Effects of light on shoot geometry and needle morphology in Abies amabilis

In some conifers, shoot geometry and needle morphology vary significantly in response to the light conditions under which they develop. We measured shoot length, silhouette area, total projected needle area, total needle weight and needle thickness on current shoots developed under a wide range of light conditions in a 36-year-old Abies amabilis (Dougl.) Forbes stand. Current light was quantified by evaluating percent openness from hemispherical photographs taken before the growing season. Unweighted total openness was correlated with shoot geometry and needle morphology better than any weighted indices tested. Needle thickness and leaf mass/area were both closely correlated with total openness (R2 = 0.86 and 0.82, respectively). The most exposed needles were 2.5 times thicker and had 3-4 times more leaf mass/area than the most shaded needles. Total projected leaf area/shoot silhouette area was also correlated with openness (R2 = 0.74) and was about twice as high in sun shoots as in shaded shoots. As a result of greater leaf mass/leaf area and greater leaf area/shoot silhouette area, a unit of intercepted light was dispersed over about 6 times as much leaf mass in a sun shoot as in a shade shoot, which presumably permits more efficient utilization of the intercepted light under high illumination with less energy wastage to light saturation. Moreover, leaf mass per unit of silhouette area was almost exactly proportional to canopy openness, as predicted by resource optimization theory if nitrogen concentration and photosynthetic capacity per unit mass are constant in new leaves. The close correlation of needle thickness and leaf mass/area with openness suggests that either parameter could be used as an index of the distribution of light or light-driven processes in an A. amabilis canopy.