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18 May 2021

From threat to opportunity – Inverewe Garden’s biochar project

Written by Maddie Geddes-Barton, Woodland Gardener at Inverewe
A smiling lady crouches down in a woodland glade, surrounded by delicate spring woodland flowers. She is holding an orange, omega-shaped sign that has the National Trust for Scotland logo on it and the text: love.
Maddie Geddes-Barton is the Inverewe woodland gardener.
Inverewe Garden faces a number of invasive species challenges. However, the team are going to use a specialised form of charcoal making, called biochar, to ensure a productive and environmentally friendly outcome to their eradication mission.

Inverewe Garden, like many other gardens on the west coast, is challenged by a scourge of an invasive species, Rhododendron ponticum. R. ponticum was originally introduced to the UK in the 1760s, as a cultivated flowering shrub, and was widely planted in the Victorian era on estates for game cover. However, it has since spread uncontrollably across the delicate habitats of the west coast of Scotland and north-west England. It is particularly well adapted to the environmental conditions of the west coast: the oceanic climate and acidic peaty soil provide the ideal conditions for this species of rhododendron to colonise. One plant can produce over a million seeds per year, and these can be spread up to 400m–1km, depending on the prevailing wind.

The problem with R. ponticum is that it is so successful that it crowds out almost all other plants, forming a dense thicket within which nothing else can grow. The native woodland flora of primroses, anemones, bluebells, and rarer wildflowers, as well as ferns, mosses and lichens, are shaded out, and insects and other creatures that live in the woodlands lose their habitat. It also means that the ongoing process of succession of woodland trees is interrupted – the shade is too great for the next generation of saplings to establish. Without human intervention, the woodland would die when the existing trees reached the end of their life, leaving a monocultural landscape of only R. ponticum.

This is worrying for us at Inverewe because we need the shelterbelt woodland to keep renewing itself – it is the shelterbelt that protects the garden from the Atlantic storms. We’re also concerned about R. ponticum spreading from the garden to the wider landscape and colonising natural woods and moorland.

A close-up view of bluebells and white ramsons growing on a woodland floor.

Scotland’s rainforest

Our work is made more pressing at Inverewe because of the high conservation status of this part of the west coast of Scotland. Our hyper-oceanic climate – principally influenced by the wet, mild air of the Gulf Stream – creates a very special habitat in which communities of rare ferns, lichens, liverworts and mosses flourish. It’s a habitat of ancient and native woodland, open glades, boulders, crags, ravines and river gorges. Surfaces drip with moisture; everywhere is covered in luxuriant growth. There is a huge richness of biodiversity – rare insects and moths can hide in the cover of the epiphytes, and these in turn provide food for birds like wood warblers and redstarts. This habitat is so distinctive that it now has a special name: ‘Scotland’s rainforest’.

Scotland is the last stronghold of this globally important, rare habitat. The required climatic conditions occur on less than 1% of the Earth’s surface and are concentrated on the west coasts of Britain, Ireland and Norway. In the rest of the UK, pollution and land-use mean that this type of rainforest has been unable to retain its biodiversity. By contrast, North-West Scotland has very clean air and its rugged landscape has protected some areas from overgrazing. This has left patches of Scotland’s rainforest that are hugely botanically rich, containing species such as the lichen Graphis alboscripta (only found in Scotland) and the lichens Pyrenula hibernica and Pseudocyphellaria spp (specklebelly lichens) that are only found in this habitat.

Because of habitat rarity, many of the species that occupy Scotland’s rainforests have special conservation status. This is particularly the case with Scottish lichens. There are about 1,850 types of lichen in Britain (40% of Europe’s lichens) and over 500 of these have been recorded in Scotland’s rainforest. This habitat supports an internationally important oceanic lichen flora, with a significant proportion of the European and/or global populations of 86 lichen species for which the UK has international responsibility. Some of these species are endemic. We have some of these very rare species in the garden at Inverewe. For example, the tree-dwelling lichen Collema fasciculare (commonly known as ‘octopus suckers’) is a Scotland’s rainforest species and internationally rare.

In North-West Scotland we have a global responsibility to look after and preserve our rainforests. But even here, land use changes over the 20th century, particularly deforestation and overgrazing, mean there is as little as 30,325 hectares left. The National Trust for Scotland is an active partner in the Alliance for Scotland’s Rainforest, which is fighting to preserve these extremely special woodlands. The challenge that confronts us is a serious one. R. ponticum can be found in 40% of rainforest sites. It threatens to choke the woodlands and prevent the distinctive rainforest flora from surviving. Scotland’s rainforest species are very slow colonisers, and the original biodiversity would take a very long time, maybe centuries, to re-establish if shaded out by R. ponticum. The invasive rhododendron must be carefully cleared, and we must prevent more seedlings escaping to the wild.

In caring for the shelterbelt woodland at Inverewe, by clearing the R. ponticum and fostering natural regeneration, we’re contributing to a critical conservation effort. We’re working both to preserve the species diversity of our own small area of woodland (some of which is semi-natural woodland with rare west coast provenance pine, self-seeded birch, rowan and hazel), and to ensure that invasive rhododendron does not escape the garden and do irreversible damage to the landscape beyond.

2020 Project Wipeout

In 2020 Inverewe was awarded a £30,000 grant from Project Wipeout – set up to tackle invasive species, and funded by players of People’s Postcode Lottery, the Nature Scot Biodiversity Challenge Fund and Baillie Gifford – to clear R. ponticum from the garden. The essence of our approach is to link this clearance project to the restoration and regeneration of the areas that have been cleared.

The problem we face in restoring areas that have been historically colonised by dense thickets of R. ponticum is that the whole ecosystem balance of the woodland has been disrupted. Forest floor ecosystems are complex, with a vast range of creatures involved – from visible beetles, worms and millipedes, through to predatory nematodes and microscopic bacteria, fungi and other soil microbes that cycle nutrients. Such is the complexity of this soil life and the sophisticated symbiotic relationships with tree roots, particularly in undisturbed woodland, that scientists are only just beginning to appreciate what is going on.

An illustrated diagram, with labels, of the soil food web, showing the cycle between organic waste, plants and birds/animals.
The soil food web

Under the monocultural regime of R. ponticum, many of these organisms lose their niche, and the resulting soil is biologically impoverished. The removal process itself can also be very damaging to the soil ecosystem. Consequently, once the woodland has been cleared of R. ponticum, our first job is to restore the balance of soil life. Increasingly, research shows that achieving a healthy soil is critical to achieving successful regeneration. Without it, the trees we plant and the saplings that self-seed will not have the right conditions to grow and flourish.


Our plan for restoring the soil health of our woodland is to convert the R. ponticum we have cleared into biochar and return it to the soil. Biochar is a specialised form of charcoal, made under very precise burn conditions that mean the pore structure of the original wood-cells remains intact. We have purchased an Exeter Retort, which uses a very low pollution technique to make biochar – burning off environmentally harmful gases, whilst retaining the wood’s carbon. This means that at least 50% of the carbon locked up in the R. ponticum can be returned to the woodland. As biochar is slow to degrade, this carbon can potentially be locked up for thousands of years, which is why this process could be considered a carbon sequestration technique.

The really exciting thing for us about biochar is its potential to help restore the soil life to areas of the woodland that have been cleared. Acidic, highly leached soil, like that at Inverewe, responds better to biochar amendment than other types of soil. Because it retains the pore structure of the original wood, biochar has a very large surface area; apparently there can be the equivalent of up to several football pitches of surfaces in a single gram of biochar. Water, air and nutrients can occupy these spongy spaces, making it easier for the soil to stay aerated, regulate its moisture levels during wet and dry periods, and retain nutrients against leaching.

Read the report in support of these findings

Importantly, beneficial organisms can also occupy these spaces, exploring, mining and storing nutrients within their bodies. In a woodland context, the microbial communities we’re particularly interested in are the fungi. These shuttle minerals and nutrients to and fro within the soil, connecting trees to one another and establishing communication and interconnected networks of mutual assistance. The more we can help these beneficial fungi – both mycorrhizal fungi that enter directly into symbiotic relationships with tree roots and free-living fungi that are indirectly helpful – the healthier and more resilient our woodland will be.

The benefits of this approach have been proven over the last few decades in a wide range of scientific experiments. At the Bartlett Tree Research Laboratories in North Carolina, America, the survival rate of beech saplings in soil to which biochar had been added was up to 50% higher than the control group. Resilient soils can also help protect woodlands from tree disease outbreaks. A three-year trial in Essex showed much greater resistance to ash dieback in tree populations with biochar-amended soils. Biochar has also been shown to improve tree resistance to Phytophthora ramorum ­– a fungal organism which plagues west coast woodlands.

A woman stands at the edge of a garden path, holding a garden spade full of compost. Before her is a newly mulched bed. Behind her are large flowering bushes, possibly rhododendrons, with clumps of daffodils.
Maddie applying compost as mulch

Composted biochar

To help speed up the process of fungal colonisation, we’ll give the fungi and other beneficial organisms a head-start by adding compost to the biochar before returning it to the woodland. The compost is made carefully, to provide the best aerobic conditions for as many beneficial micro-organisms as possible. Whilst a teaspoon of normal soil contains several metres of fungal hyphae [the branching filaments], several thousand protozoa and a few dozen nematodes, a teaspoon of well-made compost can contain up to 300m of fungal hyphae, up to 50,000 protozoa and anything from 30–300 nematodes. Because we want the greatest diversity of beneficial microbes in our biochar as possible, we’re also experimenting with collecting leaf litter from undisturbed natural woodlands on our estate and culturing the fungi and soil life. This is then added to our compost and will be used to enrich the biodiversity of the biochar which is returned to the woodland. We have a 400x microscope that allows us to see and identify the micro-organisms in our soils. Over the next few years, we’ll be using the microscope to monitor the success of the biochar in enriching the soil health of our woodland.

The shelterbelt

Ultimately, the arbiter of our success will be the progress we’re able to make over the next few decades in transforming our shelterbelt into a resilient and self-sustaining woodland. The shelterbelt is the backbone of the garden at Inverewe – it makes our more specialised horticulture possible. Establishing a species-rich, mixed-age canopy woodland, with high biodiversity, will provide shelter for the rare creatures that live in this part of Scotland – from red squirrels, pine martens and sea eagles down to the rare lichens, ferns and mosses. It will also shelter Inverewe Garden, providing it with the protection it needs to withstand the climatic pressures of the 21st century.

A mix of tall tree species grow together in a woodland, with some bushes at the lower levels also visible.

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