Appendix A. Parameterization of the SAIL light model.

To calculate the percentage light transmitted using the SAIL model we let LAI vary from 4 to 9 (the range of variation observed across size classes), but assigned constant values for all other model parameters: latitude was set to 45' N, the leaf angle parameters were calculated by pooling data from all crown positions, the mean leaf optical parameters were calculated as described below, and the proportion of direct radiation was set to the default value of 1 (resulting in a conservative estimate of PAR since direct light is attenuated faster than diffuse light). We estimated the absolute amount of radiation transmitted (photosynthetic photon flux density, or PPFD) by multiplying the percentage transmission of PAR by the estimated above-crown PPFD (93.1 mol·m^-2·d^-1) incident under clear skies at Haliburton at the mid-growing season (15 July) (Frazer et al. 1999).

Measurement of leaf optical properties

Three leaves were harvested at random from the upper crowns of a subset of sampled A. saccharum and B. alleghaniensis trees and placed in a cooler to prevent dessication. Within several hours, we measured the spectral transmittance and reflectance of each leaf (Baltzer and Thomas 2005) using an Ocean Optics Spectrometer with integrating sphere system (J. Grandison and S. Thomas, unpublished data). Measurements of the three leaves were averaged for each tree and linear regression was used to examine if tree diameter was related to total spectral leaf transmission or reflectance within each species. No significant patterns were detected, so the data for all trees were pooled to provide the input for the SAIL model. Sample sizes were 10 trees for A. saccharum and 11 for B. alleghaniensis.

LITERATURE CITED

Baltzer, J. L., and S. C. Thomas. 2005. Leaf optical responses to light and soil nutrient availability in temperate deciduous trees. American Journal of Botany 92:214–223.

Frazer, G. W., C. D. Canham, and K. P. Lertzman. 1999. Gap Light Analyzer (GLA),version 2.0: imaging software to extract canopy structure and gap light transmission indices from true-color fisheye photographs, users manual and program documentation. Simon Fraser University, Burnaby, British Columbia, Canada and the Institute of Ecosystem Studies, Millbrook, New York, USA.