INTRASPECIFIC AND DIFFUSE COMPETITION - THE RESPONSE OF NASSELLA PULCHRA IN A CALIFORNIA GRASSLAND

In inland California grasslands, the high densities of alien annual sp ecies have altered the growing environment for native perennial grasse s. Using variable-density plots, we measured the influence of intraspe cific competition (conspecifics only) and diffuse competition (mixed-c omposition neighborhoods that include conspecifics) on growth and surv ival of Nassella pulchra, purple needlegrass. We assessed the effects of intraspecific and diffuse competition in weeded plots and unweeded plots, respectively, across a density gradient of N. pulchra plants (1 6-356 plants/m(2)). We used summer fire and spring sheep grazing to re duce diffuse competition in unweeded plots. The potential effect of ro oting volume on competitive interactions was explored by establishing plots on two sites of different soil depth. Diffuse competition had an overriding influence on N. pulchra growth in all treatments. Intraspe cific competitive effects were apparent only in the absence of diffuse competition. The effects of grazing and soil depth on growth were onl y short-lived interactions with the burning treatment. Burning was a l onger-lived interaction, but only in weeded plots. Plant mortality was significantly increased by diffuse competition. Overall, N. pulchra s urvival was greatest in weeded plots, in grazed plots, and in deeper s oil plots. The growth of N. pulchra individuals was negatively affecte d by alien annual species in all treatment combinations. Our data indi cate that recruitment of N. pulchra within inland California grassland s is reduced by the adverse environment created by high densities of a lien annual species. Successful attempts to increase populations of N. pulchra through management of the grassland community must involve si gnificant modification of the biotic environment.