| How does species diversity affect ecosystem-level processes such as primary production and nutrient cycling? To investigate this, I created an experimental plant diversity gradient in restored field plots of serpentine grassland in south San Jose, CA, using four functional groups of plants: early season annual forbs (E), late seasonal annual forbs (L), perennial bunchgrasses (P), and nitrogen-fixers (N). These groups differ in growth form, phenology, and other traits relevant to nutrient cycling (e.g., rooting depth, litter C:N ratio). I planted two or three species of each type in single-group treatments, and in 2-, 3-, and 4-way combinations, giving a range of richness from 0 to 9 species. At the scale of this experiment ({dollar}alpha{dollar}-diversity and yearly production), more diverse treatments were not necessarily the most productive (Chapter 1). Relative yield totals indicated complementary resource use in some mixtures. However, absolute productivity in mixtures was not greater than the most productive monocultures because E's and L's reduced the biomass of P's (the most productive group in monoculture) much below expected levels based on planting density. Using an index that incorporates effects of plants on pool sizes of several depletable soil resources (inorganic nitrogen, available phosphorus, and water), I found a significant relationship of increasing resource use with increasing plant diversity (Chapter 2). However, leaching losses did not decrease as functional group richness increased in vegetated treatments, in part due to differences among functional groups in effects on microbial processes. E's and L's, but not P's, had significant positive effects on microbial immobilization of nitrogen, both in short-term (24 hr.; Chapter 2) and long-term (3-15 month; Chapter 3) {dollar}sp{lcub}15{rcub}{dollar}N experiments. The results indicate that (a) competition during critical parts of the growing season may prevent absolute increases in net primary production with increasing diversity, despite complementary resource use at other times of the year; (b) composition (the identity of the species present) can be as important as diversity (the number of species present) in controlling ecosystem processes; and (c) indirect effects of plants on microbial immobilization can be as large as effects of direct plant uptake for ecosystem nitrogen retention. |
