Seasonal dynamics of albedo across European boreal forests: Analysis of MODIS albedo and structural metrics from airborne LiDAR

Aarne Hovi,Eva Lindberg,Mait Lang,Tauri Arumäe,Jussi Peuhkurinen,Sanna Sirparanta,Sergey Pyankov,Miina Rautiainen

doi:10.1016/j.rse.2019.02.001

Abstract

Uncertainties in estimation of albedo-related radiative forcing cause ambiguity in evaluation of net climate effects of forests and forest management. Numerous studies have reported local relations between forest structure and albedo in the boreal zone. However, more research is needed to establish these relations for geographically extensive areas, and to examine seasonal courses of albedo to understand the effects of forest structure on mean annual shortwave energy balance. Remote sensing is a viable option for accomplishing these goals, but there are many challenges related to e.g. long periods of cloud cover and low solar elevations in high latitudes. We used the new MODIS Collection 6 (MCD43A3) daily albedo product, and analyzed MODIS albedo dependence on airborne LiDAR-based forest structure in 22 study sites in Estonia, Finland, Sweden, and Russia (57°–69° N, 12°–57° E). Wall-to-wall LiDAR data allowed us to take into account the effective spatial resolution of MODIS, which notably improved correlations between albedo and forest structure. Use of the best quality backup algorithm (magnitude inversion) together with main algorithm results in the MODIS albedo product did not reduce the correlations compared to using main algorithm only. We quantified the effects of landscape-level forest structure (forest height, canopy cover, fraction of young forest) and fraction of broadleaved deciduous forest on mean annual albedo. We showed that because the forest structure-albedo relations are the strongest in snow-covered periods, and because the snow-covered period is longest in the north, the effect of forest structure on mean annual albedo increases towards the north. On the other hand, the effect of broadleaved fraction did not show such latitudinal trend. Our results indicate that even within a single climatic zone the optimal forest management solution to mitigate climate change depends on geographic location.

Highlights

Albedo determines the shortwave radiation balance and influences the interaction of land surfaces with climate
We showed that because the forest structure-albedo relations are the strongest in snow-covered periods, and because the snow-covered period is longest in the north, the effect of forest structure on mean annual albedo increases towards the north
Effective spatial resolution of MODIS We present the results of sensitivity analysis for black-sky albedo only, noting that the results regarding white-sky albedo were similar

Summary

Introduction

Albedo determines the shortwave radiation balance and influences the interaction of land surfaces with climate. Albedo of forests depends on forest structure and species composition, which has resulted in active research and discussion considering past and future changes in albedo due to e.g. forest management or disturbances, and their influence on climate In the boreal zone, where nonradiative heat fluxes are relatively small, albedo has a notable contribution to the surface energy balance and climate (Anderson-Teixeira et al, 2012; Bright et al, 2017). To account for albedo in e.g. climate change mitigation policies, relations between albedo and forest structure need to be accurately known. A major challenge is that the boreal zone is vast, with considerable variations in forest structure and climatic conditions.

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Conclusion