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 andwind speed was examined in the field and in the wind tunnel.The field measurements were made in small blocks of Sitka sprucerespaced 15 years previously to different stand densities. Thewind tunnel measurements were made with dynamically correct20-cm tall plastic trees. The maximum bending moment on treesshowed a linear increase with the ratio of spacing to height.Estimates suggest that at the wider spacing tree diameter hasincreased sufficiently so that trees are less likely to break.However, the increase in resistance to overturning is not asrapid as the increase in wind loading and wider spaced treeswill be more vulnerable to overturning. Wind tunnel measurementswere also carried out on a range of commercial thinning practicesand showed that the critical factor in reducing stand stabilityis the size of gap made in the forest. The mechanical and dynamiccharacteristics of trees at different spacing are presented.These show that with increasing spacing the Young's modulusof trees decreases and their damping coefficient increases.This illustrates that wide spaced trees have weaker wood thanclose spaced trees but they are less reliant on the supportof neighbouring trees. The implications of the measurementson the overall stability of stands and the implications forforest managers are discussed.