A comparison of field-based and modelled reflectance spectra from damaged Pinus radiata foliage

Abstract

Accurate and cost-effective monitoring of the health and condition of Australian Pinus radiata D.Don plantations is crucial to predicting the impact of damaging agents on wood yield and, where appropriate, targeting timely intervention. Stressful agents can induce changes in the biochemical, physiological and structural integrity of pine needles and subsequently reduce tree growth and ultimately cause plant death. Three important stressful agents occurring within Australian P. radiata plantations are the aphid Essigella californica, soil nitrogen deficiency and Sphaeropsis sapinea, a fungal pathogen. Within a study site in southern New South Wales, needles were sampled from crowns exhibiting key symptoms at three levels of crown severity. Needle level spectra were measured with a field spectroradiometer and foliage samples taken to extract needle chlorophyll a and b and to determine needle moisture content. A radiative transfer model (LIBERTY) was also used to estimate theoretical needle reflectance, given changes in two of its five input parameters (needle chlorophyll and moisture content). Two specific questions were posed. First, given that most spectral indices are based on a reference or stable wavelength as well as sensitive wavelengths, what is the most effective suite of stable wavelengths for predicting of needle chlorophyll and moisture? Second, which published spectral indices best discriminated the three categories of crown-damage severity for each damaging agent? Analysis of needle samples indicated that the needles affected by E. californica were the least chlorotic compared with the other damaging agents. For all damaging agents, needles showed an increase in reflectance with a lowering of chlorophyll content in the visible region (400–700 nm), associated with increasing severity. Changes in the shape of the spectral curve in the red-edge region of the electromagnetic spectrum were minor for E. californica-affected and nitrogen-deficient needles; however, changes were significant when comparing the S. sapinea severity classes. Correlations with published vegetation indices indicated that needle chlorophyll content was most highly correlated with a number of the recently proposed indices, including the structurally insensitive simple ratio. In general, the best results were obtained with 705 nm as the chlorophyll sensitive wavelength and either 750 or 445 nm as the insensitive wavelengths to account for needle reflectance and surface properties. By varying two of the input parameters of the LIBERTY model, the estimated spectra generally matched the trends and magnitude of actual spectra. This suggests that the application of radiative transfer models, correctly parameterised, can provide important information when estimating discrimination categories of needle damage.