Multi-scale understanding and modeling of plant hydraulics

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Li, Lingcheng

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Plant hydraulics describes the mechanisms of water uptake from the soil to roots, water transport through the xylem to leaves, and water loss via stomata. Mechanistic modeling of plant hydraulics has advanced in recent decades with a demonstrated capability to simulate evapotranspiration and gross primary production, especially under water stress conditions. As these water stress conditions are expected to intensify in a warming climate, it is important to ensure land surface models (LSMs) for use in Earth system models (ESMs) are equipped with appropriate parameterizations of plant hydraulics. Most LSMs employ an idealized “big-leaf” concept to regulate water and carbon fluxes in response to soil moisture stress through empirical soil hydraulics schemes (SHSs). Such schemes have been shown to cause significant uncertainties in water and carbon simulations. This dissertation aims to better understand and simulate the role of plant hydraulics in regulating the terrestrial water and carbon cycles through observational data analysis, numerical model development, and continental-scale applications. Chapter 2 analyzes the impacts of plant hydraulics-related properties on the sensitivity of vegetation interannual variability to hydroclimatic factors such as precipitation and atmospheric water vapor pressure deficit (VPD) in North America. Compared to isohydric plants, anisohydric plants are more negatively affected by VPD and would suffer more from increasing atmospheric moisture stress under climate change. Focusing on the Noah-MP land surface model, Chapter 3 presents a novel plant hydraulics scheme (PHS) (hereafter referred to as Noah-MP-PHS), which employs a big-tree concept by considering the whole-plant hydraulic strategy. Noah-MP-PHS is evaluated using plot-level observations from UMBS, and improves water and carbon simulations, especially during dry soil conditions. Noah-MP-PHS can reproduce contrasting plant hydraulic behaviors for two species, i.e., isohydric ‘risk averse’ red maple and anisohydric ‘risk prone’ red oak. The stem water storage in PHS enables nocturnal plant water recharge and provides an important buffer to relieve xylem hydraulic stress during dry soil conditions. Chapter 4 extends the plot-level and tree-level PHS experiments to the forest regions in the continental United States (CONUS). Six experiments are conducted, including three SHS experiments and three PHS experiments. PHS_plot, PHS risk-averse, and risk-prone experiments use calibrated PHS parameters, respectively, from the plot-level, maple tree-level, and oak tree-level simulations as described in Chapter 3. The spatial sapwood area and volume indexes over the CONUS forest regions are calculated for the first time for use in land surface modeling. PHS impacts the ET partitioning to transpiration and the total water storage anomaly over the CONUS. Plant hydraulic traits play an essential role in PHS water simulations. Therefore, the implementation of plant hydraulics, along with more realistic representations of plant traits and hydraulic strategies, could reconcile observations and models of terrestrial water cycles. Noah-MP-PHS provides a useful platform to better understand the roles of terrestrial ecosystems on global carbon and water cycles and energy budget, including research of data assimilation, land-atmosphere interaction, extreme event prediction, climate projection, and so on.


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