Interspecific competition and herbicide injury influence 10-year responses of coastal Douglas-fir and associated vegetation to release treatments

Responses of competing vegetation and planted Douglas-fir ( Pseudotsuga menziesii (Mirb.) Franco var. menziesii) were studied for 10 years after six herbicide and manual release treatments in the Washington and Oregon Coast Ranges. Research objectives were to quantify regional, long-term responses of vegetation to various levels of competition, light and soil water availability, and intensity versus importance of factors influencing Douglas-fir growth. Three treatments reduced shrub cover relative to the untreated check: triclopyr in year 1, glyphosate in years 1–5, and repeated control (via several herbicide applications) in years 1–10. Reductions in woody cover from glyphosate stimulated increases in herb cover in years 3 and 5, while repeated control reduced herb cover in years 1, 2 and 5. Through year 10, Douglas-fir survival (86–99%) varied little among treatments. Visual symptoms of herbicide injury to Douglas-fir from triclopyr (45% of trees) and glyphosate (17% of trees) were associated with 0.1 – 0.2 m reductions in first-year height. After adjusting for tree size, Douglas-fir growth in stem basal area 2 years after triclopyr was less than that of the untreated check, suggesting prolonged effects of herbicide injury. Because it sustained low levels of interspecific competition, caused minimal tree injury, and prevented overtopping cover from red alder ( Alnus rubra Bong.), repeated control was the only treatment in which Douglas-fir size (9.8 m height and 21 cm basal diameter in year 10) significantly exceeded ( P ≤ 0.02) that of the untreated check (7.8 m height and 12 cm diameter).