Calluna vulgaris foliar pigments and spectral reflectance modelling

Biochemical and leaf structural state variables influencing the reflectance of Calluna vulgaris, a dwarf shrub, in the 400-950 nm spectral region have been measured over a 2 year period and were found to display considerable intra- and inter-annual variability. For the leaf structural parameters, the annual variability observed may be attributed to systematic seasonal fluctuations following Calluna growth and senescent cycle, and the inter-annual variation may be attributed to its response to each year's prevailing meteorological and solar illumination conditions. Chlorophyll-a and -b (chl-a and -b), total carotenoids and anthocyanins have been identified within the 'shoots and leaves' samples and their change over two growth and senescent cycles quantified. The LIBERTY radiative transfer model was adapted and used to estimate the Calluna chl-a and -b, total carotenoid and anthocyanin content in vivo absorption spectra. The modified LIBERTY model (LIBERTY3P), incorporating the in vivo pigment absorption spectra and parameterized with the biochemical variables measured, and leaf structural parameters determined during this work, has enabled the reflectance of Calluna 'shoots and leaves' to be modelled and compared with reflectances measured from the same samples, in time-series, through the 2005 and 2006 growth cycles. The relative error between modelled and measured reflectances was found to vary from between +/- 5% to +/- 15% dependent on wavelength and the time of year of the sampling interval. Greatest errors between measured and modelled reflectances were noted at the beginning of seasonal growth and lowest during midsummer. Differences between the chl-a and -b in vivo absorption spectrum determined during this work and the chlorophyll-a (chl-a) and chlorophyll-b (chl-b) in vivo absorption spectra, caused by a varying chl-a: chl-b ratios, are a possible cause for the inconsistencies between measured and modelled reflectance spectra.