Snow damage in Australian pine plantations

Summary

In June 1981 heavy falls of wet snow with air temperatures near 0°C and strong winds caused extraordinary damage in both pine plantations and native eucalypt forests in much of southeastern Australia, especially at altitudes between 700 m and 1200 m. Using data from a thinning trial, inventory plots, forestry records and extensive inspections of plantations and arboreta, a study was made of this damage to identify types, incidence and mechanisms of damage in Pinus radiata plantations, and their silvicultural implications.

The most widespread but least injurious types of snow damage were the bending and breakage of branches. Uprooting was important in only one plantation. The main damage was the bending or breakage of stems. Some 500 ha of pine plantations have had to be salvaged by clearfelling and a further 1700 ha by heavy thinning, most of them about 15–30 m tall.

Snow damage to P. radiata stems was found to be influenced by: (a) bending moments (due to imbalance of the loaded crowns and lean of the tree), (b) the height/diameter (H/D) ratio of the stems, (c) the shelter enjoyed by the trees below the general canopy, (d) the height/spacing ratio (and hence the chance of one tree falling on another), (e) selective removal of the weaker, less stable individuals of the stand, (f) effects of growing space on diameter growth and hence H/D ratios, and (g) effects of age on H/D ratios and the intrinsic strength of the wood. Comparisons are made with other species.

It is concluded that the H/D ratio provides a valuable indication of the risk of snow damage, and that this risk can be largely controlled by spacing-thinning strategies. In plantations of P. radiata, trees should be allowed as much growing space from an early age as is compatible with other constraints; they should be established at no more than 1000 stems/ha and thinned by removal of the more slender trees before the dominants exceed 20 m in height.