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Article

Abstract

In inland California grasslands, the high densities of alien annual species have altered the growing environment for native perennial grasses. Using variable-density plots, we measured the influence of intraspecific competition (conspecifics only) and diffuse competition (mixed-composition neighborhoods that include conspecifics) on growth and survival 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 (16–356 plants/m2). We used summer fire and spring sheep grazing to reduce diffuse competition in unweeded plots. The potential effect of rooting 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. Intraspecific competitive effects were apparent only in the absence of diffuse competition. The effects of grazing and soil depth on growth were only short-lived interactions with the burning treatment. Burning was a longer-lived interaction, but only in weeded plots. Plant mortality was significantly increased by diffuse competition. Overall, N. pulchra survival was greatest in weeded plots, in grazed plots, and in deeper soil plots. The growth of N. pulchra individuals was negatively affected by alien annual species in all treatment combinations. Our data indicate that recruitment of N. pulchra within inland California grasslands is reduced by the adverse environment created by high densities of alien annual species. Successful attempts to increase populations of N. pulchra through management of the grassland community must involve significant modification of the biotic environment.

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