The present investigation is aimed at studying wood collapse variations along the stem, and examining the possible correlations of the extent of collapse with wood traits.
The study used one plant from the following eucalypt species: Eucalyptus bicostata, E. gunnii, E. occidentalis, E. x trabutii and E. viminalis. From each tree, two adjacent discs were taken every two meters from the bottom up to the utilizable height. One disc was used for qualitative evaluation of collapse, drawn by difference between the shrinkage before and after wood reconditioning. The other disc was used to assess the following traits: basic density, moisture, fibre and lumen length and width, cell wall thickness, extractives in alcohol benzene and 1% NaOH, alpha-cellulose, lignin and hemicelluloses.
The extent of wood collapse varied considerably both among the species and within the tree, decreasing gradually with height in a well defined measure that can be expressed by a second degree polynomial with correlation coefficients ranging between 0.951 for E. bicostata and 0.998 for E. gunnii (with n = 7).
Because the extent of collapse is closely correlated to basic density (r = 0.831), the gradual decrease in collapse from the tree bottom to the top can be explained by the concomitant axial decrease in basic density. The latter is a function, in turn, of wood structural traits and, to a lesser extent, of chemical composition. Statistically significant relationships were recorded with cell wall thickness (r = 0.685) and NaOH extractive content (r = 0.855).
On the basis of the experimental results, it can be concluded that wood collapse is quantitatively appreciable when the cell wall area/lumen area ratio is less than 4.5. The effect of NaOH extractives (high molecular weight polyphenols) is probably due to an occlusion that makes water evaporation harder. Should wood traits be controlled, as is probable, by genetic factors, selection could bring about a decrease in the extent of collapse in eucalpyt wood.
Key words: wood collapse, Eucalyptus.
Correspondence: Giuseppe Ferrari, Experimental Center for Agriculture and Forestry, S.A.F.S.p.A, c p. 9079-00166, Rome, Italy