On Transient Semi-Arid Ecosystem Dynamics Using Landlab: Vegetation Shifts, Topographic Refugia, and Response to Climate

Projecting how arid and semi-arid ecosystems respond to global change requires the integration of a wide array of analytical and numerical models to address different aspects of complex ecosystems. We used the Landlab earth surface modeling toolkit (Hobley et al., 2017, https://doi.org/10.5194/esurf-521-2017) to couple several ecohydrologic and vegetation dynamics processes to investigate the controls of exogenous drivers (climate, topography, fires, and grazing) and endogenous grass-fire feedback mechanisms. Aspect-controlled ecosystems and historical woody plant encroachment (WPE) narratives in central New Mexico, USA are used to construct simulations. Modeled ecosystem response to climatic wetness (i.e., higher precipitation, lower potential evapotranspiration) on topography follows the Boyko's "geo-ecological law of distribution." Shrubs occupy cooler pole-facing slopes in the dry end of their ecoclimatic range (Mean Annual Precipitation, MAP = 200 mm), and shift toward warmer equator-facing slopes as regional moisture increases (MAP > 250 mm). Trees begin to occupy pole-facing slopes when MAP > 200 mm, and gradually move to valleys. Pole-facing slopes increase species diversity at the landscape scale by hosting relict populations during dry periods. WPE observed in the region since the middle 1800s is predicted as a three-phase phenomenon. Phase II, rapid expansion, requires the removal of the positive grass-fire feedback by livestock grazing or fire suppression. Regime shifts from grassland to shrubland are marked by critical thresholds that involve grass cover remaining below 40%, shrub cover increasing to 10%-20% range, and the grass connectivity, Cg, remaining below 0.15. A critical transition to shrubland is predicted when grazing pressure is not removed before shrub cover attains 60%.