Smooth alder

Smooth Alder (Alnus serrulata): A Comprehensive Reference

Identity and Botanical Classification

Smooth Alder is the accepted common name for Alnus serrulata (Aiton) Willd., a deciduous shrub or small tree belonging to the family Betulaceae (the birch family). It is also commonly known as Tag Alder or Hazel Alder. The species epithet serrulata refers to its finely serrated leaf margins; the leaves are finely toothed (serrulate).

Several synonyms appear in historical botanical literature. Accepted synonyms include Alnus incana (L.) Moench var. serrulata (Ait.) Boivin, A. noveboracensis Britt., and A. rugosa (Du Roi) Spreng. var. serrulata (Ait.) H.J.P. Winkl. The original basionym is Betula serrulata Ait.

Smooth Alder is a deciduous shrub growing to 4.5 m (14 ft 9 in). Other authorities describe it as capable of reaching greater stature: as a multi-stemmed shrub to 10 m, with light grey, ± smooth bark. The species is monoecious — individual flowers are either male or female, but both sexes can be found on the same plant — and is pollinated by wind. Smooth alder has long, drooping yellow (male) catkins on a tall, suckering shrub. The male flowers are yellow-brown catkins, while the female flowers are smaller and red, offering a charming display in the late winter to early spring.

Native Range and Habitat

Native to the eastern United States, Smooth Alder thrives in wet areas like streambanks and bogs, and offers a range of ecological benefits. It is documented in Connecticut, Massachusetts, Maine, New Hampshire, Rhode Island, and Vermont, found on shorelines and in swamps. A fast-growing and tolerant species, hazel alder is commonly used to restore or mitigate wetlands.

An important ecological property is the plant's capacity for symbiotic nitrogen fixation. It can fix nitrogen, significantly improving soil quality through nitrogen fixation via Frankia symbionts.

Parts Used in Traditional and Modern Preparations

Smooth Alder is a deciduous shrub native to North America, historically valued in traditional herbal medicine. Indigenous peoples and early settlers utilized various parts of the plant — especially the bark and leaves — for their purported anti-inflammatory and astringent properties. Additional plant parts with attributed therapeutic potential in the broader Alnus genus include the cones and roots. Different parts of plants like stems, barks, seeds, leaves, roots, fruits as well as tree cones, infructescences, buds, and blossoms are known to possess therapeutic potential.

Common traditional forms of preparation for A. serrulata include:

• Bark tea (decoction or infusion): A tea made from the bark is described as analgesic, astringent, blood purifier, diuretic, emetic, and purgative.
• Poultice: Poultices crafted from the leaves or bark were applied externally to alleviate skin irritations, wounds, and swelling, promoting faster healing and comfort.
• Wash or eyewash: Externally, it is used as an eye wash and a wash for hives, poison ivy rash, piles, swellings, and sprains.
• Cone decoction: A decoction of the cones is described as astringent.
• Chewing the bark: The bark was also sometimes chewed to relieve toothaches.

Traditional and Historical Use

Indigenous North American Traditions

A tea made from the bark was used by Native Americans as a treatment for diarrhea, coughs, toothaches, sore mouth, and the pain of birth. It was used by Native Americans for a range of medicinal uses including as a purgative or emetic, for eye trouble, and in treating toothache.

Native American tribes used parts of the plant to create infusions for both medicinal and ceremonial purposes. Historically, extracts from the bark of Alnus serrulata were used as a folk remedy to treat conditions such as arthritis and other inflammatory disorders.

The bark is rich in tannins and was traditionally used in poultices and washes to treat wounds, insect bites, and skin irritations. It has also been used as a treatment for acne in the form of a skin wash.

This plant has a rich history in indigenous medicine, where it was used as a general remedy for fever, inflammation, and even digestive issues. Its bark and roots are especially valued for their astringent properties.

Range of Traditional Indications

Documented traditional applications of A. serrulata bark preparations span several body systems:

• Gastrointestinal: Decoctions or infusions made from the bark were used to treat diarrhea and other ailments.
• Respiratory and oral: The bark tea was used internally as an analgesic, astringent, blood purifier, diuretic, emetic, and purgative, and specifically for diarrhea, coughs, toothache, and sore mouths.
• Fever: The tea was used to soothe sore throats and reduce fever.
• Obstetric/pain: It was used in the treatment of the pain of childbirth.
• Dermatological: Preparations were used externally to alleviate skin irritations, wounds, and swelling.

Eclectic and Early American Medical Traditions

Beyond purely indigenous use, 19th-century Eclectic physicians — American practitioners who incorporated botanical medicine extensively into clinical practice — recognized alder bark in their pharmacopoeia. Historical Eclectic texts record its use for rhus poisoning (poison ivy/oak) and for pustular and eczematous disease of the skin. Earlier Eclectic accounts describe alder as removing waste products, improving the tone of mucous structures, stimulating gastric juice and aiding digestion, and curing various forms of ulcerations in the mouth or gastrointestinal canal.

Broader Genus Traditions

Because phytochemical research has focused largely on related species, it is important to note the traditions associated with the genus Alnus more broadly, particularly where constituents are shared with A. serrulata. Members of genus Alnus are well known for their traditional uses in the treatment of various diseases like cancer, hepatitis, inflammation of the uterus, uterine cancer, rheumatism, dysentery, stomachache, diarrhea, and fever. Alnus hirsuta, distributed in Korea, China, Japan, and Russia, has been used in Oriental medicine as a remedy for fever, haemorrhages, burn injuries, diarrhea, and alcoholism. The bark of Alnus species has been used in Korean folk medicine as a remedy for fever, hemorrhage, diarrhea, and alcoholism.

Key Constituents and Active Compounds

While dedicated phytochemical studies specific to Alnus serrulata itself remain sparse, the plant belongs to a genus whose chemical constituents have been extensively characterized in closely related species. Preliminary phytochemical analyses reveal that the plant contains bioactive compounds including flavonoids, tannins, and triterpenoids. These findings are consistent with broader genus-level chemistry.

Diarylheptanoids

Phytochemical investigations of the genus have revealed the presence of diarylheptanoids, polyphenols, flavonoids, terpenoids, steroids, and other compounds. Diarylheptanoids, natural products with a 1,7-diphenylheptane structural skeleton, are the dominant constituents in the genus, whose anticancer effect has been brought into focus.

Diarylheptanoids comprise a class of natural products formed from 1,7-diphenylheptane, which appears in linear and cyclic forms. They have been regarded as the primary bioactive compounds of Alnus and have drawn attention due to their physiological activities, especially their anticancer and antioxidative activities.

In the closely related European alder (A. glutinosa), whose bark chemistry is expected to share substantial overlap with that of A. serrulata, detailed phytochemical profiling has identified a rich diarylheptanoid array. A study of secondary metabolites from the bark of Alnus glutinosa led to the isolation of fourteen diarylheptanoids: oregonin, platyphylloside, rubranoside A, rubranoside B, hirsutanonol, hirsutenone, hirsutanonol-5-O-β-D-glucopyranoside, platyphyllonol-5-O-β-d-xylopyranoside, aceroside VII, alnuside A, alnuside B, and others. Diarylheptanoids have been identified as the predominant compounds in A. glutinosa bark extract (0.65 g/g), with oregonin alone constituting 74.67% of the extract.

Ellagitannins

The plants of this genus contain terpenoids, flavonoids, diarylheptanoids, phenols, steroids, and tannins. Ellagitannins constitute a particularly important subclass. Ellagitannins were identified as the predominant phenolics in ethanol and aqueous extracts of A. glutinosa acorns. Key ellagitannins identified in genus members include pedunculagin and glutinoin.

Flavonoids

To date, 25 diarylheptanoids, 3 ellagitannins, 2 flavonoids, and 13 terpenoids have been identified in A. glutinosa. Flavonoids documented in related Alnus species include quercetin-3-sophoroside and genkwanin. Extracts derived from the seeds of A. glutinosa yielded bioactive compounds including hirsutanonol, oregonin, and genkwanin, which displayed strong DPPH radical scavenging abilities.

Terpenoids and Steroids

Phytochemical studies in closely related Alnus hirsuta have identified a diverse range of secondary metabolites, including diarylheptanoids, terpenoids, flavonoids, and phenolic compounds, many of which exhibit strong bioactivities.

Tannins and Overall Phenolic Load

The bark is rich in tannins and was traditionally used in poultices and washes to treat wounds, insect bites, and skin irritations. In laboratory analyses of A. glutinosa bark, a high content of phenolic compounds was detected, expressed as gallic acid equivalents (0.71 g GAE/g dry extract).

Mechanisms of Action

Mechanistic research on the bioactive constituents of Alnus — extrapolated from the closely related species where most studies have been conducted — points to several key pathways. It is critical to note that these mechanisms have not been formally established for A. serrulata specifically, as the species lacks dedicated pharmacological investigation.

Anti-Inflammatory Pathways

Mechanistically, diarylheptanoids modulate inflammation by inhibiting NF-κB and MAPK pathways, suppressing pro-inflammatory mediators such as COX-2, iNOS, and cytokines, while simultaneously enhancing antioxidant defenses.

Diarylheptanoids are the dominant constituents within the genus Alnus, and some have exhibited inhibitory activity against nuclear factor kappaB activation, nitric oxide, and tumor necrosis factor-α production, human umbilical vein endothelial cells, farnesyl protein transferase, cell-mediated low-density lipoprotein oxidation, HIF-1 in AGS cells, and the HIV-1-induced cytopathic effect in MT-4 cells.

Antioxidant Mechanisms

Oregonin and hirsutenone, among the most abundant diarylheptanoid compounds detected in A. glutinosa bark, showed strong ability to scavenge oxygen radicals compared with positive controls in TEAC, DPPH, and ORAC assays. Structure-activity analysis demonstrated that hirsutenone, hirsutanonol, oregonin, and rubranoside B — which possess two 3,4-dihydroxyphenyl rings — were more active against reactive oxygen species than alnuside A and alnuside B.

Astringent Mechanism (Tannin-Mediated)

The classical astringent actions traditionally ascribed to A. serrulata bark are attributable to its tannin content. Tannins precipitate proteins on mucosal surfaces and skin, creating a physical barrier, reducing secretion, and tightening tissues — actions consistent with the traditional uses for diarrhea, sore throats, and wound treatment.

Hepatoprotective Activity

Some ellagitannins in the genus showed hepatoprotective activity even at a dose of 1 mg/kg, which is ten-fold smaller compared with the dose of traditional flavonoid-based drugs.

Potential Anti-Adipogenic and Epigenetic Effects (Related Species)

Extracts of the related species A. incana acted early in the differentiation process and acted as partial agonists toward peroxisome proliferator-activated receptor gamma (PPAR-γ) activity. The diarylheptanoid glycoside oregonin was isolated and confirmed to be the active principle exerting the anti-adipogenic effect. Another study found that oregonin altered DNA methyltransferases expression and mtDNA copy numbers in mouse embryonic fibroblasts. Molecular modeling indicated that oregonin fits the catalytic site of DNMT1, suggesting it could act as an inhibitor of DNMT1 enzymatic activity. These findings substantiate the potential of oregonin to regulate epigenetic changes.

Scientific Evidence by Area of Use

It must be stated plainly at the outset of this section: dedicated clinical trials or controlled human studies specifically investigating Alnus serrulata as a dietary supplement or therapeutic agent have not been published in the peer-reviewed literature as of the time of writing. Modern scientific research on Smooth Alder is still emerging, and comprehensive clinical trials in humans remain limited. People use smooth alder for sore throat and intestinal bleeding, but there is no good scientific evidence to support any use. Where evidence is available for closely related Alnus species, this is clearly distinguished below and noted as indirect.

Anti-Inflammatory and Analgesic Effects

Evidence level: Preclinical (animal/in vitro only); no human clinical data for A. serrulata

Preliminary phytochemical analyses reveal that the plant contains bioactive compounds including flavonoids, tannins, and triterpenoids. These constituents are known for their antioxidant, anti-inflammatory, and antimicrobial activities, lending biochemical support to some of the traditional uses of Smooth Alder.

In a published animal study using the closely related Alnus nitida (Himalayan alder), extracts were evaluated for in vivo analgesic and anti-inflammatory potential; using carrageenan-induced paw edema assay, Freund's complete adjuvant-induced edema, xylene-induced ear edema, and histamine-induced paw edema models in rats, the results showed significant (p < 0.01) reduction (70–80%) in edema compared to inflammatory controls. These findings are in rat models and cannot be directly applied to A. serrulata or to human outcomes without further research.

Pharmacological investigations in closely related A. hirsuta have revealed anti-inflammatory, anticancer, antioxidant, anti-melanogenesis, hepatoprotective, lipid-regulating, and hair growth effects, with diarylheptanoids frequently implicated as major bioactive constituents.

Antioxidant Activity

Evidence level: In vitro only; no human clinical data for A. serrulata

Bark extract of the related A. glutinosa showed strong and concentration-dependent antioxidant activity in vitro, with IC₅₀ values of 0.15–12.21 µg/mL. All extracts tested from A. glutinosa bark were found to be strong 1,1-diphenyl-2-picrylhydrazyl (DPPH) free radical scavengers, exhibiting IC₅₀ values of 3.3–18.9 µg/mL, and also showed activity in inhibition of lipid peroxidation. Again, these findings are in vitro and for a different species.

Antimicrobial Activity

Evidence level: In vitro only; no human clinical data for A. serrulata

The ethanolic and methanolic extracts from the leaves of A. glutinosa showed antibacterial activity against Staphylococcus aureus, Bacillus subtilis, Escherichia coli, Klebsiella aerogenes, and Pseudomonas aeruginosa, with MIC values ranging from 125 to 250 µg/mL. The compound oregonin, produced from the methanolic extract of A. glutinosa, showed extremely high antibacterial properties against both Gram-negative and Gram-positive bacteria (Bacillus subtilis — 15.6 µg/mL, Staphylococcus aureus — 15.6 µg/mL, Proteus vulgaris — 31.2 µg/mL).

Laboratory studies have demonstrated that extracts from related Alnus species exhibit inhibitory effects against certain pathogens and oxidative processes, suggesting potential benefits for immune support and general wellness.

Anticancer Activity

Evidence level: In vitro only; no human clinical data for any Alnus species in this indication

Compounds from A. glutinosa bark exhibited strong anticancer activity compared with other diarylheptanoids from the same species and considerably higher than curcumin, which served as a positive control, in human non-small cell lung carcinoma cell lines. Structure-activity analysis revealed a high dependence of the cytotoxic action on the presence of a carbonyl group at C-3, substitution of a heptane chain on C-5, and the number of hydroxyl groups in the aromatic rings.

Platyphylloside, which has a keto-enol moiety and one hydroxyl group in the aromatic ring, showed the most potent cytotoxic activity on B16 mouse melanoma and human stomachic adenocarcinoma cells in vitro. This study suggested that the ketone or keto-enol moiety and hydroxyl groups in the aromatic rings were essential to the higher cytotoxic activity of diarylheptanoids. All of these findings remain in the stage of cell culture research and have not been validated in human clinical trials.

Anti-Angiogenic Activity

Evidence level: In vitro/in vivo model (non-human); no human clinical data

In research using A. glutinosa bark extract, promising anti-angiogenic activity was observed, inhibiting both vessel growth (IC₅₀ 23.39 µg/egg) and the release of an endogenous phosphatase alkaline enzyme (IC₅₀ 44.24 µg/embryo) in chick chorioallantoic membrane and zebrafish embryo models.

Hepatoprotective Effects

Evidence level: Animal models only; no human clinical data for A. serrulata

The closely related Alnus incana is used to treat gastrointestinal and skin ailments and for gargling during bacterial mouth and throat infections. This species has been reported to contain diarylheptanoids with different pharmacological activities including anti-inflammatory, anti-influenza, and hepatoprotective effects. Animal research findings suggest that A. incana extracts might alleviate iron overload-induced hepatotoxicity and other pathological conditions characterized by hepatic iron overload.

Dermatological Uses

Evidence level: Traditional use only; no clinical trial evidence for A. serrulata

The astringent properties of Alnus serrulata have been traditionally used to heal skin wounds, treat acne, and reduce inflammation. These applications rely on the tannin content of the bark to produce a protein-precipitating astringent effect on skin and mucosal surfaces. No controlled studies have validated these applications in humans.

Gastrointestinal and Throat Uses

Evidence level: Traditional use only; no clinical trial evidence for A. serrulata

People use smooth alder for sore throat and intestinal bleeding, but there is no good scientific evidence to support any use. Tea made from the bark or leaves of Alnus serrulata was traditionally consumed for its astringent and anti-inflammatory effects. The tea was used to soothe sore throats, reduce fever, and treat diarrhea. These are entirely traditional claims that await scientific validation.

Body Systems and Health Areas Associated with Smooth Alder

Based on the totality of traditional use records and the preliminary scientific data available for the species and its genus, the following body systems and health areas have been associated with preparations of A. serrulata or closely related alder species. The strength of association is predominantly traditional rather than clinical.

• Gastrointestinal system: Anti-diarrheal use via astringent tannins; traditional use for stomachache, dysentery, and digestive issues.
• Integumentary system (skin): Wound healing, treatment of acne, skin inflammation, hives, poison ivy rash, swelling, sprains.
• Oral and upper respiratory: Sore throat, cough, toothache, sore mouth; use as an eyewash for ocular conditions.
• Musculoskeletal system: Traditional use for arthritis and rheumatism as an anti-inflammatory.
• Reproductive/obstetric: Historical use for pain of childbirth.
• Hepatic system: Hepatoprotective properties demonstrated for related species (preclinical data only).
• Immune/anti-infective: Antimicrobial activity demonstrated in vitro for related species; traditional use as a general "blood purifier."

Dosage Forms and Reported Dosages

There are currently no standardized, clinically validated dosage protocols for Alnus serrulata as a dietary supplement. There is not enough reliable information to know what an appropriate dose of smooth alder might be.

The following dosage forms appear in traditional use records and product documentation, but are not backed by human clinical evidence of safety or efficacy at any specific dose:

• Bark tea (internal): Prepared as a decoction or infusion from dried or fresh bark. Specific quantities are not established in peer-reviewed literature for A. serrulata.
• External wash or poultice: Applied topically to affected skin using a decoction of bark or leaves; concentration and frequency not standardized.
• Homeopathic / multi-ingredient preparations: In homeopathic combination products registered with the FDA, Alnus serrulata appears as an ingredient listed for hay fever and bronchitis, used in highly diluted homeopathic potencies (12X, 30X, and LM1). The clinical evidence base for homeopathic preparations is not accepted by mainstream medical authorities.

Safety Considerations and Interactions

General Safety Status

When taken by mouth, there is not enough reliable information to know if smooth alder is safe or what the side effects might be. There are no significant documented side effects of Alnus serrulata in humans when used in traditional quantities. However, the absence of documented adverse effects primarily reflects the absence of formal safety studies, not confirmed safety.

Emetic and Purgative Actions

One of the pharmacological actions historically attributed to bark preparations of A. serrulata is emesis (vomiting) and purgation. The bark tea is described in traditional sources as emetic and purgative. These effects indicate the potential for significant gastrointestinal distress, particularly at higher doses or in sensitive individuals.

Tannin-Related Concerns

The bark's high tannin content, while responsible for its astringent therapeutic properties, carries its own safety implications. Herbs that contain tannins can decrease the absorption of certain classes of drugs, most notably alkaloidal and mineral drugs such as colchicine, ephedrine, copper, iron, and zinc; individuals with specific deficiencies (e.g., iron, zinc) may be advised to avoid tannin-rich botanicals during supplementation. This is a theoretical but pharmacologically plausible concern for any tannin-rich botanical, including A. serrulata bark preparations.

Pollen Allergy and Cross-Reactivity

A well-documented safety concern associated with all Alnus species is pollen allergenicity and its cross-reactivity with birch (Betula) pollen. Birch and alder trees are the genera commonly implicated in the allergic rhinitis processes of sensitized people. Alnus pollen causes symptoms in 9–20% of total hay fever sufferers, while 41.89% of patients present a positive skin prick test for Betula allergens. The Aln g1 and Bet v1 aeroallergens belong to the PR-10 protein family and are associated with cross-reactivity processes.

The major birch pollen allergen Bet v 1 is cross-reactive with the alder allergen Aln g 1 and hazel allergen Cor a 1, all belonging to the PR-10 protein family with approximately 70% amino acid identity. The clinical consequence of this IgE and T-cell cross-reactivity is that most individuals who are allergic to birch pollen experience symptoms when exposed to pollen from other members of the birch homologous group.

Individuals with known birch or alder pollen allergies may thus be at risk of allergic reactions not only to the pollen but potentially to plant-derived preparations, particularly if Bet v 1-related proteins are present in bark or leaf extracts. Alnus serrulata pollen is an extract used in clinical allergy testing.

Drug Interactions (Theoretical)

No specific herb-drug interaction studies have been conducted for A. serrulata. The theoretical interaction risks that can be inferred from its known constituent profile are:

• Reduced drug absorption: Herbs that contain tannins might alter the absorption of another drug or herbal constituent. This applies theoretically to iron supplements, certain alkaloid-based drugs, and minerals taken concurrently.
• Additive diuretic effects: The bark tea is described as diuretic in traditional sources, which raises the theoretical possibility of additive effects with pharmaceutical diuretics.

Pregnancy and Lactation

There is not enough reliable information to know if smooth alder is safe to use when pregnant or breast-feeding. The historical use for stimulating uterine contractions (pain of childbirth) and its emetic/purgative properties further underscore the need for caution in pregnant individuals.

Emetic Risk at Higher Doses

Because the bark of Alnus serrulata is traditionally classified as an emetic — an agent that induces vomiting — preparations at supratraditional doses carry a risk of nausea and vomiting. The threshold dose at which this effect manifests is not defined in any published pharmacological study for this species.

Summary of Evidence Strength

While direct clinical evidence regarding the efficacy of Smooth Alder in nutritional products is not yet robust, its inclusion in traditional medicine systems is supported by both historical use and promising laboratory findings in related species. The current state of the evidence can be characterized as follows:

• Traditional use: Well documented across multiple indigenous North American peoples, Eclectic medical practice, and broader genus traditions spanning North America, Europe, and Asia.
• Phytochemical characterization: Preliminary for A. serrulata specifically; extensive for closely related species (A. glutinosa, A. incana, A. hirsuta), demonstrating a plausible biochemical rationale for traditional uses.
• In vitro pharmacology: Supportive, particularly for anti-inflammatory, antioxidant, and antimicrobial activities, based on genus-level data.
• Animal studies: Positive anti-inflammatory and analgesic findings for closely related species; no published animal study specific to A. serrulata.
• Human clinical evidence: Absent. No controlled clinical trials for A. serrulata as a therapeutic agent have been published.
• Safety evidence: Insufficient; no formal human safety trials; emetic and purgative properties documented; pollen allergenicity well-established for the genus.

References

• The Genus Alnus, A Comprehensive Outline of Its Chemical Constituents and Biological Activities — PMC/MDPI Molecules (2017)
• Sati SC, Sati N, Sati OP. Bioactive constituents and medicinal importance of genus Alnus — PubMed/Pharmacognosy Reviews (2011)
• New Insights on Phytochemical Features and Biological Properties of Alnus glutinosa Stem Bark — PMC/MDPI Plants (2022)
• Evaluation of Phytochemistry and Pharmacological Properties of Alnus nitida — PMC/MDPI Molecules (2022)
• A comprehensive review of the phytochemical, and pharmacological potential of Alnus hirsuta — Inflammopharmacology, Springer (2026)
• A comprehensive review of the phytochemical and pharmacological potential of Alnus glutinosa (L.) Gaertn., (Black Alder) — Medicinal Chemistry Research, Springer (2025)
• The effect of Alnus incana (L.) Moench extracts in ameliorating iron overload-induced hepatotoxicity in male albino rats — PMC (2023)
• Chemical Characteristics of Ethanol and Water Extracts of Black Alder (Alnus glutinosa L.) Acorns and Their Antibacterial, Anti-Fungal and Antitumor Properties — PMC (2022)
• Alnus serrulata — Wikipedia
• Alnus serrulata (smooth alder) — Go Botany, Native Plant Trust
• Alnus serrulata — Trees and Shrubs Online (Baxter & McAllister, 2024)
• Alnus serrulata Smooth Alder, Hazel alder — Plants for a Future (PFAF) Database
• Smooth Alder: Overview, Uses, Side Effects, Precautions, Interactions, Dosing — WebMD Natural Medicines
• Alnus serrulata pollen — DrugBank
• Cross-reactivity between the Betulaceae family and fallout in the real atmospheric aeroallergen load — Science of The Total Environment, ScienceDirect (2020)
• Cross-reactivity between aeroallergens and food allergens — PMC (2015)
• Alnus glutinosa — Overview, ScienceDirect Topics
• Herb-Drug Interactions — NIH National Center for Complementary and Integrative Health (NCCIH)