Urban tree mortality rates: applying concepts from demography and monitoring in Oakland, CA

Background/Question/Methods

Realizing the benefits of urban tree planting programs depends on tree survival. Cost-benefit analyses for urban forest ecosystem services are sensitive to mortality rate assumptions and associated population projections. However, long-term mortality data is needed to assess the accuracy of these assumptions. Analytical tools from demography, such as life tables and mortality curves, should be used to improve our understanding of urban tree mortality rates and lifespans. This novel approach to urban tree mortality is exemplified with 2 projects that focus on street trees. First, a meta-analysis of previously reported street tree survivorship data was conducted to estimate typical annual mortality rates and lifespan metrics. 16 studies were compiled and 11 were pooled for regression analysis of cumulative survivorship vs. time since planting. Second, 6 years of monitoring data (2006-2011) in Oakland, CA were analyzed with an adapted demographic balancing equation to assess how street tree population changed over time. A complete re-census of all street trees was conducted annually in a small plot in West Oakland, with observations of newly planted, standing dead, and removed trees. The Oakland data was also used to assess the shape of the street tree mortality curve.

Results/Conclusions

Based on the meta-analysis of previous studies, typical street tree annual mortality rates ranged from 3.5-5.1% with estimated street tree population half-life (the time at which half the planting cohort has died) at 13-20 years. Mean life expectancy was 13-20 years, which is considerably longer than the 7 or 13 year average lifespan discussed in the urban forestry literature. In West Oakland, the total number of street trees in the study plot increased from 993 in 2006 to 1165 in 2011; this is a net increase of 17.3%. However, several hundred new trees were observed during the study period; many did not survive. The average annual mortality rate for all trees was 4.5%, with a Type III to U-shaped size-based mortality curve. Although there was an overall net increase in street tree population size, the newly planted trees in this plot were barely out-pacing mortality. Understanding urban tree mortality rates is important for advancing the scientific study of urban ecosystems, as well as successful implementation of tree planting programs. With more accurate population projections, urban forest managers can plan for cycles of tree planting, death, removal and replacement to achieve stable canopy cover.