Irrigated urban trees exhibit greater functional trait plasticity compared to natural stands.

Community ecology royalsocietypublishing.org/journal/rsbl Irrigated urban trees exhibit greater functional trait plasticity compared to natural stands Research Peter C. Ibsen1,2, Louis S. Santiago1, Sheri A. Shiflett3, Mark Chandler4 and G. Darrel Jenerette1 Cite this article: Ibsen PC, Santiago LS, Shiflett SA, Chandler M, Jenerette GD. 2023 Irrigated urban trees exhibit greater functional trait plasticity compared to natural stands. Biol. Lett. 19: 20220448. https://doi.org/10.1098/rsbl.2022.0448 Received: 28 September 2022 Accepted: 22 November 2022 Subject Areas: ecology, environmental science, plant science Keywords: aridity, community science, carbon gain, novel ecosystem, urban trees, water use Author for correspondence: Peter C. Ibsen e-mail: 1 Department of Botany and Plant Sciences, University of California Riverside, Riverside, CA 92521, USA Geosciences and Environmental Change Science Center, United States Geological Survey, Denver, CO 80225, USA 3 Department of Environmental Sciences, University of North Carolina Wilmington, Wilmington, NC 28403, USA 4 Earthwatch Institute, Boston, MA 02143, USA 2 PCI, 0000-0002-3436-9100; LSS, 0000-0001-5994-6122 Urbanization creates novel ecosystems comprised of species assemblages and environments with no natural analogue. Moreover, irrigation can alter plant function compared to non-irrigated systems. However, the capacity of irrigation 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 the 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 trait dynamics with no natural analogue. 1. Introduction Electronic supplementary material is available online at https://doi.org/10.6084/m9.figshare. c.6328100. Urbanization is a recognized cause of novel ecosystems, assemblages of species living in environments with no natural analogue [1,2]. Novel urban tree communities result from including species from globally distributed biogeographic provinces [3], and trees encountering factors that differ from natural stands, from soil characteristics to variable microclimates [4,5]. Another cause of novel tree communities is greater resource availability, notably irrigation, in the growing environment [6]. The effect of urban irrigation on tree functioning can alter mortality, growth and phenology [7,8], which may drive changes in important plant traits. The potential for novel functional trait distributions as an additional effect of irrigated urbanization and novel ecosystem in general is unknown. Carbon gain and water use reflect trait variation axes linked to tree physiological functioning and may change in response to urban irrigation. Trait suites associated with carbon gain and water use reflect the trade-offs between high resource use and faster growth on one end of the spectrum, and a reduced © 2023 The Authors. Published by the Royal Society under the terms of the Creative Commons Attribution License http://creativecommons.org/licenses/by/4.0/, which permits unrestricted use, provided the original author and source are credited. 2. Methods 2 (a) Study system royalsocietypublishing.org/journal/rsbl The Los Angeles Megacity comprises over 17.5 million residents and is highly urbanized from the coast to the Coachella Valley desert. Atmospheric aridity, measured as the difference between mean atmospheric-saturation water vapour pressure and actual water vapour pressure in the air (vapour–pressure deficit; VPD), and maximum summer temperatures range from approximately 1.4 kPa and approximately 24.5°C on the coast, to approximately 6.1 kPa and approximately 41.0°C in the desert [24]. We use mean VPD as a primary climate variable, as temperature and VPD are tightly correlated across this region [25]. (b) Data collection To assess trait distributions, we selected 30 tree species (electronic supplementary material, appendix S2; table S1) representing 11 biomes of origin, and included eight of the 15 most common Southern California street trees [3]. We located sample individuals via partnership with trained local community scientists. Community scientists identified potential individuals, recorded GPS location and provided a qualitative evaluation of tree condition and surrounding environment. We focused on healthy irrigated specimens by only sampling community scientists identified healthy trees with at least 65% irrigated area surrounding a 10 m radius around the base of the tree. We quantified the ‘carbon-gain’ train suite by being comprised of SLA, %N and leaf laminar thickness (LT) and the ‘water-use’ trait suite being comprised of GCL, stomatal density (SD) and WD. We calculated water-use strategy shifts as the coastal-to-desert variation between leaf water potential at predawn (ΨPD), midday (ΨMD) and their daily difference (ΔΨL). Values of ΨPD approximate night-time equilibrium with soil water potential, and values of ΨMD and ΔΨ reflect daytime plant water status, with more negative values indicating a liberal water-use strategy [26]. Sampling procedures are included in electronic supplementary material, appendix S1, and trait data are available from a Dryad dataset [27]. To compare urban carbon-gain and water-use strategies to trees in natural habitats, we obtained LES trait values (SLA and %N), and water-use traits (ΨPD and WD) for each species from their native habitats with the TRY database and primary sources and compared their linear regressions slopes [28–30]. Statistical analyses were completed in RStudio version 1.4.1106 [31]. 3. Results We found the two primary axes of the principal component analysis ordination comprised 65% of plant trait variation (axis 1 = 36.5% variation, axis 2 = 29.6% variation) (figure 1a). PC axis 1 was associated with carbon-gain traits (SLA – loading value: 0.58, %N – loading value: 0.39, LT – loading value: –0.64). PC axis 2 was mostly associated with water-use traits (GCL – loading value (...truncated)