Loïc D'Orangeville PhD Oral Defense: Global changes and the balsam fir ecosystem: Effects of increased soil temperature, nitrogen deposition and water stress on soil chemistry and xylogenesis
The nature and intensity of environmental changes projected by climate models for Eastern Canada could significantly affect the functioning of boreal forest ecosystems. In this thesis, I tested experimentally the effects of some of these changes (increased soil temperature and atmospheric N deposition, and summer droughts) in a mature boreal balsam fir stand (Abies balsamea (L.) Mill.) in the Réserve faunique des Laurentides (Quebec). The general objective of the thesis was to evaluate the adaptation of the soil/plant system to these changes.
In the first experiment, using heat-resistance cables buried in the ground and water nozzles set up above the trees, soil temperatures were increased (+4°C), spring snowmelt hastened (2-3 weeks), and higher levels of atmospheric N were deposited on the canopy (3x the natural NH4NO3 concentration in the rain) for three years. In the second experiment, a summer drought (starting in July) was simulated in two consecutive years by means of throughfall exclusion using polyethylene sheets maintained 1.3-2 m aboveground and redirecting the water outside the plots. In the soil warming – N deposition experiment, volumetric soil water content was assessed weekly, soil chemistry was monitored with ion-exchange membranes (PRS-probesTM) and soil extractions, while soil respiration was measured in a laboratory incubation experiment. Tree nutrition was assessed through leaf sampling and nutrient analyses, radial growth was monitored using dendrometers and seasonal xylogenesis was assessed using microscopic analyses of wood microcores extracted weekly from April to October each year. In the second experiment, the analysis of wood microcores was also used to assess the effects of throughfall exclusion on balsam fir xylogenesis, while the volumetric soil water content was monitored weekly.
Three years of warmer soils decreased the soil mineralizable C pool by 11-15% with limited effects on the availability of inorganic N (NH4 and NO3). The availability of K, Mg and SO4 increased by 43, 44 and 79% in the forest floor of heated plots, respectively, while Mg, SO4 and Al increased by 29, 66 and 23% in the mineral horizon, respectively. Increased nitrogen deposition did not affect soil nutrient availability. The treatments had no effects on the amount of wood produced or the foliar nitrogen concentration of balsam fir. The experimentally induced earlier snowmelt did not hasten resumption of xylogenesis, although an eight-day earlier peak in diameter growth was observed in heated plots in the second year. Finally, two consecutive years of summer drought reduced by 16.1% the diameter of tracheids and increased the thickness of their cell wall by 14.1% during both years. In addition, the treatment delayed by more than a week the tracheid differentiation process in the second year, with a concomitant decrease of 26% in the number of tracheids produced.
If the short-term responses of balsam fir observed in our studies persist over time, it should be considered relatively resilient to warmer soils and higher N deposition but responsive to drought episodes. Thus, a higher frequency and/or intensity of droughts could have major consequences on the productivity of the balsam fir ecosystem. However, other environmental factors that are expected to change in the future were not included in this thesis (e.g. increasing air temperature and atmospheric CO2) and could affect the response of the ecosystem to increased soil temperature, N deposition and water stress.