Modeling Forest Physiological and Structural Responses to Climate Extremes and Feedbacks in GISS Model E

DESCRIPTION: Global forest ecosystems cover around 30% of the land surface and store around 861 Pg carbon, more than the carbon in the atmosphere. Forests regulate climate by modulating exchanges of energy, water, and momentum between the land surface and atmosphere. However, forests are extremely vulnerable to extreme events, including droughts, heat waves, and fire, which can trigger large mortality events, altering forest composition and structure and even causing regimes shifts in forest ecosystems. Empirical studies have found that, in response to these extreme events in recent years, tree mortality rates have increased, tree sizes shrunk, and forest species composition has shifted to more opportunistic species (e.g., evergreen to deciduous in boreal and tropical regions) globally. Realistically modeling these responses are critical for understanding how forest will continue to respond to extreme events in the future, and potential feedbacks to the climate system. However, in current Earth system models, global vegetation and functional diversity are represented by only a small number of plant functional types, with physiological and demographic parameters obtained from a few dominant plant species in each biome. The functional diversity and key physiological and demographic processes, such as physiological hydraulic responses, mortality, regeneration, and compositional shifts, are required for realistically predicting forest responses to climate extremes.
The proposed research will develop a parsimonious formulation of plant hydraulics that optimizes plant physiological performance and mortality risks for investigating global forest responses to climate extremes. We will incorporate the game-theoretic modeling of plant hydraulics and mortality processes in NASA Goddard Institute for Space Studies (GISS)’s Earth system model, ModelE, to simulate forest physiological and structural responses to climate extremes (e.g., drought and heat waves). These processes will be embedded in the newly developed demographic vegetation model that explicitly simulates demographic processes (e.g., reproduction, growth, and mortality), individual-based competition for light and soil resources, and transient changes in vegetation structure and composition in ModelE. We will use this model to study forest dynamics and vegetation-climate interactions in response to climate extremes. Remote sensing data from NASA satellites related to vegetation physiological activity and structure will be used to parameterize model processes and evaluate the simulation results.
This research will last for four years. We will: 1) incorporate a new plant hydraulic module to simulate plant physiological responses to drought and heat waves; 2) develop new mortality and reproduction modules related to drought stresses to simulate forest regeneration processes and forest compositional and structural changes; and 3) explore forest feedbacks to global carbon and water cycles and climate systems within the ModelE. We will use the data from NASA satellites to validate our model simulations and understand vegetation dynamics globally in climate extremes, e.g., MODIS for global vegetation productivity and phenology, OCO-2 for global photosynthesis by measured solar-induced chlorophyll fluorescence data, and ICESat-2 and GEDI data for forest structure and biomass. We have developed an individual-based demographic vegetation model, BiomeE, coupled with GISS ModelE’s land model TerraE to simulate vegetation compositional and structural changes. The newly developed plant hydraulic and mortality processes will be embedded in this vegetation model. This proposal is in response to the research theme of Extremes in the Earth System in the Modeling, Analysis, and Prediction program. It will advance the modeling of forest physiological and structural responses to climate extremes in NASA GISS's ModelE, and improve its predictions of terrestrial carbon and water cycles in a changing climate.