Alternative conifer release treatments affect leaf area index of competing vegetation and available light for seedling growth

Leaf area index (LAI) of competing vegetation and photosynthetically-active radiation (PAR) for spruce seedling growth were measured during the first growing season after alternative conifer release treatments [brush saw, Silvana Selective, Release® (a.i., triclopyr) herbicide, Vision® (a.i., glyphosate) herbicide], and control (no treatment) were applied. Bareroot white [Picea glauca (Moench) Voss] and black [Picea mariana (Mill.) B.S.P.] spruce were planted between 1987 and 1991, and averaged 82 cm height in 1993 compared with 2.6 m for major competitors. Treatments were applied in 1993, the primary competitor being trembling aspen (Populus tremuloides Michx.). After the first growing season following treatments, LAI at seedling base differed (P = 0.002) among treatments, ranging from 2.04 (Vision®) to 3.88 (control) m2 m−2. The Vision® treatment had significantly lower LAI than the brush saw, Silvana Selective, and Release® treatments (means = 2.76,2.76, and 2.74 m2 m−2 respectively), and the control treatment had significantly greater LAI than all other treatments. LAI correlated well (r = 0.81) with measured competition levels, increasing as competition levels increased. Mean tip angle (MTA) did not differ among treatments, but was higher for the Vision® treatment, presumably because of lower deciduous tree and shrub cover. Available light (PAR) decreased as LAI increased r = −0.90). In 1994, few released seedlings were overtopped by competing vegetation, and available PAR at leader height was generally 100%. For control seedlings, available PAR at leader height varied among blocks, ranging from 20 to 50%. PAR at seedling base differed among treatments, was lowest (13%) for controls, highest (36%) for the Vision®, and intermediate (~ 26%) for all other treatments. Available PAR on a whole seedling basis (i.e. avg. of leader and base) ranged from 25% for control seedlings to 68% for seedlings released with Vision®. Key words: Fallingsnow Ecosystem Project, white spruce, black spruce, trembling aspen, Vision® (glyphosate), Release® (triclopyr), leaf area index (LAI), mean tip angle (MTA), competition indices, photosynthetically-active radiation (PAR), vegetation management, plant succession