© 2001 Heron Publishing—Victoria, Canada
Leaf area distribution and radiative transfer in open-canopy forests: implications for mass and energy exchange
Beverly E. Law (1), Alessandro Cescatti (2) and David D. Baldocchi (3)
1. Department of Forest Science, 328 Richardson Hall, Oregon State University, Corvallis, OR 97331, USA / 2. Centro di Ecologia Alpina, I-38040 Viote del Monte Bondone (TN), Italy / 3. Department of Environmental Science, Policy and Management, 151 Hilgard Hall, University of California, Berkeley, CA 94720,
USA / Received August 18, 2000
Summary
Leaf area and its spatial distribution are key canopy parameters needed to model the radiation regime within a forest and
to compute the mass and energy exchange between a forest and the atmosphere. A much larger proportion of available net radiation
is received at the forest floor in open-canopy forests than in closed-canopy forests. The proportion of ecosystem water vapor
exchange (λE) and sensible heat exchange from the forest floor is therefore expected to be larger in open-canopy forests than in closed-canopy
forests.
We used a combination of optical and canopy geometry measurements, and robust one- and three-dimensional models to evaluate
the influence of canopy architecture and radiative transfer on estimates of carbon, water and energy exchange of a ponderosa
pine (Pinus ponderosa Dougl. ex Laws.) forest. Three-dimensional model simulations showed that the average probability of diffuse and direct radiation transmittance
to the forest floor was greater than if a random distribution of foliage had been assumed. Direct and diffuse radiation transmittance
to the forest floor was 28 and 39%, respectively, in the three-dimensional model simulations versus 23 and 31%, respectively,
in the one-dimensional model simulations. The assumption of randomly distributed foliage versus inclusion of clumping factors
in a one-dimensional, multi-layer biosphere-atmosphere gas exchange model (CANVEG) had the greatest effect on simulated annual
net ecosystem exchange (NEE) and soil evaporation. Assuming random distribution, NEE was 41% lower, net photosynthesis 3%
lower, total λE 10% lower, and soil evaporation 40% lower. The same comparisons at LAI 5 showed a similar effect on annual NEE estimates
(37%) and λE (12%), but a much larger effect on net photosynthesis (20%), suggesting that, at low LAI, canopies are mostly sunlit, so
that redistribution of light has little effect on net photosynthesis, whereas the effect on net photosynthesis is much greater
at high LAIs
.
Keywords:
CANVEG, canopy architecture, carbon exchange, Pinus ponderosa, modeling.
