GROWTH DYNAMICS OF BLACK SPRUCE (PICEA MARIANA) ACROSS NORTHWESTERN NORTH AMERICA

The impacts of climate change have been widely documented around the world. One of the most rapidly changing areas is the boreal forest of North America. The extent of change has been such that there have been shifts in long-standing climate-growth relationships in many boreal tree species; while the growth of many of these high-latitude forests were formerly limited by temperature, warming has increased the evapotranspirative demands such that there is widespread drought stress limiting productivity in the boreal forest. With the importance of the boreal forest as a global carbon sink, it is imperative to understand the extent of these changes, and to predict the resilience of the boreal forest in the face of continued climate change. One area of the boreal forest that has not been extensively studied, despite some of the most extreme warming, is northwestern Canada. Black spruce, the most widespread and dominant of North American boreal trees, is particularly under-studied in this region. In my doctoral research, I have sought to fill some of these knowledge gaps regarding black spruce growth dynamics across its latitudinal extent in northwestern Canada through addressing three main objectives: (1) Investigate the effects of permafrost thaw on the growth dynamics of black spruce in a discontinuous permafrost peatland; (2) Compare the productivity and main climatic drivers of black spruce growth across the latitudinal extent of the species in northwestern Canada; (3) Quantify the degree of plasticity vs. local adaptation in determining black spruce growth responses to resource availability in a common garden study. The results of this research highlight the variability in black spruce growth dynamics across this broad climate and permafrost gradient. While productivity has increased at the northern and southern margins of the boreal forest, the mid-latitude site in the discontinuous permafrost peatland has demonstrated dramatic declines in productivity. I demonstrate that this can be attributed to the negative impacts of permafrost thaw-induced drought stress, wherein the thickening of the active layer is reducing the capacity of shallow-rooted black spruce to access the water table. Thus, decreased growth at this site is an indirect effect of warming. At the northern margins of black spruce, growth is increasing as a result of warming, likely as it can drive longer growing seasons and increased nutrient mineralization. Growth at the southern margin does not appear to be driven by either temperature or precipitation alone, however growth patterns appear to be influenced by water balance at the site as well as CO2 fertilization. The common garden study of seedlings from across the latitudinal extent of black spruce in northwestern Canada provided evidence for local adaptation in black spruce seedlings; the southern seedlings accumulated biomass rapidly at the cost of risking damage to new growth from the onset of harsh temperatures, while northern seedlings grew slowly and conservatively, reducing the risk of damage at the cost of lower growth rates than their southern counterparts. We did not see any significant effect of increased CO2 concentrations on any of the seedling traits studied, however seedlings in the high nutrient treatment showed more pronounced signs of a competitive, fast-growth strategy, which ultimately led to extensive mortality of this treatment. Given this knowledge about black spruce growth dynamics in natural forests and under controlled environment conditions, I can conclude that while the mid-latitude population on a discontinuous permafrost peatland is likely to face substantial declines in productivity and forest cover loss, the northern and southern populations have the potential to remain highly productive provided evapotranspirative demands are met by precipitation, and that no major disturbances influence competitive interactions with this species

Joint effects of climate, tree size, and year on annual tree growth derived from tree-ring records of ten globally distributed forests

Tree rings provide an invaluable long-term record for understanding how climate and other drivers shape tree growth and forest productivity. However, conventional tree-ring analysis methods were not designed to simultaneously test effects of climate, tree size, and other drivers on individual growth. This has limited the potential to test ecologically relevant hypotheses on tree growth sensitivity to environmental drivers and their interactions with tree size. Here, we develop and apply a new method to simultaneously model nonlinear effects of primary climate drivers, reconstructed tree diameter at breast height (DBH), and calendar year in generalized least squares models that account for the temporal autocorrelation inherent to each individual tree\u27s growth. We analyze data from 3811 trees representing 40 species at 10 globally distributed sites, showing that precipitation, temperature, DBH, and calendar year have additively, and often interactively, influenced annual growth over the past 120 years. Growth responses were predominantly positive to precipitation (usually over ≥3-month seasonal windows) and negative to temperature (usually maximum temperature, over ≤3-month seasonal windows), with concave-down responses in 63% of relationships. Climate sensitivity commonly varied with DBH (45% of cases tested), with larger trees usually more sensitive. Trends in ring width at small DBH were linked to the light environment under which trees established, but basal area or biomass increments consistently reached maxima at intermediate DBH. Accounting for climate and DBH, growth rate declined over time for 92% of species in secondary or disturbed stands, whereas growth trends were mixed in older forests. These trends were largely attributable to stand dynamics as cohorts and stands age, which remain challenging to disentangle from global change drivers. By providing a parsimonious approach for characterizing multiple interacting drivers of tree growth, our method reveals a more complete picture of the factors influencing growth than has previously been possible