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 measureme nts were made in small blocks of Sitka spruce respaced 15 years previo usly to different stand densities. The wind tunnel measurements were m ade with dynamically correct 20-cm tall plastic trees. The maximum ben ding moment on trees showed a linear increase with the ratio of spacin g to height. Estimates suggest that at the wider spacing tree diameter has increased sufficiently so that trees are less likely to break. Ho wever, the increase in resistance to overturning is not as rapid as th e increase in wind loading and wider spaced trees will be more vulnera ble 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 fore st. The mechanical and dynamic characteristics of trees at different s pacing are presented. These show that with increasing spacing the Youn g'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 o f stands and the implications for forest managers are discussed.