The distribution of vascular plant species and guilds in space and time along a desert gradient

We studied distribution patterns of vascular plant species and environmental variables for three years along a permanent transect traversing a closed-drainage watershed in the northern Chihuahuan Desert of south-central New Mexico, USA. The transect extended for 2.7 km from a basin floor playa (1310 m elevation, fine-textured soil), across a piedmont slope, and onto the base of a granitic mountain (1410 m elevation, coarse-textured soil). The gradients in elevation and soils across our transect, along with variable seasonal rainfall, downslope redistribution of water and organic matter, and soil texture-related variation in infiltration, water holding capacity, and moisture release characteristics, interact to generate a complex spatial and temporal gradient of available soil water and nitrogen. We grouped plant species into guilds according to growth form and photosynthetic pathway type. These guilds are spatially and temporally differentiated along the transect such that particular groups utilize particular seasonal phases or spatial regions of the gradient. We identified six distinct plant communities along the transect. C4 perennial grasses dominated the mesic/high nitrogen portion of the gradient, which occurred at the highest (upper piedmont grassland, dominated by Bouteloua eriopoda) and lowest (playa, dominated by Panicum obtusum) elevations along the transect. C3 shrubs were dominant in the xeric/low nitrogen portion of the gradient located near the middle of the transect (bajada shrubland, dominated by Larrea tridentata). C3 shrubs also dominated a narrow zone of vegetation located adjacent to the playa (playa fringe, dominated by Prosopis glandulosa). C4 perennial grasses, C3 subshrubs, and C3 and C4 perennial forbs and annuals were co-dominant in the intermediate locations along the gradient, which occurred below (mixed basin slopes) and above (lower piedmont grassland) the bajada shrubland. Life-form distribution patterns at the small scale of our study reflect some of the patterning that occurs at larger scales in response to climate gradients. The distributions of some species and guilds along the transect are apparently modified by competitive interactions.