Modeling stand recovery from fire disturbance across the North American boreal forest: Climate and soil organic layer drive forest structure and ecosystem carbon storage
The boreal forest has experienced significant changes in climate over the past century, and it is projected that warming temperatures and drought conditions will increase fire frequency and severity in the future. It has been suggested that these changes in climate and fire regime could result in an increase in deciduous dominance over evergreen species in the boreal forest. However, it is not well understood (i) how changes in forest structure will differ regionally, (ii) how changes in vegetation will feed back on soil carbon accumulation and decay, and (iii) how soil and vegetation changes will impact terrestrial carbon storage in the boreal zone. In this study, we use a numerical model featuring parameterizations for aspen and black spruce species-types, a dynamic soil organic layer, and species-specific seedling mortality dependent on the organic layer thickness. The model is constrained by eddy covariance, forest inventory, and multi-century basal area and organic layer depth chronosequence measurements taken in various locations in Alaska and Canada. Using this model, we investigate: (1) the effects of soil organic layer thickness on modeled forest structure dynamics and ecosystem carbon accumulation, and (2) the effects of projected changes in temperature on forest structure and ecosystem carbon accumulation.
Our model is able to reproduce observed vegetation dynamics and soil processes in locations throughout the North American boreal forest including multi-century basal area growth trends, inter- and intra-annual variability in monthly net ecosystem productivity, soil organic matter accumulation, and moss growth. The model is able to capture observed perennial black spruce dominance in patches with an initial thick organic layer. In patches with an initially thin (<5 cm) organic layer, aspen dominate for a period before succession to black spruce. Periods of aspen dominance result in decreased soil carbon accumulation and comparable plant carbon accumulation. In our simulated warming experiments, we find that forest structure shifts towards longer periods of aspen dominance over black spruce. Initially this shift results in a slight increase in total ecosystem carbon from increased aspen productivity. However, on a multi-century time scale, soil carbon decreases significantly with warming as a result of increased decomposition, input of more decomposable litter, and decreased moss growth. Our results highlight the important interplay between the organic layer, aboveground forest structure and belowground carbon storage in the boreal forest and how this interplay can be expected to change with a projected warming by the end of the century.