Ecophysiological adaptations of black spruce (Picea mariana) and tamarack (Larix laricina) seedlings to flooding and nutrition stress

Black spruce (Picea mariana (Mill) B. S. P.), an evergreen conifer and tamarack (Larix laricina (Du Roi) K. Koch), a deciduous conifer, are the predominant tree species of boreal peatlands of northern Alberta, Canada. The harsh boreal environment is characterized by low nutrient availability, low soil temperature and high water tables. The rise of the water table creates anaerobic conditions in the root zone where the ammonium form of nitrogen becomes more available while the availability of nitrate decreases. Microsite variation within and between years also results in variable nutrient availability to plants.; In order to understand the mechanisms of flooding tolerance, nutrient uptake pattern, and growth responses, seedlings of both species were subjected to flooding and different forms and levels of nitrogen fertilizers in a series of growth chamber experiments. Results of this project suggest that tamarack is more flood tolerant than black spruce since it maintained a higher gas exchange rate and root hydraulic conductance apparently because of its higher root respiration and greater allocation of carbohydrates to the root. Gas exchange of tamarack was also unaffected by the application of exogenous ethylene though root hydraulic conductivity increased. Additionally, tamarack was able to acclimate morphologically to flooding by producing stem hypertrophy and adventitious roots while black spruce lacked such adaptations.; While nitrate and ammonium uptake was inhibited in flooded seedlings of both species, non-flooded black spruce showed a clear preferential ammonium uptake and tamarack exhibited an equal uptake of both forms of nitrogen. Low supply of nitrogen affected gas exchange of black spruce more negatively than tamarack. On the other hand, higher nutrient availability for two consecutive seasons did not have a significant positive effect on growth in black spruce while it increased gas exchange rate and biomass production in tamarack dramatically. Maintenance of higher root respiration, root hydraulic conductance, and equal carbon allocation to roots allowed tamarack to withstand flooding while equal uptake of ammonium and nitrate, and higher growth rate in conditions of variable nutrient availability would confer a competitive advantage to tamarack to succeed in the boreal peatland environment.