The Genus Alnus

Alnus seem to have originated as a monophyletic genus, and a sister genus to Betula, within the family Betulaceae. It may have originated at or before the Late Cretaceous and developed further in the Tertiary Paleocene and Eocene. The major biodiversity focus is (now at least) in Eastern Asia. The American species, including those in Central and South America, appear to have crossed from Asia mainly via the Beringian Land Bridge. Currently there are 18 - 23 species in Asia, 4 - 5 in Europe and 9 in America. The only one that strays into Africa is the Holearctic Alnus glutinosa.

As well as the Frankia actinomycete nodules, seedlings may have AM mycorrhizae, and ECM mycorrhizae are often found throughout the life cycle. 

Red Alder, Alnus rubra, Sutton-at-Hone Burial Ground

While visiting Sutton-at-Hone Burial Ground and looking at the yews, I accidentally came across a Red Alder, Alnus rubra (Bong.), growing away in a cluttered section of the churchyard, What on earth was it doing there? There seemed to be no other Alders around, other trees had clearly been planted, had it arrived accidentally or was it deliberately placed I wonder? It was a bit tight for space, or will be in a very few years.

It is supposed to grow 25 - 40 m high and the crown is generally thought to be slender and pyramidal in shape. On the other hand the branches tend to droop a little - described as pendulous. The shoots and young twigs are angled or even triangular in section when examined closely, and as far as I know this is the only Alnus species that shows this characteristic.

This is the trunk, lovely and smooth, with relatively small ovals of pale lichen:

The poor landscaper is apparently having trouble controlling the side-shooting, with repeated tidying being necessary. Here is some of the foliage, showing the currently green catkins, neatly toothed oval leaves, and long pointed buds.

and here is a single leaf from below, and I think I can just convince myself that I can see the minutely turned-down leaf margin.

The leaf veins are deeply impressed and the catkins are shiny and green at present:

May - Alder

Young female flowers.