Discussion
On an extensive set of 122 herbaceous species covering a broad range of
water availability conditions in European temperate grasslands (from dry
through mesic to wet grasslands), we documented a coordination between a
physiological trait directly linked to water availability (turgor loss
point, πtlp) and traits commonly used as proxies of
drought tolerance: intrinsic water use efficiency (iWUE), leaf dry
matter content (LDMC), specific leaf area (SLA) and maximum plant height
(height). By investigating the relationships across species, within
major plant functional types (PFTs, forbs and graminoids) and by
accounting for plant phylogenetic relatedness, we were able to
distinguish the relationships that were strong and more general
(πtlp–LDMC and πtlp–iWUE) against
those that were weaker (πtlp–SLA) or that could even be
misleading at a first glance (πtlp–height).
πtlp is coordinated with intrinsic water-use
efficiency
Higher leaf level dehydration tolerance (more negative
πtlp) was coordinated with higher intrinsic water use
efficiency (less negative δ13C) on all levels
considered. This may suggest that though plants adjust their osmotic
potential in order to keep stomata open, the mechanism has a limited
capacity in temperate plants. This may occur when the intensity of the
environmental stress (drought expressed by soil water potential) is not
stable but fluctuates deeply below the capacity of the osmotic
adjustment. During these episodes, stomata are closed, and the
assimilated carbon has less negative δ13C (higher
iWUE). The existence of the coordination across our species can be
interpreted perhaps as a continuous run (i.e. developing osmotic
adjustment) of the drought-exposed plants out of the dehydration stress,
which they never quite win (if they won, they would keep stomata open,
i.e. lower iWUE), but which is sufficient enough to maintain growth and
ensure survival in the more stressful conditions.
πtlp is coordinated with leaf dry matter
content
Stronger leaf dehydration tolerance (i.e., more negative
πtlp) was tightly coordinated with LDMC, morphological
trait commonly used as a proxy of plant’s drought resistance. One
explanation is that because low osmotic potential is a response to
drought stress (e.g. Májeková et al. 2019), water limitation
simultaneously results in smaller cell wall expansion, smaller cells
and/or more cell walls, i.e. greater LDMC (Poorter et al. 2009).
Our results are in line with previous evidence on coordination between
these two leaf-level traits in herbaceous species: on 33 C4 grassland
species (Liu & Osborne 2015) and on 19 tallgrass prairie species
including C3 and C4 graminoids, forbs and shrubs (Griffin-Nolan et
al. 2019). The fact that we found the coordination between
πtlp and LDMC to be strong on all levels, together with
the previous evidence including also C4 species, suggests that this
coordination is biophysically-based and transferable to other systems,
plant functional types and plant families.
πtlp is only weakly coordinated with specific
leaf area
Our results imply that the mechanistic link between SLA, plant water
economics and soil water availability could not be simply assumed on any
given scale without further stratification. The coordination between
πtlp and SLA appeared only weakly (P = 0.045) in
forbs after plants were stratified by their PFTs and then more strongly
when their phylogenetic relatedness was accounted for. Our results fit
well into the so far ambiguous evidence regarding the coordination
between SLA and πtlp. While a weak relationship between
more negative πtlp and higher SLA was documented in a
big compiled tree dataset (Zhu et al. 2018), no was found on a
more local spatial scale for both trees (Maréchaux et al. 2015,
2019) and herbs (Májeková et al. 2019). Moreover, recent evidence
found no relationship between SLA and water availability gradients at
different spatial and taxonomical scales (for reviews see Bartlettet al. 2012b; Griffin-Nolan et al. 2018). This together
suggest that SLA should be considered very carefully as a proxy
reflecting the response of plants to water availability.
πtlp is not coordinated with plant height
The relationship between πtlp and plant height
represents a nice example on how a coordination between two traits could
be misinterpreted without a further stratification by PFTs or accounting
for plant phylogenetic relatedness. When considering all species without
any stratification, a weak, but significant, negative relationship
appears between πtlp and plant height, suggesting that
higher leaf dehydration tolerance (more negative πtlp)
is coordinated with taller stature. A potential explanation would offer
itself, i.e. that shorter plants growing under taller plants would be
sheltered from direct irradiance, and therefore experience less
dehydration. However, a closer examination reveals that, in our case,
the relationship is driven purely by the pronounced differences between
forbs and graminoids in both traits considered and disappears within the
PFTs and after accounting for plant phylogenetic relatedness. It seems
that the risk of embolism, driving the relationship between plant height
and hydraulic traits in woody species (Ryan et al. 2006; Liuet al. 2019), does not play a major role in the shorter,
relatively to trees, herbaceous species. Indeed, a height of 1 m
generates only –0.01 MPa of gravitational potential, which is
negligible when compared with osmotic potential. Rather, plant height in
grassland plants seems to be under different and potentially more
important selection pressures than water availability, such as
competitive ability for light (Keddy & Shipley 1989), thus resulting in
little coordination among height and πtlp.
Critical remarks: the use of single traits as proxies of
function
One needs to carefully consider that the leaf is a multifunctional
organ, and thus bulk leaf traits might not always capture exactly the
response to a single abiotic factor in question. For instance, a plant’s
response to water availability measured through SLA, a very popular leaf
economics spectrum trait, can be simultaneously confounded by light
availability, nutrient availability, or herbivory resistance (Walters &
Reich 1999; Sack 2004; Poorter et al. 2009; Markesteijn et
al. 2011; John et al. 2017). As advocated by Hodgson et
al. (2011), one needs to carefully consider how valid the use of such a
trait is as a stand-alone proxy of a single function.
Here, we would like to reinforce this view by tentatively proposing
three advices. First, we advocate the use of traits known to be directly
and mechanistically linked to the factor in question (Griffin-Nolanet al. 2018). In case of water availability, we suggest using
πtlp, which is a hydraulic trait feasible to measure on
a large number of individuals. Second, if leaf traits are measured that
could reflect multiple functions of the leaf, i.e. SLA and/or LDMC, we
suggest coupling them with traits known to be mechanistically linked to
the factor in question. In case of water availability, this would be
πtlp, intrinsic water use efficiency, or any other
hydraulic trait. This should be done on at least a subset of species, in
order to validate (e.g. by simple correlation) that the morphological
traits reflect the function in question.
Third, both approaches can be further reinforced by coupling the
quantitative measures with the plant functional types. Indeed, if a
relationship (with another trait, or environmental factor) is driven
only by the differences in functional types, but does not hold within
the types, one might ask whether the trait considered really reflects
the function being investigated. For instance, here we showed that
stratification by simple plant functional types and/or accounting for
species phylogeny might help to identify relationships that are
biophysically-based, and therefore potentially better transferable among
different systems. Such would be the coordination between
πtlp and LDMC, where the strength of the relationship
holds true within PFTs, after accounting for phylogenetic relatedness,
as well as in other functional groups such as C4 grasses (Liu & Osborne
2015). On the other hand, the need for verifying the relationship is in
our case highlighted on the relationship between πtlpand plant vegetative height, which disappeared when further stratifying
by PFTs or accounting for phylogenetical relatedness.
Conclusion
Here, we addressed the recent plea for a use of traits that would have a
more direct mechanistic link to plant drought resistance and water
availability to be considered in community trait ecology and global
change ecology (Brodribb 2017; Griffin-Nolan et al. 2018; Volaire
2018). We demonstrated the
feasibility of the use of a physiological drought tolerance leaf-level
trait, πtlp, that becomes operational via the novel
osmometry method for large-scale studies across different plant
functional types. In herbaceous plants, πtlp was tightly
coordinated with intrinsic water use efficiency and with leaf dry matter
content. However, πtlp was not coordinated with plant
height, which is the leading dimension of the global spectrum of form
and function, and only weakly to SLA, which represents the second most
important axis (Díaz et al. 2016). This implies that another
important axis related to drought tolerance and water availability has
so far been largely omitted in the trait-based ecology. Finally,
stratification either by simple plant functional types and/or by
accounting for species phylogeny might help to identify relationships
that are biophysically-based and under the same selection pressure, and
therefore potentially better transferable among different systems.