Urban tree survival in Sacramento, CA: Establishment loss and implications for ecosystem services

Background/Question/Methods

Urban forests can provide valuable ecosystem services that motivate planting campaigns, and tree survival is a key element of program success and projected benefits. In Sacramento, CA, the Shade Tree Program is designed to reduce building energy use through tree shade. This program distributes thousands of free trees to private residential properties every year. We monitored a random sample of young trees annually for five years to quantify tree losses during the establishment phase. The analysis for risk factors associated with tree death included socioeconomic, biophysical, and maintenance characteristics. In addition to field observations of tree planting status, survival, and maintenance, we also collected property ownership information (renter vs. owner-occupancy, homeowner change, and foreclosure) through the Multiple Listing Service and neighborhood socioeconomic characteristics from the U.S. Census. Planting status and survival outcomes were assessed using conditional inference trees (random forests). Subsequent survival analysis accounted for interval and right-censored observations.

Results/Conclusions

84.9% of trees were planted, with 70.9% survivorship at five years post-planting, and estimated 6.6% annual mortality. Planting rates were higher in neighborhoods with higher educational attainment and on properties with stable homeownership. Five-year survivorship was higher for properties with stable homeownership, as well as for tree species with low water use demand. In this program, residents were provided with educational materials on tree stewardship, and then had sole responsibility for planting and maintenance. Yet many residents did not adhere to recommended maintenance practices, and poor maintenance was significantly more common on properties with renter occupancy, change in owner, or foreclosure. These results illustrate the critical role of stewardship and consistent homeownership to young tree mortality on residential properties. Our findings also suggest that survival assumptions in urban forest ecosystem services models may be overly optimistic. Locality-specific field data can be used to improve the accuracy of urban forest models.