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Establishment of Optimum Species Mixtures


Approximately 32,000 hectares of broadleaves have been planted during the period 1990 – 2007 inclusive (Forest Service statistics, 2008). The Government Strategy is to maintain 30% of all annual afforestation as broadleaves.

The afforestation programme at the beginning of the project facilitated intimate species mixtures with broadleaves within Grant Premium Categories (GPCs) 5, 6 and 7. Since the project began a GPC category for pure alder (GPC 8) has been introduced which allows a 10% mix of other species. GPC 6 has also changed. Previously alternate lines of oak:conifer were to be planted. The ratio has now changed to 10:1 oak:conifer and all trees are to be planted at 2 m x 1.5 m spacing (3,300 stems per hectare). See Table 1:

Table 1. Grant Premium Categories relevant to broadleaf afforestation
GPC 5 – Broadleaves This plot is made up of acceptable broadleaves other than oak and beech
GPC 6 – Oak This plot is made up of pure oak and/or an oak/conifer mix. The only conifers acceptable for this mixture are Scots pine and European larch. Chapter 9, paragraph 9.10 of the Forestry Schemes Manual details the stocking and spacing requirements for an oak/conifer mix.
GPC 7 – Beech This plot is made up of pure beech and/or a beech/conifer mix. The only conifers acceptable for this mixture are Scots pine and European larch. Chapter 9, paragraph 9.10 of the Forestry Schemes Manual details the stocking and spacing requirements for a beech/conifer mix.
GPC 8 – Alder This plot is made up of pure alder.

Each of these categories has presented problems with regard to silvicultural performance.

In GPC 5 opportunities for mixture establishment have not been fully realised, often resulting in poor quality pure stands. Whereas the management of pure stands may be more straightforward, particularly where a low quality product is accepted, these plantations cannot provide the optimum commercial, ecological or social returns. This lack of return may become increasingly apparent in the long term. For those who wish to optimise the returns from broadleaved plantation forests, a greater range of silvicultural options need to be provided. For example, Gabriel et al. (2005) report results of a trial planted in 1955 on the North York Moors in the UK. The trial consisted of broadleaf afforestation planted as pure stands and also as a mixture with different nurse crops. After 47 years growth, it was generally found that the broadleaf crop exhibited superior growth and quality when grown in mixture relative to grown as a pure stand.

Where broadleaved mixtures have been attempted results are variable at best. Inappropriate mixtures are understandable given the lack of cohesive guidance in this area and subsequent piecemeal nature of experimentation. Conversely successes may have gone unrecorded and unpublicised. Whilst numerous mixtures have been recommended, for example by Horgan et al. (2003), for broadleaf planting, there has been little information found in the literature to substantiate them.

This is understandable however given that historical data on broadleaved species mixtures is neither well collated, nor tested within the environmental context of the Irish forestry sector. Research is required in order to address both aspects of this knowledge gap, and ultimately to feed new knowledge into practical silvicultural guidelines. A thorough review of the literature on broadleaved silviculture is required, together with a targeted survey of international best practice which is relevant to Irish forestry. This should be followed by a series of field trials designed to apply the state of the art to the national plantation forestry sector.

The above is also broadly true in relation to broadleaf / conifer mixtures – GPCs 6 and 7 – particularly with regard to the knowledge gaps. In these GPCs, mixture options are more prescriptive but no less problematic. In the case of conifer / broadleaf mixtures the problems are somewhat different and are generally associated with two situations:

  • Early loss of apical dominance due to lack of nursing effect from the conifer species
  • Suppression of broadleaved crop species due to lack of timely management input / removal of conifer species

A recent study by Christopher Guest in Ireland (Guest, 2007) highlighted the problems associated with GPC6 mixtures and concluded that research is required to:

  • Provide thinning options for GPC6 stands
  • Investigate more robust options that provide a larger window of opportunity for a silvicultural intervention without severe detriment to the remaining broadleaf crop.
  • Investigate alternative compatible nurse species.

This work package not only determined the silvicultural optimums, it also provided practical solutions relative to the likely intensity of management inputs.

The Work Package consisted of five sub-tasks:


  • Investigate silviculturally compatible mixtures for establishment of a valuable broadleaf crop.