Submitted to: Ecology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 9/9/2002
Publication Date: 1/1/2003
Citation: Wilsey, B.J., Polley, H.W. 2003. Effects of seed additions and grazing history on diversity and productivity of subhumid grasslands. Ecology. 84(4):920-931.
Interpretive Summary: Species-rich plant communities may better resist invasive plants and be more productive than species-poor communities, but processes that promote species richness or diversity are poorly understood. One view holds that the number of species present in plant communities, like grasslands, depends more on the number of species that are available in soil as seeds (seed limitation) than on processes that regulate participation in the plant community. Disturbances, like grazing, are thought to increase species diversity by increasing establishment from seeds. We tested for seed limitation of species diversity in Texas grasslands by adding seeds of 20 native species to restored grasslands that were continuously grazed or had not been grazed for 1, 2, or 6 years and to an unplowed tallgrass prairie. Contrary to expectation, the disturbance caused by current grazing did not increase seedling establishment. Greatest number of seedlings occurred in grasslands not grazed for 1-2 years where both light at the soil surface and soil water were available at moderate levels. Species diversity decreased with time since grazing. Adding seeds of species that were absent from these grasslands caused a slight increase in the number of species present in grasslands along the grazing gradient, but did not affect species diversity in the species-rich prairie. Our results are not consistent with currently-held views that species diversity is regulated primarily by seed availability and that disturbances, like grazing, increase diversity by increasing plant recruitment from seeds. Rather, our results indicate that species diversity in these grasslands usually depends more on the rate at which existing species are lost than on recruitment of new species from seeds.
Technical Abstract: Grassland species richness (density) and primary productivity may be limited by seed availability. However, it is not known how widespread this limitation is, whether it is affected by disturbance, or which mechanisms underlie any disturbance effect. We tested for seed limitation and explored the role of litter accumulation in explaining effects of grazing on seedling establishment, species diversity (density and evenness), and grassland productivity. We added seeds and removed litter in a factorial arrangement within each of five fields: currently grazed, no grazing for 1-2 , 2-3, or 6-7 years, and an unplowed prairie. Seeds from each of 20 native species were added to replicated 1 m2 plots in each field during each of two growing seasons. Seedling establishment increased with time since grazing when litter was removed, and was greatest in fields not grazed for 1-3 years when litter was present. Light availability at the soil surface decreased and soil water availability increased with time since grazing, suggesting that limits on recruitment changed from water to light following release from grazing. Aboveground productivity during the second year of the study was reduced by seed additions along the grazing gradient. Seed additions increased species density but decreased species evenness in fields along the grazing gradient, resulting in no net effect on diversity. In the species-rich prairie, seed additions had no effect on productivity or species density or evenness. Results indicate that seed availability did not limit productivity and diversity, and that local extinction may be more important than local recruitment in explaining species richness in these grasslands during most years.