On a sweltering Thursday in June, we headed to the Hall Farm Estate on the southern fringe of Dartmoor. We were a small team of two social scientists and one ecologist from Forest Research.  Our visit was an opportunity to engage with colleagues from the Forestry Commission and The Woodland Trust on the topic of natural colonisation.

Chauffeured by a Forestry Commission colleague, we weaved through country lanes, admiring overhanging trees and rolling hills before arriving at a picturesque farmhouse. The farm and its woodland were steeped in history, having been home to The Woodland Trust’s founder Ken Watkins during the Trust’s inception. In recent history the farm had been in private ownership, but has this year been bought back by the Woodland Trust. Dave Rickwood, The Woodland Trust’s site manager for Dartmoor, explained that the Trust are looking into an England Woodland Creation Offer (EWCO) grant to undertake natural colonisation on the Hall Farm Estate.

Before heading out to see natural colonisation beside the estate, we presented some of our research to date, and introduced questions we will be exploring over the next year. As well as the importance land managers place on outcome uncertainty when weighing up whether to use tree planting or natural colonisation as tree cover expansion strategies, we landed on the importance of language and communication. Recognising that uptake of grants for natural colonisation has been low, and that natural colonisation as a term is not commonly used by land managers, we asked attendees on the visit to consider how language and communication might be impacting the tree expansion strategies taken up by land managers they work with. With this provocation in place, the group of attendees, as well as three eager dogs, headed out of the barn to see some trees! 

We continued on up to the fringe of Dartmoor national park and Dave shared an interesting finding with us from a researcher at Plymouth University. This professor has estimated that for woodland expansion to occur on the commons of Dartmoor, 8 years of no grazing would need to be secured. Grazing pressure is just one of the many things landowners weigh up when deciding on woodland expansion strategies. 

Having waited in anticipation, Kevin explained to the group how long it had taken natural colonisation to reach this level of maturity. Using maps and aerial footage from the post-World War Two period, the team of spatial scientists at Forest Research had estimated that natural colonisation had taken about seventy years to reach its current state. We all agreed that while the tree cover was impressive this was quite a long time! The time it takes for natural colonisation to establish has emerged as an area of concern across land manager groups and this example certainly spoke to concerns that establishment could take longer than a human lifetime. That said, standing among the plush trees you couldn’t help but be struck by the diversity and beauty of this naturally colonised space and see the appeal of using such strategies.
 
We walked back up to the house, discussing our research and planning to follow up with some new faces we had met. It was a stimulating and thought-provoking day out, one that sparked conversation in the car back to Plymouth around what kind of site we might like to see next. We wondered about a visit to a site where natural colonisation has been given the green light but is yet to begin. Or, we thought it might be interesting to visit a site where natural colonisation had failed. We sunk back into the warm seats of the car and decided it was a decision for another day. 

​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
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].
 
WAITING TIMES
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 [3]. 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 [4]. 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.

BLENDED APPROACHES
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.  ​

​CITED LITERATURE: [1] Forestry Commission (2021) Using natural colonisation for the creation of new woodland. [2] Herbert et al. (2022) Woodland creation guide, Woodland Trust. [3] Fuentes-Montemayor et al. (2021) The long-term development of temperate woodland creation sites: from tree saplings to mature woodlands. Forestry 95: 28–37. [4] Broughton et al. (2021) Long-term woodland restoration on lowland farmland through passive rewilding. PLOS ONE.