The effects of urban warming on herbivore abundance and street tree condition.

Adam G Dale,Steven D Frank,Ben Bond-Lamberty

doi:10.1371/journal.pone.0102996

Abstract

Trees are essential to urban habitats because they provide services that benefit the environment and improve human health. Unfortunately, urban trees often have more herbivorous insect pests than rural trees but the mechanisms and consequences of these infestations are not well documented. Here, we examine how temperature affects the abundance of a scale insect, Melanaspis tenebricosa (Comstock) (Hemiptera: Diaspididae), on one of the most commonly planted street trees in the eastern U.S. Next, we examine how both pest abundance and temperature are associated with water stress, growth, and condition of 26 urban street trees. Although trees in the warmest urban sites grew the most, they were more water stressed and in worse condition than trees in cooler sites. Our analyses indicate that visible declines in tree condition were best explained by scale-insect infestation rather than temperature. To test the broader relevance of these results, we extend our analysis to a database of more than 2700 Raleigh, US street trees. Plotting these trees on a Landsat thermal image of Raleigh, we found that warmer sites had over 70% more trees in poor condition than those in cooler sites. Our results support previous studies linking warmer urban habitats to greater pest abundance and extend this association to show its effect on street tree condition. Our results suggest that street tree condition and ecosystem services may decline as urban expansion and global warming exacerbate the urban heat island effect. Although our non-probability sampling method limits our scope of inference, our results present a gloomy outlook for urban forests and emphasize the need for management tools. Existing urban tree inventories and thermal maps could be used to identify species that would be most suitable for urban conditions.

Highlights

Trees provide ecosystem services that mitigate the negative effects of urban habitats on human and environmental health [1,2,3]
M. tenebricosa, is a native armored scale insect that lives on the trunk and branches of maple trees, where it feeds on cambial parenchyma cells [28]
Our results suggest that urban warming increases scale insect abundance on street trees (Figure 2)

Summary

Introduction

Trees provide ecosystem services that mitigate the negative effects of urban habitats on human and environmental health [1,2,3]. Dale and Frank [23] found that an armored scale insect, Melanaspis tenebricosa (Comstock) (Hemiptera: Diaspididae), produced 52% more eggs per adult female with a 1.6uC increase in average site temperature This contributed to three orders of magnitude more scale insects on street trees at warmer urban sites. Our hypothesis was that urban heat increases M. tenebricosa abundance and that these two factors concurrently increase tree water stress and reduce tree condition. To test this hypothesis, we first determined how temperature affects M. tenebricosa abundance on red maple (Acer rubrum) street trees. Understanding how urban conditions reduce street tree condition is essential to our understanding of these ecosystems and managing their resources

Methods

Objective

Results

Discussion