Identifying trees: no aliens here

 

London plane trees in Berkeley Square (image by Richard Croft, CC BY-SA 2.0  (Wikipedia) 

I attended a course on tree identification at the Cambridge Botanic Gardens. The course was exceptional: it provided everything I could wish. I am a perpetual amateur when it comes to natural history: I see things, but I always forget what they are called, and all I want from a course on identifying trees is to give me some simple method for working out which tree is which, so even if I have been told the name in the past, I can re-discover it. I am surrounded by people who tell me “that’s a hornbeam”, or “that’s a sycamore”, without telling me how they reached that identification. I want to be able to work out the name for myself.

The tutor, Ros Bennett, produced a simple key to all 52 species of British trees. identifying them via their leaf structure – what could be better? To keep the experts and beginners on a level playing field, she covered up the labels of the trees we were trying to identify, and asked those who knew which tree we were looking at to keep quiet while we tried her key.

For the most part, the identification key worked. This was a major step forward for me; my interest in tree identification means I have bought several identification guides, but never found one that I could use on a regular basis. In an attempt to describe many species, perhaps they provided too many choices. 

Some of the 55 different leaf types described in the Collins Complete Guide to British Trees (2007)

This key, in contrast, worked very well for the limited species described. Like all the best keys, you were only contrasting one or two things at a time.

So, what’s to complain about? Given that the course was only one day, the tutor had to be limited in setting goals, and this course only covered  deciduous trees (although she told us there are only three native species of conifer in the British Isles). In addition,  she revealed that, although she has published her key in a recent book (Tree Spotting, 2022), she was only using one of the keys in that book. She didn’t make any use of the key to differentiating trees via their buds, which is also found in her book. In addition, we noticed that of course there are other simple clues to identifying trees, such as last year’s fruit hanging from the tree.

The final limitation, and the one that intrigues me, was that the course was restricted to “native” trees. She confidently stated there are 52 native species, but immediately relaxed that criterion to say she was including sycamore, which is not native. Yet we would not consider London plane, which is introduced, rather than native.

 Hang on a minute! As a keen amateur taxonomist, I’m always fascinated to discover classification rules that are broken as soon as they are stated. If we are including sycamore, then what exactly is the criterion for tree identification? I realise that we were sitting in the classroom of the Cambridge Botanic Garden, which is full of an incredible variety of trees from all around the world, but we know as well that a botanic garden is not a typical environment. We don’t expect our simple key to cover everything in the Garden, but I would expect it to include sycamore, and I would expect the course to include very common trees that are found in streets and gardens, such as London plane. I was also alarmed by the terms “native” and “alien”. What is going on here? Two days later, I was walking in a London street only to discover it was populated with gingko trees. Not so exotic, then, if they are widely used as street trees.

Consider the flowers in our garden. Cambridge Botanic Garden has a fascinating set of beds containing flowers that were introduced in each century. Many of these flowers are widely grown, and it seems almost irrelevant to try to distinguish which flowers were present at the last ice age and which were not.

The tutor’s definition of “native” corresponded with that in other sources:

It is widely accepted that ‘native’ trees and shrubs are those species that have occurred naturally in Britain since the last Ice Age. The more recent introductions that have established themselves in the wild are referred to as ‘naturalised’ or ‘archeophytes’. [Royal Horticultural Society]

Some other definitions allow to “natural” entry to Britain:

A native plant is either a plant that arrived naturally in Britain and Ireland since the end of the last glaciation (i.e. without the assistance of humans) or one that was already present (i.e. it persisted during the last Ice Age). [Botanical Society of Britain and Ireland]

Other definitions, from outside the UK, are wider:

A plant is considered native if it has occurred naturally in a particular region, ecosystem, or habitat without human introduction. [US National Wildlife Federation] 

Does this matter? It certainly jars when current debates in the UK Parliament are all about controlling the level of migration to the UK. Terms such as “native” have rather nasty connotations for a liberal like me. I would have thought the most appropriate definition of trees I would like to identify is: 

All trees that either were present at the time of the last ice age, or which have become widely established since that date, regardless of how they were introduced. 

I want a definition that covers the trees I am most likely to see occurring in the wild in the UK, as well as the most widely planted trees in streets. Instead of 52 species, that might go up to around 80 (the number included in the Woodland Trust’s “A-Z of British Trees”,or the 114 contained in John Kilbracken’s Easy Way to Tree Recognition (1983, but still in print). That list includes London plane, the Holm oak, and turkey oak. John Kilbracken describes the situation very well: 

With trees it’s very different [compared to birds]. Many species grow wild – by which I mean they are self-sown – but more often they have been planted.

Like it or not, the trees you see around are very frequently the result of human choices, unlike birds, for the most part. I’d like to include therefore a larger list than Bennett’s 52 species – and I’d like to change the title, while I’m about it. It’s no doubt my hyper-sensitivity, but I much prefer the idea of “trees that are common in Britain”, to any other criteria of “nativeness” that seems to me arbitrary and not linked to common knowledge. I’d like to have a more flexible definition of native – and if we applied the definition “present at the last ice age” to humans, we would get into all kinds of difficulty.