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Universitat Autònoma de Barcelona (2016)

Understanding the mechanisms of drought-induced mortality in trees

Garcia Forner, Núria

Titre : Understanding the mechanisms of drought-induced mortality in trees

Auteur : Garcia Forner, Núria

Université de soutenance : Universitat Autònoma de Barcelona

Grade : Doctoral Tesis 2016

Plants are exposed to several environmental stressors including drought and extreme temperatures that can limit their growth and survival. Water availability is considered the main limiting factor for plant productivity. Plants display a plethora of strategies to cope with drought and maintain an adequate water balance, including modifications of the leaf area, stomatal control, changes in biomass allocation, modifications of source/sink carbon balance, and resistance to xylem embolism. Despite this, drought-induced forest mortality is a widespread phenomenon with potentially large ecosystem-level implications and is expected to increase due to increasing drought events as a result of ongoing climate change. Understanding how the complex network of traits involved in drought resistance determine species’ or individuals’ to survive drought is critical to assess the vulnerability of current vegetation to changes in climate and the potential impacts on ecosystem functioning and services. In 2008, McDowell et al. summarized drought-induced mortality mechanisms in a coherent and simple hydraulic framework. They hypothesized two main, non-exclusive physiological mechanisms leading to plant death under drought : hydraulic failure and carbon starvation. Hydraulic failure is the point at which whole-plant water transport becomes blocked due to excessive cavitation resulting from critical tensions in the xylem. Carbon starvation is the situation in which carbon supply from photosynthesis, carbon stocks or autophagy fails to meet the minimum metabolic needs. According to this framework, the preponderance of one or the other mechanism depends on the drought intensity and duration and plants’ ability to regulate their water potential (Ψw). Isohydric species might be more vulnerable to carbon starvation due to earlier stomatal closure to maintain relatively constant Ψw (and avoid embolism), while anisohydric species would be more susceptible to hydraulic failure as soil dries as they operate with narrow hydraulic safety margins due to their lower Ψw. The previous framework is centered on stomatal behavior, regardless of the plethora of traits involved in plant drought responses. In addition, stomata respond to several factors besides Ψw, hence assuming that iso/anisohydric regulation of Ψw is able to fully explain stomatal behavior may be misleading. For these reasons, the main objectives in this thesis were to : (1) determine if differences in stomatal regulation between species relate to iso/anisohydric behaviors and how these are associated to different mortality mechanisms under drought, warming or both ; (2) test the assumptions that relate anisohydric behaviors with higher stomatal conductances and longer periods of carbon uptake under drought, and isohydric behaviors with stronger stomatal control and wider hydraulic safety margins ; and (3) understand how morphological and physiological traits and their plasticity in response to drought explain, and to what extent, time until death within species. To address targets (1) and (2) we studied two reference models with contrasted drought-vulnerability between species : piñon-juniper and holm oak systems. In both cases, we compared drought responses between isohydric (Pinus edulis and Quercus ilex) and anisohydric species (Juniperus mosperma and Phillyrea latifolia), emphasizing stomatal regulation and carbon and water economies. In these species, we provided evidence that more anisohydric behavior is not necessarily related with looser stomatal responses to Ψw and, thus, with higher levels of xylem embolism. Likewise, stronger regulation of Ψw (isohydric behavior) was neither associated with earlier stomatal closure under drought nor with higher carbon constrains. Both studies challenge widespread notions and warn against linking iso/anisohydry with contrasted stomatal behaviors and mortality mechanisms. At the tree level (3), sustaining carbon uptake and carbon stocks above some critical level was the key factor prolonging survival under extreme drought, even at expenses of higher water losses. Fully integrating carbon and water economies is the key challenge to advance our understanding of drought responses and mortality mechanisms in plants.

Mots clés : Mortalitat induïda per sequera ; Drought-induced mortality ; Iso ; Anisohidria ; Anisohydry ; Comportament estomàtic ; Stomatal behaviour

Présentation (Tesis Doctorals en Xarxa) )

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