Shelterwood harvesting in root-disease infected stands — post-harvest soil disturbance and compaction

Abstract

Soil disturbance and compaction are described for an integrated project, testing the use of shelterwood harvesting and root removal treatments. Sites were winter harvested and assessed the following summer using criteria from the Forest Practices Code (FPC) of British Columbia. At two installations, pushover harvesting caused a significantly greater level of counted soil disturbance and forest floor displacement than hand felling combined with ground skidding. Disturbance levels increased with basal area removal on pushover harvested treatment units. The highest disturbance was on the clearcut–pushover treatment at the Golden site, where levels averaged 36% for both variables. Soil disturbance on the Nakusp site was lower for a number of reasons. Pushover harvesting at the Golden site also resulted in over 25% of the sample points becoming calcareous at the surface, due to exposure and deposit of calcareous subsoil; as opposed to less than 5% on the hand felled treatment units. Pushover harvesting at Golden, compared to the Nakusp site, also yielded a greater proportion of the treatment units occupied by stump holes, because of larger tree size and finer soil textures. Soil compaction significantly increased total bulk density at Golden and decreased total porosity and two measures of aeration porosity at both sites. Machine disturbance not meeting the disturbance survey criteria of the FPC occupied up to 16% of treatment units at the Golden site. This uncounted disturbance was not significantly different from countable compaction for five physical properties. Compaction impacts were usually significant, irrespective of the harvesting method or basal area retention on a treatment unit. Aeration porosities for some compacted samples were at or below ‘critical’ values of 15% on both sites. Tree growth at Golden is expected to be reduced because compaction has reduced aeration porosity and push felling exposed greater levels of free lime. Site sensitivity guidelines for pushover harvesting are exceeded for the soil conditions at Golden. More recent harvesting on other sites has resulted in less disturbance than reported here. It appears that countable soil disturbance can be limited to 15% for pushover harvesting under operational conditions.