SHOOT STRUCTURE, LIGHT INTERCEPTION, AND DISTRIBUTION OF NITROGEN IN AN ABIES AMABILIS CANOPY

We studied the effects of variation in shoot structure and needle morp hology on the distributions of light and nitrogen within a Pacific sil ver fir (Abies amabilis (Dougl.) Forbes) canopy. Specifically, we inve stigated the role of morphological shade acclimation in the determinat ion of resource use efficiency, which is claimed to be optimal when th e distribution of nitrogen within the canopy is directly proportional to the distribution of intercepted photosynthetically active radiation (PAR). Shoots were collected from different heights in the crowns of trees representing four different size classes. A new method was devel oped to estimate seasonal light interceptance (SLI, intercepted PAR pe r unit needle area) of the shoots using a model for the directional di stribution of above-canopy PAR, measurements of shoot silhouette area and canopy gap fraction in different directions. The ratio SLI/SLIo, w here the reference value SLIo represents the seasonal light intercepta nce of a spherical surface at the shoot location, was used to quantify the efficiency of light capture by a shoot. The ratio SLI/SLIo, doubl ed from the top to the bottom of the canopy, mainly as a result of sma ller internal shading in shade shoots than in sun shoots. Increased li ght-capturing efficiency of shade shoots implies that the difference i n intercepted light by sun shoots versus shade shoots is much less tha n the decrease in available light from the upper to the lower canopy. For example, SLI of the five most sunlit shoots was only about 20 time s greater than the SLI of the five most shaded shoots, whereas SLIo wa s 40 times greater for sun shoots than for shade shoots. Nitrogen cont ent per unit needle area was about three times higher in sun needles t han in shade needles. This variation, however, was not enough to produ ce proportionality between the amounts of nitrogen and intercepted PAR throughout the canopy.