The effect of resident trees preferences on urban forest biodiversity in southern California

Background/Question/Methods

In arid regions the majority of urban trees are planted, and thus chosen by residents. In a previous study we surveyed over 1000 residents in southern California about their preferences for tree attributes. In this study, we compared the survey results to the actual composition of trees in southern California cities. Based on the initial survey results, we expected a greater number of trees in wealthier neighborhoods, more shade trees in hotter neighborhoods and more water use efficient trees in drier neighborhoods. We also hypothesized that we would find different patterns for public versus private trees. To test our hypotheses, we inventoried tree communities in 12-13 neighborhoods in three Southern California counties (Los Angeles, Orange, and Riverside), which span a range of environmental conditions (precipitation and temperature). In each neighborhood we surveyed 10 plots which were aggregated to the neighborhood scale for analyses. Neighborhoods were chosen to span a range of ages and incomes. We used stepwise multiple regressions to determine which socio-economic and environmental factors best explained tree community variability. 

Results/Conclusions

Urban tree communities in southern California were very diverse with 113 species identified. There was a significant interaction between county and tree type (p = 0.017), where Orange County had greater private tree richness, but same public tree richness compared with Los Angeles and Riverside. We found support for many of our predictions based on the results of the survey of resident preferences for tree attributes. There were more trees (R² = 0.257; p = 0.003) and greater tree richness (R² = 0.244; p = 0.003) in older and wealthier neighborhoods and a greater proportion of private shade trees in hotter neighborhoods (R² = 0.101, p = 0.058). We also found more water use efficient species on public property in drier and newer neighborhoods (R² = 0.274, p = 0.024), suggesting that urban forestry agencies are increasingly more concerned with water use than residents. Overall, there were different relationships between socio-economic and environmental drivers with tree communities dependent on whether the government or residents planted the trees. Ultimately our study demonstrates that the species richness and functional traits of urban tree communities are at least partially driven by residents’ preferences and perceptions of urban tree traits.