Vertical and temporal distribution of insolation in gaps in an old-growth coniferous forest

Abstract

The combination of canopy access at the Wind River Canopy Crane Research Facility, hemispherical photography, and long-term insolation data provided estimates of vertical and temporal distributions of insolation in nine canopy gaps in a 65 m tall Douglas-fir (Pseudotsuga menziesii (Mirb.) Franco) - western hemlock (Tsuga heterophylla (Raf.) Sarg.) forest. Yearly insolation (long-term data from Portland, Oreg.) exhibited a sigmoidal pattern with height, with a bright zone (>4200 MJ/m2) above 50 m, a transition zone from 45 to 30 m (2000 MJ/m2), and less rapid decrease from 30 to below 10 m (600 MJ/m2). Intergap variation peaked between 20 and 40 m. Interannual variation of yearly insolation (CV = SD/mean) was about 5% throughout the canopy. Seasonality of insolation was driven by solar angle and cloudiness. Diffuse insolation was 50% of annual above-canopy flux, increasing to nearly 70% at 1.5 m, and diffuse proportion was greater in winter and spring. Hourly simulations under clear and cloudy conditions provided an appropriate time scale for modeling photosynthesis. Estimated leaf area index peaked at 30-35 and 5-10 m but was underestimated (3.7 vs. 9.1 m2/m2 from direct measurements) because of foliage clumping. The methods documented highly variable distributions of insolation driven by forest structure, cloudiness, and seasonal changes in solar angle.