Simulating Canopy Gap Fraction of Complex 3d Forest Scenes of Picea Sitchensis (sitka Spruce)

We present an application of 3D canopy modelling for assessing the structural accuracy of complex 3D structural models of Sitka spruce. Such models can be used to simulate canopy reflectance at the landscape-scale with a minimum number of assumptions. Typically, however, the underlying canopy structural models require a large number of parameters, many of which are hard to measure and so must be estimated. How then can the resulting canopy structure be validated? We compare estimates of canopy gap fraction of modelled Sitka spruce from simulating hemispherical canopy reflectance from within the canopy, with those derived from upward-looking hemispherical photography from Harwood Forest, Northumberland, UK. Results show that gap fraction estimated from the 3D model canopies compares well with estimates derived from hemispherical photography. Where estimates disagree, such comparisons can be used to inform improvement of the structural models. We also use the detailed structural models to explore the sensitivity of hemispherical photography methods to error in estimating gap fraction in the field.