Who is the most biodiverse of them all?

Matthew Cooper

In the arb industry we can, quite rightly, become obsessed with oak trees as the bastions of biodiversity.

So imagine my surprise when reading Peter Thomas’s wonderful book Trees to find that it states, very casually, that there are more insect and mite species found on willow than oak (Thomas, 2022)! This admittedly included five willow species, rather than two oak species, but even so it made me think again about a genus that is usually only talked about as being good for a wet site.

And so began my quest to learn more about the genus of Salix in Bonny Britain. The first question I asked myself was: how many native willows are there? Easy, must be five, right? Well, no, earlier in his book Peter Thomas lists six, admittedly trees and shrubs, but some I had thought should be in there seemed to be missing. His source was Clive Stace’s Flora (2019) and my luck was in; it had just gone through the 2024 corrections, so was bang up to date.

I checked and checked again, now there are 16! This list included some Highland dwarf willows, but it was also missing some I thought should be in there like crack willow and white willow. Is neither native? Well, it turns out crack willow is now thought to be an interspecific hybrid, Salix × fragilis, and, to quote Stace, ‘(is) represented by a range of variants and cultivars linking Salix enxina with Salix alba’. Salix enxina is from Turkey and the Transcaucasus, a region in the Caucasus Mountains between the Black and Caspian Seas. OK, so what about white willow (Salix alba)? This was less clear, as although it was not listed as native, another part of the world was not mentioned as its origin. So back to old faithful: my heavily worn, much-loved Mitchell’s guide (1978). White willow and crack willow are both in the four native willow trees listed. OK, so I’m not going mad. Cross-referencing here I come. On to a newer Collins guide (Johnson & More, 2004), which lists eight species of native willow, including white willow and crack willow. The joy of nomenclature and our increasing knowledge! At least I felt less silly about thinking these two were native, as they may indeed be so.

Is anyone still reading? ‘Get on with it!’ I hear you cry. ‘Tell us something useful!’ OK, OK. Sorry – I will, I promise.

The genus name, Salix, comes from Celtic words meaning ‘near water’ (sal =near; lis = water) (Johnson et al., 2019), which fits, right? The members of the genus range from trees that can reach over 20 m in height to dwarf species that can be under 20 cm and every height in between. Their leaf forms also vary wildly from the long slim leaves of osier, crack willow and white willow (Fig. 1) through the oval-shaped leaves of the goat willow (Fig. 2) to the almost completely round leaves of dwarf willow (Fig. 3). They come from the family Salicaceae (Mitchell, 1978; Johnson & More, 2004) and are generally dioecious, that is to say the male and female parts are on different trees. As a result, they hybridise worryingly easily, which is why I said they are generally dioecious, as some hybrids – even some listed as female – can have bisexual catkins (Stace, 2019). Therefore, identification may often require the catkin and both young and mature leaves (Stace, 2019).

So which are our native willows? Mitchell (1978) lists bay willow (S. pentandra), crack willow (S. fragilis), white willow (S. alba) and goat willow (S. caprea). The later Collins guide lists those four with the addition of grey willow (Salix cinerea subsp. oleifolia), purple osier (S. purpurea), almond willow (S. triandra) and common osier (S. viminalis) (Johnson & More, 2004). Peter Thomas (2022) lists bay willow (S. pentandra), goat willow (S. caprea), grey willow (S. cinerea), purple willow (S. purpurea), tea-leaved willow (S. phylicifolia) and one interspecific hybrid, laurel-leaved willow (S. × laurina), the first three as trees and the latter three as shrubs. None of these lists is exhaustive, they give exclusions and Peter Thomas goes even further by offering sources and details of neophytes (trees introduced after 1500) and archaeophytes (trees introduced earlier than 1500), which adds two more willows.

Figure 1: The long elegant leaves of white willow (Salix alba). (© Matthew Cooper)

Entrance to a nuthatch nest, the cavity made smaller with mud to restrict access. (© Chris Knapman)

Figure 3: The delicate round leaves of the dwarf willow (Salix herbacea). (Reproduced with the kind permission of the Board of Trustees, Royal Botanic Gardens Edinburgh © RBGE)

However, the uber list is from Clive Stace’s Flora (2019). It still comes with caveats and is subdivided as follows:

1. Trees and tall shrubs with leaves that are lanceolate to narrowly elliptic and acuminate with finely or close serrate margin. That is, long and slim with a pointy tip and a fine-toothed edge to the leaf. Catkins long and slender on leafy peduncles (fruit stalk).

• bay willow (S. pentandra)

2. Shrubs or small trees with orbicular (round) to linear (straight) leaves with coarsely or irregular serrate margin (leaf edge). Catkins short and wide, sessile (without stalk) or on leafy peduncles (fruit stalk).

• broad-leaved osier (S. × smithiana)
• creeping willow (S. repens)
• dark-leaved willow (S. myrsinifolia)
• downy willow (S. lapponum)
• eared willow (S. aurita)
• goat willow (S. caprea)
• grey willow (S. cinerea)
• laurel-leaved willow (S. × laurina)
• mountain willow (S. arbuscula)
• purple willow (S. purpurea)
• tea-leaved willow (S. phylicifolia)
• woolly willow (S. lanata)

3. Dwarf shrubs with orbicular to ovate leaves with entire or coarsely serrated margins. Catkins short and wide on leafless peduncles.

• dwarf willow (S. herbacea)
• net-leaved willow (S. reticulata)
• whortle-leaved willow (S. myrsinites)

So, there you have it! We have a list and a rough guide to identification … sort of.

‘But this article was on biodiversity,’ I hear you cry. Sorry, I do like a tangent and, like Alice, can’t resist a rabbit hole.

The five willow species Peter Thomas lists as native to the UK play host to 23 leaf miners, 39 gall formers, 81 sap feeders and 307 chewers (insects and mites). These include things like the Phyllonorycter salicicolella, Stigmella salicis and Phyllocnistis saligna for the leaf miners (British Leafminers website). Gall formers like Aceria iteina, Aceria tetanothrix and Aculus laevis, which are mite galls; Eupontania bridgmanii, a sawfly gall; and Iteomyia capreae, a midge gall (British Plant Gall Society website). Sap feeders like Tuberolachnus salignus and Pterocomma salicis plus chewers like Plagiodera versicolora (Nature Spot website) are also in the mix.

This myriad of insects and mites that love and live with willow is further supported by the five-star rating (out of five) accorded by Alexander et al. (2006) for the number of organisms that use pollen or nectar as a habitat or food source. Their article splits the willows into ‘grey and goat willows’ and ‘crack, white and other rough barked willows’, puts them amongst many other tree species and rates them on how good they are from a biodiversity point of view with one star being low wildlife value and five stars being relatively high wildlife value. The table above shows how the willows fare in the rating system.

If you want to go macro with the biodiversity, our increasing number of British beavers (Castor fiber) interact with willows for many reasons, but some of these are thought to be associated with the content of salicylic acid to reduce the discomfort that can be caused by their ever-growing teeth. Willows are also useful in their dams and lodges. The fallen stems sucker and regrow, offering more food, medicine and materials. When beavers aren’t there to fell the trees, I wonder whether a wood-decay fungus associated with the genus, like willow bracket (Phellinus igniarius), fills this ecological niche, causing stems to fall but only when they are of large enough size that they will grow again. I have no evidence for this last point, but it is an interesting idea nonetheless, based solely on what I have seen while surveying.

When willows are viewed in the more complete picture of the biodiversity associated with tree species, the balance does somewhat lean back to our native oaks being more biodiverse. Nevertheless, the evidence does still show how biodiverse the willow genus is, and the range of species these trees host.

Another point arises in relation to the mycorrhizal fungi that associate with willows. It appears that they change the relationships they have with mycorrhizal fungi through their life, switching from a relationship with endomycorrhizal fungi, more commonly now called arbuscular mycorrhizal fungi (Boddy 2021), when they are young to ectomycorrhizal fungus when they are older, and they may possibly have relationships with both at stages during their life, suggesting a much more complex relationship than we currently understand. It’s also very exciting that we may have a greater insight into this when a new book on mycorrhizal fungi comes out in the near future, if Ted Green gets his way (2023).

So, what have we learnt? Well, I’ve learnt loads! Willows are more diverse than I thought, both between species, in the way they hybridise and in the diversity of life they support. And we didn’t even mention their cultural significance or, for that matter, wood-decaying fungi and their relationships with willow. Maybe in another issue.

To my fellow tree people: may you be well, happy and peaceful. Until next time.

To Martyn Dickson and Lorna Mitchell at the Royal Botanic Garden Edinburgh: thank you for your help with the image of Salix herbacea.

Matthew Cooper is a Chartered Arboriculturist and Chartered Environmentalist. He is a Fellow of the Association and a member of the Scotland Branch committee.

References

Alexander, K., Butler, J., & Green, T. (2006). The value of different tree and shrub species to wildlife. British Wildlife 18(1).

Boddy, L. (2021). Trees and Fungi: Their Complex Relationships. Arboricultural Association.

Green, T. (2023). Treetime. Arboricultural Association.

Hickey, M., & King, C. (2000). The Cambridge Illustrated Glossary of Botanical Terms. Cambridge University Press.

Johnson, A.T. et al. (2019). Plant Names Simplified: Their Pronunciation, Derivation and Meaning, 3rd edition. 5M Publishing Ltd.

Johnson, O., & More, D. (2006). Tree Guide: The most complete field guide to the trees of Britain and Europe. HarperCollins.

Rayner, A., & Boddy, L. (1988). Fungal Decomposition of Wood: Its Biology and Ecology. Wiley-Blackwell.

Stace, C.A. (2019). New Flora of British Isles, 4th edition (with 2024 corrections). C&M Floristics.

Thomas, P. (2022). Trees. HarperCollins.

Websites accessed

www.britishplantgallsociety.org/sallow-willow/, accessed 27/10/2024

www.leafmines.co.uk/html/mine-guide/sallows.htm, accessed 27/10/24

www.naturespot.org.uk/search/content?keys=willow+aphids, accessed 27/10/24

www.naturespot.org.uk/species/imported-willow-leaf-beetle, accessed 27/10/24

This article was taken from Issue 210 Autumn 2025 of the ARB Magazine, which is available to view free to members by simply logging in to the website and viewing your profile area.