Initial Abundance of Tanoak Determines 20-Year Growth Patterns of Planted Douglas-fir in Southwestern Oregon

摘要

An ongoing study of tanoak (Lithocarpus densiflorus] competition with planted Douglas-fir (Pseudotsuga menziesii) was revisited to quantify 20-year tree and stand growth responses, hi spring 1983, triclopyr herbicide (Garlon?) was applied selectivelyto individual tanoak sprout clumps growing within Douglas-fir plantations at two clearcut sites in southwestern Oregon, Glendale (age 1 yr) and Cave Junction (age 2 yr). Four tanoak densities (0, 25, 50, or 100% of the cover of non-treated plots) were created in 0.1-acre plots replicated three times at each site. Glyphosate herbicide (Roundup? was applied to additional replications of the tanoak densities to suppress shrub and herbaceous vegetation. In summer 1996 (Glendale) or 1998 (Cave Junction), federal land managers contracted a pre-commercial thinning (PCX) of Douglas-fir to a 14' spacing among selected crop trees and cutting of all hardwoods near ground level. A circular 0.08-acre plot was located centrally within each of the 39 study plots to quantify tree and stand growth responses, hi October 1999 and February 2003, diameter at breast height (dbh) was measured on each Douglas-fir rooted within the plots, and height and crown height were measured on a subset of these trees. Regression analysis was used to test relationships of Douglas-fir average dbh, height, and crown ratio and stand basal area, volume, and three-year Volume growth to tanoak density and presence versus absence of shrub and herb suppression.Each of the Douglas-fir tree and stand variables declined with increasing density of tanoak (R2=QA8 to 0.77). Douglas-fir grown in non-treated plots had about half the average dbh and height of trees grown where all tanoak had been removed, while their stand basal area and volume were only a third as large. Shrub and herb suppression stimulated increases in average tree size only where all tanoak had been removed. Reductions in dominant height (height of the 40 largest Douglas-fir per acre) from tanoak competition were less than half those observed for average height. Despite selection of the largest crop trees and temporary reduction of tanoak competition from PCT, 1999-2003 volume growth of Douglas-fir grown in non-treated plots (33 to 41 ft3 acre"1 yr'1) averaged less than half thatof trees grown where all tanoak had been removed (63 to 105 ft3 acre"1 yr"1). hi non-treated plots, Douglas-fir crown ratio was 81 to 87% versus 92 to 97% for trees grown where all tanoak had been removed. Results indicate that high densities of tanoakduring early development of Douglas-fir plantations will greatly limit the size of crop trees available for selection during PCT, as well as subsequent volume growth of the stand.