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Shoot and root responses of hybrid poplars to interspecific competition and soil fertility

Bilodeau-Gauthier, S.. 2011. l'Université du Québec à Montréal

Abstract

A forest zoning approach that includes intensive silviculture of fast-growing trees on a restricted portion of the territory would meet the demand for wood products, reduce the pressure on natural forests, and increase the surface of protected areas. Hybrid poplar is an excellent candidate for intensive silviculture as its fast growth allows harvest after 20 years. This study aimed to assess environmental conditions that maximize the growth of hybrid poplars in boreal forest soiIs. Three silvicultural techniques (mechanical soil preparation, vegetation control, and fertilization) were tested on 40 hectares of plantations in the Saguenay-Lac-Saint-Jean region. Excavations of root systems of poplars, along with incubations of soil at different depths, were conducted to explain the response to management techniques. The understanding of the behavior of hybrid poplar roots was deepened through a pot experiment where poplars were planted in the presence of grass and with a varying spatial distribution of nutrients, either homogeneous or heterogeneous. The results suggest that mechanical soil preparation should be promoted before the other forest management techniques since it provides the best gains in growth. The mounding preparation proves to be the best method to ensure the survival and growth of hybrid poplar trees. The development of the root system was greatly favored in the mounds due to the soil that was warmer and looser, the increased mineralization of nitrogen, and the low root competition, which provided access to a large volume of soil and an important source of nutrients. Poplar roots actively avoided competition, even to their detriment when the most fertile areas of land were occupied by live competitors. Yet the results of the pot experiment showed that poplar roots could still enter the soil area occupied by a competitor, if it was at least as fertile as its own starting soil area. Poplar roots explored the soil further and gained more resources, despite the presence of competition, when the nutrient distribution was homogeneous. Stem growth in this case was similar to the treatments without competition. Hybrid poplars respond to the presence of competition and the spatial distribution of nutrients by modifying root architecture, reflecting its high plasticity. This study will promote the success of hybrid poplar plantations, a winning condition of forest zoning, in addition to generating new knowledge ta better understand the behavior of plants.

Key Words

silviculture, nitrogen, plant competition, spatial nutrient distribution, fertilization, hybrid poplar, fast-growing tree plantation, mechanical sail preparation, roots, forest soils.