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Shizuoka University (2017)

DROUGHT EFFECTS ON MULTI-SCALE WATER USE AND ECOSYSTEM CARBON EXCHANGE IN A DESERT ECOSYSTEM

GU Daxing

Titre : DROUGHT EFFECTS ON MULTI-SCALE WATER USE AND ECOSYSTEM CARBON EXCHANGE IN A DESERT ECOSYSTEM

Auteur : GU Daxing

Université de soutenance : Shizuoka University

Grade : Doctoral Thesis 2017

Résumé partiel
Drought is an important factor markedly influencing biotic and abiotic processes and then have considerable effects on ecosystem key material and energy processes, such as water and carbon cycles over a wide region on the earth terrestrial surface. In arid and semi-arid lands, drought effects on ecosystem are more obvious even decisive. In the current study, drought effects on crucial water and carbon processes of typical desert ecosystem are examined at various scales based mainly on four-year field measurement via sap flow and gas exchange flux techniques. The studied ecosystem is dominated by a desert shrub, Haloxylon ammodendron and locates on the southern edge of Gurbantünggüt desert in Northwest China and Central Asia. Transpiration per leaf area and stomatal conductance for water vapor at leaf, branch and whole plant scales and their response to air drought (atmosphere vapor pressure deficit, VPD) and other climate factors are compared in chapter 2. Daytime average transpiration and stomatal conductance at leaf scale were higher than that at branch and whole plant scales. High level of transpiration at leaf and branch scales appeared proximately at midday time while that of whole plant scale appeared at morning time, inducing reduced transpiration in 79% of daytime. Transpiration at the three scales showed similar response to photosynthesis photon flux density (PPFD), increasing with enhanced PPFD at low light condition and nearly saturating under highlevel PPFD. Similarly, transpiration at three scales linearly increased with increasing VPD. Under high VPD, however, leaf- and branch-scale transpiration nearly saturated while that of whole plant-scale declined with increasing VPD. For stomatal conductance, its sensitivity to VPD (m/Gsref) decreased continually from 0.52 at whole plant scale to 0.35 at leaf scale. This showed that whole-plant average stomata was more sensitive to air drought and had more conservative water use strategy protecting whole plant water homeostasis compared with that at leaf and branch scales. The different magnitude of transpiration, stomatal conductance and their response to air drought could be only explained by canopy patchiness, which could reduce whole plant water loss and allow partial assimilation by part of the canopy under drought, which was considered as one of the key mechanisms to balance water loss and carbon acquisition at whole plant scale. Results obtained here enriched the database of multiscale water use strategy of plants in xeric ecosystems and contributed to a better understanding of plant water use strategies in these ecosystems

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