Incorporating spatial patterns into a state and transition model for arid grasslands and shrublands in southern New Mexico

Background/Question/Methods

State and transition models synthesize and communicate information about alternative states in arid rangelands and other ecosystems but often do not adequately account for processes interacting across a range of temporal and spatial scales. Grassland to shrubland transitions have occurred as patchy and asynchronous patterns in arid lands across the world and remain poorly understood beyond local-scale drivers and regional land use trends. A landscape perspective that acknowledges the role of heterogeneity in land history, soil-geomorphic patterns and processes, and other broad-scale interactions can be used to interpret and predict local state-transitions.  We ask: what governs the abundance and distribution of alternative states and what spatially structured processes are related to transition mechanisms? A broad range of historical and contemporary data was used to produce a spatial state-and-transition model for a relict basin floor landscape on the Jornada Experimental Range in south-central New Mexico, USA.  We quantified the spatial pattern of state transition from repeat aerial photography and historical land cover maps and identified temporal and spatial patterns and thresholds of grass production from historical field data, historical maps, and management records. We used repeated plot based soil-vegetation inventory spatially linked to mapped patterns of transition to identify past drivers and constraints defining current state functioning in grass-shrub-soil interactions.

Results/Conclusions

We present a spatially explicit state and transition model that addresses:  (1) spatial variation in land use driver history which explains transition pattern differences in otherwise uniform areas, (2) spatial dependence in response to drivers filtered by soils, landform, and climate, and (3) contagious processes mediated by vegetation-environment feedbacks which propagate transitions beyond the point of initial impact in a predictable way. We illustrate how variation in soil development, land-use decisions in the early 20th century, and contagious processes related to shrub spread and erosion have shaped the pattern of transition in this landscape.  We also note the implications of these patterns for management responses at different points in time and in different parts of the landscape.