A note on the G-function for needle leaf canopies

Formulas to calculate the radiation extinction coefficient or ‘G-function’ for plant canopies originally were derived under the assumption of flat leaves. The G-function in that case is obtained as the ratio of the expected value (with respect to the leaf normal distribution function) of the projected leaf area to the one-sided leaf area. Although the corresponding formulas have later been presented also for conifer needles of different shapes they have rarely been used to determine conifer specific G values for parameterization of radiation models for coniferous stands. Instead, these models have typically been based on the erroneous assumption that the G-functions for flat leaves and needles are equivalent if only the one-sided leaf area (used as denominator in G) is replaced by the vertically projected, planimetric needle area. The importance of consistent definition of leaf area index and G for conifers is discussed in this study which presents an overview of previous studies on the G-function of needles and an easily applicable approach to calculate it for needles of different shapes.