The effect of small gaps in California annual grassland on above-ground biomass production J. S. Fehmi, E. A. Laca and K. J. Rice Department of Agronomy and Range Science, University of California, One Shields Avenue, Davis, CA 95616- 8515, USA Abstract Small gaps and clumped species distributions are com- mon in grasslands. In California annual grasslands, pat- ches of Lolium multi¯orum 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 aggre- gation, 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. multi¯orum/half B. hordeaceus and two quarters L. multiflorum/two quarters B. hordeaceus (nine treatments). Plant aggregation had no signi®cant effect on above-ground production of whole plots, but individual tillers near gaps were signi®cantly 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 signi®cant (P <0á001) differ- ences in forage production as a result of the species the plots contained. Plots containing only L. multi¯orum produced 4053 kg of dry matter (DM) ha ±1 , B. hordeaceus plots produced 2448 kg of DM ha ±1 , and plots contain- ing 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. Keywords: spatial pattern, additive design, plant distri- bution, small-scale gaps, competitive effects Introduction Small gaps and patchy species distributions have been commonly observed throughout many grasslands and rangelands. Heady (1958) reported the occurrence of numerous 6á5 cm 2 bare patches in California grasslands. Pitt and Heady (1978) and Bartolome et al. (1980) reported foliar cover of 0á20±0á70 on grasslands distri- buted across California. Gaps and patchy spatial distri- butions of plants common in the annual grasslands of California may be related to resource limitation, as Bartolome et al. (1980) found that total foliar cover was well correlated with total precipitation on plots spanning California. However, aggregation of plants into mono- speci®c clumps appears to be an inef®cient strategy for resource capture by these grasses. Despite high root densities and effective exploitation of below-ground space (McConnaughay and Bazzaz, 1991), resources captured by plants should be maximized when individ- uals are well distributed (Sutherland and Benjamin, 1993; Benjamin, 1996). Grassland gaps appear to have the potential to limit the productivity of such grasslands. Among others, two introduced grasses, Lolium multi- ¯orum Lam. (Italian ryegrass; all nomenclature per Hickman, 1993) and Bromus hordeaceus L. (Blando brome; previously B. mollis per Munz and Keck, 1968), often dominate portions of the California grass- land (Wu and Jain, 1979; Heady et al., 1992) and are commonly found together (e.g. Gulmon, 1979; Fehmi, 1998). Both these grasses are preferred species and represent a signi®cant forage component of California annual grasslands (Jain et al., 1970; Young et al., 1996). We observed that patches of bare soil or plant litter often occurred between nearly monospeci®c small patches of these species in addition to other grasses. Studies are lacking that have considered the effect of gaps and spatial distribution on the production of L. multi¯orum and B. hordeaceus. Correspondence to: Dr J. S. Fehmi, ERDC-CERL, 2902 Newmark Drive, Champaign, IL 61826, USA. E-mail: j-fehmi@cecer.army.mil Mention of trade names or commercial products in this work is solely for the purpose of providing speci®c infor- mation and does not imply recommendation or endorse- ment by the US Department of Agriculture or the University of California. Received 8 September 2000; revised 19 January 2001 Ó 2001 Blackwell Science Ltd. Grass and Forage Science, 56, 323±329 323