Weak coordination between leaf drought tolerance and proxy traits in herbaceous plants

Increased drought is predicted to have a major impact on plant performance
under environmental change. Yet leaf hydraulic traits directly related to
drought tolerance, such as leaf turgor loss point (πtlp), are
underrepresented in trait-based studies and have been largely overlooked
within the main frameworks evaluating trait–trait coordination and
trade-offs: the leaf economics spectrum and the global spectrum of plant
form and function. Using 122 herbaceous species from the Central European
temperate grasslands, we investigated (1) the coordination between πtlp
and traits often used as proxies for drought tolerance: specific leaf area
(SLA), leaf area (LA), leaf dry matter content (LDMC), leaf thickness
(LT), plant height and intrinsic water use efficiency (iWUE); (2) whether
the strength of the trait–trait relationships differed across plant
functional types (PFTs: graminoids and forbs) and depended on species
phylogeny; (3) whether single or multiple traits, combined with either
PFTs or phylogenetic relatedness, provide a good prediction of πtlp. A
more negative πtlp (higher leaf drought tolerance) was coordinated with
higher LDMC and higher iWUE. This pattern was consistent among PFTs and
also after accounting for phylogenetic relatedness. However, the
coordination of πtlp with other traits was weak. For LT and height, it was
driven by the differences between PFTs. For SLA and LA, it was only
observed after accounting for phylogenetic relatedness. The most
parsimonious model predicting πtlp as a function of other traits retained
LDMC and LA (adj. R2 = 0.37). Since πtlp showed a strong phylogenetic
signal, accounting for the influence of phylogenetic relatedness further
improved πtlp prediction by 17%. In herbaceous temperate plants, there is
relatively weak coordination between leaf drought tolerance (πtlp) and
traits representing key dimensions of the leaf economics spectrum and the
global spectrum of plant form and function. None of the proxy traits
considered here, alone or in combination, provided a strong prediction of
πtlp across a large number of grassland plant species. Therefore, our work
emphasizes the need for direct measurements of leaf hydraulics when
estimating plant drought responses to better understand and predict
species responses to environmental change.