Elucidating the hydraulic vulnerability of the longest-lived Southern Hemisphere conifer to aridification

Abstract

Drier climatic conditions will be the future scenario in many regions worldwide, including southern South America. Few studies have characterized the ecophysiological vulnerability of the endemic tree species that inhabit this area, to climate change. In this study we assessed the hydraulic vulnerability of the longest-lived tree of the Southern Hemisphere, Fitzroya cupressoides, focusing on adult trees and saplings from two highly disturbed populations: the Coastal Range (AC) and Central Depression (FN) of southern Chile, which represent contrasting site conditions. This, as a basis for the design of conservation strategies to safeguard the persistence of these endangered forests in a drier future. We assessed water potentials (WP) throughout a growing season, their relationships with environmental conditions, as well as leaf and stem (branch) traits and hydraulic safety margins. Despite that the studied summer was the second driest in the last seven decades, minimum WP were not that negative (−1.3 to −1.5 MPa); which could be partly explained by a high leaf capacitance in this species. Adult trees and saplings from both sites did not significantly differ in their WP at turgor loss point, and their associated leaf safety margins, which were relatively low in all cases. However, they significantly differed in the xylem WP causing a 50% loss of stem conductivity (P50): adults AC: −5.14, saplings AC: −2.53, adults FN: −3.71, and saplings FN: −3.87 MPa. These values led to a relatively large stem safety margin (SSM) in most cases, and their variation was not explained neither by wood density, nor by tracheid size changes. Moreover, there appears to be an ontogenic adjustment in the more restrictive site AC, which was not seen in FN. Within the continuum of species strategies to cope with water stress, Fitzroya has features of the two ends of the continuum: tissues with large SSM, and tissues that sustain milder operation pressures through capacitance. Although Fitzroya appears to be relatively resistant to water scarcity, saplings from AC, seem to be the most vulnerable to the aridification trend in southern Chile. Moreover, future drying could become a significant extra threat to the highly endangered Central Depression population. Conservation actions are urgently needed to secure the future of Fitzroya forests in southern Chile.