Abstract
Recognising the importance of the timing of image acquisition on the spectral response in remote sensing of vegetated ecosystems is essential. This study used full wavelength, 350–2500nm, field spectroscopy to establish a spectral library of phenological change for key moorland species, and to investigate suitable temporal windows for monitoring upland peatland systems. Spectral responses over two consecutive growing seasons were recorded at single species plots for key moorland species and species sown to restore eroding peat. This was related to phenological change using narrowband vegetation indices (Red Edge Position, Photochemical Reflectance Index, Plant Senescence Reflection Index and Cellulose Absorption Index); that capture green-up and senescence related changes in absorption features in the visible to near infrared and the shortwave infrared. The selection of indices was confirmed by identifying the regions of maximum variation in the captured reflectance across the full spectrum. The indices show change in the degree of variation between species occurring from April to September, measured for plant functional types. A discriminant function analysis between indices and plant functional types determines how well each index was able to differentiate between the plant functional groups for each month. It identifies April and July as the two months where the species are most separable. What is presented here is not one single recommendation for the optimal temporal window for operational monitoring, but a fuller understanding of how the spectral response changes with the phenological cycle, including recommendations for what indices are important throughout the year.
Highlights
To understand the optimal timing for image acquisition of hyperspectral images of vegetation, detailed information is required about the spectral characteristics of natural species over the growing season
The analysis evaluates established narrow-waveband indices in identifying phenological change and the point in the season when plant functional groups are spectrally most distinguishable
The selection of indices used in this analysis was determined by research in the literature for changes in spectral response believed to relate to phenological change (Table 2)
Summary
To understand the optimal timing for image acquisition of hyperspectral images of vegetation, detailed information is required about the spectral characteristics of natural species over the growing season. B. Cole et al / ISPRS Journal of Photogrammetry and Remote Sensing 90 (2014) 49–58 been a number of recent studies using the phenological differences among plant species to map invasive species, in rainforests (Somers and Asner, 2012, 2013), temperate forests (Burkholder et al, 2011) and riparian areas (Fernandes et al, 2013). Cole et al / ISPRS Journal of Photogrammetry and Remote Sensing 90 (2014) 49–58 been a number of recent studies using the phenological differences among plant species to map invasive species, in rainforests (Somers and Asner, 2012, 2013), temperate forests (Burkholder et al, 2011) and riparian areas (Fernandes et al, 2013) This application requires identification of vegetation at a species level and so optimizing the spectral separability of the species through intensive monitoring of species dynamics is essential to determine the time to discriminate between species. Recommendations for the optimal time for operational monitoring are discussed
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