© 1998 Heron Publishing—Victoria, Canada
Shoot structure, light interception, and distribution of nitrogen
in an Abies amabilis canopy
Pauline Stenberg (1), Heikki Smolander (2), Douglas Sprugel (3) and Sampo Smolander (4)
1. Department of Forest Ecology, P.O. Box 24, FI-00014 University of Helsinki, Finland (PAULINE.STENSERG@HELSINKI.FI) / 2. The Finnish Forest Research Institute, Suonenjoki Research Station, FI-77600 Suonenjoki, Finland / 3. College of Forest Resources, AR-10, University of Washington, Seattle, WA 98195, USA / 4. Department of Biosciences, Division of Genetics, P.O. Box 56, FI-00014 University of Helsinki, Finland / Received January 5, 1998
Summary
We studied the effects of variation in shoot structure and needle morphology on the distributions of light and nitrogen within
a Pacific silver fir (Abies amabilis (Dougl.) Forbes) canopy. Specifically, we investigated the role of morphological shade acclimation in the determination of
resource use efficiency, which is claimed to be optimal when the 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 developed to estimate seasonal light interceptance
(SLI, intercepted PAR per unit needle area) of the shoots using a model for the directional distribution of above-canopy PAR,
measurements of shoot silhouette area and canopy gap fraction in different directions. The ratio SLI/SLIo, where the reference value SLIo represents the seasonal light interceptance of a spherical surface at the shoot location, was used to quantify the efficiency
of light capture by a shoot. The ratio SLI/SLIo doubled from the top to the bottom of the canopy, mainly as a result of smaller internal shading in shade shoots than in
sun shoots. Increased light-capturing efficiency of shade shoots implies that the difference in intercepted light by sun shoots
versus shade shoots is much less than 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 times greater than the SLI of the five most shaded shoots, whereas SLIo was 40 times greater for sun shoots than for shade shoots. Nitrogen content per unit needle area was about three times higher
in sun needles than in shade needles. This variation, however, was not enough to produce proportionality between the amounts
of nitrogen and intercepted PAR throughout the canopy.
Keywords:
morphological acclimation, Pacific silver fir, resource use efficiency.
