Measurements of photosynthetic photon flux density (PPFD; global, diffuse, and direct: 400–700 nm) and red:far-red ratio (660:730 nm) were made above and below an understory of salal (Gaultheriashallon Pursh) within three western red cedar (Thujaplicata Donn)–western hemlock (Tsugaheterophylla (Raf.) Sarg.) stands on the northern end of Vancouver Island, British Columbia. These stands were chosen to represent a wide range of canopy closure and understory salal cover. The measurements were repeated under both clear and overcast sky conditions. Within each stand, the percent cover, height, number of stems per hectare, and leaf morphology of the salal were measured. As expected, there was a decrease in both the salal cover and in the global PPFD and red:far-red ratio measured directly above salal as the percent tree cover increased. Salal leaf thickness and specific leaf weight decreased, whereas its leaf area increased, as global PPFD and red:far-red ratio decreased. The results indicate that the minimum light requirement for salal survival is between 3.9 and 11.1 μmol•m−2•s−1 (1.2 to 3.3% of global PPFD measured in an adjacent clearing) and 4.5 and 27.2 μmol•m−2•s−1 (0.3 to 1.8% of global PPFD measured in the clearing) under overcast and clear sky conditions, respectively. The global PPFD and red:far-red ratio measured 30 cm above the forest floor (beneath the salal) did not vary widely among the three stands. At this level, global PPFD values were very low, ranging from 0.15 to 0.35% and 0.58 to 0.75% of the global PPFD measured in the clearing under clear and overcast sky conditions, respectively. Red:far-red ratios at the forest floor level varied from 0.13 to 0.25 and 0.40 to 0.58 under clear and overcast sky conditions, respectively. The silvicultural implications of the results are discussed.
Daylighting design with lightscoop skylights: Towards an optimization of proportion and spacing under overcast sky conditions
