Comparison of Gap Fraction and Leaf Area Index Measurements Under Clear and Cloudy Sky Conditions in Alaskan Spruce Forests

Abstract

At northern high latitudes, warming trends have been accelerating, and it is important to understand how the terrestrial ecosystems in these regions respond to such climate change. Satellite-based monitoring of vegetation parameters such as the leaf area index (LAI) provides the diagnostic characteristics for terrestrial vegetation dynamics. Thus, an effort to assure data quality through a comparison with ground-based datasets is crucial. The objective of this study is to evaluate LAI from gap fraction measurements under clear and cloudy sky conditions. We performed gap fraction measurements using plant canopy analyzers at four spruce forest sites in interior Alaska, USA, in September to October 2011 and August 2016. The measured gap fraction was then used to compute the LAI. After correcting the scattering radiation effect on the gap fraction, we obtained an LAI (L_m) of 1.00 to 1.75. When the woody area and shoot level clumping effects were taken into account, the green LAI was estimated to range from 1.18 to 2.33. The LAIs estimated after the scattering correction were closer to the LAIs obtained in cloudy sky conditions, suggesting that the LAI obtained in clear sky conditions can be considered to have the same accuracy as that obtained in cloudy sky conditions.