Hybrid Poplar Growing in Plantations -
Growth Responses to Alternative Establishment Systems
Intensive forest plantations hold the potential of a much greater productivity over a short period of time than that of native forests and are intended to increase supply of timber, fiber and biofuels (West 2006). Globally, the demands for timber and other wood products will steadily increase, and new legislations in many countries will further restrict harvesting natural forests (Zsuffa et al. 1996). Concerns over the availability of fiber have led to the establishment of industrial-scale plantations during the past two decades and hybrid poplar plantations could become a very significant source of timber and fiber products (Weih 2004). Moreover, poplar-culture is recognized for several environmental benefits including its greater soil and nutrient conservation compared to agricultural crops, its suitability for wastewater utilization, and its potential for phytoremediation and carbon sequestration (Zsuffa et al. 1996).
To act as a long-term, sustainable supplier for the commercial production of wood or fibre, poplar plantations need to reach high profitability. Despite existing uncertainties about potential risks, poplar plantation culture can be feasible given appropriate selection and use of planting material, site and cultivation treatments (van Oosten 2006). A prerequisite for high biomass production is the use of plant material adapted to boreal, high-latitude conditions in terms of both photoperiod and temperature (Christersson 1996).
Although there has been a lot of progress on approaches to management of poplar plantations during the last two decades, the experience with poplar plantation culture is still limited and uncertainties about potential risks to plantations still exist as a result of the limited area planted with poplars (Volney et al. 2005). Volney et al. (2005) developed a framework to assess risk of poplar plantations in Canada. Interestingly, the current model suggests that even with the best available hybrid poplar selection for the prairies and appropriate management, yields to age 30 are still 23 % below their potential (Volney et al. 2005). The model points out the need of further research and understanding on plantations to increase benefits and to guide management actions. The need to maximize productivity while reducing treatment costs drives management actions (Richardson et al. 2006). Alternative methods of establishing plantations may offer both economic and environmental benefits. Despite this, little is known about the overall effect of these alternative establishment systems on tree establishment, survival and growth.
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At present, operational plantations with the targeted end product of pulpwood are managed using conventional intensive silvicultural practices, which aim for high yields by reducing competing understory vegetation. Poplar trees are typically planted at 1000-1100 stems per hectare for a rotation of 15-25 years. At this planting density 85-95% survival should be the norm (van Oosten 2006). Typical site preparation on tree plantations is a combination of mechanical and chemical methods, including the use of selective and non-selective herbicides. To improve tree survival and maximize yield it is assumed that an almost complete vegetation control must be achieved with target of a 90% or better weed control at time of planting (van Oosten 2006). The underlying assumption is that hybrid poplar performs best in a sod-free field without competing vegetation and that any belowground interaction between trees and the ground vegetation (weeds or cover crops) will negatively impact tree performance (van Oosten 2006). This leads to intensive cultivation of plantations established in fields that were previously been used for pasture or forage production. Given the rotation length of 20 years, these practices are associated with high input costs, especially for intensive site preparation during the establishment period.
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