Development of tanoak understories in conifer stands

Abstract

The stocking, age, and growth rates of tanoak (Lithocarpusdensiflorus [Hook and Arn.] Rehd.) were studied in the understory of 13 Douglas-fir (Pseudotsugamenziesii [Mirb] Franco) and mixed conifer stands, 53–240 years old, on a range of sites in Oregon and California. The understory was uneven in both age and size in all stands. Tanoak <2.0 cm diameter at breast height (dbh) ranged from 1 to 62 years old and from 5 to 216 cm tall. The stems of tanoak >2.0 cm dbh ranged from 18 to 98 years old and from 2 to 25 m tall. A negative exponential function describing diameter distributions generally explained more than 80% of the within-stand variation in numbers of tree stems (by 5-cm diameter classes). Seedlings have a single stem for about 5–12 years, then form a burl with dormant buds below ground, lose the original stem, and produce new stems. Because of frequent stem mortality and sprouting, tanoak age cannot be determined from stems above ground but can be estimated from xylem rings in the stem below the burl. Tanoak grows slowly in the conifer understory and at 50 years of age is only 80 to 160 cm tall. Sprouting potential also develops slowly. Three years after burning, 41- to 50-year-old tanoak averaged five sprouts only 30 to 68 cm tall. The establishment of a dense, new understory with the capacity for vigorous sprouting after disturbance may take well over 100 years. Tanoak control may be less costly and less frequently needed if accomplished in 30- to 75-year-old conifer stands.