Tree Flowers: April 2016: Alder

The alder is a tree that is often found along the banks of rivers, which is no surprise as the wood is resistant to rotting in water. But this isn't the only interesting fact about alder as a genus, as it can also fix nitrogen in the soil.

It can fix nitrogen due to a symbiotic relationship with a bacteria called Frankia alni that lives within the roots of trees with the genus of Alnus, or alder. The bacteria absorbs the nitrogen from the air and in exchange for sugars, that the tree provides, it makes the nitrogen available to the tree. This nitrogen is also provided to the soil around the tree, which can make them useful as nurse trees.

There are many species of alder, around 35, along with a handful of cultivated varieties.

The flowers are wind pollinated catkins, with the males being pendulous and easily swayed by the wind. The female catkins are very small upright spikes and when looked at closely are slightly reminiscent of female hazel catkins. Flowering happens before the leaves appear, which of a common strategy for flowers that rely on wind for pollination. Additionally, both the male and female flowers occur on the same tree and neither have petals.

The photo above is an attempt at focus stacking a branch of alder. This shows last year's female catkins on the right, which are still on the tree despite having released the seeds. This is followed by this year's female catkins and male catkins.

Female catkin from the previous year.

Wind pollination is a very inefficient process and much of the pollen will fall to the ground or land on plants of different species and come to nothing. But, interestingly, it also seems that a large proportion of common alder seeds are actually empty. Add this to fact that common alder seeds don't store well either and this is pretty much a disaster! Seedlings of common alder will only naturally germinate in soils that are continuously wet for up to 30 days, during April through to June, and additionally must have exposure to the sun. Clearly the adoption of moist environments is not a coincidence as the seeds have appendages similar to cork that allow them to to float for around one year without any loss of viability.

Female catkin from the current year.

While alders can endure permanently moist environments, they can also endure drought conditions, which may make them a more common tree in the future. Alder is also an important tree for holding together river banks and preventing erosion.

Male catkin.

The timber from alders is often used for furniture, paper production, charcoal, and firewood. The remains of alder charcoal is found in archaeological finds, including those of the famous Beaker people, known for the pottery they produced. Interestingly, I've also read that the trees were also traditionally used to make clogs - which the Dutch half of me finds awesome.



Resources:
Coombes, Allen J., and Zsolt Debreczy. The Book of Leaves: A Leaf-by-Leaf Guide to Six Hundred of the World’s Great Trees. S.l.: Ivy Press, 2015.
Hemery, Gabriel, and Sarah Simblet. The New Sylva: A Discourse of Forest and Orchard Trees for the Twenty-First Century. London: Bloomsbury Publishing, 2014.
Alder - Wikipedia, the free encyclopedia. 2016. Alder - Wikipedia, the free encyclopedia. [ONLINE] Available at: https://en.wikipedia.org/wiki/Alder#Classification. [Accessed 05 April 2016].