Changes in soil properties and vegetatio
Forest Ecology and Management 200 (2004) 183–193
www.elsevier.com/locate/foreco
Changes in soil properties and vegetation characteristics
along a forest-savanna gradient in southern Venezuela
Nelda Dezzeoa,*, Noemı́ Chacóna, Elio Sanojab, Gabriel Picónc
a
Centro de Ecologı́a, Instituto Venezolano de Investigaciones Cientı́ficas, Apdo. 21.827, Caracas 1020-A, Venezuela
b
Universidad Experimental de Guayana, Puerto Ordaz, Venezuela
c
Estación Cientı́fica de Parupa, CVG-Autoridad Gran Sabana, Puerto Ordaz, Venezuela
Received 13 May 2004; received in revised form 21 June 2004; accepted 21 June 2004
Abstract
Vegetation cover in the Gran Sabana highlands (southern Venezuela) appears as a complex mosaic of tall to low forests, bush
vegetation and savannas. In this study we described the changes in structure and floristic composition along a forest-savanna
gradient consisted of tall forest (TF), medium forest (MF), low forest (LF) and open savanna (S), and analyse the possible
reasons for the observed changes. The results showed no obvious differences in the soils properties along the vegetation gradient.
All sites presented shallow soils (K ffi G>F>S
LF
A (0–40)
C (>40)
4.12 0.23
4.33 0.06
1.17 0.32
0.80 0.10
12.3 4.4
14.3 9.9
11.0 8.9
16.8 9.9
76.7 8.1
68.7 25.7
Q K>G>S
Q>K>G ffi F>S
S
A (0–35)
C (>35)
4.24 0.15
4.68 0.05
0.80 0.14
0.44 0.07
4.9 3.3
8.4 4.7
9.0 1.7
9.8 1.4
86.1 2.8
81.9 4.3
Q G>F>K
Q>K ffi G>F>S
Q: quartz, K: kaolinite, G: goethite, F: ferrihydrite, S: strengite (FePO42H2O); TF: tall forest, MF: medium forest, LF: low forest, S: savanna.
properties are comparable with the results reported by
other sandy soils derived from the regionally dominant
Roraima Group (Dezzeo et al., 1997).
The major difference between the soils of the
vegetation gradient was related with the presence of
a relatively thick organic surface layer on top of the
soils of the tall and medium forests, and their absence
on the soils of the low forest and the savanna (Table 3).
This organic layer was characterized by a complex
network of fine roots mixed with litter in various
stages of decomposition. The fine root mass in the
organic layer (41.1 Mg/ha in TF and 29.6 Mg/ha in
MF, Table 3) was high in comparison with the mean
value reported for other montane forests on more
clayed soils in the study region (23.6 Mg/ha) (Dezzeo
and Fölster, 1994) and with the range of values
reported for Amazon lowland forests (13.8–
39.5 Mg/ha) (Klinge and Herrera, 1978).
The fine root mass was significantly lower (P <
0.05) within the mineral soil than in the organic
surface layer (Table 3). Total fine roots mass in the
first 30 cm of the soils (4.6–10.4 Mg/ha) was lower
than the mean value reported for more clayed soils in
other montane forests of the study region (12.5 Mg/ha;
0–30 cm soil depth) (Fölster et al., 2001) and than the
values of lowland soils in Amazon (14.7–39.5 Mg/ha;
0–30 cm soil depth) (Sanford, 1985). This lower fine
root mass in mineral soil and the already mentioned
higher amount of fine roots in the organic layer are
consequence of the very low nutritional status of the
sandy soils in the study region. High accumulation of
roots on top of the soil surface has been considered an
Table 2
Element concentration in mineral soil along the forest-savanna gradient in the Gran Sabana
Site
Depth (cm)
C
N
P
Ca
K
Mg
TF
0–10
10–20
20–30
4.55 0.94
3.49 1.35
3.29 1.31
0.27 0.04
0.17 0.04
0.17 0.03
0.006 0.001
0.002 0.000
0.002 0.000
0.040 0.020
0.021 0.005
0.023 0.006
0.072 0.034
0.029 0.007
0.022 0.004
0.050 0.017
0.022 0.006
0.020 0.004
MF
0–10
10–20
20–30
5.86 2.61
4.55 1.43
4.28 1.25
0.37 0.14
0.27 0.05
0.23 0.07
0.005 0.002
0.004 0.001
0.002 0.000
0.035 0.009
0.025 0.007
0.023 0.003
0.098 0.060
0.029 0.008
0.018 0.006
0.075 0.022
0.033 0.006
0.023 0.002
LF
0–10
10–20
20–30
2.66 0.13
3.06 0.54
2.69 0.52
0.17 0.01
0.19 0.02
0.16 0.01
0.004 0.001
0.003 0.000
0.002 0.001
0.293 0.088
0.103 0.013
0.061 0.025
0.092 0.024
0.027 0.013
0.031 0.011
0.114 0.034
0.051 0.015
0.025 0.004
S
0–10
10–20
20–30
2.83 0.81
2.12 0.30
1.08 0.26
0.14 0.05
0.10 0.01
0.05 0.01
0.002 0.000
0.001 0.000
0.001 0.000
0.029 0.016
0.017 0.011
0.015 0.006
0.052 0.040
0.025 0.015
0.020 0.010
0.026 0.004
0.014 0.001
0.007 0.003
C and N in (%), P in (mg/g), cations in (cmol/kg). TF: tall forest, MF: medium forest, LF: low forest, S: savanna.
188
N. Dezzeo et al. / Forest Ecology and Management 200 (2004) 183–193
Table 3
Organic layer and fine roots (Mg/ha) in mineral soil along the forest-savanna gradient in the Gran Sabana
TF
MF
185.7 83.3 a
41.1 24.7 a 1
226.7 104.3 a
110.6 33.3 a
29.6 19.2 a 1
139.7 43.8 a
0–10 cm
10–20 cm
20–30 cm
1.8 0.8 a
2.1 1.1 a
0.7 0.2 a
4.0 2.0 b
3.8 1.4 b
2.1 1.2 b
2.4 1.2 a
1.4 0.7 a
1.3 0.5 a
6.1 2.2 b
3.2 0.9 b
1.4 1.4 a
Total fine roots
4.6 1.3 a 2
10.2 3.8 b 2
5.6 0.9 a
10.4 2.6 b
Organic surface layer
Litter
Fine roots
Litter + fine roots
Fine roots in mineral soil
LF
S
0.0
0.0
0.0
0.0
0.0
0.0
Different letters in columns indicate significant differences between vegetation types. TF: tall forest, MF: medium forest, LF: low forest, S:
savanna. Different numbers in columns denote differences in the fine roots among organic litter and mineral soil (ANOVA, P < 0.05).
important mechanism of nutrient conservation in forests growing on acidic and nutrient-poor soils (Herrera
et al., 1978; Cuevas and Medina, 1988; Priess et al.,
1999), and has been also related with Al toxicity
within mineral soil (Fölster, 1986). According to
our results, the vegetation types that presented organic
surface layer (TF, MF) showed considerably higher
pools of C and nutrients than those that presented no
organic layer (LF, S) (Table 4). In TF and MF, around
30–50% of the pools of C and N, and more than 65%
of the pools of P, Ca, K and Mg were stored in the
organic surface layer.
Another difference between the studied soils was
related with the presence of abundant residues of large
Table 4
Pools of C (tonne/ha) and nutrients (kg/ha) in the organic layer (litter +fine roots), fine soil roots (0–30 cm) and mineral soil (0–30 cm) in the
vegetation gradient in la Gran Sabana
TF
MF
LF
S
Organic layer
C
N
P
Ca
K
Mg
145.2
8132
44.7
116.7
124.1
24.7
44.9
4301
18.3
20.1
42.0
1.1
86.4
4072
24.1
56.0
74.3
34.9
23.0
2085
5.7
7.4
21.0
4.9
Fine soil-roots
C
N
P
Ca
K
Mg
4.2
50.8
0.7
0.7
5.0
0.6
1.1
13.2
0.1
0.2
2.3
0.3
9.1
118.2
1.6
2.7
12.5
4.3
3.4
42.8
0.6
0.8
4.2
1.5
5.2
51.6
0.9
7.6
3.5
1.5
0.7
14.7
0.3
1.7
2.0
0.5
9.2
47.4
1.1
2.6
16.2
1.5
2.2
12.7
0.2
1.2
4.3
0.2
Mineral soil
C
N
P
Ca
K
Mg
138.1
7000
11.9
19.3
52.9
12.5
34.1
1200
3.5
7.5
18.5
2.6
140.4
8300
9.2
15.8
47.2
14.4
28.9
1600
1.8
2.6
15.1
2.8
90.6
5600
8.0
84.0
59.8
21.7
19.3
1100
0.7
31.7
33.1
5.3
82.3
3900
6.2
16.9
51.1
7.7
14.4
800
1.6
8.1
27.6
1.3
Total pools in soil
C
N
P
Ca
K
Mg
287.5
15182.8
57.3
136.7
182.0
37.8
TF: tall forest, MF: medium forest, LF: low forest.
235.9
12490.6
34.9
74.6
134.0
53.6
0.0
0.0
0.0
0.0
0.0
0.0
95.8
5651.6
8.9
91.6
63.3
23.2
0.0
0.0
0.0
0.0
0.0
0.0
91.5
3947.4
7.3
19.5
67.3
9.2
189
N. Dezzeo et al. / Forest Ecology and Management 200 (2004) 183–193
fallen trees on the forest floor of TF, MF and LF, and
their absence on the soils of S. These residues did not
present signs of burning in TF, while in MF several of
them were burned and in LF all of them were total
charred. This observation together with the presence
of charcoal pieces within the mineral soils of S, and
the absence of the organic layer on the soils of LF and
S indicate that fire has affected with different intensity
and frequency the studied vegetation gradient. However, our results about the soil chemical characteristics
did not reflect the fire impact. It is known that fire
changes the chemical parameters of soils as a result of
the direct input of heat and ash (Ewel et al., 1981;
Woodmansee and Wallach, 1981; Giovannini et al.,
1988; Marafa and Chau, 1999). These changes
become dissipated with the time because of leaching
(Viro, 1974; Coutinho, 1982), and many of them even
disappeared one year after the occurrence of fire
(Marafa and Chau, 1999). The soil chemical similarity
between the soils along the vegetation gradient indicate that fire did not occur recently, and therefore the
probably changes in the soil properties due to the
direct input of ash could be depleted from the burned
vegetation in the following rainy seasons, when the
ashes were washed away by runoff.
3.2. Structure and floristic composition along the
vegetation gradient
Between TF, MF, LF and S drastic changes in stem
density, basal area, species richness and floristic composition were observed (Tables 5–7). These differences were no directly related with the soil properties
but seemed to be consequence of the fire impact on the
vegetation cover. Although fire occurrence was no
quantified, the results indicate that fire frequency
and intensity have been no spatially uniform in the
study area. Under unburned condition, the forest (TF)
showed values of stem density (1060 trees/ha) and
basal area (40 m2/ha) (trees 10 cm DBH, Table 5)
that are comparable with the mean values reported for
primary montane forests growing on sandy soils
(950 trees/ha and 38.7 m2/ha) (Dezzeo et al., 1997).
This forest presented also high species richness (45
species in 0.1 ha, Table 6), which is comparable with
the values reported for primary neotropical montane
forests, excluding the Andean forests (40–65 species
2.5 cm in 0.1) (Gentry, 1995).
TF and MF showed no significant differences (P
0.9281) in the Shannon’s diversity index, and were
also similar in term of species richness (number of
species in the sampled area) (Table 6). However, these
sites showed differences related with the forest structure. The stem density of small trees (2.5–10 cm DBH)
was 33% higher, the basal area 26% lower and the
amount of dead standing trees 2.7 times higher in MF
than in TF (Table 5). The similarity in the soil conditions between both forests (Tables 1 and 2) and the
presence of burned trunks on the forest floor of MF
suggest that the observed differences are consequences of the impact of fire in MF, which should
has been of low intensity and probably of old date,
because the organic surface layer in this site showed
no signs of burning.
Vegetation structure and species richness were
clearly different between TF and LF. Stem density,
basal area and number of species (trees 2.5 cm
DBH) were 74, 91 and 38% lower in LF than in TF
(Tables 5 and 6). The number of plants in the herbaceous stratum (
www.elsevier.com/locate/foreco
Changes in soil properties and vegetation characteristics
along a forest-savanna gradient in southern Venezuela
Nelda Dezzeoa,*, Noemı́ Chacóna, Elio Sanojab, Gabriel Picónc
a
Centro de Ecologı́a, Instituto Venezolano de Investigaciones Cientı́ficas, Apdo. 21.827, Caracas 1020-A, Venezuela
b
Universidad Experimental de Guayana, Puerto Ordaz, Venezuela
c
Estación Cientı́fica de Parupa, CVG-Autoridad Gran Sabana, Puerto Ordaz, Venezuela
Received 13 May 2004; received in revised form 21 June 2004; accepted 21 June 2004
Abstract
Vegetation cover in the Gran Sabana highlands (southern Venezuela) appears as a complex mosaic of tall to low forests, bush
vegetation and savannas. In this study we described the changes in structure and floristic composition along a forest-savanna
gradient consisted of tall forest (TF), medium forest (MF), low forest (LF) and open savanna (S), and analyse the possible
reasons for the observed changes. The results showed no obvious differences in the soils properties along the vegetation gradient.
All sites presented shallow soils (K ffi G>F>S
LF
A (0–40)
C (>40)
4.12 0.23
4.33 0.06
1.17 0.32
0.80 0.10
12.3 4.4
14.3 9.9
11.0 8.9
16.8 9.9
76.7 8.1
68.7 25.7
Q K>G>S
Q>K>G ffi F>S
S
A (0–35)
C (>35)
4.24 0.15
4.68 0.05
0.80 0.14
0.44 0.07
4.9 3.3
8.4 4.7
9.0 1.7
9.8 1.4
86.1 2.8
81.9 4.3
Q G>F>K
Q>K ffi G>F>S
Q: quartz, K: kaolinite, G: goethite, F: ferrihydrite, S: strengite (FePO42H2O); TF: tall forest, MF: medium forest, LF: low forest, S: savanna.
properties are comparable with the results reported by
other sandy soils derived from the regionally dominant
Roraima Group (Dezzeo et al., 1997).
The major difference between the soils of the
vegetation gradient was related with the presence of
a relatively thick organic surface layer on top of the
soils of the tall and medium forests, and their absence
on the soils of the low forest and the savanna (Table 3).
This organic layer was characterized by a complex
network of fine roots mixed with litter in various
stages of decomposition. The fine root mass in the
organic layer (41.1 Mg/ha in TF and 29.6 Mg/ha in
MF, Table 3) was high in comparison with the mean
value reported for other montane forests on more
clayed soils in the study region (23.6 Mg/ha) (Dezzeo
and Fölster, 1994) and with the range of values
reported for Amazon lowland forests (13.8–
39.5 Mg/ha) (Klinge and Herrera, 1978).
The fine root mass was significantly lower (P <
0.05) within the mineral soil than in the organic
surface layer (Table 3). Total fine roots mass in the
first 30 cm of the soils (4.6–10.4 Mg/ha) was lower
than the mean value reported for more clayed soils in
other montane forests of the study region (12.5 Mg/ha;
0–30 cm soil depth) (Fölster et al., 2001) and than the
values of lowland soils in Amazon (14.7–39.5 Mg/ha;
0–30 cm soil depth) (Sanford, 1985). This lower fine
root mass in mineral soil and the already mentioned
higher amount of fine roots in the organic layer are
consequence of the very low nutritional status of the
sandy soils in the study region. High accumulation of
roots on top of the soil surface has been considered an
Table 2
Element concentration in mineral soil along the forest-savanna gradient in the Gran Sabana
Site
Depth (cm)
C
N
P
Ca
K
Mg
TF
0–10
10–20
20–30
4.55 0.94
3.49 1.35
3.29 1.31
0.27 0.04
0.17 0.04
0.17 0.03
0.006 0.001
0.002 0.000
0.002 0.000
0.040 0.020
0.021 0.005
0.023 0.006
0.072 0.034
0.029 0.007
0.022 0.004
0.050 0.017
0.022 0.006
0.020 0.004
MF
0–10
10–20
20–30
5.86 2.61
4.55 1.43
4.28 1.25
0.37 0.14
0.27 0.05
0.23 0.07
0.005 0.002
0.004 0.001
0.002 0.000
0.035 0.009
0.025 0.007
0.023 0.003
0.098 0.060
0.029 0.008
0.018 0.006
0.075 0.022
0.033 0.006
0.023 0.002
LF
0–10
10–20
20–30
2.66 0.13
3.06 0.54
2.69 0.52
0.17 0.01
0.19 0.02
0.16 0.01
0.004 0.001
0.003 0.000
0.002 0.001
0.293 0.088
0.103 0.013
0.061 0.025
0.092 0.024
0.027 0.013
0.031 0.011
0.114 0.034
0.051 0.015
0.025 0.004
S
0–10
10–20
20–30
2.83 0.81
2.12 0.30
1.08 0.26
0.14 0.05
0.10 0.01
0.05 0.01
0.002 0.000
0.001 0.000
0.001 0.000
0.029 0.016
0.017 0.011
0.015 0.006
0.052 0.040
0.025 0.015
0.020 0.010
0.026 0.004
0.014 0.001
0.007 0.003
C and N in (%), P in (mg/g), cations in (cmol/kg). TF: tall forest, MF: medium forest, LF: low forest, S: savanna.
188
N. Dezzeo et al. / Forest Ecology and Management 200 (2004) 183–193
Table 3
Organic layer and fine roots (Mg/ha) in mineral soil along the forest-savanna gradient in the Gran Sabana
TF
MF
185.7 83.3 a
41.1 24.7 a 1
226.7 104.3 a
110.6 33.3 a
29.6 19.2 a 1
139.7 43.8 a
0–10 cm
10–20 cm
20–30 cm
1.8 0.8 a
2.1 1.1 a
0.7 0.2 a
4.0 2.0 b
3.8 1.4 b
2.1 1.2 b
2.4 1.2 a
1.4 0.7 a
1.3 0.5 a
6.1 2.2 b
3.2 0.9 b
1.4 1.4 a
Total fine roots
4.6 1.3 a 2
10.2 3.8 b 2
5.6 0.9 a
10.4 2.6 b
Organic surface layer
Litter
Fine roots
Litter + fine roots
Fine roots in mineral soil
LF
S
0.0
0.0
0.0
0.0
0.0
0.0
Different letters in columns indicate significant differences between vegetation types. TF: tall forest, MF: medium forest, LF: low forest, S:
savanna. Different numbers in columns denote differences in the fine roots among organic litter and mineral soil (ANOVA, P < 0.05).
important mechanism of nutrient conservation in forests growing on acidic and nutrient-poor soils (Herrera
et al., 1978; Cuevas and Medina, 1988; Priess et al.,
1999), and has been also related with Al toxicity
within mineral soil (Fölster, 1986). According to
our results, the vegetation types that presented organic
surface layer (TF, MF) showed considerably higher
pools of C and nutrients than those that presented no
organic layer (LF, S) (Table 4). In TF and MF, around
30–50% of the pools of C and N, and more than 65%
of the pools of P, Ca, K and Mg were stored in the
organic surface layer.
Another difference between the studied soils was
related with the presence of abundant residues of large
Table 4
Pools of C (tonne/ha) and nutrients (kg/ha) in the organic layer (litter +fine roots), fine soil roots (0–30 cm) and mineral soil (0–30 cm) in the
vegetation gradient in la Gran Sabana
TF
MF
LF
S
Organic layer
C
N
P
Ca
K
Mg
145.2
8132
44.7
116.7
124.1
24.7
44.9
4301
18.3
20.1
42.0
1.1
86.4
4072
24.1
56.0
74.3
34.9
23.0
2085
5.7
7.4
21.0
4.9
Fine soil-roots
C
N
P
Ca
K
Mg
4.2
50.8
0.7
0.7
5.0
0.6
1.1
13.2
0.1
0.2
2.3
0.3
9.1
118.2
1.6
2.7
12.5
4.3
3.4
42.8
0.6
0.8
4.2
1.5
5.2
51.6
0.9
7.6
3.5
1.5
0.7
14.7
0.3
1.7
2.0
0.5
9.2
47.4
1.1
2.6
16.2
1.5
2.2
12.7
0.2
1.2
4.3
0.2
Mineral soil
C
N
P
Ca
K
Mg
138.1
7000
11.9
19.3
52.9
12.5
34.1
1200
3.5
7.5
18.5
2.6
140.4
8300
9.2
15.8
47.2
14.4
28.9
1600
1.8
2.6
15.1
2.8
90.6
5600
8.0
84.0
59.8
21.7
19.3
1100
0.7
31.7
33.1
5.3
82.3
3900
6.2
16.9
51.1
7.7
14.4
800
1.6
8.1
27.6
1.3
Total pools in soil
C
N
P
Ca
K
Mg
287.5
15182.8
57.3
136.7
182.0
37.8
TF: tall forest, MF: medium forest, LF: low forest.
235.9
12490.6
34.9
74.6
134.0
53.6
0.0
0.0
0.0
0.0
0.0
0.0
95.8
5651.6
8.9
91.6
63.3
23.2
0.0
0.0
0.0
0.0
0.0
0.0
91.5
3947.4
7.3
19.5
67.3
9.2
189
N. Dezzeo et al. / Forest Ecology and Management 200 (2004) 183–193
fallen trees on the forest floor of TF, MF and LF, and
their absence on the soils of S. These residues did not
present signs of burning in TF, while in MF several of
them were burned and in LF all of them were total
charred. This observation together with the presence
of charcoal pieces within the mineral soils of S, and
the absence of the organic layer on the soils of LF and
S indicate that fire has affected with different intensity
and frequency the studied vegetation gradient. However, our results about the soil chemical characteristics
did not reflect the fire impact. It is known that fire
changes the chemical parameters of soils as a result of
the direct input of heat and ash (Ewel et al., 1981;
Woodmansee and Wallach, 1981; Giovannini et al.,
1988; Marafa and Chau, 1999). These changes
become dissipated with the time because of leaching
(Viro, 1974; Coutinho, 1982), and many of them even
disappeared one year after the occurrence of fire
(Marafa and Chau, 1999). The soil chemical similarity
between the soils along the vegetation gradient indicate that fire did not occur recently, and therefore the
probably changes in the soil properties due to the
direct input of ash could be depleted from the burned
vegetation in the following rainy seasons, when the
ashes were washed away by runoff.
3.2. Structure and floristic composition along the
vegetation gradient
Between TF, MF, LF and S drastic changes in stem
density, basal area, species richness and floristic composition were observed (Tables 5–7). These differences were no directly related with the soil properties
but seemed to be consequence of the fire impact on the
vegetation cover. Although fire occurrence was no
quantified, the results indicate that fire frequency
and intensity have been no spatially uniform in the
study area. Under unburned condition, the forest (TF)
showed values of stem density (1060 trees/ha) and
basal area (40 m2/ha) (trees 10 cm DBH, Table 5)
that are comparable with the mean values reported for
primary montane forests growing on sandy soils
(950 trees/ha and 38.7 m2/ha) (Dezzeo et al., 1997).
This forest presented also high species richness (45
species in 0.1 ha, Table 6), which is comparable with
the values reported for primary neotropical montane
forests, excluding the Andean forests (40–65 species
2.5 cm in 0.1) (Gentry, 1995).
TF and MF showed no significant differences (P
0.9281) in the Shannon’s diversity index, and were
also similar in term of species richness (number of
species in the sampled area) (Table 6). However, these
sites showed differences related with the forest structure. The stem density of small trees (2.5–10 cm DBH)
was 33% higher, the basal area 26% lower and the
amount of dead standing trees 2.7 times higher in MF
than in TF (Table 5). The similarity in the soil conditions between both forests (Tables 1 and 2) and the
presence of burned trunks on the forest floor of MF
suggest that the observed differences are consequences of the impact of fire in MF, which should
has been of low intensity and probably of old date,
because the organic surface layer in this site showed
no signs of burning.
Vegetation structure and species richness were
clearly different between TF and LF. Stem density,
basal area and number of species (trees 2.5 cm
DBH) were 74, 91 and 38% lower in LF than in TF
(Tables 5 and 6). The number of plants in the herbaceous stratum (