Negative impacts of high temperatures on growth of black spruce forests intensify with climate warming

English Abstract:

An increasing number of studies conclude that water limitations may hinder the capacity of black spruce (Picea mariana (Mill.) B.S.P.) trees to grow and assimilate atmospheric carbon. But although the impact of climate warming on the carbon assimilation capacity of black spruce has been the subject of research for many decades, there is currently no scientific consensus on the future of these forests over the next century. Here we investigate broad-scale patterns of inter-annual variability in vegetation productivity of black spruce forests and associated drivers through the analyses of a new network of tree-ring data obtained from permanent plots of Canada’s National Forest Inventory and through spatially-explicit simulations of Net Primary Productivity (NPP). We find that available soil water is a significant driver of black spruce inter-annual variability in radial growth and NPP across broad areas of the western to eastern Canadian boreal forest. In addition, in the warmest regions inter-annual variability was also driven by autotrophic respiration. The impacts of available soil water and respiration on inter-annual variability in radial growth occurred, with few exceptions, during the year preceding ring formation. Results from projections suggest an increase in the importance of available soil water and respiration as constraints on NPP over the next century due to warming, but this response may vary to the extent that other factors such as carbon dioxide fertilization, and respiration acclimation to high temperature, contribute to dampening these limitations. This new information calls into question the conclusions drawn from studies in which an assumption of no dependence on available soil water was made when inferring the future response of the productivity of black spruce to climate warming.