IAP-25-053

Understanding the ecological interplay between tree planting and natural processes to create future forests

Woodland expansion is the focus of many restoration efforts because of its potential to aid biodiversity recovery, mitigate climate change, and contribute to people’s livelihoods and well-being. In the UK, woodland creation schemes have contributed to increasing woodland cover from a historic low of 5% in the early 1900s to the current figure of 13%, and the UK Government continues to have ambitious targets for woodland expansion.

Historically, tree planting has been the most common woodland expansion strategy in the UK, but this approach is increasingly questioned following failures, overestimates of benefits and poor targeting [1]. Consequently, there is growing interest in incorporating ecological processes such as ‘natural colonisation’ (i.e. allowing trees to colonise new areas naturally) into large-scale woodland expansion strategies [2]. This is partly because creating woodlands through natural processes is expected to result in more structurally diverse, locally adapted and resilient (e.g. to tree disease and pests) woodlands of higher ecological value than planted sites [3]. However, the outcomes of natural colonisation can be variable, and there is still considerable uncertainty around the ecological processes involved [4].

But whilst there is often debate and polarisation between the use of tree planting and natural colonisation, these approaches may be used in complementary ways across a landscape, depending on the local conditions and the benefits expected [4,5,6]. Additionally, planting and natural processes exist at two ends of a continuum and can also be combined in ‘hybrid’ approaches. For example, some initial degree of planting can assist natural processes by enhancing seed dispersal and ameliorating the local environment [7]; this can be done through low-density planting or by planting small clusters of trees (also known as applied nucleation) 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. However, much of this evidence is drawn from regions with quicker habitat successional rates (e.g. the tropics) and where landscapes have not been as heavily degraded as in the UK, and it’s unclear how applicable this is to temperate regions. Therefore, many knowledge gaps remain, particularly around the drivers, timeframe and trajectory of woodland development and appropriate management practices of woodlands created through natural processes and hybrid approaches in temperate regions [4].

Key research questions: The overall aim of this studentship is to investigate how tree planting and natural processes interact, and how they can most effectively be used in combination to speed up woodland creation. In particular, the project will focus on understanding the driving factors and ecological mechanisms underlying successful woodland establishment across a gradient of planted to naturally colonised sites. Specific questions to address may include:
1) What is the relative importance of landscape (e.g. amount/configuration of nearby woodland and trees as seed sources) and local (e.g. herbivory pressure, competing vegetation) driving factors on the viability of using natural processes to create new woodland? When / where is a degree of tree planting likely to be needed?
2) What are the ecological mechanisms underlying natural colonisation and establishment in woodland creation sites, e.g. does the relative importance of zoochory vs. anemochory differ in different landscapes and is this reflected in the tree species composition? How does the movement and behaviour of animal dispersers and herbivores influence colonisation rates and establishment success across different landscapes and local conditions? Is a degree of planting likely to aid/disrupt these ecological processes?
3) How does woodland creation approach influence the timescales and trajectory of woodland development? E.g. Do woodlands established through hybrid methods recruit more trees / reach higher tree densities or more closed canopies quicker than woodlands created through natural processed alone? And is this influenced by drivers / ecological processes identified in 1 and 2?

Fieldwork will be conducted in woodland creation sites established through a range of methods encompassing planting, natural colonisation and hybrid approaches combining the two. These will include sites already identified by the TreE_PlaNat project (an interdisciplinary project investigating socio-ecological consequences of treescape expansion through planting and natural colonisation; www.naturalcolonisation.co.uk) and Forest Research’s Large-scale Ecosystem Recovery Network (LERN; a series of experimental sites designed to empirically test out different woodland creation approaches and management interventions). LERN sites were set up in 2024 and are at early development stages, with comprehensive baseline monitoring underway as part of a BACI experimental design to compare sites established at varying distances from seed sources and with/without herbivore exclosures. The TreE_PlaNat network consists of older woodlands (15-45 years since establishment) selected using a systematic site selection protocol to control the effects of former land use and distance to mature woodland. These two complementary networks of sites will enable the student to investigate different spatio-temporal aspects of woodland creation. Ecological surveys will be conducted to characterise the tree species composition and demographic structure of woodland creation sites, and of the landscapes around them (i.e. potential seed sources). Local conditions likely to influence seed establishment success (e.g. presence of competing vegetation, canopy openness) at woodland creation sites will be quantitively described. The abundance and species richness of potential zoochoric vectors (e.g. ants, birds and small mammals) and mammalian herbivores will be quantified using a combination of sampling methods (e.g. pitfall traps for invertebrates, point counts and acoustic recorders for birds, camera traps for mammals). The student will benefit from existing datasets of species occurrence and diversity of various taxonomic groups (e.g. plants and birds) which have been compiled by the TreE_PlaNat and LERN teams. There is also scope for the student to investigate ecological outcomes of woodland creation, such as biodiversity benefits and resilience of new woodland areas created through planting, natural colonisation and hybrid approaches.

Year 1

Literature review; training in experimental design and field surveying techniques; initial analysis of existing datasets

Year 2

Fieldwork to collect data on potential zoochoric vectors, herbivores and tree inventory data (e.g. species composition / relative abundance / demography); data processing; training in statistical techniques; initial data analysis of new datasets

Year 3

Fieldwork to collect additional data on zoochoric vectors (if required) and/or metrics of biodiversity/resilience; further data analysis

Year 3.5

Further data analysis; paper and thesis write-up

A comprehensive training programme will be provided comprising both specialist scientific training and generic transferable and professional skills, including:

1) Fieldwork and experimental design. Training in the required field skills (e.g. woodland surveying, invertebrate and small mammal trapping, camera trapping and acoustic monitoring techniques), and sampling design.
2) Numeracy, data analysis, ecological modelling, remote sensing and informatics. These skills will be mainly gained through targeted training courses within the IAPETUS consortium (e.g. Programming and Analysis of Environmental Data in R) or external providers (e.g. for advanced statistical techniques).
3) Land-use policy and management; the student will gain insights into understanding and formulating policy relevant research questions, skills for interdisciplinary research and the process of translating research into guidance and advice, working closely with CASE partner Forest Research.
4) Complementary training in transferable skills. Training in core scientific skills (data management, presentations, paper writing).

The PhD will be based at Stirling and the student will visit Forest Research (Edinburgh) and the University of Glasgow to meet with co-supervisors and other researchers for seminars and specific training.