Climate change and stand ageing interact to affect tree species specific relative above-ground biomass accumulation in the western boreal forest
The impact of environmental change on forest ecosystem above-ground biomass accumulation remains unclear. There is some evidence for increases throughout the temperate and tropical biomes and decreases in boreal biomes. However, earlier studies overlook multiple species-specific responses and how environmental change interacts with the stand ageing process. Here, we use 1401 permanent sample plots measured between 1958 and 2013 from two provinces across the boreal forests of Canada (Alberta and Saskatchewan) to examine species-specific relative above-ground biomass accumulation responses to stand ageing and environmental change. All trees greater than 9.2 cm in diameter-at-breast height were permanently tagged and remeasured at each census (mean= 4.7 re-measures). Relative above-ground biomass accumulation was estimated as the yearly change in above-ground biomass accumulation divided by the average of total standing biomass between census periods, and multiplied by 100. Mid calendar year between censuses was used as proxy for environmental change, which included increasing atmospheric CO2, increasing temperature, and decreasing precipitation. A Markov chain Monte Carlo Bayesian hierarchical interactive slopes mixed effects model, which accounts for measurement errors and plot effects, was implemented in R statistical software using the MCMCglmm package.
We found significant relationships between stand age and relative above-ground biomass accumulation for all tree species examined. Two species were negatively affected by more poorly drained soils: Abies balsamea and Pinus contorta. One species was significantly positively affected by poorly-drained soils: Picea glauca. Three species were negatively significantly affected by calendar year: Abies balsamea, Picea mariana, and Pinus contorta. Only Pinus contorta was less affected by climate change as it aged, with this effect lessening on more poorly drained soils. The responses to growth were much stronger, with four species being negatively affected by climate change, and one species, Populus balsamifera being positively affected (P=0.062). Our study provides evidence for a strong difference in the responses of boreal tree species to the impacts of environmental changes which can be mediated by local site effects like soil drainage. The results suggest that early-successional stages may come to be dominated by species less affected by climate change such as Pinus banksiana and Populus balsamifera, while late-successional stages will continue to be dominated by unaffected Picea glauca.