title: Seed-bank structure and plant-recruitment conditions regulate the dynamics of a grassland-shrubland Chihuahuan ecotone.
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/m2), although is largely concentrated in densely vegetated areas (with peaks up to 800-1,200 seeds/m2in 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.
authors.affiliation: Department of Geography, Durham University, South Road, Durham, DH1 3LE, United Kingdom.|Institute of Environmental Assessment and Water Research (IDAEA), Spanish Research Council (CSIC), Jordi Girona 18, Barcelona, 08034, Spain.
authors.forename: Mariano
authors.initials: M
authors.lastname: Moreno-de Las Heras
authors.affiliation: Department of Geography, Durham University, South Road, Durham, DH1 3LE, United Kingdom.
authors.forename: Laura
authors.initials: L
authors.lastname: Turnbull
authors.affiliation: Department of Geography, Durham University, South Road, Durham, DH1 3LE, United Kingdom.
authors.forename: John
authors.initials: J
authors.lastname: Wainwright
mesh_terms: D003247:Conservation of Natural Resources; D003889:Desert Climate; D017753:Ecosystem; D065948:Grassland; D009516:New Mexico; D012639:Seeds; D012987:Soil; D015206:Southwestern United States
chemical_list: D012987:Soil
keywords: Chihuahuan Desert; Sevilleta LTER; black grama; creosotebush; seedling recruitment; shrub encroachment; soil seed bank