Objective
The goal of this study is to identify trees that perform well under stressors associated with climate change in California’s Central Valley, Inland Empire and Southern California Coast climate zones. By replacing highly vulnerable species with species better adapted to future conditions, we can shift the palette of trees commonly planted to species that will provide the most environmental, social and economic value in the future. We are working with growers, designers, arborists and the public to achieve our objectives.
Tree Species Selection
Two approaches were used to identify promising, underutilized species. Expert horticulturalists were asked to list the species that they recommend for testing because of their potential resilience to climate change. We also compiled existing tree inventories for 6 to 12 cities in each climate zone. From the inventory data we identified species that were relatively rare but promising because they had individuals in larger size classes. Native tree species were welcomed but not exclusively considered. California's urban areas have relatively few tree species native within a given city’s boundary (see Pawlak et al. 2023 for digital range maps for tree species native to California).
Using these two lists, we identified species to evaluate in each climate zone. Each species was evaluated on three vulnerability criteria: habitat suitability, physiology and biological interactions. These criteria reflect each specie’s adaptive capacity, defined as its potential to ameliorate exposure to climate change stressors. Each species received a rating of-1, 0, or 1 (sensitive, neutral, insensitive) for each criterion based on its adaptive capacity, as reported in the literature. Scores were summed and each species was placed into one of five vulnerability classes (low, low-moderate, moderate, moderate-high, high). Once the highest scoring species were identified their availability for planting was confirmed. Species that could not be obtained for planting were replaced with the next highest scoring candidate.
Field Testing
Field testing of 12 species in each climate zone utilized UC Cooperative Extension (UCCE) Field Station plots in Davis (Inland Valleys), Irvine (Southern California Coast) and Riverside (Inland Empire) for side-by-side comparisons under similar growing and maintenance conditions. At each reference site, four replicates were planted in a completely randomized design, with all 48 (4 replicates x 12 species in each site) trees receiving the same irrigation, pruning and other maintenance activities. Within each climate zone, an additional 96 trees were planted in 4 parks (2 replicates per park, 24 trees per park) where growing conditions and maintenance activities were more variable. Standard monitoring protocols are being followed to record tree size, health and management issues at each site. All trees are evaluated annually for the first five years after planting, and biannually thereafter. The trees are expected to remain in the ground for at least 20 years. Measured data are analyzed and updates on growth and survival are posted to this website once published.
Outcomes
This study will generate and transfer new knowledge about the performance of underutilized trees with potentially high resilience to climate change. Application of this science will result in planting of trees more likely to thrive in future urban environments. Improved tree performance will increase production of ecosystem services, improve environmental quality, promote human health and enhance the economic vitality of California communities.