Tree of the month: Alder

The moon was full last night. So according to my favourite version of the Ogham ‘tree calendar’, we’re now halfway through the Month of the Alder. You might choose to follow my ‘ogham’ tag for other posts in this series.

The ancient Celts used their Ogham alphabet for various purposes but they don’t seem to have associated a calendar with it. Here’s a debunking of that myth. I like this calendar, though. It directs attention to a different species for each lunar month. So this month, let’s admire the alder.

The common or black alder (Alnus glutinosa) is native to Britain. It’s widespread and common. Here’s a map showing that A. glutinosa grows in almost every part of these islands. If you’re in London you can visit a specimen tree of this species at the Royal Botanic Garden at Kew. If you’re near any wood in Britain, you can look for alders because there are plenty of them.

Alder doesn’t grow in every wood. Mostly in wet ones. It has symbiotic bacteria (Frankia alni) living in its roots. That is, the bacterium and the tree are mutually beneficial. Its symbionts allow the alder to thrive in damp soil. Therefore the alder is typical of riparian (that is, riverside) and alluvial (that is, flood-prone) woods. Broadly speaking, riparian woods and alluvial woods are more or less the same thing. So it’s no surprise that the Joint Nature Conservation Committee’s map of UK alluvial forest looks like the Online Atlas of British and Irish Flora’s map of alder distribution.

Trees for Life tells us why riparian woods are so special. They provide habitat for many species of vascular plants, fungi, mosses, lichens, invertebrates, fish, mammals and birds. That’s before you even start to admire the titchy little microorganisms. The Forestry Commission (FC) agrees. FC also says that riparian woods can affect water quality, therefore needing the right kind of riparian habitat management.

What if you find a band of woodland with alder and other species like those of a riparian wood, but without a stream or river? You might be looking at the line of an old watercourse. In that sense, alder is an indicator species. When the Wildlife Trusts describe indicator species whose presence suggests ancient or semi-natural woodland, alder is one of those species. The Royal Forestry Society tells us more about ancient woodland indicators. Francis Rose at British Wildlife also tells us about these species worth noticing.

If you know how to read indicator species, you’ll know better than to believe that you’re standing on a line of geopathic stress, even a ‘ley line’ where ‘spiritual energies’ flow. To complement Mary Jones’ essay debunking the Ogham ‘tree calendar’, here’s one by Dave Gamble dubunking ‘ley lines’. I’m glad to have stumbled upon Dave’s ‘Skeptical Science’ website. Dave asks, among other things, why we don’t install ‘spiritual energy’ meters to measure the magical ‘flow’.

Enough woo, woo. The Royal Horticultural Society (RHS) tells gardeners how to grow alder. RHS also says that alder can be used to reclaim ‘brownfield sites’ where buildings used to be. But Andy Moffat at the FC’s research agency, Forest Research, isn’t very impressed by that idea. He says that alien trees, including foreign alders (various Alnus spp.) often prove more suitable for land reclamation in Britain.

Choices might become more limited for people who want to plant Alnus spp. That’s because this genus is yet another kind of tree now threatened by an emerging (that is, previously rare) disease. The disease attacking alder trees is caused by a fungus-like mould called Phytophthora alni subsp. alni. It’s a new hybrid. Here’s what FC says about the Phytophthora pathogens attacking our alder trees. The National Forest website tells us more about Phytophthora diseases affecting alder. You might choose to follow my ‘Phytopthora’ tag for more about this group of plant pathogens.

Alder has a lot of different symbionts. As well as the root-nodule bacteria, it lives symbiotically with dozens of fungi as Trees for Life explains.

If you scroll down the Trees for Life page I’ve just linked to, you’ll find pictures of alder flowers (female), catkins (male), and the cones which result. After they ripen, alder seeds leave their cones to be dispersed by wind and water.

We’re having a late spring in Britain now. You might still find flowers and catkins on the alder trees. Aren’t they pretty?

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About argylesock

I wrote a PhD about veterinary parasitology so that's the starting point for this blog. But I'm now branching out into other areas of biology and into popular science writing. I'll write here about science that happens in landscapes, particularly farmland, and about science involving interspecific interactions. Datasets and statistics get my attention. Exactly where this blog will lead? That's a journey that I'm on and I hope you'll come with me.
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5 Responses to Tree of the month: Alder

  1. earlywonder says:

    This is a great post! Calendars that draw attention to specific species? Yes please. I shared it on early wonder.

  2. Pingback: Tree of the month: Alder (Rebloged from Science on the Land) | Early Wonder

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