Critical thresholds and recovery of Chihuahuan Desert grasslands: Insights from long-term data

Background/Question/Methods

Desertification and other harmful state transitions in drylands are expected to accelerate with global change. Ecologists are called upon to devise methods to anticipate critical thresholds and promote recovery of desired states. As in other drylands, transitions in semiarid grasslands of the Chihuahuan Desert involve the loss of perennial grasses and their replacement by shrubs. Dryland applications of critical threshold theory suggest that as large vegetated patches become fragmented, the breakdown of soil-resource mediated feedbacks between patch size and plant survival/reproduction will accelerate vegetation loss and preclude recovery. Thus, metrics reflecting gradual changes in vegetation patch size might be used as indicators of critical thresholds and recovery trajectories. We use 100 years of observations at the Jornada Experimental Range and Jornada Basin LTER to ask: how can critical thresholds and recovery be more effectively managed?

Results/Conclusions

Historical data indicate that the rapid loss of the dominant perennial grass (Bouteloua eriopoda) initiated instances of state change. The shift to a persistent reduced B. eriopoda state was caused by the confluence of several factors including grazing management that emphasized forage use rather than the viability of the B. eriopoda, delayed cattle destocking at the initiation of a severe, multi-year drought period, and an episode of windy conditions that produced severe soil erosion. A 13-yr pulse-perturbation experiment sought to recreate the rapid extinction of B. eriopoda via heavy cattle grazing, but failed to do so. Even when B. eriopoda cover was reduced to very low levels, critical thresholds were not crossed and recovery rates were a linear function of existing B. eriopoda cover. Sequences of wet years promote recovery. Patch size metrics did not improve the prediction of recovery rates and were highly correlated with cover. In contrast to theoretical assumptions, measurements of B. eriopoda reproduction across a range of patch sizes indicated that the largest patches did not feature the greatest rates of plant reproductive success and suggested that a balance between competition and facilitation might govern B. eriopoda recovery. The combination of historical reconstruction and experimental results indicates that 1) B. eriopoda grasslands undergo catastrophic transitions under certain combinations of drivers, 2) transitions are not associated deterministic thresholds or proposed mechanisms reflected in theoretical models; recovery can occur with grazing rest at very low cover values, and 3) cover can be an effective indicator of vulnerability to transition.