Sitka spruce: the ugly, the bad and the good

Sitka spruce is the commonest tree in Comar Wood and also the commonest tree in Scotland, very successful but also highly controversial. My feelings towards this tree have been gradually evolving so I thought I would write a bit about it here.

Sitka spruce in Scotland

Non-native trees have acquired a negative reputation amongst many groups of conservationists and countryside lovers. Sitka spruce is by far the most common non-native tree in Scotland and bears the brunt of the ill feeling (in 2017 it accounted for 58% of conifer woodland in Scotland, covering 507,000 ha; broadleaved species altogether covered 297,000 ha¹). Much of this negative reputation stems from the 20^th Century approach of planting large blocks of spruce on previously open hillsides and on the peat bogs of the Flow Country, completely changing the character of the landscape. Viewed from a distance these blocks are often ugly with conspicuously straight edges that do not blend in to the surroundings. Close up they are also ugly, being close-planted, even-aged monocultures, resulting in a dark forest with rows and rows of tree trunks and very little underneath. At the end of the rotation large areas are clear-felled, leading to what can look like, both close-up and from a distance, mass destruction. These types of commercial forest are of relatively low biodiversity value and are not naturally functioning ecosystems, rather a crop grown for timber on a rotation of a few decades.

Unfortunately, it is the way in which it has been grown in this country, rather than the tree itself, that has given Sitka spruce much of its bad reputation. However, there are some characteristics of Sitka that make it difficult to manage within a more natural mixed woodland ecosystem. Sitka spruce is semi shade tolerant, meaning its seedlings are able to grow in partial shade (i.e. under the canopies of other trees). This puts it at an advantage compared with many native species such as birch, oak and pine, which require more light (and therefore open clearings) for seedlings to grow. Spruce also has the ability to spread its seed widely using the wind, meaning it can spread into woodlands or open ground and expand its coverage. If it is not managed, Sitka spruce can therefore begin to dominate in some settings, which may be considered undesirable in existing native woodlands or certain open ground habitats like heathland or peat bogs.

Against all these negatives, there is a reason that Sitka spruce has been so widely planted in Scotland, which is because it is a very useful tree. It grows well in the climate and soils found here, including in upland areas where few other timber species will grow, and it quickly produces timber that can be put to a range of applications. Modern uses for spruce include construction timber and engineered timber, particle board, pallets, fencing and paper. Historically, spruce wood from western America (its native range) was also used for aircraft and for piano sound boards, amongst other things.  

Unfortunately, its assets, combined with 20^th Century attitudes towards land management, resulted in it being managed in the extensive monocultures that are now widely seen across much of western and southern Scotland. There is increasing recognition that such monocultures, and the associated clear-fell management, are ecologically and visually undesirable and that there are better alternatives using continuous cover forestry methods (which are commonplace in other European countries). Current forestry guidelines such as the UK Forestry Standard (UKFS) go a small way towards changing this, but in my opinion are very weak. Anyway, maybe more on the topic of clear-fell forestry another time.

Sitka spruce in Comar Wood

Comar Wood is ancient woodland that was planted with Sitka spruce during the 1980s (a common practice back then). On first acquaintance with Comar Wood, my initial reaction to the spruce was that it was shading out the native, ancient woodland and should be removed as a matter of relative urgency. I thought of it as something that should not be there. However, over time my feeling towards the spruce has softened. While the spruce remains a threat to parts of the ancient woodland and lacks habitat diversity in the dark, dense interiors of the stand, I also see benefits that it brings to the wood. I have not done any kind of scientific study, nor am I an expert in ecology, so these are just my observations and perceptions.

One of the main things I have come to realise about the spruce is that it does provide useful habitat. It gives good cover for birds and the fact that it retains its foliage in winter is a further benefit in its provision of shelter for wildlife. There are some areas where the spruce has not grown well and remains stunted and bushy, forming dense thickets. My initial thought had been to remove all these areas of spruce with no timber value as soon as possible to open up the ground beneath for regeneration by other species. I am now thinking that, in the absence of shrubby thickets of other species, the spruce thickets are providing an important element of the woodland habitat. While I have started clearing some of these areas, my plan is to also retain some at least until we start to have thickets of other species developing in the wood – once it eventually regenerates.

The spruce provides nesting opportunities for some bird species. I have noticed many nests in spruce trees over the last couple of years. I’m not sure which species they belong to, although I’m pretty sure there are blackbirds nesting in the spruce trees behind the caravan at the moment. We have song thrushes singing up in the wood (quite a few by the sounds of it) and recently I have been hearing chiffchaffs and blackcaps in scrubby spruce areas. In contrast to our spruce wood, the woodlands either side are open birch woods with lots of bracken and very few shrubby areas or thickets. There seems to be less songbird activity in these woods compared with Comar Wood (my perception), suggesting that spruce is useful habitat, even if non-native.

While I say that dense thickets are good, it appears that areas of more open spruce woodland are also favoured by birds. Since I have started thinning (see here) I’ve noticed that where the spruce are now more spaced, especially alongside our tracks and other open areas, they are being used more by birds including tree creepers, various tits, goldcrests, chaffinches, thrushes and robins, as well as bats. They can now get amongst the trees more easily and seem to prefer the lighter, more open areas to the very dense interior of un-thinned areas, where I see very few birds.

Spruce cones provide an additional and different food source for birds and other wildlife and the abundance of fungi in the wood in autumn (see here) suggests that the spruce are contributing to the variety of fungal associations found here, which cannot be a bad thing. I’m not any kind of fungus expert, but I think many of the fungi associated with the spruce would also be found in native pine woods.

A further positive about the spruce here is its potential to provide mature tree cover. Mature spruce trees, especially if left beyond normal harvesting age, start to provide similar functions to older native trees – such as hosting epiphytic plants (mosses and ferns), providing dead wood, roosting sites for bats and nesting sites for birds. When commenting on the Glen Affric forestry management plan last year (see here), my view was that at least some of the mature spruce trees in the compartments nearer Cannich should be retained to maintain cover of mature trees. I thought FLS were rushing to remove non-native trees without considering their benefits to the existing woodland habitat. This then led me to rethink my initial plans for Comar Wood.

The spruce comprise a significant proportion of the trees in Comar Wood and, apart from scattered mature/ veteran birch and alder, they are currently the biggest and most mature trees we have (although the spruce trees here are two or three decades younger than the ones up the hill planned for felling by FLS). If we were to remove the spruce over a relatively short timescale (20 years) as originally planned, Comar Wood would then have very few mature trees and lots of areas of regenerating young trees. My thinking now is that, similarly to my views on the Affric plan, it would be good to retain some spruce to provide continuity of mature tree cover for a longer period, while regenerating trees of other species become more mature.

As well as any ecological benefit, I just enjoy spending time amongst big trees. Looking up the trunk of a big conifer is always an inspiring experience. The scent of conifers like spruce is also very stimulating to the nose and the terpenes (plant aromas) they emit are thought to be beneficial to human health. I like the smell anyway.

Managing the ugly and bad, making the most of the good

There are clearly many advantages in retaining some of the spruce trees in the wood over several more decades, but this will need to be balanced with careful management of the more negative aspects. While the spruce provides beneficial habitat in some areas of the wood, dense, un-thinned spruce is currently far too dominant in general. Thinning and felling of small groups of spruce will allow the dense stand to be broken up gradually, retaining trees to become mature in some areas and initiating regeneration of different species in other areas, with a general increase in ground flora due to increased light levels throughout the woodland.

 One of the main problems of retaining the spruce over a longer period is going to be managing the prolific regeneration that will probably result. We will need to stop spruce encroaching into the existing native areas and prevent it from dominating regeneration of young trees in our future felled groups. It is already starting to regenerate here and there in some of the open areas. I will be keeping a close eye on this, but it is something that I plan to deal with and adapt to as it happens.

Another issue is that the spruce is growing ever taller, meaning the open native woodland remnants within the stand are becoming increasingly shaded. The fact that it is on a steep, south-facing hillside has helped to maintain sunlight in these areas despite them being surrounded by spruce trees, but the taller the spruce grow, the more shade they will cast. Managing the impact on these native remnants will be important to prevent further deterioration of the ancient woodland without completely removing the spruce. This will mean starting to fell spruce immediately adjacent to the native areas to give them a bit more space, particularly to their south, either as part of thinning work or as small-scale clear-fells.

I have also started removing the lower branches of edge spruce trees growing next to native areas because these branches tend to be extensive and block light from ground flora underneath. By removing the branches, more light can be provided to the edges of the native remnants, allowing ground flora to expand outwards, without removing any trees. It also helps to let light through into the spruce stand, boosting ground flora under the trees as well. My feeling is that the more we can give ground flora within the spruce stands light and help it get a head start, the quicker it will be able to recover once groups of spruce trees are eventually felled.

Despite the challenges of managing the spruce, I think it is worth doing for the various benefits it brings to the woodland at the moment. I feel it will add an extra dimension to the wood during the transition that I hope to initiate over the coming decades. I am of the opinion that we should be taking advantage of the benefits provided by the non-native tree species we have in Scotland, rather than fighting against them (see an interesting paper by Jim Knight on this topic here). There is a big issue here surrounding how non-native tree species are viewed and managed. Understanding the problems and the benefits associated with important non-natives like spruce are important if decisions are to be made for rational reasons and not just on the basis of native equals good and non-native equals bad.

The native habitat of Sitka spruce is the west coast of America from northern California to Alaska, where they are an important tree species in the temperate rainforests found along the Pacific coast. Our own maritime Atlantic climate is very similar, which is why the spruce grow so well here and have great potential to play a role in the future evolution of temperate forests on our own shores. Whether anyone likes it or not, Sitka spruce is here to stay in Scotland, not least because we need it for its timber. The question is how we manage and adapt to make the most of the opportunities that it provides for our woodlands and their ecosystems. I believe that non-native trees have a place in our future woodlands and I would like to explore how this can work through our experiences in Comar Wood. I am sure my approach and attitude towards managing the Sitka will continue to evolve during this process!

1. Forestry Statistics 2017, IFOS-Statistics, Forest Research