Transformation from even-aged plantations to an irregular forest: the world's longest running trial area at Glentress, Scotland

Abstract

The main aim of the Glentress Trial Area has been to study the transformation of even-aged plantations to a permanently irregular structure using group selection. The Trial Area was established in 1952 when most of the plantations were 20-30 years old. The 117-ha area was divided into six Blocks and the plan was to transform the area over a 60-year period by felling and regenerating groups totalling 2 ha in each Block every 6 years. The objectives of this paper are (1) to examine the design, implementation and monitoring of the process of transformation and (2) to investigate if the data collected can be used to quantify the progress of transformation to an irregular structure. The Trial Area has been driven by a clear objective but unfortunately the management plan has not been revised and there has not been a consistent approach to record keeping. This has made it difficult to relate management interventions to the development of the forest structure. An earlier analysis claimed that transformation was almost complete; this was based on a comparison of diameter distributions of the 1990 data with an exponential regression. However, the analysis in this paper includes all the data collected between 1952 and 1990 and shows that the diameter distribution of all Blocks has been similar to an exponential since the start of the Trial. The main reason for this is that the monitoring unit has been the Block, and a spatial scale of ~20 hectares is probably too coarse to detect the changes that are clear in aerial photographs.