Maintenance of forest species diversity and latitudinal gradient

The species diversity of trees maintained in tropical rain forests is much higher than in temperate, boreal, or seasonally dry tropical forests. Many hypotheses have been proposed for higher diversity in tropical rain forests, including: (i) higher specialization of resource use, (ii) different mode of disturbance, (iii) smaller opportunity for competition on oligotrophic soil, (iv) higher productivity, (v) more active specific herbivores and pathogens, (vi) evolutionary/ecological history. In this paper we report mathematical models for tree-by-tree replacement. First the analysis of random drift model shows that the effect of gap size to species diversity is not very strong. Second we study phenological segregation model, which has the following assumptions: Basic mechanism for many species to coexist in the community is assumed given by the storage effect of lottery model, as species differ in seasonality in peak fruit production and in the subsequent period of high regeneration ability. Gaps formed during unfavorable season accumulate and become available for regeneration in the beginning of the growing season. The resulting synchronization of regeneration opportunity jeopardizes the coexistence of many similar species in seasonal environments. Analysis of a mathematical model shows: (1) the existence of unfavorable season can greatly reduce the diversity of coexisting species. (2) Diversity in the equilibrium community can be high when niche width of each species is broad and resource use is strongly overlapped. (3) Equilibrium community may include several distinct groups of species differing in phenology of regeneration. Effect of unequal niche width and frequency dependent regeneration are also examined.