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Work Package 3 – Rehabilitation of poorly performing established broadleaf plantations

Introduction

Poor stem quality is a common problem within broadleaved plantations. Whilst this problem is inherent in first rotation, green field silviculture, questionable provenance and species choice may have exacerbated the situation. The best site / provenance matches are often not considered, requested or indeed available – a fundamental problem with regard to the legacy of our first rotation broadleaved forests. Species choice may sometimes be overly optimistic with regard to site conditions, or more fundamentally misguided by the current GPC hierarchy. However, while forest policy should seek to direct the fundamentals of broadleaved plantation establishment, there remains the need to apply best (innovative) silvicultural practice to already established broadleaved plantations where stem quality is generally poor.

From observation in the field during COFORD field-tours and reports from the industry, it is apparent that there is a substantial area of broadleaf plantations, particularly ash, that for whatever reasons are under-performing and/or are of poor quality. The vast majority of broadleaf plantations are privately owned and their owners require an income from the plantations. Currently it is unclear how such poorly growing plantations should be managed although some options have been put forward internationally e.g. see Figure below.

A range of treatments may be possible, e.g. coppice with standards, underplanting, free growth, etc, which may form the basis of silvicultural trials and which could present an opportunity to begin the early diversification of first rotation broadleaved woodlands. This early intervention may therefore be seen as a positive step toward the integration of certain features which allow such woodlands to better fulfil a range of functions:

  • Coppicing, underplanting and a general (early) diversity of structure will better serve ecological and landscape functions
  • An early move toward Continuous Cover Forestry again supports ecological and landscape continuity and also begins to ensure greater continuity of supply (with regard to small lots of specialist hardwood timber – in support of local markets)

Indeed a general move toward species, age class and structural diversity within plantation woodlands may help secure certain desirable features of longer established broadleaved forests, therefore enhancing the most disadvantaged of our broadleaved plantations.

Figure 1. Silvicultural options for treating poor quality broadleaf stands. Redrawn from Evans (1984)

Objective

  • To establish a minimum of three pilot studies on methods of bringing poorly growing broadleaf plantations into productive use.
  • To produce, where possible, a set of guidelines/protocols for the rehabilitation of poorly performing established broadleaf plantations.

WP3a – State-of-the-Art

  • A comprehensive literature review will be conducted, making use of the internet and available abstracts databases. The British library Document Supply service and inter-library loans will be extensively used to obtain hardcopies of the literature.
  • All literature collected will be entered into a database (EndNote) and filed.
  • A review of unpublished work undertaken in Ireland will be carried out
  • NATFOREX database will be interrogated to source any existing national trials to visit

WP3b – Establish trial sites

  • Source a minimum of 3 suitable sites
  • With regard to information gained from WP3a, establish plots (months 6 – 42)
  • Possible treatments may include free growth, thinning and underplanting, coppice with standards
  • Endeavour to use recognised experimental designs where possible to facilitate long-term research in conjunction with demonstration
  • Measure and monitor plots to end of project
  • Survival; Stem diameter; Stem height; Stem form
  • Manage the trial sites to end of project

WP3c – Protocols

Using preliminary results from WP3, a draft set of guidelines/protocols will be produced.

Progress

The literature review has been broken down into four parts and the intention is for each part to be written and published as a peer-review journal article. The first two articles were published in 2012 and are freely available here:

Two sites have been established., one near Charlestown, Co Mayo, and the other near Knock, Co Mayo. The Charlestown site consists of a 6.5 ha broadleaf plantation, planted in 1992, primarily with ash (Fraxinus excelsior) and oak (Quercus robur). The species composition and structure of the woodland falls under four general categories:

  1. Good quality pure ash crop – suitable for (late) conventional thinning
  2. Poor quality oak / ash mixture – requiring a change of silvicultural system
  3. Very poor quality pure ash – requiring heavy thinning to facilitate a change of species via underplanting
  4. Pure ash crop with considerably less than the required number of ‘Potential Crop Trees’ (PCTs) – suitable for adapted thinning

The first category above is not being considered by the B-SilvRD project. Category 2 was originally planted with a 3 oak to 1 row ash mixture configuration. The oak are of questionable (species) provenance and have faired extremely badly. The ash, being planted in single rows 8 metres apart and having much more vigour than the intervening bands of oak, had very open-grown, heavy branched form with low boles. In this area a conversion to a coppice-with-standards system is being trialled. The best ash have been selected as standards at 10-15 m spacing, pruned and allowed ‘free growth’ thereafter. Their already open-grown form should not give rise to any stability issues. Where no suitable standard exists a new standard tree may be planted or existing stem stumped back and resultant coppice later singled. The intervening oak have been stumped back and allowed to grow as coppice. It is hoped that this regrowth will show better vigour than the original planting (from developed root systems, increased shelter and better developed forest soils). Coppice regrowth could be singled in the future to create two tiered high forest or harvested for pole wood under an ongoing coppice system.

Ash (foreground on right) and oak mixture prior to intervention.

Ash and oak mixture after intervention

This plot also has significant biodiversity value as certain scrubby sections of oak / ash / heavy native understorey were left untouched to develop naturally. One small section of better oak received more of a conventional thinning and can now be left (monitored) on a relatively long cycle.

Category 3 consisted of pure ash on wet organic soils. The growth was therefore very poor (YC4) – checked with canopy not closed. Ash was an unsuitable species choice in this area at the outset, therefore a change of species via underplanting is being trialled. As the area essentially forms an extended riparian buffer (on an SAC river corridor) it is not considered appropriate to underplant with a commercial conifer; rather alder (Alnus glutinosa), a native broadleaf, has been planted.

Two methods of underplanting are being examined:

  1. Three out of four lines of ash would be removed (felled to waste*) and replanted.
  2. Two out of four lines of ash would be removed (felled to waste*) and replanted.

Ash with 2:4 lines removed and underplanted with alder

The remaining lines of ash could be lightly thinned and pruned now and in the future, (particularly as the more open conditions should partly negate the further development of canker), however the ash element of the stand should primarily remain only to maintain some broadleaf cover in the short to medium term (with associated sheltering and maintenance of woody ecosystem functions). The immediate riparian buffer has not been underplanted and the current tree cover has been restructured to enhance the river corridor habitat.

The fourth category showed reasonable vigour (YC8), but with a relatively low incidence of PCTs. When the site was originally assessed in 2008 by Dr Ian Short and Toddy Radford there were 122 PCTs per ha identified. In this case it has been proposed to identify the PCTs and allow them almost free growth via ‘halo thinning’, i.e. remove most of the surrounding competitors. Adjacent suppressed stems remain standing to help negate wind damage to PCTs. Racks have also been introduced to improve access and facilitate future extraction of (hopefully) higher value timber.

The majority of the management prescriptions that have been carried out on the site (i.e. halo thinning, underplanting, coppice-with-standards) are novel in an Irish context. The site is therefore highly suitable for demonstration days, educational visits and ongoing research in the future.

The trial at the site near Knock is in a pure sycamore (Acer pseudoplatanus) block. The sycamore was planted in 1995 in a relatively exposed site. Whilst the growth rates have been acceptable the stem form quality has been poor. The initial proposal for this site was for the sycamore to be thinned by removing lines and then later underplanted, taking advantage of the shelter provided by the remaining standing sycamore. However, once the thinning was carried out the resultant coppice growth was impressive so the silviculture has been changed to produce a 2-tiered silviculture system with pure sycamore or a sycamore coppice-with-standards system. There are four plots:

  1. Remove 3:4 lines
  2. Remove 3:4 lines
  3. Remove 1:2 lines as alternate lines
  4. Remove 2:4 lines by removing the middle 2 lines.

A decision will be made subsequently on which system (2-tier / coppice-with-standards) will be used. The intention is that the plots will be monitored into the future.

An example of the resultant sycamore coppice regrowth in the first growing season after felling