Harrington, C.A., J.C. Zasada, and E.A. Allen. 1994. Biology of red alder (Alnus rubra Bong.). Pages 3-22 in Hibbs, D.E., D.S. DeBell, and R.F. Tarrant, eds. The Biology and Management of Red Alder. Corvallis, OR: Oregon State University Press. 256 p. Biology of Red Alder (Alnus rubra Bong.) CONSTANCE A. HARRINGTON, JOHN C. ZASADA, & ERIC A. ALLEN Red alder (Alnus rubra), also called Oregon alder, can genera in this family are the birches (Betula) western alder, and Pacific coast alder, is the most and hazelnuts (Corylus) . The most conspicuous common hardwood in the Pacific Northwest. It is feature which these three genera have in common a relatively short-lived, intolerant pioneer with is the presence of male catkins (compact aggregates rapid juvenile growth, the capability to fix atmo­ of staminate flowers) (Brayshaw 1976). The seed­ spheric nitrogen, and tolerance of wet soil bearing catkins in alder and birch are similar when conditions. The species is favored by disturbance immature, but the birch catkin disintegrates as and often increases after harvesting and burning. seeds are dispersed, while the alder catkin remains Because the commercial value of alder has tradi­ intact and attached to the plant during seed dis­ tionally been lower than that of its associated persal and for a time after dispersal is completed. conifers, many forest managers have tried to elimi­ More detailed information on the taxonomy and nate the species from conifer stands. On the other evolution of the species is presented in Ager and hand, red alder is the major commercial hardwood Stettler (Chapter 6). When information is not avail­ tree species in the region; its wood is used for fur­ able on flowering or seed production and dissem­ niture, cabinets, pallets, and to make paper ination of red alder, we present information from (Harrington 1984b). Its value has increased sub­ studies of other species in Betulaceae which have stantially in recent years and interest in the similar reproductive structures. management of the species has increased accord­ ingly, which in turn has led to an increased need Genetic Variation for detailed information on the biology of the spe­ No races of red alder have been described, though cies. This chapter summarizes published informa­ they may exist, especially in the disjunct popula­ tion as well as unpublished data and observations tions or in the extremes of the range. One on the biology and ecology of red alder with em­ researcher has divided the species into three popu­ phasis on topics not covered by other authors in lations (northern, central, and southern) based on this volume and on information that has become vegetative and reproductive features from her­ available since previous summaries were published barium specimens (Furlow 1974). (Trappe et al. 1968; Briggs et al. 1978; Heebner and Geographic variation in growth rates, sensitiv­ Bergener 1983). Much of our knowledge on many ity to frost, and other characteristics has been aspects of the biology of red alder is based on lim­ reported (DeBell and Wilson 1978; Lester and ited information; thus, where it is appropriate, we DeBell 1989; Ager and Stettler, Chapter 6). In one point out subject areas where common assump­ study, provenances from areas with cold winters tions or beliefs about red alder are not supported (i.e., Alaska, Idaho, high elevations in Washington by specific data. and Oregon) had the poorest growth but the great­ est resistance to frost damage. Specific gravity did Taxonomy not differ significantly among provenances, nor was Red alder (genus Alnus) is a member of the family it correlated with growth rate (Harrington and Betulaceae. Other common western North Ameri- DeBell 1980). In another study which compared 3 families from coastal sources, it was possible to generally occurs close to sea level. Farther south, identify families with high growth rates and low scattered trees are found as high as 1100 m, but sensitivity to spring frosts (Peeler and DeBell 1987; most stands are at elevations below 750 m. Red ai­ DeBell et al. 1990). A 24-family progeny trial in der seldom grows east of the Cascade Range in western Washington also demonstrated family Oregon and Washington or the Sierra Nevada in variation in height-growth response to water-table California, although several isolated populations depth (Hook et al. 1987). exist in northern Idaho (Johnson 1968a, 1968b). Phenotypic variation between trees is also high. Differences in form and in characteristics of Climate branch, bark, and wood were assessed for eight Red alder grows in climates varying from humid stands in western Washington; only bark thickness, to superhumid. Annual precipitation ranges from a branch diameter index, branch angle, and a 400 to 5600 mm, most of it as rain in winter. crown-width index differed significantly among Summers generally are warm and dry in the stands (DeBell and Wilson 1978). Variation among southern part of the range and cooler and wetter trees in seed production has also been reported (see in the northern portion. Temperature extremes discussion below). range from -30°C in Alaska and Idaho to 46°C in A cut-leaf variety (Alnus rubra var. pinnatisecta) California. Low winter temperatures and lack of is found in a few isolated areas in British Colum­ precipitation during the growing season appear to bia, Washington, and Oregon. The cut-leaf char­ be the main limits to the range of red alder. For acteristic is caused by a single recessive gene (Wil­ good development of trees, either annual precip­ son and Stettler 1981), and thus this variety can itation should exceed 630 mm or tree roots should be used as a marker in genetic breeding studies have access to ground water. (Stettler 1978). As forest managers plant red alder on increas­ Soils and Topography ing acreage, we anticipate the need for additional Red alder is found on a wide range of soils, from information on genetic variation in the species. well-drained gravels or sands to poorly drained Preliminary recommendations are available on seed clays or organic soils. In Washington and Oregon zones for red alder (Hibbs and Agel' 1989; Agel' et it grows primarily on soils of the orders Inceptisols al., Chapter 11) and the major tree improvement and Entisols but is also found on some Andisols, options and long-term breeding prospects for the Alfisols, Uitisols, Spodosols, and Histosols species are discussed later in this proceedings (Agel' (Harrington and Courtin, Chapter 10). In British and Stettler, Chapter 6). Information, however, is Columbia, alder occurs on Brunisols, Gleysols, Or­ lacking on the variation within the species in its ganics, Podzols, and Regosols (Harrington and tolerance of low nutrient or low soil moisture con­ COUltin, Chapter 10). Best stands are found on deep ditions and on the possible interactions among alluvial soils in river and stream flood plains; how­ silvicultural practices, genotype, and wood quality ever, some excellent stands are also found on characteristics. upland sites on residual or colluvial soils derived from volcanic materials. Habitat Soil moisture during the growing season appears Native Range to influence where the species grows. Alder can tol­ Red alder occurs most commonly as a lowland spe­ erate poor drainage conditions and some flooding cies along the northern Pacific coast. Its range during the growing season; consequently, it pre­ extends from southern California (Jat. 34° N) to vails on soils where drainage is restricted-along southeastern Alaska (60° N). Red alder is generally stream bottoms or in swamps or marshes. It is not found within 200 km of the ocean and at elevations commonly found on droughty soils, however; and below 750 m. Tree development is best on eleva­ in areas of low precipitation, it seldom grows on tions below 450 m in northern Oregon, Wash­ steep south- or southwest-facing slopes. In Idaho ington, and British Columbia. In Alaska, red alder and California, stands are usually limited to bor­ ders of streams or lakes. 4 The Biology and Management of Red Alder Associated Forest Cover Forest Succession Red alder grows in both pure and mixed stands. Red alder is a pioneer species favored by high light Pure stands are typically confined to stream bot­ levels and exposed mineral soil. Its ability to fix at­ toms and lower slopes. Red alder is, however, much mospheric nitrogen permits establishment on more widely distributed as a component of mixed geologically young or disturbed sites with low lev­ stands. It is a major component of the Red Alder els of soil nitrogen. It can form pure stands on cover type (Society of American Foresters Type 221) alluvium, avalanche paths, and other disturbed and occurs as a minor component in most of the sites. Alder has been favored by harvesting and other North Pacific cover types (Eyre 1980). burning; pollen records indicate that alder stands Common tree associates are Douglas-fir are more extensive in the tvventieth century than (Pseudotsuga menziesii), western hemlock (Tsuga they were for several centuries before that time heterophylla), western redcedar (Thuja plicata) , (Heusser 1964; Davis 1973). Red alder pollen, how­ grand fir (Abies grandis), Sitka spruce (Picea ever, was also abundant between 9000 and 4800 sitchensis), black cottonwood (Populus tricho­ B.C.; this has been interpreted as indicating a carpa), bigleaf maple (Acer macrophyllum), and somewhat warmer climate accompanied by an in­ willow (Salix spp.). Occasional tree associates in­ crease in fire frequency (Cwynar 1987). clude cascara buckthorn (Rhamnus purshiana), Observations of mature forests in the Pacific Pacific dogwood (Comus nuttallii), and Oregon ash Northwest suggest that alder stands are ultimately (Fraxinus la tifolia). Western paper birch (Betula replaced by longer-lived, more tolerant conifers papyrifera var. commutata) is an occasional asso­ having sustained growth rates at older ages than ciate in the northern portion of the range of alder, does alder.