The Effects of Urban Warming on Herbivore Abundance and Street Tree Condition

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.

Early pest development and loss of biological control are associated with urban warming

“…This study documents a new mechanism for urban pest outbreaks, which damage trees and reduce ecosystem services [13]. Warming may increase pest abundance in many habitats through multiple mechanisms, including phenological mismatches that reduce biological control.…”

Harmonia axyridis (Coleoptera: Coccinellidae): Smelling the rat in native ladybird declines

“…Such habitats include not only agricultural ecosystems (Honěk, 2012), but also urban and industrial habitats (e.g., Kozlov, 2015;Akkermans, 2016;Egerer et al, 2017). Aphids are known to benefi t from the presence of humans, reaching high densities on plants suffering from the effects of pollution: this can lead to high aphid populations in urban areas (Bell et al, 1993), perhaps further exacerbated by higher temperatures in towns (Dale & Frank, 2014). Such resource availability increases the size of ladybird populations, especially of generalist species, which prefer high aphid densities (see Sloggett, 2008).…”

Urban plant physiology: adaptation-mitigation strategies under permanent stress

“…A number of studies have recently focused on plant water use and thresholds of water stress [20,21]. It has been further demonstrated that higher temperature and increased drought in urban environments significantly affected the growth of urban trees [2,22]. Usually T, [CO 2 ] and [N] increase from the suburbs to the city center, but [O 3 ] can increase or decrease because it depends on several factors such as the positioning of wind corridors, positioning of major roads, traffic intensity, and the typology of urban vegetation [14,15,23,24].…”