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Are irregular stands more windfirm?
1 Forest Research, Northern Research Station, Roslin, Midlothian EH25 9SY, Scotland
The proposed transformation of substantial areas of even-aged plantation forests in Britain to irregular structures (‘continuous cover forestry’) has raised concerns about the likely wind stability of irregular stands. A review of the literature suggests that the major difference between irregular and regular stands is the lower (i.e. more stable) height : diameter ratio associated with the dominant trees in the former. This appears to be a consequence of the greater wind loading that these dominant trees have to withstand. Wind tunnel studies show no difference in wind profile within or above the two types of canopy. There have been few comparative root investigations in the two types of stand and no differences in rooting depth have been reported, although changes in root architecture could be anticipated as a result of greater wind loading. The implications of these findings upon windthrow risk in regular and irregular Sitka spruce stands has been explored using the Forest GALES wind risk model on sites of different wind exposure. The results suggest no difference in wind risk on sheltered sites. On sites of moderate exposure, an irregular stand at close to ‘steady state’ conditions could be more wind stable than a conventionally thinned regular stand. However, this advantage disappears with increasing exposure. The conclusion is that the promotion of irregular stands may provide structures with more stable characteristics, but these cannot be considered in isolation from the prevailing wind climate and the local site type.
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