Phenological Monitoring of a UK Woodland: Results from Digital Cameras, Spectroradiometers and Satellite Data
Rowland, Clare1; George, Charles2; Gerard, France2
1CEH Lancaster, UNITED KINGDOM; 2CEH Wallingford, UNITED KINGDOM
Plant phenology is a key indicator of environmental change. Observations of plant phenology are made by citizen scientists, coarse resolution satellites and increasingly electronic devices mounted above the canopy, such as digital cameras or spectroradiometers. This paper compares the phenological information content of above-canopy spectroradiometer data, and both above and below-canopy digital photos, with MERIS Terrestrial Chlorophyll Index (MTCI).
The study area is Wytham woods in Oxfordshire, which is part of the UKs Environmental Change Network (ECN). The woodland at Wytham is a mix of ancient, secondary and plantation woodland. The ECN sites follow a range of compulsory and voluntary measurement protocols, including a voluntary weekly vegetation photo. At Wytham vegetation photos have been taken weekly (with occasional missing dates) since 25/02/1998. This data set provides a much longer time-series than the more recent data sets acquired with digital cameras or webcams and so provides a potentially valuable data source. The Wytham weekly photos were not intended to become a quantitative data source - they were originally expected to be used for visual interpretation only. The above canopy daily digital photos are provided by Mobotix m24m 'security' camera, mounted on a flux tower giving a horizontal view approximately 6m above the tree canopy for 2011-12. The spectro-radiometers are Cropscan MSR16R 16 channel instruments with wavelengths ranging from 470 - 1640nm. They are deployed above an oak (Quercus robur) and a sycamore (Acer pseudoplatanus). In the paper we calculate the excess greenness index for the digital photos, as this technique has been successfully applied to above canopy digital camera data sets from the US.
The paper presents results from the weekly photos for 1998-2012, and discusses data quality issues, which affect the images up to 2008. The analysis then focuses on comparing the above/below-canopy photos from 2009-2012 with the above-canopy spectroradiometer data, MTCI data and climate data. The paper concludes with an assessment of the strengths and weaknesses of the three measurement systems (digital camera, spectroradiometer, satellite data) and discusses how the synthesis of the three increase our understanding of vegetation phenology.