5/14/2011

Pioneering Soil-Enhancer: Red Alder

Red alder grows in pure stands, as shown, or in mixed forest settings

Red alder (Alnus rubra) is the most common deciduous tree in the Pacific coast forest. It is not a long-lived tree (usually about 60 yrs, rarely over 100), and despite rapid growth while young, these trees never reach (40 m/131 ft) the sizable proportions of the conifer stands surrounding them. However, the Red alder possesses a capacity that no other tree on the coast has.

Red alder is the only tree west of the Rockies capable of fixing atmospheric nitrogen. Red alder develops an extensive, fibrous root system, with root nodules that fix nitrogen from the air. The nodules are a symbiotic association between the tree and actinomycete, a large group of bacteria that are responsible for the characteristic "earthy" smell of freshly turned, healthy soil.

Male flowers are long, drooping, reddish catkins, and female flowers
are short, woody, brown cones.
In this 'everyone wins' relationship, nodules are formed where bacteria infect a growing root. Then the alder supplies carbon compounds to the bacteria, and the bacteria convert nitrogen from air into a form the tree can use.

When leaves or roots from the host plant decompose, available nitrogen increases resulting in richer soil and better growing conditions. There is evidence that this relationship is so beneficial to the alder that its roots produce a substance that encourages the bacteria to set up shop. Alder is also associated with mycorrhizal fungi which helps enhance nitrogen fixation.

Alder along Carmanah River, Carmanah Park
After the soil bacteria community is established, Red alder seeds germinate well on freshly disturbed, nitrogen deprived land such as slide areas, road cuts, river flood plains, or clear cut areas that have had the slash burned. It is an aggressive pioneer species, and grows quickly as a seedling in wet soils.

Height growth of the Red alder seedling can be as much as 1 m (3.3 ft) or more in the first year. Giving the Black cottonwood a run for its money, maximum annual height growth of more than 3 m (9.8 ft) a year can be achieved by 2- to 5-year-old alder seedlings. Indeed, after the Black cottonwood, the Red alder is the second fastest growing tree in the coastal forest.

Associated conifers have much slower juvenile growth, but they sustain height growth years longer than alder. On an average site, both Douglas-fir and red alder can attain the same height at 45 years. Beyond that, Douglas-fir far surpasses Red alder in height. Once overgrown by the conifers during forest succession, the sun-loving red alder will die back, leaving a legacy of rich soil.

Red alder on their way for another season of nitrogen-fixing, soil-enriching growth
Besides its usefulness to soil quality, and its visual appeal, Red alder is also a commercially harvested species along the coast. It is used in the production of solid wood products, such as furniture, cabinets, case goods, and pallets; composite products, including plywood; and fiber-based products, such as tissue and writing paper. It is also used for firewood, and is the wood of choice for smoking salmon.

Although for a while Red-alder, like Black cottonwood, was seen as a nuisance tree and cut out of mixed stands, we are now realizing the soil-healing properties this tree has. Because it can increase soil nitrogen and organic matter, it is being increasingly used to rehabilitate disturbed sites, or prepare areas for conifers such as Douglas-fir.

The Red alder is an important part of the Pacific coast forest ecosystem. Look for them along streams and rivers, as well as lake shores. Disturbed sites will also have stands of alder pioneering the new forest. Watch for a medium-sized, deciduous broad-leaved tree with a narrow rounded crown, straight, slightly tapered stem, and smooth, light gray bark.

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