Care Of Speckled Alder Trees: Learn How To Grow A Speckled Alder Tree

Care Of Speckled Alder Trees: Learn How To Grow A Speckled Alder Tree

By: Teo Spengler

Is it a tree or is it a shrub? Speckled alder trees (Alnus rugosa syn. Alnus incana) are just the right height to pass as either. Read on for more speckled alder information, including tips on how to grow a speckled alder and its care.

Speckled Alder Information

Speckled alder trees growing in the wild look a lot like shrubs. According to speckled alder information, these trees do not get above 25 feet (7.6 m.) tall, and can be much shorter. In addition, speckled alder trees usually grow with multiple slender stems like bushes. The common name comes from the fact that the stems, heavily lined with horizontally borne lenticels, appear speckled.

Both male and female alder flowers are called catkins. The males are long and conspicuous, while female flowers are reddish and smaller, and lack outer scales.

How to Grow a Speckled Alder

If you are thinking of growing speckled alders, you need to remember the very specific growth conditions these native trees require. These alder trees grow in wetlands. In fact, it has given its name to a type of wetland known as an “alder thicket.” You’ll also see speckled alder growing along streams, in roadside ditches and in swamps. For example, speckled alder trees can colonize cut-over northern conifer swamps.

To start growing speckled alders in the landscape, you’ll need wet soil. You’ll also need to live in U.S. Department of Agriculture plant hardiness zones 4 through 9, where the alders thrive.

Plant the seeds or seedlings in full sun in wet soil. If you want to start growing speckled alders from seeds, it’s easy to collect them from the tree in autumn. Each fruit is a samara with narrow wings and produced one single seed.

Care of Speckled Alder

You won’t have to invest much time or effort in care of speckled alder. These are native trees and can take care of themselves if you site them well.

Be sure the ground is wet and that the trees get some sun. If that’s the case, care of speckled alder should be easy. If you want to grow the alder to look more like a tree than a shrub, you can prune out the stems, leaving only the strongest to serve as the trunk.

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Nature's unsung heroes, speckled alders stand out this time of year


As most trees finally shed their leaves, previously obscured layers of our local forests are uncovered. The speckled alder (Alnus incana), upon losing its leaves, reveals distinctive seed and pollen catkins. The male pollen-bearing catkins hang in dangling clusters, and the erect brown seed-bearing cones (female structures) bristle from branch tips.

As most trees finally shed their leaves, previously obscured layers of our local forests are uncovered. The speckled alder (Alnus incana), upon losing its leaves, reveals distinctive seed and pollen catkins. The male pollen-bearing catkins hang in dangling clusters, and the erect brown seed-bearing cones (female structures) bristle from branch tips.

Aside from the telltale catkins, speckled alder have prominent light-colored lenticels — spongy openings used for gas exchange — speckling the bark hence the name.

Speckled alders are, in my opinion, one of the unsung heroes of our wetlands. They are fast-growing shrubs that form thickets, which provide shelter for moose, deer, rabbit, grouse and others. Beavers browse on the twigs and foliage and use speckled alder in dam and lodge construction, while goldfinch and other songbirds eat the seeds, buds and catkins. In addition, speckled alders provide erosion control, function as wind breaks, and improve nutrient levels in surrounding soil. Humans have derived a variety of medicines from the bark, the catkins are high in protein (they're fairly bitter, so best left for dire emergencies) and the wood burns fast and hot it's known as "biscuit wood" because it is perfect for baking biscuits.

You'll find alders along stream banks, marsh edges, bog, swamps, road edges and other disturbed areas. While speckled alder is a relatively common understory shrub, it does best in full sun. When the canopy is removed, through fire or logging, for example, speckled alder will undergo explosive growth.

Most New England swampy wetlands are surrounded by dense, impenetrable thickets of alder and willow. This sort of description makes an alder thicket sound menacing and unpleasant, it implies something difficult to deal with, conjures up images of explorers hacking their way through mosquito-infested swamps, unruly landscaping, a lack of order. The words swamp and thicket are almost pejorative terms, not used when describing prime real estate for human habitat. For nature, however, an alder thicket is prime real estate.

In addition to the aforementioned uses of speckled alder by a variety of wildlife, the roots form symbiotic (an ecology term that refers to close associations between different species) relationships with soil bacteria, which form nodules on the roots that fix atmospheric nitrogen. Plants can't use atmospheric nitrogen directly (neither can we), it has to be fixed, turned into a usable form. Farmers will often rotate legumes, which have nitrogen-fixing root nodules, with crops like corn, which don't, using the legumes to enrich the soil.

In the same way, alder and their nitrogen-fixing roots enrich the soil they grow in, funneling atmospheric nitrogen into local biological communities. Alder-dominated wetlands contain significantly more plant-available nitrogen than do non-alder dominated wetlands (Hurd & Raynal Hydrological Processes 18: 2004). This is really important for plant growth, since nitrogen is frequently a major limiting factor in these ecosystems. With more available nitrogen, plants can grow more rapidly, producing more biomass (food) for the system.

Unfortunately, the glossy buckthorn, a non-native aggressively invasive shrub, can out compete alder. Glossy buckthorn grows in the same sorts of habitat, but instead of enhancing the surrounding soil, it releases chemicals that inhibit the growth of nearby vegetation. Recent research suggests that buckthorn will preferentially grow in alder swamps due to the high nitrogen levels in the soil, but once there suppresses both alder growth as well as its nitrogen-fixing symbiont (Mays, 2009).

Speckled alder are relatively easy to identify this time of year. This past weekend brought freezing temperatures, and thin layers of ice began to form on my neighborhood wetlands, signaling the start of the best time of year to traipse about your local swamps.

2.  Alnus incana (L.) Moench ssp. rugosa (Du Roi) Clausen N

speckled alder. Alnus incana (L.) Moench var. americana Regel A. rugosa (Du Roi) Spreng. 
 A. rugosa (Du Roi) Spreng. var. americana (Regel) Fern. Betula alnus L. var. rugosa Du Roi 
• CT, MA, ME, NH, RI, VT. Swamps, shorelines, wetland edges, borders of low fields.

2×3. Alnus incana × Alnus serrulata → Alnus ×‌fallacina Callier is a rarely collected alder hybrid to be expected wherever the parental taxa co-occur. It is intermediate in morphology however, given the slight differences between the progenitors, it is subtle and often difficult to determine (especially on herbarium sheets).

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Gray alder is a tree or shrub, growing from 15 feet to 82 feet (4.6-25 m) tall. [120]. Bark is smooth and thin [155,348], often with conspicuous lenticels [106] (see photo in Fire Effects and Management). The wood is soft [171]. Leaves are oblong and serrated at the margins [208,355]. The inflorescences are small, naked [131] catkins. Male catkins grow in clusters of 2 to 4. They are 0.8 to 3 inches (2-8 cm) long and pendulous at maturity. Female catkins are woody and resemble cones, growing in clusters of 2 to 6 [106,120,237]. The "cones" are 0.4 to 0.8 inch (1-2 cm) long at maturity [155]. The fruits are described as either irregular samaras [120] or nutlets with small, narrow wings [106,154,171,179,242,345]. They hold 1 to 4 seeds/cone scale [137,155,318]. The seeds lack endosperms, so the cotyledons are relatively small [154]. The root system is shallow and spreading [171]. Roots are typically infected with nitrogen-fixing, actinomycete bacteria [171,205,215,261]. A review reported that thinleaf alder fixes more nitrogen than Sitka alder (Alnus crispa subsp. sinuata) and quantities similar to those of red alder [143].

Gray alder is adapted to periodic flooding in spring or other run-off periods [23,204,267], although it cannot tolerate long periods of inundation. In the laboratory, speckled alder growth and root development were "severely reduced" when water levels were at or above the root crown for 30 days or more (P Speckled alder is a spreading shrub [120] or small tree [131], growing up to 30 feet (9 m) tall [120] and 4.7 inches (12 cm) in diameter. Typically multistemmed with crooked branches, it is "very crooked" in form as a small tree [106], and only assumes tree form on high-quality sites [171]. Its common name refers to the lenticels that give a characteristic "speckled" look to the bark [81]. In a speckled alder community by a small stream in upstate New York, speckled alder stems averaged 14 years old, ranging from 7 to 31 years old. Stem density averaged 7,850 stems/ha [331]. In central New York, age of mature stems ranged from 10 to 25 years. Based on stem sprouting vs. stem mortality rates, the author estimated maximum age of speckled alder clones at about 100 years [176].

Thinleaf alder is an open, spreading shrubby tree or shrub, growing from 15 to 39 feet (4.6-12 m) tall [120,345] and usually less than 4 inches (10 cm) in stem diameter [215]. It often forms thickets along streams [179,231,242,345], although on upland sites it usually grows in discrete, shrubby clumps [208]. Thinleaf alder stems on sandbars of the Tanana River, central Alaska, averaged 14 years old [339]. Morris and others [259] provide a key for identifying thinleaf alder and other western shrubs in winter.

Thinleaf alder is frost-tolerant [23,204].

Raunkiaer [290] life form:

Genetic differences among populations are generally small in species with wind-dispersed pollen and seeds (for example, [168]), such as gray alder. Allozyme studies of speckled alder in central Quebec showed high rates of gene flow and weak genetic differentiation among 4 populations [34].

Spatially, the genetic make-up of individuals in speckled alder thickets may be random. Studies of 4 speckled alder thickets in New York showed clones were randomly distributed, and clumps of single genotypes were not aggregated. Thus, the author concluded that although speckled alder regenerates vegetatively, sexual regeneration was driving the genetic structure of these 4 populations [175].

Seed production: Gray alder first produces seed at 25 years or younger. There are usually 1 to 4 years between large seed crops (review by [154]). Thinleaf alder produces "abundant" seed (reviews by [143,256]). Mean annual seed rain of thinleaf alder in white spruce stands of interior Alaska was 745 seeds/m² [366].

Seed dispersal: Wind and water disperse thinleaf alder seeds ([81,93], reviews by [35,312]). In waterways of Alaska, thinleaf alder seeds stayed afloat for "long periods of time" (Densmore 1976 personal observation [93]). On a floodplain of Little Otter Creek in Vermont, speckled alder seed was found in floodwaters but not in seed rain deposited on soil. The surrounding plant community was a red maple-sugar maple (Acer saccharum) forest [178].

Thinleaf alder may establish from seed in crown-stored "cones" after disturbances such as fire ([256], review by [35]), road construction, logging, and mining [256].

Seed banking: Gray alder has a transient seed bank seed longevity in the field or in water is short [344]. Even in dry storage, seeds do not remain viable for more than 2 years [154]. In a Freeman maple-white ash (Acer × freemanii-Fraxinus americana) swamp in New York, speckled alder seed density was 0.01 seed/120 cm². Speckled alder had 0.9% cover in aboveground vegetation [28]. In dry Douglas-fir forests of south-central British Columbia, thinleaf alder seed was present in the soil seed bank on unlogged and unburned sites. Thinleaf alder seed was not found in the soil seed bank on adjacent sites that were either clearcut 5 or 10 years prior or burned at low or high severity 5 years prior. Thinleaf alder was not present in aboveground vegetation on any of the study sites [317].

Germination: Gray alder seeds are usually nondormant ([27,300], review by [19]) and, under favorable conditions, may germinate immediately after dispersal (review by [143]). However, some seed lots may require a stratification period, from a few days to over winter (review by [154]). Moist soil ([71], review by [143]) and temperatures from approximately 50 to 77 °F (10-25 °C) [154] are required for germination.

Gray alder seed viability is generally low (review by [143]), but preliminary studies suggest that light enhances germination ([93], review by [154]). Some laboratory studies found only 5% viability in thinleaf alder seeds (review by [143]). In greenhouse trials, speckled alder showed low seed viability (4-42%). Germination of filled seed varied from 0 to 50%, with presoaking and light increasing germination rates [27]. For thinleaf alder seeds collected near Fairbanks, germination averaged 90% for seeds cold-stratified in light and 5% for seeds cold-stratified in dark. Unstratified seed showed 100% germination in light and ≤13% germination in dark [93]. Gray alder seed may germinate without light, however. In a laboratory study, speckled alder showed no significant difference in time to germination and germination rate in dark vs. light, averaging 10 days to germination, and 36% germination, in both light and dark [246].

Seedling establishment and plant growth: Gray alder seedling establishment may be rare except in primary succession or on disturbed, open sites. In central New York, 4 populations of speckled alder showed no seedling establishment over 3 years [176]. In a Freeman maple-white ash swamp in New York, speckled alder establishment averaged 10 seedlings/100 m². One-year-old seedlings averaged 5.2 inches (13.1 cm) tall 2-year-old seedlings averaged 13.5 inches (34.3 cm) tall [29].

Soil disturbance and/or exposed mineral soil favor gray alder establishment ([71], review by [143]). In Michigan, the margins of American beaver ponds were favorable sites for speckled alder germination and establishment [222]. In a Swedish field experiment, European gray alder seedling survival was higher in mineral soil (34%) than in humus (9%) [300].

Gray alder seedling establishment on new sandbars and on banks with receding floodwaters is common, although other substrates also provide favorable establishment sites. After a flood on the Connecticut River, for example, speckled alder seedlings were noted in a sugar maple swamp in an oxbow [170]. On Lassen National Forest, thinleaf alder established most often near wide stream channels on sand- or gravelbars. Thinleaf alder seedling establishment was negatively correlated with canopy cover (P=0.02) and litter depth (P=0.002) and positively correlated with solar radiation (P=0.002) [297]. In a greenhouse experiment in France, flooded European gray alder seedlings grew fastest on moist, sandy loams that were flooded and drained daily [177]. In boreal quaking aspen-paper birch ecosystems near Slave Lake, Alberta, thinleaf alder established on decaying logs and stumps [226]. Gray alder germinants apparently tolerate slightly drier conditions than willow germinants [114].

Gray alder seedlings grow rapidly under favorable conditions ([298], review by [154]). Monsen and others [256] report that thinleaf alder seedlings are "very competitive and vigorous. Once established, few plants can grow as rapidly". In central New York, annual growth increments of 4 speckled alder populations ranged from 0 to 2.8 inches (1.1 cm). Growth rate was generally greatest in midsized stems (0.8-1.5 inches (2.0-3.9 cm) DBH, P Vegetative regeneration: Cloning is apparently the primary means of spread in established stands of gray alder (review by [143]). Gray alder sprouts from the root crown [35,38,175,208]. It can also sprout from roots, including root offsets [175,176,298], and layers. Root sprouting and layering are apparently less important than root-crown sprouting [176], although root sprouts distant from the parent plant have been noted for both speckled alder and European gray alder (review by [143]). Haeussler and others [143] report that sprouting "can be expected" after mechanical site preparation. In a Magellan's sphagnum bog in Ohio, speckled alder sprouted after cut-stump herbicide applications, growing up to 3 feet (1 m) in one growing season [9]. Gray alder does not spread rapidly, but its clones can be long-lived. In a central New York study, 4 speckled alder populations were monitored for 4 years. There was no seedling establishment on study plots during that time. Clump sprout production and stem mortality were variable within and among populations, although no clumps died out. No lateral extension of clones via root sprouting occurred. Annual sprout production averaged about 3 live stems/clump [176].

In a study comparing root anchorage of riparian species in Italy, European gray alder was less resistant to uprooting by flood than Lombardy poplar (Populus nigra) or Elaeagnus willow (Salix elaeagnos) [191]. This relative ease of uprooting may allow for vegetative spread of gray alder when root fragments are torn off, distributed downstream, and sprout.

Gray alder tolerates full sun to light shade. It is an important colonizer in primary succession. It is also successionally important after stand-replacing events such as fire and logging [108] and in canopy gap succession [209]. Its ability to fix nitrogen (see Site Characterisitics and Plant Communities) can enhance site quality for later-successional species. Plant response to fire provides information on gray alder and postfire succession.

  • Speckled alder
  • Thinleaf alder

Speckled alder
Speckled alder prefers open sites [106,171,205] but tolerates moderate shade (Shirley 1932 cited in [39]). Studies in Michigan found that it occurred on open to closed sites but was most common on lightly shaded, cool sites [16,36]. In southern-boreal, quaking aspen forests of Quebec, speckled alder was associated with relatively high light transmission at 7 to 13 feet (2-4 m) above ground level (P Thinleaf alder
Thinleaf alder is moderately shade tolerant. Thinleaf alder is adapted to nearly all types of disturbance [151], including severe disturbance [32], and is most common in early succession. It can grow in forest understories, but it is found more often on open sites. Sprouts may tolerate shade better than seedlings (review by [143]). To date (2011), most successional studies on plant communities with gray alder had been conducted on Alaskan sites in primary succession.

Thinleaf alder is an early-successional shrub in riparian zones in primary succession [93]. The willow/alder stage typically forms on bare floodplains [362]. On the Tanana River in interior Alaska, it typically establishes on bare to nearly bare, recently deposited alluvium [1]. Feltleaf and/or sandbar willow may establish first [1,55]. Thinleaf alder and balsam poplar dominate in midsuccession, after which balsam poplar and finally, white spruce, dominate the overstory [1]. A 5-year-old thinleaf alder sandbar community on the Tanana River was a "nearly impenetrable" thicket of thinleaf alder, feltleaf willow, sandbar willow, and barrenground willow. The shrubs were uniformly about 10 feet (3 m) tall, 0.8 inch (2.0 cm) in stem diameter, and averaged 49,699 stems/ha. Balsam poplar, paper birch, and white spruce grew in the understory meadow horsetail and variegated scouringrush were dominant herbs. In 15-year-old stands, feltleaf willow was overtopping thinleaf alder, and shrub density declined to 2,827 stems/ha. Groundlayer vegetation was nearly all meadow horsetail [162]. By Glacier Bay, Alaska, thinleaf alder occurs about 15 to 20 years after glacier recession, in the "late pioneer" stage. About 25 to 30 years after recession, thinleaf alder forms closed stands, presenting an "almost impenetrable barrier". At 30 to 35 years, black cottonwoods begin to establish [337].

According to a 1923 study by Cooper [76], thinleaf alder may be abundant even in late succession after glacier recession. Herbs, especially dwarf fireweed (Chamerion latifolium), dominated the northernmost, pioneer community, but thinleaf alder and willows were also establishing. Feltleaf willow-Sitka willow-thinleaf alder communities occurred in isolated patches and on midlatitude landscapes midlatitude sites had a longer period of recession than the northern sites. Thinleaf alder was "nearly everywhere dominant" and eventually overtopped the willows. Sitka spruce forests occurred in the southernmost portion of the landscape, which had the longest time since glaciation. Thinleaf alder persisted in these late-successional forests, typically at greater abundance than in the pioneer stage [76].

The pattern of willows, and sometimes cottonwoods, establishing before thinleaf alder is typical in riparian succession. On bare gravel bars of Meadow Creek on the Starkey Experimental Forest, Oregon, thinleaf alder established at lower densities (0.96 stem/50 m²) than sandbar willow and black cottonwood [60]. Along the Animas River in southwestern Colorado, narrowleaf cottonwood/thinleaf alder communities occur upland from narrowleaf cottonwood/tickle grass communities and are considered a latter successional community type than the narrowleaf cottonwood/tickle grass community [351].

Thinleaf alder shrub communities are initiated and maintained by disturbances that are often severe. Historically in northwestern Montana, disturbances have included placer mining, ice jams, log transport [32,151], and fire. On Emigrant Creek near Burns, Oregon, thinleaf alder colonized a new alluvial fan 6.5 years after deposition following an intense thunderstorm. Cattle had grazed the area for at least 30 years [145]. Thinleaf alder snowslide communities in the Blue Mountains are maintained by avalanches and soil slippage [144]. Along the San Miguel River in southwestern Colorado, thinleaf alder shrub communities occupied less area than that of later-successional, narrowleaf cottonwood/thinleaf alder communities. Thinleaf alder shrubland also had shorter flood-return intervals (averaging

Threats and conservation

Some alders in the UK have been infected by a type of fungus, Phytophthora. Diseases caused by Phytophthora are quite common on broadleaf tree species. But it was thought to be uncommon on alder until the discovery of a new hybrid strain, which causes root rot and stem lesions.

Sometimes known as alder dieback, the disease is more noticeable in summer as the leaves of affected alders are abnormally small and yellow and often fall early. Infected trees have dead twigs and branches in the crown. They may also bear an unusually large number of cones – a sign of stress. Sometimes the trees die rapidly and other times they deteriorate gradually. Symptoms include bleeding from the bark which resembles brown, rusty spots. When exposed, the reddish, mottled inner bark contrasts with the creamy colour of healthy bark.

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