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Article

Using Paleoecological Methods to Study Long-Term Disturbance Patterns in High-Elevation Whitebark Pine Ecosystems

by
Jordin Hartley
,
Jennifer Watt
* and
Andrea Brunelle
*
School of Environment, Society, and Sustainability, University of Utah, Salt Lake City, UT 84112, USA
*
Authors to whom correspondence should be addressed.
Fire 2024, 7(11), 411; https://doi.org/10.3390/fire7110411
Submission received: 1 October 2024 / Revised: 30 October 2024 / Accepted: 9 November 2024 / Published: 12 November 2024
(This article belongs to the Special Issue Effects of Fires on Forest Ecosystems)
Browse Figures
Figure 1
<p>Detailed location of Phyllis Lake, Idaho, USA. Elevation 2800 m.</p> "> Figure 2
<p>The <span class="html-italic">x</span>-axis represents time, from 0 cal y BP (1950 CE) to ~8200 cal y BP, and is divided into four zones: the gray bars indicate the periods of time with fire episodes, the Mazama Ash layer is indicated by the vertical gray line at ~7790 cal y BP. (<b>a</b>) Ratio of arboreal to nonarboreal pollen. (<b>b</b>) Fire episodes with <span class="html-italic">p</span> values of &lt;0.05. (<b>c</b>) Fire activity, highlighted by the two time periods in gray bars that include the fire episodes. (<b>d</b>) Total pollen accumulation rate (PAR), <span class="html-italic">Pinus</span> pollen, stacked to demonstrate the abundance of <span class="html-italic">Pinus</span> at Phyllis Lake.</p> "> Figure 3
<p>The <span class="html-italic">x</span>-axis represents time from when the core was collected in 2017 to 1950 CE and encompasses Zone 5 from the text: (<b>a</b>–<b>g</b>) PAR values for taxa discussed in the text. (<b>h</b>) AP:NAP (<b>i</b>) Fire episodes with <span class="html-italic">p</span> values of &lt;0.05. (<b>j</b>) Fire activity. (<b>k</b>) Total pollen accumulation rate (PAR), <span class="html-italic">Pinus</span> pollen, stacked to demonstrate the abundance of <span class="html-italic">Pinus</span> at Phyllis Lake.</p> "> Figure 4
<p>The <span class="html-italic">x</span>-axis represents time, from 0 cal y BP (1950 CE) to ~8200 cal y BP, and is divided into four zones. The gray bars indicate the periods of time with fire episodes, the Mazama Ash layer is indicated by the vertical gray line at ~7790 cal y BP: (<b>a</b>–<b>g</b>) PAR values for taxa discussed in the text. (<b>h</b>) Fire episodes with <span class="html-italic">p</span> values of &lt;0.05. (<b>i</b>) Total pollen accumulation rate (PAR), <span class="html-italic">Pinus</span> pollen, stacked to demonstrate the abundance of <span class="html-italic">Pinus</span> at Phyllis Lake.</p> ">
Versions Notes

Abstract

:
Pinus albicaulis (whitebark pine) is a keystone species, providing food and habitat to wildlife, in high-elevation ecological communities. In recent years, this important species has been negatively impacted by changes in fire regimes, increased Dendroctonus ponderosae (mountain pine beetle) outbreaks associated with human landscape and climate modification, and the continued impact of the non-native Cronartium ribicola (white pine blister rust). This research investigates changes in fire occurrence, the establishment of Pinus albicaulis, and fuel availability at a high-elevation site in the Sawtooth National Recreation Area, Idaho, USA. Charcoal and pollen analyses were used to reconstruct fire and vegetation patterns for Phyllis Lake, Idaho, USA, over the past ~8200 cal y BP. We found that significant fire episodes occurred when the pollen accumulation rates (PARs) indicated more arboreal fuel availability, and we identified that Pinus albicaulis became well established at the site ~7200 cal y BP. The high-elevation nature of Phyllis Lake (2800 m) makes this record unique, as there are not many paleorecords at this high elevation from the Northern Rocky Mountains, USA. Additional high-elevation sites in Pinus albicaulis habitats will provide critical insight into the long-term dynamics of this threatened species.

1. Introduction

It is widely accepted that the combination of climate change and disturbance (fire, insect outbreaks, and pathogens) are disproportionately impacting Pinus albicaulis (whitebark pine) ecosystems [1,2,3]. In some regions of the Northern Rocky Mountains, Pinus albicaulis populations have experienced up to 90% mortality in as little as 30 years from Dendroctonus ponderosae (mountain pine beetle) outbreaks [4,5,6,7]. This rate of mortality threatens the survival of one of western North America’s most iconic keystone species. Some of the ecosystem services provided by Pinus albicaulis include its value as a food source for wildlife, its potential for wildlife habitat provision, and its role in the impediment of snowmelt [8].
Pinus albicaulis plays an important role in ecological community development following disturbance events due to its ability to regenerate on dry, cold, high-elevation sites where regeneration of other species is slowed by such harsh conditions. The relatively low shade tolerance of Pinus albicaulis allows natural regeneration to occur preferentially after disturbance by avalanche, glacial retreat, and, most importantly, fire [8,9,10,11]. The average fire return interval in Pinus albicaulis ecosystems is 30 to 300 years [12], and high elevations can support stable communities of Pinus albicaulis for more than 1000 years [13]. As a keystone species, the loss of Pinus albicaulis on even local scales could lead to devastating consequences for the ecological communities it inhabits due to soil erosion, reductions in watershed regulation, alterations in successional pathways of vegetation, and loss of biodiversity due to decreased carrying capacity [14].
Fire histories are of great relevance to the Northern Rocky Mountains because it has long been recognized that the compositional vegetation dynamics of the region have been heavily influenced by fire for at least the past several hundred years. Fire exclusion management practices enacted in the 1930s have reduced the area with conditions suitable for Pinus albicaulis regeneration, and, until somewhat recently, detailed information on characteristics of fire history during the centuries preceding the fire-suppression era have been sparsely available [15]. In absence of fire, Pinus albicaulis is often replaced by more shade-tolerant conifers such as Abies lasiocarpa (subalpine fir), Picea engelmannii (Engelmann spruce), and Tsuga mertensiana (mountain hemlock) [16]. Understanding millennial-scale fire and vegetation change before the more recent onset of human-induced climate change will provide insight into how Pinus albicaulis responded to fire in the past. Land managers and scientists use environmental and ecological records to understand the variability of ecological systems during times when they were less influenced by humans, which is useful in providing a baseline for determining land management goals. Fire histories are especially valuable to land management because they provide detailed information about the role of fire in shaping vegetational compositions [17]. The insight gained from this research will inform land managers in assessing ecosystem risk for additional disturbances (i.e., mountain pine beetle and white pine blister rust) currently threatening Pinus albicaulis.
This study investigates changes in vegetation as a response to climate and fire disturbance at Phyllis Lake, Idaho, USA (Figure 1). Charcoal and pollen proxies recovered from lake sediments are used to reconstruct vegetation and fire episodes since ~8200 cal y BP. The objectives of this study are (1) to identify when fires occurred in the watershed and analyze impacts on vegetation assemblages; (2) to examine the role fuel connectivity plays in fire regime dynamics; and (3) to determine when Pinus albicaulis established itself as the dominant arboreal vegetation type at this location. It is hypothesized that (1) fire frequency is influenced by the type of fuels available, and (2) climate change during the Anthropocene has impacted vegetation and fire patterns at this high-elevation Pinus albicaulis site.

2. Materials and Methods

Phyllis Lake is a glacial subalpine lake located in the Salmon River drainage of the White Cloud Mountains, Custer County, Idaho, USA (Figure 1). It occupies an area of 29,542 m2 at 44.022839° N 114.648798° W, elevation 2800 m (9200 ft), and its maximum dimensions are 270 m × 158 m. The current vegetation regime immediately surrounding the shore of Phyllis Lake consists of Pinus albicaulis, Abies lasiocarpa, Picea engelmannii, and a variety of grasses, herbs, and forbs. The arboreal vegetation at Phyllis Lake is dominated almost exclusively by Pinus albicaulis. Though Pinus albicaulis is the dominant conifer at Phyllis Lake today, its abundance decreases rapidly with descending elevation as it begins to share dominance with other conifers. Pinus albicaulis environments are characterized by wet winters and dry summers [18,19], with the main water source during the summer months being stored soil moisture from snowmelt.
Two overlapping sediment cores were recovered from Phyllis Lake, Idaho, USA, in July 2017. Together, these cores extended from the water–sediment interface to a depth of 252 cm. The short sediment core (0–93 cm (PL17-A)) was extracted using a Klein coring device (7 cm diameter), while the long sediment core (82–252 cm (PL17-B)) was extracted using a modified Livingstone piston corer (5 cm diameter) allowing ~10 cm of overlap between the two cores, which were combined and used as a composite core for the remainder of the study. The short core was cut and subsampled in the field, with each 1 cm of sediment being placed into plastic Whirl-Pak bags. The long core, which was collected in 1 m segments, was wrapped in plastic wrap and aluminum foil, and placed into a wooden core box for transport to the University of Utah, School of Environment, Society, and Sustainability, Records of Environmental Disturbance (RED) lab in Salt Lake City, Utah, USA, where it was placed in cold storage. The long core was subsequently cut into 1 cm increments and placed into plastic Whirl-Pak bags.
Every other cm from the top 40 cm were sent to Dr. Michael Ketterer at Northern Arizona University for Pu analysis using magnetic sector inductively coupled plasma mass spectrometry. Peak Pu activities in sediments occur as a result of fallout from atmospheric testing of nuclear weapons in 1963/1964 [20]. 240Pu/239Pu ratios placed the plutonium peak (−13 BP or 1963 CE) at 17.5 cm, which was used as a tie point when establishing the age model for this study (Table 1). Additionally, two pollen samples [21] were sent to the Center for Applied Isotope Studies in Georgia, USA for 14C accelerator mass spectrometry (AMS) dating. Samples containing abundances of suspected volcanic ash were sent to geochemical analyst Dr. Barbara Nash, Director of the Electron Microprobe Laboratory, Department of Geology and Geophysics, University of Utah. Samples were successfully identified as Mt. Mazama tephra using an electron microprobe. Electron microprobe analyses were performed at the University of Utah on a Cameca SX-50 electron microprobe using Probe for EPMA software. Analytical conditions were 15 kV accelerating voltage, 25 nA beam current, and a beam diameter of 5 mm. Matrix corrections were made with a fr-z (PAP) correction algorithm [22]. The accepted date for the Mt. Mazama eruption, as provided by geochemical analysis of ash recovered from the Greenland Ice Sheet Project 2 (GISP2) ice core, is 7627 ± 150 cal y BP [23]. All of these dates were then utilized to generate an age model using the CLAM package in R 3.6.1 [24]. This study uses the original age model (smoothing spline) as some of this work has already been published as a master’s thesis [25]. Supplementary Figure S1 is based on the following table.
The 1 cm interval samples were then subsampled for 5 cm3 of sediment and sieved for plant and charcoal macrofossil analysis, ranging from >125 µm to >250 µm in size. The methods described in [26] were used, with the exception that due to the unconsolidated nature of the sediment, sodium hexametaphosphate was not used. Macroscopic charcoal counts were then used to reconstruct local fire episodes in the watershed, and the resulting fire history is plotted as charcoal influx [26,27]. Many iterations of CharAnalysis were run in MATLAB R2016b [28], using various parameters, to attempt the identification of peaks in charcoal. However, none of these iterations flagged fire episodes that are known to have occurred during the last century [29], so these results are not included in the analysis. The following method was used to identify significant fire episodes in the charcoal data: z-scores (Z = (x − μ)/σ) were calculated for every 1 cm of charcoal data and corresponding p values were identified using a one-tailed test. Unlike the many iterations that were run using CharAnalysis, this method succeeded in identifying known historical fires that occurred during the last century, rendering this method more reliable than CharAnalysis for determining which episodes qualify as significant throughout the Phyllis Lake record. P values of <0.05 are considered to be indicative of statistically significant deviations from the mean and are herein used to identify significant increases in the charcoal data, therefore indicating significant fire episodes.
One cm3 of sediment from the entire upper 24 cm and every 4 cm from the remaining depths were processed using standard methods for pollen processing [30]. The collection of pollen reference slides at the University of Utah RED lab were used in conjunction with Kapp’s Pollen and Spores textbook [21] to aid the identification of pollen taxa. Given that elevation maintains a negative relationship to sedimentation rates [31], and given the high elevation of Phyllis Lake, the top 24 cm were analyzed consecutively for pollen to achieve a very finely resolved record. This sampling strategy resulted in high temporal resolution for pollen data from the top 24 cm, which represents between 1 and 16 years per cm. The temporal sampling resolution of the remaining 228 cm (every 4 cm) ranges from ~80 to ~220 years per sample. The pollen data were then used to calculate pollen accumulation rates. Pollen accumulation rates (PARs) are valuable when using fossil pollen from sediment cores to draw interpretations about vegetation histories. PARs can be estimated if both sedimentation rate and the number of pollen grains/cm2 are known [32]. PARs are used herein to reconstruct the vegetation history of Phyllis Lake.
Ratios of environmentally significant pollen data were also used to interpret the pollen record. By comparing ratios of two different pollen types, it is possible to reveal changes in vegetation that are not evident in traditional pollen percentage data [33]. Ratios of arboreal to nonarboreal PARs are used to draw interpretations about opening and closure of the forest canopy. Higher ratios of arboreal pollen indicate a relatively closed canopy system, whereas higher ratios of nonarboreal pollen indicate a relatively open canopy system (Figure 2 and Figure 3).

3. Results

Of the total terrestrial pollen grains counted, 61% were Pinus, including Pinus Haploxylon (Hap), Pinus Diploxylon (Dip), and Pinus Undifferentiated. A total of 22% of all Pinus grains were identifiable as either Hap or Dip, and of those differentiable Pinus grains, 79% were identified as Pinus Hap and 21% were identified as Pinus Dip. Though the percentage of differentiable Pinus grains is relatively low, abundances of Pinus Hap grains are consistently dominant over Pinus Dip grains. For these reasons, the combination of Pinus pollen is used as an estimate of Pinus albicaulis abundance.
The Phyllis Lake record spans the Holocene and is therefore divided into five zones for ease of interpretation. Zones 1–4 span the Holocene, and Zone 5 encompasses what has been coined the Anthropocene (1950current CE). Zone 5 also has higher PARs, due to the continuous sampling interval, lack of compression in this portion of the sediment core, and relatively high sedimentation rates. Dates for Zone 5 are given in CE to ease the comparison between the data from this study and modern fire records (Figure 3). The base of the core consists of glacial till, indicating its development following deglaciation at this high-elevation site.

3.1. Zone 1 (8177–7000 cal y BP)

Zone 1 includes the earliest portion of the Phyllis Lake record, which begins at ~8177 cal y BP, during or perhaps just after the 8200 y event, a cooling event that occurred between 8250 and 8150 cal y BP [34]. Zone 1 also includes the Mt. Mazama tephra, which was deposited after its eruption at 7627 ± 150 cal y BP [23]. Fire activity is low throughout the entirety of Zone 1, with no occurrence of significant charcoal episodes (Figure 2).
PARs for all pollen types other than Alnus (alder) are relatively low throughout Zone 1 (Figure 4), and the total PAR declines to nearly 500 grains/cm2/at ~7740 cal y BP, which is the second lowest point for the total PAR on the Phyllis Lake record (Figure 4). After this occurrence of very low total PAR at ~7740 cal y BP, the total PAR increases for the remainder of Zone 1. Pinus reaches its lowest level on record at ~7740 cal y BP. The total PAR and PAR for individual taxa exhibit a general upward trend for the remainder of Zone 1 (Figure 4). More specifically, Picea and Abies both show notably high abundances toward the end of this zone. Alnus shows an upward trend throughout the majority of Zone 1, reaching its highest peak that is present in the first four zones at ~7335 cal y BP. The Other Asteraceae PAR also reaches a peak at ~7335 cal y BP, despite a generally low PAR throughout Zone 1.
The AP:NAP reaches a record low at ~7740 cal y BP. After ~7740 cal y BP, the AP:NAP exhibits an upward trend for the remainder of Zone 1 (Figure 2).

3.2. Zone 2 (7000–5200 cal y BP)

Zone 2 contains a series of seven significant charcoal episodes, including the two most significant charcoal episodes on the Phyllis Lake record (Figure 2). The largest charcoal episode on record occurs near the beginning of Zone 2 at ~6890 cal y BP (Z = 6, p < 0.00001) (Figure 2). Two additional significant charcoal episodes occur at ~5840 cal y BP (Z = 4.75, p < 0.00001) and ~5810 cal y BP (Z = 2.2, p < 0.0135). Due to resuspension, redeposition, and in wash of material from burned slopes, charcoal incorporation into deep water sediments can lag for at least 5 years following a fire episode [27]. Given the close temporal proximity of the episodes at ~5810 and ~5840 cal y BP, they are treated as one episode at ~5840 BP, since it is the older of the two and marks the base of charcoal deposition following the fire episode. The remaining significant fire episodes during Zone 2 occur at ~5480 cal y BP (Z = 2, p < 0.022), ~5430 cal y BP (Z = 2.1, p < 0.017), ~5330 cal y BP (Z = 2, p < 0.0195), and ~5230 cal y BP (Z = 1.6, p < 0.05).
Zone 2 begins with an abrupt decrease in Picea, Abies, and Amaranthaceae PARs, all of which had been exhibiting substantial increases toward the end of Zone 1 (Figure 4). The Artemisia PAR exhibits a similar decline throughout the beginning of Zone 2, albeit more gradual and less pronounced than what is shown by Picea, Abies, and Amaranthaceae PARs. Between ~6500 cal y BP and the end of Zone 2, Pinus, Abies, Picea, and Artemisia PARs all show a general upward trend. However, this trend is punctuated by drops in the PARs of these four taxa. These fluctuations are most pronounced in Abies, Picea, and Artemisia PARs, while the Pinus PAR shows marked stability. Perhaps the most notable decrease in PARs during this time is the sharp decrease in Picea and Abies at ~5720 cal y BP, which is followed by a decrease in Artemisia and Other Asteraceae PARs at ~5615 cal y BP. The total PAR exhibits a general upward trend throughout Zone 2, though this trend is punctuated by brief decreases, most notably a sustained decrease between ~5840 and ~5500 cal y BP, where the total PAR remained relatively low for several hundred years (Figure 2 and Figure 4).
The AP:NAP begins relatively low at the base of Zone 2 but exhibits a general upward trend throughout the remainder of the zone (Figure 2). However, this trend is punctuated by brief decreases in the AP:NAP. The lowest level of the AP:NAP for Zone 2 occurs at ~6650 cal y BP, and is the second lowest point for the AP:NAP on the Phyllis Lake record, surpassed only by the record minimum for the AP:NAP that occurred during Zone 1. Another more abrupt and dramatic decrease occurs at ~5730 cal y BP, where the AP:NAP sees its third lowest level on the Phyllis Lake record (Figure 2).

3.3. Zone 3 (5200–1000 cal y BP)

Charcoal background is low throughout Zone 3, and no significant fire episodes are recorded during this time (Figure 2).
All pollen types and the total PAR generally exhibit downward trends between ~5200 and ~2470 cal y BP (Figure 4). The total PAR reaches a record minimum at ~2470 cal y BP. Other than a peak in the Pinus PAR at ~5010 cal y BP, arboreal pollen types, including Pinus, Abies, and Picea, tend to exhibit a decline throughout this zone in general. As arboreal pollen types are trending downward throughout Zone 3, nonarboreal pollen types exhibit this trend to a lesser degree (Figure 2). Contrary to the decrease in many pollen types throughout the Zone 2/Zone 3 transition, Poaceae PAR steadily increases from the beginning of Zone 3 until ~4620 cal y BP, where it exhibits its highest levels since ~6800 cal y BP, before dropping abruptly and remaining relatively low for the remainder of Zone 3 (Figure 4).
After reaching its lowest levels on record at ~2470 cal y BP, the total PAR increases gradually between ~2470 and ~1700 cal y BP, at which point it sharply decreases from >1200 grains/cm/y to <700 grains/cm/y in less than 200 years between ~1700 and ~1500 cal y BP (Figure 4). Between ~1500 and ~1300 cal y BP there is an abrupt increase in many PARs, including Pinus, Abies, Artemisia, and Other Asteraceae (Figure 4). This prominent increase in PARs is especially evident in the total PAR, which increases from <700 grains/cm/y to >1850 grains/cm/y (Figure 2 and Figure 4). PARs of Pinus, Abies, Artemisia, and Other Asteraceae then rapidly decrease for the remainder of Zone 3 at ~1000 cal y BP. This rapid decrease is also evident in the total PAR (Figure 2 and Figure 4).
The AP:NAP reaches its highest point on record thus far at ~5010 cal y BP, before sharply declining to very low levels at ~4600 cal y BP (Figure 2). The AP:NAP then shows a general upward trend until ~3500 cal y BP, though sharp fluctuations between ~4600 and ~4200 cal y BP are superimposed on this general upward trend. The AP:NAP fluctuates from ~3500 to ~2000 cal y BP but shows no discernable upward or downward trend. It then shows a slight upward trend between ~2000 and ~1000 cal y BP.

3.4. Zone 4 ((1000–0 cal y BP (1950 CE))

Zone 4 is characterized by a series of five significant fire episodes occurring between ~790 and ~350 cal y BP, with three of them occurring in continuous samples between ~790 and ~710 cal y BP. These episodes occur at ~790 cal y BP (Z = 2.38, p < 0.009), ~750 cal y BP (Z = 4.4, p < 0.00001), ~710 cal y BP (Z = 3.6, p < 0.00015), ~590 cal y BP (Z = 2, p < 0.0185), and ~360 cal y BP (Z = 1.86, p < 0.0315) (Figure 2).
The beginning of Zone 4 has relatively low PARs for all pollen types, but then most PARs, including Pinus, Abies, Artemisia, Other Asteraceae, Amaranthaceae, and Poaceae, increase for the majority of Zone 4, though in the cases of Abies, Artemisia, Other Asteraceae, and Amaranthaceae, this general upward trend is punctuated by a decrease in PARs at ~490 cal y BP (Figure 4). The total PAR is <800 grains/cm/y at ~950 cal y BP and then gradually increases, peaking at ~240 cal y BP before declining again (Figure 2 and Figure 4). The AP:NAP trends upward until ~500 cal y BP when it begins to decline for the remainder of Zone 4 (Figure 2).

3.5. Zone 5 (1950–2017 CE)

A significant fire episode (Z = 3.5, p <0.001) occurs at ~1995 CE, and the most recent significant fire episode in the Phyllis Lake record occurs at ~2011 CE (Z = 2.5, p < 0.0062) (Figure 3).
Total PAR remains relatively low throughout the majority of Zone 5, until ~1980 CE, when it increases (Figure 3). This increase is especially visible in the PARs of arboreal types Pinus, Abies, Picea, and the nonarboreal pollen type Artemisia. At ~1990 CE, the total PAR reaches its highest peak in the Phyllis Lake record, exceeding 12,500 grains/cm/y. Pinus, Artemisia, and Poaceae PARs also reach their highest peaks on record during this time. After this peak in the total PAR and the three aforementioned taxa (Figure 3), PARs trend downward for the remainder of the record. Picea, Other Asteraceae, and Amaranthaceae PARs all begin to exhibit upward trends as the record terminates in 2017. The AP:NAP fluctuates sporadically throughout Zone 5, with no discernable trends occurring (Figure 3).

4. Discussion

This research contributes a unique high-elevation (2800 m) Pinus albicaulis-dominated record to the existing paleoecological sites for the Northern Rockies, USA. Climate and fuel availability were the drivers behind fire episodes, and this knowledge will help to plan for management of this newly listed species. The record is described below by zone for a more detailed discussion of the findings.

4.1. Zone 1 (8177–7000 cal yrBP)

The development of Phyllis Lake at ~8200 cal y BP occurs following the 8200 y event, which is considered one of the most abrupt shifts in climate during the Holocene [35]; it was likely influenced by changes in North Atlantic Deep Water (NADW) circulation, which affected regions far beyond the North Atlantic [36]. It is often associated with cold conditions in western North America [37,38,39].
The duration of the 8200 y event is estimated to be limited to only 100–200 years, though ecosystem responses at many sites suggest a prolonged response [34]. The pollen data from Phyllis Lake suggest a prolonged response to the 8200 y event at this location, given that the temporal resolution between pollen samples at this location in the Phyllis Lake record is >200 years and that the Alnus PAR exhibits continuous increase until ~7335 cal y BP (Figure 4), suggesting cold and effectively wet conditions long after the 8200 y event would have been resolved.
Although the 8200 y event is often associated with a drought event in central Idaho, USA [33,40], the Alnus PAR continues to exhibit unperturbed growth between ~8177 and ~7335 cal y BP (Figure 4), despite its high moisture requirements. This suggests that moisture availability was not a limiting factor for proliferation of Alnus at this site during this time. Even if the 8200 y event is associated with drought in central Idaho, USA, high Alnus abundances at Phyllis Lake suggest a return to moist conditions between ~8177 and ~7335 cal y BP. The Alnus PAR peaks at ~7335 before declining abruptly for the remainder of Zone 1 as Picea and Abies PARs exhibit rapid growth (Figure 4), suggesting the persistence of cold, wet conditions between the termination of the 8200 y event and the end of Zone 1 at ~7000 cal y BP.
The extreme cold conditions of the 8200 y event could explain the low PARs during the earliest part of Zone 1. However, extremely low PARs for many taxa and the total PAR at ~7740 cal y BP are best explained by the Mt. Mazama eruption that occurred at 7627 ± 150 cal y BP [23] (Figure 2 and Figure 4). The Pinus PAR reaches its lowest point on the Phyllis Lake record during this time. The ash fall from this large volcanic eruption covered more than 1,000,000 km2 of western North America and caused climatic cooling on the scale of 0.6–0.7 °C, which would have been especially pronounced at mid to high northern latitudes [41]. Peak cooling would have occurred in the second month after the eruption, though cooling effects lasted several years [42]. Oetelaar and Beaudoin [41] hypothesize that tephra deposition from this large volcanic eruption would also have reduced the ability for trees and shrubs to photosynthesize, adding more stress to vegetation in addition to climatic cooling, and that a massive dark cloud would have preceded the ash fall. Given that Pinus albicaulis has a relatively low shade tolerance [13], the combination of darkened skies and a thick layer of ash fall would have had a devastating impact on the Pinus albicaulis population at Phyllis Lake. Therefore, it can be inferred that ecosystem disturbance from the Mt. Mazama eruption is the most explicable cause of this record minimum in the Pinus PAR at Phyllis Lake. Furthermore, though more shade-tolerant conifers such as Abies and Picea exhibit low PARs during this sample and also experienced negative impacts from this disturbance, Pinus is the only taxon that exhibits a record minimum due to the Mt. Mazama eruption (Figure 4). The Alnus PAR trends upward from the base of the record at ~8177 cal y BP, and continues to increase through the Mt. Mazama disturbance, possibly due in part to the elimination of competitor taxa. Furthermore, Alnus is recognized as an indicator of disturbance and maintains an important role as an early seral species for recently disturbed areas due to its ability to fix atmospheric nitrogen [43]. Although Alnus, Abies, and Picea all tolerate cold and wet conditions, Alnus was eventually outcompeted as Picea and Abies recovered from the Mt. Mazama disturbance and established themselves as important vegetational components of this system (Figure 4).
The overall negative impacts of the Mt. Mazama eruption on arboreal species can be seen clearly in the AP:NAP ratio with a record minimum in the AP:NAP at ~7740 cal y BP (Figure 2). It is also supported by the lack of impact the tephra deposit had on Amaranthaceae, Poaceae, and Alnus PARs (Figure 4). Naturally, vegetation at Phyllis Lake begins to recover after this disturbance, though the timing and degree of this recovery varies among taxa. The simultaneous peaks in Other Asteraceae and Alnus PARs at ~7335 cal y BP (Figure 4) suggest effectively wet conditions and a relatively open canopy system. A relatively open canopy system is also reflected by low AP:NAP at ~7335 (Figure 2). After ~7335 cal y BP, Other Asteraceae and Alnus PARs decline as Abies and Picea PARs rapidly increase (Figure 4), indicating that Abies and Picea likely outcompeted the Other Asteraceae and Alnus vegetation types as the relatively open canopy system began to close. This high abundance of Abies and Picea still represents cold, wet climatic conditions, and that a Picea/Abies forest had been established by the end of Zone 1.
Charcoal levels are low throughout the entirety of Zone 1, indicating low fire activity, and no significant fire episodes occur (Figure 2). Low fire activity would have been conducive to proliferation of the fire-sensitive taxa Abies and Picea, both of which reach relatively high PARs near the end of Zone 1. Although the Pinus PAR is numerically much higher than that of Abies or Picea toward the end of Zone 1 (Figure 4), it should not be assumed that Pinus was necessarily more abundant than Abies or Picea in the immediate vicinity of Phyllis Lake during this time. Although all anemophilous conifers produce high abundances of pollen [44], Picea does not contribute significantly to long-distance pollen transport [45]. This is due to its weight and higher fall speed [44]. Abies pollen is also much larger than Pinus pollen [46] and thus does not travel as far. On the contrary, Pinus is very easily transported due to its light nature [47] and is, therefore, very well represented in the pollen spectra [44]. For these reasons, it can be deduced that Abies and Picea were present in relatively high abundances within the immediate vicinity of Phyllis Lake between ~7500 and ~7000 cal y BP. Therefore, the relatively high abundances of Abies and Picea pollen toward the end of Zone 1 are reliable indicators of vegetation and climate at Phyllis Lake during this time.

4.2. Zone 2 (7000 BP–5200 BP)

High levels of Abies and Picea occur during the Zone 1/Zone 2 transition, reflecting cold, wet conditions (Figure 4). Records from interior western North America often suggest wet conditions between ~7400 and ~6600 cal y BP [48,49,50,51]. Such wet conditions would have been conducive to arboreal fuel accumulation. Charcoal influx levels are representative of the amount of arboreal fuel biomass available, and this in turn is related to the amount of forest cover as well as fire severity [52]. High abundances of Abies and Picea provide a plentiful arboreal fuel source for the catastrophic fire episode at ~6890 cal y BP (Figure 4). This interpretation is consistent with the high accumulation of arboreal fuels that is reflected in the AP:NAP toward the end of Zone 1 (Figure 2). The AP:NAP also drops to a very low point between ~6890 and ~6650 cal y BP (Figure 2), as nonarboreal pollen types Poaceae and Alnus increase (Figure 4). This sharp decrease in Abies and Picea PARs and the AP:NAP ratio is due to loss of arboreal vegetation as a result of the ~6890 cal y BP fire. Abies and Picea PARs begin to decrease slightly during the Zone 1/Zone 2 transition and before the fire of ~6890 cal y BP, suggesting that conditions were growing relatively drier. Sufficient drying of the arboreal fuels provided by the Abies/Picea forest that had established itself by the end of Zone 1 led to conditions ideal for the catastrophic fire episode of ~6890 cal y BP.
Alnus is an important indicator of disturbance and plays an integral role in ecosystem succession following disturbances such as fire [53,54]. Both Alnus and Poaceae gain abundance following the fire disturbance at ~6890 cal y BP, taking advantage of the open canopy system that had resulted from this disturbance (Figure 4). This pattern of Poaceae increasing after a significant fire episode, or after a series of significant fire episodes, is evident in other portions of the Phyllis Lake record (Figure 2 and Figure 4). It is worth noting that high levels of Poaceae occur both before and after the ~6890 cal y BP fire episode. These high levels of Poaceae near ~7150 and ~6810 cal y BP suggest that Poaceae played a role in fuel connectivity before the ~6890 cal y BP fire episode and that Poaceae quickly reestablished itself after the burn, as it took advantage of the recently opened canopy.
Fuel connectivity is one of the main drivers of fire patterns in the Northern Rocky Mountains [55], and the Pinus albicaulis fire regime is characterized mostly by small spot fires due to cool, moist conditions and lack of fuel connectivity [56]. However, Poaceae likely played an important role in fuel connectivity leading up to significant fire episodes and could explain the mixed-severity fire regime that is characteristic of Pinus albicaulis sites. In other words, the absence of fine fuels will often result in small spot fires due to fuel disconnection, whereas in the presence of fine fuels, significant stand-replacing fires are more likely to occur. Increases in Poaceae leading up to significant fire episodes is discernible in other portions of the Phyllis Lake record (Figure 2 and Figure 4).
The clear increase in the total PAR, AP:NAP, and fire frequency throughout Zone 2 suggests effectively wet conditions that would permit arboreal fuel accumulation. This increase in arboreal vegetation would have provided fuel for the ~5840 cal y BP fire, which had noteworthy impacts on many PARs, the total PAR, and the AP:NAP. The Poaceae PAR increases before this large fire episode (Figure 2 and Figure 4), supporting the interpretation that fine fuels played an important role in fuel connectivity leading up to stand-replacing fires. Though Pinus shows a mild decline after the fire episode at ~5840 cal y BP, this decline is far less pronounced than the decrease shown by Abies and Picea (Figure 4). This is probably due to the combination of both Abies and Picea being fire-sensitive taxa [57] and the tendency of Nucifraga columbiana (Clark’s Nutcracker) to cache Pinus albicaulis seeds in recently disturbed settings [58], which permits regeneration after stand-replacing fires.
The total PAR abruptly declines after the ~5840 cal y BP fire (Figure 2). Additionally, given Pinus albicaulis’s low shade tolerance [13], the elimination of competition for sunlight from Abies and Picea would have been conducive to Pinus albicaulis regeneration and survival after this disturbance. The total PAR begins to recover after the ~5840 BP fire, before declining steadily through the next series of fire episodes at ~5480, ~5430, ~5330, and ~5230 cal y BP (Figure 2). This increase in fire activity could be attributed to insolation-induced aridity that occurred between ~7000 and ~5000 cal y BP, which was particularly pronounced at high-elevation sites [55]. Poaceae again exhibits an upward trend leading up to this series of fire episodes (Figure 4), playing a role in fuel connectivity. The AP:NAP ratio trends upward between ~5840 cal y BP and the end of Zone 2 at ~5200 cal y BP (Figure 2). This upward trend in arboreal pollen abundance and the corresponding series of fire episodes during this time further support the interpretation that the fire regime in this region is largely controlled by the availability of fuel provided by arboreal vegetation.

4.3. Zone 3 (5200–1000 cal y BP)

The total PAR and PAR for individual pollen taxa begin a downward trend for the first several hundred years of Zone 3, with the exception of Pinus. The Pinus PAR experiences its highest peak on record thus far at ~5010 cal y BP, coincident with the highest peak on record thus far for the AP:NAP, suggesting that this ratio is largely controlled by Pinus abundances. Increasing abundances of Poaceae between ~5010 and ~4620 cal y BP may be explained by the tendency for Poaceae to gain abundance following significant, stand-replacing fire episodes such as the ones occurring toward the end of Zone 2. This would have allowed varieties of Poaceae to thrive in earlier stages of ecological succession. These high Poaceae abundances are accompanied by sustained fires between ~5200 BP and ~4500 BP. However, none of these fires pass the one-tailed p value test and are likely representative of surface fires fueled by high Poaceae abundances. A similar pattern is followed by Amaranthaceae between ~5010 and ~4620 cal y BP. Rising abundances of Poaceae and Amaranthaceae are suggestive of relatively dry conditions.
The total PAR trends downward after ~5200 cal y BP and reaches its lowest point on record at ~2470 cal y BP, likely due to large scale and widespread drought. Doerner and Carrara [33] define the Early to Middle Holocene as the period from ~9800 to ~3200 cal y BP, and describe a west-central Idaho, USA, record that is indicative of warm, dry conditions during this time. This portion of their record is also characterized by a significant decline in PARs, much like the record from Phyllis Lake. Other Northern Rocky Mountain records from Lost Trail Pass Bog, Montana, USA [32], Mary’s Frog Pond, Montana, USA [59], and Burnt Knob Lake, Idaho, USA [60], suggest that late summer and winter temperatures during the Middle Holocene were warmer than today [60].
An abrupt shift in climate at ~4500 cal y BP, suggestive of a pivotal change in the climate system, occurs in numerous central North American paleoclimatic records, and this shift is associated with severe, widespread drought [61,62]. Drought and warm temperatures characteristic of the mid-Holocene are reflected at Phyllis Lake by the low total PAR between ~5200 and ~2470 cal y BP (Figure 2). Furthermore, the authors of [61] describe a megadrought more severe and persistent than any of the past millennium occurring on all Northern Hemisphere continents within one or two centuries of ~4200 BP.
The lack of significant, large-scale fire episodes throughout Zone 3 (Figure 2) can be attributed to fuel scarcity, which is reflected by the low total PAR and decreases in the AP:NAP ratio throughout this zone (Figure 2). Low fire activity that is coincident with low PARs and relative decreases in arboreal taxa lend further support to the interpretation that the fire regime at Phyllis Lake is influenced largely by fuel provision from arboreal vegetation types. Although there are no significant fire episodes during this zone that pass the one-tailed p value test used in this analysis, the constant presence of low levels of charcoal indicate that low-severity fires continued to burn throughout this time.
After the downward trend of most pollen taxa and the total PAR between ~5200 and ~2470 cal y BP, all pollen taxa and the total PAR show a general upward trend between ~2470 and ~1700 cal y BP before abruptly declining between ~1700 and ~1500 cal y BP (Figure 4). This brief decline in PARs may be suggestive of a brief return to mid-Holocene conditions. This sudden and notable decrease is most prominent in Pinus, Abies, and Artemisia PARs. All PARs and the total PAR then abruptly increase between ~1500 and ~1355 cal y BP, when fuels increased rapidly, likely due to increased precipitation and cooler temperatures. Increased precipitation and cooler temperatures would have promoted vegetation growth after prolonged mid-Holocene drought. Such increases may be indicative of a shift from the Middle Holocene thermal maximum and widespread drought to effectively cool, wet conditions better suited for arboreal fuel accumulation. Climatic cooling during the Late Holocene is described by other records from the Northwestern US, and, more specifically, the Northern Rocky Mountains and central Idaho, USA [33,40,52,55,63,64]. Tree ring data from the Sierra Nevada, USA, suggest that temperatures decreased in western North America between ~1200 and ~1000 cal y BP [65,66].
After PARs peak at ~1355 BP, all pollen taxa and the total PAR decrease for the remainder of Zone 3 (Figure 4). Such trends in the pollen data from Phyllis Lake are noteworthy, given the synchronous response of the most abundant taxa, and likely indicate short-term returns to Middle Holocene conditions that punctuated the cooling trend that is characteristic of the Late Holocene.

4.4. Zone 4 (1000 BP–0 BP (1950 CE))

It has long been recognized that the past ~1200 years of Earth’s climate are characterized by two distinct periods known as the Medieval Climate Anomaly (MCA), and the Little Ice Age (LIA). The MCA is a well-known climate event characterized by shifts in atmospheric circulation patterns in the Northern Hemisphere [67], which lasted from ~1100 BP–650 cal y BP. These drought periods are evidenced in the Phyllis Lake record by frequent and significant fire activity between ~790 and ~590 cal y BP (Figure 2). The relatively cool, moist conditions of the Late Holocene would have led to canopy closure and fuel accumulation [52] leading up to the MCA. Substantial increases in the total PAR at Phyllis Lake after ~1000 cal y BP support this interpretation (Figure 2).
Large fire episodes are known to have occurred in the Northern Rockies between ~1000 and ~750 cal y BP during the most arid portion of the MCA [40]. MCA fire episodes at Phyllis Lake occur at ~790, ~750, and ~710 cal y BP (Figure 2). The most striking aspect of this evidence of the MCA in the Phyllis Lake record is that these episodes occur in three contiguous samples covering 80 years, serving as a strong signal of sustained drought and warmer temperatures. The MCA and the fire episodes associated with it have a notable impact on Abies abundances (Figure 4), because Abies lasiocarpa prefers cold temperatures, high moisture [68], and maintains a higher sensitivity to fire than Pinus albicaulis [56]. After this prolonged drought and corresponding series of fire disturbances, the Abies PAR reaches very low abundances between ~955 and ~490 cal y BP, even lower than what was reached at ~2470 cal y BP, when most PARs exhibited very low levels and the total PAR reached a record low (Figure 4). Pinus does not exhibit any negative impact from the MCA or its associated disturbances, and rather takes advantage of the loss of its more fire sensitive competitor Abies (Figure 4). Pinus continues to show tremendous growth throughout the MCA and into the LIA before reaching what is by far its highest PAR on record for the first four zones at ~240 cal y BP. A very similar pattern is exhibited by the total PAR, given that Pinus is the dominant pollen type on record at this site (Figure 2). Poaceae reach what are by far its highest abundances during the first four zones at ~240 cal y BP (Figure 4). This can be attributed to the tendency of Poaceae to inhabit recently burned areas as an early successional vegetation type. An alternative to the interpretation that these three consecutive fire episodes are a result of MCA impacts is that the charcoal distributed across contiguous samples may be part of the same local watershed-scale wildfire and a result of surface runoff and increased post-fire erosion rates [69]. After ~240 cal y BP, the total PAR and PARs for all taxa shift rapidly downward (Figure 4). This decrease is probably due initially to the fire disturbance at ~360 cal y BP, but is likely augmented by climatic fluctuations toward cold temperatures during the LIA, including what has been described as “the most severe Northern Hemisphere cold snap of the past 600 years” at ~350 cal y BP [70].

4.5. Zone 5 (1950 CE–2017 CE)

To establish a high-resolution record of both fire and vegetation for the Anthropocene, pollen was sampled and analyzed contiguously at 1 cm intervals for the time period encompassed in the Anthropocene. Zone 5 begins with low levels of charcoal influx and relatively low PARs for all taxa (Figure 3). Fire activity in the Northern Rocky Mountains was low during the mid-20th century, but sharply increased during the late 20th century [71]. Morgan et al. [71] attribute this sharp increase in fire activity during the late 20th century partly to changes in climate, including warm springs and dry summers, and partly to fire-suppression practices that permitted the accumulation of fuels. Fire activity in the Northern Rocky Mountains has increased markedly in recent decades and this increase is strongly linked to warmer temperatures and longer fire seasons [72].
All PARs and the total PAR experience a rapid increase between ~1970 and ~1990 CE, providing an abundance of fuel for the significant fire episode occurring at ~1995 CE. This significant increase in charcoal is likely indicative of the 1988 fire season, which is known for the many large fires that burned in the Greater Yellowstone Area and other regions of the Western US. PARs for all taxa and the total PAR decline after this fire disturbance. Another significant fire episode occurs at ~2011 CE, according to the Phyllis Lake age model. This fire is likely indicative of the Valley Road Fire of 2005 CE, which burned ~161 km2 [29]. The errors between the dates in the age model and the dates of these two known episode are almost identical: 7 years and 6 years, respectively, providing support for linking the episodes from this study to the two fires that are known to have occurred. The temporal sampling resolution is ~3 years per sample during this portion of the record. Furthermore, this small discrepancy between ages from the CLAM output and known episodes fits well within the margins of error from the dates used in the model, given the error in the two 14C dates (±20 and 25 years) and the error in the Mt. Mazama Ash tephra date (±45 years) (Figure 3).

5. Conclusions

The objectives of this study were to (1) identify when fires occurred in the watershed and analyze impacts on vegetation assemblages; (2) examine the role fuel connectivity plays in fire regime dynamics; and (3) determine when Pinus albicaulis established itself as the dominant arboreal vegetation type at this location. We specifically hypothesized that (1) fire frequency is influenced by the type of fuels available, and (2) climate change during the Anthropocene has impacted vegetation and fire patterns at this high-elevation Pinus albicaulis site.
We were able to identify when fires occurred in the watershed using a z-score analysis of the charcoal influx data and then analyzed the impacts of this disturbance event on the vegetation. The fire occurrence was closely linked to fuel connectivity as reconstructed through PAR. These results allowed us to then identify when Pinus albicaulis became the predominant conifer in the watershed. These data also allowed us to detect increases in fire occurrence associated with effective moisture decreases and temperature increases associated with climate change in conjunction with impacts of Euroamerican land management such as fire suppression.
After the 8200 y event, climate conditions at Phyllis Lake remained cold and likely wet, which is evidenced by the increasing Alnus PAR and the rapidly decreasing AP:NAP. The most devastating impacts from the Mt. Mazama eruption were suffered by coniferous arboreal taxa, and more specifically by Pinus albicaulis. Relatively cold, wet conditions persisted until ~7000 cal y BP when an Abies/Picea forest was established. The frequency of significant fire episodes at this location in the Northern Rocky Mountains is largely controlled by the availability of arboreal biomass, which is influenced by moisture availability. Varieties of Poaceae play an important role in fuel connectivity leading up to significant fire episodes, and the role fine fuels play in fire/fuel connectivity may help to explain the mixed-severity fire regime that is characteristic of Pinus albicaulis sites. Zones 4 and 5 also support our hypothesis that fuel types influence the presence of fire. From 1000 cal y BP to the present, there is an increase in PAR from the previous, and this time period is dominated by arboreal pollen. The presence of fire episodes returned during this period, coinciding with the increase in fuel availability. Fire activity continued during the late 20th and early 21st centuries, likely due to fuel buildup as a result of fire-suppression practices, warmer springs, and drier summers due to climate change.
Humans could have potentially used this area over the last 8200 years. However, the elevation and climate of the site would make it unsuitable for year-round occupation. Seasonal use and the resources available at this elevation would suggest that it was unlikely that fire was applied as a landscape management tool.
Paleoecological studies provide invaluable insight into past environmental dynamics, including responses to changes in climate, and should be utilized to inform land management techniques. Land managers are advised to monitor the health of Pinus albicaulis populations in the face of Earth’s changing climate in order to preserve the ecological roles this keystone species plays in the Northern Rocky Mountains and other regions of western North America.

Supplementary Materials

The following supporting information can be downloaded at: https://www.mdpi.com/article/10.3390/fire7110411/s1, Figure S1: The age/depth model for Phyllis Lake was produced in CLAM [24] and based on Table 1 in the manuscript.

Author Contributions

Conceptualization, J.W., J.H. and A.B.; methodology, J.W. and J.H.; formal analysis, J.H.; writing—original draft preparation, J.W.; writing—review and editing, A.B. and J.H.; supervision, J.W.; funding acquisition, J.W. All authors have read and agreed to the published version of the manuscript.

Funding

This research was funded by the Redd Center Off Campus Faculty Research Award.

Data Availability Statement

Pollen and charcoal data will be contributed to Neotoma upon publication.

Acknowledgments

The authors would like to thank our entire field crew (Georgie Corkery, Jessica Spencer, Alexander Watt, Marv and Emily Hoyt) that hauled gear several miles in a garden wagon. We would also like to thank our exceptional reviewers who provided invaluable feedback on the first draft of this manuscript.

Conflicts of Interest

The authors declare no conflict of interest.

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Figure 1. Detailed location of Phyllis Lake, Idaho, USA. Elevation 2800 m.
Figure 1. Detailed location of Phyllis Lake, Idaho, USA. Elevation 2800 m.
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Figure 2. The x-axis represents time, from 0 cal y BP (1950 CE) to ~8200 cal y BP, and is divided into four zones: the gray bars indicate the periods of time with fire episodes, the Mazama Ash layer is indicated by the vertical gray line at ~7790 cal y BP. (a) Ratio of arboreal to nonarboreal pollen. (b) Fire episodes with p values of <0.05. (c) Fire activity, highlighted by the two time periods in gray bars that include the fire episodes. (d) Total pollen accumulation rate (PAR), Pinus pollen, stacked to demonstrate the abundance of Pinus at Phyllis Lake.
Figure 2. The x-axis represents time, from 0 cal y BP (1950 CE) to ~8200 cal y BP, and is divided into four zones: the gray bars indicate the periods of time with fire episodes, the Mazama Ash layer is indicated by the vertical gray line at ~7790 cal y BP. (a) Ratio of arboreal to nonarboreal pollen. (b) Fire episodes with p values of <0.05. (c) Fire activity, highlighted by the two time periods in gray bars that include the fire episodes. (d) Total pollen accumulation rate (PAR), Pinus pollen, stacked to demonstrate the abundance of Pinus at Phyllis Lake.
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Figure 3. The x-axis represents time from when the core was collected in 2017 to 1950 CE and encompasses Zone 5 from the text: (a–g) PAR values for taxa discussed in the text. (h) AP:NAP (i) Fire episodes with p values of <0.05. (j) Fire activity. (k) Total pollen accumulation rate (PAR), Pinus pollen, stacked to demonstrate the abundance of Pinus at Phyllis Lake.
Figure 3. The x-axis represents time from when the core was collected in 2017 to 1950 CE and encompasses Zone 5 from the text: (a–g) PAR values for taxa discussed in the text. (h) AP:NAP (i) Fire episodes with p values of <0.05. (j) Fire activity. (k) Total pollen accumulation rate (PAR), Pinus pollen, stacked to demonstrate the abundance of Pinus at Phyllis Lake.
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Figure 4. The x-axis represents time, from 0 cal y BP (1950 CE) to ~8200 cal y BP, and is divided into four zones. The gray bars indicate the periods of time with fire episodes, the Mazama Ash layer is indicated by the vertical gray line at ~7790 cal y BP: (a–g) PAR values for taxa discussed in the text. (h) Fire episodes with p values of <0.05. (i) Total pollen accumulation rate (PAR), Pinus pollen, stacked to demonstrate the abundance of Pinus at Phyllis Lake.
Figure 4. The x-axis represents time, from 0 cal y BP (1950 CE) to ~8200 cal y BP, and is divided into four zones. The gray bars indicate the periods of time with fire episodes, the Mazama Ash layer is indicated by the vertical gray line at ~7790 cal y BP: (a–g) PAR values for taxa discussed in the text. (h) Fire episodes with p values of <0.05. (i) Total pollen accumulation rate (PAR), Pinus pollen, stacked to demonstrate the abundance of Pinus at Phyllis Lake.
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Table 1. Age reported at 1 cm was the year the core was collected, and age reported at 14 cm represented the plutonium peak identified by Michael Ketterer at Northern Arizona University, Flagstaff, Arizona [20]. Age at depth of 223 cm was Mazama Ash tephra [23]. Cal y BP refers to calibrated years before present. 14C refers to uncalibrated radiocarbon dates.
Table 1. Age reported at 1 cm was the year the core was collected, and age reported at 14 cm represented the plutonium peak identified by Michael Ketterer at Northern Arizona University, Flagstaff, Arizona [20]. Age at depth of 223 cm was Mazama Ash tephra [23]. Cal y BP refers to calibrated years before present. 14C refers to uncalibrated radiocarbon dates.
Depth (cm)CoreLab NumberMaterialAge (14C y BP)Age (cal y BP)
1PL17-ASurface −67 (2017 CE)
9PL17-APlutonium extrapolationBulk sediment −40 (1990 CE)
18PL17-APlutonium peakBulk sediment −13
93PL17-BCAIS * 39123Pollen2810 ± 202875
245PL17-B Ash6845 ± 457790
252PL17-BCAIS * 39124Pollen7450 ± 258177
* Center for Applied Isotope Studies.
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Hartley, J.; Watt, J.; Brunelle, A. Using Paleoecological Methods to Study Long-Term Disturbance Patterns in High-Elevation Whitebark Pine Ecosystems. Fire 2024, 7, 411. https://doi.org/10.3390/fire7110411

AMA Style

Hartley J, Watt J, Brunelle A. Using Paleoecological Methods to Study Long-Term Disturbance Patterns in High-Elevation Whitebark Pine Ecosystems. Fire. 2024; 7(11):411. https://doi.org/10.3390/fire7110411

Chicago/Turabian Style

Hartley, Jordin, Jennifer Watt, and Andrea Brunelle. 2024. "Using Paleoecological Methods to Study Long-Term Disturbance Patterns in High-Elevation Whitebark Pine Ecosystems" Fire 7, no. 11: 411. https://doi.org/10.3390/fire7110411

APA Style

Hartley, J., Watt, J., & Brunelle, A. (2024). Using Paleoecological Methods to Study Long-Term Disturbance Patterns in High-Elevation Whitebark Pine Ecosystems. Fire, 7(11), 411. https://doi.org/10.3390/fire7110411

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