SIMULATED INFLUENCE OF LEAF GEOMETRY ON SUNLIGHT INTERCEPTION AND PHOTOSYNTHESIS IN CONIFER NEEDLES

The light interception capabilities of individual conifer needles are governed by their cross-sectional geometry and their orientation to su nlight. Leaf cross sections typical of conifer tree species were model ed to quantify the interception of direct sunlight over a range of inc ident light angles. The needle shapes exhibited by Abies nordmanniana Spach, Picea asperata Master, Pinus cembra L., P. monophylla Torr. & F rem., and P. sylvestris L. were selected because they are representati ve of the range of geometric shapes found in conifer tree species. Cal culated light interception values were compared to corresponding predi ctions for a laminar broadleaf. Estimates of carbon gain were derived from computed incident light integrated over the leaf cross section an d a representative curve of conifer photosynthetic response to light. Flat leaf cross sections (e.g., Abies nordmanniana) with high surface area to volume ratios (> 6) intercepted more light per unit area at hi gh angles of incidence than thick leaves. Thick leaves (e.g., Pinus ce mbra) intercepted more light at low angles of incidence than at high a ngles of incidence. Needles of Pinus monophylla had no angular depende nce for light interception because of their circular cross section. La rge differences in estimated CO2 assimilation occurred among the speci es, especially when CO2 uptake was expressed on a unit volume basis. A maximum uptake of 67.9 mmol CO2 m-3s-1 was predicted for A. nordmanni ana compared to a minimum of 39.7 mmol m-3s-1 for P. monophylla. A gre ater angular dependence occurred for estimates of CO2 uptake than for estimates of light interception.