This brings new challenges for the forest owner as the management of mixtures requires a different approach than for monocultures and, at least at first, may appear rather more daunting.
Different species of tree have different requirements, characteristics and growth rates and all these need to be taken into account.
That said, there is a compelling argument that uneven-aged, mixed forests are healthier than monocultures and, unquestionably, they are areas of greater biodiversity.
While the various species have different requirements, the management of broadleaves is fundamentally different again to that for conifers.
Mixed plantations can still be pure conifer or pure broadleaf, but of two or more species, or may contain both conifers and broadleaves.
Broadleaves and mixtures
In any young plantation of broadleaves, there is likely to be greater variation in stem form and vigour than is the case with conifers.
An essential part of broadleaf management is to identify from an early age the best examples from which ultimately the 'potential crop trees' (PCTs) can be chosen.
Therefore, from the first thinning (or tending, as the operation is more usually called), the business of identifying superior specimens as PCTs begins and these need to be clearly marked, eg with spray paint, and the future management concentrated on these.
Tending should start when the top height of the stand is about 8m and normally involves the removal of every seventh row in its entirety. The identification and marking of 350 to 500 PCTs per hectare, and the removal of the immediate competitors to the PCTs or 'wolves' (large, malformed trees), and any weak or diseased trees.
In broadleaf/conifer mixtures, lines of conifers should be removed at 12m intervals for extraction purposes. And, bearing in mind that the conifers will have been planted as a nurse crop for the broadleaves, up to one-third of the most vigorous of the remaining conifers should also be removed.
Thereafter, the treatment of the broadleaves will be the same as for a pure broadleaf plantation.
The selection of the PCTs demands some care as these will ultimately be the most valuable element of the crop.
They should be selected on the basis of vigour, good stem form and even distribution throughout the stand and should show no signs of disease.
They will also need to be regularly pruned, but care must be taken to ensure that sufficient live crown remains.
Here it is important to determine whether one of the species is to be favoured to form the final crop, or whether two or more species will be given equal weight.
A first thinning should be carried out as for a single species crop – ie racks removed every 12m and a selective thinning between the racks – but thereafter the growth and form of the different species should be monitored and management tailored accordingly.
Teagasc and the Forest Service are holding an event focusing on the management and thinning of oak and conifer mixtures tomorrow, July 10.
This event is to be opened by Tom Hayes TD, the new Minister of State with responsibility for forestry, at the Kilruane McDonagh GAA Centre, Cloughjordan, Co Tipperary.
All are welcome and further details are available from Tea-gasc offices and the website.
William Merivale is the national secretary of PEFC Ireland and a forestry consultant based in Cork. Contact him at firstname.lastname@example.org