The current environmental fashion is in favour of tree planting. Scottish Ministers have a target of 10,000 hectares of new forests per year, increasing to 15,000 hectares per year by 2024.
This is a worthwhile initiative and Scotland is ahead of the rest of the UK, with 84% of all new planting happening in Scotland.
There are questions however about whether the aim of long-term carbon lock-up can be best achieved by tree planting, where best to plant new forestry and which species of tree is most suitable. The debate has also become confused with the high-profile topic of rewilding.
In recent years there has been agreement among experts that non-native trees such as commercial conifers are more effective at storing carbon because of their fast growth, compared to slower growing deciduous and native trees.
However, for this to be valid then stored carbon has to stay in solid form, for example as a roof truss or furniture. A report commissioned by the RSPB in March 2020 found that commercial tree plantations may not be so effective at storing carbon because more than half of the harvested timber is used in short-life products or packaging (i.e. it lasts for less than 15 years) and a quarter of it is burned. The carbon is therefore released into the atmosphere within a relatively short timeframe.
Furthermore, the RSPB report also makes the point that the location and siting of new woodland plantations is crucial. In most cases, moorlands are not suitable for commercial forestry (even shallow peat moorlands) primarily because of poor drainage, exposure and poor access.
Another factor is the impact on biodiversity. Some rare bird species such as the curlew or golden plover need open moorland for breeding – they will not nest if there are forests close by. It is important to consider the impact on our birdlife, particularly for rare species and not to sacrifice birds’ habitat if there is doubt about the carbon lock-up value of the replacement land use.
Plantation forestry releases carbon from the soil when the trees are planted (due to drainage and drying out of peat soils by the trees), and again when the trees are harvested and processed or burnt for fuel, so it may only be carbon neutral in the long term.
In Scotland, moorland management is a very important environmental project and makes a significant contribution to carbon sequestration in its own right.
Well managed moorlands have a special assemblage of species and biodiversity, arguably richer and more important than those on scrub woodlands, which is what would grow for many decades if the land were given over to ‘rewilding’.
Bird species which thrive on open moorland include many which are unique to the UK, rare and in steep decline elsewhere – including red grouse, curlew and golden plover. By contrast, woodland bird species are relatively common. Where tree plantations are sited adjacent to open moorland, predator species such as foxes and crows multiply, and ground nesting birds are lost.
The peat soils under moorland lock up carbon all the time. They are the ‘tortoise’ of carbon sequestration, constantly building up the peat layer below and preventing carbon dioxide release.
It is also worth bearing in mind that well-managed muirburn only burns off the top layer of vegetation and does not damage the peat layer below. Actually, carbon-sequestering sphagnum mosses and heather are boosted after muirburn.
In addition, it is not widely known that once a native woodland reaches maturity its carbon sequestration capability diminishes, whereas peat soils just keep on getting deeper and the carbon stays buried. The UK’s peatlands lock up more carbon today than the UK’s forests.
Woodlands can be categorised as the ‘hare’ of carbon lock up – a quick fix but they run out of steam. They are useful for medium term carbon storage but trees on open moorland fundamentally alter a unique and precious environment of which the UK has 75% of the world’s resource.
It is heartening to see that the report published by the RSPB recommends ‘protection and restoration of peatlands’ over and above ‘carbon sequestration through tree growth’. We must keep a balance of different habitats to benefit wildlife and achieve carbon capture.