Canopy structure and light environment of an old-growth Douglas-fir/western hemlock forest

The three-dimensional light environment within the canopy of a tall coniferous forest was sampled to quantify its variation and localize the sites of radiation absorption. Broadband visible (PAR) and ultraviolet radiation (UVB) were measured around midday in midsummer in an old-growth Douglas-fir/Western Hemlock forest in the Cascade Range in southern Washington using sensors suspended from the gondola of a large tower crane. Patterns of vertical transmittance varied greatly between locations and showed abrupt transitions from bright to dark conditions at varying heights. The average light field in this canopy with trees to 60 m was resolved into three functional zones. Above 40 m from the ground is a "bright zone," where light was reliably intense and predominantly in the direct beam component, and below 12 m, a "dim zone," where light was reliably low and mostly diffuse. Between these levels is a "transition zone," with a steep vertical gradient in light transmittance, high horizontal variation, and a mixture of beam and diffuse components. The pattern of UVB light was very similar to that of PAR. From the general transmittance profile the vertical structure of the canopy was estimated to have a peak density of foliage at 12 m (less than one quarter of the stand height) with declining densities above and below. The "bottom-heavy" canopy structure found in this study differs markedly from the "top-heavy" profiles reported from managed or young stands.