Constraining vegetation turnover rates in Terrestrial Biosphere Model using L-band ALOS PALSAR backscatter

An improved representation of the carbon and water cycle dynamics in terrestrial ecosystems underpins a large uncertainty reduction in modeling Earth system dynamics. The climate sensitivity of ecosystem processes controls land-atmosphere interactions and the overall atmospheric carbon uptake and release dynamics across scales. Local and Earth observations of vegetation dynamics are key for the evaluation of our understanding and support the quantification of process representation in model development. Previous research has shown the importance in undermining equifinality using multi-variate observation constraints, focusing water and carbon fluxes and stocks.

Long-wavelength radar backscatter provides unique insights into the dynamics of plant water and carbon dynamics when compared to optical EO products, as such, embeds the potential for constraining various parameters controlling local climate vegetation responses. In this study, we present an approach for assimilating L-band ALOS PALSAR backscatter data along with carbon and water fluxes measured at FLUXNET sites into a terrestrial ecosystem model to improve estimates of vegetation parameters turnover rates. A semi-empirical radiative transfer model, the Water Cloud Model (WCM), is employed as the observation operator linking modeled plant water content to L-band backscatter. Multiple model–data integration experiments are conducted to assess the added value of radar constraints across different model structures, including configurations with and without plant hydraulic schemes, and across temporal scales ranging from sub-daily to monthly.

Our results indicate that assimilating L-band backscatter observations improves estimates of aboveground biomass and strengthens constraints on foliage and woody turnover rates. However, persistent equifinality between plant water and carbon cycle processes remains, highlighting the need for improved estimates of the WCM parameters. Ultimately, this study highlights the potential of L-band backscatter to enhance vegetation carbon cycle modeling, emphasizes the added value of the newly launched ESA BIOMASS mission, and underscores the importance of integrating vegetation water dynamics into carbon models.