Coordination of hydraulic and morphological traits across dominant grasses in eastern Australia

1. Leaf hydraulic traits characterize plant drought tolerance and
responses to climate change. Yet, plant hydraulics are biased towards
northern hemisphere woody species. We collected rhizomes of several
perennial grass species along a precipitation gradient in eastern
Australia and grew them in an experimental garden to investigate potential
tradeoffs between drought tolerance and plant morphology. 2. We measured
the following leaf hydraulic traits: the leaf water potential (Ψleaf) at
50% and 88% loss of leaf hydraulic conductance (P50Kleaf and P88Kleaf),
the Ψleaf at 50% loss of stomatal conductance (P50gs), leaf turgor loss
point (TLP), leaf dry matter content (LDMC), leaf modulus of elasticity
(ε), and the slope of the relationship between predawn and midday Ψleaf.
We also measured basal area, tiller density, seed head density, root
collar diameter, plant height, and aboveground biomass of each individual.
3. As expected, grass species varied widely in leaf-level drought
tolerance, with loss of 88% hydraulic conductance occurring at a Ψleaf
ranging from -1.52 to -4.01 MPa. However, all but one species lost leaf
turgor, and most reached P50gs before this critical threshold. Taller more
productive grass species tended to have drought vulnerable leaves
characterized by low LDMC and less negative P88Kleaf. Species with greater
tiller production experienced stomatal closure and lost turgor at more
negative Ψleaf. Although our sample size was limited, we found no
relationships between these species’ traits and their climate of origin.
4. Overall, we identified important hydraulic and morphological tradeoffs
in Australian grasses that were surprisingly similar to those observed for
woody plants: (1) xylem of taller species was less drought tolerant and
(2) turgor loss occurs and stomatal closure begins before significant loss
of Kleaf. These data build upon a small yet growing field of grass
hydraulics and may be informative of species responses to further drought
intensification in Australia.