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

Abstract

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.

Wide climatic niche breadth and traits associated with climatic tolerance facilitate eucalypt occurrence in cities worldwide

“…Functional traits have proved to be useful determinants of the likely tolerance of a large panel of urban tree species to heat and drought stresses (Esperon‐Rodriguez et al., 2020; Hanley et al., 2021; Ibsen et al., 2023; Marchin et al., 2022; Sjöman et al., 2018). Here we found evidence that species with small leaves, high δ 13 C, high leaf N per dry mass and high wood density—which are typical characteristics of heat and drought tolerant species—are planted in cities experiencing warmer and drier conditions than in their native range in Australia.…”

“…To date, several studies have been conducted on a local or regional scale, focusing on few species or common garden experiments (e.g. Esperon‐Rodriguez et al., 2020, 2021; Hanley et al., 2021; Ibsen et al., 2023; Ilyas et al., 2021), which limits their broad applicability and capacity to describe global patterns of urban tree distribution.…”

Exploring potential trade-offs in outdoor water use reductions and urban tree ecosystem services during an extreme drought in Southern California

“…We acknowledge that this approach cannot fully account for spatial heterogeneity in species traits given uncertainty in remote sensing data (as well as limited spatial coverage). Nor does this approach account for potential longer term adaptation of species traits with climate change (Ibsen et al, 2023). Nonetheless it allows us to account for observed species differences from the Santa Barbara setting.…”

“…Our parameterization and modeling approach also ignore the potential for some ecophysiologic traits to adapt to local conditions and a changing climate (Ibsen et al, 2023). Few ecohydrologic models incorporate this plasticity but as our understanding of trait adaptation matures, future models may incorporate these dynamics.…”

“…Irrigation inputs have caused some urban trees in Los Angeles to acclimate and rely on irrigation along with natural water sources (Bijoor et al, 2012;Chen et al, 2015). Tree water use and drought tolerance depend on not only how much irrigation is applied but also species ecophysiological traits, including regulation of stomatal conductance in response to the environment (McDowell et al, 2008;Ibsen et al, 2023). Drought tolerant species or native species may require less irrigation.…”