Abbey Leigh Marcotte

Peatlands are well-known for their carbon storage capabilities; however, they also provide several valuable ecosystem services including atmospherically derived pollutant sequestration, water quality regulation, and water flow and flood regulation. Climate change-driven increases in drought and wildfire risk threaten these ecosystems, leading to erosion, runoff, nutrient, and metal contamination, and carbon release to the atmosphere. The risk of increasing natural disturbances shifting northwards illustrates the need to enhance our current understanding of interactions and feedbacks between climate change, peatland fires, ecohydrology, and geochemical responses in temperate peatlands. This thesis specifically addresses knowledge gaps regarding ash characteristics, legacy metal and nutrient mobility, and hydrological responses in two wildfire-impacted ombrotrophic peatland systems in northwestern Europe. With this knowledge, we can guide future research and also better inform pre-fire management and post-fire mitigation efforts.

Through a series of field observations and laboratory experiments, chapter 2 assessed spatial patterns of smoldering occurrence and ash production, ash and soil pH, and potential impact of ash pH on underlying peat soils in a burnt Dutch peatland (Deurnese Peel wildfire, April 2020). Extent of smoldering was not widespread throughout the entire landscape and was highly spatially variable, likely influenced by pre-fire field conditions such as vegetation composition and ground water stand. Ash sampled ~2 months following the wildfire (‘aged ash’) had an average pH of ~6, whereas fresh ash produced in an experimental smolder box (‘fresh ash’) was alkaline (pH ~9–10). We also found that soil pH in both smoldered and non-smoldered sampling locations was acidic (pH ~3–4). Taken together, this suggests that ash produced from the wildfire initially had a high pH but alkaline components may have been leached by the time of field sampling. This demonstrates that the ash ‘aging’ process may quickly modulate ash chemistry, and also that impact of alkaline ash addition to peaty soils at the study area may be restricted to a relatively short time frame (i.e., 1-2 months) following this wildfire event.

In chapter 3 we investigated how severe drought and wildfire impact the hydrological and biogeochemical responses in a heavily contaminated blanket peatland in the UK (Saddleworth Moor wildfire, June 2018). Stream chemistry changes were quantified in a 9-month timeframe following the wildfire during five discreate rainstorm events. Concentration-discharge (C-Q) responses were evaluated during storms to determine solute mobilization and transport within the catchment following the wildfire. Nutrients in the streamflow were highest immediately following the wildfire, and decreased in the spring storm events (~9 months post fire) and showed primarily rapid mobilization patterns. Metal concentrations began increasing in autumn (~3 months post-fire) and into the spring storms, and consistently exhibited delayed mobilization. Metals may most likely be sourced distal headwaters rather than being predominately sourced from near the downstream reaches (i.e., proximal sources), as suggested by our findings for nutrients and other base cations. Seasonal re-wetting and rejuvenated hydrologic connectivity of the catchment following extreme drought were a dominating factor controlling source zone activation, mobilization, and transport of solutes in our study catchment.

Moving to the landscape scale, chapter 4 quantified potentially toxic metals through source (hillslope) to sink (reservoir) monitoring following the Saddleworth Moor wildfire. Specifically, we quantified metal concentrations in ash and peat deposits from different burn severities, and the transport of metal-bound particulate deposits during the rainstorm events (chapter 3) to the receiving reservoir. Chemical composition of hillslope ash and peat soil samples were spatially variable, but notably metal concentrations were generally higher than reported baseline values from regional studies. Metal concentrations measured in eroded hillslope material were highest in the summer and autumn ca. ~3 months post-wildfire and generally declined moving to the springtime (~9 months post-fire), with metal concentrations were higher in the extreme burn severity areas compared to lower burn severity areas. Metals in stream suspended sediment were highest in the observed spring storm events compared to immediately post-fire. Given the spatiotemporal heterogeneity in both hillslope metal-rich burnt materials and transport, this points to a potential chronic re-working and transport of metal-rich materials within the system several months after wildfire occurrence, potentially having adverse effects for the ecosystem and human health.

My research has advanced our knowledge of the interactions between wildfire, ash, metal mobility, and hydrological responses in two wildfire-impacted temperate peatlands. We show primarily that wildfire burn severity and combustion types are highly spatially heterogeneous, and suggest that these responses depend on pre-fire landscape conditions. The resulting biogeochemical and hydrological impacts are therefore also spatio-temporally complex. Some impacts and processes identified from this thesis share similarities with other peatland systems, such as burn patterns and nature of hydrologic functioning. There were, however, large contrasts when comparing ash chemistries to other ecosystems, for example. Most pronounced post-fire changes were apparent in the initial months post-wildfire (~2-3 months), and chronic metal contamination may exist when burn residuals are stored in the catchment thus vulnerable to further leaching and/or re-mobilization. Overall, keeping peatlands wet is seemingly one of the most beneficial management and restoration strategies to ensure optimal peatland health and functioning, thus protecting peatlands from impacts extreme climate events, such as carbon and pollutant release and extreme precipitation, drought, and wildfire events.