The water transport system is considered to be a primary site of adaptation involved in determining the ability of woody plants to survive under different rainfall regimes and to recover after water stress. Most of our current understanding about the variability and interaction of hydraulic traits comes from interspecific comparisons. However, studies examining variation within species remain scarce despite their importance to defining species’ tolerance ranges and predicting species response to climate change. In this study we assess the variability of drought related leaf and stem traits across population of two species of Hakea, a genus endemic to Australia. Hakea leucoptera is widely distributed in dry and xeric areas whereas Hakea dactyloides is restricted to wet and temperate areas. Traits measured included wood density, vulnerability to embolism, hydraulic efficiency, leaf capacitance, turgor loss point and cuticular transpiration, leaf carbon isotope discrimination (Δ13C), leaf carbon and nitrogen content, and specific leaf area. The main objective is to assess how traits vary with aridity and identify potential tradeoffs.
Our results show that the adjustment of the hydraulic system to climate dryness in these two species occurs primarily through reductions of the leaf area index and Huber value and leaf level traits: thicker with more stomatal control, lower turgor lost point and lower cuticular transpiration which result in greater capacity to supply water to the leaves and limit the drop in water potential. In contrast, between-population variability in vulnerability to embolism appears to be limited. These results suggest that both species limit the risk of hydraulic failure by adjusting the minimum water potential to a relatively constant value which remains within the current safety margins of the xylem, i.e. the difference between the minimum water potential experienced by the plant and the level of water stress that is likely to induce rapid loss of vascular function caused by embolism. The low phenotypic variation in vulnerability to embolism compared with ongoing rapid changes in the environment such as the increased occurrence of extreme droughts and heatwaves with the concomitant reduction of water potential will pose serious risk if individuals are not able to adjust the minimum water potential sufficiently to maintain safety margins within a range preventing mortality.