Irish forestry relies heavily on introduced tree species; what do you consider as the major challenges and opportunities of introduced tree species in Irish forestry?
The major opportunity of the introduced species is their high productivity, especially compared to that of the native conifers (Scots pine (Pinus silvestris) and yew (Taxus baccata)), and their ability to grow well on marginal agricultural land. Also important is the quality of their wood that allows for a wide range of utilisation. The major challenges are related to the social and environmental acceptability of introduced species. The risks associated with the use of introduced species, in terms of potential diseases and pests, should be taken into account, especially for a species such as Sitka spruce (Picea sitchensis (Bong.) Carr.) which makes up 60% of Irish forests. On the other hand, we are currently experiencing a major outbreak of dieback disease in our native species ash (Fraxinus excelsior).
Bearing in mind climate change, do you think the proportion of introduced tree species in Irish forests will change over time or remain similar?
I am not sure that climate change will influence the overall proportion of introduced species, but it will impact on the individual proportions of the introduced species. For instance, it is likely that Sitka spruce will be used less widely, as it is drought sensitive and most climate predictions indicate drier, warmer summers, especially in the southeast of the country. It is possible that Sitka will be replaced by Douglas fir (Pseudotsuga menziesii) in these situations, but only on the better sites. It is also possible that different provenances of existing species or additional introduced species will be used that are very suited to the changed climatic conditions and that have similar timber properties.
How would you define an ‘ideal’ balance between introduced and native species in order to improve future resilience of Irish forests?
For me, the balance between introduced and native species is not the most important factor in determining resilience. What is important is that any species is selected and used according to its site requirements. This needs to be done on a much finer scale than currently is the practice, by not planting large blocks of single species but by selecting combinations of species based on detailed site assessments. These assessment should be based on traditional knowledge, such as the natural vegetation variability and soil characteristics, and on information from remote sensing sources. Of course an over-reliance on any one species, introduced or native, should be avoided or reduced.
Do you think that invasive potential of introduced tree species in general is considered at the time of their introduction?
Certainly in the past this was not the case. The criteria that were used related directly to the suitability of the species to the sites and to their productive capacity. In terms of forest tree species, we have been rather lucky in Ireland and no serious invasions have occurred. In the west of Ireland, there is some issue with the natural regeneration of lodgepole pine (Pinus concorta var. latifolia) on protected peatlands. On the other hand, the ability of the introduced species to regenerate naturally will allow us to move away from using only clearfelling and planting, and to introduce continuous cover forestry systems for these species.
Do you think some countries are more open to species introductions than others?
Yes, there are clear differences between countries, mainly based on their forest history and the role forest play in their culture. Societies in countries which have maintained their forest cover to a large extent and where ‘forest culture’ is strong are less likely to accept introduced species, while in countries where forest cover had largely been destroyed and where it had to be re-established by large-scale reforestation projects, the choice of species was largely determined by economic considerations and often introduced species performed better than native ones. However, in most countries conflicting views will exist, and opinions also change over time, both for and against the use of introduced species.
Prof. Maarten Niuewenhuis
Head of the Forestry Department
University College Dublin