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Hydrological tipping points and desertification of semi-arid woodlands



Titre : Hydrological tipping points and desertification of semi-arid woodlands.

Organismes NSF : Division of Environmental Biology DEB

Durée : April 1, 2016 - March 31, 2019

Forest mortality caused by drought has recently increased and is now of global concern. This project is designed to improve the ability of ecologists to predict the impact of significant tree die-off in semi-arid, forested ecosystems. The study focuses on piñon-juniper woodlands. In the Southwest U.S., where they occur, higher temperatures and decreased precipitation have increased drought severity, reducing tree health, and triggering widespread tree death across the region. This research is motivated by previous findings showing that piñon pine mortality unexpectedly leads to woodlands becoming hotter and drier, potentially altering the environmental conditions that control future plant growth and ecosystem recovery

The project will test the overarching hypothesis that tree density in semi-arid biomes may represent a tipping point, such that a decrease in tree density due to drought-induced mortality can trigger a potentially irreversible drying of the system, and transition to a new ecosystem state. The project will address : 1) specific mechanisms that explain why plant-available soil moisture and remaining juniper and small piñon pine tree function decreases following piñon mortality (decrease of hydraulic redistribution, altered energy balance, or reduced infiltration due to hydrophobic litter inputs) ; and which process, if any, is dominant ; 2) if there are specific tree density thresholds or species-specific community compositions that facilitate aridification following tree mortality ; 3) if there are soil structure and/or precipitation regimes in semi-arid woodlands that facilitate aridification following tree mortality and 4) what is the likelihood that these disturbed woodlands will return to their previous state or progress to a new ecosystem state. The experimental design is centered on nine large plots, three with no disturbance that will act as controls, three in which all piñon pines are girdled, and three in which all juniper are girdled, which will allow the research team to examine species-specific roles of piñon and juniper in regulating the hydrology of these woodlands. Measurements include the response of hydraulic redistribution and sapflow in the remaining species, soil moisture profiles, and surface energy balance components, which will be used to constrain a land surface model, the Community Land Model (CLM 4.5), modified to include hydraulic redistribution. The proposed work will greatly enhance the ability to predict how and why coniferous mortality in semi-arid biomes may trigger aridification, advance our understanding of the links between tree mortality and hydrology in semi-arid systems, and quantify the role of hydraulic distribution at the ecosystem scale.

Partenaires : Marcy Litvak mlitvak (Principal Investigator) William Pockman (Co-Principal Investigator)

Financement : $1,012,904.00

Présentation (National Science Foundation)

Page publiée le 2 mars 2017, mise à jour le 12 octobre 2017