IAP-24-061

The genetic diversity and adaptive potential of mountain birch in Scotland for high-altitude nature recovery

Globally, around two in five plant and fungi species are threatened with extinction as a result of increasing pressure from habitat fragmentation, emerging pests and pathogens, and climate change. Montane woodland habitats have undergone extensive anthropogenic degradation and fragmentation in recent centuries linked particularly to overgrazing by large herbivores. The contraction of these ecosystems makes populations more vulnerable to extinction by random events such as landslides or disease, and negative genetic effects such as reduced diversity and inbreeding. The threats to montane woodlands are of significance given their intrinsic biodiversity value and the associated services they provide , such as: the potential to mitigate the impacts of climate change, acting as a habitat refuge for rare species, stabilisation of slopes, and their vital role in flood mitigation.

Montane woodland is broadly defined as communities of tree and shrub species growing at high-altitude or in arctic-alpine regions, forming a mosaic between continuous cover woodland and more open vegetation such as grasslands and heaths. Birches (Betula spp.) are a key component in Europe, along with willows (Salix spp.), Pinus sylvestris, Juniper communis, and Sorbus aucuparia. In Scandinavia where mountain woodland is widespread, progression towards the altitudinal treeline reveals a gradual transition from silver birch (Betula pendula) to downy birch (Betula pubescens) and dwarf birch (Betula nana). In contrast, mountain birch in Britain is mainly found in tiny, scattered and fragmented populations of Betula pubescens in upland locations across Scotland, sometimes with Betula nana . These species are known to hybridise and are challenging to identify based on morphology alone (see Amphlett, 2021), though their different ploidy (number of complete chromosomes) makes them amenable to identification via cytometry. Previously the distribution of mountain birch may have been more extensive in Britain, prior to divergent land-use histories compared to Scandinavia – however to what extent this occurred remains unknown.

Genetic diversity within forests underpins their overall biodiversity. It is key to the survival, adaption and evolution of species under changing environmental conditions. Despite the importance of genetic diversity, relatively little is known about the amount and distribution of adaptive and neutral variation of montane woodland species in Britain, especially for birches. By studying how differentiated populations of the remaining mountain birch stands are in terms of their neutral genetic variation, we can explore whether this habitat may have been more widespread and contiguous in the past. This knowledge will inform land management regarding the need, or otherwise, to improve genetic connectedness. Testing for the presence of inbreeding in Scotland and comparing genetic diversity to that of Norwegian birch belt populations will inform restoration. Action in Scotland is seeking to revive a vibrant, diverse and resilient mountain woodland mosaic featuring birch that will tackle the nature and climate emergencies on a national scale, integrated with applied conservation genetics.

In situ field observations indicate that there is considerable variation in the morphology and phenology of birch trees (e.g. height, number of stems, leaf and catkin size and shape etc.) growing at different altitudes and in contrasting ground conditions. Such morphological traits are often used to distinguish between species; however, it is not understood if these traits are determined genetically through long term adaptation to the conditions of growing at high altitude or whether birch trees have a high degree of phenotypic plasticity, and their growth form is purely a response to the local growing conditions. Cytometry, ecological assessments and measurements of a common garden trial should help to inform these questions.

For mountain birch populations to be sustainable in the long term across their natural range and to survive the multiple stressors they face, their adaptive potential must be realised – a process that will inform forest and land managers regarding landscape restoration. Practitioners in upland nature recovery have questioned which sources of Betula pubescens would be most suitable for propagating towards future planting projects; for example if seed should only be collected from stands specifically adapted to the high-altitude environment, or whether a broader range of provenances could be included to improve genetic diversity, survival and growth rates. The molecular, morphological, physiological and ecological diversity between and within mountain birch populations needs to be quantified, to underpin management strategies with science.

This project will address the following key questions in relation to Scottish mountain birch populations:

• Are there genetic and morphological differences between higher and lower altitude populations?
• What is the state of genetic diversity?
• How much adaptive diversity is present and how is it distributed?
• What is the role of environmental and ecological drivers in determining population differences in adaptive variation?

This PhD project will work alongside the Mountain Birch Working Group (MBWG), a subgroup of the Mountain Woodland Action Group (MWAG). The group was established in December 2023 to guide, conduct, collate and disseminate research on Mountain Birch and birchwoods in Scotland. Members consist of ecologists, conservation organisations, plant scientists, land managers and tree growers all of whom have a collective interest in high altitude birch. The close collaboration with the MBWG will ensure that the results of this study and their implications will be integrated into future policy and practice. The project will focus on two of the three birch species in Britain i.e. Betula pubescens and Betula nana.

1. Morphological identification of the two birch species and their hybrids can be problematic and efficient flow cytometry methods are now available to distinguish between the tetraploid Betula pubescens and the diploid, Betula nana. The UKCEH partner is experienced in the use of this method.
2. Intra and inter-population differences in neutral genetic variation will be assessed at the RBGE using microsatellite and/or single-nucleotide polymorphism (SNP) markers.
3. Variation in a range of adaptive traits (e.g. height, phenology) in high and low altitude Betula pubescens will be assessed in a pot based common garden trial established at Forest Research in advance of the project start. These plants will be authenticated to be Betula pubescens using cytometry and will eventually be established along an altitudinal gradient as a reciprocal transplant trial. Forest Research and UKCEH have considerable experience in the methodology for this area of work. NTS and Corrour will assist in setting up reciprocal transplant trials.
4. Fine scale local distribution of the two Betula species and introgressed hybrids will be determined, including detailed evaluations of the local environment. Individual plants will be mapped and sampled for genotyping and cytotype determination via flow cytometry. Their seed production and viability will also be assessed.

Year 1

1. Develop a detailed project plan
2. Literature review
3. Carry out assessments of adaptive traits in glasshouse grown high and low altitude Betula pubescens progenies at Forest Research facilities.
4. Finalise reciprocal transplant experiment design and establish common gardens (high and low altitude) in collaboration with CASE partners.
5. Identification of an appropriate high altitude study area where B. pubescens and B. nana exist in proximity and selection of ecological traits to measure across the study area.
6. Complete leaf sampling fieldwork of high altitude Betula pubescens individuals for genotyping.
7. In autumn 2026, assess survival in high and low altitude reciprocal transplant trials
8. Gain understanding of laboratory methods for cytometry and genotyping.

Year 2

1. Genotype all collected individuals at Royal Botanic Gardens Edinburgh (RBGE) laboratory.
2. Assess adaptive traits in high and low altitude reciprocal transplant trials.
3. Conduct ecological site assessment where the two birch species coexist.
4. Fieldtrip to high altitude study sites in Norway to compare ecological conditions in high altitude birch populations in Scotland and Norway. Collect ecological data at sites in Scotland and Norway.

Year 3

1. Analyse molecular data.
2. Assess adaptive traits in high and low altitude reciprocal transplant trials.
3. Analyse data on adaptive traits in and low altitude birch progenies.
4. Analyse ecological data.
5. Present findings at a relevant conference.

Year 3.5

1. Thesis write up.

The student will receive training in the following methods: cytometry, DNA extraction and molecular genotyping, assessment of adaptive traits, ecological site assessment. Full training will be provided in collection and analysis of molecular, ecological and adaptive trait data and in scientific writing skills.