Little information on fine roots that play an important role in nutrient cycling was available in tropical rainforests. Distribution of fine root production and effects of selective logging on root biomass and nutrient content change were studied in the tropical rainforests of south-western Cameroon. Twenty five root samples were excavated in each of two soil depths (0-10 and 10-25 cm) and in each of three undisturbed forests (Ebom, Ebimimbang and Nyangong) and one disturbed forest by logging, using a square metallic frame of 25 x 25 cm and 30 cm high. Root samples were categorized in three diameter classes: fine (<2 mm), small (2-5 mm) and medium roots (5-20 mm). Root biomass of three diameter classes and nutrients (N, Ca, Mg, K, Na and P) of the two first diameter classes were determined. Results have shown that total root biomass varied from 9.62 (Ebom) to 29.88 t ha-1 (Ebimimbang); those of fine roots decreased from Nyangong (7.43 t ha-1) to Ebom (1.74 t ha-1). In the top soil, the fine root biomass was 2 to 4 time lower in Ebom (1.43 t ha-1) than in other undisturbed forests (3.28 and 5.87 t ha-1). From 70 to 80% of fine root biomass were produced in the first 10 centimeters of soil depth. Nutrient amount in fine root biomass were also lower in Ebom than in Ebimimbang and Nyangong, except the P amount (kg ha-1) which remained high in Ebom. On the contrary, no pattern of nutrient content (g kg-1) changes among forest were found, except N and P that decreased from Ebom to Nyangong. Six years after logging, its effects on root biomass and nutrient changes were not significant and during this period, root production of disturbed forest was high due to rapid regeneration of forest particularly heliophilous tree species.
The tropical rainforests of southern Cameroon as African rainforests are subjected to deforestation due to shifting cultivation, commercial timber and forest exploitation (
Because of their high nutrient absorption capacity and quick turnover, fine roots (0.5 to 2 mm diameter) play a key role in nutrient cycles (
Most of the research on fine roots has been carried out in the temperate region (
The aim of the present work is to assess the change of fine root biomass and their nutrient content in three undisturbed forests, developed on different soil types according to soil Taxonomy-USDA and differed also by soil chemical characteristic as K, Mg and Ca amount (t ha-1) and available P and water content (
Study sites are situated in the TCP (Tropenbos Cameroon Programme) research area, located in the western part of the Biafran Atlantic forest of south Cameroon (
The vegetation of the TCP research area is part of this rainforest. The western and central portions of this area, with an altitude of slightly less than 700 m a.s.l., are covered by a evergreen forest characterised by tall trees that reach heights of about 60 m as well as by the presence of
In the TCP research area, three experimental sites were chosen, at Ebimimbang, Ebom and Nyangong, corresponding respectively to low (100 m a.s.l.), median (about 440 m) and high altitude (> 600 m) forest zones. Moreover, these forests differ in soil types (Ultisols in Ebimimbang, Oxisols/Ultisols in Ebom and Oxisols in Nyangong) as well as in soil nutrient content like available P (
In each of the three sites, five plots of 100 m2 were chosen at random. To evaluate the impact of logging on the variation of fine root biomass, five other plots of 100 m2 were chosen in a part of the Ebom forest logged 6 years before. In each plot, five sub-plots of 1 m2 were selected, 1 at the 4 corners and other one in the middle of the plot (
Root sampling was carried out during the long dry season of 2001 (january - early march). All the sub-plots were carefully cleared manually of grass and litter, taking care not to damage the superficial roots. A root pre-sampling was carried out in another part of the forest and it showed that most of the fine roots are located in the upper 10 cm of the soil. In each of the 100 sub-plots of 1 m2, a block of soil was excavated in the middle at two different depths (0-10 and 10 - 25 cm) using a metal frame of a 25 cm side and 30 cm high, making a total of 200 samples. The soil blocks (cores) containing these roots were transported to the river and washed over a sieve with a 0.5 mm mesh. These roots samples were then collected by hand and stored in labelled paper bags and transported to the laboratory at Kribi, where they were dried in the oven at 60°C for 48h. They were then separated according to diameter classes in keeping with the model of Vogt & Persson (
The 25 samples of each of the first two diameter class (fine and small roots) by each depth of each forest type were lumped in three main replicates. This gave 12 samples (3 replicates x 2 diameter classes x 2 depths) to chemical analyse per forest type. The medium category (5 - 20 mm diameter) was not included in the chemical analysis because only very few roots were collected here. Powder samples, obtained after grounding in a
Before performing any statistical analysis, all variables were tested for normality and if necessary transformed. The comparison among undisturbed forests for the root biomass and the nutrient quantity was carried out by using ANOVA, followed by Scheffe’s test at 5% if ANOVA is significant. A Student’s test was used to compare soil depths, root diameter classes as well as the disturbed and undisturbed parts of the Ebom forest. These tests were conducted using SX statistical software (version 4.0, Analytical software 1992).
The root biomass did not differ significantly between diameter classes, except at Nyangong, at overall depth of 0-25 cm, and at Ebimimbang, at depth of 0-10 cm and 0-25 cm (
Root biomass varies among forest types for each diameter classes and soil depth (
Due to the large variation per site (high S.E.), root biomass is not significant different between the disturbed and undisturbed forest, except the total amount of the fine root biomass, which is significantly higher in the disturbed than undisturbed forest (
Nutrient contents (g kg-1) of fine and small roots at two soil depths of three forests are given in
There is a significant difference in the total nutrient amounts (kg ha-1) between the three forests (
Generally, the N, P, K and Ca contents are higher in the undisturbed than in the disturbed forest (
The nutrient amounts in the roots vary with the soil depth and the root diameter classes in the disturbed forest (
On the whole, the nutrient amount of the roots do not differ significantly between the disturbed and undisturbed forest, except for Mg (t2 = 2.8, P < 0.05) and Na amount (t2 = 6.50, P < 0.001) whose quantity in fine roots is significantly higher in the disturbed than undisturbed forest.
The root biomasses found in the present study fall in the same range of data from other rainforests (
Data from the literature on the nutrient contents of root differ from our results in respect of on the nutrient considered and soil type (
The distribution of the fine roots, as well as the nutrient amount of the three undisturbed forests decrease significantly with soil depth. Eighty two, 79 and 70% of the fine root biomass are produced in the top layer (0-10 cm), respectively at Ebom, Nyangong and Ebimimbang. This vertical distribution is in conformity with those reported in other tropical forests (
Soil texture can equally constitute a factor that is likely to explain this distribution in depth in our study, since the clay percentage increases with soil depth in the three undisturbed forest types (
The distribution of fine root biomass in the topsoil layer varies in the three forest types. They constitute 54, 34 and 12% of the total biomass respectively at Nyangong, Ebimimbang and Ebom. The share of small roots is equal in these forests, with 33, 32 and 35% respectively at Nyangong, Ebimimbang and Ebom. A comparison of different rainforests has shown that a higher root biomass production occurs in sites that are poor in nutrients (
Likewise, the three undisturbed forests differ in the nutrient contents in roots. In fact, the variation trend of N and P contents among the forests are similar; with these elements contents tending to decrease from Ebom to Ebimimbang respectively for the two diameter classes for a depth of 10-25 cm and for small roots for a depth of 0-10 cm (
Van Gemerden & Hazeu (
In this study, differences between the disturbed and undisturbed forests of Ebom are not significant with regard to root biomass and nutrient amounts, except for Mg and Na which are significantly higher in the disturbed than undisturbed forest. This suggests that the impact of selective logging on root biomass and their nutrient loss in Ebom is relatively small and recovery of fine roots was relatively fast. In fact, Van Gemerden & Hazeu (
Our study shows that root biomass in the Cameroonian rainforest - especially of fine roots - are among of the lower values found in other tropical rainforests, growing on the similar soils type as the Oxisols/Ultisols. Fine root biomass decreases with soil depth. Spatial distribution of fine root biomass varies between forest types, especially in the upper of soil layers. Ebom forest has a lowest root production and highest available P amount in the root, while Nyangong forest has highest root production and lowest available P amount and Ebimimbang is intermediate according its root production and available P amount. Mg was found in low quantities in root biomass, especially at Ebom forest. Logging activities provoked nutrient deficit, especially N, P and Ca that is generally present in low amounts in soil, and an increase of the root biomass, especially those of fine roots for landing to nutrient deficits in soils. But, fast fine root recovery, less than 7 years and low extraction of logs from a stand and relative small effects of logging systems on the site lead to insignificant nutrient losses.
This work has been done in Tropenbos Cameroon Programme for sustainable management forest of south west of Cameroon, with financing of European commission. The authors thank MAB Ayangma, C Kana and MM Mva for their field and laboratory assistance and technicians of the
Location of TCP research area in southwest Cameroon.
Location and geomorphologic, pedologic and climate characteristics of study sites. (*): mean annual rainfall from 1996 to 2000; (**): soil depth: 0-10 cm. Source: van Gemerden & Hazeu (
Characteristics | Ebimimbang (B) | Ebom (A1) | Nyangong (N) |
---|---|---|---|
Location | 3°03’N, 10°28’E | 3°05’N, 10°41’E | 2°58’N, 10°45’E |
Altitude (m a.s.l.) | 100 | 440 | > 600 |
Rainfall (mm) * | 1817 | 2115 | 1983 |
Relief intensity (m km-1) | low (< 30) | moderate (30-80) | high (120-250) |
River density | high | moderate | low |
Vegetation | lowland forest | lowland forest | sub-montan forest |
Land uses | high logging and agriculture | low logging and agriculture | logging absent and low agriculture |
Soil types | ultisols | ultisols/oxisols | oxisols |
Clay (0-20 cm - %) | 0-25 | 20-50 | 35-70 |
Clay (20-60 cm) | 20-45 | 35-60 | 50-80 |
Sandy (%)** | 60-90 | 40-60 | 10-40 |
pH (water) | 6.1 | 4.7 | 4.3 |
Carbon (%)** | 2 - 3.5 | 4 - 8 | 4 - 9 |
Nitrogen (%)** | 0.15 - 0.35 | 0.25 - 0.50 | 0.25 - 0.40 |
Mean root biomass (standard errors in parenthesis) in t ha-1 of dry mass in the three undisturbed forest types and a disturbed forest of southern Cameroon. Different letters indicate that the values are significantly different. a et b for comparison of diameter classes of the roots (vertical) and α and β for comparison of three forest types: A1, B and N (horizontal).
Soil depths (cm) | Diameter classes of roots (mm) | Undisturbed forests | Disturbed Forests | ||||
---|---|---|---|---|---|---|---|
FBNA1 | Ebimimbang (B) | Nyangong (N) | Ebom (A1) | Ebom (A2) | |||
0-10 | <2 | 4.23* | 3.28 (1.62)b αβ | 5.87 (3.31) β | 1.43 (1.22) α | 2.78 (0.69) | 1.93ns |
2 - 5 | 3.29ns | 3.13 (0.97)b | 3.47 (2.04) | 1.15 (0.83) | 1.51 (0.49) | 0.75ns | |
5-20 | 3.71* | 16.38 (10.34)a β | 9.55 (7.96) αβ | 1.95 (1.31)α | 1.94 (1.70) | 0.01ns | |
- | F | 7.86** | 1.80ns | 0.51ns | 1.39ns | - | |
10-25 | <2 | 2.89ns | 1.42 (0.91) | 1.56 (1.02) | 0.31 (0.30) | 0.61 (0.17) | 1.74ns |
2 - 5 | 0.81ns | 1.08 (0.92) | 0.78 (0.29) | 3.42 (6.26) | 3.49 (6.21) | 0.02ns | |
5-20 | 1.46ns | 4.60 (3.08) | 3.88 (3.48) | 1.36 (1.63) | 0.73 (0.20) | 0.76ns | |
- | F | 5.09 ns | 2.95ns | 0.71ns | 0.82ns | - | |
Total (0-25) | <2 | 5.25* | 4.70 (1.98)b α β | 7.43 (3.79)ab β | 1.74 (0.95)α | 3.39 (0.85) | 2.59* |
2 - 5 | 0.01 | 4.20 (1.25)b | 4.25 (1.99)b | 4.57 (6.65) | 5.00 (6.24) | 0.10ns | |
5-20 | 6.82* | 20.98 (8.58)a β | 13.43 (7.90)a αβ | 3.31 (2.23) α | 2.67 (1.61) | 0.47ns | |
- | F | 17.26*** | 4.04* | 0.48ns | 0.40ns | - | |
Total general | - | - | 29.88 | 25.11 | 9.62 | 11.06 | - |
Nutrient contents (g kg-1) of the fine and small roots of three undisturbed forests and disturbed forest in southern Cameroon. Soil depth (cm); RDC: root diameter classes (mm).
Forest: | Ebimimbang forest (B) | Nyangong forest (N) | Undisturbed Ebom forest (A1) | Disturbed Ebom forest (A2) | ||||||||||||
---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
Depth: | 0 - 10 | 10 - 25 | 0 - 10 | 10 - 25 | 0 - 10 | 10 - 25 | 0 - 10 | 10 - 25 | ||||||||
RDC: | <2 | 2-5 | <2 | 2-5 | <2 | 2-5 | <2 | 2-5 | <2 | 2-5 | <2 | 2-5 | <2 | 2-5 | <2 | 2-5 |
N | 13.65 | 8.75 | 7.35 | 6.65 | 11.62 | 13.09 | 12.25 | 13.30 | 18.73 | 20.83 | 17.15 | 21.70 | 14.35 | 18.25 | 15.75 | 16.80 |
P | 0.66 | 0.39 | 0.39 | 0.39 | 0.53 | 0.53 | 0.66 | 0.66 | 1.95 | 1.37 | 1.37 | 1.80 | 1.82 | 1.68 | 1.41 | 1.41 |
K | 4.36 | 1.33 | 0.80 | 1.15 | 3.72 | 2.94 | 3.04 | 1.05 | 1.59 | 1.06 | 0.99 | 1.21 | 0.79 | 1.27 | 0.99 | 1.04 |
Ca | 4.93 | 1.13 | 2.63 | 7.51 | 4.83 | 7.17 | 5.89 | 6.23 | 1.08 | 1.64 | 2.28 | 2.30 | 0.36 | 1.01 | 2.26 | 2.21 |
Mg | 0.48 | 0.25 | 0.24 | 0.30 | 0.78 | 0.50 | 0.46 | 0.90 | 0.53 | 0.29 | 0.49 | 0.78 | 0.74 | 0.25 | 0.63 | 0.63 |
Na | 0.01 | 0.06 | 0.07 | 0.02 | 0.04 | 0.02 | 0.02 | 0.04 | 0.03 | 0.02 | 0.02 | 0.03 | 0.04 | 0.02 | 0.02 | 0.02 |
Comparisons of nutrient amounts (kg ha-1) of roots between soil depths (0-10 and 10-25 cm), among three undisturbed forests (B, N and A1) and between undisturbed (A1) and disturbed (A2) Ebom forests of southern Cameroon. Standard error in parentheses. FBNA1
Kind | Nutrients | Depth (cm) | Undisturbed forests | Disturbed forest | ||||
---|---|---|---|---|---|---|---|---|
FBNA1 | Ebimimang (B) | Nyangong (N) | Ebom (A1) | Ebom (A2) | Student t2 | |||
Fine Roots | N | 0-10 | 1.4ns | 44.72 (22.18) | 68.27 (38.42) | 35.66 (17.51) | 39.91(9.87) | 0.4ns |
10-25 | 1.93ns | 10.47 (6.68) | 19.11 (12.50) | 6.99 (4.66) | 9.54 (2.68) | 0.9ns | ||
Student t1 | 3.31* | 2.72* | 2.74ns | 5.94** | - | |||
P | 0-10 | 1.06ns | 2.16 (1.07) | 3.09 (1.74) | 3.72 (1.83) | 5.06 (1.25) | 1.2ns | |
10-25 | 1.29ns | 0.56 (0.36) | 1.03 (0.67) | 0.56 (0.37) | 0.85 (0.24) | 1.3ns | ||
- | Student t1 | 3.16* | 2.48* | 2.94* | 6.60*** | - | ||
K | 0-10 | 4.10ns | 14.29 (7.09) | 21.86 (12.30) | 3.03 (1.49) | 2.21 (0.55) | 1.1ns | |
10-25 | 5.80* | 1.14 (0.73)b | 4.74 (3.10)a | 0.40 (0.27)b | 0.60 (0.17) | 1.2ns | ||
Student t1 | 4.13** | 3.02* | 3.01* | 5.61** | - | |||
Ca | 0-10 | 5.14* | 16.15 (8.01)ab | 28.36 (15.96)a | 2.06 (1.01)b | 0.99 (0.25) | 2.1ns | |
10-25 | 4.30* | 3.75 (2.39)ab | 9.19 (6.01)a | 0.93 (0.62)b | 1.37 (0.38) | 1.2ns | ||
Student t1 | 3.32* | 2.51* | 1.65ns | 1.65ns | - | |||
Mg | 0-10 | 5.49* | 1.57 (0.78)b | 4.55 (2.56)a | 1.00 (0.49)b | 2.07 (0.51) | 2.8* | |
10-25 | 2.60ns | 0.34 (0.22) | 0.71 (0.46) | 0.20 (0.13) | 0.38 (0.11) | 2.0ns | ||
Student t1 | 3.40** | 3.30* | 2.73ns | 6.46*** | - | |||
Na | 0-10 | 6.49* | 0.06 (0.03)b | 0.24 (0.13)a | 0.05 (0.02)b | 1.03 (0.26) | 6.5*** | |
10-25 | 4.99* | 0.09 (0.06)a | 0.03 (0.02)b | 0.01(0.01)b | 0.01 (0.003) | 1.1ns | ||
Student t1 | 0.99ns | 3.36* | 2.89* | 8.00*** | - | |||
Small Roots | N | 0-10 | 1.31ns | 27.37 (8.52) | 45.39 (26.73) | 31.94 (8.12) | 27.48 (8.90) | 0.7ns |
10-25 | 1.93ns | 7.16 (6.09) | 10.39 (3.88) | 98.70 (155.1) | 78.19 (18.55) | 0.2ns | ||
Student t1 | 4.32** | 2.90* | 0.74ns | 4.95* | - | |||
P | 0-10 | 3.03ns | 1.23 (0.38) | 1.83 (1.08) | 2.64 (0.67) | 2.54 (0.82) | 0.2ns | |
10-25 | 2.07ns | 0.42 (0.36) | 0.51 (0.19) | 8.17 (12.85) | 6.55 (9.93) | 0.2ns | ||
Student t1 | 3.43** | 2.69* | 0.75ns | 0.81ns | - | |||
K | 0-10 | 5.36* | 4.16 (1.30)ab | 10.18 (6.00)a | 1.63 (0.41)b | 1.91 (0.62) | 0.7ns | |
10-25 | 1.51ns | 1.24 (1.05) | 0.82 (0.31) | 5.50 (8.64) | 4.85 (7.35) | 01ns | ||
Student t1 | 3.91** | 3.49** | 0.77ns | 0.74ns | - | |||
Ca | 0-10 | 7.53* | 3.54 (10.95)ab | 24.85 (14.63)a | 2.52 (1.64)b | 1.53 (0.49) | 2.2ns | |
10-25 | 0.42ns | 8.08 (6.88) | 4.87 (1.82) | 10.48 (16.48) | 10.28 (15.58) | 0.02ns | ||
Student t1 | 4.69** | 3.03* | 0.84ns | 0.51ns | - | |||
Mg | 0-10 | 4.26* | 0.78 (0.24)b | 1.72 (1.01)a | 0.44 (0.11)b | 0.38 (0.12) | 0.7ns | |
10-25 | 1.96ns | 0.32 (0.03) | 0.70 (0.26) | 3.52 (5.54) | 2.91 (4.41) | 0.2ns | ||
Student t1 | 6.85*** | 2.18ns | 0.97ns | 0.49ns | - | |||
Na | 0-10 | 11.15* | 0.19 (0.06)a | 0.08 (0.05)b | 0.04 (0.01)b | 0.03 (0.01) | 1.1ns | |
10-25 | 1.60ns | 0.02 (0.02) | 0.03 (0.01) | 0.15 (0.24) | 0.08 (0.12) | 0.5ns | ||
Student t1 | 6.15*** | 2.27ns | 0.83ns | 0.69ns | - |
Root biomass (t ha-1) and their nutrient contents (g kg-1) of tropical rainforests. (*): Source: Noij et al. (
Country | TRP | DC (mm) | FRphytomass | N | P | K | Ca | Mg | Sources |
---|---|---|---|---|---|---|---|---|---|
Oxisols/Ultisols (infertile soils) | |||||||||
Cameroon | 9.6-29.91 | - | - | - | - | - | - | - | This study |
- | < 2 | 1.7-7.4 | 7.4-18.7 | 0.4-2.0 | 0.8-4.4 | 1.1-5.9 | 0.2-0.8 | This study | |
- | 2-5 | 4.2-4.6 | 6.7-21.7 | 0.4-1.8 | 1.1-2.9 | 1.1-7.5 | 0.3-0.9 | This study | |
- | 5-20 | 3.3-21.0 | - | - | - | - | - | This study | |
44.52 | - | - | - | - | - | - | - | Kanmegne 2004 | |
Ghana (Kumasi) | - | 5/6 | - | 10.00 | 0.14 | 0.69 | 1.10 | 0.62 | Greenland & Kowal 1960 |
Brazil | 32.2 | 6 | 14.57 | - | - | - | - | - | Klinge et al. 1975 |
Venezuela | - | 6 | 2.0 | - | - | - | - | - | Jordan & Escalante 1980* |
- | 2 | 15.4 | - | - | - | - | - | Sanford 1980* | |
56 | 6 | 32 | - | - | - | - | - | Stark & Pratt 1977* | |
Other soil types | |||||||||
Costa Rica | - | 2 | 2.7 | - | - | - | - | - | Raich 1980* |
14.4 | 2 | 2.9 | - | - | - | - | - | Jordan 1985* | |
Venezuela | - | 6 | 32.9 | - | - | - | - | - | Herrera & Klinge 1978* |
132.2 | 6 | 92.5 | - | - | - | - | - | Klinge et al. 1975 | |
Ghana (Kade) | 22.1 | 5/6 | 4.45 | 8.1 | 0.8 | 5.6 | 8.8 | 2.0 | Greenland & Kowal 1960 |
Nouvelle Guinée | 40.0 | 5 | 2.8 | 7.5 | 0.3 | 3.9 | 7.1 | 6.1 | Grubb & Edwards 1982* |
Porto Rico | - | 5/6 | - | 7.0 | 0.3 | 4.2 | 5.6 | 1.8 | Ovington & Olsen 1970* |
Venezuela | - | 5/6 | - | - | 1.2 | 7.1 | 9.3 | 2.7 | Hase & Folster 1982* |
- | 5/6 | - | 10.0 | 0.1 | 0.7 | 1.1 | 0.6 | Golley et al. 1980* | |
Tropical forests | - | 5/6 | - | 6.7 | 0.6 | 2.9 | 1.6 | 1.4 | Vitousek & Sanford 1986 |
- | 5/6 | - | 10.7 | 0.3 | 1.1 | 1.2 | 1.5 | Vitousek & Sanford 1986 |
Variation of soil available P, total K, Mg, Ca and N, and soil water content among three undisturbed forests. Values in kg ha-1. Source: van Gemerden & Hazeu (
Elements | Ebom | Ebimimbang | Nyangong |
---|---|---|---|
Total N | 8900 | 7000 | 12500 |
Available P | 28 | 24 | 8 |
K | 755 | 370 | 360 |
Mg | 175 | 165 | 195 |
Ca | 1785 | 1810 | 1065 |
Water content available (%)* | 17 | 11 | 10 |