Abstract
Satellite remote sensing is an important method for forest phenological studies at continental or global scales. Sentinel-1 (S1), a polar orbit satellite with a spatial resolution of 10 m, provides an opportunity to observe high-resolution forest phenology. The sensitivities of S1 C-band backscatter measurements to vegetation phenology, such as crops, meadows, and mixed forests, have been discussed, whereas their performance for different forest types has not yet been quantitatively assessed. It is necessary to evaluate accuracy before adapting S1 datasets in forest phenological studies. This study discusses the seasonal variations in S1 backscatter measurements and assesses the accuracy of S1-based forest phenological metrics in two types of typical forests: deciduous and coniferous. S1 C-band SAR dual-polarization backscatter measurements for the period 2017–2019 were used to extract forest phenology metrics using the Fourier transform (FT) and double logistic (DL) functions. Phenological metrics from the ground-based PhenoCam dataset were used for evaluation. The S1 backscatter VV-VH signal peaks for deciduous and coniferous forests occur in the winter and summer, respectively. The S1 backscatter could reasonably characterize the start of season (SOS) of deciduous forests, with R² values up to 0.8, whereas the R² values for coniferous forest SOS were less than 0.30. Moreover, the retrieved end of season (EOS) was less accurate than the SOS. The differences in accuracy of S1 backscatter phenological metrics between deciduous and coniferous forests can be explained by the differences in seasonal changes in their corresponding canopy structures. To conclude, S1 C-band backscatter has a reasonable performance when monitoring the SOS of deciduous broadleaf forests (R² = 0.8) and relatively poor performance when extracting EOS of deciduous broadleaf forests (R² = 0.25) or phenology of evergreen needleleaf forests (R² = 0.2).
Highlights
Forests are important ecosystems that produce natural biological resources for humans [1,2]
The end of season (EOS) of broadleaf temperate deciduous forests of the eastern USA was widely delayed by 0.8 days per year from 1988 to 2008 [6]; an advanced trend was found at the start of season (SOS) of North American species [7], both clearly indicating the impact of global warming
VV polarization displayed no obvious seasonal variation, while the VH polarization showed a decrease in spring
Summary
Forests are important ecosystems that produce natural biological resources for humans [1,2]. Forest phenology is a key variable in conducting scientific management of forestry and has become important in climate change studies [3,4]. Forest phenology refers to the sensitive reactions of forest vegetation biological events to seasonal variations in environmental variables such as temperature and humidity [5]. These reactions make forests the most sensitive indicator of environmental conditions [3,4]. Accurate and detailed forest phenological measurements are essential for forestry management and for climate change studies. It is necessary to efficiently and economically monitor forest phenology
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