The impact of boreal wildfires on carbon and nitrogen dynamics: the interplay between biotic and abiotic processes

Purpose and aims

Wildfires are a natural phenomenon but human activities are altering both the driving factors (climate) and the vulnerability (land-use factors) of ecosystems, increasing both frequency and severity of fire impacts. This is an issue of concern given that wildfires play a major role in the global carbon cycle by affecting carbon and nitrogen storage in ecosystems. Yet, our knowledge of early post-fire carbon (C) and nitrogen (N) (hereafter abbreviated as CN) dynamics has been severely limited by the lack of cross-scale (from soil to plant to ecosystem) and cross-landscape (wetlands to uplands, managed and unmanaged land) studies. Understanding the mechanisms causing variability in CN dynamics (e.g., CN accumulation) , in heterogeneous landscapes, is critical for predicting changes in C and N storage with more frequent disturbance. Given this immediate research need, I propose an ambitious research program to investigate the impact of wildfires on the C and N cycle in the boreal landscape, capitalizing on a recent stand-replacing wildfire in Sweden. With an array of paired pre- and post-fire data, which is rare in wildfire ecosystem research, I aim to address whether pre-disturbance and initial post-disturbance conditions can be used to formulate predictions of post-disturbance ecosystem development. I will employ a novel multidiciplinary framework, which integrates ecological process, like plant community development, into the biogeochemical processes. This much needed integration makes it possible to improve and add new mechanisms to current ecosystem models and to answer under what conditions is the system is most vulnerable to change under frequent and severe wildfires. Three question-based work packages are described below as the basis of this wildfire research program:

  1. CN losses. CN losses. Where in the landscape do the largest C and N losses occur, and what factors control losses? How large are CN combustion losses relative to C transformed into charcoal and hydrologically-exported CN following fire?

  2. CN pool development. What is the relative importance of abiotic (e.g. soil moisture, temperature) and biotic (e.g. plant traits) factors in generating variation in post-fire recovery rate of C and N pools at different spatial scales?

  3. Vegetation development. What controls species and trait assembly post-fire? What is the role of niche-based processes (abiotic effects: environmental filtering, and biotic effects: legacy effects, regeneration traits) in contrast to neutral processes (stochasticity, priority effects)?

Survey of the field

Boreal systems and the role of wildfire

Wildfires have a large impact on the boreal carbon and nitrogen cycle (Bond-Lamberty 2007, Smithwick 2005). Large CO2 emission to the atmosphere from more frequent and larger wildfires can have a positive climate change feedback if the C is not swiftly re-sequestered. Furthermore, the boreal forests are N limited (Tamm 1991) and fire induced N losses may provide a strong control on post-fire productivity, and consequently long-term C storage (soil, standing biomass). Land-use practice, like drainage and forestry, may further increasing the vulnerability of the ecosystem to lose CN during and after fires, possibly leading to regime shifts (Kettridge 2015). Thus, there is a pressing need for ecosystem models to accurately capture C and N losses due to wildfire, and C and N build-up during the post-fire succession.

Despite recent efforts to study wildfire effects on biogeochemistry and vegetation, few studies have attempted to connect biotic and abiotic processes and their contribution to the landscape complexity that emerges after fire (Hollingsworth 2013). To understand these processes, and how they affect resilience and stability of the ecosystem, cross-scale and cross-landscape are need to capture the “functional mosaic” of the landscape (Turner 2010, Holling 1973). However, most studies on post-fire ecosystem development have relied on chronosequence designs (space for time substitution), which are unable to resolve fine-scale processes and underlying mechanisms (Kashian 2013). Studies have suggested that burn severity (i.e. CN loss) largely regulates fine-scale processes (Schimmel 1996, Johnstone 2010), however, linking CN loss following fire, with plant assembly processes and ecosystem functions like CN dynamics, remains an unexplored line of research.

The need for an integrative research approach is particularly true for soil processes and soil organic C, which accounts for a striking 85% of the total boreal C stock (Deluca 2012). Furthermore, despite the majority of C being stored in peatlands, cross-landscape studies covering both peatlands and uplands are surprisingly rare. Current process-based ecosystem models (e.g., Terrestrial Ecosystem Model, TEM)) are still lacking or fail to include these important abiotic-biotic interactions (Yi 2009). As a result, there can be estimation errors of the initial post-fire C stock and unaccounted differences in the early recovery phase, which can incorporate large biases in long-term C balance predictions generated by modelling (Kelly 2016). The present research program will address these knowledge gaps and in addition, advance fire-related ecosystem research by linking processes over various scales.

C and N loss following fire

The majority (up to 85%) of fire-emitted C in boreal upland forests originates from the organic soil layer and mosses/lichens (Turetsky 2011). Recent research shows that C-loss from moss dominated peatlands can be substantial, often in par with uplands, but few studies have included peatlands in their C-loss estimates after fire, or assumed an average C loss value (Turetsky 2011). Ignoring peatlands in C emission calculations can give large errors, particularly in areas with drained peatlands. My research suggests that drained peatlands are vulnerable to deep burns; yet, field data on C-losses from drained peatlands in Scandinavia and Russia (which contain >95% of the worlds drained boreal peatlands) is still lacking. In addition to combustion, C can be lost through run-off water. These lateral losses are of vital importance to the net ecosystem C balance, but to my knowledge, post-fire C-losses through waterways (e.g. DOC) have not been empirically compared to losses through combustion at a catchment scale. It has been hypothesized that DOC export can increase immediately after fire, but there is little evidence supporting this claim (Evans sub).

Unlike C-emissions following fire, few studies have quantified fire-related N losses.In boreal systems, deep soil N-pools are less important for N dynamics compared to the top layer organic soil, which supplies plants with the majority of N (Tamm 1991). Hence, if a fire consumes the organic layer, it can influence ecosystem productivity by reducing plant available N. Fire can also transform organic N to inorganic forms th