Fire is the principal enemy of individual tanoak trees (3). Ground fires, as well as crown fires, are sometimes fatal. More often, however, fires leave long vertical wounds reaching from 1.2 to 3.0 in (4 to 10 ft) up the trunks. Although the bark of mature trees is at least 3 to 8 cm (I to 3 in), and occasionally 10 or 13 cm (4 or 5 in) thick, some trees are burned badly.
Fire injuries to small trees often heal over, but fungi usually enter the wounds on older trees. The exposed wood on these larger trees rots and the wounds do not heal. If decayed wood catches fire it burns readily and the original wound is enlarged. Sometimes one-third to one-half the diameter of the tree is destroyed as a result of repeated fires and decay.
Until injured by fire, tanoak is relatively free from insect attacks and fungal diseases and is windfirm (3). Injury to the trunk, however, allows fungi to enter. Wind and heavy snows eventually fell many trees originally injured by fire and subsequently weakened by decay.
Fire and fungi cause tanoak to be fairly defective. One study based upon cubic volume in 90 trees showed that the amounts of saw log cull were 39 percent in cull trees, 8 percent in noncull trees, and 13 percent in all trees.
Fungi found in living trees are the beefsteak fungus (Fistulina hepatica), which causes a brown cubical rot; the weeping conk (Inonotus dryadeus), a white root rot; and a necrophyte (Schizophyllum commune), which causes a sap rot on injured areas of standing trees. Tanoak is susceptible to the shoestring root disease (Armillaria mellea). The fungus Ceuthocarpum conflictum causes a commonly seen leafspot on tanoak (10).
Several insects have been found feeding on tanoak but, generally, the damage is not economically significant. Two of these are armored scales identified as the greedy scale (Hemiberlesia rapax) and the oak scale (Quernaspis quercus). The greedy scale chiefly infests the bark but also feeds on leaves. The oak scale feeds on the undersides of leaves. Another insect, the crown whitefly (Aleuroplatus coronatus), resembles soft unarmored scales and feeds on the undersides of leaves, sometimes causing the leaves to fall prematurely. Ehrhorn's oak scale (Mycetococcus ehrhorni) is found on stems and the white sage mealybug (Pseudococcus crawi) on stems and leaves (5).
In 1957, the California oakworm (Phryganidia californica) completely destroyed that year's foliage of tanoaks growing on Hennessey Ridge, near Salyer, Trinity County, CA. This damage was localized and was not observed at other places nearby. Usually, the California oakworm causes little damage but irregularly becomes epidemic over large areas.
Other insects work under the bark. Adults of the Pacific oak twig girdler, Agrilus angelicus, feed on foliage, but its larvae mine spiral galleries that girdle twigs, small limbs and trunks, or sprouts. Adults of a false powderpost beetle (Mela1gus confertus) prune twigs by boring at the fork of small branches (5).
Decline of tanoak sprout vigor was observed in mixed conifer-hardwood forests in the central Sierra Nevada (18). Affected clumps were wider and denser, but only one-fifth as tall as unaffected clumps. Reason for the decline is not known.
Tanoak is avoided by livestock if better feed is available. Mule deer rarely browse it. The current year's growth of tanoak leaves and twigs is protected by abundant stellate trichomes, which are unpleasant to inhale.