Changes in spatial variance during a grassland to shrubland state transition

State transitions are changes in ecosystem structure and self-reinforcing feedbacks that are initiated when an exogenous driver variable crosses a threshold. Reversing state transitions is difficult and costly. While some state transitions are relatively rapid, many take years to decades. Outside of theoretical models, very little is known about slower state transitions and how they unfold in time and space.
We quantified changes in spatial variance as a mesic grassland ecosystem shifts to a shrub-dominated state, using long-term experiments and simulations that maintain grasslands with annual fires or initiate a state transition to shrub dominance by decreasing fire frequency.
In the experiments, the susceptibility to state transitions varied substantially in space. In the less frequent fire treatment, some plots became shrub-dominated around year 20 and grass extirpations began in year 25, but a third of the plots were still grass-dominated in year 37. Variable rates of state transi...

生态系统状态转换（state transitions）是指当外源驱动变量突破阈值时，引发的生态系统结构改变与自强化反馈过程。逆转此类状态转换不仅难度极大，且成本高昂。尽管部分状态转换过程相对迅速，但多数状态转换需耗时数年至数十年。除理论模型外，学界对缓慢状态转换及其在时间与空间维度上的演化过程仍知之甚少。
本研究依托长期实验与模拟手段，针对中生草原（mesic grassland）向灌木主导状态的转变过程，量化了其空间方差的变化；实验与模拟分别设置两类处理方案：一类通过每年实施火烧以维持草原生态系统，另一类通过降低火烧频率启动向灌木主导状态的转换。
实验结果显示，生态系统对状态转换的易感性存在显著空间异质性。在低火烧频率处理组中，部分样地于第20年左右转为灌木主导状态，草地完全消失的现象始于第25年，但截至第37年仍有三分之一的样地保持草原主导状态。状态转换速率存在显著差异……