Abstract
Large areas of desert grasslands in the southwestern United States have shifted to sparse shrublands dominated by drought‐tolerant woody species over the last 150 yr, accompanied by accelerated soil erosion. An important step toward the understanding of patterns in species dominance and vegetation change at desert grassland–shrubland transitions is the study of environmental limitations imposed by the shrub‐encroachment phenomenon on plant establishment. Here, we analyze the structure of soil seed banks, environmental limitations for seed germination (i.e., soil‐water availability and temperature), and simulated seedling emergence and early establishment of dominant species (black grama, Bouteloua eriopoda , and creosotebush, Larrea tridentata ) across a Chihuahuan grassland–shrubland ecotone (Sevilleta National Wildlife Refuge, New Mexico, USA ). Average viable seed density in soils across the ecotone is generally low (200–400 seeds/m 2 ), although is largely concentrated in densely vegetated areas (with peaks up to 800–1,200 seeds/m 2 in vegetated patches). Species composition in the seed bank is strongly affected by shrub encroachment, with seed densities of grass species sharply decreasing in shrub‐dominated sites. Environmental conditions for seed germination and seedling emergence are synchronized with the summer monsoon. Soil‐moisture conditions for seedling establishment of B. eriopoda take place with a recurrence interval ranging between 5 and 8 yr for grassland and shrubland sites, respectively, and are favored by strong monsoonal precipitation. Limited L. tridentata seed dispersal and a narrow range of rainfall conditions for early seedling establishment (50–100 mm for five to six consecutive weeks) constrain shrub‐recruitment pulses to localized and episodic decadal events (9–25 yr recurrence intervals) generally associated with late‐summer rainfall. Re‐establishment of B. eriopoda in areas now dominated by L. tridentata is strongly limited by the lack of seeds and decreased plant‐available soil moisture for seedling establishment.