Water availability drives urban tree growth responses to herbivory and warming

1. Urban forests provide important ecosystem services to city residents, including pollution removal and carbon storage. Climate change and urbanization pose multiple threats to these services. However, how these threats combine to affect urban trees, and thus how to mitigate their effects, remains largely untested because multi-factorial experiments on mature trees are impractical. 2. We used a unique urban warming experiment paired with a laboratory chamber experiment to determine how three of the most potentially damaging factors associated with global change for urban and rural trees—warming, drought, and insect herbivory—affect growth of Quercus phellos (willow oak), the most commonly-planted large shade tree in the southeastern US, which is known for its resilience to these potential stressors. 3. In a previous study, we found that the urban heat island effect was associated with reduced growth of Q. phellos and higher abundance of Parthenolecanium scale insects, key pests of oaks in cities. Here, we tested the hypothesis that tree water stress is the mechanism for these effects of warming. We found evidence that water stress is a major, interactive factor reducing urban tree growth, but found no evidence that water stress is associated with Parthenolecanium survival or abundance. Warming and Parthenolecanium only reduced growth in Q. phellos saplings that were simultaneously water stressed. 4. Synthesis and applications. Across many temperate cities worldwide, urban trees grow less than rural trees. Our results point to water stress as the most likely driver for this pattern. Importantly, we found that water stress both reduces tree growth on its own and exacerbates effects of warming and insect pests on tree growth. Therefore, management strategies targeted at increasing tree hydration in cities may reduce effects of these three key stressors that are expected to intensify with further urbanization and climate change.

1. 城市森林为城市居民提供重要的生态系统服务（ecosystem services），包括污染物清除与碳固存。气候变化与城市化进程正对这些服务构成多重威胁。然而，由于成熟林木的多因子实验难以实施，这些威胁如何共同作用于城市树木，以及如何缓解其影响，在很大程度上仍未得到验证。
2. 本研究借助独特的城市增温实验与实验室箱室实验，探究全球变化背景下对城乡树木危害最显著的三大因子——增温、干旱与昆虫食草作用——对柳栎（*Quercus phellos*，willow oak）生长的影响。柳栎是美国东南部种植最广泛的大型遮荫树种，以对上述潜在胁迫因子的抗逆性著称。
3. 此前本团队的研究发现，城市热岛效应（urban heat island effect）与柳栎生长受抑以及城市橡树主要害虫——扁平蚧属介壳虫（Parthenolecanium scale insects）种群丰度升高存在关联。本研究针对"树木水分胁迫是增温产生上述影响的作用机制"这一假说开展验证。结果表明，水分胁迫是抑制城市树木生长的关键交互因子，但未发现水分胁迫与扁平蚧属介壳虫的存活或种群丰度存在关联。仅当柳栎幼树同时处于水分胁迫状态时，增温与扁平蚧属介壳虫才会抑制其生长。
4. 综合与应用。全球诸多温带城市中的城市树木生长状况均差于乡村树木。本研究结果表明，水分胁迫是造成这一现象的最可能驱动因子。值得注意的是，水分胁迫不仅会直接抑制树木生长，还会加剧增温与虫害对树木生长的负面影响。因此，针对提升城市树木水分供给的管理策略，或可缓解上述三大关键胁迫因子的影响——而随着城市化进程与气候变化的进一步推进，这些胁迫因子的危害预计将持续加剧。