Field spectroscopy and spectral reflectance modelling of Calluna vulgaris
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Date
12//2/25/0Author
MacArthur, Alasdair Archibald
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Abstract
Boreal peatlands store carbon sequestered from the atmosphere over millennia and
the importance of this and the other ecosystem services these areas provide is now
widely recognised. However, a changing climate will affect these environments
and, consequently, the services they provide to the global population. The rate
and direction of environmental change to peatlands is currently unclear and they
have not yet been included in many climate models. This may in part be due
to the ecological heterogeneity and spatial extent of these areas and the sparse
sampling survey methods currently adopted. Hyperspectral remote sensing from
satellite platforms may in future offer an approach to surveying and do so at the
high spectral and spatial resolutions necessary to infer ecological change in these
peatlands. However, work is required to develop methods of analysis to determine
if hyperspectral data can be used to measure the overstorey vegetation of these
areas. This will require an understanding of how annual and inter-annual cyclical
changes affect the peatland plant canopy reflectances that would be recorded by
hyperspectral sensors and how these reflectances can be related to state variable
of interest to climate scientists, ecologists and peatland managers.
There are significant areas of peatland within Scotland and, as it is towards the
southern extreme of the boreal peatlands, these may be an early indicator of
environment change to the wider boreal region. Calluna vulgaris, a hardy dwarf
shrub, is the dominant overstorey species over much of these peatlands and could serve as a proxy for ecological, and consequently, environmental change. However,
little has been done to understand how variations in leaf pigments or canopy
structural parameters influence the spectral reflectance of Calluna through annual
and inter-annual growth and senescence cycles. Nor has much work been done to
develop methods of analysis to enable images acquired by hyperspectral remote
sensing to be utilised to monitor change to these Calluna dominated peatlands
over time.
To advance understanding of the optical properties of Calluna leaves and canopies
and develop methods to analyse hyperspectral images laboratory, field and
modelling studies have been carried out in time series over a number of years. The
leaf and canopy parameters significantly affecting reflectance have been identified
and quantified. Differences between published Chlorophyll(a+b) in vivo absorption
spectra and those determined were found. Carotenoids and Anthocyanins were
also identified and quantified. The absorption spectra of these pigments were
incorporated into a canopy reflectance model and this was coupled to a Calluna
growth model. This combined model enabled the reflectance of Calluna canopies
to be modelled in daily increments through annual and inter-annual growth and
senescence cycles. Reasonable results were achieved in spectral regions where
reflectance changed systematically but only for homogeneous Calluna stands.
However, it was noted during this research that the area of support for the
spectral measurements appeared to differ from that assumed from the specification
provided by the spectroradiometer manufacturers. The directional response
functions (DRFs) of two spectroradiometers were investigated and wavelength, or
wavelength region, specific spatial dependences were noted. The effect that the
DRFs of the spectroradiometers would have on reflectances recorded from Calluna
canopies was investigated through a modelling study. Errors and inaccuracies in
the spectra that would be recorded from these canopies, and commonly used
biochemical indices derived from them, have been quantified.