Abstract
Plant phenology timings, such as spring green-up and autumn senescence, are essential state information characterizing biological responses and terrestrial carbon cycles. Current efforts for the in situ reflectance measurements are not enough to obtain the exact interpretation of how seasonal spectral signature responds to phenological stages in boreal evergreen needleleaf forests. This study shows the first in situ continuous measurements of canopy scale (overstory + understory) and understory spectral reflectance and vegetation index in an open boreal forest in interior Alaska. Two visible and near infrared spectroradiometer systems were installed at the top of the observation tower and the forest understory, and spectral reflectance measurements were performed in 10 min intervals from early spring to late autumn. We found that canopy scale normalized difference vegetation index (NDVI) varied with the solar zenith angle. On the other hand, NDVI of understory plants was less sensitive to the solar zenith angle. Due to the influence of the solar geometry, the annual maximum canopy NDVI observed in the morning satellite overpass time (10–11 am) shifted to the spring direction compared with the standardized NDVI by the fixed solar zenith angle range (60−70°). We also found that the in situ NDVI time-series had a month-long high NDVI plateau in autumn, which was completely out of photosynthetically active periods when compared with eddy covariance net ecosystem exchange measurements. The result suggests that the onset of an autumn high NDVI plateau is likely to be the end of the growing season. In this way, our spectral measurements can serve as baseline information for the development and validation of satellite-based phenology algorithms in the northern high latitudes.
Highlights
The rising temperature in the northern high latitudes is abrupt and greater than the global mean [1,2,3]
We present the in situ spectral measurements in an open black spruce forest
(1) Canopy scale normalized difference vegetation index (NDVI) varied with solar zenith angle, and the slope of NDVI-SZA varied seasonally from positive in fully snow-covered understory to weak positive in the summer
Summary
The rising temperature in the northern high latitudes is abrupt and greater than the global mean [1,2,3]. One of the key questions in the Arctic and world communities is how boreal ecosystems respond to such climate and environmental changes. It has not been answered clearly because of difficulties in untying the complex positive and negative feedbacks [5,6,7,8,9]. NIR, and SWIR are reflectances in red, near infrared, and shortwave infrared spectral regions, respectively These phenology-like variables, such as the start of the growing season (SOS) and the end of the growing season (EOS) may or may not be related to actual plant phenology. The accuracy of SOS depends on the degree of synchronism in snow melt and leaf emergence timings
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