The physiological thresholds set by plant hydraulic traits are thought to constrain the environmental conditions under which a plant can survive. Water stress associated with severe drought can result in hydraulic failure of trees and has the potential to cause large scale forest dieback impacting, not only species distribution, but community structure, hydrology and associated ecosystem services.
We aim to address the questions: How do hydraulic thresholds compare for tree species from different climatic regions? Is there coordination of hydraulic traits? What can such coordination tell us about the strategies employed by plants to resist water limitation? And how vulnerable are different forests types to hydraulic failure from severe drought events?
In this research, we characterize evergreen forest and woodland communities from across Australia by the hydraulic traits and thresholds of the dominant tree species. We measure xylem and leaf vulnerability to drought induced embolism at six sites, representative of major Australian biomes. Measurements of current water status (seasonal minimum in-situ water potentials) are used to assess risk of drought-induced hydraulic failure. In addition, we examine coordination of leaf and stem hydraulic parameters within and across species and sites.
Our results show a strong relationship between climate variables, such as mean annual precipitation, and hydraulic traits. We found coordination between stem hydraulic vulnerability to drought induced embolism (specifically the water potential at which 50% conductivity is lost due to embolism, P50) and the water stress experienced for different species across sites (as described by minimum water potential, Psi min). This result indicates that climate imposes limitations on species distribution and that species are finely tuned and adapted to their environment.
By comparing leaf traits (turgor loss point, osmotic potential at full turgor, and leaf vulnerability to embolism) with stem vulnerability to drought induced embolism, we see clear coordination between plant organs (leaves and stem), such that stomata are expected to close before the induction of cavitation in the stem.
Hydraulic safety margins, defined as the difference between P50 and Psi min, were smaller among tropical rainforest species (average 1.6MPa); species located at more arid sites maintained larger safety margins (2.7MPa). This suggests that rainforest species are operating closer to their hydraulic threshold and could be more vulnerable to future climate change.