Hybrid Poplar Growing in Plantations -
Growth Responses to Alternative Establishment Systems
Study area
Figure 1. Location of study area.
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The study area was located in the Dry Mixedwood Natural Subregion (Natural Regions Committee 2006) in central Alberta, Canada near Boyle (54°90′N, 112°85′W, 570 m above sea level) (Figure 1). This area is situated at the interface of the agriculturally dominated Central Parkland to the south and the Central Mixedwood Natural Subregion, dominated by upland forests, to the north. The study was established by Alberta-Pacific-Forest Industries Inc. on research sites that were naturally mixedwood boreal forest and converted to agronomic perennial forages most recently used as hayland. The site is characterized by a gradual transition from a well-drained upland to an adjacent poorly drained lowland site.
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Experimental Design
This study was established in a strip-plot design with fifteen blocks, four vegetation control treatments, and two hybrid poplar clones (Populus spp.). Ten blocks represented an upland site and five blocks a peaty lowland site (Figure 2). The four treatments were randomly assigned within a block, each to one of four plots 11m x 28m in size. Each clone was randomly assigned to one of two strips orthogonal to the four treatment plots within each block to avoid interspecific competition. In the beginning of June 2012, 20 individuals of both poplar clones ‘Walker’ and ‘Okanese’ were hand-planted at 2.8 m spacing in 4 X 5 clonal blocks within each treatment plot, including a buffer row. Within each clonal block, the six center trees were individually marked for data collection.
Figure 2. Experimental design showing 15 blocks, each including four different treatments and two different clones.
The four applied treatments were (Figure 3):
1) Business-as-usual as the control treatment, using conventional repetitive cultivation and herbicide spraying (‘Bau’),
2) a full year of mechanical and chemical fallow prior to planting (‘Fal’),
3) sowing of a cover crop mixture into conventionally fallowed fields between tree rows after tree planting (‘Coc’); and
4) planting of poplars into untilled fields following localized vegetation suppression using herbicide (‘Not’).
Figure 3: Four vegetation control treatments.
Treatment Application
All four treatments included extensive pre-planting herbicide application and cultivation, except the no-till treatment which used herbicide application and deep-ripping of strips only (Table 1). Treatment application started in June 2011 exclusively for fallow plots. Broadcast herbicide applications of glyphosate (5 L ha-1) during spring and early summer 2011, followed by monthly cultivation until fall of 2011 were used to attain an extended one year period of herbaceous vegetation control. Final site preparation in the form of cultivation and herbicide application occurred prior to planting of trees in early summer 2012. Treatment application for all three other treatments started in September 2011 with an initial glyphosate application (5 L ha-1). For both business-as-usual and cover crop plots, initial herbicide application was followed by cultivation of the full plot whereas in no-till plots only the strips designated for tree rows were deep-ripped, without cultivation of interrow areas. After tree planting and interseeding of cover crops in the cover crop plots, both the tree-rows and alleys were sprayed with pre-emergent herbicide (Lorox) for business-as-usual, fallow and no-till plots whereas only tree-rows were sprayed with Lorox in the cover crop plots to allow establishment of interseeded cover crop mixture in the alleys. The cover crop mixture included White Clover (Trifolium repens L.), Creeping Red Fescue (Festuca rubra L.), Courtney Tall Fescue (Festuca arundinacea Schreb. ‘Courtney’) and Perennial Ryegrass (Lolium perenne L.) at a low seeding rate. Following planting, silvicultural treatments were business-as-usual and similar between treatments. Both business-as-usual and fallow plots achieved herbaceous vegetation control through repeated cultivation. Cover crop and no-till plots were mowed annually to reduce litter accumulation and habitat for rodents that might feed on tree seedlings. For cover crop plots, mowing was also used as a means of supporting the establishment of cover crops in the first growing season.
Table 1: Plantation establishment systems used in this study, grouped by site preparation, planting and maintenance methods.