Using Dynamic Vegetation Modeling to Explore Boreal Forest Canopy-cover Shifts Under Water Stress and Changing Climate

Tuesday, December 10, 2019 - 17:30
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Changes in high-latitude forests have strong implications to regional and global climate, and water and carbon cycling. Shifts in canopy cover (i.e., abundance and shifts between evergreen and deciduous species) will alter albedo, carbon, and water fluxes. For example, soil water deficits can lead to decreases in ecosystem productivity in deciduous boreal forests, and strongly influences the water uptake and storage between snowmelt and leaf out. Deciduous trees transpire 21–25% of available snowmelt water, while coniferous trees transpire <1%. A shift to deciduous trees therefore reduces groundwater recharge, and potentially leads to more storms and lightning-induced fires. All these climate-related interactions will affect plant competition, survival, and ultimately community distribution and carbon storage. To be able to accurately predict and model these complex ecological processes we are using a new demographic vegetation model (FATES; Functionally-Assembled Terrestrial Ecosystem Simulator) that is coupled to ELMv1, the land surface model in the global Earth System Model - E3SM. A new continuous soil-root-plant plant hydraulic scheme has been included within FATES, allowing dynamic plant mortality and growth from water stress. We use FATES-Hydro to quantify the impacts on water cycling (e.g., water use efficiency, latent heat, soil water storage) and carbon fluxes (NEE) under transitions between boreal evergreen and deciduous trees.

To evaluate changes in high-latitude water and carbon cycling as a result of climate-vegetation interactions, we performed a parameter sensitivity analysis using a Latin hypercube approach to sample the parameter space of 15 main vegetation parameters, over a 100-member ensemble run. In addition, leaf and wood allometry parameters for boreal plants have been updated based on observational data from the BAAD Database. Initial tests of FATES at a boreal Alaska site found strong biomass sensitivity to soil moisture stress. Application here of the newly developed plant hydraulic scheme (FATES-Hydro) allows us to simulate the impacts of precipitation and soil moisture changes on shifting boreal evergreen and deciduous tree cover.

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