Agronomy Department, University of Florida, McCarty Hall B, Gainesville, Florida, 32611, USA.
Research and Conservation Department, Denver Botanic Gardens, 909 York Street, Denver, Colorado, 80206, USA.
Ecology. 2019 Dec;100(12):e02859. doi: 10.1002/ecy.2859. Epub 2019 Sep 6.
Global change stressors such as drought and plant invasion can affect ecosystem structure and function via mediation of resource availability and plant competition outcomes. Yet, it remains uncertain how native plants respond to drought stress that co-occurs with potentially novel resource conditions created by a nonnative invader. Further, there is likely to be temporal variation in competition outcomes between native and nonnative plant species depending on which resources are most limiting at a given time. Interacting stressors coupled with temporal variation make it difficult to predict how global change will impact native plant communities. To address this knowledge gap, we conducted a 5-yr factorial field experiment to quantify how simulated drought, plant invasion (by cogongrass, Imperata cylindrica), and these stressors combined, affected resource availability (soil moisture and light) and competition dynamics between the invader and native longleaf pine (Pinus palustris), a foundation species in southeast U.S. forests. Drought and invasion mediated the survival and performance of pine seedlings in temporally dynamic and unexpected ways. Drought and invasion alone each significantly reduced pine seedling survival. However, when the stressors occurred together, the invader offset drought stress for pine seedlings by maintaining high levels of soil moisture, humidity, and shade compared to uninvaded vegetation. This facilitative effect was pronounced for 2 yr, yet shifted to strong competitive exclusion as the invasion progressed and the limiting resource switched from soil moisture to light. After 3 yr, pine tree survival was low except for pines growing with uninvaded vegetation under ambient precipitation conditions. After 5 yr, pines experiencing a single stressor were taller and had greater height to diameter ratios than pines under no stress or both stressors. This outcome revealed a filtering effect where poorly performing trees were culled under stressful conditions, especially when pines were growing with the invader. Together, these results demonstrate that although drought and invasion suppressed a foundation tree species, the invader temporarily moderated stressful drought conditions, and at least some trees were able to survive despite increasingly strong competition. Such unpredictable effects of interacting global change stressors on native plant species highlight the need for additional long-term studies.
全球变化胁迫源，如干旱和植物入侵，可以通过资源可用性和植物竞争结果的调节来影响生态系统结构和功能。然而，目前还不确定本地植物如何应对与非本地入侵物种创造的潜在新资源条件同时发生的干旱胁迫。此外，由于资源在特定时间内的限制，本地和非本地植物物种之间的竞争结果可能会随时间而变化。相互作用的胁迫源加上时间变化，使得很难预测全球变化将如何影响本地植物群落。为了解决这一知识差距，我们进行了一项为期 5 年的因子田间实验，以量化模拟干旱、植物入侵（由狼尾草，Imperata cylindrica）以及这些胁迫源的组合如何影响资源可用性（土壤水分和光照）以及入侵物种和本地长叶松（Pinus palustris）之间的竞争动态，长叶松是美国东南部森林的基础物种。干旱和入侵以暂时动态和意外的方式影响了松树幼苗的生存和表现。单独的干旱和入侵都显著降低了松树幼苗的存活率。然而，当胁迫源同时发生时，与未入侵植被相比，入侵物种通过维持高水平的土壤水分、湿度和遮荫来抵消干旱对松树幼苗的压力。这种促进作用在 2 年内很明显，但随着入侵的进展和限制资源从土壤水分转变为光照，这种促进作用转变为强烈的竞争排斥。3 年后，除了在环境降水条件下与未入侵植被一起生长的松树外，松树的存活率都很低。5 年后，经历单一胁迫源的松树比无胁迫或两个胁迫源下的松树更高，且高度与直径的比值更大。这一结果表明存在一种过滤效应，即在压力条件下，表现不佳的树木被淘汰，特别是当松树与入侵物种一起生长时。总之，这些结果表明，尽管干旱和入侵抑制了基础树种，但入侵物种暂时缓解了压力性干旱条件，并且尽管竞争日益激烈，至少一些树木能够存活下来。这种相互作用的全球变化胁迫源对本地植物物种的不可预测影响突出了需要进行更多的长期研究。
