The 2011 flash drought caused substantial tree mortality in the Memorial Park of Houston, one of the largest urban parks (573.9 ha) in the US, with significant impacts on the ecosystem services provided by the park. Some estimated that up to 80% trees in forested patches (mostly pine stands) died since 2011. Over 30,000 dead trees had been removed and additional 5,000 trees (mostly red oak and loblolly pine) were tagged for mechanical removal (NRCS-TX). A better understanding of the extent and spatial pattern of the tree mortality and how it was influenced by different environmental factors is critical to assess the impacts and design of ecosystem restoration efforts. This research aims to quantify the tree mortality in the Memorial Park subsequent to the 2011 drought and explore the spatial patterns of the mortality and potential processes regulated them. We used high resolution aerial imagery in 2004-2013 to develop tree canopy covers (0.30-m resolution) and examine the spatial patterns of tree mortality before and after the drought and the influence of soil types, over a 418 ha area of forested landscape within the park's boundaries.
The tree mortality rate during 2011 was estimated about 33.0%, in contrast to the average tree mortality rate of 4.3% per year prior the drought (2004-2009). The mortality rate continued to be high in the two years after the drought (2011 and 2012), at about 10.4%. There were substantial spatial variation in tree mortality rate in association with soil types. During 2011, the highest mortality rate (38.6%) was observed in the very fine sandy loam soils, followed by intermediate mortality rates in silty clay (31.7%) and loam (30.4%), and the lowest in fine sandy loam (25.0%). Interestingly, the mortality rates prior to the drought were similar for very fine sandy loam soils (4.7%) and the fine sandy loam soils (4.5%). The fine sandy loam soils are often in close proximity to the streams and drainages, which might helped alleviate the stress during the drought. More in-depth exploration of spatial patterns of tree mortality rates and the potential influences of soils, topography, hydrography, and adjacent anthropogenic features (impervious surface and irrigation systems) can yield important insights for guiding the spatial design for the restoration of the forest ecosystems.