Shrubs Of The Great Basin: A Natural History

Mountain Alder

Mountain Alder Alnus tenuifolia But the Mountain Alder must have its roots in eternal streams whose waters it helps, with its shade and its humus, to keep cool and pure. So this Alder passed into the true legends of the West as the friend of the explorer and settler. -Donald Cuiross Peattie, A NATURAL HISTORY OF WESTERN TREES LIKE MOST of its cousins the world over, our mountain alder frequents waterways in the mountains throughout the Great Basin. Its roots help bind the banks together. Its shade provides a comfortable habitat for those ferns and wildflowers which prefer a moist environment. Ordinarily the mountain alder varies between 2 and 3 meters high, but on rare occasions it may be~ come 8 meters tall. The trunk is smooth and gray or sometimes reddish brown. The dark green, rather coarse leaves are oval to nearly round and from 4 to 10 centimeters long. At first glance, they look a little like elm leaves, but they lack the oblique base of elm leaves and, in the case of our mountain alder, they are edged with both large and small teeth. There is a prominent midrib, and the lateral veins which run at an angle out to the larger teeth are nearly parallel with one another. The leaves begin to drop with the first frosts, frequently without even turning brown. Alders are easy to recognize even in the winter, or perhaps we should say especially in the winter, because of the clusters of three to seven small, pineconelike structures at the ends of some of the smaller branches. Each "cone," about a centimeter long, consists of the hard, woody bracts which enclosed the winged fruits before they were shed. In addition to last summer 's empty cones, elongated staminate catkins, several centimeters long and scheduled to open next spring, are also very obvious at the ends of those 37 BETULACEAE branches which developed during the past summer. These branches have characteristic elliptical, reddish brown buds about 6 millimeters long. If one looks carefully, near the base of the dormant staminate catkins will be seen several immature female cones only 3 or 4 millimeters long. It will also be evident that the empty cones from last summer were produced on branches developed during the previous summer. These are located at the base of the most recent growth, and sometimes the cones from two summers ago may still be seen. Last summer's staminate cones, unlike the female cones, are discarded soon after the pollen is released. The staminate catkins have very small, stamen,bearing flowers borne at the base of the bracts making up the catkin. Each of these tiny flowers con, sists of four sepals and three or four stamens but no petals. The female cones have two pistillate flowers at the base of each scale; each pistillate flower has a single pistil but no sepals or petals. The flowers are wind,pollinated, and, as is typical of such plants, an overabundance of pollen is produced. The alder is often first to come into bloom in spring, frequently as early as Febru, ary, if there have been a few warm days. Subsequent to blooming, the leaves rapidly enlarge and reach maturity. Each pistil matures to produce a single small nutlet bordered by a thin wing which aids its dispersal by wind or water. One notable feature of alders is their ability to fix atmospheric nitrogen. Most people are familiar with this ability as a characteristic of the bean fam, ily. In the latter, specialized nodules which contain a certain bacterium oc, cur on the roots. The bacteria are able to trap free nitrogen from the air and assimilate it into living protoplasm. Eventually, this nitrogen becomes avail, able to the host plant. This same bacterium, incidentally, is unable to cap, ture nitrogen when it occurs as a free'living form in the soil. Thus, the bacterium gets nourishment from the host plant, while the host in tum benefits from the nitrogen captured by the "parasite" on the roots. This kind of mutually dependent cooperation goes by the name of symbiosis among biologists. At any rate, nodules, like those in the bean family, occur on the roots of alders. But these nodules contain a fungus known as an actinomycete, rather than the bacterium common to members of the bean family. This fungus, like the bacterium, is able to assimilate atmospheric nitrogen. AI, ders, then, aside from the other benefits they confer on their habitats, help...