Drought and hydraulic thresholds: Predicting the mortality of woody plants of Patagonian forests and deserts

Background/Question/Methods

Droughts are expected to increase in the next 50 years in temperate areas of South America, particularly in the western slopes of the southern Andes covered by Nothofagus and conifers species and in the driest Patagonian ecosystems (steppes). Selective tree dieback has been observed in these temperate forests during one of the most severe drought of the 20th century (1998-1999) in which Austrocedrus chilensis trees survived whereas trees of the co-occurring evergreen species (Nothofagus dombeyi) experienced symptoms of water stress, such as leaf wilt and abscission before die-back occurred. Similarly, some evergreen shrubs in the steppe exhibited dieback during unusual summer droughts while other shrub species survived. The mechanisms and plant traits underlying or mitigating specie-specific mortality under future climate change scenarios are poorly understand. Studies were done to understand drought responses of trees and shrubs and to link hydraulic traits with mortality providing a tool to predict the degree of species-specific vulnerability within and across these ecosystems with dry summers and cold winters. We asked whether different stem and leaf hydraulic traits exhibited by the two forest species and by seven steppe species could help explain the contrasting survivorship rates.

Results/Conclusions

Species experiencing die-off under severe drought have wide stem safety margins and minimum stem water potentials (Ψ) far from the threshold of irreversible hydraulic failure (P₈₈), but the minimum leaf Ψ are close to leaf P₈₈, which probably are reached during anomalous droughts. Massive mortality of N. dombeyi and of shrubby species appears to be the consequence of the total loss of leaf hydraulic conductance (K_leaf) which triggers leaf senescence and drop. Drought occurs during the summer and it is highly likely that these species cannot recover its photosynthetic surface to produce carbohydrates required to avoid tissue injury before subzero temperatures occur. Species surviving the drought have wide leaf safety margins as a consequence of high leaf-and-stem capacitance, tight stomatal control, ability to recover K_leaf and deep roots. Drought hydraulic limitation to carbohydrate production more than plant hydraulic failure per se triggers die-off in N. dombeyi which could favor the dominance of conifers in these temperate forests under predicted climate changes. We suggest that strong hydraulic segmentation inducing leaf drop during drought may not favor survivorship of some species because carbon assimilation and osmotically active carbohydrate synthesis during the summer season are required to avoid freezing damage during winter.