Modelling Bark Thickness in the tree in relation to silviculturaltreatment

Based upon a sampling of 186 trees of Norway spruce, Sitka spruce and Douglas fir from Belgium (24 Norway spruce), Denmark (24 Norway spruce and 24 Sitka spruce), France (24 Norway spruce), Germany (24 Douglas fir), Great Britain (24 Sitka spruce) and Italy (24 Douglas fir) specially cut for the Project, this study which is a part (Task 6) of a larger ECC Project, has shown the effects of the tree, its Social position in the stand, the thinning intensity and the site productivity on bark thickness and bark volume.

All along the stem, bark thickness has a particular profile characterized by a steep decrease from the bottom up to a given height variable from species to species, due to a bottom effect, then being relatively constant up to the living crown base level and finally decreasing slightly in the living crown part. The most important bottom effect has been observed on Douglas fir.

Referring to a variance components analysis, variability in bark thickness is mainly due to the tree itself and the stand factor which globalized the cumulated effect of thinning intensity and site productivity. However, it appears that the trees selected by couple for each Social position in each stand are similar whatever the species could be, and may be considered, in fact, as true replicates.

For each species, the Social position of the tree in the stand influences bark thickness in the same way. In all cases, dominant trees have indeed the thickest bark, whilst suppressed trees are always characterized by the thinnest bark and co-dominant trees range to this respect in an intermediate position.

The effect of site productivity is not clear through this sampling, due to interferences of different factors, mainly the differences in tree age which let appear a significant effect of the country (differences in growing conditions and between productivity classes). Nevertheless, when tree age is almost the same in different countries, the general trends are going in the direction of a decrease of bark thickness when the site productivity becomes lower.

The same general trends are observed in connection with the thinning intensity because bark thickness generally decreases when thinnings are less intense.

In fact, bark thickness is far to be constant according to the stand selection criteria (site productivity, thinning intensity) but is noticeably affected by the tree selection criteria (Social position).

Bark thickness, bark volume and amount of bark are mainly depending on the growing conditions with a major effect of the Social position compared to the thinning intensity and the site productivity.

Unfortunately, a general model was unsuitable for an accurate prediction of bark thickness. The best predicted values of bark thickness was obtained in using common models, linear ones for Norway spruce and Sitka spruce but non linear ones for Douglas fir.