Transforming even-aged plantations to CCF

Introduction

Most current Irish forests are young, single-aged monocultures. To be managed as permanent forests, a transformation into a more complex, stable structure is required. Transformation involves crown thinning, where some dominant and co-dominant trees are removed from the upper canopy.

Transformation is achieved as a planned and progressive series of stand interventions that emulate the successional stages in natural woodland. The principles of CCF transformation are well defined and the practice of CCF is the standard approach to forest management in many parts of Europe.

Four transformation stages

There are four well-defined stages (see also diagram below), summarised as follows:

Stage 1: Preparation

Stands should be identified early in their development based on their suitability for thinning and productivity. Poor quality trees (inferior phenotypes) should be removed and better quality trees (superior phenotypes) should be retained.

Individual tree stability can be increased through the thinning process (i.e., modifying height: diameter ratios) and the selection process can then continue with frequent light thinnings.

Identifying good quality stems, across a range of diameter classes, will by default lead to greater irregularity in the canopy strata. This is best achieved with “crown” thinning as opposed to “low” thinning, which is the norm in most even-aged plantations. This approach allows the forest manager to concentrate stand increment on high quality stems.

Stage 2: Regeneration

As the stand matures and trees start to produce seed, consideration can then include the regeneration process. Thinning should aim to reduce basal area and enable appropriate levels of light to reach the forest floor. Threshold basal areas for regeneration, and separately for sustained growth, are now well recognised for most of our productive species, and this information can act as a guide for thinning prescriptions.

Avoiding uniform removal of the overstorey trees, and maintaining a degree of “clumping”, will allow natural regeneration to become established in small cohorts at irregular spacings throughout the stand.

Stage 3: Structural development

In this stage, tree selection focuses on removing high quality (crop) trees at their desired target size, and maintaining good quality smaller trees (i.e. future crop trees) from across a broad range of diameter classes. Ideally, a maximum of 20% of basal area and a volume no greater than the stand increment will be removed at an intervention.

Stage 4: Structural maintenance

Finally, the stand will be transformed to an irregular structure where the objective is to maintain a sustained yield of high value trees while promoting the development of the understorey strata.

Trees should be marked for removal at their optimum economic value, often called the target size, or if they are damaged and not contributing to the development of the stand structure.

Wind damage, if it occurs, becomes part of the management system and generally understorey trees are released to fill the gaps created by blowdown of larger individuals.

The transformation process can be summarised as follows:

Relevant research

• Transformation of Sitka spruce to CCF research project