Changes in Plant Community Diversity and Floristic Composition on Environmental and Geographical Gradients

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Abstract
Trends in community composition and diversity of neotropical forests as measured by a series of samples of (1) plants .gtoreq. 2.5 cm dbh in 0.1 ha, (2) plants over 10 cm dbh in 1-ha plots, and (3) complete local florulas are analyzed as a function of various environmental parameters. These trends are also compared with those found in similar data sets from other continents. Altogether the basic 0.1-ha data sets are reported for 87 sites in 25 countries on six continents and several islands. New data from ten 1-ha tree plots in upper Amazonia are also compared with each other and with similar data from the literature. Some noteworthy trends include: (1) Lowland neotropical plant species richness is generally far more tightly correlated with precipitation than with edaphic factors. (2) The nearly linear increase of lowland neotropical plant species richness with precipitation reaches an asymptote (community saturation?) at about 4,000 mm of annual rainfall. (3) Although the species represented in adjacent forest types on different substrates may change dramatically, diversity tends to change relatively little in upper Amazonia. (4) The species present at different sites are very different but the families represented and their diversities are highly predictable from environmental parameters. (5) On an altitudinal gradient in the tropical Andes there is a sharp, essentially linear decrease in diversity from about 1,500 m to near the upper limit of forest above 3,000 m. (6) There is no indication of a "mid-elevation bulge" in diversity, at least not in the sampled habit groups. (7) Even near timberline, montane tropical forests are as diverse as the most diverse temperate forests. (8) Moist subtropical forests are markedly less diverse than their inner-tropical equivalents, but dry subtropical forests in Mexico are apparently richer in species than inner-tropical dry forests. (9) Central African forests are about as species rich as neotropical forests with similar amounts of precipitation, but forests in tropical West Africa are relatively depauperate. (10) Tropical Australasian forests are no more diverse than equivalent neotropical forests; the world''s highest tree species diversity is in upper Amazonia, not Southeast Asia. (11) Contrary to accepted opinion, equivalent forests on the three continents are similar in plant species richness and (with a very few notable exceptions) floristic composition but are markedly different in structure. The predictability of the floristic compositions and diversities of tropical forest plant communities seems strong, albeit circumstantial, evidence that these communities are at ecological and perhaps evolutionary equilibrium, despite indications that certain aspects of their diversity are generated and maintained stochastically.