Details of the thinning regimes
Conventional stand management
In sustainable forestry, thinning is required to maintain control over the growth and development of the forest stand. Stand interventions take place every three to five years, depending on site productivity and exposure, from stands aged 15-20 years.
In Ireland, the conventional form of thinning is called 'low' thinning (Figure 4). This removes small and poor-quality trees at each stand intervention; remaining trees are more uniform in size and quality, and have adequate space for continuing growth.
In the alternative thinning approaches, a balance must be achieved between timber production and the ecological requirements for natural regeneration. Rather than uniformity, suitable trees must be nurtured for seed production and small gaps must be created for seedling establishment.
Crown and graduated density thinning
The two alternative regimes being assessed in the TranSSFor project are 'crown' and 'graduated density' thinning. The key difference between these and 'low' thinning is that trees with superior quality attributes are selected after the first and second thinning interventions, in the 'crown' and 'graduated density' regimes, respectively.
These are called Q-trees (Figure 5). The trees marked for removal are generally those that are competitors to Q-trees. By removing competitors, the Q-trees are given additional space into which their crowns can expand. Providing space around the Q-trees leaves other areas where fewer trees are removed. This promotes a more irregular spacing between trees (Figure 6) and different tree size distributions in the forest stand.