Introduction
Climate change is predicted to increase the occurrence of extreme drought events, altering the water available for plant growth (Dai 2013; IPCC 2014). Mechanistic understanding of plant growth and survival, as well as changes in community assembly and species distribution in response to changes in water availability can be understood through physiological ‘response’ traits directly linked to plant water economics (also termed ‘hydraulic traits’; Engelbrecht et al. 2007; Suding et al. 2008; Reich 2014; Griffin-Nolan et al. 2018). Unfortunately, those physiological traits have been mostly omitted in large-scale species distribution and community-scale trait studies because their measurement is time- and labor-intensive, and unfeasible under field conditions (for reviews see Bartlett et al. 2012b; Griffin-Nolan et al.2018). To this end, different morphological response traits (e.g., specific leaf area) have been used as proxies. However, the relationship between these proxies and precipitation (as a dominant measure of water availability for plants) on the community scale is weak, highlighting the need for a more careful trait selection (for review see Griffin-Nolan et al. 2018).
A turning point could arise from a recent development of a fast and feasible osmometry method to determine leaf osmotic potential at turgor loss point (πtlp), one of the key physiological response traits. The osmometry method, originally developed on woody species (Bartlett et al. 2012a) has been subsequently validated for herbaceous species (Griffin-Nolan et al. 2019; Májeková et al. 2019), and thus can be applied across a wide range of functional types. Leaves maintaining a more negative πtlp remain turgid at lower soil water potentials allowing them to maintain critical physiological processes (growth and photosynthetic assimilation of CO2) under drier conditions (Scholander et al.1965; Schulze et al. 1987; Kubiske & Abrams 1990; Kramer & Boyer 1995; Koide et al. 2000; Mitchell et al. 2008; Bartlett et al. 2012b). This leaf-level dehydration tolerance (sensu Volaire 2018) trait can be scaled-up to a whole-plant drought tolerance (sensu Noy-Meir 1973), a link well documented in woody species (Lenz et al. 2006; Baltzer et al. 2008; Bartlett et al. 2012b, 2016b). It is important to note that we are referring to drought tolerance as the ability of plants to maintain growth under decreased water availability (sensu Noy-Meir 1973), as opposed to drought avoidance and drought escape). πtlp also clearly relates to other leaf-level hydraulic traits, such as leaf water potential at 50% loss of hydraulic and stomatal conductance (Bartlett et al. 2016a; Farrell et al. 2017; Griffin-Nolan et al. 2019).
A question that remains open is the degree of coordination between πtlp as the trait mechanistically linked to drought tolerance and functional traits commonly used as proxies for drought tolerance in trait-based and global change ecology. Coordination among physiological and morphological aspects of plant’s phenotype on both leaf and whole-plant level is important for understanding the mechanisms governing plant’s responses to water availability and their adaptive nature. To this end, we have selected four traits spanning a spectrum of ‘hard’ to ‘soft’ traits on the leaf- and whole-plant level: intrinsic water-use efficiency (iWUE) captured by carbon stable isotope δ13C, leaf dry matter content (LDMC), specific leaf area (SLA, inverse of leaf mass per area, LMA) and plant height.
Water use efficiency represents a ratio of net CO2assimilation to stomatal conductance (Farquhar et al. 1989; Seibtet al. 2008). If water is the main limiting factor, iWUE should increase with decreasing water availability (Araya et al. 2010). The carbon stable isotope δ13C in leaves has been often used as a surrogate of iWUE (Farquhar et al. 1989; Seibtet al. 2008). Leaf δ13C is an integrated, long-term measure of the ratio between internal and ambient CO2 concentrations (Ci/Ca) that reflects the stomatal conductance to CO2 and thus also drought-induced stomatal closure. This is because plants preferentially assimilate lighter 12C and increasingly use heavier13C when CO2 (and thus the abundant lighter isotope) is less available. Such a situation typically occurs in leaves with closed stomata under drought stress (Farquhar et al.1989).
Specific leaf area (SLA; inverse of leaf mass per area, LMA), has been the most commonly used proxy in relation with water availability gradients, especially in large-scale studies along very long gradients (Westoby 1998; Westoby et al. 2002; Wright et al. 2004; Díaz et al. 2016; Griffin-Nolan et al. 2018). It also represents the second axis of the global spectrum of form and function (Díaz et al. 2016), an important trait of the leaf economics spectrum (Wright et al. 2004), as well as a prominent trait for assessment of plant strategies (Westoby 1998; Pierce et al. 2013). Decrease in SLA and increase in leaf dry matter content (LDMC) under decreasing water availability can be explained by the decrease in leaf expansion rates achieved by formation of smaller cells and/or tighter packed cells with less air space in between and/or thicker cell walls in order to reduce water requirements under drought (Garnier et al. 2001; Poorter et al. 2009). Though LDMC and SLA are often considered interchangeable (e.g. Pierce et al. 2013), when leaf thickness as the third player is being considered (Vile et al. 2005), their relationship is hyperbolic (Garnier et al. 2001; Vendraminiet al. 2002), potentially resulting in SLA and LDMC diverging in their response to the stress considered (e.g. Hodgson et al.2011).
Plant height is the leading dimension of the global spectrum of form and function (Díaz et al. 2016), a prominent trait defining plant strategies (Westoby 1998; Pierce et al. 2013) and as Westobyet al. (2002) states: “[…] the quantitative trait that has been adopted by virtually everyone doing comparative plant ecology”. A decrease in plant height along gradients of decreasing water availability relies on two assumptions that act together in the field conditions. First is the simple premise that plant growth and biomass production are reduced under water limitation (Schulze et al. 1987). Second is the ‘hydraulic limitation hypothesis’ (Ryanet al. 2006), which states that with increasing height increases also the difficulty to supply leaves with water and the risk of embolism, leading to stomata closure, decrease in photosynthesis, and less carbon available for growth.
An important factor to be considered when investigating the coordination among different traits possibly related to the same function (i.e. water availability) is whether such a relationship is biophysically based, and thus transferable to all plant species, or whether it is reflecting the adaptations typical for a certain plant functional type (PFT) or a phylogenetic clade. If two traits are related in the same way within different PFTs, or the relation holds when accounting for phylogenetic relatedness between species, it would suggest that whenever there was an evolutionary change in one trait in one direction, it coincided with an evolutionary change of a second trait in the same direction due to adaptations to the same selection pressure (‘selective correlation’; Stebbins 1950; Felsenstein 2004).
Here, we screened 122 temperate grassland species from different plant functional types (forbs and graminoids) across a broad range of water availability conditions in European temperate grasslands (from dry through mesic to wet grasslands). We assessed the relationships between πtlpand other traits related to plant drought tolerance on the leaf and the whole-plant level. We hypothesized that higher physiological leaf-level dehydration tolerance (more negative πtlp) will be coordinated with (1) physiological trait represented by higher intrinsic water use efficiency (iWUE) measured as δ13C; (2) lower values of the specific leaf area (SLA) associated with less acquisitive species within the leaf economics spectrum; (3) leaf-level morphological trait represented by higher leaf dry matter content (LDMC); (4) plant stature represented by smaller maximum vegetative height. We further investigated whether the aforementioned relationships are biophysically-based, and therefore general, or driven by the differences inherent in the major plant functional types and/or inherent in different phylogenetic clades.