What is a Safe Site?: Neighbor, Litter, Germination Date, and Patch Effects

Abstract

To investigate some of the factors that determine the fate of a seedling, censuses of seedlings of two native grasses, Aristida longiseta and Bouteloua rigidiseta, were made at frequent intervals in a Texas grassland for 2 yr. Seedlings of both species with neighboring seedlings or juvenile plants of either within 2 cm had higher rates of survival and growth than those without such neighbors, implying that the effects of aggregation in favorable microsites outweighed the effects of competition among these plants. Neighboring adults of these species, and neighboring juveniles of other species, had positive effects in some instances, negative in others. Litter, even in amounts too small to cover the surface, reduced the survival rates of seedlings of both species, while surface rocks may have increased them. Overall, seedlings that germinated earlier in the autumn were more successful. The rank order of quadrat favorableness for seedlings varied in a complex fashion between species, between years, among census intervals, and among germination cohorts. This variation in patch favorableness must prevent these populations from reaching a stable spatial distribution and from reaching a stable age distribution in any given patch.