Thresholds vs. gradients in a semi-arid grassland: Long-term grazing treatments induce slow, continuous, and reversible vegetation change
Semiarid ecosystems can exhibit non-reversible shifts among alternative stable ecosystem states (thresholds and hysteresis), but can also be characterized by slow, continuous, and reversible changes in plant composition (successional gradients). Conceptual state-and-transition models (STMs) attempt to describe both types of ecosystem dynamics by including phase shifts (easily reversible changes in species composition) and state transitions (community changes not reversible without substantial input or effort) as possible consequences of management practices and environmental variability. Grazing management is purported to be the primary driver of state transitions in current STMs for North American grasslands, but empirical evidence for hysteretic responses to grazing is limited. In a northern mixed-grass prairie in Wyoming, USA, we examined plant community responses to (1) long-term (33-year) grazing intensity treatments (none, light, moderate and heavy stocking rates) and (2) new experimental treatments in which pastures grazed heavily for 25 years were subjected to light stocking or no grazing for 8 years.
Long-term grazing treatments were associated with distinct, but not stable, plant communities. From year 22 to 33, heavier stocking rates decreased cover of dominant C3 grasses and increased cover of the dominant C4 grass (Bouteloua gracilis). For dominant C3 grasses, changing stocking rates from heavy to light or no grazing resulted in reversal of changes induced by the prior heavy stocking. However, reversal rates were slow, and B. gracilis showed no evidence of reversal. In this semiarid rangeland, different long-term grazing intensity treatments caused continuous, directional changes with important management implications, but did not appear to induce alternative stable states. In grazing-adapted rangelands and other resilient ecosystems, quantifying the timescales and compositional gradients associated with key phase shifts may be more important than identifying thresholds between alternative stable states.