Variability in seasonal patterns of water use efficiency and growth of non-native urban tree species in the Los Angeles region using leaf sugars

Background/Question/Methods

There is growing interest in understanding water and carbon cycle processes of urban trees and forests, in order to quantify benefits associated with maintaining or expanding urban tree cover. However, despite the relevance to ecosystem function, services and management, urban tree physiology has rarely been measured. In Southern California, moderate temperatures combined with widespread landscape irrigation have created a highly diverse urban forest, with trees native to many regions of the world. In order to understand the variability in water use efficiency and growth of these non-native urban trees, this study used a common garden approach to compare seasonal carbon isotope discrimination (δ¹³C), leaf sugar concentrations and stem growth across 24 commonly planted tree species in the Los Angeles Basin. These species come from a variety of native habitats, climates, and phenological groups (conifer, temperate deciduous, tropical deciduous, semi-deciduous and evergreen), providing the opportunity to explore how water use efficiency and growth in a common environment correlates with other plant traits and native habitat conditions.

Results/Conclusions

We found δ¹³C values to differ dramatically across the 24 species, with evergreens generally having lower values than other phenological groups. δ¹³C values varied seasonally, matching the pattern of evaporative demand. Concentrations of water-soluble sugars in leaves also varied seasonally and across species, with the seasonal pattern inversely related to stem growth (i.e. sink strength). Species differed in seasonal stem growth patterns, with many species concentrating most of their stem growth into late spring and early summer, despite favorable temperatures and water availability year-round. Leaf phenology and biome of origin did not fully explain differences in seasonal patterns of stem growth. Better understanding of urban tree physiology can inform urban tree management and future species selection, in order to optimize the balance between urban tree costs and services. Additionally, urban ecosystems offer unique experimental opportunities, creating “common garden” assemblages of species that would never co-occur naturally.