Trees planted near streets and highways are subject to elevated insect damage due to the adverse effects of vehicle pollution and urban heat, according to newly published research led by urban landscape entomologist Emily Meineke of the UC Davis Department of Entomology and Nematology.
The article, “Vehicle Pollution Is Associated with Elevated Insect Damage to Street Trees,” is published in the Journal of Applied Ecology. It received the Editor’s Choice Award.
“This research reveals strong effects of vehicle pollution on insect damage to trees,” said Meineke, who conceived the idea for the project, funded by the department. “Trees next to highways are exposed to multiple stressors, including urban heat, pollution, and insects, all of which affect one another and tree health. Our research strongly suggests planting trees that are less susceptible to herbivory near highways.”
Her team included her colleague, UC Davis distinguished professor Richard “Rick” Karban, who co-wrote the manuscript, and junior specialist David Eng, then of the Meineke lab. The study targeted vehicle pollution in the Sacramento Valley “and adds to a now growing chorus of studies demonstrating the scientific value of intra-urban gradients of particular variables (heat, pollution, surrounding vegetation),” they wrote.
They suspect that “vehicle pollution depresses defensive pathways within trees and reduces the concentrations of key compounds that protect against herbivore damage.”
“Vehicle pollution is a pervasive aspect of anthropogenic change in both urban and rural habitats,” they pointed out in their abstract. “Plant-insect herbivore interactions are highly contingent on environmental variables associated with pollution, including the availability of carbon and nitrogen in leaves.”
The researchers demonstrated that leaf damage to a native oak species (Quercus lobata), known as the valley oak,” is “substantially elevated on trees exposed to vehicle emissions.”
“Together, our studies demonstrate that the heterogeneity in vehicle emissions across cities may explain highly variable patterns of insect herbivory on street trees,” they wrote. “Our results also indicate that trees next to highways are particularly vulnerable to multiple stressors, including insect damage. To combat these effects, urban foresters may consider installing trees that are less susceptible to insect herbivory along heavily traveled roadways.”
The valley oak is a deciduous, long-lived tree that can reach up to 98 feet in height and live up to 600 years. It is known to tolerate wildfires.
The authors noted that urban trees provide ecosystem services, such as local cooling, air purification, and runoff reduction; are important to human health and wellbeing; and serve as foundational species that support urban biodiversity. However, urban trees “are under threat from stressors associated with both urbanization and climate change, but the extent to which insect damage may also shift over time is poorly characterized.”
“Past studies hint at the potential role of vehicle pollution as a driver of leaf nutritional quality for chewing herbivores. At one site in the United Kingdom, trees within 100 meters of motorways were much more likely to be severely defoliated than trees at further distances. Elevated herbivory was attributed to elevated nitrogen dioxide (NO2) along highways.” Another study in the Los Angeles Basin, showed that “herbivore communities on oak trees at more polluted natural areas tended to be more dominated by chewing herbivores compared to less polluted natural areas.”
The UC Davis researchers wrote that their results “demonstrate that highly polluted, highway-adjacent habitats are associated with shifts in plant-insect interactions and that this topic may be ripe for future research into how roadside environments may affect insect conservation and plant performance in cities.”
Their study highlights the importance of planting decisions along major roadways. “The concept of ‘right tree, right place’ has long stated that tree selection should be aimed at maximizing the performance in urban areas,” they wrote. “Quercus lobata and other species that are highly susceptible to herbivores may provide ecosystem services sub-optimally along highways, and may have shorter lifespans due to chronic damage promoted by on-road pollution trees.”
“Identifying tree species that are robust to pollution, and resistant to insects that may benefit from pollution, could be a novel consideration in planting decisions. This consideration may become even more important as many cities become drier and hotter, and insect herbivores have disproportionate impacts on tree growth. Because city-owned trees are planted and cannot themselves evolve in response to climate change, we may be required to develop new cultivars to promote robust trees along roadways.”