Effects of stocking and thinning on wind damage in plantations

Examination of relations between stocking or thinning and the incidence of wind damage by uprooting or the bending and breakage of stems showed that stocking and thinning strategies can reduce the risk of damage in plantations of Pinus radiata D. Don. Except on sites where root growth was very restricted, plantations raised at lower stocking generally experienced markedly less wind damage. The risk was increased, however, immediately after a thinning, especially if the retained trees were tall and slender, and the thinning had been heavy and had removed dominants. These influences were explained by the long-term effect of stocking on tree development and by the immediate effect of thinning in opening the canopy to the wind. Reduced stocking greatly increased stem diameter and crown and root growth, but had little or no influence on the height of the dominant trees, except at extremely high or extremely low stockings. The height/diameter (H/D) ratio was thus greatly reduced at lower stocking. The H/D ratio, calculated from the mean height and diameter at breast height (1.3 m) over bark (d.b.h.o.b.) of the 200 largest diameter trees/ha, was identified as a valuable index of the risk of wind damage, at least with respect to stem failures. It was found that H/D values are influenced by growth at various stockings and after various thinning regimes. The most important conclusion reached is that trees should be allowed as much growing space from as early in their life as can be reconciled with other silvicultural and economic constraints