Differential Responses of Dipterocarp Seedlings to Soil Moisture and Microtopography

Niche diversification is prominent among the mechanisms proposed to explain tropical rain forest tree diversity, with many studies focusing on trade-offs among shade tolerance and growth. Less obvious is the impact of occasional, ephemeral and often minor disturbances on tree seedling survival. We propose that differential tolerances to soil waterlogging can contribute to the distribution of tree seedling communities along microtopographical gradients. We test this hypothesis experimentally by evaluating survival and performance of planted seedlings across microtopographical gradients in a periodically inundated tropical rain forest environment. Survival and relative growth rates were assessed for six Shorea (Dipterocarpaceae) species in Sepilok Forest Reserve (Sabah, Malaysia) over a 2-yr period, during which seedlings were subjected to two brief flooding events. The species were selected on the basis of soil habitat affinities, with two species being primarily associated with low-lying alluvial flats subject to inundation, two being associated with non-flooded mudstone hills, and two species occurring in both habitats. Seedling performance was related to microtopographic elevation within and among plots and to soil moisture among plots. The faster growing species, Shorea argentifolia, Shorea leprosula and Shorea parvifolia, tended to be more vulnerable to high soil moisture in terms of mortality than the three species with lower growth rates. Within plots, soil moisture was inversely correlated with microelevation, and seedlings located at higher microelevations had an increased probability of survival. Microtopographical differences in seedling position could therefore contribute to species assembly processes through differential mortality, particularly in areas subject to minor and ephemeral flooding events.