Natural colonisation occurs when tree seed reaches a site and establishes where woodland has not recently existed. This differs from natural regeneration where new trees establish within existing woodland or where woodland has recently been located.
Supporting woodland expansion through such natural processes, is another method with potential to contribute to current tree cover establishment targets. Natural colonisation has several potential benefits including reduced costs, increased biodiversity value and natural matching of trees/genetic stock to sites (Hutchings and Quine, 2021). However, evidence of the full range of outcomes associated with natural colonisation across different sites, and over different timescales is limited. Interdisciplinary studies investigating and quantifying the benefits and dis-benefits of natural colonisation, alongside the attitudes of landowners and managers towards this expansion strategy are lacking.
Ecology of natural colonisation
Interdisciplinary fieldwork aims to establish a robust methodology to assess the efficacy of ‘natural colonisation’ as an approach to establish new woodland and its impacts on biodiversity and above and below ground carbon.
Key questions being evidenced are:
- Which tree species have colonised and established via natural colonisation?
- What is the density and height of the colonising trees at different spatial scales across the sites?
- What is the corresponding impact of natural colonisation on above ground biomass and below ground carbon?
- Is there an impact of adjacent or nearby tree planting, established at a similar time to the natural colonisation, on the colonisation and establishment processes?
Methodology
Areas of natural colonisation established within agriculturally dominated landscapes and initiated with the previous 10-20 years were selected. Each of the sites contained three distinct areas of woodland:
- an area of mature existing woodland (as a potential source woodland)
- an adjacent area of natural colonisation
- an area of supplementary tree planting (planting adjacent to natural colonisation) or control planting (planting nearby, but not adjacent to the natural colonisation).
At least three rectangular transects were used to capture spatial variation across the sites. All sampling took place within these transects: the whole transect was divided into sub-transects for terrestrial laser scanning and photogrammetry flights (results to be presented in the next phase of the project) and plots positioned along the transect were used to manually sample tree size and species, as well as other vegetation cover. Soil samples were taken across the same transects.
Findings and Recommendations
Preliminary findings year 1 (2021/22)
Trees appear to be colonising the studied sites in three main ways, related to tree dispersal mechanisms, potentially ground preparation and browsing pressure:
- Steadily creeping out from adjacent mature woodland as hypothesised e.g. oak and beech, but especially evident at one site where the site was seeded to grass from arable and there is very low herbivore pressure.
- Wind dispersed species dominating sites e.g. willow and ash. Willow is particularly dominant in areas with low competition from other species such as grass.
- Vector dispersed species e.g. blackthorn appearing not only adjacent to mature woodland but also influenced by nearby tree planting.
However, this first year pilot study has demonstrated the high degree of variability within and between the sites which need to be investigated further to be able to provide generalisable evidence.
Latest Updates
Work in year 2 (2022/23) will expand the sample of sites to include a wider range of ages and starting land use.
Social dimensions of natural colonisation
This research aims to understand why different kinds of land managers may or may not be likely to utilise natural colonisation as a tree expansion strategy.
Work conducted in 2021/22 characterised different land managers, their understandings of and attitudes towards natural colonisation, barriers to uptake, and support and information needs.
The research found variation in how natural colonisation is understood by different land managers, as well as in the language used to refer to it as an expansion strategy. Further, it highlighted land managers’ need for knowledge and support in the form of successful examples, guides on best practice, and information about grants.
Following on from these findings, work is being conducted in 2023/24 to explore the language and messaging surrounding natural colonisation, to support more effective engagement and uptake of natural colonisation with different kinds of land managers. Work is also underway to identify which incentive schemes supporting natural colonisation have or have not been taken up by which kinds of land managers and the reasons behind this, to inform future scheme design and delivery.
This is being achieved by addressing the following questions:
- How can language and messaging promoting natural colonisation to different kinds of land managers (including farmers and foresters) be more enfranchising and engaging?
- Which natural colonisation grants (e.g. EWCO, Woodland Trust, Community Forests) have different land managers taken or not taken up?
- What factors have impacted natural colonisation incentive uptake, e.g., message framing, communication strategies, quality of delivery and the influence of advisors had on natural colonisation grant uptake?
Methodology
- Workshops with stakeholders to validate land manager characterisation; to understand language and communication around natural colonisation; to understand grant landscape
- Semi-structured interviews with different land managers to further explore language, messaging, awareness and engagement with grants
- Thematic and content analysis of data using NVivo software
Findings and Recommendations
Year 1 Research
Objectives
In 2021/22 the research aimed to understand and characterise different kinds of land managers and why they may or may not be likely to utilise natural colonisation as an expansion strategy. It achieved this by answering the questions:
- What are land manager attitudes to, and perceptions of the risks and benefits associated with natural approaches to woodland creation and expansion?
- In what contexts do land managers believe natural regeneration/colonisation could be a successful method for creating woodland and adapting to a changing climate and other pressures?
- Are there any specific barriers to uptake of these approaches including uncertainty and risk perception, knowledge gaps, timescales, and unmet support needs?
Findings and Recommendations from year 1
The majority of land manager across all types understand natural colonisation as an effective strategy for the provision of biodiversity/natural habitat and nature restoration, rather than to achieve other benefits of woodland creation (e.g. carbon sequestration, timber production).
Land managers were found to either select sites for natural colonisation close to existing areas of woodland, or undertake ‘enrichment’ planting of trees into areas of natural colonisation, as a strategy to spread risk and ensure their objectives for their land were achieved.
Land managers and their advisors require tools and guidance, particularly to support site selection and assess the likely outcomes of natural colonisation processes, relevant to their land holding type and landscape context.
Latest Updates
Research in 2023/24 will expand on initial work further understanding those factors affecting different land managers decisions to use natural colonisation as a viable woodland expansion strategy.
Soil and carbon science of natural colonisation
A review of the literature found very little information on how carbon in woodland natural colonisation areas compares to planted areas of similar age. This part of the wider study set out to provide estimates of both tree biomass and soil carbon on natural colonised sites compared to adjacent tree planting of similar age.
Methodology
Areas of natural colonisation established within agriculturally dominated landscapes and initiated with the previous 10-20 years were selected. Each of the sites contained three distinct areas of woodland:
- an area of mature existing woodland (as a potential source woodland)
- an adjacent area of natural colonisation
- an area of supplementary tree planting (planting adjacent to natural colonisation) or control planting (planting nearby, but not adjacent to the natural colonisation).
At least three rectangular transects were used to capture spatial variation across the different areas. All sampling took place within these transects: the whole transect was divided into sub-transects for terrestrial laser scanning and photogrammetry flights (results to be presented in the next phase of the project) and plots positioned along the transect were used to manually sample tree size and species, as well as other vegetation cover. Soil samples were taken across the same transects.
Research Objectives
- To quantify the above ground and below ground carbon sequestration potential of woodland expansion through natural colonisation
Latest Updates
Preliminary findings and Latest updates to June 2022
Soil samples have been collected and are currently being analysed. Vegetation biomass has been quantified along each transect using traditional methods and terrestrial laser scanning and photogrammetry flights have been undertaken
Findings and Recommendations
The research began in 2021/22. A review of existing evidence was completed in 2021 and indicated that:
- Where existing mature woodland and hedgerows or isolated trees provide suitable seed sources and the site conditions are favourable, then natural colonisation provides the potential to expand tree cover relatively quickly (within five years). In other circumstances, there is a risk of little apparent colonisation progress in the short to medium term and the result can be a scrubland rather than woodland habitat. Therefore, natural colonisation depends primarily on choice of site, ground conditions and suitable seed source.
- Natural colonisation by trees and scrub is progressive and, as well as contributing towards increased woodland cover, it can potentially provide complex habitat mosaics of high biodiversity value. The projected biodiversity gains from natural colonisation processes (after first excluding grazers) include highly diverse transitional habitat that could be expected to last for 20 years or more.
- There may also be benefits to long-term carbon sequestration or storage. However, these are yet to be quantified. The unpredictability of natural colonisation may result in it being judged as less reliable in achieving rapid carbon storage when compared to tree planting or direct seeding systems.
Other results from the project are described for each of the science workstreams.
defra
Forestry Commission