A 3.6 ha experimental fire was conducted in a black spruce peatland forest that had undergone thinning the year prior. After 50 m of spread in a natural stand at 35–60 m min⁻¹, the crown fire (43,000 kW m⁻¹ intensity using Byram’s method) encountered the 50% stem removal treatment; spread rates in the treatment were 50–60 m min⁻¹. Fuel consumption in the control (2.75 kg m⁻²) was comparable to the treatment (2.35 kg m⁻²). Proxy measurements of fire intensity using in-stand heat flux sensors as well as photogrammetric flame heights had detected intensity reductions to 30–40% of the control. Crown fuel load reductions (compensated by higher surface fuel load) appear to be the most significant contributor to the decline in intensity, despite drier surface fuels in the treatment. The burn depth of 5 cm in moss and organic soil did not differ between control and treatment. These observations point to the limited effectiveness (likely reductions in crown fire intensity but not spread rate) of stem removal in boreal black spruce fuel types with high stem density, low crown base height and high surface fuel load. The observed fire behaviour impacts differ from drier conifer forests across North America.

We aimed to assess how peatland drainage altered the spatiotemporal variability in forest cover, aboveground biomass, and tree productivity and how these changes related to the spatial variability in peat burn severity. We studied a black spruce and birch dominated boreal peatland in Parkland County, Alberta, Canada, which was drained in 1987 and burned in 2021. Using remote sensing techniques (historical imagery and LiDAR), we determined that forest cover increased by 180% following drainage and aboveground tree biomass decreased from 26.1 kg m⁻² adjacent to the nearest drainage ditch to 2.8 kg m⁻² 95 m away from the nearest ditch. Field surveys and a LiDAR‐based analysis were conducted to measure the spatial variability in peat burn severity. Drained peatland margins experienced the greatest peat burn severity with a mean depth of burn of 26.9 ± 12.6 cm (18.2 ± 10.1 kg C m⁻²) compared to natural middles at 15.3 ± 6.2 cm (3.9 ± 2.1 kg C m⁻²), where peat burn severity increased with proximity to ditches and greater aboveground biomass. We present a conceptual model outlining the increases in aboveground and peat fuel loads following drainage and suggest that the area around a ditch that is impacted by drainage, which is commonly assumed to be 30 m, likely increases through time in forested peatlands due to the afforestation feedback. Drained peatlands represent a severe fire risk for communities and fire management agencies. Peatland restoration should be integrated into fuel management strategies to reduce the risk that drained peatlands pose.

As climate change increasingly threatens the northern peatland net carbon sequestration function, there is a pressing need to better understand the limits of ecohydrological regulatory mechanisms. This is especially urgent for shallow peatlands (< 40‐cm average peat depth), which consistently experience water stress with greater intensity, frequency and duration than deep peatlands and may represent sentinels for climate change. In this ‘part 2’ paper, we review the peatland hydrological feedbacks originally proposed a decade prior in Hydrological Feedbacks in Northern Peatlands ‘part 1’ (Waddington et al. 2015) to investigate the strength of feedback mechanisms as a function of peat depth. We show that in some hydrogeomorphic and hydroclimatic settings there are differences in hydrophysical properties and vegetation cover between shallow and deep peatlands. These structural characteristics influence the strength of the fast (i.e., function on a timescale of seconds to days) hydrological feedbacks (moss surface resistance and albedo, transmissivity, peat deformation and specific yield). In contrast, the slow feedbacks (i.e., operating on the scale of months to decades) related to vegetation community change and peat decomposition directly impact peatland physical characteristics (patterns and composition of vegetation, bulk density, etc.). We discuss how the vulnerability of shallow peatlands arises from the interactions between regulatory (negative) and destabilizing (positive) ecohydrological feedbacks.

Peatlands are critical for global climate regulation storing approximately 500 Gt of carbon and accounting for 33% of global soil organic carbon. Regionally, these ecosystems provide essential wildfire resilience and are important pollutant sinks but degradation puts these key ecosystem services at risk. Smelting operations in Sudbury, ON, Canada, released approximately 12 000 t of particulate copper and nickel into the atmosphere between 1883 and 1969. Toxic metal and sulphur deposition on peatlands from smelting activities caused the widespread decline of keystone peatland moss species (i.e., Sphagnum) and altered peat properties. The changes in peat hydrophysical properties due to historical metal contamination likely reduce peatland resilience to drought and wildfires, thereby increasing the potential for toxic heavy metal remobilisation; however, these peat properties changes have yet to be quantified. We determine 1) how historical smelter pollution impacts peat hydrophysical properties by measuring bulk density, saturated hydraulic conductivity and soil water retention in the upper 40 cm of both undisturbed (located ~160 km outside the deposition region) and smelter‐impacted peatlands, 2) use these data to explore the vulnerability of these peatlands to wildfires and drought and 3) assess the potential for natural Sphagnum moss recovery. Smelter‐impacted peat had a significantly higher bulk density, lower macroporosity and saturated hydraulic conductivity that drove large differences in modelled soil water tension profiles during simulated drying events. These differences in soil water tension and retention profiles resulted in the smelter‐impacted peat having a far greater proportion of the peat profile that would be susceptible to smouldering combustion than the undisturbed peat. Additionally, the smelter‐impacted peat properties likely contributed to the limited Sphagnum moss recovery, while concurrently increasing drought and wildfire risk. As such, we argue that contaminated peatland restoration is necessary to enhance Sphagnum moss recovery to mitigate toxic metal remobilisation risk from drought and wildfire.

Northern peatlands provide important ecosystem services and while these ecosystems are facing large increases in the frequency and severity of climate-mediated disturbances (e.g., wildfire, drought), they are generally resilient to these disturbances. Numerous autogenic feedbacks operate within peatlands that regulate their response to changes in seasonal water deficit. However, our recent research has determined that shallow peatlands have greater water table variability and drawdown rates, moisture stress and depths of burn than deeper peatlands. Moreover, we found that peatland carbon sequestration was significantly lower during periods when the water table became hydrologically disconnected from near-surface peat, which occurs more often in shallow peatlands. This suggests that shallow peatlands are less resilient to disturbance due to the limited capacity of their autogenic ecohydrological feedback mechanisms to mitigate disturbance, when compared to deeper peatlands.We explore how several autogenic feedbacks change in sign and strength with increasing peatland depth and argue that shallow peatlands represent sentinels for climate change; acting as a bellwether for deeper peatlands in a future with more frequent, prolonged, and intense water deficits. We suggest that an explicit quantification of peatland feedback mechanisms across a gradient of hydroclimatic settings, and the thresholds and constraints they operate under, will help identify systems at greatest risk for loss of function or catastrophic degradation under climate change. Furthermore, this work provides insight into peatland restoration and peatland evolution as all deep peatlands were, at one point, shallow and perhaps at the height of their vulnerability.