Post-fire variability in Siberian alder in Interior Alaska: distribution patterns, nitrogen fixation rates, and ecosystem consequences

Abstract

The circumpolar boreal forest is responsible for a considerable proportion of global carbon sequestration and is an ecosystem with limited nitrogen (N) pools. Boreal forest fires are predicted to increase in severity, size, and frequency resulting in increased losses of N from this system due to volatilization. Siberian alder (Alnus viridis ssp. fruticosa) N-fixation is a significant source of N-input within the interior Alaskan boreal forest and likely plays a pivotal, though poorly understood, role in offsetting losses of N due to fire. This study disentangles the effects of fire severity, post-fire age, and environmental variables on Siberian alder N-input across the upland boreal forest and quantifies the landscape-level implications of Siberian alder N-input on N pool balance. Stand types of an early- and intermediate-age burn scar were determined by relevé plot sampling, hierarchical clustering, and indicator species analysis. Alder growth traits (density, nodule biomass, nodule N-fixation, and other traits) were sampled across all stand types, burn scars, and a fire severity gradient. Pre- and post-fire landscape-level N-fixation inputs were quantified within the early-age burn scar by scaling-up Siberian alder growth traits to the stand-level and then mapping the total area of pre- and post-fire stand types. Results show that fire severity shares a complex relationship with Siberian alder N-input in black spruce stands, wherein moderate fire severity has a negligible effect on Siberian alder N-input, moderate to high fire severity increases Siberian alder N-input, and high fire severity reduces Siberian alder N-input. Fire likely limited alder vegetative propagation in post-fire black spruce trajectory stands but enhanced propagation in post-fire deciduous trajectory stands that experienced moderate severity. Following the 2004 Boundary Fire, Siberian alder N-input showed an overall increase across the landscape, mostly within post-fire deciduous stand types. Future increases of fire severity and subsequent conversions of stand type from black spruce to deciduous dominance have the potential to increase total short-term N-input on the landscape, but a majority of those gains will be concentrated within a small proportion of the post-fire landscape (i.e. deciduous trajectory stand types). In the boreal forest, the temporal and spatial pattern of ecosystem processes that rely on N fixation inputs is dependent on the recruitment and growth of Siberian alder, which is in turn dependent on a complex relationship between fire severity, stand type, and post-fire age.