Seed vs. microsite limitation for seedling emergence in the perennial grass Stipa tenacissima L. (Poaceae)

This paper examines the relative influence of seed and microsite availability in the seedling emergence of Stipa tenacissima, a perennial grass inhabiting semi-arid Mediterranean steppes. A correlative approximation was used to establish 21 (10 × 10 m) plots along a climatic gradient in the Northern limit of the geographic distribution in East Spain. Seed production, seed removal by ants, ant nest density, and the relative cover of S. tenacissima, other perennial plants and bare ground, as explanatory variables, were measured in each plot. Multiple linear regression with forward stepwise selection procedure was used to analyze the relationships between seedling emergence and the explanatory variables. There was large variation between and within plots of all the studied variables, but cover of perennial plants was the only explanatory variable included in the regression model when all the plots were used for the analysis. We suggest that this result was the consequence that high annual precipitation on the Northernmost plots had on the cover of perennial plants and it is consistent with the experimental evidence that shadowing by perennial plants negatively affects germination and performance of the S. tenacissima seedlings. Once the Northernmost plots were removed from the model of regression then seed production, seed removal by ants and ant nest density significantly affected seedling emergence. We concluded that microsite quality, defined in relation to the shadowing capacity of perennial species, was more influential than seed and microsite availability at the Northern limit of the geographic distribution of S. tenacissima.