COS 51-5 - GEDI-constrained Predictions of Current and Future Terrestrial Carbon Dynamics

Abstract: Canopy structure and composition are strong predictors of terrestrial carbon, water, and energy fluxes. Accurate representations of their initial states with process-based ecosystem and biosphere models are critical for accurate predictions of current and future ecosystem states and fluxes. In this study, we analyze ecosystem dynamics in California Sierras over the coming century by assimilating measurements of forest structure derived from Global Ecosystem Dynamics Investigation (GEDI) spaceborne waveform lidar measurements and composition derived from imaging spectrometry data into a cohort-based terrestrial biosphere model. The study region encompasses a diversity of forest and woodland ecosystems that are structurally and compositionally complex, spanning across a gradient of climate, soils, and elevation. Analysis of contemporary carbon fluxes indicates markedly stronger carbon uptake (gross primary productivity and net primary productivity) in dense-canopy mid-elevation mixed forests compared to both lower-elevation woody savannahs and high-elevation conifer forests due to the sparse tree densities in these areas. Due to the diversity in canopy structure, composition, and physical environment, the ecosystems respond differently to drought conditions: mid-elevation forests are strongly negatively impacted by the drought, experiencing significant declines in gross primary productivity, aboveground biomass), and basal area during the 2012-2015 drought period. In contrast, the high-elevation forests are impacted far less, reflecting a combination of greater resilience to water stress and post-drought ecosystem structure. Forward model simulations over the period 2020-2100 indicate that there are significant long-term legacies of canopy structure on the model predictions of ecosystem change. While the impacts of initialization gradually decline, the initialization state has continuing impacts on model predictions into the latter half of the century. These findings demonstrate the importance of above-ground canopy structure to predictions about how on-going human activities are altering the composition and structure of terrestrial ecosystems and their biogeochemical and biophysical dynamics. More generally, this research shows how remotely sensed estimates of ecosystem structure can be assimilated into process-based terrestrial ecosystem and biosphere models to analyze and predict patterns of regional fluxes and terrestrial ecosystem responses to climate variability and change.