How do arctic shrub canopies respond to changes in the diffuse light fraction of solar radiation?

The aim of this study is to understand how tall (>50cm) arctic shrub canopies, such as those dominated by the shrub species Betula nana and Salix pulchra, function under different light conditions. Previous studies have demonstrated that, despite reduced total irradiance, net production can be greater under diffuse light conditions compared with bright sun (direct irradiance). This is because under diffuse conditions, such as occur when clouds obscure the sun, there is reduced shading and increased light scattering, enabling photons to penetrate further into a canopy and illuminate leaves that would otherwise be shaded. To study this, we sampled B. nana and S. pulchra  shrub canopies near the ARC LTER sites at Toolik Field Station using 1x1m² enclosed chambers with an infrared gas analyzer and irradiance sensors to measure the rate of net ecosystem production in conjunction with the direct:diffuse light fraction. We also made detailed canopy descriptions including several estimates of leaf area, light penetration under direct and diffuse conditions throughout the canopy, and measures of photosynthetic capacity for top, mid, and understory shoots within the community. We found that as predicted by theory, the shrub canopies achieved higher photosynthetic rates under diffuse conditions (diffuse fraction > 0.7) than they did under equivalent photon flux densities of direct light (diffuse fraction <0.3). These findings demonstrate the necessity of incorporating diffuse light fraction into photosynthesis measurements, models, and predictions of ecosystem response as both the diffuse light fraction and predominance of arctic, shrub vegetation are expected to increase in coming decades.