21

— Red Alder: A State of Knowledge —

Biology and Ecology of Red Alder

Constance A. Harrington1

1

Constance A. Harrington is research forester, USDA Forest Service, Pacific Northwest Research Station, 3625 93rd Ave. SW, Olympia, WA 98512-9193, 

email: charrington@fs.fed.us

Abstract

Red alder is the most common hardwood in the 

Pacific Northwest with a range stretching from coastal 

southeast Alaska to southern California and east to isolated 

populations in Idaho. Soil moisture during the growing 

season influences where it grows and its growth rates; it 

can tolerate poor drainage but not droughty, hot sites. Due 

to its tolerance of wet soil conditions, alder is common in 

riparian areas. Alder can be injured by spring and fall frosts 

and is not found at elevations above 1100 m anywhere in 

its range. The species produces small, very light seeds that 

disperse over long distances; it is favored by disturbance 

and often increases in abundance after logging or burning. 

Alder establishment via seed is not assured, however, as 

drought and heat injury, pathogens, animals and other 

factors often destroy seedlings. Alder has nitrogen-fixing 

nodules on its roots that directly and indirectly increase 

nitrogen in forest ecosystems. Alder usually has a spreading 

fibrous root system when young and can root deeply if soil 

aeration is not limiting. When grown in dense stands, its 

shade intolerance results in rapid mortality of shaded stems 

and lower branches. It is a relatively short-lived, intolerant 

pioneer with rapid juvenile growth. This pattern of height 

growth—very rapid when young and slowing quickly at a 

relatively young age—means that if thinning is delayed, 

alder can not rapidly build crown and increase in diameter 

growth. Although alder wood stains and decays rapidly after 

death, live trees compartmentalize decay very efficiently, 

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.

and pruning and thinning are feasible operations. Stem rots 

do not result in high volumes of damage overall but can be 

locally important; ring shake can also be a serious problem 

in some stands. Insects and diseases are not generally 

a problem in young stands although insect defoliators, 

Nectria cankers, and alder bark beetles can cause problems. 

Meadow mice, voles, and beaver can also hinder stand 

establishment in some areas. Deer or elk may rub small 

alder saplings during the fall with their antlers but do not 

usually browse its leaves and twigs. Sapsucker damage is 

sporadic, but if stems are repeatedly damaged, their future 

log value will be greatly reduced. Mortality and top damage 

have been observed after ice or early snowstorms. Red alder 

is now being managed for a wider range of objectives and 

on many more sites than in the past; this is due to recent 

increases in wood value of alder relative to other species as 

well as its ability to fill specialized niches (e.g., add nitrogen 

to forest ecosystems, immunity to laminated root rot) and 

produce sawlogs on relatively short rotations. Management 

experience with the species, however, is still limited to a 

fairly narrow range of sites and management scenarios. 

Thus, information on the biology and ecology of the species 

should help guide managers until more direct experience is 

available.

Keywords: Alnus rubra, biology, ecology, damaging agents, 

growth, red alder, regeneration
22

Introduction

Red alder (Alnus rubra), also called Oregon alder, 

western alder, and Pacific coast alder, is the most common 

hardwood in the Pacific Northwest. It is a relatively 

short-lived, intolerant pioneer with rapid juvenile growth, 

the capability to fix atmospheric nitrogen, and tolerance 

of wet soil conditions (although best growth is not on 

poorly drained sites, see below). The species is favored 

by disturbance and often regenerates after harvesting 

and burning. Because the commercial value of alder has 

traditionally been lower than that of its associated conifers, 

many forest managers have tried to eliminate the species 

from conifer stands. On the other hand, red alder is the 

major commercial hardwood tree species in the region; 

its wood is used for furniture, cabinets, pallets, and to 

make paper (Harrington 1984b). Its value has increased 

substantially in recent years which has led to greater 

interest in the management of the species, and thus, to a 

need for more information on its biology. This chapter is 

a revised version of Harrington et al. 1994; it summarizes 

published information as well as unpublished data and 

observations on the biology and ecology of red alder with 

emphasis on topics not covered by other authors in this 

volume and on information that has become available since 

previous summaries were published (Trappe et al. 1968, 

Briggs et al. 1978, Heebner and Bergener 1983, Hibbs et al. 

1994).

Taxonomy and Genetics

Red alder (genus Alnus) is a member of the family 

Betulaceae. The most conspicuous feature that the North 

American genera in this family have in common is the 

presence of male catkins (compact aggregates of staminate 

flowers) (Brayshaw 1976) that disintegrate after pollen 

shed. The seed-bearing catkins in alder remain intact and 

attached to the plant during seed dispersal and for a time 

after dispersal is completed. More detailed information on 

the taxonomy and evolution of red alder is presented in Ager 

and Stettler (1994). 

Genetic Variation

No races of red alder have been described, though 

they may exist, especially in disjunct populations or in the 

extremes of the range. One researcher divided the species 

into three populations (northern, central, and southern) 

based on vegetative and reproductive features from 

herbarium specimens (Furlow 1974).

Geographic variation in growth rates, sensitivity to 

frost, and other characteristics has been reported (DeBell 

and Wilson 1978, Lester and DeBell 1989, Ager and 

Stettler 1994). In one study, provenances from areas 

with cold winters (i.e., Alaska, Idaho, high elevations in 

Washington and Oregon) had the poorest growth but the 

greatest resistance to frost damage. Specific gravity did 

not differ significantly among provenances, nor was it 

correlated with growth rate (Harrington and DeBell 1980). 

In another study that compared families from coastal 

sources, it was possible to identify families with high 

growth rates and low sensitivity to spring frosts (Peeler 

and DeBell 1987, DeBell et al. 1990); work on laboratory 

techniques to predict frost tolerance of alder families is 

ongoing (Johnson and Herman 1995). A 24-family progeny 

trial in western Washington also demonstrated family 

variation in height-growth response to water-table depth 

(Hook et al. 1987).

Phenotypic variation between trees is also high. 

Differences in form and in characteristics of branch, 

bark, and wood were assessed for eight stands in western 

Washington; only bark thickness, a branch diameter 

index, branch angle, and a crown-width index differed 

significantly among stands (DeBell and Wilson 1978). 

Variation among trees in seed production has also been 

reported (see discussion below).

A cut-leaf variety (Alnus rubra var. pinnatisecta)

(fig. 1) was first reported near Portland, OR (Starker 

1939); the cut-leaf variety has since been found in several 

other isolated areas in British Columbia, Washington, 

and Oregon. A single recessive gene causes the cut-leaf 

characteristic (Wilson and Stettler 1981); thus, this variety 

can be used as a marker in genetic breeding studies 

(Stettler 1978).

As forest managers plant red alder on increasing 

acreage, the need will increase for additional information 

on genetic variation in the species. Preliminary recommendations are available on seed zones for red alder 

(Hibbs and Ager 1989, Ager et al. 1994) and the major tree 

improvement options and long-term breeding prospects 

for the species have been discussed (Ager and Stettler 

1994). Information, however, is lacking on the variation 

within the species in its tolerance of low nutrient or low 

soil moisture conditions and on the possible interactions 

among silvicultural practices, genotype, and wood quality 

characteristics.

Habitat

Native Range

Red alder occurs most commonly as a lowland species 

along the northern Pacific coast. Its range extends from 

southern California (lat. 34° N) to southeastern Alaska 

(60° N). Red alder is generally found within 200 km of the 

ocean and at elevations below 750 m. Tree development 

is best at elevations below 450 m in northern Oregon, 

Washington, and British Columbia. In Alaska, red alder 

generally occurs close to sea level. Farther south, scattered 

trees are found as high as 1100 m, but most stands are at 

much lower elevations. Red alder seldom grows east of the 
23

Cascade Range in Oregon and Washington or the Sierra 

Nevada Mountains in California, although several isolated 

populations exist in northern Idaho (Johnson 1968a, 1968b).

Climate

Red alder grows in climates varying from humid to 

superhumid. Annual precipitation ranges from 400 to 

5600 mm, most of it as rain in winter. Summers generally 

are warm and dry in the southern part of the range and 

cooler and wetter in the northern portion. Temperature 

extremes range from -30°C in Alaska and Idaho to 46°C 

in California. Low winter temperatures and lack of 

precipitation during the growing season appear to be the 

main limits to the range of red alder. For good development 

of trees, either annual precipitation should exceed 630 mm 

or tree roots should have access to ground water.

Soils and Topography

Red alder is found on a wide range of soils, from welldrained gravels or sands to poorly drained clays or organic 

soils. In Washington and Oregon it grows primarily on soils 

of the orders Inceptisols and Entisols but is also found on 

some Andisols, Alfisols, Ultisols, Spodosols, and Histosols 

(Harrington and Courtin 1994). In British Columbia, alder 

occurs on Brunisols, Gleysols, Organics, Podzols, and 

Regosols (Harrington and Courtin 1994). Best stands are 

found on deep alluvial soils in river and stream flood plains; 

however, some excellent stands are also found on upland 

sites on residual or colluvial soils derived from volcanic 

materials.

Soil moisture during the growing season appears 

to influence where the species grows. Alder can tolerate 

poor drainage conditions and some flooding during the 

growing season; consequently, it is common on soils where 

drainage is restricted—along stream bottoms or in swamps 

or marshes. It is not commonly found on droughty soils, 

however; and in areas of low precipitation, it seldom grows 

on steep south- or southwest-facing slopes. In Idaho and 

California, stands are usually limited to borders of streams 

or lakes.

Associated Forest Cover

Red alder grows in both pure- and mixed-species 

stands. Pure stands are typically confined to stream 

bottoms and lower slopes. Red alder is, however, much more 

widely distributed as a component of mixed stands. It is 

a major component of the Red Alder cover type (Society 

of American Foresters Type 221) and occurs as a minor 

component in most of the other North Pacific cover types 

(Eyre 1980).

Common tree associates are Douglas-fir (Pseudotsuga 

menziesii), western hemlock (Tsuga heterophylla), western 

redcedar (Thuja plicata), grand fir (Abies grandis), Sitka 

spruce (Picea sitchensis), black cottonwood (Populus 

trichocarpa), bigleaf maple (Acer macrophyllum), and 

willow (Salix spp.). Occasional tree associates include 

cascara buckthorn (Fangula purshiana), Pacific dogwood 

(Cornus nuttallii), Oregon ash (Fraxinus latifolia), bitter 

cherry (Prunus emarginata), and Pacific silver fir (Abies 

amabilis). Western paper birch (Betula papyrifera var. 

commutata) is an occasional associate in the northern 

and eastern portions of the range of alder, and redwood 

(Sequoia sempervirens) in the southern portion.

In the western hemlock and Sitka spruce zones, 

alder is most vigorous in plant associations dominated 

in the understory by western swordfern (Polystichum 

munitum) or redwood sorrel (Oxalis oregana) (Henderson 

et al. 1989). Common associations in British Columbia 

are: alder-salmonberry (floodplain), alder-swordfern 

(more upland) and alder-sedge (Carex). Common 

species in these plant associations include salmonberry 

(Rubus spectabilis), red elderberry (Sambucus 

racemosa), thimbleberry (Rubus parviflorus), red-osier 

dogwood (Cornus stolonifera), false lily-of-the-valley 

(Maianthemum dilatatum), oval-leaf huckleberry 

(Vaccinium ovalifolium) (especially on decaying woody 

debris), youth-on-age (Tolmiea menziesii), Siberian 

miner’s lettuce (Claytonia sibirica), lady fern (Athyrium 

filix-femina), foamflower (Tiarella trifoliata), devil’s 

club (Oplopanax horridum), and coastal leafy moss 

(Plagiomnium insigne) (D. Peter, P. Courtin, pers. 

comm. 2005). Salal (Gaultheria shallon), especially 

on raised surfaces, logs or stumps, is common in 

these plant zones, but increasing amounts on mineral 

soil indicate progressively drier and poorer sites for 

red alder. Increasing amounts of devil’s club, skunk 

cabbage (Lysichiton americanum), and sedge indicate 

progressively wetter sites with greater potential for 

windthrow and ultimately poorer growth. Windthrow 

is not common in alder stands but can occur on poorly 

drained sites or where the root system has been undercut.

At higher elevations red alder is restricted by snow 

breakage. Increasing snow breakage potential is indicated 

by an increase in the occurrence of silver fir throughout 

much of the range of red alder with the notable exception 

of coastal rainforests in Washington and southern British 

Columbia. To the south, red alder is increasingly restricted 

to the Sitka spruce, redwood, and the wettest portions of the 

western hemlock zones, and especially to riparian areas. In 

these areas salmonberry is considered an especially good 

indicator of potential alder habitat (Atzet et al. 1996).

As is true in many plant communities worldwide, 

alder communities often contain non-native plants. This 

is particularly true in riparian areas as they are more open 

(many non-native plants that become problems are intolerant 

of shade) and water may serve as a transport path for seeds 

and other plant propagules. In addtion, riparian areas are 

often disturbed by flooding, animals accessing water, and 
24

in some areas by people recreating, so conditions may be 

favorable for non-native plant establishment. Japanese and 

giant knotweed (Fallopia (syn. Polygonum)) (fig. 3) and 

old man’s beard (Clematis vitalba) are examples of three 

non-native species that can be very invasive in riparian 

areas. Disturbed or harvested alder sites are also favorable 

for rapid spread of nitrogen-loving species such as thistles 

(Cirsium spp.), grasses including bentgrass (Agrostis spp.) 

and velvet grass (Holcus lanatus), and groundsels (Senecio

spp.). Other exotic plants that are common and vigorous 

on disturbed sites include cat’s ear (Hypochaeris radicata), 

foxglove (Digitalis purpurea), evergreen blackberry (Rubus 

laciniatus) and Himalayan blackberry (Rubus discolor). 

Many exotic species are shade-intolerant and decline as the 

tree canopy closes, but particularly in riparian areas where 

flood disturbance may be chronic and the canopy more 

open, exotics such as the knotweeds may remain vigorous.

Alder is host to many epiphytes, especially lichens. 

Alder bark is brown, reddish brown, or greyish brown 

with white lenticels visible on the twigs; however, the bark 

often appears to be white or very light gray and patchy 

in color due to the presence of crustose lichens that can 

completely cover the bark and mask the underlying color 

(fig. 4). Common genera of crustose lichens on alder bark 

that give it a white or grey appearance are: Arthothelium, 

Graphis, Lecanora, Lecidea, Ochrolechia, Rinodina, and 

Thelotrema (T. Goward, F. Rhoades, pers. comm. 2005). 

Many other lichen genera are also present on alder trunks 

and branches; common species include: Parmelia sulcata, 

Figure 1—(above) A cut-leaf alder growing in Capitol State Forest near 

Olympia, WA.

Figure 2—(below) Understory vegetation in alder stands can vary 

substantially. Shown are: A) swordfern understory with a red huckleberry 

just leafing out visible on a stump in the background; B) understory 

dominated by lady fern and devil’s club on a fairly wet site; c) red 

elderberry and swordfern; and D) salmonberry in flower and indian plum 

on a riparian site.
25

that those alder stands will be replaced by shrubs and that 

without disturbance a shrub-dominated community may 

persist for an extended period of time (Newton et al. 1968, 

Carlton 1988, Tappeiner et al. 1991, O’Dea 1992). Clonal 

shrubs, particularly salmonberry, but also thimbleberry and 

vine maple, often form a dense shrub canopy which makes it 

difficult for conifers to invade and become established from 

seed. These shrub species can expand rapidly by vegetative 

reproduction as space becomes available due to death of 

the alder overstory (Tappeiner et al. 1991, O’Dea 1992, 

Zasada et al. 1992).

Experience suggests that alder replacement by conifers 

will have a high degree of spatial and temporal variation if 

the process is left to proceed naturally. The density of alder 

stems and the presence and abundance of shade-tolerant 

tree species are obviously important in influencing which 

successional trajectory will be followed; however, the 

specific ecological factors that determine the successional 

sequence in alder stands are not known. This topic is 

discussed in more detail in Deal (2006).

Life History

Reproduction and Early Growth

Flowering and fruiting. Red alder reaches sexual 

maturity at age 3 to 4 years for individual trees and at age 

6 to 8 for most dominant trees in a stand (Stettler 1978). 

The species is generally monoecious, with separate male 

and female catkins developing on the previous year’s twigs 

(Hitchcock et al. 1964). Staminate catkins, which occur 

in pendulous groups and are usually in a terminal position 

on a short shoot, elongate in late winter, changing from 

green to reddish brown and from 2 to 3 cm long to about 7 

or 8 cm. Pollen grains are small (20 to 23 μM in diameter, 

Owens and Simpson, N.d.), lightweight, and produced in 

abundance. Several pistillate catkins are borne per floral 

bud and are commonly located on a bud proximal to the 

staminate catkins. They are 5 to 8 mm long and reddish 

green when receptive. Both male and female catkins usually 

occur in groups of three to six; variation in the ratio of 

male to female catkins in terms of numbers and weights 

has been described for three elevational transects (Brown 

1986). Flowering occurs in late winter or early spring; peak 

shedding of pollen generally precedes peak receptivity by a 

few days but synchrony in pollen shed and receptivity have 

been observed in some trees (Brown 1985). Pistillate catkins 

are upright at the time of flowering but become pendulous 

as they mature. Most alder seed is probably the result of 

outcrossing, but some self-pollination occurs (Stettler 1978).

Limited information is available on the effects 

of cultural practices on flowering of red alder. In an 

experimental plantation on a well-drained site near Olympia, 

Washington, both male and female flower production 

were decreased by irrigation during the growing season 

Menegazzia terebrata, Ramalina farinacea, Usnea wirthii, 

Hypotrachyna sinuosa, Evernia prunastri, Hypogymnia 

enteromorpha, Peltigera collina, and Usnea cornuta

(these were the 9 most common species out of 160 species 

of mostly foliose and fruticose species found on alder 

in western Washington, western Oregon, and northern 

California, L. Geiser and A. Ingersoll, pers. comm. 2005). 

Many lichen species are sensitive to air quality (Nash 1996, 

Stolte et al. 1993) and the bark of alder growing near pulp 

mills, coal-fired power plants or even in urban areas can be 

strikingly different in appearance from the bark of alders 

growing in areas of higher air quality (fig. 4). 

Other Associates

General associations or interactions between red 

alder and wildlife species have been discussed previously 

(McComb 1994) and were discussed at this conference 

(McComb 2005). Interactions between red alder and 

animals, insects, and fungi that can result in tree damage 

are discussed later in this chapter.

Forest Succession

Red alder is a pioneer species favored by high 

light levels and exposed mineral soil. Its ability to fix 

atmospheric nitrogen permits establishment on geologically 

young or disturbed sites with low levels of soil nitrogen. 

It can form pure stands on alluvium, avalanche paths, 

and other disturbed sites. Harvesting and burning have 

favored alder; pollen records indicate that alder stands 

are more extensive in the twentieth century than they 

were for several centuries before that time (Heusser 1964, 

Davis 1973). Red alder pollen, however, was also abundant 

between 9000 and 4800 B.C.; the mix of species present 

in the pollen record for that time has been interpreted as 

indicating a somewhat warmer climate accompanied by an 

increase in fire frequency (Cwynar 1987).

Observations of mature forests in the Pacific 

Northwest suggest that alder stands are ultimately replaced 

by longer-lived, more tolerant conifers that have more 

sustained growth rates at older ages than does alder. This 

is undoubtedly true in most cases (see discussion below 

for exceptions), but the time required for this to occur in 

unmanaged forests is not well documented. Rapid growth 

and high stem densities of alder in younger stands make it 

difficult for conifers (especially shade-intolerant species) 

to regenerate and grow if they do not become established 

at the same time or shortly before alder invades a disturbed 

area. Douglas-fir can be easily eliminated in dense young 

alder stands while more tolerant species (western hemlock, 

western redcedar, and Sitka spruce) can survive and over 

time grow into the alder canopy and ultimately dominate 

the site.

Many alder stands in western Oregon have few 

associated conifers, leading some researchers to conclude 
26

(Harrington and DeBell 1994); this is consistent with 

observations for other species that moisture stress increases 

flowering (Owens 1991). Within an irrigation regime, flower 

production was generally concentrated on the larger trees; 

thus, the percentage of trees flowering was greatest at the 

widest spacing, and within each spacing, the percentage of 

trees flowering increased as tree size increased. 

Seed production and dispersal. Seeds are small, 

winged nutlets borne in pairs on the bracts of woody, conelike strobili (Schopmeyer 1974). The seeds are without 

endosperm and contain only small cotyledons (Brown 

1986). The strobili are 11 to 32 mm long and 8 to 15 mm 

wide.

Red alder is believed to be a prolific and consistent 

producer of seed; however, there are no long-term records 

of red alder seed production. Seed production in red alder 

varies substantially among trees; Brown (1985) reported 

production rates for 45 mature trees of similar size from 

0 to 5.4 million seeds per tree. She reported within-stand 

variation to be much greater than variation among stands. 

Based on a two-year study of seedfall on two sites in British 

Columbia, McGee (1988) reported substantial variation 

between sites and years. Maximum total production was 

1550 seeds/m2; in the more productive year, production 

was four to seven times greater than in the less productive 

year. In the more productive year, one site produced 1.8 

times more seed than the other, while in the less productive 

year seedfall was similar at both sites. Seed-crop quality 

(percent viable seeds) was similar between sites with 40 to 

50 percent of the seeds viable in the good year and less than 

10 percent in the poorer year. Worthington (1957) concluded 

that moderate seed crops are produced almost annually and 

bumper crops occur every three to five years; however, 

no specific studies were cited. Complete failure of a seed 

crop is rare, but after a severe freeze in 1955, almost no 

seed was produced in 1956 (Worthington 1957). Additional 

information on variation in red alder seed production is 

available (Ager et al. 1994). Information on seed production 

in other members of the Betulaceae (Alnus and Betula) is 

summarized in Harrington et al. 1994.

The annual pattern of red alder seed dispersal and how 

it varies over its range is not well documented. In general it 

is believed to begin in the middle of September in the center 

of its range and slightly earlier or later to the north and south 

respectively. McGee (1988), working in British Columbia, 

found that small amounts of seed were dispersed in 

September, but that the major dispersal events occurred from 

November to February. Lewis (1985) reported that major 

seedfall of red alder in Washington occurred during winter 

and spring, but that some seedfall was observed throughout 

the year. Major dispersal events occurred in consecutive 

months as well as in months separated by several months of 

low dispersal, a pattern similar to that in other members of 

the Betulaceae (McGee 1988, Zasada et al. 1991).

The nature of the catkin suggests that the timing of seed 

dispersal is regulated by factors similar to those regulating 

the release of seeds from the cones of conifers; that is, 

once catkins are mature, dispersal is determined by the 

occurrence of weather which dries them, thus opening the 

scales and allowing the seeds to be released. In general, wet 

weather keeps catkins closed and wet weather following dry 

weather closes catkins, thus terminating a dispersal event. 

Nonetheless, heavy seedfall can occur during wet weather 

under certain catkin conditions (Lewis 1985) and dispersal 

will not occur if ice freezes the seed in the catkin (Lewis 

1985, Brown 1986). The dispersal patterns reported for red 

alder (Lewis 1985, McGee 1988) are consistent with the 

hypothesized mechanisms, but more frequent observations 

of seed dispersal in relation to weather and catkin condition 

need to be made. In coastal Alaska, drying trends brought 

by high-pressure weather systems are important to seed 

dispersal of Sitka spruce and western hemlock (Harris 

1969), two common associates of alder. In addition, 

generally similar patterns of seed dispersal have been 

observed for alder and hemlock growing in the same stand 

in western British Columbia (McGee 1988).

Red alder seeds are very light, numbering 800 to 3000/g,

and wind dissemination is effective over long distances. 

Lewis (1985) documented dispersal of red alder seeds for 

a two-year period and found amount of seed, seed weight, 

percentage of filled seed, and viability all to be inversely 

correlated with distance from the seed source. Amount of 

seedfall 100 m from the edge of an alder stand was 2 to 3 

percent of the seedfall density inside the stand. 

Although seeds are dispersed primarily by wind, some 

dispersal may occur by water (Brown 1986) and by birds 

or other animals. Birds are commonly seen around catkins, 

and alder seeds have been shown to be an important source 

of food for some species (White and West 1977). Birds 

play both passive and active roles in seed dispersal. Passive 

dispersal occurs simply by movement of the catkins as birds 

work in the crown of alders; active dispersal occurs as birds 

extract seeds from the catkins while feeding.

Germination, seedling survival and development. In 

normal germination of red alder seed, the radicle or root 

elongates first, followed by hypocotyl growth (hypogeal 

germination); this means that that the root emerges first 

from the seed coat (fig. 5). When seed has been stored for 

an extended period of time, this process can be reversed and 

hypocotyl elongation occurs first, the cotyledons emerge 

from the seed coat and then root elongation begins (epigeal 

germination) (pers. obs., J. Kraft).

Seed germination in clearcut environments in the 

central Oregon Coast Range began in late February and 

early March and was completed by mid-April (Haeussler 

1988, Haeussler and Tappeiner 1993). Differences between 

north and south aspects were small compared to the forested 

environments where the onset of germination was delayed, 
27

Figure 4—(above) The lichen community on alder bark can result in very 

different appearances from smooth brown bark to brown bark with shaggy 

spots to white or light gray bark. Many lichen species are sensitive to 

air pollution; the tree on the left with almost no lichens on its bark was 

growing in the vicinity of a coal-fired power plant near Centralia, WA.

Figure 5—(right) Alder seed germination is rapid and 

hypogeal; the radicle develops root hairs almost immediately. 

Photo by J. Kraft, PNW.

Figure 3—(below) Exotic species can be a particular problem in riparian 

areas where light levels are high and disturbance from flooding or animal 

access occurs. This view shows Japanese knotweed (yellow-brown foliage) 

overgrowing Himalayan blackberry (shrub with dark green foliage) in a 

black cottonwood and red alder stand on the Snoqualmie River in western 

Washington. Photo by S. Reutebuch, PNW.
28

relative to that in clearcuts, for a month and continued into 

June. On average, the number of germinants emerging 

was higher on disturbed than on undisturbed seedbeds. 

There was no clear difference in germinant appearance 

between forested and clearcut environments for either 

seed-bed type. A positive relationship between spring 

soil moisture conditions and germinant appearance was 

stronger in the clearcut environment than in the modified 

light and temperatures prevailing under forested conditions 

(Haeussler 1988).

Seedling establishment. The number of seeds required 

to produce a seedling one growing-season old differed 

dramatically between westside and eastside Coast Range 

environments (Haeussler and Tappeiner 1993). Under the 

drier conditions on the eastside of the Coast Range, no 

seedlings survived through the growing season in either 

of the years of study. On the north aspect of the westside 

coast site, 1 seedling was produced per 32 seeds sown; on 

an adjacent south aspect, 1 seedling was produced per 181 

seeds (Haeussler 1988). In another study on a southwestfacing coastal site, sowings in each of two years on newly 

created mineral soil seedbeds at a rate of 1000 to 1500 

seeds/m2 failed to produce any surviving seedlings at the 

end of one growing season (J. Zasada, pers. comm. 1994). 

In a third study, sowing of alder seed on dry Coast Range 

sites similar to those studied by Haeussler (1988) resulted 

in germinants but no surviving seedlings after one growing 

season (J. Tappeiner and J. Zasada, pers. comm. 1994). 

These three studies suggest that alder establishment is not 

assured even when large quantities of seeds are sown on 

what are believed to be desirable seedbeds. 

A number of environmental factors result in high 

mortality of seeds and seedlings between the time seeds 

arrive on the seedbeds and the end of the first growing 

season (Haeussler 1988), and these certainly contribute to 

the temporal and spatial variation in alder regeneration. In 

unprotected microsites, seedling emergence was 75 percent 

on disturbed seedbeds and 38 percent on undisturbed 

seedbeds on protected microsites. Loss of seeds to soil 

biota was greater under forest conditions than in clearcuts. 

On undisturbed and mineral soil microsites, 60 and 20 

percent of the seed population were destroyed by soil 

organisms, respectively. Causes of seedling mortality 

included drought and heat injury, pathogens, animals, 

erosion, frost and smothering by organic debris. Drought 

and heat-related mortality were the major causes of 

mortality in clearcuts, whereas damping-off fungi and other 

pathogens were most important under forest conditions 

(Haeussler 1988).

Alder seeds are most commonly described as having 

little or no dormancy. This is based on studies which 

have shown that germination of stored seeds under 

optimum germination temperatures is not improved by 

stratification (Radwan and DeBell 1981, Berry and Torrey 

1985); however, one provenance from British Columbia 

was reported as having a physiological dormancy that 

was released by stratification (Elliott and Taylor 1981a). 

Germination under sub-optimum temperatures, such as may 

prevail at the time of germination under field conditions, is

enhanced by stratification (Tanaka et al. 1991). Based on 

germination of lots stored for several years, the need for 

stratification is not affected by storage but seed viability will 

decline over time.

Bormann (1983) and Haeussler (1988) demonstrated 

that alder seeds do not germinate in the dark and that the 

phytochrome system is very sensitive (i.e., germination 

is inhibited by exposure to far-red light). A field study by 

Haeussler (1988) strongly suggests that alder germination is 

controlled by light quality and that a type of light-enforced 

dormancy may prevent seeds from germinating when other 

conditions appear optimal; thus, a persistent alder seedbank 

may be present under some conditions. A buried seedbank 

is probably not important in alder because of the high seed 

mortality rate caused by soil organisms (Haeussler 1988); 

however, it can’t be ruled out without study since seeds of 

some species of Betula remain viable in the soil for much 

longer than would be expected based on seedcoat structure 

and general seed germination characteristics (Granstrom 

1982, Perala and Alm 1989).

Assuming that site conditions are suitable, red alder 

can initially be regenerated by any silvicultural system that 

provides moderate sunlight and exposed mineral soil. The 

species is an aggressive pioneer on avalanche paths, road 

cuts, log landings, skid trails, or other areas where mineral 

soil has been freshly exposed to seedfall. For example, 

shortly after a heavy thinning (removal of 50 percent of 

the basal area) in a 62-year-old Douglas-fir stand, an alder 

understory became established and grew rapidly (Berg and 

Doerksen 1975). Clearcutting and large-group selection 

are feasible regeneration systems. During harvesting or in 

a subsequent site preparation treatment, the site must be 

disturbed sufficiently to expose mineral soil. Fire probably 

can substitute for mechanical disturbance on most sites. 

To exclude red alder from the next rotation stand, some 

forest managers try to reduce the supply of alder seed by 

cutting possible alder seed trees in the vicinity before or at 

the time of final harvest; to avoid creating favorable seedbed 

conditions, they also disturb the site as little as possible 

during logging and, if feasible, do not burn the logging 

slash (Lousier and Bancroft 1990).

Artificial regeneration can be accomplished with either 

bare-root or containerized seedlings, and guidelines for 

producing planting stock are available (Berry and Torrey 

1985, Radwan et al. 1992, Ahrens 1994). Survival and 

growth of planted seedlings are usually excellent (Radwan 

et al. 1992), but can vary significantly with slope, slope 

position, and aspect within a given clearcut. For example, 

Zasada (pers. comm. 1994) followed the fates of seedlings 

planted on different sites within a clearcut and observed 

nearly 100 percent survival on steep north aspects over a 
29

three-year period while immediately adjacent south-facing 

and stream bottom sites (with higher soil moisture stress 

and a higher probability of early season frosts, respectively) 

suffered as much as 60 percent mortality.

Height growth of red alder seedlings is generally 

rapid. On favorable sites, seedlings can grow 1 m or more 

the first year, and on all but the poorest sites, seedlings 

surpass breast height (1.3 m) the second year (Smith 1968, 

Harrington and Curtis 1986). Maximum annual height 

growth of more than 3 m a year can be achieved by 2- to 

5-year-old seedlings (Harrington and Curtis 1986).

Seasonal growth of red alder is under strong climatic 

control and consequently quite variable. The timing of radial 

growth is similar for red alder and its common associate 

Douglas-fir; in the Puget Sound area of Washington State, 

growth begins about mid-April and continues until midSeptember (Reukema 1965). Substantial height growth 

(i.e., height growth other than that associated with initial 

bud break) begins slightly later in the season than radial 

growth. Red alder has indeterminate height growth; thus, 

height growth continues through the growing season until 

soil moisture, temperature, or light conditions become 

unfavorable (see DeBell and Giordano 1994). The specific 

environmental conditions that control root and shoot 

growth have not been determined. 

Vegetative reproduction. Red alder sprouts vigorously 

from the stump when young. It can be repeatedly coppiced 

on short cycles but rootstock mortality increases with each 

harvest (Harrington and DeBell 1984). The likelihood 

of obtaining stump sprouts and the vigor of the sprouts 

are influenced by age, time of year, and cutting height 

(Harrington 1984a). Stumps sprout best when trees are 

cut in the winter and when stump height exceeds 10 cm. 

Older trees rarely sprout and coppice regeneration cannot 

be expected after pole-size or sawlog-size material 

is harvested (Harrington 1984a). Because of reduced 

vigor of sprouting, manual cutting of alder as a means 

of competition control in conifer plantations can be an 

effective vegetation management practice (DeBell and 

Turpin 1989); however, results from cuts at different times 

during the summer can be variable (Pendl and D’Anjou 

1990).

Other vegetative methods of propagation that have 

been successfully used include: rooting greenwood cuttings 

from young trees (Monaco et al. 1980); rooting of cuttings 

of succulent new spring growth from shoots of young trees 

and epicormic sprouts from 30-year-old trees (Radwan et 

al. 1989), and rooting of sprouts propagated by mound 

layering (Wilson and Jewett, unpub. data, see Harrington et 

al. 1994 for a more complete description.). 

Sapling and Pole Stages to Maturity

Growth and yield. Alder growth form is strongly 

excurrent during the period of rapid height growth. Crown 

form becomes moderately to strongly deliquescent as the 

trees mature. Growth of vegetative shoots is primarily 

monopodial (e.g., branching with the apical bud forming a 

persistent leader and new branches arising laterally below 

the apex, Swartz 1971); however, shoots producing flowers 

exhibit sympodial growth (e.g., the terminal bud withers and 

the main axis of branching is made up of a series of lateral 

branches, Swartz 1971). 

Alder produces three types of branches: sylleptic 

(developed from a bud formed during the current year), 

proleptic (developed from a bud formed the previous year), 

and epicormic (developed from suppressed buds). Sylleptic 

branches are important in rapid development of leaf area but 

are short lived. Proleptic branches are persistent and form 

primarily from buds at the upper end of the height increment 

for the year; thus, the pattern of proleptic branches on the 

main stem can often serve as a rough method of determining 

past height increment. Epicormic branches are produced 

when suppressed buds are triggered to develop by injury 

or a change in plant environment. They are commonly 

seen after top breakage, a major increase in exposure (e.g., 

road construction, logging), or in response to injury (e.g., 

flooding, girdling). The physiological factors that determine 

the amount of apical control on branch growth and angle 

have not been studied for alder. 

Growth of primary shoots of alder can be phototropic 

(i.e., differential elongation of cells resulting in growth 

toward light; Zimmerman and Brown 1971). Alder trees 

can exhibit substantial amounts of lean when grown in 

irregularly spaced stands or when located along roads, 

streams, stand boundaries, or other areas with unequal light 

distribution on all sides of the tree; the lean is probably 

caused primarily by greater crown development on the 

sunnier sides of the tree rather than by phototropism per se 

(DeBell and Giordano 1994). Other changes in stem form 

may occur as the result of heavy snow or if gravity causes 

all or part of the tree to shift abruptly (e.g., as a result of soil 

slumping or high winds when soils are saturated). If juvenile 

red alder is grown at wide and fairly even spacing, however, 

lean and sweep will be minimized (Bormann 1985, DeBell 

and Giordano 1994).

Red alder has rapid juvenile growth. Among its 

associates, only black cottonwood grows as much or more 

during the juvenile phase. On good sites, alder trees may be 

9 m at age 5, 16 m at age 10, and 24 m at age 20. One tree 

was 9.8 m tall and 16.3 cm in dbh 5 years from seed (Smith 

1968).

Growth slows after the juvenile stage, the decrease 

beginning much sooner on poor sites. Site index, as 

determined at base-age 20 years, ranges from 10 to 25 m 

(Harrington and Curtis 1986); at base age 50, it ranges from 

18 to 37 m (Worthington et al. 1960). Associated conifers 

have much slower juvenile growth, but they sustain height 

growth years longer than alder. On an average upland site, 
30

both Douglas-fir and red alder can attain the same height at 

about age 45 (Williamson 1968). Beyond that age, Douglasfir surpasses red alder in height. Because the two species 

have different site tolerances, their relative performances 

will be site-specific as well as age-specific (Deal 2005, 

Harrington and Courtin 1994).

Red alder is a relatively short-lived species, maturing 

at about 60 to 70 years; maximum age is usually about 

100 years (Worthington et al. 1962). On favorable sites, 

trees can be 30 to 40 m tall and 55 to 75 cm in diameter. 

The American Forests Big Trees national champion red 

alder in Oregon measured 198 cm in dbh (www.odf.state.

or.us; last updated Aug.25, 2004) and the largest red alder 

in Washington measured 158 cm in diameter (Van Pelt 

1996), but trees over 90 cm in diameter are rare. In pure 

stands on good sites, it has been estimated that red alder 

can achieve mean annual cubic volume growth rates of 21 

m3/ha in pulpwood rotations of 10 to 12 years, and 14 m3/ha 

in sawlog rotations of 30 to 32 years (DeBell et al. 1978). 

Most of the existing alder volume is in naturally regenerated 

mixed-species stands where growth and yield are variable. 

Several reports have indicated that maximum cubic volume 

in alder stands is about 500 m3/ha at age 50 to 70 (DeBell 

et al. 1978, Worthington et al. 1960, Chambers 1983); 

however, volumes of 700 m3/ha and even 1000 m3/ha have 

been measured on small inventory plots in fully stocked 

natural stands 70 years or older in the Queen Charlotte 

Islands or islands in the Johnston Straits (N. Hughes, pers. 

comm. 2005). These extremely high volumes are very 

unusual and may not be possible on larger areas.

Rooting habit. Red alder forms extensive, fibrous root 

systems. Root system distribution is primarily controlled 

by soil drainage, soil structure, and compaction. In poorly 

drained soils, most rooting is surface-oriented, and rooting 

is often prolific in the boundary between the lower organic 

layer and the uppermost mineral horizon. In wet soils, 

the uppermost mineral horizon usually is rooted heavily, 

as is the lower part of the surface organic layer if it is 

thick enough. On well-drained sites, root distribution is 

strongly influenced by water availability; increased rooting 

is common at horizon boundaries where changes in soil 

texture slow downward water movement through the profile. 

Because rooting also follows the path of least resistance, 

it is greater in old root channels or, especially if the soil 

is compacted and soil structure well developed, between 

units of soil structure. Root system extent is a function 

of soil characteristics and tree size. Smith (1964) showed 

tree diameter and average root length to be significantly 

correlated; larger trees also tended to have deeper roots than 

smaller trees. 

Red alder, especially when young, forms adventitious 

roots when flooded. In a greenhouse study, alder seedlings 

previously growing under well-drained conditions produced 

adventitious roots when the soil was saturated (Harrington 

1987). These roots emerged at or near the root collar and 

grew on top of the saturated soil surface; when the soil was 

drained, the root exteriors suberized and many of the longer 

roots turned downward into the soil where they continued to 

grow. Minore (1968) also reported formation of adventitious 

roots when seedlings in pots were flooded, noting that 

seedlings that did not form adventitious roots did not 

survive. Although it has not been documented, formation 

of adventitious roots may be an important adaptive trait on 

floodplain sites.

The sensitivity of red alder root growth to 

environmental conditions is not well known, but recent 

studies provide some information (also see Shainsky et 

al. 1994). Under soil moisture stress, red alder saplings 

shifted carbon allocation from leaf and stem biomass to root 

biomass (Chan 1990). In a companion study, root biomass 

decreased with increasing density of alder stems (Shainsky 

et al. 1992). Ratios of root to shoot were significantly 

affected by stand density; however, most of the variation in 

root biomass was directly attributable to variation in shoot 

biomass. When grown in pots in a growth chamber, these 

ratios were decreased by fertilization and were lower in sandy 

soil than in loam or sandy loam (Elliott and Taylor 1981b).

Red alder roots are commonly ectomycorrhizal, 

although only a few species of fungi form ectomycorrhizal 

associations with alder. Fungal symbionts include alderspecific fungi and fungi capable of mycorrhizal associations 

with other hosts (Molina 1979, Molina et al. 1994).

Red alder also has root nodules that fix atmospheric 

nitrogen (fig. 6). The nodules are a symbiotic association 

between the tree and an actinomycete (Frankia spp.). 

In natural stands nodulation occurs soon after seed 

germination; root systems of seedlings a few months old 

commonly have dozens of visible nodules. Nodule sizes on 

young trees range from the size of a pinhead up to 25 mm 

in diameter. Mature trees have nodules on both the large 

woody roots and the smaller new roots. The large compound 

nodules found mature trees can be 80 or 90 mm in diameter. 

Rates of nitrogen fixation and the effects of these nitrogen 

additions on soil chemistry have been discussed by others 

(Binkley et al 1994, Bormann et al. 1994).

Reaction to competition. Red alder requires more light 

than any of its tree associates except black cottonwood and is 

classed as intolerant of shade (Minore 1979). Light quality 

has been shown to be important in germination (Bormann 

1983, Haeussler 1988); however, its role in seedling 

development has not been documented. Young seedlings 

can withstand partial shade for a few years but will grow 

very little; if not released, they will die. The only trees that 

survive are those that maintain dominant or codominant 

crown positions. Self-thinning or mortality caused by 

competition is rapid in red alder stands; densities in natural 

stands may be as high as 124,000 seedlings/ha at age 5 

(DeBell 1972) and fully stocked stands at age 20 averaged 

1665 trees/ha (Worthington et al. 1960).
31

Red alder also self-prunes extremely well when grown 

in dense stands. Shaded lower branches rapidly die and fall 

off, resulting in clear and slightly tapered boles. Live crown 

ratios in crowded, pure stands are very low, and narrow, 

domelike crowns are characteristic. As would be expected 

for a shade-intolerant species, branch retention and crown 

shape are strongly related to light levels in the canopy. 

Trees grown at low densities develop large lower branches 

(c.f., fig. 7) that live longer and take much longer to decay 

after death than do branches that develop under higher 

stand densities.

Early control of spacing is necessary to keep live 

crown ratios high enough to maintain good growth beyond 

the juvenile phase. Sawlog yields might be maximized 

on short rotations by combining early spacing control 

with pulpwood thinnings (DeBell et al. 1978). Thinnings 

in previously unthinned stands are most effective in 

stimulating growth of residual trees if done before height 

growth slows—about age 15 to 20 (Warrack 1949, Olson et 

al. 1967, Smith 1978). Thinning in older stands can salvage 

mortality and help maintain the vigor of residual trees, but 

usually does not accelerate diameter growth (Lloyd 1955, 

Warrack 1964).

Epicormic branching has been reported after thinning, 

especially when thinning has been late or drastic (Warrack 

1964, Smith 1978). If epicormic sprouting occurs after 

thinning, it is most common on the south or west side 

of stressed trees; however, trees drastically opened up 

(e.g., via clearcutting or construction activities) may have 

epicormic branches on any or all sides. Epicormic branches 

appearing after early thinning may be ephemeral, but this 

has not been documented and is likely to dependent on 

stand density. Epicormic branches were reported after 

pruning 21-year-old trees (Berntsen 1961), but those trees 

were heavily thinned and overstory conifers had been 

girdled at the same time as pruning (H. Rapraeger, 1949 

report on thinning and pruning alder at Cascade Head 

Experimental Forest, on file at Olympia Forestry Sciences 

Laboratory) so it was not possible to separate the effects 

of pruning on epicormic branches from those of the other 

cultural practices. Very few epicormic branches developed 

after pruning of young trees (Brodie and Harrington 2006).

Red alder can be managed in pure stands or as part of 

a mixture with other intolerant species, such as Douglasfir and black cottonwood (or Populus hybrids), or with 

more shade-tolerant species, such as western redcedar, 

western hemlock, or Sitka spruce. Knowledge of sitespecific growth rates and relative shade tolerances of 

each component in a mixture is critical to achieving the 

potential benefits from mixed stands. Alder must be kept 

in the upper canopy to survive in mixed stands. Even if 

alder is shaded out in a mixed stand, however, it may make 

substantial contributions to soil nitrogen prior to that time 

(Berg and Doerksen 1975, Tarrant and Miller 1963). The 

benefit of alder to the other species is dependent on site 

conditions as well as the relative density of the tree species 

(Courtin and Brown 2001, Deal el al. 2004, Miller 2005, 

Miller and Murray 1978, Miller et al. 1993, Miller et al. 

1999). The proportion of alder and Douglas-fir in mixed 

plantations has been shown to influence the stem quality of 

both species (Grotta et al. 2004). In addition, the richness 

of shrubs, ferns, herbs, and mosses in mixed-species stands 

is increased over that found in conifer stands (Deal 1997). 

Reaction of alder to competition is influenced by many 

factors including the size, species composition, and density 

of the competing vegetation (other alder stems, non-alder 

stems in the upper canopy, and plants in the understory) 

as well as soil and site factors. For example, growth 

of closely spaced, dominant alder was decreased with 

increasing density of subordinate Douglas-fir (Shainsky 

and Radosevich 1991, Shainsky et al. 1994). Rectangularity 

of spacing also influences growth and development of alder 

(DeBell and Harrington 2002).

Damaging Agents

There are relatively few instances where damaging 

agents kill enough red alder trees to result in large 

openings in a natural stand. Frost pockets or unseasonable 

heavy frosts may be an exception. Forest managers may 

also be concerned, however, at lower levels of mortality, 

when growth rates are depressed, or tree form or wood 

quality is affected. In addition, problems will likely 

increase as management is intensified, particularly in 

nurseries and plantations.

Leaf and Stem Fungi. Red alder is fairly free 

from most disease problems, especially when young 

and uninjured (Worthington et al. 1962, Hepting 1971). 

Many species of fungi have been reported growing in 

association with alder (Lowe 1969, Shaw 1973, Farr et al. 

1989), but few have been shown to cause ecologically or 

economically important levels of damage in natural stands. 

Nectria distissima can be a serious bark canker in young 

stands (Fig. 8) and Cytospora spp. can develop following 

sunscald or winter damage (W. Littke, pers. comm. 

2005). Several other canker-causing stem diseases—

Didymosphaeria oregonensis, Hymenochaete agglutinans 

and Botytis cinerea—cause some damage but overall 

their impact is slight. Nonetheless, the potential exists 

for more damaging levels of disease should conditions 

occur that favor their rapid development and spread. For 

example, naturally occurring weak pathogens have been 

used as biocontrol agents for juvenile red alder (i.e., to kill 

unwanted alder) (Dorworth 1995), thus, indicating their 

ability to cause mortality under certain conditions. 

The primary disease of concern in nursery production is 

Septoria alnifolia, a disease that causes leaf spots and stem 

cankers (some experts believe that Septoria alnifolia only 

causes a leaf spot and another species or another stage of 

the same species with a separate name is responsible for the 
32

canker). Although some infected seedlings grow over stem 

cankers caused by S. alnifolia (see note above), on other 

seedlings the cankers result in top dieback, stem breakage, 

reduced growth, or mortality. Thus, infected nursery stock 

should be graded out as cull (i.e., discarded). This disease 

can be controlled with monthly applications of Benlate® (a 

fungicide) and by locating alder nursery beds in areas not 

adjacent to alder stands (W. Littke, pers. com. 1992).

Fungi diseases of alder catkins (Taphrina occidentalis 

and T. alni) cause enlargements of the bracts of female 

catkins (Mix 1949), which prevent or hinder normal 

fertilization and seed development. Although currently these 

fungi are not important economically, they could become 

so if alder seed orchards or seed production areas were 

established.

Compared with other hardwood species, living red alder 

trees have very little decay. In a study on Vancouver Island, 

Allen (1993a) examined 383 alder trees ranging from 20 

to 120 years old. Decay losses of merchantable volume 

were less than 4 percent in all trees sampled. The incidence 

of decay (number of decay columns per tree) was not 

correlated with age in the Vancouver Island study and thus, 

susceptibility to decay in older trees does not appear to be as 

severe as suggested in previous reports (Johnson et al. 1926, 

Worthington 1957). 

Much of the decay present in living alder results from 

injury to standing trees due to broken tops and branches, 

or scars from falling trees (Allen 1993a). Once trees are 

injured, decay organisms gain entry through the damaged 

tissue. Alder, however, is very efficient in its ability to 

compartmentalize decay, and most decay events do not 

spread much beyond the injured tissue. For example, the 

dead tissue of stubs formed from self-pruned branches was 

colonized by fungi and sometimes developed into a decay 

column in the main stem. Most branch stubs, however, 

were overgrown by healthy wood with no further decay 

development (see additional information on this topic in 

DeBell et al. 2006). In general, individual decay columns 

were not large, with a median volume of 0.0024 m3.

The wood of red alder is a light, creamy-white color 

prior to exposure. Once trees are cut or the wood is exposed 

by breakage, the exposed wood develops a reddish orange 

or reddish brown stain. This stain is the basis for the species 

name (rubra meaning red). It is not known definitively what 

causes the color change but a compound, oregonin 

(a diarylheptanoid xyloside) was extracted from alder bark 

and demonstrated potential for stain development in the 

presence of peroxidase and hydrogen peroxide; i.e., the stain 

appears to be the result of oxidation but its exact mechanism 

is not known (Karchesy 1974). In addition, a more intense 

red stain can develop in response to fungal infection by 

Ceratosystis picea (a vascular wilt fungus) (Morrell 1987). 

Fungal spores land on freshly exposed surfaces of alder 

logs, germinate and grow into the wood, and the response 

of the living cells is an intense red stain (Allen 1993b). The 

rate of stain development is influenced by season of harvest 

and conditions after harvest (Allen 1993b). Wood producers 

use several techniques to reduce the stains due to both 

oxidation and those due to fungal infection; in addition, new 

techniques are being evaluated to determine their efficacy to 

reduce stain development in logs and sawn wood products. 

A number of decay fungi have been isolated from living 

alder trees in British Columbia, including Heterobasidion 

annosum, Sistrotrema brinkmannii, Pholiota adiposa, 

Trametes sp., and Meruliopsis corium. From a log buyer’s 

standpoint, “redheart” and “black knot” are common 

defects but multiple fungal species could be associated 

with these general terms; one species that causes red 

heart rot in alder is Stereum sanguinolentum (Omdal, pers 

comm. 2005). A previous report suggested that white heart 

rot, caused by Phellinus (syn. Fomes) igniarius, is the 

most destructive disease of living alder trees (Worthington 

1957). This statement seems to have originated from 

Johnson et al. (1926), who made a similar claim that 

was unsupported by data or reference citation. Phellinus 

igniarius has been found only rarely on living alder in 

British Columbia, although the pathogen may be more 

common in other parts of the alder range. As indicated 

above, these decay fungi do not appear to result in serious 

losses in most living trees. 

Wood stain and decay proceed rapidly in cut alder 

trees. Losses due to stain resulting from fungal infection 

that occurs during the time between harvesting and milling 

are much greater than losses from decay in living trees 

(Allen 1993a). For this reason, logs should be processed 

as soon as possible after harvest, particularly in warm 

summer months. The development of stain and decay is 

retarded in winter months and in logs stored in fresh water 

(Worthington 1957).

In the past, some foresters have suggested that 

red alder would have to be managed on short rotations 

due to increasing disease problems with age (so-called 

pathological rotations) or that thinning and pruning were 

risky due to the increased probability of inducing stem 

damage; based on our current information on alder’s 

ability to compartmentalize decay, these suggestions 

appear unwarranted. During intermediate cuts, however, 

care should be taken to avoid injuring residual trees. There 

are good reasons for short rotations, such as the species’ 

growth pattern with age and economics; however, sensitivity 

to decay does not appear to be a reason to suggest short 

rotations.

Root rots. The root rot pathogen Heterobasidion 

annosum has been observed growing on alder in mixedspecies stands in which both alder and conifers were 

present (Allen 1993a). Since the fungus infects both 

hardwoods and softwoods, it is possible that alder could 

become infected when planted on sites previously occupied 
33

by infected conifers. In addition, infected alder could 

serve as an inoculum source for subsequently planted 

conifers. Thus, preharvest surveys for root-rot fungi should 

be considered for sites where alder is a possible species to 

manage as well as on sites where conversion from alder to 

other species is being considered.

Red alder is subject to root rot by Armillaria mellea but 

it is not considered to be a major disease (Hepting 1971).

All hardwoods are immune to Phellinus weirii (a 

widespread conifer root rot) and red alder occurs naturally 

and has been planted on sites where P. weirii infection levels 

are high. The absence of susceptible species will eventually 

“starve-out” the fungus as suitable substrates decay and 

are not replaced. It has been hypothesized that red alder 

alters the soil environment to the detriment of P. weirii 

by enhancing the growth of microbes (e.g., Trichoderma) 

antagonistic to the pathogen (Nelson et al. 1978) and/or by 

inhibiting the growth of the fungus (Li et al. 1969, 1972; 

Hansen 1979).

Many foresters, following the suggestion that alder 

may serve as a biological control agent for Phellinus weirii 

(Trappe 1972, Nelson et al. 1978), have recommended 

planting alder as a root-rot control measure. This may 

have contributed to increased planting of red alder on 

some lands, but the interactions between red alder and 

P. weirii are not fully understood and therefore caution 

should be exercised in making management decisions. For 

example, Hansen (1979) observed vigorous and extensive 

development of P. weirii ectotrophic mycelium on infected 

roots of very large, old-growth Douglas-fir stumps in a 20-

year-old alder stand that established following logging of the 

Douglas-fir. He concluded that red alder apparently “does 

not shorten the time required to reduce the inoculum.” 

Long-term trials are underway to quantify the longterm effects of alder stands on Phellinus weirii. At this 

time all I can say is that alder should be considered as one 

of several immune species to plant on sites with high levels 

of P. weirii. Foresters planting red alder on sites with high 

levels of P. weirii inoculum that are considered poor or 

unsuitable sites for alder should expect alder growth to be 

poor and problems with damaging agents to increase.

Insects. Numerous insects have been reported feeding 

on or associated with red alder (Furniss and Carolin 1977, 

Gara and Jaeck 1978, Dolan 1984). Insect pests are not 

usually a major concern, but serious outbreaks of some 

defoliators can cause growth reductions. The forest tent 

caterpillar (Malacosoma disstria), western tent caterpillar 

(M. californicum), alder woolly sawfly (Eriocampa ovata), 

striped alder sawfly (Hemichroa crocea), the alder flea 

beetle (Altica ambiens), and a leaf beetle (Pyrrhalta 

punctipennis) have caused substantial damage, but reports 

of mortality are rare (Worthington et al. 1962, Furniss and 

Carolin 1977, Briggs et al. 1978). Mortality, however, has 

been observed when tent caterpillar outbreaks overlapped 

drought periods (Russell 1991, K. Ripley pers. com. 2005) 

and the mortality from the combined stresses was probably 

substantially greater than would have occurred if only one 

stress had been present.

A flatheaded wood borer (Agrilus burkei) can kill twigs 

and branches (Furniss and Carolin 1977, Briggs et al. 

1978). The alder leaf miner, Lithocolletis alnicolella, can 

cause necrotic spots up to 30 mm in diameter on leaves 

but does not apparently affect growth (W. Littke, pers. com. 

1992). An epidemic of grasshoppers was reported to only 

slow growth slightly (Russell 1986). The fall webworm 

(Hyphantria cunea) skeletonizes or consumes leaf blades, 

but its damage is usually minor (Furniss and Carolin 1977). 

The alder bark beetle (Alniphagus aspericollis) breeds 

primarily in slash and in young stressed trees; however, 

healthy trees can be attacked when bark beetle populations 

are high (Gara and Jaeck 1978).

The alder aphid (Pterocaulis alni) feeds on tender 

shoots (Furniss and Carolin 1977) and on foliage with 

high nitrogen content (Dolan 1984). Aphids are common 

associates in many young alder stands and generally are not 

considered to cause much damage, although severe aphid 

epidemics have been reported in young alder plantations 

planted on droughty soils (Dolan 1984, DeBell pers. comm. 

2005). 

Ambrosia beetles (Gnathotrichus retusus, 

Trypodendron lineatum, Xyleborus saxeseni) attack logs 

and slash left on the ground, thus, log quality can rapidly 

degrade. Insect holes can also serve as entry sites for fungi. 

Merchantable material should be removed rapidly, and 

large accumulations of slash should be avoided.

Animals. In general, animals cause only minor 

damage in alder stands; however, under some 

circumstances animal damage can be significant. Alder 

is not a highly preferred browse species for black-tailed 

deer (Odocoileus hemionus columbianus) or Roosevelt 

elk (Cervus elaphus roosevelti) during most of the year. 

Young trees are occasionally browsed by deer and elk, 

especially during the late summer and fall (Brown 1961), 

and browsing begins earlier in the summer when weather 

conditions are dry or when other food sources are not 

available.

Abscising or freshly abscised leaves were documented 

as being a major component of deer and elk diets in oldgrowth forests on the Olympic Peninsula (Leslie et al. 1984) 

and penned black-tailed deer have been observed eating 

freshly abscised alder leaves in the fall when other food 

sources were available (D. L. Campbell, pers. com. 1992). 

Seasonal changes in deer and elk browsing may be related 

to changes in foliar chemical composition; alder foliage 

in the fall is higher in crude fat content and lower in total 

phenols than during the summer (Radwan et al. 1978). 

Elk repeatedly browsed red alder planted on a debris 

flow associated with the 1981 eruption of Mt. St. Helens 
34

(Russell 1986) when alternative food sources were limited. 

Most browsed trees resprouted vigorously and very little 

mortality was associated with the heavy browse damage; 

however, the repeated browsing resulted in trees with 

shrub-like forms.

Deer and elk can cause stem deformation, reduce 

growth, and provide entry sites for decay organisms when 

they rub their antlers against tree trunks; in localized 

pockets this type of damage can be common. In a young 

spacing trial near Centralia, Washington, the incidence 

of stem rubbing was greatest in the narrowest spacings, 

presumably because the closer spacings had higher rates of 

branch mortality that resulted in easier access to the main 

stems. Others, however, have observed greater damage in 

wide spacings (Newton and Cole 1994). The relationship 

between spacing and damage may change with age or other 

factors. Deer and elk occasionally strip and eat alder bark, 

especially during winter and spring.

Mountain beaver (Aplodontia rufa) clip small alder 

stems and branches; only the bark is eaten from stems 5 

to 20 mm in diameter while smaller pieces are consumed 

whole (D. L. Campbell, pers. com. 1992). Although 

mountain beaver only clip small-diameter pieces, they climb 

trees and can continue to clip branches and terminals as 

trees increase in size. In an artificial feeding trial in which 

several plant species were made available at the same 

time, mountain beaver consistently selected alder stems 

all months of the year (data on file, USDA APHIS Animal 

Damage Research Unit, Olympia, Washington). Thus, alder 

appears to be a regular item in mountain beaver diets and 

problems in stand establishment should be anticipated on 

sites with established mountain beaver populations (D. L. 

Campbell, pers. com. 1992). Mountain beaver use of alder 

foliage for food is minor except when other food sources are 

not available or in late September when use is fairly heavy 

(Voth 1968).

Observations of other animals damaging red alder are 

limited. Beaver (Castor canadensis) will cut any species of 

tree near their ponds to support their construction activities. 

As a food source, beaver prefer red alder over Douglasfir, but other plants are selected before alder if they are 

equally available (D. L. Campbell, pers. com. 1992). In 

years of high populations, meadow mice (Microtus sp.) 

girdle young stems; this type of damage has been most 

commonly observed in grassy or very wet areas. Deer mice 

(Peromyscus maniculatus) eat alder seed from the surface 

of snowpacks when other food is difficult to obtain (Moore 

1949); however, alder seed is not usually a preferred food 

source. Individual trees can be heavily damaged (fig. 9) by

red-breasted sapsuckers (Sphyrapicus ruber); if the damage 

encircles all or most of the stem, the top may break off 

during periods of wind or snow. Ring shake can create a 

serious problem of log quality in some stands. The causes of 

ring shake in alder are not known but it has been linked to 

sapsucker damage in eastern species (Baumgrass et al. 2000, 

Shigo 1963).

Extremes in physical factors. Extremes in 

temperature, wind, or fire can damage red alder. Mortality 

and top damage have been documented in natural stands 

after ice storms or unseasonable frosts (Duffield 1956, 

Worthington et al. 1962). Widespread cold damage (terminal 

dieback and mortality) has been observed in bare-root 

nurseries in several years after unseasonably cold frost 

events in October and early November. Frost protection in 

nursery beds via watering is effective in preventing damage 

and the stems that bend over with the ice resulting from 

watering usually recover well (N. Khadduri, pers. com. 

2005). Recently planted trees are also susceptible to cold 

damage; late spring frosts and early fall frosts have caused 

top dieback and mortality (DeBell and Wilson 1978, Peeler 

and DeBell 1987). When grown at a common location, alder 

sources from northerly locations or higher elevations set 

bud and became cold tolerant earlier in the fall than sources 

from more southerly locations or lower elevations (Cannell 

et al. 1987, Ager and Stettler 1994); geographic variation 

in spring budbreak and frost hardiness were more complex. 

Variation in budbreak has been predicted from growing 

season thermal sums (Ager 1987, Ager and Stettler 1994). 

The winter dormancy requirement for red alder has not 

been studied, however, and the causal factors controlling 

timing of spring budbreak are not known. Presumably, once 

chilling requirements (if any exist) are met or day length 

is permissive, budbreak is temperature dependent. This 

assumption is consistent with the observation of Peeler and 

DeBell (1987) that cold damage occurred when late frosts 

followed a period of warmer-than-normal temperature.

Other temperature-related problems observed on alder 

are sunscald and frost cracks. As is generally true for other 

species, this type of damage is most common on the south 

and west side of exposed trees and both sunscald and 

frost cracks occur during the winter. As noted above, stem 

cankers may develop after this type of damage.

Fire is rarely a damaging agent because of the scarcity 

of flammable debris in alder stands; in fact, the species 

has been planted as a firebreak to protect adjacent conifers 

(Worthington et al. 1962). Alder bark is thin but sufficiently 

fire-resistant to prevent damage during light surface fires 

(Worthington 1957). Alder stands are also likely to be 

somewhat more fire resistant than most conifer stands 

because many alder stands are in riparian areas or on 

generally moister microsites.

Windthrow is not common in alder because of the 

intermingling of roots and branches, the absence of leaves 

during winter storms when soils can be waterlogged, and 

the relatively deep-rooting habit of the species on welldrained soils. Uprooted trees are most commonly observed 

along cutting boundaries or where flooding or erosion has 

undercut established root systems. High winds, heavy snow, 

and ice storms will break alder tops and branches, but these 

problems are generally less for alder than for associated 

species that are foliated during the winter. Exposed windy 
35

Figure 6—(above) Nitrogen-fixing nodules on the root system of a red 

alder seedling. Photo by J. Kraft, PNW.

Figure 8—(right–top) Nectria distissima can cause cankers on young alder. 

Photo by W. Littke, Weyerhaeuser Company.

Figure 9—(right–bottom) Red-breasted sapsuckers can cause substantial 

damage to individual alder stems. This tree near Olympia, WA was 

repeatedly damaged by sapsuckers several years after it was pruned.

Figure 7—(below) Alder trees grown at low density, such as this tree near 

Cape Disappointment, WA, will develop large-diameter branches that do 

not die or abscise quickly. Note the difference in stem form, branching and 

bark color between this tree and those shown in figs. 2 and 4.
36

sites, however, such as those near the ocean or mountain 

passes, have top breakage and reductions in height growth 

consistently enough to reduce site index (Harrington 1986). 

Top breakage due to snow, ice, or wind can be a serious 

problem under some conditions. For example, young 

trees re-spaced from high to lower densities can be very 

susceptible to breakage after thinning (P. Courtin, pers. 

comm. 2005). This occurs because the lower crown ratios 

that resulted from the higher density growing environment 

result in a small stem diameter at the base of the live 

crown and the thinning changes wind sway (or snow or ice 

loading) from a whole-stand phenomenon to one where 

individual trees primarily sway or flex with the base of the 

live crown as the pivot point. Damage is particularly severe 

if wet snow or ice occur in the late fall when the foliage is 

still on the branches.

Red alder has evolved to survive in climates with low 

summer rainfall. The greater stomatal control of red alder 

as compared to black cottonwood (Harrington 1987) is 

probably a key feature that allows the species to grow on 

upland sites. In general, however, red alder is probably 

not as drought tolerant as most of its coniferous associates 

(Shainsky et al. 1994).

During the summer of 1987, rainfall in the Puget 

Sound area of Washington was less than one-third of 

normal; for red alder this resulted in widespread leaf 

yellowing and premature abscission, terminal dieback, 

and, on droughty sites or new plantings, mortality (Russell 

1991). Prior to 1987, the Puget Sound area experienced 

several decades without back-to-back dry summers and 

many years of above-normal rainfall. Combining these 

weather patterns with high levels of harvesting activity that 

created seedbed conditions favorable to alder establishment 

may have increased the percentage of alder stands growing 

on drought-sensitive sites (K. Russell, pers. com. 1992). 

Every summer (1 June to 30 September) from 1987 

through 1992, the Puget Sound region had below-normal 

precipitation. Thus trees stressed by the extreme drought in 

1987 may have been further stressed in subsequent years; 

presumably these back-to-back dry summers are one of the 

causes of the wide-spread instances of alder top dieback and 

mortality in the Puget Sound region in the late 1980s and 

early 1990s (K. Russell, pers. com. 1992).

The sensitivity of red alder to stress factors other than 

those discussed above is not documented. Alder is found on 

sites close to the ocean and presumably is fairly tolerant 

of salt spray. Alder has also been observed adjacent to pulp 

mills and other industrial plants and thus exhibits tolerance 

for at least some components of air pollution.

Future Research Needs

More than 25 years ago Minore (1979) commented 

on the surprising lack of information on autecological 

characteristics of red alder. Although available information 

has increased since then, much of our knowledge of 

the biology of red alder is still based on casual or shortterm observations and not on detailed life histories 

or controlled experiments. Additional research on the 

biology and ecology of red alder is warranted to provide 

a firm knowledge base from which to make management 

recommendations. Specific topics of interest include 

the physiological or ecological factors that control: 

(1) forest succession, (2) alder seed production, dispersal 

and germination, (3) spatial distribution, timing of, and 

interrelationships between root and shoot growth, (4) 

tree responses to changes in light, nutrients, moisture, 

or temperature regimes, and (5) the occurrence and 

significance of biotic damaging agents. 

Acknowledgments

I thank D. Campbell (retired Project Leader, USDA 

Animal, Plant and Health Inspection Service, Animal 

Damage Research Unit, Olympia, Washington), P. Courtin 

(Regional Pedologist, British Columbia Ministry of Forests, 

Nainimo, BC), D. DeBell (retired Research Forester, 

Pacific Northest Research Station, Olympia, WA), L. 

Geiser (Ecologist, U.S. Forest Service, Corvallis, OR), 

T. Goward (Curator of Lichens, University of British 

Columbia, Vancouver, BC), A. Ingersoll (Air Resource 

Database Manager, U.S. Forest Service, Corvallis, OR), 

N. Khadduri (Nursery Scientist, Department of Natural 

Resources, Olympia, WA), W. Littke (Research Pathologist, 

Weyerhaeuser Company, Federal Way, WA), D. Omdal 

(Pathologist, Washington Department of Natural Resources, 

Olympia, WA), F. Rhoades (Curator, University Fungal 

Herbarium, Western Washington University, Bellingham, 

WA), K. Ripley (Forest Health Specialist, Washington 

Department of Natural Resources, Olympia, WA), K. 

Russell (retired Forest Pathologist, Washington State 

Department of Natural Resources, Olympia, WA), and 

D. Peter (Ecologist, Pacific Northwest Research Station, 

Olympia, WA) for sharing unpublished observations and 

S. Reutebuch (Pacific Northwest Research Station, Seattle, 

WA) and W. Littke (Weyerhaeuser Company, Federal Way, 

WA) for sharing photos.
37

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45

— Red Alder: A State of Knowledge —

Red Alder Stand Development 

and Dynamics

Robert L. Deal1

1Robert L. Deal is a research forester, U.S. Department of Agriculture, Forest Service, Pacific Northwest Research Station, 620 SW Main Street, Suite 400, 

Portland, OR 97205, email: rdeal@fs.fed.us

Abstract

This paper synthesizes information on the development 

of natural pure red alder stands and dynamics of mixed 

alder-conifer stands. Early research on red alder growth and 

yield focused on developing stand volume and normal yield 

tables for alder in the Pacific Northwest. Recent site-index 

estimation and height-growth curves were developed on a 

20-year site base age. These height-growth and site-index 

curves were a significant improvement over earlier work and 

are widely used today. Red alder exhibits rapid early height 

growth with much more rapid height growth than for conifer 

associates. On good sites, trees may be 9 m at age 5, 16 m 

at age 10 and 24 m at age 20. Height growth then quickly 

declines, and by age 15 alder will reach more than half its 

total height and nearly all of its mature height by the age of 

40. Alder height growth essentially stops by age 50. 

Long-term successional sequences of red alder stands 

are not well understood. Red alder frequently occurs in 

mixed stands with conifer associates including Douglas-fir, 

western hemlock, Sitka spruce and western redcedar. Mixed 

species dynamics is more complex and variable than for 

pure stands and depends on a number of factors including 

forest composition, establishment and survival of understory 

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.

conifers, timing of alder mortality, abundance and 

composition of shrubs, and geographic location. Red alder 

is short lived (approximately 100 years) compared with its 

conifer associates. In some sites, alder may succeed to brush 

dominated communities with few if any conifers to replace 

the alder. In other sites, succession to more shade tolerant 

trees such as western hemlock and Sitka spruce is certain 

but the sequence of events is unknown. On some nitrogen 

deficient sites red alder has been shown to increase growth 

of conifer associates but in other cases red alder has been 

shown to suppress and even kill less shade tolerant conifers. 

Other conifer species such as western hemlock, Sitka spruce 

and western redcedar can survive as lower strata species and 

eventually grow above the alders but it may take several 

decades for this to occur. Height growth and long-term stand 

dynamics in mixed stands are highly variable and long-term 

succession is difficult to predict. More work is needed to 

understand the long-term successional sequence of events. 

Keywords: Alnus rubra, red alder, stand development, stand 

dynamics, mixed alder-conifer stands, forest succession
46

Introduction

Red alder (Alnus rubra Bong.) has become an 

increasingly important species both as a commercial 

forest product and for it’s recognized ecological role in 

forests. However, many questions remain about the growth 

potential, stand development, and long-term succession of 

alder stands. Forest land owners and managers have only 

recently begun to manage alder plantations and preliminary 

results suggest that these plantations have different growth 

potential and possibly shorter rotations than natural stands. 

These alder plantations also appear to have different tree 

height and diameter growth patterns than natural alder 

stands. In natural stands, red alder frequently occurs in 

species mixtures and common conifer associates include 

Douglas-fir (Pseudotsuga menziesii (Mirb.) Franco), 

western hemlock (Tsuga heterophylla (Raf.) Sarg.) and Sitka 

spruce (Picea sitchensis (Bong.) Carr.). These mixed stands 

are quite different than pure stands of either red alder or 

conifer species.

Red alder has often been considered as a weed species 

and in the past alder was commonly eliminated from conifer 

stands. However, the potential value of red alder as a 

commercial forest product has been recognized for decades. 

An article published in the 1930s (Munger 1938) discussed 

the increase of red alder in cut over and burned areas of 

the Pacific Northwest and the emergence of sawmills that 

were using the wood of red alder as a substitute for other 

hardwoods such as birch, mahogany or walnut. Munger 

described the wood of red alder as “among other desirable 

qualities it turns well, takes glue and paint satisfactorily, 

and does not incline to warp, check or shrink if properly 

handled.” Other papers entitled “Alder comes of Age” 

(Clark 1955), and “Don’t call red alder a weed!” (Morse 

1967) highlight the difficulty for alder to become recognized 

as an important commercial species in the Pacific 

Northwest. Despite this information on the potential of 

red alder, forest managers continued to discourage alder in 

managed forests for decades. However, the recent premium 

value for alder sawlogs has led to widespread interest in red 

alder as a commercial forest product. Current sawlog prices 

for red alder now equal or exceed those for Douglas-fir and 

red alder is now recognized as a premium value tree. In 

a region that has witnessed many saw mill closures and a 

major reorganization of the timber industry, new sawmills 

are being opened that are focusing almost exclusively on 

red alder. The commercial value of red alder now appears 

firmly established in the Pacific Northwest, with increasing 

demand for domestic lumber, furniture supply and a 

significant increase in lumber exports to countries such as 

China (Forestry Source 2005). 

Red alder’s important ecological role in both aquatic 

and terrestrial systems is also widely recognized. Red alder 

has been shown to increase the abundance of invertebrates 

for fish and other aquatic resources in its immediate vicinity 

and in downstream reaches (Wipfli 1997, Piccolo and 

Wipfli 2002). Within red alder stands, vegetation for both 

invertebrate and vertebrate herbivores was increased by 

the presence of red alder, an observation consistent with 

previous work in southeastern Alaska (Hanley 1996, Hanley 

and Barnard 1998, Deal 1997, Wipfli 1997, Piccolo and 

Wipfli 2002). Increased prey availability likely affects many 

more species, such as bats and other small mammals, birds, 

and other wildlife in these young forested ecosystems. 

Results suggest that red alder has an important ecological 

function in forested ecosystems (Pechanec and Franklin 

1968, Tarrant et al. 1983, McComb et al. 1993, Deal 1997, 

Wipfli 1997, Hanley and Barnard 1998). Rather than 

viewing red alder as an undesirable, early-successional tree 

and managing it accordingly, managers are planting alder or 

manipulating existing vegetation to favor the maintenance 

of red alder. 

Most available information on red alder growth and 

yield comes from natural unmanaged red alder stands 

(Worthington et al. 1960, Curtis et al. 1968, Harrington 

and Curtis 1986). New preliminary information from alder 

plantations suggest different stand and tree growth patterns 

in these plantations as compared with natural stands (Bluhm 

and Hibbs 2006). Little information is available on longterm stand development and forest succession of red alder, 

and most of the literature is from descriptive studies with 

speculation on successional sequences of red alder stands. 

Mixed alder-conifer stands are also very different than either 

pure red alder or conifer forests and show highly variable 

development patterns and differences among species 

compositions such as red alder – Douglas fir and red alder 

– Sitka spruce stands. This paper will briefly summarize 

some of the key findings and synthesize the state of 

knowledge about red alder stand development and dynamics 

of mixed alder-conifer stands. The major objectives of this 

paper include:

• Describe stand development and growth of natural 

pure red alder stands, and compare and contrast 

development with common conifer associates 

including Douglas-fir, western hemlock and Sitka 

spruce,

• Describe stand dynamics of mixed red alderconifer forests focusing on red alder/Douglas-fir 

stands and red alder/Sitka spruce/western hemlock 

stands.

Finally, forest succession of red alder stands will be 

discussed and some specific future research needs for red 

alder management will be identified. 
47

Development of Natural 

Pure Red Alder Stands

Some clarification of terms is necessary to define the 

scope and content of this section. Pure stands are those 

stands that are essentially composed of only red alder 

and are contrasted with mixed species stands which will 

be described in a later section. Natural stands are those 

stands that established following natural disturbances or 

harvesting without artificial aid and have never received 

any management practices including planting, vegetation 

control, thinning or pruning. These natural stands are 

contrasted with artificial plantations and/or stands that have 

received weeding, thinning or other density management or 

stand-tending practices.

Red alder growth and yield

Early research on red alder growth and yield focused 

on developing stand volume and normal yield tables for 

alder in the Pacific Northwest. Tree taper equations were 

developed (Curtis et al. 1968), site curves constructed 

(Johnson 1949, Bishop et al. 1958, Skinner 1959) and 

yield tables produced (Worthington et al. 1960, Chambers 

1973). This research provided baseline information on 

red alder volume, growth and yield of natural unmanaged 

stands. Height growth models in the past were based on 

site-index equations (Bishop et al. 1958, Worthington et 

al. 1960). These equations were developed to predict site 

index from stand height, however, and were probably not 

the best means to predict stand height (Curtis et al. 1974). 

Another concern for using these equations was using a site 

index based on the top height at age 50 when alder rotations 

were becoming increasingly shorter in length. To address 

these concerns, Harrington and Curtis (1986) expanded the 

database to include younger stands and developed separate 

height growth equations for red alder. They sampled twenty 

three natural red alder stands in western Washington and 

Oregon across a range of geographic locations, site quality 

and soil conditions. They supplemented this data with red 

alder stem analysis data from earlier red alder height growth 

curves (Johnson and Worthington 1963). Harrington and 

Curtis (1986) developed site-index estimation curves for 

red alder and developed new height-growth curves based on 

a 20-year site-base age (fig. 1). The height growth and site 

index curves they developed were a significant improvement 

over earlier work and are still widely used today. 

Most available information on growth and yield comes 

from natural unmanaged stands. A number of variables 

including initial stand density, mortality, site factors such 

as soils, moisture and nutrients, and management regimes 

such as planting density, thinning, and vegetation control 

can significantly affect stand growth. Some important 

considerations for growth and yield are summarized by 

Puettmann (1994) and include information on height and 

diameter growth, mortality, site-index and height-growth 

equations, yield tables, growth models and stand density 

guides. Red alder plantations appear to have different 

tree height, diameter and stand growth than natural 

stands (Bluhm and Hibbs 2006) and the need for further 

investigation of plantations led to the establishment of 

the Hardwood Silviculture Cooperative (HSC). The first 

plantations from the HSC were established in 1989 and 

efforts are underway to use this information to develop new 

stand growth models for red alder. In particular, there is a 

need to use new tree taper data from plantations to develop 

volume equations and growth models more appropriate for 

red alder plantations.

Height growth of red alder stands

Red alder exhibits rapid early height growth and then 

growth slows considerably after age 15-20 (fig. 1). Height 

growth of seedlings is exceptionally rapid and on favorable 

sites seedlings can grow 1 m or more the first year, and on 

all but the poorest sites, seedlings surpass breast height (1.3 

m) by the second year (Harrington 1990). Maximum annual 

height growth of more than 3 m per year can be achieved by 

2- to 5- year-old seedlings. On good sites, trees may be 9 m 

at age 5, 16 m at age 10 and 24 m at age 20 (Harrington and 

Curtis 1986, Harrington 1990). Height growth rate decreases 

substantially beyond age 20. Dominant tree heights range 

from 10-25 m at age 20 and at age 50 range from 18-37 m 

(Worthington et al. 1960). By age 15, red alder has reached 

more than half of its total height and height growth attains 

nearly all of its mature height before age 40 (Newton and 

Cole 1994). 

Total height and total age possible for red alder is 

unknown. Red alder height can reach 25-30 m on many 

sites and the tallest red alder is listed as 32 m (http://

en.wikipedia.org/wiki/Alnus_rubra). As previously 

mentioned, alder height growth essentially stops by age 

40 or 50. Red alder is a short lived tree but individuals can 

Figure 1—Red alder height-growth curves based on a 20-year site base age 

(Figure modified from Harrington and Curtis, 1986).

 5

10

15

20

25

30

35

Total age (years)

10 20 30 40 50

Index age 20

S20

S25

S15

S10

0

0

Height (m)
48

live to be more than 100 years old, however, pure red alder 

stands frequently begin to “break up” at around age 80 

(Newton and Cole 1994, Smith 1968). Red alder is shade 

intolerant and highly susceptible to suppression. When 

slower growing but longer lived conifers eventually overtop 

alders, these alders can die, and alders are not reestablished 

without new disturbances. Long-term succession for 

alder stands is not clearly understood and depends on the 

establishment of conifer associates, shrub development, 

geographic location and specific site factors. More 

discussion on succession is presented below in the section 

on mixed species stand dynamics. 

Potential site productivity or site quality for alder is 

indicated by height-growth and site-index curves. Site 

index or height at a specified age is the most commonly 

used index of site quality. Site index estimates will be 

most reliable when stand age is within 10 years of the 

index age and estimates are unreliable for stands less than 

5 years of age or at stands much older than 50 years. Siteindex curves were developed by Harrington and Curtis 

(1986) and later adapted to a 50 year base to compare with 

conifer associates (Mitchell and Polsson 1988, Thrower 

and Nussbaum 1991). Site quality is also closely associated 

with other measures of productivity other than tree height. 

Site quality was related to several environmental factors 

and published by Harrington (1986) and summarized 

later by Harrington and Courtin (1994). Harrington found 

that elevation, physiographic position, and other soil site 

characteristics could be used to predict potential site index. 

This information has been useful to predict potential site 

index for red alder from soil data and other environmental 

factors when reliable tree height data was not available. 

Management activities also can influence apparent site 

quality as measured by tree height. Spacing experiments 

have indicated that alder height growth is influenced by 

initial density and maximum height growth occurs at 

intermediate spacings (DeBell and Giordano 1994, Knowe 

and Hibbs 1996). Recent findings from the Hardwood 

Silviculture Cooperative indicate that plantations established 

at high densities appear to be growing faster than natural 

plantations with more rapid height, diameter and volume 

growth than expected (Bluhm and Hibbs 2006, figs. 2 and 

3). Density appears to have an important effect on early 

height growth with most rapid height growth in stands 

of moderate to moderately high densities. The site index 

and height growth curves developed from natural stands 

(Harrington and Curtis 1986) may underestimate potential 

height and volume growth of plantations at moderate to 

moderately high densities. 

Height growth and 

stand development of 

conifer associates

This section compares early and later height growth 

of pure conifer stands with pure red alder stands. It is 

important to make the distinction between height growth 

of pure conifer and pure alder stands, and the development 

of mixed species stands. Competition between tree species 

with different growth development patterns produces more 

variable and complex mixed species stands and will be 

described in the last section on dynamics of mixed alderconifer stands. This section will describe juvenile and 

mature height growth of common conifer associates of red 

alder including Douglas-Fir, Sitka spruce, western hemlock 

and western redcedar (Thuja plicata Donn ex D. Don). 

Young red alder height growth is more rapid than all 

other tree associates with the exception of black cottonwood 

(Populus trichocarpa Torr. and Gray) (Harrington 1990). 

The available height growth information for conifers is 

much better than for red alder and the data for DouglasFigure 3—Nine-year-old red alder plantation planted and thinned later to 

about 246 trees ha-1. Figure 2—Four-year-old red alder plantation planted at about 2950 trees 

ha-1.
49

fir is particularly robust. Red alder juvenile height growth 

is much more rapid than any of its conifer associates. On 

the best sites, Douglas-fir may be 12 m at age 10, 18 m at 

age 15, and 24 m at age 20 (King 1966). Even on the best 

Douglas-fir sites, juvenile height growth of red alder is more 

rapid than for Douglas-fir, and total height of dominant alder 

are greater than dominant Douglas-fir until about age 20. 

It is probably more useful to compare height growth of 

alder and conifers on average sites. Alder has significantly 

greater early height growth than any of the common conifer 

associates. On average sites, Douglas-fir height is only 1 m 

at age 5, 2 m at age 10 and 4 m at age 15 compared with 5 

m at ate 5, 11 m at age 10 and 15 m at age 15 for red alder 

(fig. 4). Green and Klinka (1994) report that early height 

growth is slightly higher for western hemlock followed by 

Douglas-fir and Sitka spruce, with the slowest height growth 

for western redcedar. Other researchers (Omule 1987) report 

Douglas-fir with the most rapid early height growth, with 

redcedar height growth significantly slower than these other 

conifers. However, differences in early height growth are 

small among the four most common conifer associates with 

each species lagging behind red alder (Omule 1987, Green 

and Klinka 1994). 

Longer-term height growth and stand development 

shows very different height growth patterns for alder and 

conifers. As previously reported, alder height growth rapidly 

declines after age 20 whereas height growth increases and 

is sustained for Douglas-fir, hemlock and spruce. Coastal 

Douglas-fir attains the largest height increments between 

ages 20-30, and has the ability to maintain a fairly rapid 

rate of height growth over a long period (Hermann and 

Lavender 1990). On average sites, Douglas-fir, Sitka spruce 

and western hemlock all surpass red alder at about age 50 

(fig. 5, Green and Klinka 1994). Another study (Williamson 

1968) reports that Douglas-fir and red alder attain the same 

height at about age 45. Western redcedar height growth 

is somewhat slower and may take as long as 80 years to 

achieve the same height as red alder (Green and Klinka 

1994). Again, it is worth noting that these projections are 

for pure stands and not for mixed alder-conifer stands where 

tree species interact. Mixed stands can be quite different and 

have extremely variable growth patterns.

Dynamics of mixed red 

alder – conifer stands

Growth and development of mixed red alder-conifer 

stands is different than patterns observed in either pure alder 

or pure conifer stands. Mixed species stand development is 

more complex and variable than for pure stands and depends 

on a number of factors including establishment and species 

composition, early height growth, species shade tolerance, 

initial species dominance and the spatial distribution of 

species. This section will discuss dynamics of mixed alderconifer stands using some examples of mixed red alder/

Douglas-fir stands and mixed red alder/Sitka spruce/western 

hemlock stands. Discussions of stand dynamics will focus 

on stand growth and development and some of the effects 

of mixed stand structure on other forest resources including 

understory plants.

Figure 4—Comparison of juvenile height growth of red alder and 

associated conifers to age 15. Height curves are based on median values 

for conifers on Site Class II (King 1966). (Figure modified from Green and 

Klinka 1994).

Figure 5—Comparison of height growth of red alder and associated 

conifers to age 100. Height curves are based on median values for conifers 

on Site Class II (King 1966). (Figure modified from Green and Klinka 

1994).

0

5

10

15

20

25

30

35

40

45

0 10 20 30 40 50 60 70 80 90 100

Height Growth Comparison

Alder S127

Douglas-fir S130

Hemlock S126

Redcedar S118

Sitka spruce S129

Source: Green and Klinka, 1994

Height (m)

Age (yr from seed)

Juvenile Height Growth 

Height (m)

Age (yr from seed)

1 2 3 4 5 6 7 8 9 10

Alder S127

Douglas-fir S130

Hemlock S126

Redcedar S118

Sitka spruce S129

Source: Green and Klinka, 1994

0

4

6

8

12

14

16

10

2

11 12 13 14 15
50

Mixed red alder – Douglas-fir stands

A well-documented study of mixed red alder-conifer 

stands at Wind River in southwestern Washington compared 

a nitrogen-deficient stand of mixed Douglas-fir and red 

alder with adjacent pure stands of Douglas-fir (Tarrant 1961, 

Tarrant and Miller 1963, Miller and Murray 1978). After 48 

years, mixed alder-conifer stands had significantly greater 

stand growth with overall wood production nearly double 

that of adjacent pure Douglas-fir stands. Douglas-fir site 

index was increased by an average of 6.4 meters. Improved 

soil fertility was suggested to be largely responsible for the 

improved growth of these stands (Bormann et al. 1994). 

Other authors have indicated that on poor sites alder may 

increase growth and production in Douglas-fir plantations 

(Atkinson et al. 1979, Tarrant et al. 1983).

Research from a mixed alder-conifer stand at Cascade 

Head in coastal Oregon reported different results in 

productivity. At Cascade Head, researchers found no 

appreciable difference in productivity between the mixed 

alder-conifer stand and an adjacent pure conifer stand. 

Both biomass and net primary productivity were higher in 

the pure conifer stand than in the mixed stand (Binkley et 

al. 1992). On fertile sites such as this stand, where growth 

was less limited by nitrogen, availability of nitrogen 

also increased, but with no corresponding increase in 

productivity (Binkley et al. 1992). The mixed alder-conifer 

site at Cascade Head has also been used to investigate 

effects of alder on stand structure and understory vegetation 

(Franklin and Pechanec 1969, Pechanec and Franklin 1968). 

Franklin and Pechanec found that the species richness 

and cover of herbaceous plants was greatest under pure 

alder and least under pure conifer, with mixed stands 

intermediate. 

Several studies suggest that red alder can inhibit 

Douglas-fir growth (Newton et al. 1968, Miller and Murray 

1978, Miller et al. 1999). In a reconstruction study of red 

alder and Douglas-fir in the Pacific Northwest, Newton et al. 

(1968) assessed height growth of red alder and Douglas-fir 

in 39 nearly pure red alder stands in western Oregon. They 

determined that red alder remained dominant for 25 to 35 

years on most sites, and 40 years or more on wetter sites. 

Emergence of Douglas-fir from beneath red alder depends 

on its ability to grow while being suppressed and the 

intensity of suppression. Newton et al. (1968) reported that 

under all environmental conditions the initial growth of red 

alder was substantially better than for Douglas-fir. On some 

sites they suggested that Douglas-fir could not be expected 

to reach the overstory unless it was established 3 to 8 years 

before the alder. Newton concluded that Douglas-fir would 

be unable to maintain height growth while being suppressed, 

but more shade-tolerant species might be able to do so. In 

another study, Miller and Murray (1978) assessed growth in 

four mixed red alder — Douglas-fir stands. Conifers showed 

reduced growth in heavily stocked mixed alder — Douglasfir stands on higher quality sites. Douglas-fir generally 

emerged from the alder canopy and attained dominance in 

mixed stands by 25 to 35 years on drier sites and about 10 

years later on wetter sites. They suggested that to attain full 

stocking of Douglas-fir, the stocking and distribution of red 

alder would need to be controlled either by thinning the 

alder and recommended uniformly stocking the stand with 

50 to 100 alder trees ha-1.

Mixed red alder – Sitka spruce – western 

hemlock stands

A series of studies was established in 45-year-old 

mixed red alder/Sitka spruce/western hemlock stands on 

Prince of Wales Island, Alaska (Wipfli et al. 2002, Deal et 

al. 2004, Orlikowska et al. 2004). Nine stands were sampled 

across a compositional range from 0-86% alder. Alder 

height growth was initially rapid then slowed considerably, 

whereas, conifer height growth was initially slow then 

rapidly increased. Dominant conifers emerged from the 

alder overstory in 18-25 years after logging, and at age 45, 

conifers were 4-9 m taller than the associated alders (Deal 

et al. 2004). Red alders were relatively evenly distributed 

with most trees in a narrow diameter and height range. 

Conifers were more variable in size than the alders and 

included numerous small trees and a few of the tallest trees 

in the stand. Stands containing both red alders and conifers 

provided a broader and more even tree-size distribution 

than is typically found in pure conifer stands. These mixed 

alder-conifer stands created a multi-layered forest canopy 

with a few tall overstory conifers, a mid-canopy level of red 

alder and a lower canopy level of small diameter conifers. 

This pattern of development has also been reported in other 

mixed red alder/Sitka spruce/western hemlock stands in 

Alaska (Hanley and Hoel 1996, Deal 1997). Overall, these 

mixed alder-conifer stands provided more heterogeneous 

structures than is typically found in pure conifer stands of 

the same age.

Stand development in these mixed alder-conifer stands 

is highly dynamic and tree height and stand structure is 

different then it was at earlier stages. In mixed alder-conifer 

stands in Alaska (Deal et al. 2004), dominant spruces 

and hemlocks emerged from the slower growing alder 

overstory at about 18-25 years after logging. This species 

change in overstory canopy position has been reported 

by other authors (Newton et al. 1968, Miller and Murray 

1978, Stubblefield and Oliver 1978) in mixed red alder 

and Douglas-fir stands in the Pacific Northwest. Increased 

Sitka spruce height growth, however, occurred earlier and 

appeared to release more quickly then for Douglas-fir. 

Other conifer species such as western hemlock and western 

redcedar survived as lower strata trees and eventually grew 

above the alders, however, it took several decades for these 

conifers to release (Stubblefield and Oliver 1978). 
51

Forest succession of 

red alder stands 

Red alder is relatively short-lived compared to its 

associated conifers (Smith 1968, Harrington 1990). 

Worthington et al. (1962) suggested a maximum age for 

red alder of about 100 years. Smith (1968) notes that by 

50 years of age, pure alder stands on highly productive 

Douglas-fir sites in British Columbia were “breaking up.” 

Individual trees showed dieback as early as 40 years of age. 

On less productive sites, mortality occurred at stand age 60 

to 70, with few red alder stands remaining intact beyond 

100 years. In Washington, Oregon, and British Columbia, 

mortality of red alder increases rapidly in stands over 90 

years old, and little alder remains by the age of 130 years 

(Newton and Cole 1994). 

Successional sequences of alder stands are not well 

understood. Observation of mature forests in the Pacific 

Northwest suggests that where seed sources are present, 

alder is replaced by longer-lived, more shade-tolerant 

conifers that sustain growth rates for longer periods. Alder’s 

rapid early growth and high stem densities make it difficult 

for other shade-intolerant species to regenerate and grow 

if they do not become established at the same time alder 

invades a disturbed area. Douglas-fir can be eliminated in 

dense, young alder stands while more shade-tolerant species 

such as western hemlock, Sitka spruce and western redcedar 

can survive, grow into the canopy, and ultimately dominate 

the site (Harrington et al. 1994). Other scenarios have 

been reported where following initial dominance by alder, 

understory shrub cover increases after about age 20, and in 

the absence of understory conifers, shrubs may completely 

dominate the site after senescence of alder at age 80-100 

(Peterson et al. 1996, Hibbs and Bower 2001). 

Long term stand dynamics of mixed alder-conifer 

forests are uncertain. Red alder is expected to die out of 

these stands but it is unclear how long this will take. The 

mechanism of alder mortality is also unknown but it is 

likely due competition with conifers and/or tree disease. 

Red alder is short lived and some researchers have reported 

that by 50 years most alder stands were breaking up and 

few stands remain intact beyond 100 years (Smith 1968). 

Newton and Cole (1994) report for stands in Oregon that 

the last alder will succumb in less than 130 years. It is 

unlikely that any of the alder stands in southeast Alaska will 

be replaced by shrubs as has been observed in some sites 

in the Oregon Coast Range (Newton et al. 1968, Hibbs and 

Bower 2001). The abundance of shade tolerant conifers 

such as Sitka spruce and western hemlock suggests that 

a treeless shrub succession is highly unlikely. Also, when 

alders die out they may create more gaps in the canopy and 

lead to a later stage of stand development that is either more 

open or invaded by a new cohort of conifers. Where alder 

forms a more continuous canopy with few conifers, alder 

trees compete with one another, often forming a continuous 

canopy height, and they may persist for much longer. In 

summary, the successional sequences of alder depend on 

a combination of factors including forest composition, 

establishment and survival of understory conifers, timing of 

alder mortality, abundance and composition of shrubs and 

geographic location. 

Future Research Needs

Most of the available information for red alder on 

stand development, growth and yield comes from natural 

unmanaged stands. Red alder plantations, however, appear 

to have different tree height and diameter and stand growth 

than natural stands. As new information becomes available 

from red alder plantations it will be essential to use these 

data to construct new tree taper and volume equations and 

develop growth models based on plantations. A number 

of factors including initial planting density, mortality, 

thinning, vegetation control and other management practices 

significantly affect stand growth. Preliminary results suggest 

that early height growth of alder plantations is closely 

related to initial stand density. Longer term data is needed 

to determine if this reported early height growth in red alder 

plantations results in shorter rotations and/or increases in 

overall stand growth. If so, then new growth models will 

need to be developed that reflect these changes in stand 

growth projections. Growth and development of mixed 

red alder-conifer stands is quite different than patterns 

observed in pure stands and successional sequences of 

alder stands are not well understood. Stand development 

of mixed-species stands is highly variable and depends 

on a number of factors including species composition, 

establishment, initial species dominance, shade tolerance, 

disturbance frequency, geographic location and other factors 

that influence long-term successional sequences. Although 

it would be difficult to control experimentally, better 

information is needed on the relative importance of these 

factors and their role for successional sequences of different 

alder-conifer compositional mixtures.

Acknowledgements

David Hibbs, Paul Hennon and Glenn Ahrens reviewed 

an earlier draft and provided helpful comments to improve 

the clarity of the manuscript. I gratefully acknowledge the 

organizers of the Symposium for Red Alder: A state of 

Knowledge from which these papers were produced.
52

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