Document: JEF-3-38-11

Radial dispersion of neighbors and the small-scale competitive impact of two annual grasses on a native perennial grass.

FEHMI, J.S.*, E.A.LACA, K.J.RICE and D.BARTELL

University of California, One Shields Ave., Davis, CA 95616-8515 ¹

Abstract:
Annual grasses from the Mediterranean region of Europe now dominate many of California's grasslands possibly taking over from native perennials such as Nassella pulchra. Among other invaders, two grasses Lolium mutiflorum (Lomu) and Bromus hordeaceus (Brho), commonly dominate portions of the grassland and are also found together in varying proportions. We hypothesized that small-scale differences in spatial distribution and plant species composition of these alien annuals, on the order of 300 cm² (i.e. pattern at scales < 10 cm radius) could be important determinants of Nassella pulchra fitness and performance. The two grasses were planted in 20 cm diameter circular plots at a constant rate of 1 seed per cm² surrounding a Nassella plant. The plots were divided to allow for three spatial distributions of the two annuals: No divisions (whole plots planted), one division (half planted, half empty), and two divisions (two opposing quarters planted, two empty). Each species was planted in each distribution and they were combined as mixed, half Lomu/ half Brho, and two quarters Lomu/ two quarters Brho (nine treatments). At the center of each plot, a single Nassella plug was was planted. In January 1999, 5 replications of each treatment were established in randomized complete blocks. Outside the plots, we planted an equal mix of Lomu and Brho (1 seed per cm²) to fix the size of the gaps to the designed size. Nassella pulchra showed no effect of the species of competitor or from the distribution of the competitors. Both interspersion of patches of open resources (bare ground) and separation of competitors into patches or their removal within 20 cm did not benefit Nassella. This implies that at small scales, Lomu and Brho compete equally as well with Nassella and that Nassella doesn't benefit from the aggregation of competitors both in terms of the species composition and in terms of limited amounts of unoccupied soil at the plant base.

Keywords: additive design, neighbor effect, spatially mediated interaction, asymmetric competition, plant interference

This abstract is being presented at: 11:30 AM in session:
Oral Session #72: Plant Competition.