© 2001 Heron Publishing—Victoria, Canada
Shoot structure and photosynthetic efficiency along the light gradient in a Scots pine canopy
Pauline Stenberg (1), Sari Palmroth (1), Barbara J. Bond (2), Douglas G. Sprugel (3) and Heikki Smolander (4)
1. Department of Forest Ecology, University of Helsinki, P.O. Box 27, FI-00014 University of Helsinki, Finland / 2. Department of Forest Science, Oregon State University, Corvallis, OR 97331, USA / 3. College of Forest Resources, University of Washington, Seattle, WA 98195, USA / 4. Finnish Forest Research Institute, Suonenjoki Research Station, FI-77600 Suonenjoki, Finland / Received Augutst 18, 2000
Summary
We examined the effects of structural and physiological acclimation on the photosynthetic efficiency of Scots pine (Pinus sylvestris L.) shoots. We estimated daily light interception (DLI) and photosynthesis (DPHOT) of a number of sample shoots situated
at different positions in the canopy. Photosynthetic efficiency (ε) was defined as the ratio of DPHOT to the potential daily
light interception (DLIref) defined as the photosynthetically active radiation (PAR) intercepted per unit area of a sphere at the shoot location. To
calculate DLIref, DLI and DPHOT, the radiation field surrounding a shoot in the canopy was first modeled using simulated directional distributions
of incoming PAR on a clear and an overcast day, and estimates of canopy gap fraction in different directions provided by hemispherical
photographs. A model of shoot geometry and measured data on shoot structure and photosynthetic parameters were used to simulate
the distribution of PAR irradiance on the needle surface area of the shoot.
Photosynthetic efficiency (ε) was separated into light-interception efficiency (εI = DLI/DLIref) and conversion efficiency (εPHOT = DPHOT/DLI). This allowed us to quantify separately the effect of structural acclimation on the efficiency of photosynthetic
light capture (εl), and the effect of physiological acclimation on conversion efficiency (εPHOT). The value of ε increased from the top to the bottom of the canopy. The increase was largely explained by structural acclimation
(higher εI) of the shade shoots.
The value of εPHOT of shade foliage was similar to that of sun foliage. Given these efficiencies, the clear-day value of DPHOT for a sun shoot
transferred to shade was only half that of a shade shoot at its original position. The method presented here provides a tool
for quantitatively estimating the role of acclimation in total canopy photosynthesis.
Keywords:
light interception, photosynthesis, Pinus sylvestris, shade acclimation.
