The comparison of different methods to measure leaf area index of forests in Maoershan Mountain, Northeastern China

Different optical instruments are currently available for measuring LAI such as LAI 2000 Plant Canopy Analyser (LAI-2000), Tracing Radiation and Architecture of Canopies (TRAC) and Digital Hemispherical Photography (DHP). Their applicability varies in different ecosystems. This study was devoted to compare LAI measured using four different methods (LAI measured by DHP, LAI measured by TRAC, LAI calculated using effective LAI measured by LAI-2000 and clumping index measured by DHP, and LAI calculated using effective LAI measured by LAI-2000 and clumping index measured by TRAC) in the Maoershan experimental forest farm of Northeast Forestry University located in Shangzhi city of Heilongjiang province. Methods used to measure LAI have considerable effects on observed LAI. The means of LAI measured by four different methods are 3.15, 4.73, 3.97, and 4.24 and corresponding standard deviations are 1.54, 2.39, 1.82, and 1.75, respectively. According to previous studies, the combination of LAI-2000 with TRAC can give the most reliable measurements of LAI. Therefore, DHP tends to underestimate LAI at this area, especially for dense canopies while TRAC tends to overestimate slightly LAI for dense canopies. The fitting of LAI measured using four different methods with normalized difference vegetation index (NDVI) and reduced simple ratio (RSR) calculated from TM data acquired on June 24, 2009 indicated that RSR is a better predictor of LAI than NDVI in this study area. The agreements between measured and estimated LAI using the best fitted models are 58%, 70%, 57% and 68% for these four different methods. Corresponding root mean square errors (RMSE) are 0.80, 0.85, 0.88, and 0.75, respectively. The regional means of LAI retrieved using the empirical models established on the basis of RSR and LAI measured with four different methods are 3.47, 5.26, 4.31, and 4.68, respectively, indicating that if DHP is used as a surrogate of TRAC and LAI-2000, LAI might be underestimated by about 25.7% in this area.