Seed Production and Dissemination
Tanoak is a heavy seeder (3). In general, viable seeds are borne in abundance after the 30th to 40th year (8), although 5-year-old sprouts also have produced fairly heavy crops. A long dry period at pollination time helps the setting of acorns. Trees are heavily laden almost every alternate year, and complete seed crop failures are rare. "Jayhawking"-peeling the bark from standing trees-has shown that girdling produces excessively large acorn crops before the trees die. Scanty crops generally are caused by frosts or by a dry year.
Mature trees produce the most acorns. One estimate places annual acorn production of a veteran tanoak 76 cm (30 in) in d.b.h. at about 454 kg (1,000 lb). Because about 110 acorns weigh 0.45 kg (I lb), this production is more than 110,000 acorns. Other estimates showed that trees between 46 and 61 cm (18 and 24 in) d.b.h. produced 3,900 to 4,600 acorns.
Insects destroy a significant number of acorns. One study found insect larvae infesting 51 percent of the acorns. The insects identified were the filbert weevil (Curculio uniformis) and the filbertworm. (Melissopus latiferreanus). Other insect larvae that have been found in tanoak acorns are from the families Gelechiidae and Pyralidae (5).
Many immature acorns have been seen on the ground as early as August 25, but these were probably knocked down by heavy rains. Mature tanoak acorns drop between September 20 and November 15. The first acorns to fall are usually insect infested, whereas those falling later are usually sound. Indians in California placed a taboo on collecting acorns for food until their medicine women held a ceremonial festival that celebrated the falling of sound acorns.
Because the acorns are large-2.5 to 5.1 cm (1.0 to 2.0 in) long and 15 to 18 min (0.6 to 0.7 in) in diameter-and heavy, most of them fall straight to the ground and are found under the tree crowns. Only a few bounce outward when dropping onto lower branches or roll for short distances on steep slopes. In one small study, acorns were counted under trees 46 to 61 cm (18 to 24 in) in diameter at ,rates of 194,000 to 226,000/ha (78,400 to 91,500/acre) (24).