Growth rate, crown development and wood quality of Sitka spruce on upland sites in Scotland, with particular reference to nursing mixtures
Sitka spruce from three upland sites in Scotland was studied in order to discover how growth rate and quality was affected by site and management. Two of the sites, established in the late 60s, contained nursed and fertilised treatments so spruce in intimate mixture with lodgepole pine and larch could be compared with spruce which had been annually or periodically fertilised with N. Spruce (P46) from the third site had been planted in row mixture with Scots pine on three different types of cultivation. A detailed analysis of growth rates in diameter, height and volume was carried out. Crown form and the dependent characteristics - branchiness, stem taper and ellipticity - were compared between treatments. Wood quality was assessed through density, knot content and the proportions of juvenile and compression wood. It was clear that nursing was an effective means of raising a crop of Sitka spruce, even on a deep peat site, but that lodgepole pine was most likely to reduce branchiness, taper and ellipticity and therefore would promote the growth of better quality timber. Repeated applications of N did not encourage volume production but rather immoderate branching and poor stem form. On the older site complete ploughing had increased growth rates during the first 20 years but in the long term there was no gain. Analysis of horizontal and vertical trends in density showed that spruce from all sites were producing timber of desirable strength properties but that the presence of knots, compression wood and, in some treatments, the extent of the juvenile core greatly reduced timber quality. A survey was carried out to test the use of the pilodyn to estimate density and the results were encouraging.