Alnus rubra

"Red alder" redirects here. For the Southern African tree, see Cunonia capensis.

Red alder
Red alder leaves
Scientific classification
Kingdom:	Plantae
(unranked):	Angiosperms
(unranked):	Eudicots
(unranked):	Rosids
Order:	Fagales
Family:	Betulaceae
Genus:	Alnus
Subgenus:	Alnus
Species:	A. rubra
Binomial name
Alnus rubra Bong.
Natural range of Alnus rubra
Synonyms
Alnus incana var. rubra (Bong.) Regel Alnus oregana Nutt. Alnus rubra var. pinnatisecta Starker Alnus rubra f. pinnatisecta (Starker) Rehder

Alnus rubra, the red alder, is a deciduous broadleaf tree native to western North America (Alaska, Yukon, British Columbia, Washington, Oregon, California, Idaho and Montana).

Description

Male catkins with tiny female catkins above

It is the largest species of alder in North America and one of the largest in the world, reaching heights of 20 to 30 m (66 to 98 ft). The official tallest red alder (1979) stands 32 m (105 ft) tall in Clatsop County, Oregon (USA). The name derives from the bright rusty red color that develops in bruised or scraped bark. The bark is mottled, ashy-gray and smooth, often draped with moss. The leaves are ovate, 7 to 15 centimetres (2.8 to 5.9 in) long, with bluntly serrated edges and a distinct point at the end; the leaf margin is revolute, the very edge being curled under, a diagnostic character which distinguishes it from all other alders. The leaves turn yellow in the autumn before falling. The male flowers are dangling reddish catkins 10 to 15 cm (3.9 to 5.9 in) long in early spring, and female flowers are erect catkins which develop into small, woody, superficially cone-like oval dry fruit 2 to 3 cm (0.79 to 1.18 in) long. The seeds develop between the woody bracts of the 'cones' they are shed in the autumn and winter.

Distribution

Alnus rubra grows from southeast Alaska south to central coastal California, nearly always within about 200 km (120 mi) of the Pacific coast, except for an extension 600 km (370 mi) inland across Washington and Oregon into northernmost Montana.

Ecology

Red alder, western hemlock and bigleaf maple forest

In southern Alaska, western British Columbia and the northwestern Coast Ranges of the United States, red alder grows on cool and moist slopes; inland and at the southern end of its range (California) it grows mostly along the margins of watercourses and wetlands.

Commonly associated trees

Red alder is associated with coast Douglas-fir Pseudotsuga menziesii subsp. menziesii, western hemlock Tsuga heterophylla, grand fir Abies grandis, western redcedar Thuja plicata, and Sitka spruce Picea sitchensis forests.

Along streambanks it is commonly associated with willows Salix spp., red osier dogwood Cornus stolonifera, Oregon ash Fraxinus latifolia and bigleaf maple Acer macrophyllum.

In marginal habitat

To the southeast of its range it is replaced by white alder (Alnus rhombifolia), which is closely related but differs in the leaf margins not being rolled under. In the high mountains it is replaced by the smaller Sitka alder (Alnus viridis subsp. sinuata), and east of the Cascade Mountains by thinleaf alder (Alnus incana subsp. tenuifolia).

As pioneer species

In moist forest areas Alnus rubra will rapidly cover a former burn or clearcut, temporarily preventing the growth of conifers but also improving soil fertility for future growth of conifers. It is a prolific seed producer, but the seeds require an open area of mineral soil to germinate, and so skid trails and other areas disturbed by logging or fire are ideal seedbeds. Such areas may host several hundred thousand to several million seedlings per hectare in the first year after landscape disturbance (Zavitkovski & Stevens, 1972).

Role as wildlife fodder

Twigs and buds of alder are only fair browse for wildlife, though deer and elk do browse the twigs in fall and twigs and buds in the winter and spring. Beavers eat the bark. Several finches eat alder seeds, notably common redpoll and pine siskin, and as do deer mice.

As soil enricher

Alnus rubra is also very valuable for playing host to the nitrogen fixing actinomycete Frankia. It is this ability which allows alder to grow in nitrate-poor soils.

Uses

Broken branch showing red weathered bark

Red Alder leaf, showing the typical discolouration caused by ozone pollution.

As dye

A russet dye can be made from a decoction of the bark and was used by Native Americans to dye fishing nets so as to make them less visible underwater.

Traditional medicine usage

Native Americans used red alder bark (Alnus rubra) to treat poison oak, insect bites, and skin irritations. Blackfeet Indians used an infusion made from the bark of red alder to treat lymphatic disorders and tuberculosis. Recent clinical studies have verified that red alder contains betulin and lupeol, compounds shown to be effective against a variety of tumors.

In restoration

Alnus rubra is an important forestry tree. Its rapid growth makes it useful in covering disturbed land, such as mine spoils. Alder leaves, shed in the fall, decay readily to form a nitrogen-enriched humus. It is being considered as a rotation crop to discourage the conifer root pathogen Phellinus weirii (Laminated root rot).

Alnus rubra are planted as ornamental trees and will do well in wet swales, riparian areas, or on stream banks.

If used domestically they should be planted well away from drainpipes, sewage pipes, and water lines, as the roots may invade and clog the lines. Alnus rubra are known for growing easily in burned or destroyed land, and are used as "pioneering" or "reclamation" trees.

In woodworking

Alder lumber is not considered to be a durable option for outdoor applications, but due to its workability and ease of finishing it is increasingly used for furniture and cabinetry. Because it is softer than other popular hardwoods such as maple, walnut and ash, historically alder has not been considered of high value for timber. However it is now becoming one of the more popular hardwood alternatives as it is more economically priced when compared to other hardwoods. In the world of musical instrument construction, red alder is valued by some electric guitar / electric bass builders for its balanced tonality. Alder is frequently used by Native Americans for making masks, bowls, tool handles, and other small goods.

The appearance of alder lumber ranges from white through pinkish to light brown, has a relatively soft hardwood texture, and has medium luster. It is easily worked, glues well, and takes a good finish.

In fish smoking

Because of its oily smoke, A. rubra is the wood of choice for smoking salmon.

As an environmental indicator

Additionally, red alder is often used by scientists as a biomonitoring organism to locate areas prone to ozone pollution, as the leaves will react to the presence of high ozone levels by developing red to brown or purple discolourations.

Forestry

The vigorous growth has in the past earned it the designation of a "trash tree" by the timber industry. Herbicide spraying of red alder over large areas of coastal Oregon and Washington has resulted in a number of lawsuits, with the claim this spraying has caused health problems, including birth defects and other side effects.

Increasing value of the wood, combined with a better understanding of the species' benefits to other trees, has largely led to a cessation of this practice.

Genetic Diversity

For a population to adapt to changes in the environment, genetic diversity must exist. Endangered species tend to have a smaller amount of individuals in the population. There may be a positive correlation between population size and the genetic diversity in a population, suggesting that endangered species show lower genetic diversity. Therefore, the endangered Red Adler has less genetic diversity than unthreatened species.

Red alders have a narrow latitude range, around 52°, that they thrive in without maladaptation and where they have the highest ability to spread seeds and reproduce. Since genetic diversity in a population is greater in species with broader ranges, the Red Adler has insufficient space to thrive and faces low levels of genetic diversity.

The low levels of genetic diversity in Red Adler populations are further evidenced by minimal variation between some traits. For example, the lean, stem form index, crown width index, clear bole index, branch angle, branch diameter index, and wood density show few differences in diversity, with only three traits having a moderate amount of genetic diversity. Low levels of genetic diversity make it difficult to determine potential environmental effects on important traits in a population. Furthermore, inbreeding in Red Adler populations can also partially explain the low levels of genetic variation.

Genetic variation in populations can greatly influence, and be influenced by, surrounding ecological processes. Plants that show greater genetic diversity attract a greater amount of insect populations to feed because they offer a wider variety of choices to eat from and live in. Since the Red Alder does not have high genetic diversity; the ecosystem around it will hardly be affected and contain a lower amount of populations.

Further references and external links

• Flora of North America: Alnus rubra
• Plants of British Columbia: Alnus rubra
• Jepson Flora: Alnus rubra
• Interactive Distribution Map of Alnus rubra