Urbanization creates novel ecosystems comprised of species assemblages and environments with no natural analog. Moreover, irrigation can alter plant function compared to non-irrigated systems. However, the irrigations capacity to alter functional trait patterns across multiple species is unknown but may be important for the dynamics of urban ecosystems. We evaluated the hypothesis that urban irrigation influences plasticity in functional traits by measuring carbon-gain and water-use traits of 30 tree species planted in southern California, USA spanning a coastal-to-desert gradient. Tree species respond to irrigation through increasing the carbon-gain trait relationship of leaf nitrogen per specific leaf area compared to their native habitat. Moreover, most species shift to a water-use strategy of greater water loss through stomata when planted in irrigated desert-like environments compared to coastal environments, implying that irrigated species capitalize on increased water availability to cool their leaves in extreme heat and high evaporative demand conditions. Therefore, irrigated urban environments increase plasticity of trait responses compared to native ecosystems, allowing for novel response to climatic variation. Our results indicate that trees grown in water-resource-rich urban ecosystems can alter their functional traits plasticity beyond those measured in native ecosystems, which can lead to plant traits dynamics with no natural analog.

Leaves were sampled during the summers (July-Sept) of 2016-2018 from southern exposed portions of the tree crown to minimize differences in light availability among samples. Leaf samples were placed in plastic bags with a damp paper towel to prevent desiccation and kept in a dark cooler for transport. Fresh leaves were measured with a micrometer to determine LT, and stomatal peels and subsequent microscopic measurements at 400´ magnification were used to determine SD and GCL. Fresh leaves were scanned then analyzed with ImageJ (http://rsbweb.nih.gov/ij/) software to calculate leaf area, dried in a 60 °C oven for three days and weighed to determine dry matter content to express SLA as cm2 g-1. All leaf measurements were averaged (n=3) to the individual tree for analysis and the mean of 3 to 5 individuals for each site was calculated for further analyses. Wood cores were sampled from tree trunks using an increment borer (Cornelissen et al. 2003), and mass was determined by volumetric displacement, with cores subsequently dried in a drying oven at 60 °C for four days to express WD as the ratio of wood core volume to dry weight. Water-use strategies were derived by sampling ΨPD and ΨMD of southern exposed leaves with a Scholander Pressure chamber (PMS Instruments, Corvallis, OR USA) in coast and desert locations (n = 3 individuals/species/location for 21 species). Any sampled individual that did not have a full set of associated traits was removed from the analysis. We applied a Box Cox transformation to each category of functional trait and applied a z-transform to confirm to assumptions of normality.