Ecophysiology of the coexistance between deciduous and coniferous trees in the boreal zone

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While most ecologists predict that coniferous trees with long lived needles will dominate in boreal and subarctic climates, deciduous trees are common in most boreal zones and often form treelines in arctic and alpine areas. The coexistence and competition between these plant groups is important since climate change and changing forestry regimes are changing the Canadian forests. The proposed research programme investigates the coexistence of deciduous and evergreen trees using the species pair paper birch and black spruce. We try to determine the relative importance of three key theories that can potentially explain the success of deciduous trees in boreal and subarctic regions: the resource limitation theory, the sink limitation theory and the frost desiccation theory. The research will be done along a climatic gradient of four sites from a subarctic alpine treeline (Schefferville or Caniapiscau) to the transition zone of boreal and temperate forests (Laurentides). 1) To address differences in nutrient limitations between the species we will compare the litter decomposition and nutrient mineralization rates under birch and spruce cover along the climatic gradient. 2)Differences in the carbon limitations will be investigated by measuring photosynthesis, sapflow of birch and spruce stands along the gradient. We will explicitly try to understand indirect costs of leaf area by analysing biomass partitioning using the pipe model theory (i.e. analyse the costs of tissues required to support foliage)3)The importance of frost desiccation will be investigated by manipulating spring soil temperature in the field. This will be done by changing snow depths during the winter.4)A larger manipulation experiment including defoliation, debudding, fertilisation and soil temperature manipulations will be used to discriminate between the different theories. 5)Process based modelling will be used to understand the quantitative importance of the results The research will result in four master theses and one doctoral thesis.