The effects of spacing on root anchorage and tree stability.

摘要

The risk of windthrow of forest trees is one the most important limitations to the application of Continuous Cover Forestry (CCF) systems to British forests. It is therefore important to examine the stability of trees grown at the wide spacings often used for CCF so that decisions may be made as to where CCF systems can be applied. Thirty-four-year-old Sitka spruce trees grown on an exposed (DAMS 18) upland site with a shallow mineral soil, and respaced twelve years after planting to 500, 1,000 and 2000 stems ha-1, were mechanically overturned to compare their root anchorage. Their anchorage characteristics and dimensions were then input into the ForestGALES wind risk model to provide predictions of the critical wind speeds required to overturn trees at each stand density. The relationship between root anchorage and stem weight was not changed by stand density. The critical wind speed required for stem breakage increased with spacing, but the wind speed required for uprooting decreased, due to increasing exposure and crown size. The return-time for wind speeds that would damage this stand were predicted to be 14 years, 72 years or >200 years if it had 500, 1,000 or 2,000 stems ha-1 respectively. Scenarios were modelled in which the same trees were located on sites with a range of wind exposures (DAMS 14 to 20). Results of this exercise indicated that, while on exposed sites (DAMS 18 and above) these trees would be particularly vulnerable to wind damage at 1,000 stems ha-1 or less, on less exposed sites (DAMS 14 or less) the risk of wind damage would not be increased appreciably. However, the trees in this study had been pre-commercially thinned, and we would expect a greater increase in risk to trees of this age if recently respaced to the wider spacings. The results highlight the need for an assessment of wind risk before deciding which stands are appropriate for conversion to CCF.