Declaration: This is a long piece of in-depth writing; a white paper in support of Eucalyptus. Carefully researched so as to be sufficiently comprehensive to serve as a resource, whilst avoiding superficiality.

Are eucalyptus trees good for biodiversity?

This is a question we frequently encounter, and one which inspires some heated reactions! Eucalyptus have an unfairly earned reputation as being detrimental to biodiversity and wildlife, but this is far from the truth.

Are we biased?

That is a fair question. After all, we specialise in eucalyptus and could be expected to advocate for it. Our backgrounds are varied, but share a common theme – resilience. I have an MSc in Forestry and arrived at Grafton Nursery because of an interest in climate change resilient tree species – driven by first-hand experience of the waste and losses that are accepted in UK forestry. Hilary has spent decades growing almost every plant that will grow in the British Isles, and is now perhaps the most experienced Eucalyptus grower in the UK if not Europe. We chose to grow Eucalyptus here at Grafton Nursery because we believe that they are an incredibly useful family of species, with great potential for the UK. Climate change is an unfortunate reality, along with other threats to our biodiversity such as novel pests and pathogens. Either we adapt quickly, or we lament our hesitance in the future. It is essential that we use appropriate non-native species, supported by extensive research and sensible implementation. This demands that we be guided by science, not emotions.

What is biodiversity?

Biodiversity refers to the variety of plant and animal life in a habitat, and as you might expect, a higher value is usually better. An ancient woodland is an example of an environment with high biodiversity (Schmidt et al, 2014), and a heavily mown and weeded lawn is a biodiversity poor environment (seriously, let your grass grow wild and sow some wildflowers!). The key is variety, not just quantity. A chicken farm has a very high density of organisms, but it isn’t particularly species rich (or good for the chickens).

Biodiversity also doesn’t just consider the obvious species like trees and large fauna, but everything down to fungi, invertebrates and bacterial life. Biodiversity is important for many reasons, including a healthy functioning ecosystem, cultural value, recreation, science, and our economy. It is also important for the simple reason that every species has the right to exist (except Horseflies), and every species that we lose is gone forever.

Above: A biodiversity rich environment, our local Grafton Wood nature reserve. Note the diversity of both species and habitat structure, woodland rides like this are essential habitats for many pollinators.

What makes something bad for biodiversity?

In short, anything that diminishes the variety of organisms in an ecosystem can be viewed as detrimental to biodiversity. This can take many forms; such as land-use change, unsustainable land-management practices, and the introduction of invasive species. In the UK, for example, the destruction of ancient woodlands, removal of hedgerows, and building housing on greenfield sites are all examples of how biodiversity might be reduced. Species introduction is another prominent concern in the UK, in terms of species that both displace and destroy. A notable example of the former is Rhododendron ponticum; the beloved evergreen shrub that adorns so many gardens. An extremely well adapted survivor, it has escaped from gardens and parks across the UK and now resides within many of our woodlands. Shade-tolerant, fast-spreading, and hardy, R. ponticum will out-compete most of our native plants, suppressing them and reducing the number of plant species (and their associated animal species) that are present in a forest understorey. It is particularly established in the South-West and Scotland, where ongoing programmes of eradication are being undertaken (Forestry and Land Scotland, 2022).

Rhododendron also harbours diseases, which are an example of biodiversity destruction. Ash dieback, Emerald Ash Borer, Dothistroma needle blight, Phytophthora ramorum, Xylella, Chestnut Leaf Miner; the list of invasive pests and diseases that affect our native trees is seemingly endless and unfortunately shows no signs of diminishing. As these pathogens take root in ecosystems, they reduce biodiversity by killing and predating existing species.

The plight of our native Ash trees is perhaps the most high-profile example; Hymenoscyphus fraxineus is killing many (up to 85% (Coker et al., 2018)), and those that survive may well be erased by the arrival of the Emerald Ash Borer (currently making its way across Europe from Asia). Not only is this a problem for ash trees, it is also an existential threat to the over 950 animal and insect species that rely upon ash for at least part of their life cycle – including 44 species that are only found on ash (Mitchell et al., 2014). Pests and pathogens are also one of the primary drivers for the introduction of new species; in something of a paradox, the diseases introduced by new species mean that we have to introduce more new species to maintain sufficient biodiversity amongst our tree populations. You can see where this becomes a problematic cycle.

Above: A likely case of ash dieback. Diamond shaped lesions are a tell-tale indicator, initially presenting as a purple discolouration, and progressing to open wounds like this.

Are Eucalyptus trees bad for biodiversity?

Not especially, and certainly not moreso than almost any other non-native tree species. I am certain that there are many who would disagree, largely because Eucalyptus is such an inimitably exotic family of trees. Their appearance and biology are so inherently unusual that I believe they draw a lot of flak that might otherwise be better directed.

I recently undertook a piece of research for my MSc thesis, assessing the perceptions of forestry stakeholders towards non-native species. The results were unambiguous; individuals perceive some species as more exotic, and their willingness to consider their use is directly related to this perception. The more exotic a tree is considered to be, the more negative its impacts are expected to be. This is in spite of the fact that more exotic species are often less ‘risky’ than species that appear more similar to our ‘native’ trees. To put it simply:

• Eucalyptus trees don’t displace existing tree species. They aren’t invasive and don’t grow naturally in the UK; I have seen references to wild populations of E. gunnii in the South-East, but I’ve not been able to confirm this. Our climate and their population density simply doesn’t allow them to spread in the manner of, for example, Rhododendron, Cherry beech, Douglas fir, or Turkey oak. This doesn’t mean that all non-native species that can spread naturally in the UK are a problem for biodiversity, some are extremely valuable for biodiversity and the economy, and will become more so as climate change kicks in. It is also worth noting that many commonly accepted species in the UK are ‘non-native’, including sweet chestnut, horse chestnut, and sycamore.

• Eucalyptus trees don’t hybridise with anything that isn’t a eucalyptus. Our native oak species can easily hybridise with, for example, Turkey oak or Downy oak. Hybridisation is detrimental to biodiversity, because it merges genetically distinct species and populations (Todesco et al, 2016).

• As far as we are aware, Eucalyptus are not host to specific pests or diseases that affect anything native. In their native ecology they have pests and pathogens that affect them, but we have been fortunate in that very few have made it to the UK. Those that do will naturally only affect eucalyptus trees. Responsible producers who grow their trees from seed such as ourselves also reduce the risk of imported pests and diseases.

• A eucalyptus plantation is a much brighter environment than many non-native plantation species, with obvious benefits for biodiversity. The leaves of some species can suppress undergrowth development in sufficient density, but this is no different to species such as beech and pine.

• Despite a popular perception to the contrary, the eucalyptus species grown in this country aren’t toxic unless eaten in large quantities, and have no detrimental effects on native wildlife that we know of. If they did, our resident deer would have probably stopped chewing their bark!

Biological Deserts

An allegation sometimes levelled at non-native species in general – and Eucalyptus in particular – is that they create ‘biological or ecological deserts’. The implication here is that owing to their non-native status, they are unable to support native wildlife and plants. This is inaccurate, and the problematic definition of nativeness is something that will be explored in a future blog post. Examples of ecological deserts are heavily mown and treated lawns, or excessively grazed upland pastures. Anywhere that a lack of diversity in both species and habitats precludes the presence of species that might otherwise be found there.

Are Tree Plantations Biological Deserts?

Nope! Contrary to fairly popular perception, plantations aren’t as bad for biodiversity as one might think. The classic example of this in the UK are the Sitka spruce (Picea sitchensis) plantations that blanket much of our uplands in Scotland and Wales. Sitka spruce is our most common tree species (Mason, 2007), prized for its growth rate and useful timber. Many commentators have claimed that Sitka spruce plantations are ecological deserts, devoid of biodiversity, and this has in turn fuelled antipathy towards them in regions where it is most commonly used. A Norwegian study by Håvard Øyena and Nygaard (2020) reviewed 75 biodiversity studies from Sitka Spruce plantations, finding that studies from the UK demonstrated a mixture of both positive and negative impacts on biodiversity. This is supported by the work of Quine and Humphrey (2010), who found that there was no significant difference in species richness between native and non-native UK forest stands. Why is this? Many species are generalists and will occupy varied environments quite happily. Think of a magpie (Pica pica), a grey squirrel (Sciurus carolinensis), or brambles (Rubus). Generalists like this are resilient and adaptable, which is also why they are sometimes (wrongly) viewed as pests. Other species are specialists, and can only exist in very specific ecological niches. Epiphytes that can only grow on the surface of ancient trees are a good example, as are the many invertebrates that have evolved to exist in or upon different plants. The Brown Hairstreak butterfly (Thecla betulae) only lays eggs on young Blackthorn (Prunus spinosa) shoots. Coincidentally a population also happens to live just across the valley from our nursery in Worcestershire. A particularly interesting example is that of our beloved red squirrel, which thrives in coniferous woodland. Some of the last few refugia where it can be found in the UK are characterised by exotic coniferous plantations.

This doesn’t mean that a plantation of non-native species is going to have the same species richness as an ancient woodland, but it does mean that we can grow commercial tree species without necessarily compromising biodiversity – certainly not when compared to intensive agriculture or building houses. In an ideal world we would have both environments in abundance; carefully protected and managed ancient woodlands as biodiversity rich havens, and well planned productive plantations to meet our requirements for sustainable energy and resources. The UK has one of the lowest levels of tree cover in Europe (BBC, 2017), so get planting!

Above: A Eucalyptus glaucescens plantation in Yorkshire.

Diversity

Does this mean that monocultures are a good idea? Definitely not! Diversity is the best means of ensuring resilience and biodiversity, and even-aged monocultures are a high risk proposition. Just look at the ash plantations after the spread of ash dieback, or larch with phytophthora, or pine with Dothistroma needle blight, and so on. Putting your proverbial eggs in one basket means that if a new disease appears or the climate changes, then you’re in trouble. The key to maintaining the health of our tree populations in the future is to ensure that we have as wide a range of viable species as possible. This will minimise the impacts of new threats, as well as bolstering biodiversity. Diversity doesn’t just refer to species either; structural diversity is an important consideration. Compare an even-aged plantation where the canopy is sixty feet in the air, and an uneven-aged forest where an ever-shifting assemblage of trees ranging from seedlings to mature and standing deadwood exist in an intimate mixture. This has implications for both forest health and biodiversity, because different organisms often favour tree habitats at different ages and heights.

So how do Eucalyptus trees help with biodiversity?

Firstly, I would like to start with some anecdotal observations. They aren’t very scientific, but this isn’t something that has been researched in any great depth in the UK. We have a great many Eucalyptus species planted in and around the nursery both to test their hardiness and also because we are rather fond of them. In the past year I have seen; a flock of goldfinches using the evergreen canopy of a Eucalyptus neglecta as shelter overnight, swarms of pollinators enjoying the flowers on our Eucalyptus glaucescens, E. nicholii and E. kybeanensis, deer browsing the bark on our Eucalyptus globulus, and insects munching away on the foliage of a Eucalyptus cinerea. We have a robin called Reginald who likes to nest in our snow-gums (and he doesn’t particularly welcome the nursery team being in his tunnel). I also visited a plantation of Eucalyptus glaucescens in Yorkshire where the high levels of light on the forest floor allowed for a wealth of understorey species to thrive that would otherwise be shaded out in the coniferous plantations to which I am accustomed.

There are also many examples of successful mixed ornamental and forestry plantings, where a eucalyptus has a thriving understory of shrubs, perennials and grasses: Hillier Arboretum in Hampshire, Batsford Arboretum, the Eden Project, Beth Chatto’s Garden, RHS Rosemoor, RHS Harlow Carr, RHS Wisley, along with a number of our customers.

Most of the scientific research into Eucalyptus and biodiversity has been undertaken in China which is one of the most prodigious users of Eucalyptus in the world. Liang and Zong-Qiang (2009) assessed the impact of eucalyptus on indigenous biodiversity. They found that indigenous plant species in the majority of plantations were less than those in indigenous plantations, but more than in other exotic species plantations.

Concerns around the impacts of eucalyptus on biodiversity also fuelled a study by Williams (2015), which observed that planting eucalyptus can restore biodiversity on lands that have previously been used for intensive agriculture. A note of caution was sounded by the author, observing that the location and management style of different plantations has a significant impact on whether the plantations encourage or suppress understorey regeneration. This touches upon the significance of management on the impact that a plantation has on biodiversity and other environmental considerations. Are you growing a dense, dark plantation with a single species that is then to be clear-felled a few decades later? Or are you creating a diverse, multi-aged plantation that is managed through selective felling (removing trees as individuals or small groups to preserve an uneven-aged structure)?

Specific Considerations

The physiology of eucalyptus trees means that there are several biodiversity relevant considerations:

Light

Eucalyptus trees are excellent at screening, but they create the sort of dappled screen that is rather unlike our ubiquitous and often resented conifer hedging. Visit a eucalyptus plantation and you’ll be amazed at just how much light reaches the forest floor when compared to a Sitka spruce or beech stand. This is largely because of the morphology of eucalyptus trees, in that their mature foliage is pendulous and hangs down. This is to reduce the amount of light that the trees are exposed to; whilst it sounds counterproductive, it is actually an adaptation to cope with the extreme heat of their native environment by reducing transpiration. The practical consequence is that the ground beneath a eucalyptus is better lit than under many other species, which means that organisms which might otherwise be shaded out can grow there. Visit a mature stand of Sitka spruce or beech and you’ll see very little in the way of ground cover and understorey plants compared to a young mixed forest with pioneer species like birch or eucalyptus.

Flowers

Eucalyptus trees flower in the UK, producing delicate masses of male stamens once the “cap” of the flower-bud falls off. You may have seen eucalyptus honey for sale in the supermarket, and our bees seem particularly enthralled with the flowers. Eucalyptus trees often also flower in the early and late winter, which provides a useful source of food for pollinators when other sources are limited. Overwintering bees can wake up on warm days, and ensuring they have available food sources is essential.

Foliage

As evergreen trees, eucalyptus always have foliage present. If you’re an animal that needs shelter then this can be rather useful. We have flocks of finches living in our Eucalyptus neglecta and the resident robins have built their nests in the branches of our snow-gums.

Conclusions

You will note that this research primarily concerns the use of eucalyptus in plantations, as opposed to planting an individual tree in your garden. In the latter context, I can only anticipate that the presence of a eucalyptus would increase biodiversity by virtue of creating a tree where there previously was none. Every tree that you plant is going to provide a home for wildlife, along with sequestering carbon and a host of other benefits. Just make sure you choose the right tree for your requirements and growing environment, and give it the care it needs to become established.

So what is the lesson here? As with so many questions, there is no easy answer. Are eucalyptus trees are good or bad for biodiversity? The answer is that depending on the context, eucalyptus trees can be either good or bad for biodiversity – just the same as any tree species, including ‘natives’. The key isn’t the trees themselves; it is how we utilise and manage them. The popular narrative that eucalyptus trees are always detrimental to biodiversity is both wrong and dangerous. We need novel tree species if we are to adapt to our changing world, and eucalyptus will play a part in that. Whether it takes the form of carefully researched and implemented species, or a haphazard and reactionary response is entirely up to us. The alternative is for us to stand around futilely trying to protect our ever dwindling surviving ‘native’ trees, watching our forests fade away.

References:

BBC. 2017. Online https://www.bbc.co.uk/news/science-environment-41551296

Coker, T.L.R., Rozsypalek, J., Edwards, A., Harwood, T.P., Butfoy, L. and Buggs, R.J.A. 2018. Estimating mortality rates of European ash (Fraxinus excelsior) under the ash dieback (Hymenoscyphus fraxineus) epidemic, Plants People Planet. https://doi.org/10.1002/ppp3.11

Forestry and Land Scotland. 2022. Online https://forestryandland.gov.scot/what-we-do/biodiversity-and-conservation/habitat-conservation/woodland/rhododendron

Øyen, B.H. and Nygaard, P.H. 2020. Impact of Sitka spruce on biodiversity in NW Europe with a special focus on Norway – evidence, perceptions and regulations, Scandinavian Journal of Forest Research, 35, pp. 117-133.

Mason, W. L. 2007. Changes in the management of British forests between 1945 and 2000 and possible future trends. IBIS, 149 (2), pp.41-52.

Liang, P. and Zong-Qiang, X. 2009. Effects of introducing Eucalyptus on indigenous biodiversity, Yingyong Shengtai Xuebao. 20 (7), pp. 1765-1774.

Quine, C.P. & Humphrey, J.W. 2010. Plantations of exotic tree species in Britain: Irrelevant for biodiversity or novel habitat for native species?, Biodiversity and Conservation, 19 (5), pp.1503-1512.

Schmidt, M., Molder, A., Schonfelder, E., Engel, F., Schmiedel, I. and Culmsee, H. 2014. Determining ancient woodland indicator plants for practical use: A new approach developed in northwest Germany, Forest Ecology and Management. 330, pp. 228-239.

Todesco, M., Pascual, M. A., Owens, G. L., Ostevik, K. L., Moyers, B. T., Hübner, S., Heredia, S. M., Hahn, M. A., Caseys, C., Bock, D. G., & Rieseberg, L. H. 2016. Hybridization and extinction. Evolutionary applications. 9 (7), 892–908. https://doi.org/10.1111/eva.12367