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Species diversity a sturdier option than monocultures


SURPRISE: The second National Forest Inventory, taken in 2013, found that 14.2pc are conifers

SURPRISE: The second National Forest Inventory, taken in 2013, found that 14.2pc are conifers

SURPRISE: The second National Forest Inventory, taken in 2013, found that 14.2pc are conifers

The common perception is that the bulk of Irish forests are monocultures comprising mainly Sitka spruce. However, the second National Forest Inventory, published in December 2013, shows that this is not actually the case.

While conifer forest account for 74.2pc of all woodlands and remains the dominant forest type, Sitka spruce at 52.5pc only accounts for just over half of all species. Interestingly enough, less than 30pc of all our forests are in fact single species.

It is quite true that Sitka has been, and is likely to remain for some time, the dominant species as it is only during the last 20 years or so that a greater range of tree types has been planted.

Moreover, it must be said that Sitka has served Irish forestry extremely well. In Irish conditions it is a remarkably adaptable and forgiving tree, and is suited to a wide range of sites and soil types.

However, as is more evident than ever from the outbreaks of disease, first of Phytophthera ramorum in Japanese larch, and then last year Chalara fraxinea in ash, placing too much reliance on just a few species could potentially spell disaster.

It is now well established that mixed species of uneven-aged forests are healthier, less prone to storm damage and ultimately more productive than monocultures.

These type of plantations therefore make sound economic as well as environmental sense. It is likely that the proportion of single species forest in Ireland will reduce significantly over the coming years and forest managers will have to adapt accordingly.

The Forest Biodiversity Guidelines published by the Forest Service emphasise the importance of species diversity at the property, compartment and stand level, and stipulate that where non-native varieties are being planted the dominant species should account for no more than 80pc of the mix.

In common with the UK Woodland Assurance Standard (UKWAS), the PEFC Irish Forest Certification Standard goes a bit further and requires the proportions of replanting in certified forests to be no more than 65pc primary species, and at least 20pc secondary species, 10pc open space and 5pc native or naturalised broadleaf.


While the FSC Irish Forest Stewardship Standard is not as specific with regard to the respective percentages on this point as the PEFC Standard, nevertheless it too includes a number of requirements on species diversity, and a planned move towards uneven-aged stands, and with a greater proportion of native species.

Chapter 9 of the Forestry Schemes manual, entitled 'Silvicultural Standards', (available to download from the Forest Service website) contains much valuable information on species selection as well as a list of the acceptable species for grant aid.

The list is perhaps longer than some might realise. Following the removal of Japanese and hybrid larch, and ash because of the diseases mentioned above, the list still includes 17 varieties of conifer and 11 of broadleaves, albeit the latter includes pedunculate, sessile and red oak.

It should be noted, however, that a number of the acceptable species, for example Leyland cypress, Monterey pine, Serbian spruce and Lime, are rarely, if ever, included in grant-aided planting schemes.

Monterey pine (P. radiata) is New Zealand's 'Sitka', exhibiting even faster growth in that country than Sitka here, producing fine quality, joinery grade timber often in less than 30 years.

It has been said that every generation of Irish foresters rediscovers this species, and while it can also grow very rapidly here, it is now generally accepted that Ireland is at the northern extreme of its productive range.

It is both difficult to establish and tends to produce a high incidence of "wolf" trees, that is stems of poor form, coarse and heavily branched, hence of low value.

'The right tree in the right place' is a mantra that bears frequent repetition. Quite rightly the grant schemes over the last 20 years have favoured the establishment of more diverse plantations, but, unfortunately, from time to time we see examples of questionable species selection.

While this can only be determined with the benefit of hindsight, as we continue to plant several thousands of hectares of new forest every year it is prudent to take great care over selection and to ensure we learn from past mistakes.

Cherry is a case in point. For a few years in Britain, then in Ireland in the mid-1990s, some regarded Cherry as the new Sitka for farm forestry, and in many respects with good reason.

On fertile, sheltered brown earths it is relatively easy to establish, can grow very quickly, and in a fairly short rotation can produce high value joinery and veneer-grade timber.

However, in the wild in both Britain and Ireland it tends to grow either individually or in small clumps and it became apparent after much large-scale planting that this is its preferred state.

It is a very disease-prone species and valuable lessons were learned. Perhaps we have now gone too far the other way as today Cherry is hardly planted at all, which is a shame as it remains a fine tree and still has its place as an infill species.

A number of factors need to be considered before final selection is made. These can be broadly grouped under two main headings: site characteristics, and soil type.

Site characteristics include factors such as height above sea level, degree of exposure, and susceptibility to spring frosts and salt spray.

With regard to soil types, in addition to whether the soil is a brown earth, a surface water gley, or a blanket peat, pH, fertility, soil moisture and drainage characteristics all have a bearing.

Finally, while they provide a number of important goods, services and other benefits, it must not be forgotten that the primary purpose of plantation forestry is the production of timber. With this in mind, in a forthcoming article I will look at the characteristics and requirements of the principal species grown in Ireland.

William Merivale is national secretary of PEFC Ireland and a forestry consultant based in Cork. Email:

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