During our Big Tree Measure in October, one of the volunteers asked about the conifer trees we were walking past. Glandwr Forest is primarily a broadleaf woodland, with a research study nestled at its centre made up of both broadleaf and conifer trees. Whilst the broadleaf parts of our carbon study are a mix of species, including birch, cherry, oak and aspen, the conifer planting blocks are all made up of a single species: Sitka Spruce (Picea sitchensis). Why did we plant just one conifer species as part of our research study, and what is special about this tree?
The short answer is that Sitka Spruce is the most widespread conifer species planted in the UK for timber production, and therefore it made sense for our Carbon Community field study to focus on this species alongside our broadleaf trials. For us it was important to research both conifer and broadleaf on the same site, to understand the impact of the treatments in the same conditions on tree resilience, tree growth, and ecosystem carbon. Our aim is that the learnings from our study will help inform all types of forestry.
How and why did Sitka become so popular in commercial forests?
Originating in North America’s temperate rainforests along the Pacific coast, the Sitka Spruce first arrived in Europe in the early nineteenth century, introduced by plant hunters as a specimen tree to be nurtured in arboreta. It wasn’t until the mid-late twentieth century however that the trees started to spread rapidly across the British landscape, largely in response to timber shortages during and following the First World War. Timber was in desperately short supply, particularly for use as pit props in coal mines.
The challenge for commercial foresters was how to grow trees quickly enough. Conifers grow more quickly than broadleaves, which means they can be harvested earlier – plus they tend to grow straight and tall which is desirable for timber. Whilst you can harvest a spruce tree after 40-50 years, you have to wait for 120-160 years to harvest an oak.
Foresters in the twentieth century experimented with various species, but growing conditions were restrictive. Enter Sitka Spruce! Fast-growing and resilient, able to survive in a cool wet climate, and produce what is known in forestry terms as high ‘yields’, Sitka became the conifer of choice and has been improved in the UK to grow tall straight trees without many knots to improve the resulting timber.
Sitka plantations boomed in the late twentieth century. In the focus on speed and efficiency of timber production at that time, other landscape features were sometimes overlooked – protecting peatland, or preserving the character of a landscape for example. The phrase ‘right tree, right place’ is now common when it comes to tree planting, emphasising the need to carefully select where you plant, and which species.
To many, the Sitka spruce now represents an intrusion on the British landscape with its non-native origin and often managed under a clearfell regime which results in all trees being harvested for timber and replanted in one go.
And yet – we depend on wood products for our day-to-day human existence, from building houses to making furniture. The UK currently imports 73% of its timber1, which puts us in a position of dependence. As a sustainable, carbon-storing material, shoring up our domestic timber supply brings huge benefits, including the opportunity to find innovative new markets, for instance using timber from felled ash trees affected by ash dieback. A middle ground is possible in which the wider landscape and environment is respected, whilst still enabling forests to produce crops of essential timber.
In 1998, the Forestry Commission published the UK Forestry Standard (UKFS), which is one of the most robust standards for sustainable forest management in the world. The UKFS sets out best practice guidelines for growing and managing forests, ensuring that a wide range of factors are taken into account, from biodiversity and climate change, to water and soils, to people and the historic environment. For example, you are not allowed to plant trees on peat. It is a handbook for balancing environmental, economic and social objectives, and every UK planting scheme is assessed against UKFS criteria. In the last year, the 5th edition of the UKFS has been launched, which includes a new maximum species cap of 65% for any single species. This means that monoculture forests cannot be planted in the UK going forward.
Forestry practices have also changed considerably over time, for instance reducing soil disturbance during planting to reduce the amount of carbon that is released from the soil. There is also a growing movement of continuous cover techniques, a management method whereby you harvest some trees within a forest but not all, maintaining a continuous canopy rather than clearfelling.
So much comes down to the objectives of a given forest. If you are planting in order to produce timber quickly, conifer species make more sense. If you are planting for longer term hardwood timber or primarily to support biodiversity, you would consider broadleaf. But either way, your choice of species and management methods, and your consideration for wider environmental features and factors, is what can make all the difference.
Despite its bad press, the Sitka spruce tree itself is quite remarkable. It is hugely resilient, it grows quickly and captures a huge amount of carbon along the way, and is very versatile. It is certainly growing successfully in our field study so far!
However, it is under threat from the eight toothed spruce bark beetle, Ips typographus. Primarily found in Europe, and having a devastating effect on European forests, outbreaks of the beetle have been found in the UK in recent years. A changing climate may also affect how well Sitka fares in the UK over the decades to come.
In our corner of Wales at the Carbon Community’s Glandwr Forest, we’ll be carefully monitoring our Sitka spruce as they continue to grow healthily, doing our best to understand how we can help these trees to capture even more carbon as they head skywards. Through our research programme, we are looking at the impact of enhanced rock weathering and soil microbiome inoculation on the carbon capture of both broadleaf and conifer species. One day, we hope the findings of our study will have the power to inform and transform tree planting projects and their carbon capture potential not only in the UK but beyond – whether for mixed broadleaf, mixed conifer, or commercial forests with Sitka spruce!
Photo: Paul Box
