Webb (1968) noted that although rainforests typically favour nutrient-rich or eutrophic soils, this correlation is not causal in the sense that rainforest trees have a physiological dependence upon high levels of soil nutrients. Rather, he argued that the occurrence of rainforest on eutrophic soils is more a consequence of the overarching effect of fire on the growth and survival of rainforest trees. Thus soil fertility is not merely determined by the underlying geology of a site but by fire history, nutrient cycling and topography. For instance, Webb argued that, in seasonally dry and geologically infertile environments, rainforest is restricted to topographically fire-protected sites, and that the absence of burning allows rainforest to enrich soils by nutrient cycling. The purpose of this chapter is to critically consider the role of nutrient cycling, fire and topography in creating differences in soil fertility between rainforest and non-rainforest vegetation.

Nutrient cycling and rainforest soil fertility

The view that differences in nutrient cycling contribute to the higher soil fertility of rainforest compared with adjacent non-rainforest is supported by the available data. For instance, Chandler and Lamb (1986) provide data on differences in nutrient use by rainforest and non-rainforest vegetation using the index of ‘nutrient use efficiency’ developed by Vitousek (1984).