No growth stimulation of Canada's boreal forest under half century of combined warming and CO2 fertilization
Significance Limited knowledge about the mechanistic drivers of forest growth and responses to environmental changes creates uncertainties about the future role of circumpolar boreal forests in the global carbon cycle. Here, we use newly acquired tree ring data from Canada's National Forest Inventory to determine the growth response of the boreal forest to environmental changes. We find no consistent boreal wide growth response over the past 60 y across Canada. However, some southwestern and southeastern forests experienced a growth enhancement, and some regions such as the northwestern and maritime areas experienced a growth depression. Growth climate relationships bring evidence of an intensification of the impacts of hydroclimatic variability on growth late in the 20th century, in parallel with the rapid rise of summer temperature. Considerable evidence exists that current global temperatures are higher than at any time during the past millennium. However, the long term impacts of rising temperatures and associated shifts in the hydrological cycle on the productivity of ecosystems remain poorly understood for mid to high northern latitudes. Here, we quantify species specific spatiotemporal variability in terrestrial aboveground biomass stem growth across Canada's boreal forests from 1950 to the present. We use 873 newly developed tree ring chronologies from Canada's National Forest Inventory, representing an unprecedented degree of sampling standardization for a large scale dendrochronological study. We find significant regional and species related trends in growth, but the positive and negative trends compensate each other to yield no strong overall trend in forest growth when averaged across the Canadian boreal forest. The spatial patterns of growth trends identified in our analysis were to some extent coherent with trends estimated by remote sensing, but there are wide areas where remote sensing information did not match the forest growth trends. Quantifications of tree growth variability as a function of climate factors and atmospheric CO2 concentration reveal strong negative temperature and positive moisture controls on spatial patterns of tree growth rates, emphasizing the ecological sensitivity to regime shifts in the hydrological cycle. An enhanced dependence of forest growth on soil moisture during the late 20th century coincides with a rapid rise in summer temperatures and occurs despite potential compensating effects from increased atmospheric CO2 concentration.
Proceedings of the National Academy of Sciences