| Under
favourable conditions Alnus glutinosa can reach
heights of up to 40m (Clapham, et al, 1987) but is more
usually of lower stature up to 20m and is common in shrub
form. With age the crown becomes typically arched composed
of crooked branches; the young tree may be identified
by unusual stalked, purplish buds (Gamlin, 1985). The
young branches are smooth, shiny and green and are covered
in resin warts, and the older branches become dark grey
and rugged. The roots develop nodules that contain the
mycelia of a fungus, Actinomyces alni, that allows
the tree to utilise the free nitrogen in the air. In swamp
areas, Alnus can create the appearance of standing on
stilts. This is actually the supporting root system that
grows out of the stem in to the soil. Often the stumps
may rot away whilst the vigorous supporting root system
remains (Vedel and Lange, 1978). The leaves remain green
in autumn almost until they fall and are 3-9cm in length
(Clapham, et al. 1987). They are serrate and oval, the
upper sides are dark green and slightly sticky and the
lower are lighter green. Alnus forms male and female
catkins and cones which remain on the trees after seed
dispersal (Gamlin, 1985). The seeds are flat, reddish
brown nuts, and are edged with air-filled webbing that
enables them to float on water therefore achieving a wide
distribution, especially as Alnus is suited to
damp ground near to streams and lakes (McVean 1955, Vedel
and Lange, 1978). The seeds can also germinate on the
surface of the water and take root when washed ashore
(Gamlin, 1985). |
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