Effects of respacing on young Sitka spruce crops

The practice of conventional silviculture with regular removal of stems in production thinnings from about age 20 has resulted in windthrow of many forest crops in upland Britain. Once started, windthrow tends to spread to adjacent areas and frequently results in the need for premature clearfelling of the site. Harvesting costs at such sites are large and since relatively few stems are of large sawlog sizes, the net result is that the profitability of the enterprise is jeopardized. In consequence, many plantation forests in the UK have been designated ‘no thin’ because thinning would reduce their rotation length and hence overall volume production. Nevertheless, the final assortment of materials available from unthinned forests will contain a much smaller proportion of logs in the valuable large sizes, than considered likely at establishment. Moore (1973) described a silvicultural system termed ‘oceanic forestry’ which seemed capable of overcoming the problems of windthrow and loss of volume production. Moore asserted that by early heavy thinning to about 1000 stems ha–1 (before the thinnings were of commercial sizes), the remaining trees would be inherently more wind stable and that timber volume production would be as great as from unthinned stands. In