— Red Alder: A State of Knowledge — Biology and Ecology of Red Alder Constance A. Harrington1 Abstract Red alder is the most common hardwood in the and pruning and thinning are feasible operations. Stem rots Pacific Northwest with a range stretching from coastal do not result in high volumes of damage overall but can be southeast Alaska to southern California and east to isolated locally important; ring shake can also be a serious problem populations in Idaho. Soil moisture during the growing in some stands. Insects and diseases are not generally season influences where it grows and its growth rates; it a problem in young stands although insect defoliators, can tolerate poor drainage but not droughty, hot sites. Due Nectria cankers, and alder bark beetles can cause problems. to its tolerance of wet soil conditions, alder is common in Meadow mice, voles, and beaver can also hinder stand riparian areas. Alder can be injured by spring and fall frosts establishment in some areas. Deer or elk may rub small and is not found at elevations above 1100 m anywhere in alder saplings during the fall with their antlers but do not its range. The species produces small, very light seeds that usually browse its leaves and twigs. Sapsucker damage is disperse over long distances; it is favored by disturbance sporadic, but if stems are repeatedly damaged, their future and often increases in abundance after logging or burning. log value will be greatly reduced. Mortality and top damage Alder establishment via seed is not assured, however, as have been observed after ice or early snowstorms. Red alder drought and heat injury, pathogens, animals and other is now being managed for a wider range of objectives and factors often destroy seedlings. Alder has nitrogen-fixing on many more sites than in the past; this is due to recent nodules on its roots that directly and indirectly increase increases in wood value of alder relative to other species as nitrogen in forest ecosystems. Alder usually has a spreading well as its ability to fill specialized niches (e.g., add nitrogen fibrous root system when young and can root deeply if soil to forest ecosystems, immunity to laminated root rot) and aeration is not limiting. When grown in dense stands, its produce sawlogs on relatively short rotations. Management shade intolerance results in rapid mortality of shaded stems experience with the species, however, is still limited to a and lower branches. It is a relatively short-lived, intolerant fairly narrow range of sites and management scenarios. pioneer with rapid juvenile growth. This pattern of height Thus, information on the biology and ecology of the species growth—very rapid when young and slowing quickly at a should help guide managers until more direct experience is relatively young age—means that if thinning is delayed, available. alder can not rapidly build crown and increase in diameter growth. Although alder wood stains and decays rapidly after Keywords: Alnus rubra, biology, ecology, damaging agents, death, live trees compartmentalize decay very efficiently, growth, red alder, regeneration This paper was published in: Deal, R.L. and C.A. Harrington, eds. 2006. Red alder—a state of knowledge. General Technical Report PNW-GTR-669. Portland, OR: U.S. Department of Agriculture, Pacific Northwest Research Station. 150 p. 1Constance A. Harrington is research forester, USDA Forest Service, Pacific Northwest Research Station, 3625 93rd Ave. SW, Olympia, WA 98512-9193, email: [email protected] 21 Introduction greatest resistance to frost damage. Specific gravity did not differ significantly among provenances, nor was it Red alder (Alnus rubra), also called Oregon alder, correlated with growth rate (Harrington and DeBell 1980). western alder, and Pacific coast alder, is the most common In another study that compared families from coastal hardwood in the Pacific Northwest. It is a relatively sources, it was possible to identify families with high short-lived, intolerant pioneer with rapid juvenile growth, growth rates and low sensitivity to spring frosts (Peeler the capability to fix atmospheric nitrogen, and tolerance and DeBell 1987, DeBell et al. 1990); work on laboratory of wet soil conditions (although best growth is not on techniques to predict frost tolerance of alder families is poorly drained sites, see below). The species is favored ongoing (Johnson and Herman 1995). A 24-family progeny by disturbance and often regenerates after harvesting trial in western Washington also demonstrated family and burning. Because the commercial value of alder has variation in height-growth response to water-table depth traditionally been lower than that of its associated conifers, (Hook et al. 1987). many forest managers have tried to eliminate the species Phenotypic variation between trees is also high. from conifer stands. On the other hand, red alder is the Differences in form and in characteristics of branch, major commercial hardwood tree species in the region; bark, and wood were assessed for eight stands in western its wood is used for furniture, cabinets, pallets, and to Washington; only bark thickness, a branch diameter make paper (Harrington 1984b). Its value has increased index, branch angle, and a crown-width index differed substantially in recent years which has led to greater significantly among stands (DeBell and Wilson 1978). interest in the management of the species, and thus, to a Variation among trees in seed production has also been need for more information on its biology. This chapter is reported (see discussion below). a revised version of Harrington et al. 1994; it summarizes published information as well as unpublished data and A cut-leaf variety (Alnus rubra var. pinnatisecta) observations on the biology and ecology of red alder with (fig. 1) was first reported near Portland, OR (Starker emphasis on topics not covered by other authors in this 1939); the cut-leaf variety has since been found in several volume and on information that has become available since other isolated areas in British Columbia, Washington, previous summaries were published (Trappe et al. 1968, and Oregon. A single recessive gene causes the cut-leaf Briggs et al. 1978, Heebner and Bergener 1983, Hibbs et al. characteristic (Wilson and Stettler 1981); thus, this variety 1994). can be used as a marker in genetic breeding studies (Stettler 1978). Taxonomy and Genetics As forest managers plant red alder on increasing acreage, the need will increase for additional information Red alder (genus Alnus) is a member of the family on genetic variation in the species. Preliminary recom- Betulaceae. The most conspicuous feature that the North mendations are available on seed zones for red alder American genera in this family have in common is the (Hibbs and Ager 1989, Ager et al. 1994) and the major tree presence of male catkins (compact aggregates of staminate improvement options and long-term breeding prospects flowers) (Brayshaw 1976) that disintegrate after pollen for the species have been discussed (Ager and Stettler shed. The seed-bearing catkins in alder remain intact and 1994). Information, however, is lacking on the variation attached to the plant during seed dispersal and for a time within the species in its tolerance of low nutrient or low after dispersal is completed. More detailed information on soil moisture conditions and on the possible interactions the taxonomy and evolution of red alder is presented in Ager among silvicultural practices, genotype, and wood quality and Stettler (1994). characteristics. Genetic Variation Habitat No races of red alder have been described, though they may exist, especially in disjunct populations or in the Native Range extremes of the range. One researcher divided the species Red alder occurs most commonly as a lowland species into three populations (northern, central, and southern) along the northern Pacific coast. Its range extends from based on vegetative and reproductive features from southern California (lat. 34° N) to southeastern Alaska herbarium specimens (Furlow 1974). (60° N). Red alder is generally found within 200 km of the Geographic variation in growth rates, sensitivity to ocean and at elevations below 750 m. Tree development frost, and other characteristics has been reported (DeBell is best at elevations below 450 m in northern Oregon, and Wilson 1978, Lester and DeBell 1989, Ager and Washington, and British Columbia. In Alaska, red alder Stettler 1994). In one study, provenances from areas generally occurs close to sea level. Farther south, scattered with cold winters (i.e., Alaska, Idaho, high elevations in trees are found as high as 1100 m, but most stands are at Washington and Oregon) had the poorest growth but the much lower elevations. Red alder seldom grows east of the 22 Cascade Range in Oregon and Washington or the Sierra spruce (Picea sitchensis), black cottonwood (Populus Nevada Mountains in California, although several isolated trichocarpa), bigleaf maple (Acer macrophyllum), and populations exist in northern Idaho (Johnson 1968a, 1968b). willow (Salix spp.). Occasional tree associates include cascara buckthorn (Fangula purshiana), Pacific dogwood Climate (Cornus nuttallii), Oregon ash (Fraxinus latifolia), bitter cherry (Prunus emarginata), and Pacific silver fir (Abies Red alder grows in climates varying from humid to amabilis). Western paper birch (Betula papyrifera var. superhumid. Annual precipitation ranges from 400 to commutata) is an occasional associate in the northern 5600 mm, most of it as rain in winter. Summers generally and eastern portions of the range of alder, and redwood are warm and dry in the southern part of the range and (Sequoia sempervirens) in the southern portion. cooler and wetter in the northern portion. Temperature extremes range from -30°C in Alaska and Idaho to 46°C In the western hemlock and Sitka spruce zones, in California. Low winter temperatures and lack of alder is most vigorous in plant associations dominated precipitation during the growing season appear to be the in the understory by western swordfern (Polystichum main limits to the range of red alder.