Millennial-Scale Fire and Vegetation Change from a Rare Mid-Latitude Permafrost Fen (Beartooth Plateau, WY)

Abstract

Long-term fire histories are well-documented across most North American temperate forest systems, yet the fire regimes of high-alpine treeline environments remain poorly understood. Here, we present a millennial-scale fire history from the Sawtooth Fen Palsa (SFP), a rare permafrost fen palsa located in the high-alpine treeline ecotone of the Beartooth Plateau, Wyoming, a permafrost system now unraveling due to recent decades of rapid warming. Analysis of paleoenvironmental proxies from peat sediments overlying the permafrost reveals a multi-century peak in fire activity at the beginning of the record, ca. 10,000 cal yr BP, coinciding with the afforestation of newly deglaciated, ice-free sites. This initial surge in high-severity fire activity was followed by a decline when solar-orbitally driven increases in growing-season temperatures likely promoted forest opening and more surface fire activity within the SFP watershed. High-severity fire activity increased again during the mid-Holocene (ca. 5800–5000 cal yr BP), when effective moisture increased, favoring subalpine forest expansion and increased connectivity of woody biomass (sagebrush and forest), enhancing the potential for canopy fire spread. Only two small fire episodes were recorded in recent millennia when a rapid change in the sedimentation rate may indicate a partial loss of the sediment record. Rapid warming in recent decades has triggered the formation of dozens of thermal collapse ponds across the fen palsa. The frequency of these features has more than doubled since 2000 CE, underscoring the degradation of underlying permafrost in response to changing climatic conditions. Continued warming is expected to cause the complete loss of the permafrost lens and alter ecosystem dynamics, disturbance regimes, and carbon and nutrient cycling in alpine systems throughout the Rocky Mountains.

1. Introduction

1.1. The Vulnerability of High-Elevation Ecosystems

Ecosystems throughout the western United States are responding to rapid warming, with higher-latitude alpine environments warming at greater rates than lowland environments [1,2]. These high-elevation systems serve as critical sentinels for climate change, yet their long-term ecological baselines are often obscured by a lack of paleoenvironmental data. Ecosystem models predict latitudinal and altitudinal shifts in forest systems in response to warming temperatures [3,4], though local site conditions play an important role in influencing the directional outcomes [5]. For example, while rising isotherms typically drive upward tree migration, subalpine forests could become moisture-limited rather than exhibiting an expected positive growth and recruitment response to continued warming temperatures [6,7].

The current rate of climatic change driving ecosystems at high elevations towards possible extinction events related to mountaintop traps highlights a need to better understand the magnitude of paleoclimatic and paleoecological changes in the subalpine treeline [8]. Species adapted to the uppermost limits of alpine systems face greater risks, as mountain-top elevations limit the upper range of climatic envelopes that support their suitable habitat. Additionally, records focusing on how subalpine treeline position and species composition have fluctuated across multiple timescales are needed to clarify how changes in fuel conditions can lead to novel fire regimes in high-elevation systems. Disentangling these drivers is essential for forecasting future fire risk, as the controls of temperature and moisture on fuel aridity and fuel connectivity in alpine systems are often decoupled from adjacent lowland systems [9].

While the vegetational history of the Greater Yellowstone Ecosystem (GYE) since the last glacial period has been studied extensively through the analysis of pollen, charcoal, and other proxies from lakes and wetlands [10,11,12,13,14], there are few records of fire activity and environmental change from high-elevation alpine ecosystems at the subalpine forest treeline to alpine tundra ecotone [7,15,16]. This gap is particularly evident in relation to the interaction between fire and permafrost features, where fire is typically thought to be limited or absent [16].

1.2. Alpine Permafrost, Palsa Dynamics and Changing Fire Regimes

Alpine permafrost features are relatively rare features in the contiguous US, with undocumented estimates of ca. 100,000 km² in alpine settings in the western US [17], within the Rocky Mountain cordillera. Permafrost landscapes support unique high-elevation peatlands, including bogs and fens, and function as important reservoirs of carbon and methane. However, warming temperatures are extending the latitudinal and elevational extent of permafrost degradation of alpine permafrost that occur in the coterminous western United States [18].

Fen palsas, peat-covered permafrost mounds that form within wetland basins underlain by permafrost, are even rarer landforms commonly associated with the subarctic and subalpine permafrost systems at higher latitudes (>50° N) [18,19]. The Sawtooth fen palsa is the only fen palsa documented in the contiguous US [19], likely because their formation and persistence require a combination of climatological conditions that more typically occur at higher latitudes. These conditions include cold, dry continental climates with 500–1500 thawing degree days (sum of daily mean temperature above 0 °C totaled over a year), 500–4000 freezing degree days (sum of the daily mean temperature below the freezing point 0 °C totaled over a year), around 300 mm of rainfall, and high soil moisture accumulation [20].

Following several decades of rapidly warming temperatures and the reduced frequency of freezing degree days and cold, dry conditions, permafrost features are unraveling throughout North America [18]. Since ~2000 CE, permafrost degradation threatens to destabilize many of the fen-palsa systems, with significant implications for alpine ecosystems, disturbance regimes, and carbon cycling. Fire activity in these cold, high-elevation settings with short fire seasons was thought to be low except during centennial- to millennial-scale climate-driven warm periods [7,16].

Historical fire activity is poorly documented in fen palsa systems, particularly at lower latitudes, although changes in fire regimes under a warming climate in high-latitude permafrost systems are well-documented [21,22,23]. Persistent warming in permafrost systems can initiate a self-reinforcing feedback in which the active layer thickness increases and surface fuel drying promotes fire spread when ignitions occur. Although fire-induced active-layer deepening is typically more pronounced in forested permafrost systems, slow-moving smoldering fires in treeless peatlands can consume dry peat and remove the insulating layer that protects underlying permafrost [21,24]. Post-fire processes may further accelerate instability because drying peatlands promote fuel accumulation, sometimes including woody fuels, which can intensify subsequent fires under continued warming [21,22]. Permafrost thaw and associated changes in fire activity are therefore producing cascading ecosystem effects, with significant consequences for carbon storage, nutrient cycling, and hydrology [25].

1.3. Research Objectives

Here, we investigate the long-term environmental change and fire history of a high-alpine wetland area by analyzing pollen and charcoal from a rare mid-latitude c. 8-hectare alpine fen palsa, the Sawtooth Fen Palsa (SFP), located at c. 2950 masl on the high-alpine Beartooth Plateau, Wyoming. First identified in 1961 [26], the SFP is characterized by an underlying 0.5–2.0 m thick permafrost lens that has been melting due to recent decades of rapid warming. This has resulted in the instability of the fen palsa, with the desiccation of several meters of peat overlaying sections of permafrost causing a shift from a stable fen palsa mound to a matrix of thaw depression ponds interspersed by pillars of dried peat. The number of depression ponds has been increasing in number since the late 20th century [27].

Using a multiproxy comparison, we evaluate past fire activity (macroscopic charcoal influx), vegetation change (pollen), and climatic conditions (lithology, sedimentation rates) to reconstruct a long-term record of fire activity and environmental change at a unique alpine fen palsa in the Northern Rocky Mountains to address the following questions: (1) How stable was this low-latitude permafrost feature throughout the Holocene? (2) What was the frequency and character of fire activity at this alpine site? (3) How did vegetation respond to changes in fire activity? (4) How is this SFP changing in response to recent decades of warming?

This paleoenvironmental record offers a rare, long-term perspective on alpine fire–climate–permafrost interactions, and understanding these dynamics is essential for anticipating ecosystem shifts under continued warming.

2. Materials and Methods

2.1. Study Site Description

The Beartooth Mountain Range in south-central Montana and northwest Wyoming is the highest mountain range in the state of Montana (3904 masl at its highest point, Granite Peak). The mountain range lies within the Custer, Gallatin, and Shoshone National Forests. The vegetation zonation along the elevation gradient includes Pinus contorta (lodgepole pine) and Pseudotsuga menziesii (Douglas fir) forests at lower elevations, subalpine forests of Picea engelmannii (Engelmann spruce), Abies lasiocarpa (subalpine fir), and Pinus albicaulis (whitebark pine), and alpine tundra vegetation dominates above approximately 3000 masl, characterized by Poaceae (grass), Carex (sedge), Salix (willow), and other herbaceous vegetation [27].

The climatology of the Beartooth Plateau is governed by large-scale atmospheric circulation patterns: most precipitation is received during the cool season (October–April) from westerly storm tracks originating over the Pacific Ocean, and a secondary spring (April–June) precipitation pulse can deliver substantial moisture to the region and typically occurs due to regional recycling of winter moisture and upsloping easterlies laden with plains moisture that may extend from as far away as the Gulf of Mexico [28]. These spring precipitation events can occur as convectional storms or snowfall at high elevations. The late summer season is typically dry due to the presence of the Pacific subtropical high-pressure system, which redirects moisture away from the Northern Rocky Mountains [29].

The SFP (44°53′32.54 N, 109°27′34.82 W) is located in a glacially carved basin south of Sawtooth Peak along the Beartooth Uplift in Wyoming (Figure 1). This is thought to be the southernmost palsa feature in North America, over 1000 km south of the closest known fen palsa in Canada [19]. A survey of the SFP in 1961 reported the depth to permafrost as approximately 38–46 cm [26].

The SFP vegetation is primarily composed of grasses (Festuca brachyphylla, Deschampsia caespitosa, Agrostis mertensii), sedges (Carex scopulorum, C. canescens, C. aquatilis), herbs (Rumex paucifolius, Antennaria lanata), and a mix of mosses [27]. The basin surrounding the palsa consists of sedges (Carex scopulorum, C. praeceptorum, C. illota, and C. aquatilis), grasses (Deschampsia caespitosa), small shrubs (Salix planifolia, Rosa woodsii), and herbaceous plants typical of alpine tundra (Artemisia scopulorum, Carex sp., Geum rossii, Phlox caespitosa, Polygonum bistortoides, Potentilla diversifolia, Gentiana algida, Phyllodoce empetriformis) across the broader Beartooth alpine ecosystem [27]. The treeline ecotone in this basin is diffuse with an approximate upper elevation of ~3060 m asl, about 0.3 km from SFP. The closest dense subalpine forest stand of whitebark pine and Engelmann spruce is located ~0.5 km away at ~3000 m asl.

Interpolated climate normals for 1991–2020 (PRISM Climate Group, 2022) indicate the basin receives an average of ~800 mm of annual precipitation. Most precipitation is received in the winter and spring months as snow (DJF = 175 mm, MAM = 275 mm), and the least in the summer months (JJA = 165 mm). Temperatures are highest in July (average of 12 °C), and lowest in December (average of −9.5 °C). The average annual temperature is −0.1 °C.

2.2. Lithology, Sediment Sampling, and Chronology

Our 2022 surveys of the depth to permafrost varied from 30 cm to nearly 200 cm at different locations on top of the palsa dome, indicating the peat depths above the permafrost vary. We cored a location where sediments overlaying the permafrost surface were approximately 150 cm. Peat sediments were collected using a D-section peat coring device, which is designed to retrieve soft sediments without significant compression or distortion of the stratigraphy. Three sediment cores, each 50 cm in depth (150 cm length in total), were retrieved in the field. The cores were wrapped in plastic wrap, secured in split-PVC tubes, and stored in cold storage at Montana State University for analysis. The last 50 cm sediment core interfaced with the beginning of the permafrost layer at ~150 cm in total depth. Thus, the 150 cm of sediment was bounded and underlain by a permafrost layer.

In the laboratory, the lithology of the SFP cores was described based on visual inspection of color (Munsell), grain size, and organic content. Nine bulk sediment samples spanning the three cores were submitted to the NOSAMS Woods Hole Oceanographic Institute for Accelerator Mass Spectrometry (AMS) radiocarbon dating (Table S1). Two macrofossils found within the top 4 cm of the permafrost layer, at the boundary of the peat sediments and the surface of the permafrost, were also radiocarbon dated to constrain the onset of peat accumulation above the permafrost. The AMS radiocarbon ages were converted to calibrated ages using CALIB Radiocarbon Calibration Program 8.2 [30].

To develop a robust chronology for the sediment record, an age–depth model for the SFP was created using BACON program [31] (Figure 2). BACON utilizes a Bayesian statistical approach to reconstruct accumulation rates. This method uses millions of Markov Chain Monte Carlo (MCMC) iterations to estimate age–depth relationships, accommodating the variable sedimentation rates typical of dynamic peatland environments.

2.3. Pollen Analysis

The SFP sediments were processed for pollen following the standard procedures of Bennett and Willis [32]. Sediment samples of 1 cm³ were taken at regular intervals and treated with hydrochloric acid (HCl) to remove carbonates, potassium hydroxide (KOH) to digest humic acids, hydrofluoric acid to remove silicates, and acetolysis to remove cellulose. A tablet with a known concentration of Lycopodium spores was added to each sample prior to chemical processing to calculate pollen concentration (grains/cm³) and accumulation rates (grains cm⁻²yr⁻¹).

A minimum of 300 pollen grains were counted per sample at 400–1000x magnification and identified to the lowest possible taxonomic level using reference collections at Montana State University and pollen manuals [33,34,35]. Pollen results were separated into stratigraphic zones designated through cluster analysis performed by the CONISS program, which uses a stratigraphically constrained incremental sum of squares method to identify significant changes in assemblages [36]. Tree species and several upland herb species that were present in very small percentages were grouped together in “other trees” and “other shrubs and herbs” categories.

2.4. Charcoal Analysis

Fire history was reconstructed using macroscopic charcoal analysis. Contiguous sediment samples of 2 cm³ were taken at 1 cm intervals throughout the SFP cores. These samples were processed following the procedures outlined by Whitlock and Larsen [37]. Samples were soaked in a deflocculant and washed through a 125 µm sieve. Charred particles retained on the sieve (>125 µm) were counted under a stereomicroscope. This size fraction has been shown to represent local-to-watershed scale fire events, as larger particles do not travel far from the source of combustion [38].

Charcoal counts were converted to charcoal accumulation rates (CHAR, particles cm⁻² yr⁻¹) using the sedimentation rates derived from the age–depth model. Fire events and changes in fire frequency over time were identified using the CharAnalysis software for MatLab [39]. Background charcoal accumulation rates (BCHAR), which represent the regional charcoal pool and secondary depositional processes, were calculated using a 500-year Lowess smoother. This window width was selected to optimize the signal-to-noise ratio, effectively separating local fire peaks from the background noise. Peaks exceeding a locally defined threshold were identified as fire episodes (i.e., one-to-several fire events occurring within sample peaks).

3. Results

3.1. Lithology and Chronology

Sediment lithology for each of the three 50 cm length cores consisted of sedge peats within an organic fine-grained sediment matrix. There were no distinct changes in lithology or grain size across the three cores. The age–depth chronology indicates the peat sequence spans ca. 10,250 cal yrs BP (Figure 2). Radiocarbon-dated macrofossils found within the permafrost ice lens indicate the boundary layer between the peat sediments and the surface of the permafrost ice occurs in this same time interval, ca. 10,000 cal yr BP. This confirms that the permafrost feature established after deglaciation and has persisted throughout the Holocene.

The interpolated age–depth chronology derived from BACON shows several distinct changes in sedimentation rate (sediment deposited per year cm/yr⁻¹). The sedimentation rate is relatively rapid (>0.05 cm/yr⁻¹) from ca. 10,250–8500 and ca. 5800–4700 cal yr BP. Conversely, sedimentation rate is relatively slow (<0.01 cm/yr⁻¹) from ca. 8500–5800 cal yr BP and ca. 4970 cal yr BP—to present day. Changes in the sedimentation rate did not correlate with visible changes in sediment color, grain size, or organic material. The consumption or loss of sediments (via fire or erosion) during these intervals of slow sedimentation may explain the absence of noticeable lithological changes.

3.2. Fire Activity

Analysis of macroscopic charcoal influx (charcoal particles cm²/yr⁻¹) as a proxy for past fire activity/woody biomass burning suggests the SFP experienced high-severity fires at different periods of the Holocene. A pronounced decrease in the rate of sediment/peat deposition and hiatuses in sediment/peat deposition indicate climatic and environmental conditions at the fen palsa led to loss of some part of the paleoenvironmental record via erosive processes or consumption by fire (Figure 3).

Early Holocene (ca. 10,250–7500) Fire Activity: Fire activity and biomass burning of woody vegetation was high during the beginning of the early Holocene. During this period, CHAR consistently exceeded two particles cm²/yr⁻¹, and seven high-severity fire events were recorded.

Mid-Holocene (ca. 7500–5000) Transition: After a decline in peak fire events ca. 8500 yrs ago, low levels of charcoal influx persisted. Increased seasonality and warmer summer conditions could have led to drying of the peat surface and loss of charcoal through surface fire, or erosive processes that consumed or removed charcoal from the record. Peat accumulation increased near the end of the mid-Holocene (ca. 5800–5000 cal yr BP), recording another period of high fire activity (CHAR > 2 particles cm²/yr⁻¹). This period of high woody biomass burning coincides with increases in winter/spring temperatures recorded by adjacent sites [6,35,36,37].

Late-Holocene (ca. 5000-present) Absence of Fire Activity: The late Holocene fire record indicates minimal biomass burning at the fen palsa. Two fires were recorded at ca. 2900 cal yr BP and ca. 200 cal yr BP. The initial onset of this decline in fire activity parallels the onset of cool and wet climatic conditions at adjacent high-alpine settings when snow and ice cover increased year-round at high elevations of the Beartooth Plateau [6,40]. The pronounced decrease in late-Holocene peat accumulation suggests that much of the record of biomass burning following ca. 2900 cal yr BP was likely removed by erosive processes (aeolian or water) or consumed by fire.

3.3. Vegetation Change

Pollen data indicate that for millennia the SFP basin was dominated by sedges and grasses with surrounding uplands dominated by sagebrush, forbs, and forests of pine, spruce, and fir (Figure 3 and Figure 4).

Early Holocene (ca. 10,250-c. 7500): Pollen percentages of Cyperaceae (sedges) were highest (18–23%) from ca. 10,250–9400 cal yr BP and then decreased (to 7%) by ca. 7500 cal yr BP, suggesting hydrologic conditions supported extensive sedge wetlands during the early Holocene. Poaceae and Amaranthaceae levels were also high, reflective of the surrounding open basin. Pinus levels were moderate (62–66%) between ca. 10,250–9000 cal yr BP, then increased to the highest levels of the record (75–83%) between ca. 9000–7500 cal yr BP. Picea and Abies levels were also moderate (6 and 2%, respectively), and Populus levels were highest (2–4%) at the base of the core from ca. 10,250–9000 cal yr BP. This suggests pine/spruce/fir/aspen parklands and open forests likely established along the uplands surrounding the wetland from ca. 10,250–9000 cal yr BP. Higher pollen percentages of Pinus (subgenus Strobus type) between ca. 9000–7500 cal yr BP indicate forest densification of what is likely pollen from whitebark pine.

Mid-Holocene (ca. 7500–5000 cal yr BP): Artemisia increased during the mid-Holocene (from 8 to 14%), at the expense of Pinus (back down to 64%), although Picea and Abies pollen levels increased (6–10% and <1–3%, respectively). Cyperaceae also remained at lower levels, likely due to a contraction in the wetland margin extent.

Late Holocene (ca. 5000–2500 cal yr BP): Pollen percentages of Pinus subg. Strobus show a distinct peak ca. 5500 cal yr BP (35%), then return to moderate levels (11–13%). Other than the spike in Pinus subg. Strobus, total Pinus, Picea, and Abies remained consistent (60–68%, 5–9%, 4–2%, respectively). Following this, sediment accumulation rates slow, or possibly cease completely, ca. 5000 cal yr BP–present day indicating the loss of the top of the sediment core, or climatic conditions (cold temperatures or possibly ice accumulation) prevented the accumulation of organic materials. During this decline in sediment accumulation, Salix pollen increased (to 8%), and Poaceae and Artemisia levels remained moderate (9–13%, 3–7%, respectively).

Late Holocene/Modern (ca. 2500 cal yr BP—present day): Pollen percentages remain relatively stable and consistent with the previous zone for the last two millennia, other than a decrease in Salix (to 0%), and decrease in Cyperaceae (from 10 to 5%).

4. Discussion

The few high-elevation long-term records of fire and vegetation change from the US Rocky Mountains provide important insights into alpine ecosystem dynamics, upslope and downslope shifts in treeline, and the ecological response to changes in fire activity [4,6,7,41]. This fire and vegetation history from the only known fen palsa in the lower latitudes of North America [19] suggests high-severity canopy fires were a common occurrence at high elevations during the early- and mid-Holocene. Rapid changes in sedimentation also indicate the SFP experienced periods of instability leading to the loss of peat layers overlying the palsa dome. Recent warming is leading to an unraveling of the permafrost underlying the fen system, likely signaling a change in fuel conditions that will promote increased fire activity and changing carbon dynamics in coming decades.

4.1. Early Holocene Fire Regimes

The SFP experienced several high-severity fire events during the early Holocene, when spruce, fir, aspen, and grasses dominated the high-alpine environment (Figure 3, Figure 4 and Figure 5). These early Holocene fires and high-elevation spruce and fir forests coincide with regional records showing the upwards expansion of forests in the alpine environments of the Northern Rocky Mountains [6,7,41,42,43]. This synchrony in increased fire activity across the region was likely in response to climate variability driven by changes in solar orbital forcing (high summer insolation), which created drier and warmer summers but cooler winters. These conditions allowed woody biomass to expand to high elevations, while simultaneously drying fuels sufficiently during the summer fire season to facilitate canopy fire spread. The SFP record suggests that, however sparse, the treeline forests surrounding the fen palsa burned readily once vegetation was established and seasonal conditions led to adequate fuel drying.

4.2. The Mid-Holocene Shift and the Whitebark Pine Anomaly

Macroscopic charcoal influx declines ca. 8500–5800 cal yr BP, coinciding with a pronounced reduction in the sedimentation rate, suggesting a period of aridity that resulted in slow peat accumulation, if not the total desiccation of the peat surface and subsequent loss of surface peat (and therefore charcoal) due to degradation, erosion, or fire, which may also have consumed the peat surface. Continued warm, dry summers due to high seasonality likely facilitated the occurrence of fires during this period [41], yet colder winters may have shortened the fire season, and less dense forest and fewer connected canopy fuels after 7000 cal yr BP likely prevented the generation of large charcoal peaks typical of stand-replacing events. The possible loss of the part of the fire record obscures a complete record of fire activity during this interval.

As rapid peat/sediment accumulation resumed, charcoal influx increased, indicating high-severity fires occurred for several centuries (ca. 5800–5000 cal yr BP). We interpret this increase in fire activity forested uplands surrounding the SFP to be related to a pronounced period of warming evident in adjacent climate records [40]. An increase in Pinus strobus type pollen (likely Pinus albicaulis—whitebark pine) during this interval suggests whitebark pine benefited from increased fire activity. Under certain fire severities and frequencies, whitebark pine trees can confer an advantage over subalpine arboreal competitors as fires reset succession, allowing whitebark pine seedlings to establish in open sites following fires [44]. The increase in whitebark pine pollen coincides with other records suggesting the upward movement and establishment of whitebark pine stands across the Beartooth Plateau [6,7].

4.3. Late-Holocene Cooling and Hiatus in Peat Accumulation

Only two small fires (low peak magnitude fire events) were recorded from when seasonality decreased, summers became cooler and wetter, and alpine glaciers began re-advancing in northern Montana ca. 4000–2000 cal yr BP [45]. A pronounced decline in peat accumulation occurred during this period, coinciding with cooler climatic conditions [6,40,41]. The initial decline or hiatus in accumulation could be related to a general decline in primary productivity and delivery of organic material into the SFP due to cooler conditions. Alternatively, a substantial increase in the accumulation of snow and ice recorded at nearby sites at similar elevations after ca. 4000–3000 cal yr BP could indicate the fen palsa was covered by snow and ice for much of the year, restricting the accumulation of organic sediments and preventing fire activity within the watershed [6,7,40].

After ca. 3000 cal yr BP, the pronounced slowdown in peat accumulation likely indicates the top peat layers were lost to desiccation, degradation, erosive processes, or that the surface peats were consumed by fire. High-magnitude oscillations between wet and dry periods and extended droughts such as the Medieval Climate Anomaly (ca. 1000–750 cal yrs BP) [46] could have led to fires that consumed desiccated peat surface or desiccation alone, provided conditions whereby aeolian or water erosion could have removed any record of fire and paleoenvironmental change for the last several millennia.

4.4. Modern Fen Palsa Dynamics and Degradation

While small permafrost features have been identified across high-elevation systems throughout North America, fen palsas are rare features more commonly associated with permafrost zones at higher latitudes (>50° N). Climatic conditions supporting the development and persistence of fen palsas are declining. Warmer temperatures, decreasing number of freezing days, and drying soils are driving a rapid collapse of permafrost features, especially in the discontinuous permafrost zone and alpine permafrost features at lower latitudes. These changes are occurring in the high alpines of the US Rocky Mountains as evidenced by the rapid melting of persistent ice patches and small permafrost wetlands [6,40]. Recent decades of warming and subsequent permafrost degradation have destabilized the SFP system; and with continued warming, the SFP permafrost lens will likely disappear.

Images show a more than double-fold increase in the number of thermal depression ponds at the SFP basin between 1994 and 2022 [19] (Figure 6A,B). A field survey of the SFP in 1982 notes the presence of raised peat deposits and a small number of thermal depression ponds and polygonal cracking of the fen palsa surface peats resulting from freeze–thaw dynamics, but images from this survey depict a largely intact palsa system, contrasting sharply with the system’s current instability.

Increases in fire activity at the SFP during the early- and mid-Holocene, a period characterized by higher seasonality and warm summers, may foreshadow how recent warming could precondition the SFP watershed and surrounding forests for fire spread. Pronounced warm-season drying of peats and woody vegetation along with elevated temperatures that persist later into the autumn months will likely provide ample fuel drying to allow for larger fires across an expanded fire season. The rapid increase in thaw depression ponds since 2000 has led to almost complete drying of the fen peat matrix surrounding the collapse ponds, providing ample connected, dry fuels for fire spread. We expect that the thawing of the permafrost beneath the fen system, which has persisted throughout the Holocene, will result in the complete loss of the SFP permafrost feature within decades.

5. Conclusions

Despite its location in the high-alpine environment characterized by a short growing season and snowpack persisting through much of the year, high-severity fire activity was recorded at the SFP in the early- and mid-Holocene under intensified seasonality (colder winters and warmer summers) and warmer fire-season conditions. Pronounced fluctuations in peat accumulation suggest mid-Holocene and late-Holocene conditions led to the degradation and loss of parts of the paleoenvironmental and paleofire records via erosive processes or consumption by fire. The paleofire record indicates high-severity fires occurred once every few hundred years during times when peat accumulation was consistent. This suggests that the climatic conditions associated with productive peat accumulation were also conducive to increased canopy cover and fire spread. Analysis of multiple proxies from the SFP sediments provides insights into our primary research questions.

• Stability of the SFP: The long-term paleoenvironmental record from the SFP suggests this low latitude fen palsa system experienced at least two periods of instability, when high seasonality and/or desiccation and possible increase in surface fires led to degredation of the fen palsa.
• Frequency and character of fire activity: High-severity fire activity was pronounced during the early- and mid-Holocene, whereas fire activity was likely obscured by partial loss of the sediment record during periods of instability of the SFP system. Desiccation and/or surface fires likely degraded or consumed portions of the peat surface, complicating the reconstruction of a complete fire and environmental record, as indicated by slow sedimentation rates, ca. 6000–8000 and 5000–0 cal yrs BP.
• Vegetation response to climate and fire: Vegetation was dominated by alpine conifers, sagebrush grasses and sedges and exhibited relatively stable response to climate variability and fire activity. Forest opening and increases in sagebrush and grasses were most evident during the high seasonality of the early- to mid-Holocene transition.
• Recent changes: Consistent with patterns observed in higher-latitude permafrost peatlands, recent decades of rapid warming have accelerated SFP degradation. Continued warming is likely to result in system collapse within decades. The loss of the SFP and change in the climate regime responsible for its formation will likely have far-reaching consequences for ecosystem dynamics, disturbance regimes, carbon storage and nutrient cycling in alpine environments throughout the Rocky Mountain cordillera.

Evaluation of the multiproxy paleoenvironmental record from the SFP site offers a rare, long-term perspective on alpine fire–climate interactions. High-severity fire activity was a common feature in this high-alpine watershed during the early- and mid-Holocene. Instability of the SFP system highlights the vulnerability of low-latitude permafrost sites to climate warming. Although the SFP represents a relatively rare feature in lower-latitude alpine settings, monitoring the rate and character of permafrost loss will be important for anticipating cascading effects of a warming climate on carbon storage, hydrology, fire activity, and ecological dynamics in high-alpine environments.