Urban tree health in the aftermath of a hurricane: Chronic impacts of hurricane Sandy on New York City's red maple and London plane street trees
Hurricane Sandy was the most destructive hurricane of 2012, and is thought to be the second most costly hurricane in United States history. Sandy hit New York City on the evening of October 29, 2012, causing major flooding, wind damage, and loss of life. The New York City Department of Parks & Recreation documented over 20,000 fallen street trees in the storm due to the physical impact of wind and debris. Salt water flooding appears to have caused additional stress to approximately 48,000 street trees located in the storm’s inundation zone. Knowledge about the severity and duration of this unexpected stress is important for urban forest managers to consider when restoring coastal zones which are susceptible to flooding in the future. In order to quantify the level of stress to urban street trees caused by salt water flooding, we chose to sample two species - red maple, because of its hardiness and general resilience to stressors, and London Plane because of its importance as a street tree in NYC. All sample trees were in the borough of Queens.
We used a multivariate tree health assessment tool to quantify tree stress over the course of two growing seasons post flooding for Red Maple and for only the second growing season post Sandy for London Plane trees. At the end of the first growing season post Sandy we saw significant differences in several measures of tree crown stress, with the flooded red maple trees exhibiting a greater number of stress symptoms than unflooded trees. Two growing seasons after Hurricane Sandy flooded and unflooded red maples are equally health suggesting a trajectory of recovery for the flooded trees. However flooded red maples experienced 6% mortality. Two years post Sandy, the London Plane trees show significantly poorer heath in flooded zones. In addition, flooded London plane trees had higher levels of stress than flooded red maples, suggesting a higher sensitivity to salt water inundation. The results of this study show that red maple are recovering from stress caused by salt water flooding while flooded London plane street trees exhibit high levels of physiological stress for at least 2 years after inundation. These results suggest that urban forest managers faced with restoring coastal forests or replanting street trees in coastal zones impacted by Hurricane Sandy should consider planting salt tolerant tree species or genotypes.