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.
