Data from: Thresholds and gradients in a semi-arid grassland: long-term grazing treatments induce slow, continuous and reversible vegetation change

1. Temporal changes in semi-arid ecosystems can include transitions between alternative stable states, involving thresholds and multiple domains of attraction, but can also include relatively continuous, symmetric and reversible shifts within a single stable state. Conceptual state-and-transition models (STMs) describe both types of ecosystem dynamics by including state transitions (plant community changes difficult-to-reverse without substantial input or effort) and phase shifts (easily reversible community changes) as 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 there is limited empirical evidence from these grasslands showing that grazing can cause difficult-to-reverse transitions between alternate stable states. 2 .In a northern mixed-grass prairie in Wyoming, USA, we examined plant community responses to (i) long-term (33-year) grazing intensity treatments (none, light, moderate and heavy stocking rates) and (ii) 8 years of light or no grazing in pastures that were grazed heavily for the previous 25 years. 3. 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. 4. Reversing stocking rates from heavy to light or no grazing resulted in reversal of changes induced by prior heavy stocking for dominant C3 grasses, but not for B. gracilis. For both groups, rates of change following grazing treatment reversals were consistent with rates of change during the initial years of the experiment (1982–1990). 5. Synthesis and applications. In a semi-arid rangeland with a long evolutionary history of grazing, different long-term grazing intensity treatments caused slow, continuous, and directional changes with important management implications, but did not appear to induce alternative stable states. For this and similar ecosystems, quantifying the time-scales and compositional gradients associated with key phase shifts may be more important than identifying thresholds between alternative stable states.

1. 半干旱生态系统的时间动态可包含两类变化模式：一是不同替代稳定态（alternative stable states）间的转换，涉及阈值与多重吸引域；二是单一稳定态内部相对连续、对称且可逆的位移。概念性状态转换模型（state-and-transition models, STMs）通过纳入状态转换（无大量投入或干预则难以逆转的植物群落变化）与相位位移（极易逆转的群落变化），来刻画这两类生态系统动态，二者均由管理措施与环境变异驱动。当前针对北美草原的STMs研究中，放牧管理被认为是驱动状态转换的首要因子，但来自这些草原的实证证据十分有限，尚无充分数据证明放牧可引发替代稳定态间难以逆转的转换。
2. 在美国怀俄明州的北部混合草草原中，我们针对两类处理开展了植物群落响应研究：（i）为期33年的长期放牧强度处理（无放牧、轻度放牧、中度放牧与重度放牧放牧率）；（ii）对此前25年持续重度放牧的样地实施为期8年的轻度放牧或休牧处理。
3. 长期放牧强度处理与特征鲜明但并非稳定的植物群落显著相关。在实验第22至33年期间，更高的放牧率会降低优势C3禾草的盖度，同时提升优势C4禾草格兰马草（Bouteloua gracilis）的盖度。
4. 将放牧率从重度调整为轻度或无放牧时，优势C3禾草的群落变化可得到逆转，但格兰马草（Bouteloua gracilis）的变化则无法逆转。两类禾草群落在放牧处理逆转后的变化速率，均与实验初始阶段（1982–1990年）的变化速率保持一致。
5. 总结与应用：在具备长期放牧演化历史的半干旱牧场中，不同长期放牧强度处理仅引发了缓慢、连续且定向的群落变化，这一结果具有重要的管理意义，但并未诱导出替代稳定态。针对该生态系统及同类生境，量化与关键相位位移相关的时间尺度与组成梯度，或许比识别替代稳定态间的阈值更为重要。