et al. (1984). The
significant increase in microbial C/P ratios in the
gaps and tree-cut plots is due to a relatively sharper
decline in microbial P.
5.5. N-mineralization
pattern
The increase in the concentrations of ammoniumN and nitrate-N in the field-incubated soils reflects
the ammonification and nitrification processes. Ammonification rate was higher than nit&?&on
rate in
all the four sites studied. Similar findings have also
been reported for tropical forests by several other
workers (Schimel and Patton, 1986; Singh et al.,
1991). The net N-mineralization rates (see Table 8)
of the subtropical pine forest soils were within the
range reported by Singh et al. (1991) for soils from
tropical India (0.007-0.767 p,g g- ’ day- ’ ). The low
N-mineralization
rate in soils of the understorey
could be due to rapid immobilization of the mineralized nitrogen by the microbes as well as plant roots.
A. Arunachalam
et al./Forest
Ecology
This could be a N-conserving mechanism by which
the nutrient loss is checked. On the other hand, rapid
mineralization of N in the disturbed plots signals
potential loss of available N from the system, owing
to a little dense vegetation. This might have resulted
in a lower concentration of ammonium-N in soils of
the tree-cut stand and soil heaps.
5.6. Relative importance of litter, fine roots and
microbes in C and N dynamics
Fine roots play more important role in soil C
dynamics than the litter and microbial biomass. On
the other hand, microbial biomass contributes much
to the soil N pool compared with the two detrital
fractions (see Table 7). The insignificant role of
these three biological processes in the soil heap
compared with the understorey indicates the extremity of soil degradation, and the site may not be easily
restored in a short time. An increase in the contribution of litter, fine roots and microbial biomass to C
and N pools in soils of the slightly and mildly
disturbed plots is a measure of nutrient retention on
the forest floor, in spite of reduced soil organic C
and total N stocks, and would thus sustain the future
uptake and productivity of the regrowing forest community.
6. Conclusions
The present study concludes that the community
structure, dynamics of litter, fine roots and microbial
biomass, and N-mineralization
have been substantially altered due to perturbations. Natural gap formation did not alter the soil nutrient status, microbial
nutrients and detrital stock significantly when compared with the understorey. Stem harvesting through
selective logging and soil degradation by whole-tree
harvesting have exposed the soil to direct insolation
and excessive leaching; thereby the soil fertility level
declined. Fine-root mass and microbial C, N and P
declined with the decrease in water-holding capacity,
concentrations of organic-C, total nitrogen and available-P in the soil. It was also concluded that fine
roots play a crucial role in C-cycling, and microbial
biomass in N-cycling, of the disturbed pine forest.
This indicates the dynamic nature of C and N circu-
and Management
88 (1996)
273-282
281
lation on the forest floor, as these two biological
processes have a rapid turnover, and thus the fine
roots and microbes are important for nutrient conservation and maintenance of the disturbed secondary
forest. It is therefore suggested that the dynamics of
fine roots on the forest floor in relation to microbial
biomass and nutrient mineralization should receive
more attention from ecologists and forest managers.
Based on the ecology of secondary forest succession and nutrient cycling processes, the natural and
human-impacted Pinus kesiya forest ecosystem can
be sustained for at least a fairly high level of productivity by adopting appropriate forest management
practices and also through the planting, in gaps and
selectively logged plots, of selected tree species
which would conserve nutrients and have low nutrient-use efficiency. Although difficult in practice,
microbial management can also help in restoring the
soil fertility, especially N-status, through synchronization of nutrient mineralization and its uptake by
plants, resulting in fairly high-level productivity of
the Pinus kesiya forest.
Acknowledgements
Special recognition is given to Dr. A.K. Das and
Professor P.S. Ramakrishnan, TSBF, SARNET, for
providing us with the TSBF Methodology Book. The
authors thank CSIR, Government of India, for financial assistance. The first author is grateful to Professor K.B. Misra, Director, NERIST, for providing the
necessary facilities. The authors thank the two
anonymous referees for their useful comments for
improvement of the paper.
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