Effect of stem bending and soil moisture on the incidence of resin pockets in radiata pine

Background

Mechanical bending stress due to tree sway in strong winds and water stress during drought are thought to contribute to the formation of resin pockets, but it is unclear if these are linked and whether the initiation of resin pockets is influenced by the water status of the trees at the time of stem bending.

Methods

The effect of stem bending on the formation of resin pockets was evaluated under various soil moisture conditions. The stems of 12-year-old radiata pine (Pinus radiata D.Don) trees were bent mechanically in spring or summer when the soil was water deficient, and in summer after rehydration. After the completion of the growth season, a selected sample of trees was felled and stem discs were assessed for the presence of resin pockets, using disc photos and image analysis. All stem bending treatments were compared with control trees.

Results

Stem bending in spring or summer was found to increase the number of Type 1 resin pockets, but had no effect on the number of Type 2 resin pockets. The soil moisture conditions at the time of stem bending had no effect on the number of Type 1 or 2 resin pockets.

Conclusions

The Type 1 resin pockets occurred in the inner part of the early wood, adjacent to the growth ring boundary. This suggests the Type 1 resin pockets were initiated in the mature wood, behind the cambium and zone of differentiation, and were not influenced by the water status of the tree stems at the time of stem bending.