The Impact of Fire on Boreal Forest Nitrogen Stocks and Consequences for Post-Fire Recovery

Abstract:

The North American boreal forest is among the largest terrestrial biomes and a key terrestrial carbon store. It is also among the most rapidly warming regions of the globe. Wildfire is the primary agent of disturbance in the boreal forest and is expected to increase in frequency and severity as climate change progresses. Fire in this ecosystem has major effects on nutrient cycling, which in turn have implications for vegetation recovery, long-term future forest productivity and, therefore, the boreal forest carbon balance. However, despite the potential importance of these nutrient-mediated effects, they are not well understood.

To assess the effect of fire on nutrient cycling and understory productivity paired burned and unburned spruce forest sites were established in the Yukon Territory and North West Territories, within the Canadian boreal region. At each site, vegetation surveys and destructive leaf area determination were carried out, and vegetation and soil samples were collected. Measurements of net ecosystem CO₂ exchange (NEE) were taken on the understory vegetation at each site using a LiCor LI6400 portable photosynthesis system and a large Perspex chamber.

These data allow determination of the consequences of fire for total ecosystem nitrogen (TEN). In combination with estimated rates of nitrogen accumulation, this will indicate whether increased fire frequency could deplete boreal forest nitrogen stocks, enhancing progressive nitrogen limitation. While wildfire may reduce TEN in the long term, a short-term increase in the abundance of plant-available forms of nitrogen is often seen after fire. Quantification of soil inorganic nitrogen and foliar nitrogen will seek evidence of this, while understory NEE measurements will assess whether fire-induced changes in nitrogen dynamics can affect carbon uptake in understory vegetation. In addition, these measurements will further our understanding of how the different light regimes experienced by understory vegetation in recently burned and mature forest affect carbon uptake by this vegetation. These findings give an insight into the long and short-term impacts of fire on boreal forest nutrient cycling and their implications for vegetation recovery and carbon uptake.