Abstract: Broadband vegetation indices (BVIs) derived from routine radiation measurements on eddy flux towers have the advantage of high temporal resolutions, and thus have the potential to obtain detailed information of dynamics in canopy leaf area index (LAI). Taking the temperate broadleaved deciduous forest around the Maoershan flux tower in Northeast China as a case, we investigated the controlling factors and smoothing method of four BVI timeseries, i.e., broadband normalized difference vegetation index (NDVI_B), broadband enhanced vegetation index (EVI_B), the ratio of the near-infrared radiation reflectance to photosynthetically active radiation reflectance (SR_NP), and the ratio of the shortwave radiation reflectance to photosynthetically active radiation reflectance (SR_SP). We compared the seasonal courses of the BVIs with the LAI based on litterfall collection method. The values for each BVI were slightly different among the three calculation methods by Huemmrich, Wilson, and Jenkins, but  showed similar seasonal patterns. The diurnal variations in BVIs were mainly influenced by the solar elevation and the angle between the solar elevation and slope, but the BVIs were relatively stable around 12:30. The noise of daily BVI timeseries could be effectively smoothed by a threshold of clearness index (K). The seasonal courses of BVIs for each time of day around the noon had similar patterns, but their thresholds of K and the percentages of remaining data were different. Therefore, the daily values of BVIs might be optimized based on the smoothing and the proportion of remaining data. The NDVI_B was closely correlated linearly with the LAI derived from the litterfall collection method, while the EVI_B, SR_NP, and SR_SP had a logarithmic relationship with the LAI. The NDVI_B had the advantage in tracking the seasonal dyna-mics in LAI and extrapolating LAI to a broader scale. Given that most eddy flux towers had equipped with energy balance measurements, a network of monitoring canopy LAI could be readily achieved if the reflectance of photosynthetically active radiation was measured synchronously.

Key words: broadband vegetation index, clearness index, leaf area index,