Glentress Trial Area


The main aim of the Glentress Trial Area is to study the transformation of even-aged plantations to a permanently irregular structure using group selection.  It was established in 1952 by Professor Mark Anderson, who was then Head of the Forestry Department at the University of Edinburgh, and Forestry Commission Scotland.

Black and white photo from the 1960s taken at the south end of management Block A. The photo shows a mature mixed conifer stand on a slope to the right and a pole stage plantation to the left

When the Trial Area was established in 1952 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 two hectares in each Block every six years.

A map of the 117ha trial area at Glentress. Six management Blocks are highlighted. In the top right of the image there is an inset map of Scotland showing the location of the trial area (approximately 25 miles south of Edinburgh)

The initial species choice was to plant European silver fir, Norway spruce and European beech into the groups in an attempt to emulate the forests of the Swiss Jura, which had inspired Professor Anderson’s thinking (explained in Anderson, 1960). Whilst all these species are still represented within the Trial Area, only Norway spruce now makes up a significant proportion of the stand. This was due to difficulty sourcing European silver fir and browsing of European beech by initially sheep and then later deer. 

Copy of the original plan for the Trial Area (PDF-806K)
(We think this was written by Professor Anderson in 1955).

Progress since 1952

Arial photograph of the Glentress trial area taken in 1968. The photo clearly shows the small groups felled within the trial area

The original plan for the area was the University of Edinburgh would produce an annual plan of work for the area, which would be carried out by the Forestry Commission.  Unfortunately, detailed information on these plans and their implementation were not recorded and only general trends, such as the changes in the species being planted, have been recorded.  The plan also involved the University assessing the Trial Area to measure progress of the transformation.  There have been three distinct periods of assessment in the Trial Area.  From 1952 to 1964 every tree above 12.5 cm DBH was assessed every six years.  It was not possible to maintain this level of intensity and from 1964 to 1989 few measurements were made.  In 1990 funding from the Scottish Forestry Trust was used to establish 240 permanent sample plots in the Trial Area.  All subsequent research in the Trial Area has been based on this network of plots.

An important development during the early 1990s was the increase in recreational development in Glentress forest and the Trial Area. The forest is now one of the major mountain biking centres in Britain with over 250,000 visitors a year.

An airborne cyclist passing through Block F of the Trial Area

Present position and future plans

Since 2008, Forest Research has assumed responsibility for the research aspects of the Trial Area and work closely with Forestry Commission Scotland on this project. A review of progress (Kerr, 2010) between 1952 and 1990 helped clarify progress and a recent PhD study by Hamish Mackintosh has updated this work.  The following is a summary of the main findings in the Trial Area so far; for more information see the publications listed below.

A large European larch with two tree fellers beneath its canopy Naturally regenerated Sitka spruce seedlings and sapling growing within a gap in the Sitka spruce canopy
  • Progress towards transformation has been good but Professor Anderson’s initial aim of having a fully developed irregular structure throughout the Trial Area in 2012 has not been achieved.
  • The main method of transformation since 1952 has been group felling and planting. However, recently the approach has changed with a more balanced approach between felling trees (from throughout the diameter distribution using mainly crown thinning) and regeneration (using planting in open/poorly stocked areas and some natural regeneration elsewhere).
  • Analysis of the most recent data on species composition shows that the most abundant species are Sitka spruce and Norway spruce and the main concern about future species composition is the domination of Sitka spruce.
  • An earlier analysis of the 1990 data claimed transformation was almost complete based on comparisons of the 1990 diameter distributions with ideal target structures (reverse-J curves).  Past analysis using this method has been carried out at too coarse a spatial scale (~20 ha). However, the most recent analysis has shown the diameter distributions are very similar to the ideal target structures at much smaller spatial scales (2-11 ha).
  • Forest operations in some parts of the Trial Area have been hampered by difficult access.
  • Long-term data from the permanent sample plots has shown only 37% of saplings (trees >1.3 m and < 7cm DBH) develop into trees.  This is important for forest management using continuous cover methods as a key requirement is to obtain enough young trees to ensure the forest develops into the structure and species composition required. 

A new management plan for the Trial Area was been agreed between Forestry Commission Scotland and Forest Research in June 2010.


Continuous Cover Forestry at Glentress (Scottish Forestry Trust)


Anderson, M.L. (1955). High elevation experimental area: Glentress Forest. Glentress archive, Forest Research, Alice Holt Lodge, Farnham, Surrey, GU10 4LH, UK.

Anderson, M.L. (1960). Norway spruce-silver fir-beech mixed selection forest: Is it possible to reproduce this in Scotland? Scottish Forestry, 14:87-93.

Kerr, G., Tracey, C. and Edwards, C. (2010). Glentress Research Trial Area management plan 2010-2024. Glentress archive, Forest Research, Alice Holt Lodge, Farnham, Surrey, GU10 4LH, UK.

Kerr, G., Morgan, M., Blyth, J.F. and Stokes, V. (2010). Transformation from even aged plantations to an irregular forest: the world's longest running trial area at Glentress, Scotland. Forestry 83, 329-344.

Mackintosh, H. (2012). “Developing the silviculture of Continuous Cover Forestry: using the data and experience collected from the Glentress Trial Area”, PhD thesis, University of Edinburgh, Edinburgh.

Further reading

Kerr, G. and Mackintosh, H. (2012). Long-term survival of saplings during the transformation to continuous cover. Forests 3, 787-798.

Malcolm, D.C. (1992). The development of a transformation from even-aged plantations to an irregularly-structure forest. Report to the Scottish Forestry Trust.  School of Forestry, Institute of Ecology and Resource Management, University of Edinburgh, Edinburgh.

McIver, H.W., Blyth, J.F. and Malcolm, D.C. (1992). The application of group selection working in an upland forest in south Scotland.  Scottish Forestry 46, 202-211.

Wilson, E. R., McIver, H. W. and Malcolm, D. C. (1999). Transformation to irregular structure of an upland conifer forest. Forestry Chronicle 75, 407-412.