A technique to identify annual growth rings in Eucalyptus grandis using annual measurements of diameter at breast height and gamma ray densitometry

Abstract

Many eucalypt species do not show distinct growth rings because cambial activity does not show a strong response to seasonal variation in climate. Eucalyptus grandis, one of the most important commercial hardwood species in South Africa, is one such example of a species that does not have well-defined growth rings. The light and dark bands visible on the cross-section of the wood of E. grandis do not always correspond with the growing season, which makes it difficult to resolve wood property data into annual increments. A method was developed to separate growth rings on wood density profiles of E. grandis into annual increments using bark-to-pith density profiles and annual measurements of diameter at breast height (DBH) from permanent sample plot (PSP) datasets. Using the PSP data, it was possible to assess the annual pattern of stem diameter growth at a compartment level by calculating the radial increment (RI) per year and expressing that value as a percentage of the radius at the end of the increment for that year. Mean radial increment percentage (%MRI) was calculated for each year and used to predict annual RI at an individual tree level. Predicted RI values for each tree were expressed as cumulative distances from the bark end and superimposed onto their respective density profiles. Predicted RI corresponded well with latewood density peaks and these separation points were considered a reliable guide to divide the density profile into annual increments closer to the bark end and into broader age classes closer to the pith. By assessing the pattern of variation in radial density within the context of the growth history of a compartment by means of annual PSP data, it was possible to confirm that growth rings on density profiles of E. grandis closer to the bark end can serve as a reliable representation of annual growth.