Interactions between silvicultural treatment, geological deposit and climate in controlling black spruce growth in Québec (Canada)

Abstract : In managed boreal black spruce forests of Québec, an ecosystem-based approach to forest management is carried out using silvicultural treatments that maximize productivity and achieve specific stand objectives, such as merchantable wood volume for a given area of forest. Stand level management is guided by calculations of annual allowable cut (AAC), a calculation that historically uses retrospective measurements to predict future forest growth. In order to improve the accuracy of forest growth models and guide forest management through an evolving climate, a better understanding of ecological mechanisms and their interactions between silvicultural treatment and different ecological regions is required to ensure sustainable black spruce productivity for a thriving forest economy. To do so, this research project explored multiple two-way interactions between silvicultural treatments, geological deposit and climate in controlling mid-rotation aged black spruce growth in Québec. In Chapter 2, the objective was to test the resource availability and physiological response of black spruce to scarification in two climatic regions of Québec. These were quantified by measuring needle gas exchange and nutrient use efficiency in black spruce stands that had undergone scarification 18 years earlier in two contrasting climate regions. Scarification increased tree height and relative growth rate equally across both warm-dry and cool-wet climate regions. As a result, greater evapotranspiration demand from taller trees increased moisture stress on both site types. Black spruce maintained growth by decreasing stomatal conductance in order to increase water use efficiency, as determined by stable isotope ratios. This suggests that on relatively warm-dry sites, warmer temperatures and a longer growing season maximize tree growth potential after scarification treatment. On the cool-wet site, scarification improved soil nutritional quality, perhaps by reducing competition with ericaceous shrubs and promoting a positive feedback loop of taller trees contributing high quality litterfall to the soil. In Chapter 3, the objective was to explore the mechanisms by which pre-commercial thinning (PCT) increases black spruce growth in different climate and soil types. We explored ecological mechanisms that could be responsible for greater diameter of residual stems on black spruce PCT stands that had been treated approximately 20 years earlier. PCT reduced stand basal area while increasing diameter at breast height of residual stems equally across three contrasting site types. On warm-dry sites, PCT increased soil water availability on till deposits but decreased microbial biomass on clay deposits. On cool-wet sites, PCT reduced rates of soil N mineralization. Compared to non-thinned stands PCT increased canopy openness, which was negatively correlated to residual stem diameter across all site types. PCT therefore likely increases radial growth of residual stems by reducing intraspecific competition. PCT should be avoided on excessively wet sites where it could reduce soil fertility. Instead, it should be prioritized on water-limited sites where it may help mitigate moisture stress in response to climate change. In Chapter 4, the objective was to determine if PCT could mitigate drought-stress of black spruce stands on sites of varying climatic regime and geological deposit. We tested the growth response of black spruce to variations in site water balance using dendrochronology. Interannual variability of black spruce ring width was correlated to historical standardized precipitation-evapotranspiration index across three contrasting site types. These relationships were used to test the relative effect of PCT treatment for black spruce radial growth across sites with varying water balance since time of treatment and during years of climate anomalies. On relatively dry sites, greater interannual variability of radial growth was positively correlated to soil water availability; whereas on wet sites, the opposite trend was found. These findings suggest that black spruce stands on low quality or low fertility sites, or in southern regions with relatively warmer growing seasons, are most at risk to reduced productivity due to rising temperatures. By comparison, northern black spruce stands likely to benefit from warmer temperatures in the future. In the 15 years after treatment, PCT had the greatest benefit to radial growth of black spruce trees on dry sites but was equally beneficial across all site types during anomalously warm and dry growing seasons. Our findings imply that silvicultural treatment should be applied on a site-specific basis to ensure site quality and maximum harvestable wood volume of black spruce. For this reason, it is recommended that scarification treatment be performed in cool-wet climates to reduce competition with ericaceous shrubs. This will help to maintain increased growth rates as well as high quality soil nutrition for future forest rotations. On the other hand, PCT treatment is recommended for moisture-limited sites, particularly with fast-draining till soils, whereby moisture stress can be reduced as a result of lower stand density. Following such recommendations, silvicultural treatments can maximize black spruce growth and site quality while mitigating the negative effects of climate change. Results and recommendations from this research project will hopefully be considered by forest planners and managers and incorporated into future strategic planning and sustainable management strategies to ensure black spruce productivity.

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