Department of Biology, University of New Mexico, Albuquerque, NM, 87131, USA.
Division of Biology, Kansas State University, Manhattan, KS, 66506, USA.
Ecol Lett. 2021 Apr;24(4):636-647. doi: 10.1111/ele.13676. Epub 2021 Jan 14.
Hysteresis is a fundamental characteristic of alternative stable state theory, yet evidence of hysteresis is rare. In mesic grasslands, fire frequency regulates transition from grass- to shrub-dominated system states. It is uncertain, however, if increasing fire frequency can reverse shrub expansion, or if grass-shrub dynamics exhibit hysteresis. We implemented annual burning in two infrequently burned grasslands and ceased burning in two grasslands burned annually. With annual fires, grassland composition converged on that of long-term annually burned vegetation due to rapid recovery of grass cover, although shrubs persisted. When annual burning ceased, shrub cover increased, but community composition did not converge with a long-term infrequently burned reference site because of stochastic and lagged dispersal by shrubs, reflecting hysteresis. Our results demonstrated that annual burning can slow, but not reverse, shrub encroachment. In addition, reversing fire frequencies resulted in hysteresis because vegetation trajectories from grassland to shrubland differed from those of shrubland to grassland.
滞后现象是交替稳定状态理论的一个基本特征，但滞后现象的证据很少。在湿润草地中，火频度调节着从草本占主导到灌木占主导的系统状态的转变。然而，增加火频度是否能逆转灌木扩张，或者草本-灌木动态是否存在滞后现象还不确定。我们在两个很少被火烧的草地中进行了每年的火烧，而在两个每年被火烧的草地中停止了火烧。由于草的迅速恢复，每年的火烧使草地的组成趋向于长期每年被火烧的植被，尽管灌木仍然存在。当停止每年的火烧时，灌木的覆盖面积增加，但群落组成并没有与长期很少被火烧的参照点相吻合，因为灌木的随机和滞后扩散，反映了滞后现象。我们的结果表明，每年的火烧可以减缓，但不能逆转，灌木的入侵。此外，由于从草地到灌木地的植被轨迹与从灌木地到草地的植被轨迹不同，因此改变火频度会导致滞后现象。
