Evaluation of structure description requirements for predicting gap fraction of vegetation canopies

Abstract

Gap fraction in homogeneous crop canopies is usually computed using the Poisson law. For heterogeneous canopies, gap fraction may be computed by assuming the canopy to be made of contiguous cells, in which the Poisson law is used. However, the implicit hypothesis of random phyto-element position is frequently violated, because of plant structure and canopy planting pattern. This may empirically be taken into account by introducing an additional dispersion parameter. However, we are unable, from a deterministic approach, to predict such a parameter. Most experimental works to study the relations between gap fraction and canopy structure suffer from the limited accuracy of the measurements. A laboratory experiment was therefore conducted on artificial canopies, which enabled an accurate description of both structure and gap fraction. The usual turbid medium approach, with random leaf dispersion, but an accurate description of spatial variations of canopy structure, enabled us to predict the directional gap fraction of the artificial canopies with a maximum error of about 0.06. The accuracy of simulations remained fairly good when using simplified descriptions of structure, provided that they took into account most of the spatial variations of leaf area index. The residual error of 0.06 indicates however a significant departure from the Poisson law. Analysis of spatial and directional dependence of gap fraction measurements showed that fitting simulated and experimental data required the introduction of a dispersion parameter incorporating spatial, zenith, and azimuth variations.