Socioeconomic drivers of urban forest structure and diversity in the semi-arid San Joaquin Valley of central California

Background/Question/Methods:

Human actions directly affect the structure and composition of the urban forest, but the precise nature of this relationship remains poorly understood. Urban land and water management decisions result from dynamic interactions between institutional, individual, and ecological factors that determine the characteristics of the urban forest. The semi-arid Fresno-Clovis Metropolitan Area in California’s central valley has significant socioeconomic variability and provides an ideal system to study the human-urban forest relationship. The recent onset of residential water metering in Fresno also provides an opportunity to study the impact of an institutional policy change on human water use and how it affects the urban forest. Using a multivariate model-based inference approach, we ranked models of the relationships between socioeconomic factors (home value, building age, population density, irrigation ate, and substrate cover types) and three primary characteristics of the urban forest (tree density, tree species richness, and canopy cover). Here we present results from the pre-metering phase, and some preliminary results from ongoing research to study the longer-term impact of metering as residents alter their home garden landscapes to reduce water costs.

Results/Conclusions:

All primary characteristics of the urban forests were significantly positively influenced by home value, which indexes with socioeconomic status. Tree density was best explained by home value, followed by building age and percent impervious substrate (R²=0.459, best fit model AIC_c=366.2). Tree species richness was best explained by home value, followed by percent impervious substrate, percent grass substrate and population density (R²=0.585, AIC_c=259.18). Canopy cover was best explained by home value, followed by population density and percent building substrate (R²=0.313, AIC_c=-20.95). Secondary characteristics of the urban forest were not related to home value: Percent volunteer trees (%) was influenced by percent soil/mulch substrate alone (R²=0.201, AIC_c=-32.23); percent trees bearing edible fruits was influenced by percent grass substrate, percent building substrate, and building age (R²=0.340, AIC_c=-47.01).

The greater cost of now metered water is causing many Fresno residents to reconsider their landscaping priorities. The impact on deep-rooted species is currently unclear, but comparisons of traditional lawn yards with waterwise gardens illustrate an increase in herb species richness and density, and increases in rocks and artificial turf cover. Effects on urban forest structure will take longer to document.