Changes in July albedo and its relationship with EVI over the last two decades in the Swedish alpine region

Abstract: Vegetation in the subarctic is expected to respond to climate warming as its growth becomes less temperature limited. This has previously been recorded in the form of treeline advances and increasing shrub cover. Changes in vegetation cover may accelerate warming due to a decrease in summer albedo, in what is termed surface albedo feedback. A quantification of the relationship between vegetation change and its corresponding change in surface albedo is therefore important to better understanding of the surface energy balance and its consequent impacts on the climate. This study focuses on the Swedish portion of the Scandes mountain range and its surrounding area and set out to investigate whether the shortwave albedo in July has changed significantly in this region between the years 2000 to 2022, and whether this corresponds to a change in vegetation. This was done by using imagery from the Moderate Resolution Imaging Spectroradiometer (MODIS). Vegetation changes were quantified using the Enhanced Vegetation Index (EVI). Changes in both shortwave albedo and EVI were quantified using a linear regression model and their relationship was assessed through correlation analysis. This study found, contrary to expectations, that the shortwave albedo for July had mainly been stable or increased. Significant (p<0.05) trends in shortwave albedo changes were found for 44% of the area, of which 79% were increasing trends. Out of the area with significant changes in shortwave albedo, 38% had also a significant (p<0.05) change in EVI, most of which (83%) were increasing trends in EVI as well. The correlation between shortwave albedo and EVI for the entire area was, however, quite weak (ρ=0.227) but showed varying correlation strength when considering vegetation classes separately. Vegetation below the treeline had a positive relationship between shortwave albedo and EVI, whereas vegetation above the treeline in some areas had a negative trend in EVI, indicating possible vegetation browning, along with a corresponding increasing trend in shortwave albedo. The mechanisms behind the changes in shortwave albedo throughout the Swedish alpine region remain ambiguous, and the differences in trends for the different vegetation classes suggest a need for a closer look at specific species responses within each subregion of the Scandes vegetation classes.