Alnus rhombifolia Nutt.

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Credits

Tim Baxter & Hugh A. McAllister (2024)

Recommended citation
Baxter, T. & McAllister, H.A. (2024), 'Alnus rhombifolia' from the website Trees and Shrubs Online (treesandshrubsonline.org/articles/alnus/alnus-rhombifolia/). Accessed 2024-05-26.

Genus

Common Names

  • White Alder
  • California Alder

Synonyms

  • Alnus californica hort. ex Winkler
  • Alnus rhombifolia var. typica Callier
  • Alnus rhombifolia var. bernardina Munz & I.M.Johnst.

Glossary

acute
Sharply pointed.
alluvial
Sediments deposited by rivers or soils derived from such material.
apex
(pl. apices) Tip. apical At the apex.
included
(botanical) Contained within another part or organ.
indigenous
Native to an area; not introduced.
ovate
Egg-shaped; broadest towards the stem.
perianth
Calyx and corolla. Term used especially when petals and sepals are not easily distinguished from each other.
pubescent
Covered in hairs.
revolute
Rolled downwards at margin.
staminate
Male referring to male plants (dioecy) or flowers (monoecy) or the male parts of a hermaphrodite flower.
sympatric
With the same distribution as another taxon (or with overlapping distribution). (Cf. allopatric.)

Credits

Tim Baxter & Hugh A. McAllister (2024)

Recommended citation
Baxter, T. & McAllister, H.A. (2024), 'Alnus rhombifolia' from the website Trees and Shrubs Online (treesandshrubsonline.org/articles/alnus/alnus-rhombifolia/). Accessed 2024-05-26.

Open-crowned, often multi-stemmed tree to 25 m. Bark light grey, smooth. Branchlets green to red-brown, lustrous, not differentiated into long and short shoots, resinous, with white circular lenticels. Buds elliptic-obovate, 3–9 mm long, scales two, resinous, stalk 3–5 mm, glabrous to sparsely pubescent, highly resinous. Leaves ovate, elliptic, or rhombic, (3–)8.5–12 × 2.5–6.5 cm, apex acute, obtuse, or rounded, base broadly cuneate to rounded, glandular, adaxially glabrous to sparsely pubescent, medium green, abaxially sparsely pubescent to velutinous, light green, finely serrate, serrulate to lobulate, resinous, glabrous apart from tomentose on veins abaxially, craspedodromous with 9–12 pairs of veins, petioles 7–15 mm long, villous to velutinous. Stipules elliptic, apex acute, 6–11 mm long. Pistillate inflorescences 3–6 in erect raceme, ovate to elliptic, at anthesis 3–6 mm. Staminate catkins in multiple clusters of terminal racemes, 3–7, pendulous, at anthesis 3–10 cm, peduncles 2–12 mm long. Fruit ovoid, ellipic, or cylindrical, 10–22 × 7–10 mm, bracts woody and usually very resinous, 3–3.5 mm, terminal lobe larger than others, acute and extended. Seed broadly elliptic, 2–2.5 × 1.5–2 mm, styles persistent, wing narrow. Diploid, 2n=28. (Furlow 1979; De Langhe 2018; T. Baxter pers. obs. ).

Distribution  United States S Washington and adjacent W Idaho, southwest to N California and south to the Mexican border

Habitat On rocky streambanks and adjacent slopes in the mediterranean climatic zone, 100–2400 m asl. It can be found on various soil types, but mostly on alluvial soils in gravelly or rocky ground.

USDA Hardiness Zone 6-9

RHS Hardiness Rating H7

Conservation status Least concern (LC)

Alnus rhombifolia is a medium-sized tree of 15–25 meters with smooth pale grey bark, becoming scaly with age. It is a common tree throughout the dry mediterranean climatic zone of the western United States. It has been suggested there was an extant forest flora including alders in western North America from the Tertiary onwards which included species such as A. maritima; later, in the Miocene, species included A. rhombifolia, A. oblongifolia and A. rubra (Furlow 1979) . These species are most closely related to the central and southern American taxa and represent populations that may have given rise to these, or else are remnants of populations that did not migrate north during the Pliocene. White Alder grows along open, rocky stream banks and adjacent slopes from low altitude, and along gullies in upland areas. Mexican populations have never been confirmed but might be expected in adjacent Baja California.

White Alder is most closely related to Alnus oblongifolia, from which it can be reliably distinguished by the smaller, more even teeth and generally broader leaves; A. rubra is readily distinguished by grey mottled bark and ovate, coarsely-toothed leaves (sometimes lobulate) with margins that are recurved or revolute; from A. incana subsp. tenuifolia it can be distinguished by having staminate catkins with two stamens longer than the perianth, and buds which are acute at the apex and which do not entirely enclose the immature leaves. A. incana subsp. tenuifolia and A. rhombifolia are sometimes sympatric at higher altitudes and towards the north and west of the latter’s range, and hybrids are known. A variable tree across its range, A. rhombifolia now includes several previously recognised forms including var. bernardina, distinguished at one time on account of its very pubescent leaves but this is now accepted as part of the natural variation of the species (Plants of the World Online 2022).

In cultivation Alnus rhombifolia is a useful, fast-growing upright tree with attractive pyramidal form, a straight trunk and a tolerance of both heat and exposure. Best grown in full sun or dappled shade in moist, especially alluvial soils, it can also tolerate the wettest conditions. It is not a common tree in cultivation outside its native range and remains rare in Europe. It is usually not offered by nurseries in the eastern United States but is commonly grown in some areas of California. In Britain it is grown in a number of collections including Ventnor, RBG Edinburgh, and at the late Maurice Mason’s garden near Kings Lynn, Norfolk, with the tallest recorded being 13 m trees at Kew and Bute Park, Cardiff (Tree Register 2021). White Alder makes a useful park or occasional street tree, especially in California, but has also been known to damage pavements and underground services through aggressive rooting. It has been variably used by indigenous peoples for the treatment of diarrhoea and burns, as a blood purifier, and to aid childbirth (Shaw et al. 2014). In its native California, stressed White Alder can be attacked and killed by the Alder Flatheaded Borer, Agrilus burkei.