Five-year growth responses of Douglas-fir, western hemlock, and western redcedar seedlings to manipulated levels of overstory and understory competition

Abstract

Douglas-fir (Pseudotsuga menziesii (Mirb.) Franco), western hemlock (Tsuga heterophylla (Raf.) Sarg.), and western redcedar (Thuja plicata Donn ex D. Don) seedlings were planted in March 2001 within three clearcut-harvested, shelterwood, or thinned stands of mature Douglas-fir near Olympia, Washington. From 2002 to 2005, areas of vegetation control of 0, 4.5, or 9 m2 were maintained with herbicides around a total 162 seedlings per species. Photosynthetically active radiation (PAR) was 34%, 62%, and 100% of full sunlight in thinned stands, shelterwoods, and clearcuts, respectively. Effects of overstory level and vegetation control on seedling growth and resource availability generally were additive. Seedling stem volume index in clearcuts averaged four to eight times that observed in thinned stands, and with vegetation control, it averaged two to four times that observed without it. In thinned stands, relative growth rate of seedling stem volume index had a positive linear relationship with PAR (R2 = 0.38). Foliar nitrogen content of Douglas-fir explained 71% of the variation in relative growth rate. Factors explaining the most variation in foliar nitrogen content differed between thinned stands (PAR, R2 = 0.34) and clearcuts or shelterwoods (midday water potential, R2 = 0.63), suggesting that light and root competition, respectively, were the primary growth-limiting factors for these overstory levels.

Forest vegetation management under debate: an introduction

“…Harrington (2006) found that overstorey density and vegetation control had additive effects on Pseudotsuga menziesii seedling responses and did not interact. However, in a previous study, Groot (1999) observed interactions between overstorey treatment (clearcut versus shelterwood) and vegetation control.…”

Juvenile growth of hybrid poplars on acidic boreal soil determined by environmental effects of soil preparation, vegetation control, and fertilization

“…Given that hybrid poplars have high needs for resources, they are also known to be particularly sensitive to competition (Stanturf et al, 2001;Kabba et al, 2007Kabba et al, , 2009. Competition control generally has positive effects on early development of seedlings because the first few years are the most critical for survival (Morris et al, 1993;Löf, 2000;Harrington, 2006). The control of competing vegetation typically proves beneficial to hybrid poplars (Stanturf et al, 2001), although experimental results may diverge, with some pointing towards effectiveness of the removal of aboveground vegetation only (Czapowskyj and Safford, 1993) while others insist on the need to target belowground plant parts (Coll et al, 2007).…”

The role of salal in forest regeneration problems in coastal British Columbia: problem or symptom?

“…Salal is capable of gaining near complete occupancy of a site (Haeussler et al 1990) and is considered to be a vigorous competitor with trees for water and nutrients (Fraser et al 1993). On dry sites, salal is believed to be a serious competitor for moisture, and several field studies have reported increases in growth and moisture status of Douglas-fir following removal of salal (Kelliher 1985, Vihanek 1985, Price et al 1986, D'Anjou 1998, Harrington 2006. In contrast, Sabhasri (1961) reported that salal removal reduced growth of Douglas-fir in western Washington.…”