Woody life-form composition and association on rainfall and soil fertility gradients in Ghana

The hypothesis that treelets/shrubs are more sensitive than trees to drought and low soil fertility was tested by analysing species densities (species per unit area) in 155 tropical forest plots (0.0625 ha) in Ghana, West Africa. Species densities of both life forms decreased with increasing dryness and with increasing soil fertility, which were correlated. Goodness of fit tests detected species preferences for four combinations of rainfall (dry: < 1500 mm yr(-1), wet: greater than or equal to 1500 mm yr(-1)) and soil fertility (two classes based on a composite variable with high weightings for total exchangeable bases and pH), and a meta-analysis of these results compared life forms. The results contradicted the hypothesised response of life forms in two ways: (i) species densities of treelets/shrubs were comparably or relatively less reduced by dry conditions than were tree species densities, and (ii) treelet/shrub species did not show a more frequent preference for wet or fertile sites than did tree species. Light availability and rooting depth are discussed as explanations for the sensitivity of understorey plants to water and nutrient stress, and for the data's failure to support the hypothesis.