The urgent need to tackle the climate crisis has inspired a global imperative to plant trees and restore lost forest cover. Although the drive to plant trees has resulted in the creation of many monoculture plantations with non-native species, there is growing pressure from researchers and ecologists to use trees native to the area. Native species are commonly associated with higher levels of animal and plant biodiversity and are the species best adapted to local growing conditions. By planting native tree species, forest creation projects can restore entire woodland ecosystems, and contribute towards tackling the global biodiversity loss crisis as well as sequestering carbon.
There is a long history in the UK of planting non-native conifers for timber production, as they are fast-growing and provide good quality wood. Given that the UK has one of the lowest levels of forest cover in Europe at 13% and the fact that our net-zero policy relies on new forests to absorb some of the residual emissions, is there still a role for fast-growing, non-native trees? Can non-native species also contribute to supporting biodiversity or are our conifer plantations ecologically impoverished?
Native species are commonly defined as those which occur naturally within an area, without having been translocated by human intervention. Establishing which species are native to a region is difficult and a source of much debate between ecologists and natural historians. The main challenge is determining the historical point at which a species should have been present, to be classed as native. For the British Isles this is defined as those that became established following the end of the last glacial period 11,000 years ago, before we were separated from the continental landmass 8,000 years ago.
Although evidence suggests that the Romans introduced a number of species including rabbits, most non-native species in the UK were introduced after 1500 when intercontinental travel by sea became more common. Species such as collared doves (Streptopelia decaocto) have colonised Britain naturally, although it may have been wider human influence on the landscape or climate that facilitated their arrival. Today there are more than 3,000 non-native species in the UK, most of which are plants. Exotic species can cause serious problems for native wildlife, at which point they become classed as an invasive species.
Invasive plant species in the UK such as Rhododendron ponticum and Japanese knotweed (Reynoutria japonica) can cause considerable ecological and economic damage, and invasive animals such as signal crayfish (Pacifastacus leniusculus) and grey squirrels (Sciurus carolinensis) can threaten their native counterparts by spreading disease. Invasive species can precipitate the collapse of entire ecosystems as in Lake Victoria, where introduced fish species have decimated native populations and destroyed the ecological systems within the lake. It has been estimated that invasive species cost the UK economy upwards of £1.7 billion annually, representing a substantial economic burden. Not every non-native species becomes invasive, however, and it is estimated that only 1% of non-native species can survive in the UK without human intervention and only 0.1% of non-native species become invasive. On other islands which have been separated from mainland continents for longer such as New Zealand, invasive species are much more of a problem due to higher levels ofspecialised endemic species.
Depending on how species are defined, there are between 31 and 85 native species of tree in the UK, and a further 53 that were introduced before 1900. The early introductions include most of the commercial conifers such as Sitka spruce (Picea sitchensis, introduced in 1831) and Norway spruce (Picea abies, introduced in 1548). Most of the UK native species are broadleaved and there are only three native coniferous species, Scots pine (Pinus sylvestris), juniper (Juniperus communis), and yew (Taxus baccata). UK woodland is generally dominated by the two oaks, pedunculate (Quercus robur, countrywide) and sessile (Quercus petraea, mainly in the north and west of the country), beech (Fagus sylvatica, in the south east), and ash (Fraxinus excelsior). In Scotland the native forests were dominated by Scots pine and silver birch (Betula pendula).
Although commercial forestry has favoured the planting of conifers for timber, this has predominantly been in Scotland, where only 26% of the woodland is broadleaved. By contrast, 51% of the woodland in Wales is broadleaved and 74% of the woodland in England. Not all our broadleaved species are native, however, and some of Britain’s most familiar broadleaved species are non-native, such as horse chestnut (Aesculus hippocastanum) and sycamore (Acer pseudoplatanus). According to a recent inventory, native species make up 70% of woodland and woodland fragments in England, 50% in Wales, but only 33% in Scotland. Of the native woodland in the UK around a third is ancient woodland, defined as persistent woodland cover since 1600 in England and Wales, or 1750 in Scotland. Ancient woodland is the richest and most complex terrestrial habitat in the UK, and it is characterised by indicator species such as bluebells, wild garlic, and wood anemones.
According to a recent report by the Woodland Trust, only 7% of the native woodland in the UK is in good ecological condition, with a diverse mix of tree species and ages, a good number of veteran trees, open spaces for butterflies and insects, and standing deadwood for invertebrates and birds. Much of our native woodland exists in isolated fragments, which are low in biodiversity and not large enough to sustain woodland specialist species. All of our woodlands are facing a barrage of threats to their health including poor management, climate change, invasive species, disease, deer browsing damage, and destruction from development. The report calls for increased protection and management of our existing native woodland, and a dramatic increase in the creation or restoration of native woodland.
There is growing pressure from scientists to use native trees in planting projects, with guidelines such as the Oxford Principles for Net Zero Aligned Carbon Offsetting advocating a focus on restoring ecosystems rather than on planting trees. The recommendations advocate that creating diverse, native forests would increase biodiversity, could sequester 40 times more carbon, and would offer more ecosystem services to people than non-native plantations. There is, however, a distinct lack of empirical studies demonstrating the benefits of creating native woodland for biodiversity and other ecosystem services within the UK, which is surprising considering how strong the recommendations are to plant native species.
It is estimated that the UK has lost 50% of its native biodiversity and we have low numbers of endemic species that are found nowhere else, so it can appear that preserving woodland biodiversity within the UK is less important than in other countries. However, the UK has a unique moist oceanic climate that has created rare temperate rainforest habitat and our limited number of native trees have high genetic diversity within each species, making them particularly resilient to environmental changes. This combination of low numbers of tree species and a wet climate has led to the development of unique communities of highly adapted ferns, liverworts, mosses, and lichens.
Although non-native woodland does provide habitat for other species, our native woodland is associated with highly specialised species that cannot live elsewhere. Our native trees can support extraordinary numbers of species, with oak woodlands in particular associated with 2300 species including 555 species of fungi, invertebrates, and lichen that depend entirely on oak. Compare this to the non-native horse chestnut, which has only 4 associated insect species, although non-native spruce species have 37. The high biodiversity found in native woodland is primarily thought to be due to the long association between the species, a co-evolution over millennia between host plants, and their predators, pollinators, and epiphytes. For example, as plants evolved more elaborate defence mechanisms to deter attack from phytophagous (plant-eating) insects, the insects evolved more specialised mechanisms to bypass them.
A multitude of empirical and theoretical studies have found that high species diversity within an ecosystem gives it more stability in the face of fluctuating environmental conditions. This is particularly true of assemblages of native species because such ecosystems demonstrate a high degree of complementarity, where there is a complex balance between species that have evolved alongside each other. In other words varying environmental conditions will favour different species at different times. This means that if one species is struggling, another with a similar ecological niche may be more successful and will keep the ecosystem functioning. A wide range of native species can even counteract the appearance of an invasive predator such as the Nile perch in Lake Victoria, although additional environmental pressures can lead to a tipping point that results in ecosystem collapse.
Ecosystems with high biodiversity are also about twice as productive as monocultures, and this is also true of forests globally, where timber production is higher in diverse forests. Although monoculture plantations are more efficient for harvesting, it is interesting to note that a diverse forest will grow faster and sequester more carbon. This means that there is an economic value to biodiversity for its own sake as well as for the resilience it provides to other ecosystem services, and it is essential that we preserve and restore our native biodiversity.
The UK is heavily dependent on non-native conifer trees to supply our timber requirements, and even then we import 81% of our wood products. These non-native trees grow faster than our native broadleaves and require less processing as they form dense stands of straight trunks. Monoculture plantations are less effective than diverse native woodland for carbon sequestration as they are associated with a shorter lifespan as timber, and lower levels of carbon absorption. They may also release carbon back into the atmosphere faster through their interactions with native insects and soil organisms. In terms of biodiversity, however, non-native forests can provide useful habitat for other species.
Studies looking at the number of species in a forest (species richness) have found that there is little difference between non-native and native conifer plantations or between conifer plantations and broadleaved woodland. Indeed some fungi and spiders are found in higher numbers in Sitka spruce plantations than surrounding native woodland. There are certain bird species such as crossbills, siskins, redpolls, goshawks, and crested tits who are conifer specialists and rely on non-native conifer plantations for habitat. Red squirrels are also reliant on Sitka spruce forests as grey squirrels cannot use them as habitat, so these forests provide a haven where the red squirrels can avoid disease transmission, although their preference is mixed conifer plantations. The diversity and abundance of bird species found in plantations is related to the structure of the forest more than the species of trees, and it is important to include new growth and a shrub layer around the edges of forest habitat.
Although non-native plantations can provide useful habitat for our native species, the way in which forests are planted and managed is critical to maintain biodiversity. Conifer plantations are associated with lower ground flora diversity but the density of the canopy has a greater effect on this than the species used. For this reason, the UK Forestry Standard stipulates that only 75% of larger forests can consist of 1 species. The remaining 25% should consist of 10% open ground, 10% other tree species, and 5% native broadleaved trees or shrubs. The UKFS also requires that forests be harvested selectively to create a diversity of age classes within a plantation. This age variation increases the diversity of beetle and bird species using the forest, and improves habitat for reptiles. Structural diversity within forests can be further enhanced by leaving deadwood standing, which provides habitat for birds, bats, and invertebrates. Due to their intensive management, conifer plantations have more standing deadwood than many of our native woodlands.
Non-native tree species may offer a solution to some of the issues caused by warming temperatures related to climate change. If UK temperatures become too warm then some of our native species may struggle to adapt and we may need to plant species that are sourced from warmer climates. There are risks associated with using non-native species, however, in terms of introduction of pests or disease. A comprehensive review concluded that although there is an argument for exploring the use of non-native trees in forestry (with strict control measures), there is no rationale for doing so where trees are required for conservation purposes. Another alternative is to use our native species but plant trees with seed stocks from warmer climates, although there is currently little evidence of the wider impact of doing this.
Both native and non-native tree species are needed to supply the ecosystem services required by the UK from woodland. From the evidence it is startlingly clear that when creating woodland it is imperative to consider the structure and maintenance of the woodland to maximise biodiversity and productivity. Our existing woodlands are also in urgent need of management work to improve their condition, increase biodiversity, and assure their long-term future in the face of climate instability.