Biology Department, Austin Peay State University, Clarksville, Tennessee, USA.
Department of Ecology, Evolution and Marine Biology, University of California at Santa Barbara, Santa Barbara, California, USA.
Ecol Appl. 2023 Jun;33(4):e2841. doi: 10.1002/eap.2841. Epub 2023 Apr 5.
Forest removal for livestock grazing is a striking example of human-caused state change leading to a stable, undesirable invasive grass system that is resistant to restoration efforts. Understanding which factors lead to resilience to the alternative grass state can greatly benefit managers when planning forest restoration. We address how thresholds of grass cover and seed rain might influence forest recovery in a restoration project on Hawai'i Island, USA. Since the 1980s, over 400,000 Acacia koa (koa) trees have been planted across degraded pasture, and invasive grasses still dominate the understory with no native woody-plant recruitment. Between this koa/grass matrix are remnant native Metrosideros polymorpha ('ōhi'a) trees beneath which native woody plants naturally recruit. We tested whether there were threshold levels of native woody understory that accelerate recruitment under both tree species by monitoring seed rain at 40 trees (20 koa and 'ōhi'a) with a range of native woody understory basal area (BA). We found a positive relationship between total seed rain (but not bird-dispersed seed rain) and native woody BA and a negative relationship between native woody BA and grass cover, with no indication of threshold dynamics. We also experimentally combined grass removal levels with seed rain density (six levels) of two common understory species in plots under koa (n = 9) and remnant 'ōhi'a (n = 9). Few seedlings emerged when no grass was removed despite adding seeds at densities two to 75 times higher than naturally occurring. However, seedling recruitment increased two to three times once at least 50% of grass was removed. Existing survey data of naturally occurring seedlings also supported a threshold of grass cover below which seedlings were able to establish. Thus, removal of all grasses is not necessary to achieve system responses: Even moderate reductions (~50%) can increase rates of native woody recruitment. The nonlinear thresholds found here highlight how incremental changes to an inhibitory factor lead to limited restoration success until a threshold is crossed. The resources needed to fully eradicate an invasive species may be unwarranted for state change, making understanding where thresholds lie of the utmost importance to prioritize resources.
森林砍伐用于放牧是人类引起的状态变化的一个显著例子，导致稳定的、不受欢迎的入侵草系统，该系统抵制恢复努力。了解哪些因素导致对替代草状态的恢复力，可以极大地帮助管理者在规划森林恢复时做出决策。我们在美国夏威夷岛的一个恢复项目中探讨了草盖度和种子雨阈值如何影响森林恢复。自 20 世纪 80 年代以来，超过 40 万棵相思树（koa）被种植在退化的牧场上，而入侵的草仍然占据着林下，没有本地木本植物的繁殖。在 koa/草矩阵之间是剩余的本地桃金娘（‘ōhi'a）树，在这些树下自然繁殖本地木本植物。我们通过监测 40 棵树（20 棵 koa 和‘ōhi'a）的种子雨，测试了在这两种树种下，是否存在加速本地木本植物繁殖的本地木本植物下木基底面积（BA）阈值。我们发现，总种子雨（但不是鸟类传播的种子雨）与本地木本 BA 之间呈正相关，而本地木本 BA 与草盖度之间呈负相关，没有迹象表明存在阈值动态。我们还在 koa（n=9）和剩余‘ōhi'a（n=9）下的实验中，将草去除水平与两种常见林下物种的种子雨密度（六个水平）相结合。尽管添加的种子密度比自然发生的高出两到 75 倍，但在没有去除草的情况下，很少有幼苗出现。然而，一旦去除了至少 50%的草，幼苗的繁殖率就会增加两到三倍。自然发生的幼苗的现有调查数据也支持草盖度的阈值，低于该阈值幼苗才能建立。因此，不需要去除所有的草就能达到系统的响应：即使是适度的减少（~50%）也可以增加本地木本植物的繁殖率。这里发现的非线性阈值突出表明，对抑制因子的渐进式改变会导致恢复成功有限，直到达到阈值。完全根除入侵物种所需的资源可能对于状态变化来说是不必要的，因此，了解阈值的位置对于优先分配资源至关重要。
