Modeling vegetation dynamics among Chihuahuan Semi-desert Grassland ecological groups as part of the Integrated Landscape Assessment Project (ILAP)

Background/Question/Methods

As part of the Integrated Landscape Assessment Project (ILAP), we developed probability-based state and transition models for a suite Chihuahuan Semi-desert Grassland groups per the U.S. National Vegetation Classification (USNVC).  ILAP explores the dynamics of broad-scale, multi-ownership landscapes over time by evaluating and integrating information about fuel and potential fire conditions, wildlife and aquatic habitats, economic values, and projected climate change across all lands in Arizona, New Mexico, Oregon and Washington.  Underpinning these objectives is a need for robust ecological dynamics models integrated with wall-to-wall spatial information on current and potential conditions under various disturbance scenarios.  Accordingly, we used the Vegetation Dynamics Development Tool (VDDT) as the modeling framework to examine the role of various disturbance agents in ecosystem dynamics (e.g., fire, grazing, invasives, and drought) at a landscape scale for the Chihuahuan Desert ecoregion.  Using available long-term monitoring and experimental data, each model addresses probability of changes in vegetation structure and composition within groups of plant communities.   Model outcomes under various disturbance scenarios are computed at watershed levels corresponding to HUC 10, which allows for ready visualization at regional scales.

Results/Conclusions

We developed four VDDT models that correspond to USNVC Groups: Chihuahuan Semi-Desert Lowland Grassland, Semi-Desert Piedmont Grassland, Semi-Desert Foothill Grassland, and Semi-Desert Sandy Plains Grassland.  Each model had a base structure of four tiers reflecting early, mid, and late succession plus exotic invasion as driven by fire, grazing, invasives, and drought.  Within tiers, transitions from grassland to shrubland states were driven by primarily by grazing and drought.  Models differed among groups with respect to speed of transition to shrubland with Sandy Plains Grassland the most sensitive with minimum transition times under 40 years (primarily to mesquite duneland with wind erosion).  Piedmont Grassland had intermediate transition times based on long-term sheet erosion and lower inherent speeds of shrub establishment.  Foothill Grassland and Lowland Grassland had the lowest rates of transition to shrubland as function of relatively stable edaphic processes.  Invasive species impacts can be significant but localized with respect to contagion.   Equilibrial models were made available for testing various alternative management scenarios (e.g., prescribed fire) and climate change effects (e.g., increased duration and frequency of droughts), and built into the ILAP overall project area landscape-scale spatial analysis.