Heterogeneity in ecological state transitions at multiple spatial scales in the northern Chihuahuan Desert

Ecological state transitions are characteristically heterogeneous in arid grasslands as well as in other systems. The causes of this heterogeneity emerge from hierarchical and overlapping scales of drivers and constraints including the spatial variation of drivers, spatial variation in response to drivers filtered by soils and landforms, and spatial contagion of transitions due to vegetation-environment feedbacks. Understanding the spatial and temporal patterns of shrub encroachment and state change are crucial for understanding ecological processes and prioritizing geographically-based management objectives. Three studies were conducted to quantify the spatial heterogeneity of ecological state transitions from regional to patch scales. Shrub encroachment was mapped at the regional scale across gravelly and sandy ecological sites, and modeled as a function of spatial variation in precipitation and topography. Landscape scale patterns of shrub encroachment were quantified and related to soil characteristics, historical management legacies, and historical perennial grass type. Patch scale patterns of bunchgrass seedling establishment and subsequent cover increase following a high precipitation period were modeled as a function of soil, topographic, and vegetation characteristics. At regional scales, patterns of shrub encroachment varied by ecological site. Shrub encroachment on gravelly soils was higher in areas receiving higher amounts of winter precipitation and on eastern slopes, indicating that wetter soil conditions contributed to historical shrub encroachment. Conversely, shrub encroachment on sandy sites was higher in drier locations featuring an increased likelihood of drought and south facing slopes. The strength of the relationships between encroachment and historical precipitation varied spatially in region-wide trends. At the landscape scale, patterns of shrub encroachment were caused primarily by patterns of contagious spread mediated by land use and grass growth form. Shrublands spread to a greater extent in bunchgrass-dominated patches than in patches dominated by stoloniferous grasses. Differences in grazing and brush management history mediated shrubland spread between pastures. Patch scale analysis of perennial bunchgrass response to a high precipitation period revealed that seedling establishment was positively correlated to soil water holding capacity and large bare ground patches. Increases in cover following establishment were more likely in areas dominated by shrubs. Collectively, these results demonstrate the explanatory power of viewing ecological state transition processes in a spatially-explicit manner.