Trends in street tree survival, Philadelphia, PA
Challenges to street tree survival and the resulting short lifespans are frequently discussed but inadequately quantified. This study addresses trends in street tree mortality by reviewing published literature and by assessing street trees in Philadelphia 2-10 years after planting. Proposed causes of mortality and decline in street trees include poor soil and atmospheric conditions, natural disturbances, and direct human actions. Street tree mortality studies carried out to date have measured mortality using a wide variety of methodologies and have shown a wide range of results. Reported survival rates from thirteen published studies in temperate climates are compared. Annual survival rates are estimated for each study, and range from 34.7 to 99.7%. Linear regression analysis of eleven of these studies reveals a significant relationship between survival and time period since planting (estimated r2 = 0.69, estimated annual survival 94.2%). Methods for estimating average lifespan are also discussed. Street tree survival in Philadelphia was assessed for comparison to published literature. Select street trees species planted in the years 1995-2003 in North Philadelphia were surveyed to assess survival rates, and to identify trends in mortality and tree size. These trees were planted by the Philadelphia Green program of the Pennsylvania Horticultural Society. The overall survival of street trees 8-10 years after planting is 57%. Planting year specific survival rates for 8, 9, and 10 years after planting are 44%, 58%, and 63%, respectively. These values are comparable to those predicted by the analysis of previously published studies. Survival of trees 8-10 years after planting varies significantly by species, zip code, and U.S. Census indicators. Current tree size varied significantly by exposed soil area and sidewalk condition. One select species, hedge maple, was chosen for additional study to assess temporal trends in street tree mortality. Hedge maples 2-10 years after planting in North Philadelphia were surveyed. Time period since planting was significantly correlated to survival (estimated r2 = 0.75, estimated annual survival 95.5%). Conclusions about the causes of mortality in the study area are suggestive, but limited. Power analysis of tree diameter and site vehicular disturbance indicates that a sample size upwards of 1000 would be needed to detect difference. Recommendations are made for future data collection of street tree studies in Philadelphia and elsewhere.