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.