Selective tree harvesting (removal of selected trees rather than clear-cutting) is undertaken in some areas in an attempt to manage forests for both timber production and species conservation. This Canadian study evaluated the effect of selective harvesting operations on understorey plants of mature deciduous forest in the Haldimand-Norfolk Region of southern Ontario (42°30'N, 80°30'W).
The study was undertaken in two areas with mature forest (generally oak Quercus spp. dominated) scheduled to be selectively cut. The experiment comprised removal of 0, 33 or 66% of the basal area of canopy trees from plots 12.5, 25 or 50 m in diameter. Three replicates of the nine treatments (i.e. 3 cutting intensities x 3 plot diameters) were established at one site and two replicates of the nine treatments at the other.
Trees were cut in winter (November-April) and commercially valuable trunks removed using a mechanical skidder. Unwanted trunks and tree tops were left. Before and after trees were cut, understorey vegetation in the plots was surveyed along two (2-m wide) transects. The presence of newly exposed soil and tree slash was also recorded when spring-flowering forbs were resurveyed in May. Surveys were also repeated later in the growing season (July-August).
The percentage of species lost from plots before and after cutting increased with cutting intensity; the most heavily cut plots lost significantly more species than plots cut at an intermediate level or uncut (average percentage of species lost per plot ranged 3 to 19% for the nine treatments).
There was no statistically significant difference in the percentage of species lost according to plot diameter. Ground surface disturbance increased with cutting intensity and appeared to account for most species loss. Understorey woody plants (damage to above ground parts) were more affected than herbs (underground perennating organs protected below ground during harvesting).
Herbs and woody species present at low density (i.e. 1-8 stems/plot) were more likely to be lost than more frequent species. To reduce losses of uncommon species during harvesting, the authors suggest that it would be more effective to minimise surface disturbance than cutting intensity or the area cut.
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