Water availability drives urban tree growth responses to herbivory and warming

Abstract

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. 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. 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. 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.

How do herbivorous insects respond to drought stress in trees?

“…Drought effects are also likely to be more severe in urban than natural areas because of vegetation fragmentation (Savard, Clergeau & Mennechez, ). Recent findings indicate that water stress in urban areas may not only cause reduced tree growth, but also exacerbate the effects of warming and insect pests (Meineke & Frank, ). Investigations of the impact of drought on tree defences frequently have focused on saplings rather than older trees as saplings are often more convenient for experimental studies (Rice et al ., ; Slik, ). Unfortunately this creates a knowledge bias because the investment in tree growth differs in saplings compared to adult trees (Thomas & Winner, ).…”

“…Drought effects are also likely to be more severe in urban than natural areas because of vegetation fragmentation (Savard, Clergeau & Mennechez, 2000). Recent findings indicate that water stress in urban areas may not only cause reduced tree growth, but also exacerbate the effects of warming and insect pests (Meineke & Frank, 2018).…”

Urbanization Shapes the Ecology and Evolution of Plant-Arthropod Herbivore Interactions

“…Two aspects of urbanization, soil compaction and widespread impervious surface coverage, can prevent the absorption of nutrients and water into host plants and render them more susceptible to herbivore colonization (Speight et al, 1998;Huberty and Denno, 2004). Decreased water absorption may also cause plants to have concentrated phloem, with higher amino acid content than watered plants, which can be beneficial to some phloem-sucking insects such as aphids (White, 1984(White, , 2009Schmitz, 1996), though there is conflicting evidence regarding the effects of chronic water stress on leaf-chewing and phloem-sucking arthropods (Mattson and Haack, 1987;Huberty and Denno, 2004;Meineke and Frank, 2018). Elevated temperatures caused by the urban heat island effect further induces water stress in plants, which makes them grow less and become more susceptible to herbivory in some circumstances (Meineke and Frank, 2018).…”

“…Decreased water absorption may also cause plants to have concentrated phloem, with higher amino acid content than watered plants, which can be beneficial to some phloem-sucking insects such as aphids (White, 1984(White, , 2009Schmitz, 1996), though there is conflicting evidence regarding the effects of chronic water stress on leaf-chewing and phloem-sucking arthropods (Mattson and Haack, 1987;Huberty and Denno, 2004;Meineke and Frank, 2018). Elevated temperatures caused by the urban heat island effect further induces water stress in plants, which makes them grow less and become more susceptible to herbivory in some circumstances (Meineke and Frank, 2018). Water stress also induces stomatal closure, which conserves water but decreases photosynthesis, potentially reducing herbivory from phloemsuckers (Schaffer and Mason, 1990;Hawkes and Sullivan, 2001).…”

Urbanization drives unique latitudinal patterns of insect herbivory and tree condition

“…Due to the different possible scaledependent mechanisms of how impervious surface could affect tree condition and herbivore abundance (e.g. tree water stress, natural enemy abundance, scale fitness (Dale and Frank 2014a, Meineke and Frank 2018), we predicted that the effects of impervious surface on tree condition would vary based on spatial extent and estimation technique. We predicted that a smaller, field-based estimate would be more robust, given its ability to account for the conditions that directly affect a tree and, thus, its herbivores than a larger, remotely-sensed estimate.…”