Field and wind tunnel assessments of the implications of respacing and thinning for tree stability

The change in wind loading on trees due to tree spacing and wind speed was examined in the field and in the wind tunnel. The field measurements were made in small blocks of Sitka spruce respaced 15 years previously to different stand densities. The wind tunnel measurements were made with dynamically correct 20-cm tall plastic trees. The maximum bending moment on trees showed a linear increase with the ratio of spacing to height. Estimates suggest that at the wider spacing tree diameter has increased sufficiently so that trees are less likely to break. However, the increase in resistance to overturning is not as rapid as the increase in wind loading and wider spaced trees will be more vulnerable to overturning. Wind tunnel measurements were also carried out on a range of commercial thinning practices and showed that the critical factor in reducing stand stability is the size of gap made in the forest. The mechanical and dynamic characteristics of trees at different spacing are presented. These show that with increasing spacing the Young's modulus of trees decreases and their damping coefficient increases. This illustrates that wide spaced trees have weaker wood than close spaced trees but they are less reliant on the support of neighbouring trees. The implications of the measurements on the overall stability of stands and the implications for forest managers are discussed.