IAP-24-088

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

‘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 has pledged to plant an additional 30,000 ha of trees per year up to 2050.

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 it is assumed that naturally created forests will be cheaper, better adapted to local environmental conditions and more resilient (e.g. to tree disease and pests) than planted sites. Naturally colonised sites are also perceived to lead to more heterogenous and complex woodlands of higher ecological value than planted woodlands [3]. 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.

Current evidence indicates that both tree planting and natural colonisation have the potential to play important complementary roles in woodland cover expansion as alternative strategies [4,5,6]. 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 [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, there is a need to understand how tree planting and natural colonisation processes interact, and how they can most effectively be used in combination to speed up woodland creation and the accrual of biodiversity benefits and resilience to environmental change.

Key research questions: The overall aim of this studentship is to investigate the interplay between tree planting and natural colonisation. In particular, the project will focus on understanding the ecological mechanisms underlying woodland establishment, as well as the ecological consequences of woodland expansion through i) planting, ii) natural colonisation, and iii) ‘hybrid’ approaches that combine attributes of each method. Specific questions to address may include:
1) What is the relative importance of landscape (e.g. seed source availability) and local (e.g. herbivory pressure, competing vegetation, site exposure) 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 woodland expansion in woodland creation sites along a natural-to-planted gradient, e.g. does the relative importance of zoochory vs. allochory differ and is this reflected in the tree species composition?
3) What are the key determinants of the movement and behaviour of animal dispersers and herbivores in a landscape, and how do these affect natural colonisation processes?
4) How does woodland creation method (i.e. planted / natural / hybrid) affect the biodiversity value, ecological function and resilience of woodland ecosystems?

Fieldwork will be conducted in woodland creation sites established through a range of approaches encompassing planting and natural colonisation methods (and a combination of these). These will include sites already identified by the TreE_PlaNat project (an interdisciplinary project investigating stakeholder perceptions and socio-ecological consequences of treescape expansion through planting and natural colonisation; https://www.naturalcolonisation.co.uk/) and Forest Research’s Woodland Creation & Expansion group, following a systematic site selection protocol to control the effects of woodland age and proximity to seed sources (i.e. distance to mature woodland). Ecological surveys will be conducted to characterise the tree species composition and demographic structure of woodland creation sites and the landscapes around them (i.e. potential seed sources). 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, moths and bats) which have been compiled by the TreE_PlaNat team. There is also scope for the student to investigate how other woodland biodiversity (taxonomic group(s) of their choice) utilise new woodland areas created through planting, natural colonisation and hybrid approaches. Additionally, remote sensing methods and datasets of canopy dynamics will be combined with tree inventory data to investigate how weather, tree species composition and woodland structure interact to determine the resilience of woodlands along a natural-to-planted gradient to expected future droughts.

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 species composition / relative abundance; data processing; initial data analysis of new datasets

Year 3

Fieldwork to collect additional data (e.g. to include other biodiversity groups); remote sensing data processing; 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 Newcastle University to meet with co-supervisors and other researchers for seminars and specific training.