Geographical Variation in Fertility, Phenology, and Composition of the Understory of Neotropical Forests

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Abstract
We compare levels of flowering and fruiting in 55 samples of Neotropical forest understory from 13 sites in 6 countries. Each sample consists of a census of fertile understory plants along a transect. Changes in species richness and density of fertile understory plants are correlated with rainfall and soil fertility. Areas with weak (or no) dry seasons and intermediate to rich soils average 64 fertile plant species and 174 individuals per sample, whereas areas with poor soil and a strong dry season average only 5 fertile species and 8 fertile individuals. Areas with either strong dry seasons and good soils or weak dry seasons and very poor soils have intermediate values. Taxonomic composition of the understory also changes predictably with rainfall and soil fertility. In increasingly stressed forests changes are found in understory structure, with sequential loss of terrestrial herbs, epiphytes, understory shrubs, and lianas. The understory of the poorest soil site consists almost entirely of young trees. The effects of seasonal differences at a given site are small compared with between-site differences. We suggest that the level of understory fertility may provide a simple indicator of overall ecosystem productivity.