Trees are one of the most effective current solutions to absorbing excess carbon dioxide (CO2) from the atmosphere and helping us to limit global warming to the 1.5°C target set as part of the Paris Agreement. There is international recognition of the need to protect our existing forests and expand forest cover to sequester more carbon and this has led to a plethora of tree planting campaigns such as the Trillion Trees initiative and the Bonn Challenge. The declaration of a climate change emergency has led to a prioritisation of fast-growing species and a focus on planting a defined number of individual trees. This is not creating diverse forests, however, and recent evidence suggests that more natural planting is better at sequestering carbon, or even that natural forest regeneration could form a major part of our tree planting requirements. Examining the different methods of planting trees can help us appreciate the scope of the afforestation challenge ahead and how we can maximise the resources and space available.
Allowing woodland to regenerate naturally can be an effective technique to create new forest, but it does rely on there being a source of tree seeds in the area. Given that seed dispersal mechanisms vary between different tree species, it is important to incorporate seed dispersal strategies into any woodland planning. Species of birch, willows, elms, field maple, limes and ash rely on dispersal by wind (anemochory) and the seeds can travel a maximum of 100 to 200 metres. Other species such as beech, hazel, holly, oaks, wild cherry, and yew have heavier fruits that rely on dispersal via mammals or birds (zoochory), with an average maximum dispersal distance of 20 metres. This means that colonisation by trees will be slow or non-existent if there is not a nearby seed source from mature hedgerows or existing woods. In order to achieve a similar tree density to managed planting, it is safest to assume a seed dispersal distance of one to two tree heights, meaning that woodland edges are the most suitable area.
In some years there is a tenfold increase in the amount of seed produced, resulting in a seed fall of millions per hectare, which can substantially increase the speed of natural regeneration. These ‘mast years’ occur every three to five years in most broadleaved species and the abundance of seeds can overcome the predation pressure from mammals which can otherwise dramatically reduce the success of natural regeneration projects. Effective natural regeneration projects require a receptive seedbed, such as moss or thin litter and matted grass, bracken, heather, or bramble can slow down the process.
The advantages of using natural regeneration as a reforestation technique are that it is easy, that growth can be fast, that it is cheap compared to planting projects, and that it results in a woodland with locally adapted trees and good structural diversity. The disadvantages are that the success is unpredictable, that it can only occur in close proximity to existing woodland, that high seed predation can dramatically slow the process down, that it can result in low genetic diversity if there are only a few parent trees and that the pioneer species can dominate and outcompete even planted trees.
Natural regeneration of forests relies on an ecological process called succession, where land is naturally colonised by plant species to reach a notional stable or climax ecosystem. In the UK the climax vegetation would be mostly broadleaf deciduous woodland, except in the Highlands of Scotland where it is scots pine and juniper. The development of forest on any area with existing soil, such as agricultural land, is termed secondary succession. The process starts on bare soil with colonisation by grasses, then follows the establishment of scrub (e.g. hawthorn and blackthorn) and pioneer saplings whose seeds disperse on the wind and are capable of germinating in full sunlight. Once the pioneer species such as willows and birches get taller, the canopy closes and the shade-tolerant species start to develop, outcompeting the pioneer species.
When the pioneer trees die, the shade-tolerant species replace them, leading to mature woodland. A similar process can be seen when there is a break in the tree canopy of established woodland when a tree dies or is felled. Saplings of the light-loving pioneer species regenerate quickly from seed and root suckers, the herbaceous ground vegetation also flourishes and benefits from seed dispersal via birds perching on saplings, the canopy closes again and the shade-tolerant species start to develop.
It is possible to create woodland by planting seeds directly into prepared soil, a technique which is commonly used to accelerate reforestation after wildfire damage. It is a cost-effective solution that can be mechanised by using machines or drones to sow seed or even by using trained dogs to distribute high densities of seed. Local trees can be used as a source for the seed, to ensure that trees are of a local variety and adapted to the environment. In a temperate climate the seeds need to be exposed to cold weather to break their dormant state, so can be sown either in autumn or pre-treated with cold and sown in the spring. The percentage of seeds that will survive to form seedlings at the end of the first year is highly variable and nurse crops such as wheat or barley are sometimes used to protect the growing seedlings from competition from weeds. Predation of seeds by birds and small mammals is a considerable problem and direct seeding is not recommended near existing woodland for this reason. Fencing is often needed to protect the growing seedlings from rabbits and deer.
Direct seeding does have advantages, among them early canopy closure at 3 to 5 years, as opposed to 10 years in a plantation. The trees grow faster and the forest develops more quickly as a result. There is a lower planting cost compared to transplantation and the dense saplings reduce the need for herbicides. The timber is also better quality as the saplings grow straighter. The primary disadvantage is unpredictability and complete reseeding may be required if it is unsuccessful due to high predation or seed rotting on wet soils. Only oak, ash and sycamore have been extensively tested in the UK, so other species may not be suitable for this method.
The most common afforestation technique is using bare-rooted seedlings or transplants that are grown in a nursery and then lifted and transferred to site. The young trees are usually 1 to 3 years old and are supplied as ‘whips’ of 40 to 60cm tall, ‘feathereds’ of 80 to 150cm or ‘standards’ of 200 to 400cm. The advantage of using younger whips is that they are less expensive and easier to transport, but using older trees jump starts the woodland creation process if they are handled carefully. The larger standards may need to be supported with stakes. The trees are first grown in seedbeds in nurseries and transferred to fields, then the roots of the trees are often undercut during the lifting process to ensure that they have a compact root ball. They may be dipped in a mycorrhizal treatment to accelerate the development of the fungi mycelium network that is a fundamental part of a thriving woodland ecosystem.
The trees are lifted, wrapped in plastic bags to preserve moisture, and shipped. The trees must be handled very carefully to avoid desiccation and damage to roots and are planted by trained tree planters, who can plant up to 4000 saplings a day. Tree planters have purpose designed spades and planting bag harnesses that allow them to carry hundreds of young trees at a time. Trees are normally planted during their dormant period between November and March because they are less likely to get damaged.
The density of trees varies between sites and depends on the aims of the project and the species of tree being planted. Conifer plantations usually have a density of 1100 (3 x 3m spacing) to 2500 (2 x 2m spacing) trees per hectare, but broadleaves can vary from 900 to 4500 per hectare for oak to 1500 to 6000 for beech. The standard spacing is 2m for conifers and 2.5m for broadleaved woodland, giving 2500 and 1600 trees per hectare respectively.
There are three main techniques for planting the tree in the ground, pit planting, slit planting and T notch planting. Pit planting involves digging a shallow area and placing the tree in the hole then replacing the soil. It has the advantage of ensuring maximum contact between roots and soil but is a slow technique. Slit planting is what professional tree planters use and is extremely fast and suitable for most types of ground. The tree planter paces out the necessary gap, selects a good planting site, inserts their spade, levers it to create a narrow slit, drops the young tree in and presses down with their foot to close the hole. T notch planting is effective on grass and involves making an initial cut with the spade, a second cut at right angles to form the T shape, levering from the original cut to bring the turf upwards and placing the tree in the gap between the two chunks of turf. It is then gently levered back down. In all cases the collar (i.e. the mark on the stem showing the depth to which the tree was buried previously) must be level with the top of the soil.
Newly planted saplings require protection from mammals including voles, rabbits, and deer as they will all damage bark or eat leaves. Grey squirrels also pose a serious threat to trees over 7 years old as they strip the bark, damaging and sometimes killing the tree. The most common form of protection against smaller mammals is tree guards or tree shelters, which come either in spiral or tube form. Spiral guards are wrapped around the tree to reduce bark stripping, primarily by rabbits and voles, and a cane may be used to provide support. Tree shelters are transparent polypropylene tubes that are placed over the growing tree for protection but also alter the microclimate around the tree, raising temperatures to promote growth. Tree shelters can increase survival of the saplings to 89%, offsetting the initial extra cost of their use. Young Trees also need to be protected from deer, who can cause devastation and decimate hundreds of trees in a plantation overnight. Tree guards offer some protection, but it may be necessary to install deer fencing to exclude them from the area.
Although using guards significantly increases survival, trees also need protecting from competition with weeds and grasses. Weeds compete for light, moisture, and nutrients with the growing trees until their root system is established, inhibiting growth, and decreasing their survival chances. The most effective point to clear the area around the trees is mid-May to June, when the trees are growing at their fastest rate, and it is commonly carried out for up to 5 years after planting. The area weeded around each tree can be as small as a 1m diameter circle around each tree. Mulches, protective sheeting and mowing can assist in hindering weed growth and improving survival rates, as can using dense planting techniques such as the Miyawaki Method.
The process of replacing trees that have died in the years following planting is referred to as ‘beating up’. In woodland planted for nature this is not as critical as in plantations where a contracted number of trees is required. The replacement trees can be at a disadvantage so often older saplings are used. In order to create a varied canopy structure, it may be necessary to carry out ‘cleaning’ if the focal trees are being crowded out by the naturally regeneration of shrub or pioneer species, although this is not usually necessary when creating woodland for wildlife. Thinning by removing individual trees can also aid in creating a varied canopy or opening up clearings to increase ground flora and encourage butterflies. It is usually carried out when the canopy has closed and tends to remove trees that are not thriving. The deadwood from thinning processes can be left onsite to provide habitat for invertebrates and fungi.
It is clear that there are distinct situations where reforesting with natural regeneration as opposed to manual tree planting can be effective. Natural regeneration works along woodland edges and particularly in mast years such as 2020. Although the mechanism that determines what triggers a mast year is not entirely clear, they are common across different forest biomes, occur at predictable intervals at the population level and are thought to be linked to warm, dry spring conditions. Mast years may be a mechanism to control predator numbers (the predator satiation strategy), restricting their food source in most years and providing a superabundance in the mast years to maximise seedling survival prospects.
In terms of planting trees to address climate change, a recent paper highlighted that the estimate for the global potential of natural forest regrowth to capture carbon may have been underestimated by 32% and that growth rates of secondary forest are also faster than thought. Allowing forest to regenerate naturally could sequester another 23% of global CO2 emissions without impacting on food production, on top of the 30% that forests currently absorb. If Brazil were to cease clearing of secondary forest and allow it to regenerate, they could meet their commitments to restore 30 million acres of forest with no additional planting. Secondary forest can establish itself remarkably quickly and the tree biodiversity of secondary tropical forest regrowth recovers to 80% of primary forest after 20 years, although it takes centuries to match the species composition. Natural reforestation could, therefore, be a powerful tool to fight climate change and restore biodiversity.
The scope of the climate and biodiversity crises necessitates urgent action, however, and rapid reforestation of areas away from existing forest or unsuitable soils requires tree planting or direct seeding. The UK is aiming to create 30,000 hectares of forest per year as part of climate action targets and these scales will require forest to be created by planting. There is little rigorous evidence of the advantage of natural regeneration over tree planting, as there have been few comparative studies and there in an inherent bias in the literature when choosing natural regeneration sites. Plantation sites tend to have poorer soil conditions and can require more intervention prior to planting. Tree planting techniques are based on extensive evidence by research agencies such as Forest Research in the UK and designed to increase sustainable harvesting of timber and other resources. Recently there has been a renewed focus on increasing biodiversity and maximising carbon sequestration in the UK, resulting in an increase in broadleaf planting.
Tree planting mitigation for climate change globally has previously resulted in large monoculture plantations, but an increasing awareness of the value of biodiversity for both ecosystem functioning and increasing carbon sequestration is encouraging more diverse planting of native species. Combining ecologically sound tree planting with natural regeneration where possible has the potential to restore our wild spaces, help fight climate change and create thriving forests for the future.