Impact of understorey on overstorey leaf area index estimation from optical remote sensing in five forest types in northeastern China

The presence of understorey vegetation is a recognised problem that limits the accuracy of the satellite-estimated forest leaf area index (LAI). To detect the influence of the understorey, the relationships between the effective overstorey LAI (L-eo), gap fraction, effective understorey LAI (L-eu) and remote sensing were explored in five forest types: a mixed broadleaved-Korean pine (Pinus koraiensis) forest (BKPF), a spruce-fir valley forest (SVF), a secondary birch (Betula platyphylla) forest (SBF), a Korean pine plantation (KPP) and a Dahurian larch (Larix gmelinii) plantation (DLP) in northeastern China. L-eo and L-eu were obtained from hemispherical photographs of temporary sample plots. By examining these relationships, we determined that the remote sensing spectral signal was influenced by both the understorey and the gap fraction. In particular when L-eo was smaller than 2.5, the understorey substantially influenced the remotely sensed LAI estimates. The relationships between Leo and remote sensing information and between L-eo and L-eu were both strengthened by considering the effect of gap fraction, although the latter relationship was still poor compared to that of the former. To further explore the relationships between effective and "true" LAI in different forests, "true" overstorey LAI values based on litter collection and hemispherical photographs in permanent sample plots and "true" understorey LAI values through harvesting in temporary sample plots were also determined. The effective LAI of both the overstorey and understorey exhibited good agreement with the "true" LAI. L-eo underestimated L-to by 53.7%, 35.1%, 14.6%, 43.9% and 36.0% for BKPF, SVF, SBF, KPP and DLP, respectively. Both L-eo and L-eu varied less than L-to in different forest types. Our study demonstrates that the remote sensing of biophysical properties of the canopy layer in the forested part of this region is hindered by the dominant role of the understorey with the gap fraction in the spectral signal, and different formats of LAIs have different responses to the degree of disturbance in the forests. (C) 2014 Elsevier B.V. All rights reserved.