Density and species diversity of trees in four - Makerere University

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G. Eilu et al. Table 5 Canonical coefficients and interset correlations between the site scores on the first four constrained axes and environmental variables generated by canonical correspondence analysis of 120 subplots. The strongest correlations are indicated with asterisks Environmental DISCUSSION Canonical coefficients variable Interset correlations variable axis 1 axis 2 axis 3 axis 4 axis 1 axis 2 axis 3 axis 4 Altitude (m)* 0.296 −0.336 −0.170 0.214 0.009 −0.703 0.022 0.312 Rainfall (mm)* 0.150 −0.152 0.504 −0.146 −0.041 −0.285 0.869 0.099 pH* 0.622 −0.067 0.076 −0.046 0.675 0.472 0.025 −0.120 Exc. Al −0.268 −0.147 0.082 −0.181 −0.673 −0.431 −0.110 0.024 Av P (mg/kg) 0.100 −0.143 0.018 0.026 0.185 −0.288 −0.171 −0.018 Organic C (%) 0.211 0.038 0.013 0.145 −0.073 −0.488 −0.457 −0.137 Total N (%) 0.077 −0.003 −0.010 0.009 0.018 −0.459 −0.491 −0.168 K (me/100 g) −0.005 −0.103 0.016 −0.277 0.448 −0.100 −0.263 −0.436 Na (me/100 g) 0.029 −0.035 0.012 −0.061 0.181 0.154 −0.128 −0.239 Slope (°) −0.030 −0.021 0.042 −0.022 −0.026 −0.039 0.525 −0.043 Silt (%) 0.053 0.050 0.003 0.075 −0.122 0.037 −0.137 0.369 Total P (%) −0.017 0.048 0.027 0.075 0.294 −0.136 −0.061 0.228 Loss on ignition (%) 0.000 −0.178 −0.071 −0.260 −0.099 −0.477 −0.506 −0.204 Canopy density (%) −0.061 0.013 0.008 −0.054 −0.064 0.000 0.360 −0.108 Marginal effects Conditional effects Variable* λ 1 Variable λ A P F pH 0.39 pH 0.36 0.005 7.02 Exc. Al (me/100 g) 0.37 Altitude 0.26 0.005* 4.99 Rainfall (mm) 0.23 Rainfall 0.23 0.005* 4.37 Altitude (m) 0.22 Loss on Ignition 0.13 0.005* 2.58 K (me/100 g) 0.21 K 0.09 0.005* 1.82 Loss on Ignition (%) 0.19 Av. P 0.08 0.020* 1.59 Organic C (%) 0.18 Slope 0.08 0.025* 1.48 Total N (%) 0.17 Canopy density 0.06 0.080 1.28 Slope (Deg.) 0.11 Total N 0.06 0.180 1.12 Av. P (mg/kg) 0.11 Organic C 0.05 0.380 1.03 Total P% 0.11 Exc. Al 0.05 0.420 0.99 Canopy density (%) 0.08 Silt 0.05 0.445 1.00 Na (me/100 g) 0.08 Total P 0.04 0.585 0.91 Silt (%) 0.08 Na 0.05 0.790 0.84 *Variables are arranged in descending order of variance explained by each variable. λ 1 shows variance explained without considering other variables; λ A shows variance explained after successively selecting the most important variables. Compared with other studies from tropical forests, the Albertine rift forests are considered ‘low’ in species richness (Turner, 2001). The Neotropical wet forests such as those in Ecuador and Colombia are richer than the present study sites. The present results emphasise the importance of soil physicochemical properties in influencing tree species richness. There were slight differences in tree species composition from south to north with species in Bwindi being tolerant to lower pH through Kasyoha, to Kibale and Budongo with the least acid soils. This may explain the postulated south to north gradient in species Table 6 Variables explaining the tree species-environment relation selected by Canonical Correspondence Analysis using automatic forward selection composition in Uganda proposed by Hamilton (1974, 1982) but attributed to the existence of a Pleistocene refugium close to forests of south-western Uganda. Hall & Swaine (1976) noted that in closed canopy forests of Ghana, soil factors such as pH and richness in bases that showed notable variation with species distribution were dependent on rainfall. Importance of pH in forest soils is in terms of influence on the availability of phosphorus, calcium, magnesium, and trace elements. Tanner (1977) noted that the limiting factor in one of four montane rain forests of Jamaica (Mor Ridge forest) was the extremely low pH, a factor that may be considered limiting in our study sites. 308 Diversity and Distributions, 10, 303–312, © 2004 Blackwell Publishing Ltd
A number of studies (e.g. Wolf, 1994; Lieberman et al., 1996; Vázquez & Givnish, 1998; Lovett et al., 2001) showed the influence of altitudinal gradients on tree species distribution. Although the altitude gradient in the present study was short (530 m), it played a major role in distribution of tree species demonstrated by the P-value of 0.005 in the automatic forward selection by canoco. This is mainly through its relationship with rainfall and pH. Rainfall was highest in Bwindi (with the highest elevation) while pH conformed to the reverse trend over this altitudinal range. Hall & Swaine (1976) noted that altitude was important in influencing tree species distribution, but mainly in the wetter forests. Friis (1992) showed that critical altitudes existed for groups of taxa (based on trees) in north-eastern tropical Africa and Hamilton (1975) noted that the number of species and families of trees declined with altitude. Friis (1992) noted that interaction between rainfall and altitude made correlation with diversity complex. The correlation between altitude and rainfall of 0.394 in the present data set was not particularly strong. Dry season length at the study sites ranged between a total of 4–7 months and may influence the results, but the magnitude of this influence could not be established because of the slight variation between sites. The southernmost sites in this study were the wettest progressing towards the drier sites north. There were many species, particularly in Bwindi, that were favoured by high rainfall, while in Budongo some species tolerated lower levels of rainfall. Hall & Swaine (1976) found that tree species were most related to the rainfall gradient. Decrease in precipitation northwards explains part of the floristic impoverishment in forests of western Uganda along a south–north gradient. Effects of regional variables such as annual rainfall and altitude on distribution have been proved in many studies and are not discussed further. When effects of pH, altitude and rainfall are accounted for (Table 6), loss of organic carbon on ignition, K, available P and slope contributed significantly to the model of already included variables. In individual forests geographical position (Plumptre, 1996) influenced tree species composition possibly as a reflection of major gradients in environmental factors. Walaga (1993) noted that forest compartments of Budongo had differences in tree species composition attributed to differences in exchangeable potassium, nitrogen, sodium, and sand. The differences were attributed to accumulation of organic matter, mineral decomposition, leaching losses and erosion of topsoil. In the present study, the Budongo plots and Kibale 3 had species (occurring in Cynometra forest) that tolerate relatively high levels of exchangeable potassium, high pH (less acid soils), gentle slopes, relatively low elevations and low rainfall. This cluster of plots included tree species requiring relatively high levels of potassium as suggested by Walaga (1993). According to Walaga (1993), 11 soil variables explained 8.9% of the variation in tree species distribution. The present study explained about twice this percentage, emphasising the relative importance of environmental factors recorded. The present study did not assess the influence of disturbance but considered only sites of intact forest where impacts of human disturbance were minimal. Trees in Ugandan tropical forests Environmental factors measured therefore only partly explained tree species richness and diversity in forests of the Albertine rift. For example, Hall (1977) noted that soil type was the basis for the main division of forest into groups, with discontinuous forest variation reflecting the presence of two contrasting major soil units in Nigerian forests. Tree species richness and diversity is therefore influenced by geographical position (Plumptre, 1996), soil depth and composition, length and frequency of rainfall (or dry season), successional stage, and management history. Soil depth may be limiting because of inadequate support provided by very shallow soil (Tanner, 1977). In conclusion, local environmental variables (e.g. soil factors) as well as regional environmental factors (e.g. rainfall and altitude) played important roles in influencing tree species distribution in the Albertine rift forests. Differences in tree species distribution observed over the soil nutrient gradients and over gradients of other environmental factors should be taken into account when designing management strategies. The most important variable in forests of the Albertine rift is therefore soil pH which is correlated with mechanical soil properties and gross climate. Major environmental gradients cut across forests and strategies for managing forests of the Albertine rift should be designed at the landscape level. ACKNOWLEDGEMENTS The study was funded by the Danish International Development Agency (DANIDA) under the ‘Biodiversity Research and Training in Tanzania and Uganda’ project through the Makerere– Copenhagen University Enhancement of Research Capacity (ENRECA) project. I. Friis, J.M. Kasenene and A.D. Poulsen supervised the work. M. Rejmánek and three anonymous referees provided comments that greatly improved the manuscript. Various staff at the Kew herbarium, particularly K. Vollesen, B. Verdcourt, M. Lock and D. Bridson helped with identification of specimens. D.N. Nkuutu, J. Kyomuhendo and S. Kimuli collected the specimens. REFERENCES Brokaw, N. & Busing, T. (2000) Niche versus chance and tree diversity in forest gaps. Trends in Ecology and Evolution, 15, 183–188. Burnham, K.P. & Overton, W.S. (1979) Robust estimation of population size when capture probabilities vary among animals. Ecology, 60, 927–936. Clark, D.B., Palmer, M.W. & Clark, D.A. (1999) Edaphic factors and the landscape-scale distributions of tropical rain forest trees. Ecology, 80, 2662–2675. Colwell, R.K. & Coddington, J.A. (1994) Estimating terrestrial biodiversity through extrapolation. Philosophical Transactions of the Royal Society of London B, 345, 101–118. De Carvalho, L.M.T., Fontes, M.A.L. & De Oliviera-Filho, A.T. (2000) Tree species distribution in canopy gaps and mature forest in an area of cloud forest of the Ibitipoca range, southeastern Brazil. Plant Ecology, 149, 9–22. Diversity and Distributions, 10, 303–312, © 2004 Blackwell Publishing Ltd 309
Page 1 and 2: Diversity and Distributions, (Diver
Page 3 and 4: Table 1 Tree species numbers and di
Page 5: Figure 1 Canonical Correspondence A
Page 9 and 10: Trees in Ugandan tropical forests A

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