Tree planting has been the most common woodland expansion strategy in the UK for more than a century, but recently there has been a renewed interest in incorporating natural colonisation as an alternative or complementary approach. But it is not always clear under what circumstances natural colonisation is likely to be and appropriate woodland creation method, or what the likely outcomes of this approach are and how do these compare to those of tree planting. In this article, we explore how, where and for whom these different strategies can be used to scale-up woodland creation on agricultural land.
Woodland expansion is widely regarded as a key part of the solution to counteract climate change and aid biodiversity recovery. This is because woodlands and trees can sequester carbon from the atmosphere and also provide habitats for many wildlife species. Expanding tree cover has therefore made its way to the forefront of the current environmental agenda, nationally and internationally. In the UK, tree planting has been the most common woodland expansion strategy for more than a century and is expected to continue at an accelerated rate in the decades to come, with the UK Government pledging to plant an additional 30,000 ha of trees per year up to 2050.
Tree planting is generally a quick and reliable method for establishing woodland. However, there are challenges associated with scaling-up tree planting at the level required to meet current targets, for example due to insufficient availability of locally sourced seeds/planting stock and a shortage of skills in the forestry sector. But planting is of course not the only way to create and expand woodlands, and recently there has been a renewed interest in incorporating natural colonisation – allowing trees to colonise new areas naturally – as a complementary strategy for large-scale woodland expansion in the UK (natural colonisation not to be confused with natural regeneration – the establishment of trees from seeds germinated in-situ – which is a key process underlying the long-term continuity of existing woodlands). However, we still have a poor understanding of when and where natural colonisation could be a viable and appropriate strategy for creating woodlands, and of what the relative benefits and drawbacks of these alternative strategies might be.
TO PLANT OR NOT
In general, planting seedlings/saplings is considered a quicker and more reliable method for establishing woodland than allowing an area to be naturally colonised by trees. The success of natural colonisation is largely context-dependent and strongly influenced by a variety of factors. One of these is proximity to seed sources such as nearby mature woodland or other trees; the density of naturally established trees typically peaks at 20-30 m from seed sources; it then progressively declines at longer distances up to ~150 m, being mostly dominated by wind-dispersed seeds of species such as birch and willow. Bird-dispersed seeds of species such as rowan and cherries can still colonise in large numbers at long distances from existing seed sources. Species with heavier seeds dispersed by gravity or mammals (e.g. beech) are usually slower to colonise. Local soil conditions to enable germination are another important factor; in general, natural colonisation is more readily achieved on sites with nutrient-poor and well-drained soils where competing vegetation is sparse (unlike agriculturally improved sites with fertile soils where competitive vegetation can hinder the germination and on-going growth of woody species). Herbivory pressure also plays an important role; whilst this is strongly landscape-dependent, in most instances, grazing and browsing pressure from livestock and other animals such as deer and rabbits will have to be substantially reduced at least during the early stages of natural colonisation to achieve sufficient levels of tree establishment [1,2].
In planted sites, canopy closure typically occurs within 15 years. But despite initial tree establishment happening relatively quickly, it can take decades or even centuries for planted woodlands to eventually develop some vegetation characteristics similar to those of mature ancient woodlands . The length of time for trees to establish via natural colonisation on abandoned agricultural land is far more variable. Some studies have shown that natural colonisation can happen relatively quickly, but that if it does not take place within the first 5-10 years then the process can be extremely slow, taking 30-70 years for a sparse and patchy cover of trees to establish. A recent study documenting the natural colonisation of two former agricultural sites in eastern England reported that vegetation establishment was relatively rapid, with 86% tree cover averaging 3m height within 23 years, and 100% tree cover averaging 13m height after 53 years, when the vegetation structural characteristics of one of the sites were approaching those of nearby ancient woodlands. However, the tree species composition of these naturally-colonised sites was still different, being dominated by clusters of animal-dispersed species such as oak and hawthorn and wind-dispersed species like ash occurring near adjacent woodland . In general, it is expected that pioneer species such as birch, willow and hawthorn are likely to dominate in the early years of natural woodland establishment, with a gradual transition to more shade-tolerant species as the site develops. In contrast, planting allows more control over the mixture of species, the density and spatial arrangement of trees present on a site from the onset.
THE RELATIVE BENEFITS
Existing evidence on the consequences of woodland creation in the UK is dominated by studies of coniferous plantation woodlands, and much less is known about the effects of planting semi-natural woodlands on agricultural land (which covers over 70% of the UK and holds the greatest potential for woodland creation). The limited evidence that we do have shows that the benefits of tree planting include enhanced habitat connectivity, biodiversity value, ecosystem service provision such as flood mitigation, and economic paybacks derived from timber or carbon credits. Knowledge about the ecology of naturally colonised woodlands is even more limited, apart from a handful of examples showing that tree densities are lower and habitat development slower than in planted woodlands. Despite the scarcity of scientific evidence, there is a fairly generalised assumption that naturally created woodlands will be more structurally diverse and ecologically complex than planted sites, which usually have a more uniform structure (e.g. with trees of similar ages and sizes planted in evenly-spaced rows). Natural colonisation also maintains local tree genotypes and high genetic diversity which can increase resilience to future changes in environmental conditions. Additionally, naturally established woodlands are more exposed to natural selection processes than planted trees which have often been commercially grown and kept in more controlled environments. For these reasons, natural colonisation is likely to enable local adaptation to climate change and confer higher resilience to other disturbances such as tree disease and pests.
Th benefits and challenges of planting and natural colonisation suggest that these alternative approaches are each likely to be better suited to particular objectives; for example, tree planting would likely be the preferred approach for land managers interested in timber production, whilst the more naturalistic appearance resulting from natural colonisation may be more appealing to those interested in restoring biodiversity and ecosystem functioning. But whilst there is often debate and polarisation between the use of these approaches, tree planting and natural colonisation exist at two ends of a continuum and may be used in complementary and blended combinations across a landscape, depending on the local conditions and the benefits expected. For example, some initial degree of planting can assist natural processes by enhancing seed dispersal and ameliorating the local environment; this can be done through low-density planting or by planting small clusters of trees (also known as nucleation planting) to provide perches for seed-dispersing birds and a future local seed source once the trees themselves reach seed-bearing age. In naturally colonised woodlands, supplementary or enrichment planting can also be used to increase stocking densities and to introduce species which are otherwise slower to colonise. Combining approaches will bring attributes of each method. The balance between approaches should depend on site factors and management objectives. The Woodland Trust – the largest UK woodland conservation charity – recommend to always include natural colonisation in woodland creation plans wherever possible, either as primary method or as a component alongside planting. However, much of the existing evidence on blended approaches is drawn from regions with quicker habitat successional rates such as the tropics, where landscapes have not been as heavily degraded as in the UK.
PERCEPTIONS AND OBJECTIVES
Attitudes of land managers towards woodland creation are central to meeting government woodland expansion targets, as most land is in private ownership and managed primarily to meet their objectives. However, we have a poor understanding of land managers’ attitudes towards woodland creation approaches other than tree planting, and it is not clear which kinds of land managers do, or would, engage with woodland creation through alternative approaches incorporating natural colonisation, and why. This is likely to be influenced by the objectives that different land managers hold for their landholdings and woodlands, by their cultural identities (what they see themselves as, e.g. conservation manager, farmer, forester), and by their level of understanding of the likely outcomes and risks associated with different woodland expansion approaches.
THE TREE_PLANAT PROJECT
TreE_PlaNat (Treescape Expansion through Planting and Natural Colonisation) is an interdisciplinary project comprising ecologists and social scientists at the Universities of Stirling, Edinburgh and Royal Holloway, and Forest Research, in partnership with the Woodland Trust and National Forest, charities at the forefront of practical woodland creation and management activities. Our overall aim is to assess stakeholder perceptions and socio-ecological consequences of woodland expansion approaches spanning the planting to natural colonisation continuum.
Specifically, in TreE_PlaNat we will explore the attitudes (e.g. perceived risks and benefits) of a diverse range of land managers towards woodland creation strategies spanning the planting to natural colonisation continuum, and explore how these vary with their objectives, socio-economic and ecological context. We will do this through a combination of broad surveys and in-depth interviews encompassing a diversity of agricultural land managers within the private (e.g. farmers, foresters and woodland managers), public (e.g. Forestry England) and third sectors (e.g. environmental NGOs).
We will also quantify various socio-ecological outcomes of woodland creation approaches spanning the planting to natural colonisation continuum, which we have identified as of key interest to agricultural land managers during our scoping work and in consultation with our project partners. These include for example biodiversity benefits, ecological function, resilience, people’s well-being and income potential. We will quantify these using a combination of state-of-the-art technologies (such as LiDAR and acoustic monitoring) and more conventional field surveying methods in 35 woodlands created in recent decades through a range of approaches: purely using natural colonisation, combining natural colonisation and planting, or predominantly through planting alone (we have identified these sites using a combination of historic maps, Woodland Grant Scheme datasets and aerial photography).
Finally, we will integrate our socio-ecological findings to better understand the links between land managers' perceptions and objectives, the approaches they use to create woodlands, and the socio-ecological outcomes derived from woodland creation spanning the planting to natural colonisation continuum. We will also deliver a coordinated timetable of activities and outputs – including webinars, blogs and capacity building events – to demonstrate how tree planting and natural colonisation can be used in combination to scale-up woodland expansion for a range of objectives on agricultural land. We will also assemble of a board of knowledge users to discuss knowledge needs, reflect on the relevance of our findings, provide feedback on planned knowledge exchange activities, and identify opportunities for quick wins and strategies for sustained impact.
Stay tuned to keep up to date with our journey!
CITED LITERATURE:  Forestry Commission (2021) Using natural colonisation for the creation of new woodland.  Herbert et al. (2022) Woodland creation guide, Woodland Trust.  Fuentes-Montemayor et al. (2021) The long-term development of temperate woodland creation sites: from tree saplings to mature woodlands. Forestry 95: 28–37.  Broughton et al. (2021) Long-term woodland restoration on lowland farmland through passive rewilding. PLOS ONE.