© 2001 Heron Publishing—Victoria, Canada
Importance of needle age and shoot structure on canopy net photosynthesis of balsam fir (Abies balsamea): a spatially inexplicit modeling analysis
Pierre Y. Bernier (1), Frédéric Raulier (1), Pauline Stenberg (2) and Chhun-Huor Ung (1)
1. Natural Resources Canada, Canadian Forest Service, Laurentian Forestry Centre, 1055 du P.E.P.S., P.O. Box 3800, Sainte-Foy,
Québec, G1V 4C7, Canada / 2. Department of Forest Ecology, P.O. Box 24, FI-00014 University of Helsinki, Finland / Received August 18, 2000
Summary
We have developed a spatially inexplicit model of canopy photosynthesis for balsam fir (Abies balsamea (L.) Mill.) that accounts for key processes of light–shoot interaction including irradiance interception by the shoot, spatial
aggregation of shoots into branches and crowns, the differential propagation of diffuse and direct light within the canopy,
and an ideal representation of penumbra. Also accounted for in the model are the effects of the average radiative climate
and shoot age on needle retention, light interception, and photosynthetic capacity. We used reduced versions of this model
to quantify the effects of simplifying canopy representation on modeled canopy net photosynthesis. Simplifications explored
were the omission of direct beam transformation into penumbral light and the use of different constant shoot properties throughout
the canopy. The model was parameterized for a relatively dense balsam fir stand (leaf area index of 5.8) north of Québec City,
Canada, and run using hourly meteorological data obtained at the site. The overall performance of the complete model was satisfactory,
with maximum values of canopy net photosynthesis of 23 μmol (m2 ground)–1 s–1 (83 mmol m–2 h–1), and a near-saturation of the canopy at a photosynthetically active radiation photon flux density of about 750 μmol m–2 s–1 (2.7 mol m–2 h–1). The omission of penumbral effects through the use of unattenuated direct (beam) radiation at all layers of the canopy,
as used for broad-leaved species, reduced canopy net photosynthesis by 3.7%. Analysis of the results show that the small impact
of penumbra on canopy net photosynthesis stems from the high proportion of diffuse radiation (73%) estimated from our meteorological
data set; single-hour results under clear sky conditions approach theoretical bias values of about 30%. Use of mean shoot
photosynthetic, light capture and light transmission properties throughout the canopy biased canopy net photosynthesis by
less than 3%. However, simulations carried out based on properties of 1-year-old shoots throughout the canopy overestimated
canopy net photosynthesis by 9%. Use of the shoot as our smallest functional unit was a potential source of bias because the
differential absorption of direct and diffuse radiation within the shoot could not be factored into the model. Other sources
of potential bias are discussed.
Keywords:
diffuse radiation, direct beam radiation, leaf mass per unit area, penumbra, STAR.
