The effect of small gaps in California annual grassland on above-ground biomass production

Small gaps and clumped species distributions are common in grasslands. In California annual grasslands, patches of Lolium multiflorum Lam. and Bromus hordeaceus L. are often separated by gaps. These gaps potentially limit the productivity and associated resource use of these grasslands. The effect that differences in spatial aggregation, gap distribution and species mixing on 20-cm-diameter plots has on overall forage production by these two grasses was tested. There were three levels of aggregation: whole plots planted; half planted/half empty; two opposing quarters planted/two empty. Each species was planted in each distribution, and they were combined as mixed, half L. multiflorum/half B. hordeaceus and two quarters L. multiflorum/two quarters B. hordeaceus (nine treatments). Plant aggregation had no significant effect on above-ground production of whole plots, but individual tillers near gaps were significantly larger than others. Plasticity in the growth of individual annual grasses effectively buffered against variation in average productivity resulting from variations in plant distribution. There were significant (P < 0.001) differences in forage production as a result of the species the plots contained. Plots containing only L. multiflorum produced 4053 kg of dry matter (DM) ha^-1, B. hordeaceus plots produced 2448 kg of DM ha^-1, and plots containing both species produced 4712 kg of DM ha^-1. At small scales, spatial distribution was less important than species composition in determining annual grassland productivity.