Comparative physiology of plant adaptation: resource acquisition and constraints of growing season length in balsam poplar from temperate, boreal and arctic environments

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There is a trend toward higher photosynthetic rates in northern populations of several angiosperm tree species that may help compensate for short summers. We recently reported that greater photosynthesis in high latitude balsam poplar results from increased leaf mass and nitrogen per area, supported by an enhanced mesophyll conductance (gm). Mesophyll conductance is a measure of the ease with which CO2 travels through leaves, but its estimation is very uncertain and must be verified. An MSc student will use an on-line isotope discrimination technique to confirm and extend these observations in controlled north-south crosses of balsam poplar. We will compare balsam poplar with black cottonwood, which also shows enhanced photosynthesis in northern genotypes, but supported by a higher stomatal conductance (gs). Increased gs can improve nitrogen-use efficiency but reduces water-use efficiency. In contrast, increased gm should improve both. These different strategies may reflect differences in water supply, which is less limiting for black cottonwood from the northwest coast than for balsam poplar from the continental interior. Along other lines, we have established common gardens of a range-wide collection of balsam poplar at Vancouver and at similar latitude in Saskatchewan. Regardless of origin, all genotypes flush, grow and set bud ~5 weeks earlier in Vancouver. Consequently, although boreal trees can take advantage of earlier springs, they may capture less of the summer. At very high latitudes, these inherent responses could lead to strong negative impacts on tree growth if climate warming is sufficient to induce flushing a few extra weeks before the solstice. We will acquire data from field and chamber experiments to model these relationships. Finally, a PhD student will test assumptions that allow information on nitrogen uptake efficiency, sites of assimilation and ion flux to be extracted from N isotope discrimination data. He will examine phytogeographic patterns in source preference, discrimination and transcript abundance (nitrate and ammonium assimilation and transport gene families) in balsam poplar from warmer, high pH, nitrate-dominated soils vs. trees from colder, low pH, ammonium-dominated soils.