The roles of community biomass and species pools in the regulation of plant diversity

Considerable debate has developed over the importance of community biomass and species pools in the regulation of community diversity. Attempts to exp lain patterns of plant diversity as a function of community biomass or prod uctivity have been only partially successful and, in general, have explaine d only a fraction of the observed variation in diversity. At the same time, studies that have focused on the importance of species pools have led some to conclude that diversity is primarily regulated in the short term by the size of the species pool rather than by biotic interactions. In this paper , I explore how community biomass and species pools may work in combination to regulate diversity in herbaceous plant communities. To address this pro blem, I employ a simple model in which the dynamics of species richness are a function of aboveground community biomass and environmentally controlled gradients in species pools. Model results lead to two main predictions abo ut the role of biomass regulation: (1) Seasonal dynamics of richness will t end to follow a regular oscillation, with richness rising to peak values du ring the early to middle portion of the growing season and then declining d uring the latter part of the season. (2) Seasonal dieback of aboveground ti ssues facilitates the long-term maintenance of high levels of richness in t he community. The persistence of aboveground tissues and accumulation of li tter are especially important in limiting the number of species through the suppression of recruitment. Model results also lead to two main prediction s about the role of species pools: (1) The height and position of peak rich ness relative to community biomass will be influenced by the rate at which the species pool increases as available soil resources increase. (2) Variat ions in nonresource environmental factors (e.g. soil pH or soil salinity) h ave the potential to regulate species pools in a way that is uncorrelated w ith aboveground biomass. Under extreme conditions, such nonresource effects can create a unimodal envelope of biomass-richness values. Available evide nce from the literature provides partial support for these predictions, tho ugh additional data are needed to provide more convincing tests.