Shadow brightness and shadow fraction relations with effective leaf area index: importance of canopy closure and view angle in mixedwood boreal forest

Abstract

This research examines the impacts of varying canopy closure and view angle on relations of high-resolution digital camera shadow fraction and shadow brightness with mixedwood boreal forest effective leaf area index (LAIe). Results from linear regression analyses revealed weak and insignificant positive relations between shadow fraction and LAIe. Considerable scatter was observed in the relationship and was attributed to differences in canopy closure among plots, as only forest with greater than 80% closure showed a positive relationship between shadow fraction and LAIe. These results emphasized that gap size and frequency were important factors in determining the shadow fraction. Relations between shadow brightness and LAIe were significantly improved over those with the shadow fraction because of decreased data point scatter when closure was less than 80%. Shadow brightness was not adversely affected by canopy closure, while remaining sensitive to species composition. The association of shadow fraction with LAIe was understood to exist through links to the projected shadow area of tree crowns; however, this relation became unstable in more open canopies. With shadow brightness, surrogate information on LAIe was implicitly linked to differences in the transmission of light through deciduous and coniferous tree crowns. An evaluation of view angle geometry effects suggested that bidirectional reflectance impacts on the shadow fraction ‐ LAIe relations were strongest in the forward-scattering direction, but had less effect on regression analysis in the backscattering direction and at nadir.