Scottish Forestry describes building forest resilience as managing age diversity, species, and structure through restocking to reduce the risk of changing environmental, economic, and market conditions in the future.
This sounds like a sensible approach but what does it really mean in practice?
Climate change is likely to increase the risk of forest pests and diseases in the future. Predictions also suggest that drought will become an increasingly important factor in tree health, particularly in the east of the country, causing increased susceptibility to fungal diseases.
Dry summers, higher winter temperatures and extreme weather events are also likely to increase the impact of forest pests such as aphids and bark beetles by accelerating their propagation rates and weakening host trees.
In the past decade, woodland owners and managers have been confronted as never before with a variety of new forest pests and diseases.
Since it was ﬁrst found on larch in 2009, the impact of Phytophthora ramorum, particularly in south-west Scotland, has been well documented.Ash dieback, Hymenoscyphus fraxineus, appeared three years later and has since become a widespread and potentially devastating threat to one of our two main native broadleaved species.
Since the mid-2000s Dothistroma (red band) needle blight, a previously minor disease in Corsican pine, has become common throughout the UK aﬀecting stands of lodgepole pine, Scots pine and Corsican pine, particularly in the east and north of Scotland. Over the same period, green spruce aphid Elatobium abietinum, has become common in eastern Scotland, placing Sitka spruce under increased risk of severe defoliation in the future.
Further aﬁeld, the European spruce bark beetle, Ips typographus, has become increasingly destructive in commercial spruce stands in continental Europe, particularly Germany and the Czech republic. In late 2018, a breeding population of Ips typographus was discovered in Kent by the Forestry Commission, the ﬁrst time the pest has been found in the wider UK environment.
The potential rise of drought related pests and diseases on susceptible sites suggests that matching tree species to site conditions will become increasingly important, ensuring that drought susceptible species such as Sitka and Norway spruce are planted on sites with suitable soil moisture.
The increase in outbreaks of hostspeciﬁc pests and diseases has highlighted the fact that the UK forestry and timber processing industries rely heavily on Sitka spruce and a small number of other productive conifers. This has focused attention on planting a wider range of tree species to promote resilience and minimise the potential risk from unknown future threats. Until recently, however, there has been a lack of evidence about how diversiﬁcation might impact on the future marketability of timber.
The properties of Douglas ﬁr and Scots pine are well known. On sheltered, well drained sites, Douglas ﬁr provides an excellent alternative to Sitka spruce. Scots pine oﬀers longer rotations on drier soils with more limited applications as a structural timber.
Larch is no longer a viable option so the other main alternatives are noble ﬁr, Norway spruce, western red cedar and western hemlock. Forest research recently compared the structural timber properties of these species with Sitka spruce, concluding that all species were capable of meeting the strength classes required for use in construction, with western red cedar the least desirable. High yields of C16 timber were obtained from all species with Norway spruce and western hemlock producing acceptable yields of C18.
All species have strengths and weaknesses. For example, Norway spruce is susceptible to butt rot, vulnerable to drought and less tolerant of a windy climate than Sitka spruce. However, these ﬁndings help to support more widespread use of alternative conifers already well-known in British forestry. The use of more obscure conifer species has also been proposed but there is little conﬁdence in species such as macedonian pine, giant sequoia or silver ﬁr that have never been grown at a forest scale in the UK. Their widespread planting is not recommended until appropriate species and provenance trials have been carried out.
Birch is among the strongest and densest timbers grown in Britain and capable of producing high-quality sawn timber if properly managed. With rotations of around 60 years, it is worthy of consideration as an alternative timber species on free draining soils at lower elevations.
Considering a wider range of tree species when replanting and matching them to site conditions is not radical in itself but will help to promote more robust woodlands in the future.
However, although Sitka spruce can be established without protection, the more palatable alternatives usually require deer fencing to get them away in most areas. The main challenge to building forest resilience is therefore likely to be widespread acceptance of fencing as an additional cost.