Tree function and ecosystem services in the semiarid Los Angeles Basin urban forest

Background/Question/Methods Trees in urban ecosystems provide valuable ecosystem services. However, in arid regions which are not naturally forested, the irrigation required to sustain urban trees constitutes an ecosystem disservice.  A better understanding of the factors that control urban tree water use, growth, water use efficiency and drought resistance is necessary to maximize ecosystem services and minimize costs. To address this issue, a variety of physiological measurements were made (including sapflux, isotope, leaf level photosynthesis, growth rate, stem vulnerability to cavitation, water potential, and leaf nutrient measurements) spanning more than 35 species in the Los Angeles Basin. These data were used in combination with tree inventory data to examine patterns in urban tree and forest function. We wished to answer the questions: What is the relative importance of climate, species composition, and management in determining plant function? How do patterns of tree function scale to patterns of urban forest ecosystem services and disservices? What is the role of species composition in influencing urban forest ecosystem services and disservices?

Results/Conclusions Climate and management (especially whether trees received irrigation) had measurable effects on tree function (e.g. up to a 100% difference in water use), but the major driver of differences in forest function was species composition. Inventory data showed that the artificial Los Angeles Basin urban forest is more diverse than the natural shrubland ecosystem, and contains species native to virtually all regions of the world. Consequently, trees present in the Los Angeles Basin urban forest show a wider range of water use, growth rate, water use efficiency, phenology and drought resistance than in a natural ecosystem. Of the 15 most commonly observed species (representing ~40% of measured trees), many do not maximize desired ecosystem services (e.g. they have low water use efficiency and/or small or open crowns which provide little shade) or they have high disservices (such as high water use). Thus, species composition has a strong influence on ecosystem services and disservices. Depending on the services of interest, it is possible to improve the balance between services and disservices by changing forest community composition in new plantings.