Alnus rhombifolia Nutt.

white alder / California alder

Betulaceae

The Basics

USFS Plant Database

Native American Ethnobotany, University of Michigan)

Flora of North America

The scientific name of white alder is Alnus rhombifolia Nutt. (Betulaceae). White alder hybridizes with thinleaf alder in southwestern Idaho.

White alder grows along permanent streams and adjacent slopes. On 4 sites across western Oregon, its presence was positively correlated with streamside, and on the Lassen National Forest, California, white alder was negatively correlated with distance from streams. It is mostly restricted to flood zones and becomes infrequent farther upland. The most well-developed white alder forests are along rapid, well-aerated perennial streams with high spring runoffs. In dry years, white alder is a better indicator of moist soils than either cottonwoods or willows. White alder is most common along rivers and third-order streams. In the Siskiyou Mountains of California and Oregon, it was most frequent (90%) on the most mesic sites by large streams. It grows along small streams in the Coast Ranges and the Sierra Nevada, however. For example, white alder is most common on upper stream reaches in the Central Coast Ranges above Carmel Valley. (Plant Database)

White alder has numerous ethnobotanic applications. Traditionally, Native Americans formulated many medicines using different parts of the tree. Decoctions (boiled extractions) made with dried bark were used as treatments for diarrhea, dermatological issues, consumption, blood purifiers, and facilitation of childbirth. Dried wood was also used to make poultices for burns.

The bark can also be used to make a reddish dye, customarily used to paint fishermen’s bodies and as a dye for basketsand deerskins. When burned, the bark smoke could tan white buckskin yellow. White alder roots were harvested to make baskets, and young shoots served as material to make arrows. (Native American Ethnobotany)

Identification

Alnus rhombifolia is deciduous and has a medium growth form. Individuals range from 16 to 82 feet tall, with DBH ranging from 11 inches to 21 inches. One tree may have several trunks with spreading, open crowns. The bark is smooth and a light-gray color. As the tree ages, the bark darkens and becomes scaly and lenticels turn inconspicuous (lenticel: slightly corky, raised striations most often seen on young stems). White alder leaves are slightly elliptic (wider in the middle than at either end) and range from 4-9 centimeters long and 2-5 centimeters wide. The leaf base is cunneate, meaning it is wedge shaped and triangularly tapers. The margins are flat and serrate or serrulate (a serrate edge with additional smaller, sub-serrations). The abaxial side of the leaf (underside) can be slightly or densely hairy (sparsely pubescent to villous). (Flora of North America)

Threats

Most white alder riparian zones see moderate fire return intervals of 50-70 years. White alder is maladapted to dealing with fire; its thin bark offers little protection. Though fuel loads tend to be high, discontinuous canopy impedes crown fire from spreading. Post-fire seedling establishment and sprouting is weak. Although it is primarily a primary and secondary successional species, sprouting from other hardwoods tend to out-compete white alder. However, fire is less of a frequent concern due to the nature of riparian areas; flood and streambank failure more frequently set back succession.

Foraging from cattle, mule deer, feral pigs, and jackrabbits remove many fine fire fuels. Direct impacts of cattle are more readily seen in the surrounding ecosystem, rather than white alder stands alone. (Plant Database)

Reproduction

White alder is monoecious. The summer before flowering, white alder forms catkins (a dense spike or raceme of non-petaled, unisexual flowers). After fertilization, pistillate catkins become cone-shaped and woody. The following spring, flowering occurs before stem and leaf growth. White alder is wind pollinated; its fruits are narrow-winged samaras containing a small nut. Seed banks stay viable for a few months, but lack an endosperm and are prone to being washed away. Germination occurs in well-sunned areas made of wet mineral soils. White alder may be able to reproduce through layering, though this is rarely seen. (Plant Database)

Species Distribution

Alnus rhombifolia is mainly found in California, the Pacific Northwest, and parts of Idaho and Nevada. It primarily grows in riparian conifer forests and riparian oak woodlands. Riparian areas with white alder are often densely vegetated and play host to a wide variety of species. White alder is restricted to flood zones; it roots just above floodplains and tidal zones. Its range is negatively correlated with distance from streams. Though commonly found in steep canyons with narrow riparian corridors, white alder only lives in montane areas of low elevation: there are a few areas where white alder will grow up to 8,000 feet, but it is more likely to be found around 3,000 feet.

White alder is native to the western United States; it is the most common alder in the mediterranean region of the West. It is distributed from San Diego County, California, north to Chelan County, Washington, and east to Idaho County, Idaho. Its core distribution lies in the foothills of northern California and southwestern Oregon. Isolated populations occur in extreme western Nevada and western Montana. Some report that its southern distribution probably extends to northwestern Baja California Norte, but this is undocumented and refuted by others. (Plant Database)

Citation

USDA Plant Database http://plants.usda.gov/characteristics.html USDA, NRCS. 2016. The PLANTS Database (http://plants.usda.gov, 4 February 2016). National Plant Data Team, Greensboro, NC 27401-4901 USA.

Flora of North America http://www.efloras.org/flora_page.aspx?flora_id=1 Flora of North America Editorial Committee, eds. 1993+. Flora of North America North of Mexico. 19+ vols. New York and Oxford.

Silvics of North America Burns, R.M., and B.H. Honkala. 1990. Silvics of North America (Volume 1: Conifers, Volume 2: Hardwoods). USDA Forest Service Agricultural Handbook 654.

Intermountain Herbarium http://intermountainbiota.org/portal/collections/harvestparams.php Consortium of Intermountain Herbaria. 2016. http//:intermountainbiota.org/portal/index.php. Accessed on February 04.

Burke Museum Plant Image Collection The plant image collection at the Burke Museum, University of Washington.

Jepson Manual http://ucjeps.berkeley.edu/eflora/ The Jepson Manual: Vascular Plants of California. B.G. Baldwin, D.H. Goldman, D.J. Keil, R. Patterson, T.J. Rosatti, and D.H. Wilken [editors]. 2012. 2nd edition, thoroughly revised and expanded. University of California Press, Berkeley, CA., hardcover; 1600 pages. ISBN-13: 978-0520253124.

USGS Plant Species Range Maps http://esp.cr.usgs.gov/data/little/ Critchfield, W.B., and Little, E.L., Jr., 1966, Geographic distribution of the pines of the world: U.S. Department of Agriculture Miscellaneous Publication 991, p. 1-97. Little, E.L., Jr., 1971-1978, Atlas of United States trees, volume 1,3,13,17, conifers and important hardwoods: U.S. Department of Agriculture Miscellaneous Publications.

Photos ©Susan McDougall. Trees Live Here.