Influence of gap size and soil propertie
Plant and Soil 223: 185–193, 2000.
© 2000 Kluwer Academic Publishers. Printed in the Netherlands.
185
Influence of gap size and soil properties on microbial biomass in a
subtropical humid forest of north-east India
A. Arunachalam∗ and Kusum Arunachalam
Restoration Ecology Laboratory, North Eastern Regional Institute of Science and Technology, Nirjuli-791109,
Arunachal Pradesh, India
Received 10 August 1999. Accepted in revised form 19 April 2000
Key words: microclimate, microbial biomass, soil, subtropical humid forest, treefall gaps, understorey
Abstract
We examined the effects of treefall gap size and soil properties on microbial biomass dynamics in an undisturbed
mature-phase humid subtropical broadleaved forest in north-east India. Canopy gaps had low soil moisture and low
microbial biomass suggesting that belowground dynamics accompanied changes in light resources after canopy
opening. High rainfall in the region causes excessive erosion/leaching of top soil and eventually soil fertility declines in treefall gaps compared to understorey. Soil microbial population was less during periods when temperature
and moisture conditions are low, while it peaked during rainy season when the litter decomposition rate is at its peak
on the forest floor. Greater demand for nutrients by plants during rainy season (the peak vegetative growth period)
limited the availability of nutrients to soil microbes and, therefore, low microbial C, N and P. Weak correlations
were also obtained for the relationships between microbial C, N and P and soil physico–chemical properties. Gap
size did influence the microbial nutrients and their contribution to soil organic carbon, total Kjeldhal nitrogen
and available-P. Contribution of microbial C to soil organic carbon, microbial N to total nitrogen were similar in
both treefall gaps and understorey plots, while the contribution of microbial P to soil available-P was lower in
gap compared to the understorey. These results indicate that any fluctuation in microbial biomass related nutrient
cycling processes in conjunction with the associated microclimate variation may affect the pattern of regeneration
of tree seedlings in the gaps and hence be related with their size.
Introduction
Natural treefalls are the major small-scale disturbances in mature forests in the humid tropical and
subtropical regions, and the gaps created in the forest
canopy by this event are an important factor in maintaining the high species diversity by increasing environmental heterogeneity and altering abundances and
distribution of abiotic and biotic resources (Clark,
1990; Connell, 1978; Denslow, 1987). Most studies
of gaps have addressed vegetation dynamics, regeneration through seedling establishment, effect of microclimatic variables on the revegetation, etc. and in
general have concentrated on aboveground processes
(see Meer et al., 1994). Relatively few studies have
∗ FAX No: 360244307. E-mail: aa@nerist.ernet.in
addressed belowground effects of canopy gaps such
as soil-related aspects and their effects on the revegetation process after disturbance (Arunachalam et al.,
1996).
The mass of litter and fine roots on the forest
floor is generally less under canopy gaps than more
in closed canopy site (Bariket al., 1992; Ostertag,
1998), while Sanford Jr. (1989) observed no difference
in fine root biomass between canopy gap and understorey. Earlier, we also reported insignificant changes
in dry matter, carbon (C) and nitrogen (N) accumulation in litter and fine roots and in microbial biomass
between natural gaps and understorey plots of a Pinus
kesiya Royle ex. Gordon forest in north-eastern India
(Arunachalam et al., 1996). Studies on soil microbial biomass, a ‘sink’ and ‘source’ of plant nutrients
in treefall gaps of tropical forests have not attracted
186
adequate attention of foresters and ecologists. Nevertheless, there have been a few studies evaluating the
dynamics of this important component in varieties of
ecosystems exposed to varied disturbances (e.g. Arunachalam et al., 1994; Hernot and Robertson, 1994;
Maithani et al., 1996). The present study was designed
(1) to determine the effects of canopy gaps on microbial carbon (C), -nitrogen (N) and -phosphorus (P),
and (2) whether or not the consequences of canopy
gap formation are dependent on soil properties, and
(3) to evaluate the effects of gap size on soil and microbial nutrient dynamics in a ‘sacred grove’, the climax
vegetation of the humid subtropics of north-east India.
Methods
Study area
The study was conducted in a ‘sacred grove’ (subtropical wet hill forest) at Mawphlang (1700 m
asl), 30 km south-east of Shillong (25◦ 34′ N,
91◦ 56′ E) in Meghalaya, north-eastern India. The
climate of the area is monsoonic with an average
annual rainfall of 2500 mm, distributed over five
months (May–September) of the year. The periods
October–November, December–February and March–
April represent autumn, winter and spring seasons,
respectively. The mean minimum and maximum air
temperatures during the study period were 3 ◦ and 16 ◦
C. The soils derived from underlying gneisses, schists
and granites and is grouped under the latosol (oxisol)
type (Pascoe, 1950).
The forest stand covers an area of about 150 ha.
It has been left undisturbed due to religious believes
of the local people and represents the climax vegetation of the area. Gap formation is frequent in this
forest mainly by uprooted trees (Rao et al., 1990).
The forest canopy is dominated by Quercus griffithii Miquel, Lithocarpus dealbatus (Miquel) Rehder
(=Quercus dealbata), Quercus glauca Thumb. Bl.
and Schima khasiana Dyer, while Symplocos chinensis (Lour) Druce and Daphne shillong Banerjee are
the main shrub components. The diameter at breast
height (DBH) of the trees in the forest ranged from
10 to 60 cm. In a study, Rao et al. (1990) reported
16 tree species, 16 shrubs and 28 herbaceous species
in this forest. The density of tree species was 1050
stems ha−1 , while for shrubs it was plants 560 ha−1
and 274 000 plants ha−1 for herbs. There is a heavy
growth of large number of species of epiphytic orchids, mosses, ferns and lianes in the forest. Further
details may be obtained from Rao et al. (1990, 1997)
and Barik et al. (1992).
Gap identification and experimental layout
The gap was considered as an opening in the forest
extending down through all foliage levels to an average height of 2 m above the ground (Brokaw, 1982).
Barik et al. (1992) reported that the distribution of
20–100 m2 gaps were more frequent in this forest.
About 58% of the total number of gaps they identified
were out of single treefall, 17% out of multiple tree
falls, 25% by branch fall. However, on an area basis,
multiple treefalls occupied 55% of the total gap area
(3040 m2 ) and it was observed that the mean number
of gaps ha−1 was 0.24 and the gap size was 253.5
m2 . For this study, all the gaps larger than 20 m2 ,
originating from either single or multiple treefalls or
branch falls were identified. The gap sizes were calculated following Simpson’s rule as given in Rao et
al. (1997): Area = 1/3h [(Yo+Yn)+4(Y1+Y3. . . +Yn1)+2(Y2+Y4. . . +Yn-2)], where Y is the length of the
chord, h is the distance between the chords and n is
the number of chords. A chord represents the length
between two points lying opposite to each other on
the canopy edge in a gap. In the present study, an
even number of parallel chords were laid at a predetermined distance (0.5 m) in each gap and their length
was measured to determine the gap area. Six gaps of
different sizes identified in the experimental area were
selected for the present study and have been numbered
in ascending order of their size. Area of each gap was
as follows: Gap 1 – 35.3 m2 (3–4 years old), Gap 2 –
70.3 m2 (4–8 years old), Gap 3 – 144.7 m2 (5–10 years
old), Gap 4 – 306.9 m2 (5–10 years old), Gap 5 – 793.1
m2 (>10 years old) and Gap 6 – 981.8 m2 (>10 years
old). Each gap site was paired with a closed canopy
understorey site, located 25–30 m from an edge of the
treefall gap.
Measurement of microclimatic variables
The microclimate in the gaps and adjacent understories were studied by measuring light intensity, relative humidity, air temperature, thickness of litter
layer, soil temperature during January, April, July
and October 1998, the months representing winter,
spring, rainy and autumn seasons. All the parameters
were measured randomly at five places, close to the
ground surface at 12.00 h. in each gap and adjacent
understorey. Light intensity was measured using a lux
meter (LUTRON, LX-101), while relative humidity
187
and air temperature were measured using a thermohygrometer (TFA, Germany). Soil temperature was
measured using a soil thermometer (ELITE) and litter
thickness using a millimeter scale.
Study of soil characteristics
Five replicated samples of soil were collected from
0 to 10 cm depth in each gap during winter, spring,
rainy and autumn seasons in each gap and its adjacent understorey using a steel corer (6.5 cm inner
diameter). The replicated samples of a given gap and
the understorey were mixed separately thoroughly to
obtain composite samples for further analysis. The
soil samples were sieved through 2 mm mesh screen
and divided into two parts, one part was used in field
moist condition to determine microbial C, N and P,
and soil moisture, pH and available-P and the other
part was air-dried for the determination of texture,
organic-C (SOC), total Kjeldhal-N (TKN) and total-P.
Physico-chemical properties of the soil were determined following standard procedures given in Allen et
al. (1974) and Anderson and Ingram (1993). Microbial
N and P were estimated by chloroform-fumigationextraction method (Brookes et al., 1982, 1984), while
microbial C by the chloroform-fumigation-incubation
method (Jenkinson and Powlson, 1976) with some
minor modifications as suggested by Srivastava and
Singh (1988). The data presented are the means of
three replicate determinations on seasonal basis and
the data have been expressed on an oven-dry weight
basis.
Soil samples were also collected asceptically in
sterilized polythene bags from the top 0 to 10 cm layer
in all gaps and understories and were used for the
isolation of bacteria and fungi within 24 h. Soil bacterial population was estimated by Waksman’s (1952)
method using the nutrient agar medium at 105 dilution. Fungal population was estimated by dilution
plate technique (Johnson and Curl, 1972) using Martin
Rose Bengal agar medium at 103 dilution in deionized
water. The inoculated Petri-dishes were incubated at
30±1 ◦ C for 24 h and at 25±1 ◦ C for 5 days for
bacteria and fungi, respectively.
Statistical analysis
Tukey’s test was carried out to compare the mean values of physico-chemical properties and microbial C,
N and P between gaps and understories. LSD at 0.05
level was worked out to determine the variations in
different parameters studied between gaps and understorey. Linear regressions were worked out following
Zar (1974) to find out the influence of gap size on microbial C, N and P, the relationships between microbial
biomass and soil properties and also among microbial
C, N and P.
Results
Light intensity and air temperature were significantly
(P
© 2000 Kluwer Academic Publishers. Printed in the Netherlands.
185
Influence of gap size and soil properties on microbial biomass in a
subtropical humid forest of north-east India
A. Arunachalam∗ and Kusum Arunachalam
Restoration Ecology Laboratory, North Eastern Regional Institute of Science and Technology, Nirjuli-791109,
Arunachal Pradesh, India
Received 10 August 1999. Accepted in revised form 19 April 2000
Key words: microclimate, microbial biomass, soil, subtropical humid forest, treefall gaps, understorey
Abstract
We examined the effects of treefall gap size and soil properties on microbial biomass dynamics in an undisturbed
mature-phase humid subtropical broadleaved forest in north-east India. Canopy gaps had low soil moisture and low
microbial biomass suggesting that belowground dynamics accompanied changes in light resources after canopy
opening. High rainfall in the region causes excessive erosion/leaching of top soil and eventually soil fertility declines in treefall gaps compared to understorey. Soil microbial population was less during periods when temperature
and moisture conditions are low, while it peaked during rainy season when the litter decomposition rate is at its peak
on the forest floor. Greater demand for nutrients by plants during rainy season (the peak vegetative growth period)
limited the availability of nutrients to soil microbes and, therefore, low microbial C, N and P. Weak correlations
were also obtained for the relationships between microbial C, N and P and soil physico–chemical properties. Gap
size did influence the microbial nutrients and their contribution to soil organic carbon, total Kjeldhal nitrogen
and available-P. Contribution of microbial C to soil organic carbon, microbial N to total nitrogen were similar in
both treefall gaps and understorey plots, while the contribution of microbial P to soil available-P was lower in
gap compared to the understorey. These results indicate that any fluctuation in microbial biomass related nutrient
cycling processes in conjunction with the associated microclimate variation may affect the pattern of regeneration
of tree seedlings in the gaps and hence be related with their size.
Introduction
Natural treefalls are the major small-scale disturbances in mature forests in the humid tropical and
subtropical regions, and the gaps created in the forest
canopy by this event are an important factor in maintaining the high species diversity by increasing environmental heterogeneity and altering abundances and
distribution of abiotic and biotic resources (Clark,
1990; Connell, 1978; Denslow, 1987). Most studies
of gaps have addressed vegetation dynamics, regeneration through seedling establishment, effect of microclimatic variables on the revegetation, etc. and in
general have concentrated on aboveground processes
(see Meer et al., 1994). Relatively few studies have
∗ FAX No: 360244307. E-mail: aa@nerist.ernet.in
addressed belowground effects of canopy gaps such
as soil-related aspects and their effects on the revegetation process after disturbance (Arunachalam et al.,
1996).
The mass of litter and fine roots on the forest
floor is generally less under canopy gaps than more
in closed canopy site (Bariket al., 1992; Ostertag,
1998), while Sanford Jr. (1989) observed no difference
in fine root biomass between canopy gap and understorey. Earlier, we also reported insignificant changes
in dry matter, carbon (C) and nitrogen (N) accumulation in litter and fine roots and in microbial biomass
between natural gaps and understorey plots of a Pinus
kesiya Royle ex. Gordon forest in north-eastern India
(Arunachalam et al., 1996). Studies on soil microbial biomass, a ‘sink’ and ‘source’ of plant nutrients
in treefall gaps of tropical forests have not attracted
186
adequate attention of foresters and ecologists. Nevertheless, there have been a few studies evaluating the
dynamics of this important component in varieties of
ecosystems exposed to varied disturbances (e.g. Arunachalam et al., 1994; Hernot and Robertson, 1994;
Maithani et al., 1996). The present study was designed
(1) to determine the effects of canopy gaps on microbial carbon (C), -nitrogen (N) and -phosphorus (P),
and (2) whether or not the consequences of canopy
gap formation are dependent on soil properties, and
(3) to evaluate the effects of gap size on soil and microbial nutrient dynamics in a ‘sacred grove’, the climax
vegetation of the humid subtropics of north-east India.
Methods
Study area
The study was conducted in a ‘sacred grove’ (subtropical wet hill forest) at Mawphlang (1700 m
asl), 30 km south-east of Shillong (25◦ 34′ N,
91◦ 56′ E) in Meghalaya, north-eastern India. The
climate of the area is monsoonic with an average
annual rainfall of 2500 mm, distributed over five
months (May–September) of the year. The periods
October–November, December–February and March–
April represent autumn, winter and spring seasons,
respectively. The mean minimum and maximum air
temperatures during the study period were 3 ◦ and 16 ◦
C. The soils derived from underlying gneisses, schists
and granites and is grouped under the latosol (oxisol)
type (Pascoe, 1950).
The forest stand covers an area of about 150 ha.
It has been left undisturbed due to religious believes
of the local people and represents the climax vegetation of the area. Gap formation is frequent in this
forest mainly by uprooted trees (Rao et al., 1990).
The forest canopy is dominated by Quercus griffithii Miquel, Lithocarpus dealbatus (Miquel) Rehder
(=Quercus dealbata), Quercus glauca Thumb. Bl.
and Schima khasiana Dyer, while Symplocos chinensis (Lour) Druce and Daphne shillong Banerjee are
the main shrub components. The diameter at breast
height (DBH) of the trees in the forest ranged from
10 to 60 cm. In a study, Rao et al. (1990) reported
16 tree species, 16 shrubs and 28 herbaceous species
in this forest. The density of tree species was 1050
stems ha−1 , while for shrubs it was plants 560 ha−1
and 274 000 plants ha−1 for herbs. There is a heavy
growth of large number of species of epiphytic orchids, mosses, ferns and lianes in the forest. Further
details may be obtained from Rao et al. (1990, 1997)
and Barik et al. (1992).
Gap identification and experimental layout
The gap was considered as an opening in the forest
extending down through all foliage levels to an average height of 2 m above the ground (Brokaw, 1982).
Barik et al. (1992) reported that the distribution of
20–100 m2 gaps were more frequent in this forest.
About 58% of the total number of gaps they identified
were out of single treefall, 17% out of multiple tree
falls, 25% by branch fall. However, on an area basis,
multiple treefalls occupied 55% of the total gap area
(3040 m2 ) and it was observed that the mean number
of gaps ha−1 was 0.24 and the gap size was 253.5
m2 . For this study, all the gaps larger than 20 m2 ,
originating from either single or multiple treefalls or
branch falls were identified. The gap sizes were calculated following Simpson’s rule as given in Rao et
al. (1997): Area = 1/3h [(Yo+Yn)+4(Y1+Y3. . . +Yn1)+2(Y2+Y4. . . +Yn-2)], where Y is the length of the
chord, h is the distance between the chords and n is
the number of chords. A chord represents the length
between two points lying opposite to each other on
the canopy edge in a gap. In the present study, an
even number of parallel chords were laid at a predetermined distance (0.5 m) in each gap and their length
was measured to determine the gap area. Six gaps of
different sizes identified in the experimental area were
selected for the present study and have been numbered
in ascending order of their size. Area of each gap was
as follows: Gap 1 – 35.3 m2 (3–4 years old), Gap 2 –
70.3 m2 (4–8 years old), Gap 3 – 144.7 m2 (5–10 years
old), Gap 4 – 306.9 m2 (5–10 years old), Gap 5 – 793.1
m2 (>10 years old) and Gap 6 – 981.8 m2 (>10 years
old). Each gap site was paired with a closed canopy
understorey site, located 25–30 m from an edge of the
treefall gap.
Measurement of microclimatic variables
The microclimate in the gaps and adjacent understories were studied by measuring light intensity, relative humidity, air temperature, thickness of litter
layer, soil temperature during January, April, July
and October 1998, the months representing winter,
spring, rainy and autumn seasons. All the parameters
were measured randomly at five places, close to the
ground surface at 12.00 h. in each gap and adjacent
understorey. Light intensity was measured using a lux
meter (LUTRON, LX-101), while relative humidity
187
and air temperature were measured using a thermohygrometer (TFA, Germany). Soil temperature was
measured using a soil thermometer (ELITE) and litter
thickness using a millimeter scale.
Study of soil characteristics
Five replicated samples of soil were collected from
0 to 10 cm depth in each gap during winter, spring,
rainy and autumn seasons in each gap and its adjacent understorey using a steel corer (6.5 cm inner
diameter). The replicated samples of a given gap and
the understorey were mixed separately thoroughly to
obtain composite samples for further analysis. The
soil samples were sieved through 2 mm mesh screen
and divided into two parts, one part was used in field
moist condition to determine microbial C, N and P,
and soil moisture, pH and available-P and the other
part was air-dried for the determination of texture,
organic-C (SOC), total Kjeldhal-N (TKN) and total-P.
Physico-chemical properties of the soil were determined following standard procedures given in Allen et
al. (1974) and Anderson and Ingram (1993). Microbial
N and P were estimated by chloroform-fumigationextraction method (Brookes et al., 1982, 1984), while
microbial C by the chloroform-fumigation-incubation
method (Jenkinson and Powlson, 1976) with some
minor modifications as suggested by Srivastava and
Singh (1988). The data presented are the means of
three replicate determinations on seasonal basis and
the data have been expressed on an oven-dry weight
basis.
Soil samples were also collected asceptically in
sterilized polythene bags from the top 0 to 10 cm layer
in all gaps and understories and were used for the
isolation of bacteria and fungi within 24 h. Soil bacterial population was estimated by Waksman’s (1952)
method using the nutrient agar medium at 105 dilution. Fungal population was estimated by dilution
plate technique (Johnson and Curl, 1972) using Martin
Rose Bengal agar medium at 103 dilution in deionized
water. The inoculated Petri-dishes were incubated at
30±1 ◦ C for 24 h and at 25±1 ◦ C for 5 days for
bacteria and fungi, respectively.
Statistical analysis
Tukey’s test was carried out to compare the mean values of physico-chemical properties and microbial C,
N and P between gaps and understories. LSD at 0.05
level was worked out to determine the variations in
different parameters studied between gaps and understorey. Linear regressions were worked out following
Zar (1974) to find out the influence of gap size on microbial C, N and P, the relationships between microbial
biomass and soil properties and also among microbial
C, N and P.
Results
Light intensity and air temperature were significantly
(P