Planting decisions for mitigation and adaptation

Claire Thirlwall

Claire Thirlwall is a director of Oxfordshire-based landscape practice Thirlwall Associates. Her book “From Idea to Site: a project guide to creating better landscapes” for RIBA Books was published in January 2020.

As part of a regular series, chartered landscape architect and author Claire Thirlwall explores tools and guidance available to help our professional understanding of this issue’s topic.

It has been surprisingly difficult to find useful resources for landscape architects for this topic, something I hadn’t anticipated when I started my research. Most of the guidance I’ve found is from outside our sector, showing that we have issues in common with forestry, horticulture, agriculture and geology, and that they are ahead of us in this area. To make informed decisions, and to persuade clients of the value of new techniques, we need accurate and relevant data.

Beech Gardens and The High Walk, Barbican, London (1)

The Barbican – the Brutalist arts, conference and housing complex built on a site devastated by bombing of the City of London during World War Two – is a challenging site for planting. Most of the landscape areas are “podium landscapes”, above street level with uses beneath. The surrounding tall buildings create shade, and the raised locations limit soil depths.

View of the new planting.

© Nigel Dunnett

These constraints make the planting design by Nigel Dunnett, Professor of Planting Design and Urban Horticulture (and recently elected Fellow of the Landscape Institute) all the more impressive. The design was created to require low levels of irrigation, with species selected to deal with future climate change.

The design is made up of three designed plant communities to match the different microclimates around the site – steppe planting, shrub steppe and light woodland. The planting mixes are not recreations of natural plant communities, but they are selected to recreate the processes of natural or wild plant ecosystems. The planting is designed for year-round interest and seasonal change (important considerations for a residential site), and to provide colour and visual delight.

Barbican Plan.

© Nigel Dunnett

The scheme was awarded the ‘LI Fellows’ Award for Creating Healthy Places’, and the ‘Planting Design, Horticulture and Strategic Ecology Award’ at the LI Awards 2018.

Planting decisions and carbon

Carbon is a major component of plants – they take, or sequester, atmospheric carbon dioxide and convert it into the sugars that make up their leaves, stems and roots. Once a plant dies and decays that carbon is released back into the atmosphere. Plants on land have taken up approximately 25% of the carbon dioxide that humans have emitted. (4) When selecting plants and trees, volume and lifespan are two main factors to consider – a large long lived tree will retain more carbon than a short lived small perennial. If a tree is felled, the carbon will be retained.

Sequoiadendron giganteum or Giant Redwood – 1 acre of old-growth redwood forest can store up to 890 tonnes of carbon (2) the equivalent of driving 8 million miles in an average passenger car. (3)

© iStockphoto

Guidance on plant selection by carbon sequestration is an area I’ve not found data for, but the new UK Woodland Carbon Code (WCC) provides some information that may help with calculating the potential carbon sequestration for woodland planting. (5)

The WCC is the new standard for forestry projects for the mitigation of climate change, which allows woodland owners to estimate the carbon stored to allow carbon accounting, with the potential for payment. The toolkit is designed for those seeking registration, but it does allow for the calculation of the potential sequestration, including the impact of site clearance and soil carbon. The reference tables included in the toolkit are a useful summary.

How the soil is managed during construction, and through the life of a project, also impacts on the level of carbon stored. Soils are among the planet’s largest reservoirs of carbon, and improved land management can increase the level of carbon stored. (6)

Some interesting work by researchers at Newcastle University is exploring the potential for brownfield sites to store carbon, via the process of carbonation. On sites with high levels of calcium from demolition, waste, atmospheric carbon combines with the calcium to form calcium carbonate. (7) Unlike peatlands, the rate of sequestration is rapid, with one hectare of urban soil sequestering up to 85 tonnes of atmospheric carbon a year, equivalent to 165 hectares of forest. (8)

Over the lifespan of a plant there are many factors to consider. High water use, fossil fuel burning maintenance, and even the way it is transported to site, all offset the carbon storage. More work is needed to help landscape architects assess the best combination of plants to maximise climate change mitigation.

Royal Horticultural Society – Gardening in a Challenging Climate (9)

This 2017 RHS technical report is an excellent primer on climate change and horticulture. It includes detailed information about the potential impacts of climate change on planting decisions, as well as predictions as to how the use of garden space may change.

The report explores the link between gardens and climate change, lists the implications of climate projections for horticulture and considers the ways we can adapt to and mitigate for climate change, looking at risks and opportunities. Points for landscape pro fessionals to note include an increase in invasive organisms, reduced soil health and the impacts of resource use, such as water or fertilisers.

A particularly interesting section is the ideas for garden design in a changing climate – cross-sections through three UK gardens in 2100, presuming a mean temperature ranging between 2° and 5°C warmer than current. Resilient plant and tree species are listed, as well as ideas to help manage storm water surges and using plants for localised cooling. The reference list is also a useful resource.

References

1. ‘Barbican’, in Nigel Dunnett, 2019, <https://www.nigeldunnett.com/a-barbican/> [accessed 31 July 2020].

2. S Francisco, ‘Research from Save the Redwoods League and Humboldt State University Confirms Significant Role of Redwood Forests in California’s Climate Fight’, 3.

3. O US EPA, ‘Greenhouse Gas Equivalencies Calculator’, in US EPA, 2015, <https://www.epa.gov/energy/greenhouse-gas-equivalencies-calculator> [accessed 18 August 2020].

4. ‘A Global Garden: Plants Storing Carbon’, NASA Earth Observatory, 2011, <https://earthobservatory.nasa.gov/ images/51289/a-global-garden-plants-storing-carbon> [accessed 4 August 2020].

5. ‘Home – UK Woodland Carbon Code’, <https://www.woodlandcarboncode.org.uk/> [accessed 3 August 2020].

6. ‘Soil Carbon Sequestration | FAO SOILS PORTAL | Food and Agriculture Organization of the United Nations’, <http://www.fao.org/soils-portal/soil-management/soil-carbon-sequestration/en/> [accessed 4 August 2020].

7. ‘Turning urban wastelands into carbon capture gardens’, in ScienceDaily, <https://www.sciencedaily.com/ releases/2016/12/161213074347.htm> [accessed 3 August 2020].

8. F and RA (Defra) Department for Environment, ‘Environmental Value Look-Up Tool (EVL) Tool’, , 2015, <http://sciencesearch.defra.gov.uk/Default.aspx?Menu=Menu&Module=More&Location=None&Completed= 0&ProjectID=19514#Description> [accessed 8 January 2020].

9. E Webster, R Cameron & A Culham, Gardening in a Changing Climate, UK, Royal Horticultural Society, 2017, <www.rhs.org.uk/climate-change>.