IAP2-23-108

Understanding the role of complexity and diversity in riparian woodland resilience to extreme events

Maintaining the quality of freshwater habitats is essential for the health and wellbeing of people and wildlife. Native woodlands within the riparian zone (riparian woodlands) play multiple roles in the functioning of freshwater ecosystems. For example, they help to regulate water temperature by providing shade, provide food for animals below through falling leaves and insects, prevent bank erosion, are important sources and storage sites for nutrients, and reduce flood risks (Cole et al. 2020).

Extreme flood and drought events are becoming increasingly frequent and severe due to climate change and pose a threat to the health and functioning of riparian and freshwater habitats (Lawrence et al. 2014). Degradation of riparian zones, for example by agricultural activity or drought, can create many challenges including erosion, sediment deposition, reduced water quality and a weakening of the ability of riparian zones to reduce flood risk downstream (Gumiero et al. 2013).

Diverse and complex communities are widely considered to be more resilient to disturbance and thus more likely to be able to withstand and recover from the pressures of extreme events while retaining their ability to perform important functions (Biggs et al. 2020; Isbell et al. 2018). However, although the relationship between diversity and resilience is well established theoretically and has been tested in a variety of systems, little is known about how this relationship works in riparian woodlands (Lovanovska et al. 2018). In this context, several measures are relevant: structural complexity – the variety of the spatial distribution of biomass in three dimensions which can include measures of tree basal area, canopy structure and leaf area, diversity – the numbers and relative abundance of species existing in an area, and functional diversity – the diversity of functional traits present in a community, functional traits being characteristics of species (e.g. leaf area, wood density) that affect their fitness.

Scotland’s Forestry Strategy 2019-2029 (The Scottish Government 2019) has a vision to expand forest and woodland cover and given their importance for biodiversity and flood risk reduction, planting trees next to rivers and streams is a major focus, with grants available to landowners to boost riparian planting. Working with our network of practitioners this project will inform best practice when restoring and planting riparian zones to ensure their long-term resilience and function.

The aims of this project are to quantify the effects of extreme events (flooding and drought) caused by climate change on riparian woodland resilience, to investigate the role complexity and diversity play in their resilience, and to understand the implications of this for riparian woodland restoration.

This project will answer the following research questions:
1) How does the functional diversity and structural complexity of riparian woodlands impact their resilience to extreme climate events?
2) What are the implications of extreme events for the long-term functioning of riparian woodlands in the UK (including impacts on other species and ecosystem services)?
3) How do restoration/planting strategies impact riparian woodland resilience and how can accounting for complexity and diversity improve the success of new and restored woodlands in terms of their stability and function?
4) How do riparian woodland managers perceive complexity and how can the outcomes of the project be best communicated and incorporated into future management strategies?

Methodology

This project will be carried out across central Scotland in collaboration with Loch Lomond and Trossachs National Park, Forth Rivers Trust and Forth Climate Forest.

Sites in central Scotland will be selected based on recent flood and drought events from climate and gauging station data and will be guided to include catchments with recent and planned afforestation/restoration work.

The student will employ the following methods:
1) Dendroecology – The growth of riparian trees and the impact of recent heatwaves and drought on tree growth will be studied using tree ring data and climate growth analyses.
2) Field surveys – Seedling and sapling density and growth surveys will be employed across a range of drought and flood conditions (with a varied time and intensity of recent events and sites including established, restored and newly planted riparian woodlands).
3) Field surveys –Structural complexity and functional diversity of woodlands will be quantified using field measurements including terrestrial laser scanning (TLS), hemispherical and drone imagery and biodiversity surveys (birds, ground flora, invertebrates).
4) Experimental work – Using species mixes in a greenhouse setting, the growth and mortality response to flooding and drought of sapling communities of different composition will be studied.
5) Modelling and GIS – Taking a landscape scale approach, connectivity modelling will be applied to understand the long-term stability and connectivity of riparian woodlands and the impact on flood prevention.
6) Surveys and interviews – These will be applied to understand perceptions of resilience and complexity, how these are understood and implemented by managers and what the implications of this are for riparian restoration and function.

Project Timeline

Year 1

Literature review and/or meta-analysis to assess riparian ecosystem health globally and to identify indicators of this, with collection of quantitative data if possible (months 1-4); Preparing for fieldwork, including initial consultation with managers and mapping study sites (month 5); Tree core collection (months 6-8); Field Studies Council ID course (month 8); Field surveys (month 9-10); Tree core preparation and analysis (months 11-13).

Year 2

Set up for experimental greenhouse work (14); Woodland manager surveys and interviews (15-16), GIS data collection and analysis (17-18); R training course (19); Field surveys (20-22); Data collection for glasshouse work (23).

Year 3

Data analysis and writing (24-26); Conference preparation and attendance at British Ecological Society Annual Meeting (27); Data analysis and writing (28-32); Field surveys (33-34), Data analysis and writing (35-36).

Year 3.5

Writing publications and thesis submission (months 37-42).

Training
& Skills

The student will receive training from an interdisciplinary supervisory team, including key skills noted by NERC as being most wanted:
• Fieldwork: this PhD includes a large fieldwork element, and the student will be become experienced in a wide variety of sampling techniques, including remote sensing-based fieldwork and the use of TLS and hemispherical imagery.
• Taxonomic Identification: to understand wider biodiversity impacts the student will learn identification skills for plants, birds and invertebrates, further supported by attendance on a field course.
• Data management, modelling and numeracy: the student will attend an R training course and will learn a variety of statistical, GIS and modelling techniques, including spatially explicit modelling and mapping, supported by the supervisory team and wider expertise at the University of Stirling.
• Translating research into practice: The output of this PhD will inform new riparian woodland planting and restoration schemes and the student will receive training in science communication to multiple audiences (e.g. policy makers, non-governmental organisations (NGOs) and the wider public).
• Interdisciplinarity: the project includes and integrates both biological and social sciences to inform riparian restoration/function/resilience and uses tools from multiple disciplines across different spatial and temporal scales.

References & further reading

Biggs, CR., Yeager, LA., Bolser, DG., et al. (2020). Does functional redundancy affect ecological stability and resilience? A review and meta-analysis. Ecosphere 11: e03184. 10.1002/ecs2.3184.

Cole, LJ., Stockan, J. and Helliwell, R. (2020). Managing riparian buffer strips to optimise ecosystem services: A review. Agriculture, Ecosystems and Environment 296: 106891.

Gumiero, B., Mant, J., Hein, T., Elso, J. and Boz, B. (2013) Linking the restoration of rivers and riparian zones/wetlands in Europe: Sharing knowledge through case studies. Ecological Engineering 56: 36-50.

Isbell, F., Cowles, J., Dee, LE., et al. (2018). Quantifying effects of biodiversity on ecosystem functioning across time and places. Ecology Letters 21: 376-778.

Lawrence, DJ., Stewart-Koster, B., Olden, JD. et al. (2014). The interactive effects of climate change, riparian management, and a nonnative predator on stream-rearing salmon. Ecological Applications 24: 895-912.

Lovanovska, I., Ferriera, MT., Segurado, P. and Aguiar, FC. (2018) Limited resilience in hotspots of functional richness: the Mediterranean riparian shrublands. Aquatic Sciences 80: 25.

The Scottish Government (2019). Scotland’s Forestry Strategy 2019-2029. https://www.gov.scot/publications/scotlands-forestry-strategy-20192029/

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