Natural regeneration of Sitka spruce (Picea sitchensis (Bong.) Carr.) under partial canopy
This study set out to explore the relationships between overstorey canopy and regeneration with a view to recommending silvicultural prescriptions for transforming even-aged Sitka spruce stands to uneven-aged (irregular) stands and managing such stands. An experiment was established in Kingdrogan Forest, Perthshire in which six small (20 m diameter) circular gaps were created in a stand of 32 year olds Sitka spruce. Eight plots were established along a north south transect through the gap and which extended under the canopy on either side of the gap. Four plots were in the gap, two under the canopy north of the gap and two under the canopy south of the gap. Each plot contained three sub-plots which contained the following treatments: planting of Sitka spruce nursery grown transplants, artificial seeding and control. Germination, survival and growth of seedlings were monitored over subsequent growing seasons. Environmental monitoring was carried out for two years: namely canopy openness, temperature and soil moisture. Germination rates were fond to be highest in the light shade in the southern half of the gap although growth was highest in the sunny northern half of the gap. Almost no seedlings survived under the canopy. A complementary survey of older advance regeneration was carried out on two sites, Comlongan Forest in southwest Scotland and Tarenig in mid Wales. The survey looked at canopy density, characterised by canopy openness and basal area, and density and growth of natural regeneration. Canopy openness and basal area were not found to be significantly related. Basal area was a better indicator of both regeneration density and growth than canopy openness. It appeared that in order to encourage growth of the natural regeneration the stand should be kept at a basal area of 30m2ha-1 or less. This is less than the value (38m2ha -1) for a fully stocked stand. Tentative recommendations are made for the transformation of regular, even-aged stands of Sitka spruce into irregular stands.