Phenology-Driven Changes in Controls of the Surface Energy Balance Across Different Extratropical Ecosystems

Land surface phenology, which describes the seasonal dynamics of vegetated land, plays a crucial role in regulating the seasonality of water and energy exchange between the land and atmosphere. Changes in key phenological metrics, such as the start of season (SOS) and end of season (EOS), have been observed in extratropical ecosystems using satellite data, and are largely attributed to climate change. These changes can have significant impacts on the surface energy balance, affecting the exchange of heat, moisture, and radiation between the land and atmosphere. However, the magnitude and spatial distribution of these impacts are not yet well understood. 

This study aims to investigate the sensitivity of surface energy balance variables, including turbulent latent and sensible heat fluxes, evaporative fraction, surface albedo, and surface temperature, to changes in SOS and EOS in the extratropical northern hemisphere during the period 2001-2021. We develop a method to quantify the sensitivity of surface energy balance variables to changes in SOS and EOS, using a linear regression approach to extract the slope of the relationship between the phenological indicators and the surface energy balance variables. Our analysis integrates multiple datasets, including ERA5 reanalysis, MODIS remote sensing estimates, GLEAM and FLUXCOM-X observation-guided data products for the water and energy fluxes, and a land-cover type map, to provide a comprehensive assessment of the impacts of phenological changes on the surface energy balance across different plant functional types (PFT). 

Our results show that an earlier SOS is associated with increased turbulent heat fluxes, evaporative fraction, and surface temperature during the time around SOS, while later EOS has similar but less pronounced effects during the time around EOS, with spatial variability and differences among PFTs (for both phenological indicators within and between the different datasets of the surface energy balance variables). These spatial sensitivity patterns are generally consistent across multiple datasets, except for the sensible heat and evaporative fraction sensitivity to SOS, which exhibit considerable variability across datasets. Our analysis by PFT reveals a higher sensitivity of all surface energy balance variable to shifts in SOS in forests, compared to cropland, grassland, and shrubland. Finally, we place our findings on biogeophysical phenology impacts into perspective by comparing them to the effect strength of biogeochemical impacts, providing a comprehensive assessment of the relative importance of these two types of impacts at the land-surface.