Urban outbreaks of herbivores: Determining the effects of nutrients and temperature on herbivore abundance in urban forests
Forests in urban cities provide many services for citizens such as promoting health and wellness and sequestering more carbon from the atmosphere to mitigate climate change. Phenomena such as the “urban heat island effect” in urban systems, however, are a detriment to tree health and promote temperature and drought stress in urban forests. During stress, trees may increase concentrations of nutrients (amino acids and digestible carbohydrates), which may benefit herbivores. Increased herbivory due to the nutritional benefits of stress may further harm urban forests. In Raleigh, North Carolina, herbivores commonly outbreak on heat stressed red maples (Acer rubrum) and willow oaks (Quercus phellos), and have significant impacts on tree health. With increasing urban temperatures and drought with climate change, understanding the mechanisms behind herbivore outbreaks and the effects of temperature and herbivores is critical to preserving urban forests. In this study, herbivore abundance was surveyed on red maples and willow oaks in Raleigh in 80 different sites throughout the city. We measured herbivore abundance, turgor pressure (water pressure) in trees to determine stress severity, and concentrations of amino acids and carbohydrates. Our goal was to use temperature and concentrations of nutrients to predict herbivore abundance on stressed urban trees.
We found that higher temperatures were associated with lower concentrations of amino acids in trees compared to that of trees in cooler areas of the city for both species. Concentrations of carbohydrates were not associated with temperatures or tree species. The abundance of several herbivores, such as scales and aphids, varied in their response to concentrations of nutrients and temperature, suggesting species-specific responses to changes in urban trees under stress. Despite lower concentrations of amino acids in warmer trees and greater in cooler trees, herbivores varied highly in their response to nutrients and temperature. The abundance of these herbivores were more strongly associated with changes in nutrient concentrations on red maple trees compared to willow oaks. Higher temperatures in Raleigh, therefore, may lead to increases in scale and aphid abundance on red maples rather than on willow oaks. Our study suggests that nutrients influence herbivore abundance on stressed trees and considerations of nutrients must be made when predicting herbivore abundance on stressed trees in urban forests.