Warmer and drier conditions are reducing forest productivity around the globe, while tree mortality is on the rise due to extreme drought and heat stress. These new climate conditions bear significantly on tree physiology and metabolism, particularly in temperate forests where these severe events are relatively novel. Assessing the ability of trees to withstand particularly dry and warm conditions is crucial in order to predict their survival under stress and to understand the overall ecological consequences of climate change. Thus, evaluating the structural and functional aspects of tree water transport, as well as leaf resistance to heat stress, constitute key research needs that will advance our knowledge of forest resilience not only in Chile’s forests, but also in temperate forests in the Southern Hemisphere. Indeed, while much is known about species’ resistance to climate change in Europe and North America, our understanding of Latin America’s forests pales by comparison.
The decreasing precipitation and warmer temperatures that are projected for south-central Chile may have a strong impact on forest ecosystems. However, there is almost no information about the vulnerability of native tree species to climate change in the country. To fill this gap, the proposed research will focus on one of the most important tree species in south-central Chile, Nothofagus obliqua, commonly known as roble. N. obliqua mainly grows between El Maule and Los Lagos Regions in the Andean and Coastal Ranges, as well as in the Central Depression of Chile. This pioneer species is broadly distributed, ecologically important and has tangible economical relevance for the small and medium landowners in Chile. The main objective of this proposal is to evaluate the hydraulic vulnerability to water scarcity and tolerance to heat stress of six different N. obliqua provenances (populations) growing in a common garden and in the field, and to assess the main leaf and wood traits related to their response. The existence of a 20-year-old N. obliqua common garden opens a unique opportunity to evaluate if there is any genetic differentiation in its vulnerability to climate change. Moreover, the evaluation of the species vulnerability to drought and heat stress throughout its latitudinal range will reveal whether there is any plasticity in their response to warmer and drier conditions. The selected study provenances are Vilches, Quirihue, Recinto, Colico, Llifén and Llancacura, which cover the latitudinal transect and part of the longitudinal distribution.
The sensitivity of stem xylem to water scarcity will be measured using the vulnerability curve, which describes the decline in xylem water transport efficiency in response to progressive dehydration. Leaf and stem vulnerability curves will be generated using the optical and pneumatic methods, respectively. These will allow to derive the water potential at which 50% of hydraulic conductivity is lost (P50) in each population. Furthermore, since chlorophyll fluorometry can provide good estimates of thermal stability under heat stress, leaf thermotolerance will be assessed through a leaf warming experiment to determine the temperature threshold at which the maximum quantum yield of the photosystem II declines to 50% of its healthy value (T50). Safety margins for hydraulic vulnerability and heat tolerance will be estimated measuring the minimum water potentials in each population and the maximum temperatures experienced in the field, respectively. Leaf traits (e.g. leaf area, leaf mass per area, leaf nutrients), as well as wood anatomical traits (e.g. vessel area, vessel diameter, theoretical specific conductivity) will be also determined to assess differences between populations and to define which of the traits are more related to the species´ resistance to warming and water scarcity. Finally, tree growth will be evaluated in the common garden and the field, in order to disentangle if there is any trade-off between growth and resistance to embolism in the species.
Results from this study will help to identify if any Nothofagus obliqua provenance is better suited to withstand warmer and/or drier conditions, or if the species has sufficient plasticity to adapt to these conditions. These data will inform management recommendations for national forestry adaptation and mitigation plans. Assessing the variability in the above-mentioned traits for a South American temperate tree species would be an enormous contribution, not only for the country, but also for the international scientific community, because comprehensive studies such as this are still scarce.

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FONDECYT de Iniciación

INFOR

Alejandro Cárdenas (Asistente de Investigación), Danny Carvajal (investigador colaborador), Robert Skelton, Jarmila Pittermann (Colaboradores Internacionales)