Plasticity in plant hydraulic traits: An evaluation of a common-taxa experiment across a climatic gradient in the Western U.S.

Societal Impact Statement: Due to climate change, plants are experiencing both prolonged drought events and increasingly variable water availability, prompting the need for a better understanding of potential impacts on plant performance, as well as the identification of low-water-use plants. This study utilized a research network in three climatically distinct areas of the Western U.S. to test the same horticultural taxa for key plant functional traits under water deficit treatments. Plant traits varied depending on location and taxa; however, reducing irrigation volumes did not significantly impact plant performance, demonstrating a potential to reduce urban water use for landscape plants across a climatic gradient. Summary: Drought impacts plant productivity, ecosystem function, and the global carbon cycle, with many species-level responses remaining unknown. Responses to water availability vary among plant species and across biomes. This research utilized a coordinated water deficit experiment of horticultural taxa across three sites in the Western U.S. to assess taxa-level plasticity to water availability and location. Four taxa (Cercis occidentalis, Cercis canadensis, Physocarpus opulifolius Diabolo and P. opulifolius Little Devil) were measured for physiological and morphological traits affecting plant hydraulic conductivity under two water deficit treatments. Full gas exchange, specific leaf area, vessel diameter, theoretical hydraulic conductance, and Φ_PSII were collected, and water use efficiencies were calculated for each taxon at each location. Impacts of site, treatment, and taxa were analyzed on this suite of traits, and phenotypic plasticity was calculated. Results show differences in taxa performance by climatic location, between closely related species and cultivars, and distinctions in the plasticity of functional traits. However, the irrigation treatments had limited effects on physiological performance. These findings highlight the benefits of common-taxa trials over multiple geographic locations to evaluate and identify the climate suitability of plants. Utilizing a unique research framework, our results provide evidence that landscape irrigation can be reduced substantially and fill critical knowledge gaps for assessing water needs in urban and other managed landscapes.