Relationships between single-tree mountain birch summertime albedo and vegetation properties

Abstract

Understanding and quantifying the influence of tree structure and ground vegetation on albedo in the boreal-alpine treeline is urgently needed to assess and predict the biophysical climatic feedback effect of forest- and treeline expansion. Fine-spatial resolution in-situ radiation measurements sensitive to small-scale variability enable precise albedo estimation for complex heterogeneous landscapes. In this study, horizontally measured single-tree albedo of mountain birch and their spatially consistent tree- and ground vegetation properties were collected in the boreal-alpine treeline over a period of 14 days in summertime. The aim was to identify properties of tree structure and ground vegetation driving single-tree mountain birch albedo. In addition, it was of interest to analyze the relationship between the vegetation properties and a slope-estimated albedo when a simplified correction of slope and aspect of the terrain was applied to the horizontally measured incoming shortwave radiation. Both properties of tree structure and ground vegetation were strongly related to albedo. The results imply that expansion of mountain birch forests at the expense of the prevalence of bright-colored lichens, bare rock, graminoids and mosses will reduce summertime boreal-alpine treeline albedo. Taller trees with wider tree crowns will absorb more solar radiation than smaller trees and hence also reduce albedo. Overall average difference of albedo of sample plots with and without presence of mountain birch was 0.06, corresponding to 27% of the albedo for plots without birch. Horizontally measured albedo was more strongly correlated with the vegetation properties than when corrected for terrain slope and aspect. The findings show that the appropriateness of horizontally measured albedo of single trees and tree clusters in open sloping terrain, depends on the spatial size of the footprint of the downward-looking radiation sensor relative to the size of the tree subject to observation.