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Effects of simulated climate change on post-disturbance Populus tremuloides - Picea mariana ecosystems in northwestern Quebec

Dabros, Anna. 2008. McGill University, PhD

Abstract

In the mixedwood-boreal transitional forest of northwestern Quebec, the establishment of trembling aspen (Populus tremuloides Michx.) has been observed at the extremes of their regional distribution, in areas previously dominated by black spruce ( Picea mariana (Miller) BSP). Our main objective was to explore how climate change could affect the growth and performance of aspen and black spruce. Climate change simulation was provided by the installation of twenty open-top chambers (OTCs) and twenty control plots in the summer of 2005 at three disturbed sites (post-fire, logging road and logging). Each plot enclosed a pair of aspen and spruce seedlings. In comparison to control plots, the conditions in the OTCs were marked by higher air temperatures (2-3oC), drier soil (up to 10% volumetric moisture content) and cooler soil (up to 2.6oC), lower supply rates of Ca and Mg, and slower decomposition of aspen litter. Warm weather and high rainfall were likely responsible for increased height growth and advanced spring bud burst of aspen growing in the OTCs during the 2006 growing season, but not during the cooler and drier season of 2007. Leaf calcium concentration was higher, and beetle leaf herbivory was lower for OTC aspen in comparison to control plot aspen. Spruce was not affected by OTC treatment in terms of height growth, but its final dry biomass was higher, and spring bud burst was advanced by 2-3 days in the OTCs compared to control plots. Both species showed trends of higher root tip number and lower % ectomycorrhizae (ECM) colonization in the OTCs, and vice versa in the control plots. Aspen appeared to be more dependent on ECM colonization; therefore, potential effects of climatic changes on ECM would have a larger impact on aspen than spruce. Overall, aspen may be more responsive (positively and negatively) than spruce to erratic inter-seasonal weather conditions often associated with climate change. A less sensitive, more stable response to weather variations may give spruce an advantage over aspen in the long term, as the years in which the conditions are less favourable towards the physiological requirements of aspen may be detrimental to their regional populations.