Quantifying the effect of canopy architecture on optical measurements of leaf area index using two gap size analysis methods

In recent years, the methodology in ground-based optical measurements of leaf area index (LAI) of plant canopies has been substantially improved after the introduction of canopy gap size analysis methods. In this paper, the two methods by Chen and Black (1992) and Chen and Cihlar (in press) are compared for four boreal conifer stands located near Prince Albert, Saskachewan, and Thompson, Manitoba, Canada. The data used in the analysis were obtained from a new sunfleck-LAI instrument, the TRAC (Tracing Radiation and Architecture of Canopies), which measures the photosynthetic photon flux density along transects beneath the overstory at a rate of 100 samples per meter. It is confirmed in this study that the needle shoots of conifer trees can be treated as the basic foliage units (elements) for radiation interception considerations. The effect of foliage clumping at scales larger than the shoots is quantified using an element clumping index. This is necessary for indirect measurements of LAI based on the gap fraction principle using optical instruments such as the LI-COR LAI-2000. The values of element clumping index derived from these two methods agree within 17% for all stands investigated. However, the values obtained using Chen and Black´s method are consistently smaller than those calculated using Chen and Cihlar´s method. The difference results from a negative bias introduced in the method of Chen and Black which requires the assumption for a random spatial distribution of foliage clumps (tree crowns). The method of Chen and Cihlar makes no assumption of foliage distribution patterns and is therefore more reliable. Yet, Chen and Black´s method allows the derivation of several canopy architectural parameters which are useful for modeling radiative regimes in forest canopies. It is concluded that for remote sensing and other studies, a large quantity of ground truth LAI data can be acquired quickly and accurately using a combination of indirect optical measurements by the LAI-2000 for the foliage angular distribution and the TRAC for the foliage spatial distribution