Biomass Removal, Soil Compaction, and Vegetation Control Effects on Five-Year Growth of Douglas-fir in Coastal Washington

Abstract

Abstract Sustainable forest production requires an understanding of the effects of site disturbance on tree growth and the consequences of soil amelioration and vegetation control practices. We assessed the impacts of biomass removals at harvest, soil compaction and tillage, and vegetation control on early growth of Douglas-fir in coastal Washington. Harvest treatments included removal of commercial bole only (BO), bole only up to 5-cm top diameter (BO5), total tree (TT), and total tree plus all legacy coarse woody debris (TTP). Vegetation control (VC) effects were tested in BO, while soil compaction (BO/SC) and compaction plus tillage (BO/SCT) were imposed in BO/VC. Five years after planting, biomass removal and soil compaction/tillage effects on tree growth were relatively small. At year 5, mean stem basal diameter in BO was greater than in TT, whereas mean height was similar in BO and BO/SC, but increased in BO/SCT. Control of competing vegetation markedly increased tree growth. At year 5, mean tree stem diameter at 1.3-m height (dbh) and height in BO/no vegetation control (NVC) were 34 mm and 308 cm, respectively, compared to 45 mm and 357 cm in BO/VC (P <0.01). Mean relative growth rate in stem diameter for trees in BO/VC was greater than in BO/NVC in years 2–4, but this trend reversed in year 5, suggesting that tree intraspecific competition may be greater in BO/VC than in BO/NVC. Vegetation control also increased second-flushing (i.e., the flushing of set buds in late summer) in years 4 and 5. Vegetation competition effects on tree growth appeared to be related to changes in available soil water rather than to competition for nutrients.