Application of narrow-band digital camera imagery to plantation canopy condition assessment

Abstract

Ensuring forest plantations remain in optimum health and condition is critical to minimizing adverse losses in productivity. A health monitoring program capable of accurately assessing the extent and severity of symptoms of canopy strain could permit forest managers to take a proactive course of action to minimize losses in productivity and tree mortality. Across a range of factors associated with tree stress and defoliation (a fungal pathogen Sphaeropsis sapinea, low soil nitrogen (N) availability, and an aphid Essigella californica), we compared field-based observations of canopy condition with coincident imagery obtained in September 2002 and 2003 from digital camera technology fitted with selected narrow-band (10 nm) spectral interference filters. From these wavelengths a number of chlorophyll and red-edge spectral indices were derived at 50 cm spatial resolution. In the case of S. sapinea where infection is significant and results in necrotic breakdown of needle tissue, the slope of the upper red-edge was the variable most highly correlated with crown attributes (r2 = 0.76 and 0.88 for the 2 years), with an independent classification accuracy of over 90%. Feeding by E. californica is commonly associated with needle chlorosis and defoliation and was predicted at a lower level of accuracy with a simple chlorophyll index (67% overall accuracy). The results indicate that narrow-band digital camera imagery can be used to derive indices of chlorophyll sensitivity and red-edge wavelengths. Comparison of predictions over a 2 year period indicate that the red-edge-based indicators can detect differences in canopy condition and that these relationships appear robust. The results indicate the chlorophyll-based indices were less robust through time, possibly due to interactions with needle defoliation.