Blue reflectance provides a surrogate for latewood density of high-latitude pine tree rings

In conifers growing at high latitudes and altitudes, the maximum density of tree rings is closely related to climate but is difficult and very expensive to measure. Here we compare maximum density with reflectance results obtained from Pinus sylvestris trees from northern Finland. Red, green, and blue reflectance spectra were obtained using a flatbed scanner and ultraviolet and infrared by scanning monochrome photographs at those wavelengths. A region-growing algorithm used to delimit the latewood of each ring automatically excludes resin ducts and cracks, and the minimum, mean, and maximum reflectance within the region were recorded. Blue minimum reflectance is most closely correlated with maximum density in both raw (r=-0.98) and detrended (r=-0.95) data, and as wavelength increases the correlation declines. The technique is probably most sensitive to lignin content, since the light energy absorbed by lignin declines as wavelength increases. Reliable results are obtained even from very thin (<0.1 mm) latewood rings and the use of a scanner, rather than a micro scope-mounted system, allows viewing magnification to be changed without altering reflectance values. The simple chemical pretreatment routinely applied to X-ray samples is sufficient to remove the color difference between heartwood and sap wood in the blue band.