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Tanoak
Tanoak acorns (U.S. 5 cent coin, 21 mm diameter, for scale)
Conservation status
Data deficient
Scientific classification
Kingdom: Plantae Division: Magnoliophyta Class: Magnoliopsida Order: Fagales Family: Fagaceae Genus: Lithocarpus Species: L. densiflorus
Binomial name
Lithocarpus densiflorus (Hook. & Arn.) Rehd.

The Tanoak or Tanbark-oak (Lithocarpus densiflorus) is an evergreen tree in the beech family Fagaceae, native to the western United States, in California as far south as the Transverse Ranges and north to southwest Oregon. It can reach 40 m tall (though 15–25 m is more usual) in the California Coast Ranges, and can have a trunk diameter of 60–190 cm.

Although currently included in the genus Lithocarpus, genetic evidence (Manos et al. 2001) suggests it is only distantly related to the rest of the genus (all found in southeast Asia).

The leaves are alternate, 7–15 cm long, with toothed margins and a hard, leathery texture, and persist for 3–4 years. At first they are covered in dense orange-brown scurfy hairs on both sides, but those on the upper surface soon wear off, those on the under surface persisting longer but eventually wearing off too.

The seed is a nut 2–3 cm long and 2 cm diameter, very similar to an oak acorn, but with a very hard, woody nut shell more like a hazel nut. The nut sits in a cup during its 18-month maturation; the outside surface of the cup is rough with short spines. The nuts are produced in clusters of a few together on a single stem. The nut kernel is very bitter, and is inedible for people without extensive leaching, although squirrels eat them. Some California Native Americans prefer this nut to those of many Quercus acorns because it stores well due to the comparatively high tannin content.

Members of populations in interior California (in the northern Sierra Nevada) and the Klamath Mountains into southwest Oregon are smaller, rarely exceeding 3 m in height and often shrubby, with smaller leaves, 4–7 cm long; these are separated as Dwarf Tanoak Lithocarpus densiflorus var. echinoides. The variety intergrades with the type in northwest California and southwest Oregon. Tanoak does grow on serpentine soils as a shrub.

The name Tanoak refers to its tannin-rich bark, used in the past for tanning leather before the use of modern synthetic tannins.

Tanoak is one of the species most seriously affected by Sudden Oak Death (Phytophthora ramorum), with high mortality reported over much of the species' range.

• Flora of North America: Lithocarpus densiflorus
• Manos, P. S., Zhou, Z-K., & Cannon, C. H. (2001). Int. J. Plant Sci. 162(6): 1361–1379 pdf file