Effects of diversity on forest production and wood properties and implications for forest management practices

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Forest management decisions take many factors into account. One of these factors is how the forest will react to the decision taken in terms of growth. More fundamentally, even if tree growth and development have been studied for over a century, a lot of questions remain on how trees interact. For example, several studies have shown that forests with several tree species can grow more than forests composed by only one of the species. If we can understand why multi-species forests grow better, management practices could be redefined in order to take advantage of the increased growth. Researchers have however only proposed hypotheses to explain these observations, without thoroughly testing them. The long term objective of this research programme is thus to improve management practices in multi-species forests by understanding how trees interact. Among the ressources used by trees, light is important as it determines the amount of carbon dioxide gas that is transformed into sugars which are used for growth. The amount of light a tree receives is influenced by its neighbours’ size and species. For example, the crown of deciduous species such as sugar maple will not intercept light in the same way that evergreen species like balsam fir do. Trees also need nutrients and water that are picked up by the roots, the availability of both which are influenced by the soil processes. These processes are affected by the forest tree composition since the forest litter, made up by tree leafs and other organic material, will differ in nutrient composition between single species forests and multi-species forests. Within this research programme, the interaction between trees will be studied by looking at both aboveground and belowground dynamics. Using terrestrial laser scanners, which yield high precision 3D information, the interaction between trees will be quantified in order to assess how efficiently trees transform light to biomass growth, and how this efficiency changes from forests with only one species to multi-species forests. Nutrient availability and cycling will also be investigated as to evaluate the differences between single and multi-species forests. The light use efficiency will then be related to soil characteristics in order to fully understand why mixed forests have better growth. Finally, the growth allocation between each compartment of a tree (e.g. stem, branches, foliage, roots) will also be examined within this programme since the forest industry has historically been interested in the stem of the tree, and not the parts such as the branches, foliage or roots. With the development of other industries such as biofuels and biochemicals using the forest as a supply sources, there will be an increase in the need for knowledge on how much biomass is available from each compartment. The findings on aerial interactions, soil processes and allocation will be synthesized into a modeling framework based on the metabolic theory of ecology and biomechanics. The theory relates the metabolic rate of an organism to its size. When applied to forest ecosystems, the metabolic theory does not seem to hold. The underlying hypotheses of the theory will be adapted in light of the results obtained from each component of the programme. This adaptation will then be included in a tree growth simulator which will be developed in order to help forest managers take sound decisions when managing multi-species forests. These results will thus help the Canadian forest industry stay competitive. Furthermore, the programme will help to identify the processes which explain why multi-species stands have better growth, and if the increased biomass growth of multi-species stands can lead to increases in forest industry timber supply.