Functional adaptations and trait plasticity of urban trees along a climatic gradient

Highlights

• Urban trees presented functional adaptations in response to local climate.

• Urban trees had different levels of trait plasticity along a climatic gradient.

• Turgor loss point was the best surrogate for changes in precipitation and maximum temperature.

• Leaf-level traits showed a greater plasticity across species and sites.

Abstract

In urban environments, long-term tree survival and performance requires physiological tolerance or phenotypic plasticity in plant functional traits. Knowledge of these traits can inform the likely persistence of urban forests under future, more severe climates. We assessed the plasticity of morphological and physiological traits of tree species planted along an urban climatic gradient in the Greater Sydney region during a severe, multi-year drought in eastern Australia. We selected four sites along a ∼55 km east-west transect, ranging from the cool/wet coast to the warm/dry inland. We assessed five tree species (four natives, one exotic) with different predicted climatic vulnerability based on climate-origins, estimating functional traits indicative of drought tolerance: carbon isotope composition (δ¹³C), Huber value (HV), specific leaf area (SLA), wood density (WD), and leaf turgor loss point (π_tlp). Broadly, trees planted in warm/dry sites had more negative π_tlp, higher WD, δ¹³C and HV, and lower SLA than cool/wet sites, indicating phenotypic plasticity to drought. The leaf-level traits π_tlp, δ¹³C and SLA were more strongly correlated with temperature and precipitation, compared to HV and WD. Species differed in the extent of their trait shifts along the transect, with greater plasticity evident in the exotic Celtis australis and the more temperate cool-climate Tristaniopsis laurina, compared to the more tropical, warm-climate Cupaniopsis anacardioides, which showed limited plasticity and lower drought tolerance. Our findings reveal adaptive capacity of urban trees to climate via plasticity in drought tolerance traits, which can direct species selection to improve urban forests resistance to climate change.

Introduction

Urban forests are fragmented landscapes where challenging environmental conditions, such as wind canyons, restricted rooting volumes in compacted soils, and high temperatures, can be aggravated due to the urban heat island effect (Corburn, 2009; De Sherbinin et al., 2007). Such conditions increase species’ vulnerability to climate change and lead to higher mortality rates relative to rural forested areas (Escobedo et al., 2016; Roman et al., 2014; Smith et al., 2019). Further, global climate change is likely to exacerbate urban species’ risk of mortality.

Nevertheless, species growing outside their natural range and under harsh urban conditions indicate that there are underlying aspects facilitating survival of these trees in cities. These include biological and environmental factors (e.g. local adaptation) or human management (e.g. irrigation). Active irrigation and fertiliser management of urban trees can improve their performance. However, this is often limited to a defined, relatively short time period after initial establishment (Pauleit et al., 2002; Roman et al., 2013), leaving mature trees exposed to local climate without these interventions.

The long generation time of most trees means that their capacity to adapt to abrupt environmental changes via genetic filters on subsequent generations is limited. Species’ long-term survival, therefore, depends on their capacity to cope with these changes through physiological tolerance to extremes (such as heatwave and drought events) provided via genetic adaptation and phenotypic plasticity (Mellilo et al., 1996). It is, therefore, important to understand how species respond to environmental variability via local adaptation (i.e. trait plasticity).

Species’ plasticity (i.e. changes in behaviour, morphology and physiology in response to the environment; Price et al., 2003) can be assessed using functional traits. These are plant attributes that impact fitness via their effects on physiology, growth, reproduction and survival (Violle et al., 2007). Plant functional traits can be classified as morpho-anatomical or physiological and have been shown to be affected by climate (Floret et al., 1990; Orsham, 1989). Physiological traits are tightly linked to plant functions (Cornelissen et al., 2003) and can be indicative of drought tolerance (i.e. functional traits that act to confer resistance or avoidance of acute or prolonged water deficits). The leaf turgor loss point or wilting point, for example, is recognized as an indicator of drought tolerance and is correlated with water availability within and across biomes (Bartlett et al., 2012, 2014). Morpho-anatomical traits can also be correlated with water availability and with physiological traits (Esperón‐Rodríguez et al., 2018; Sack et al., 2012). For instance, leaf morphotypes often change in response to prolonged water-limited conditions, resulting in smaller leaf size and reduced specific leaf area (Ackerly et al., 2002). Furthermore, drought tolerance has been shown to increase under hotter, drier conditions (Blackman et al., 2017; Asao et al., 2020). The level of phenotypic plasticity varies among species, such that the ability to respond to different environmental conditions found in urban forests.

Functional traits can be used to inform species selection in urban forestry. Decisions over which species to plant can be based, for example, on aesthetic and social factors, disease resistance and, crucially, expected changes in climate (Conway and Vander Vecht, 2015; Esperon‐Rodriguez et al., 2019; Gerstenberg and Hofmann, 2016; Sæbø et al., 2003; Sjöman et al., 2018; Sjöman and Nielsen, 2010). To take into account the risks associated with climate change, urban planning should aim to incorporate species that are resistant to future changes in temperature and precipitation patterns. Unfortunately, knowledge of climate sensitivity and trait plasticity is limited or unknown for urban forest species. Assessing plant performance via trait plasticity along climatic gradients can provide valuable information on plant responses to climate change and help to direct species selection in urban forests. Thus, the objective of this study was to assess species’ plasticity in terms of variations of functional traits related to drought tolerance as a demonstration of climate adaptation.

We evaluated a suite of leaf- and branch-level functional traits relate to drought tolerance, allocation and efficiency among five urban tree species along a climatic gradient to determine: (1) species tolerance related to climate-origin; (2) trait plasticity related with climatic variables; and (3) differential responses among species to the climatic gradient. For this case study, we selected a climatic gradient from the relatively cool, wet coastal region to the warm, dry inland region of Greater Sydney, New South Wales, Australia, during a severe, multi-year drought. In this region, future climate change projections indicate that average temperatures will continue to increase in all seasons with greater maximum and minimum temperatures in the future (Webb and Hennessy, 2015). Winter and spring rainfall is projected to decrease, along with predictions of longer intervals between rainfall events and an increase in the intensity of rainfall events (Webb and Hennessy, 2015). Temperatures in some parts of Greater Sydney can exceed 45 °C during heatwave events (Australian Bureau of Meteorology 2019; <www.bom.gov.au/>), with expectations of longer, more frequent periods of extreme temperatures in the future (Webb and Hennessy, 2015). These harsh conditions will undoubtedly impact urban forest performance, such that climate tolerance and trait plasticity should be key considerations for species selection for future urban plantings.