COS 7-3
Impacts of severe drought events on urban forest composition in Oklahoma City, Oklahoma
During the 2011 and 2012 growing seasons, the Central Great Plains of the US experienced severe drought conditions, with the largest precipitation deficits observed in over 100 years of measurements. The effects of this drought period were far reaching, ranging from billions of dollars in crop loss to devastation of stream ecosystems. At the same time, mandatory water restrictions were imposed in cities across the region, limiting irrigation of urban landscapes. By analyzing aerial images from just before and after the drought period, coupled with ground based surveys, we investigated the impact of the recent droughts on urban vegetation cover and composition in Oklahoma City, Oklahoma. Our objectives were to 1) assess mortality of urban trees as a result of the drought, including identifying most vulnerable species and size classes; 2) to determine whether the drought led to shifts from one type of vegetation, or suite of species, to another. We hypothesized that two years after the end of the drought there would be an observable shift towards a more drought tolerant/resilient landscape.
Results/Conclusions
Existing tree surveys reveal many non-native trees, and trees that are considered to have moderate to high water needs. In our assessment, many randomly selected plots in Oklahoma City residential areas showed significant reductions in tree counts (~20%) between spring 2011 and 2013. Many trees lost due to drought effects were larger than 10 m, suggesting large potential changes in ecosystem services as well. Two years after the end of the drought damaged trees are still being replaced, and many have been replaced with species that are vulnerable to storms or have moderate to high water needs. This suggests that despite the severity of the drought and its associated impacts, there hasn’t been a significant shift to more drought tolerant species and landscapes. Assessing the impacts of extreme events on vegetation structure and composition is challenging in urban settings, as changes can result from the combination of direct climatic effects on vegetation survival, as well as subsequent human management and planting decisions, which may be responsive to previous extreme events. However, better understanding of the role of extreme climate events in shaping urban forests is critical to assessing the resilience of urban ecosystems and service provisioning.