Continuous Cover Forestry at Glentress

Summary Description:

The Glentress Trial Area is one of the longest running Continuous Cover Forestry (CCF) trials in the world. It was established in 1952 by Professor Mark Anderson, who was then the head of the forestry department at the University of Edinburgh. He managed to persuade the Forestry Commission of the value of long term trial areas and was given 117 ha of Glentress forest. The overall aim of the Trial Area was to transform from even-aged stands to an irregular structure using a group selection system over a 60 year transformation period. The transformation involved felling two hectares, made up of multiple group selections between 0.1 and 0.2 ha in size in any one year. Therefore over a 60 year period, the entire Trial Area would have been felled and the groups would be at varying stages of development.

The PhD project aimed to assess forest structure in the Trial Area which was last updated in 1990. The project utilized simple models (the reverse-J distribution and the Equilibrium Growing Stock (EGS)), seedling physiology and a hybrid gap model in an effort to gain a better understanding of the effects of management on stand structure.

Project Outcomes:

At present a lack of knowledge regarding CCF management is a constraint limiting its implementation in the UK. Output from the project has contributed to the developing knowledge-base of CCF in the UK and has been used to help guide the future management at the Glentress Trial Area.
The main findings from the project were:

• Sitka spruce has become the dominant species in the trial area between 1990 and 2008 and there have also been increases in shade tolerant conifers such as Western hemlock. Light demanders such as Scots pine and Larch have declined over the same period.
• Basal area for the trial area appears low at just 25.3 m2 ha-1 in 2008. However, low basal areas are essential for successful natural regeneration.
• Seedling numbers have increased significantly between 1990 and 2008, reflecting the fact that many of the trees are reaching the age of maximum seed production. However, saplings have declined over the same period with sapling survival at only 37%. Whilst the causes of mortality are likely many, deer browsing is though to be a likely cause.
• The reverse-J curve, which is a diameter-frequency distribution, was observed at smaller scales than previously investigated (~2-11 ha) indicating an irregular stand structure and the parameters from the curve fit well with those found in continental Europe. Additionally, ideal reverse-J curves were produced using stand parameters which were then used to help inform thinning interventions.
• The EGS has two main components: standing volume and the distribution of that volume across three broad diameter classes. The standing volume in the trial area in both 1990 and 2008 was much lower than those found on the continent. The volume distribution commonly observed on the continent is 20/30/50 percent across the small, medium and large diameter classes. In the Trial Area there is too much percentage volume found in the small diameter class and too little in the large diameter class.
• The results from the seedling physiology study showed that basal area is not a good indicator of light transmittance in CCF. Hemispherical photography provides a much more reliable estimate.
• On a seedling a leader to lateral ratio greater than one indicates that a seedling is growing in a light environment suitable for sustained growth.
• The hybrid gap model, PICUS v1.4, was used to measure the effects of varying management over two periods of time (1954-2008 and 1954-2075) on an area of the trial area.  The model was parameterized for Sitka spruce grown under UK conditions. Weather data was gathered from local weather stations and for future predictions a medium emissions scenario from the UKCP09 was used.  The output for the group selection scenarios was realistic relative to the stand measurements recorded in 2008. The model output indicated that thinning to a low basal area (25 m2 ha-1) or a group selection with underplanting results in a stand structure most similar to CCF.