Monday, 12 October 2015

Tree Identification № 4: Alder

Common Alder
The Alder is a complicated tree that many people seem to struggle with. Even if the struggle is that they muddle Alder with Elder. Easily done! Of course if you know the Latin names Alder is called Alnus and Elder Sambucus so that removes one confusion. And then of course Alder is a proper tree, they grow up to 20m, and Elder is usually classed as a shrub.

Anyhow, we have three different kinds of Alder that grow well in this part of North London. And there are plenty of them. About 25 to 30 years ago (goodness, can it be so long?) the climate in these parts got a lot wetter. And convinced that this was an ongoing trend, a lot of tree officers (working for the local council and responsible for planting street trees) planted Alders because they knew these trees cope well with the wet conditions.

Common Alder
First there is the Common Alder (Alnus glutinosa) which is native to the British Isles, then we have Grey Alder (Alnus incana) and Italian Alder (Alnus cordata). The Grey and Italian have what you might call leaf-shaped leaves. Not a technical term, but the shape a child would draw if you asked her to draw a leaf. The leaves of the Common Alder look rather as though someone had chopped the pointy end off each leaf. It's very distinctive and the experts call it broad-cuneate (remember I'm not an expert).

So far so good. What, I think, bothers people about Alders is the pine cones. And the catkins. Catkins on a tree that isn't a Hazel? And cones on a tree that isn't a pine? It's confusing.

Well, OK, the catkins are flowers. Alders have male catkins about 20 - 30mm long, and shorter female catkins 3mm long. Everyone knows that Hazels produce catkins (the male flowers) but so do Birch trees, and even Oaks (again the male flowers). And the Alder flowers are on the trees for months at a time. I took these photographs in October, but the flowers don't open until March or even April. And, (which the tree officers obviously failed to consider) when the catkins/flowers are over they chuck themselves all over the pavement in a very messy way.

Italian Alder
Also shown in the second photograph are fresh fruits (the green ovoids - sorry it's a technical term) and the ripe fruits (which look like miniature pine cones. So it's a very busy tree with flowers and fruits all at once. And you can bet that people don't like it much when the local Alder chucks ripe fruits all over the pavement because they're crunchy and hard and you might slip on them.

Not a successful street tree then, but great in slightly soggy places. My favourite Alders are in a green space alongside a brook. They grow very happily in the squashy grass.

Italian Alders come from guess where? Southern Italy and Corsica. According to my tree book they are uncommon in this country, but can be found (somewhat specifically) on the Basingstoke by-pass. What? Well, you can also find them around here. And although you can see the fruits are certainly very similar to those on the Common Alder, the leaves look quite different. Leaf-shaped and glossy. The tree book says the leaves have large tufts of pale orange hairs on the underside. I need to go back and check. It's amazing what you don't see if you don't know it's there. Which, I guess is why most people never notice that Oaks produce catkins. And yes, I went back and checked, The orange tuft really are there.

Grey Alder
Also growing locally is the Grey Alder, and why grey I really don't know unless this refers to the bark.

Again the leaves are leaf-shaped but they are quite clearly toothed where the Italian Alder leaves are pretty smooth along the edges.

I suppose the leaves are a little bit greyish on the underside - but not very. I wonder if someone just got stuck for a name. Anyway, Grey Alders also have catkins for flowers and tiny little cone-like fruits. But if they didn't, you might struggle to believe this tree was related to the Common or Italian Alder.

People often look at trees in the wrong way. Here we have three trees with rather different leaves so these trees can't be related can they? And then we have three trees which all feature catkins and little fruits that look a bit like pine cones. So maybe they are related after all.

Well, we know they are - I know they are related because I looked it up in a tree book. A proper expert has done all the hard work for me. But next time you look at a tree and try and work out what it is; look at what's the same. Not at what's different.

Grey Alder
Let's be honest, Alders aren't terribly glamorous trees. They don't feature pretty flowers, or brightly coloured fruits, they don't smell wonderful, but they are useful. You can plant them in soggy wet places and they'll grow where not much else will, and the wood (I read) has a very useful ability to withstand rot under water. So it has been used for boat building, for building up the sides of waterways (presumably all the canals in Britain), and much of Venice is built on piles of Alder wood. A glamorous result from a not very exciting tree.

Common Alder by the Mutton Brook in late November

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