Late Holocene vegetation and fire history at the southern boreal forest margin in Alberta, Canada

Palaeogeography, Palaeoclimatology, Palaeoecology 164 (2000) 263–280 www.elsevier.nl/locate/palaeo

Late Holocene vegetation and fire history at the southern boreal forest margin in Alberta, Canada I.D. Campbell a,b, *, C. Campbell c a Canadian Forest Service, 5320-122 Street, Edmonton, Alb., T6H 3S5, Canada b Department of Earth and Atmospheric Sciences, University of Alberta, Edmonton, Alb., T6G 2H4, Canada c Department of Biological Sciences, University of Alberta, Edmonton, Alb., T6G 2H4, Canada Accepted for publication 16 March 2000

Abstract A transect of three shallow ponds across the Aspen Parkland/Boreal Forest transition in Elk Island National Park, Alberta, Canada, shows changes in late Holocene vegetation and disturbance dynamics, and consequent carbon storage, in response to climate change and to recent anthropogenesis. Pollen and charcoal analyses reveal unexpected changes in fire regime. Although there is the expected decline in fire activity during the historic period, presumed due to agricultural clearance around the park, there is also a prehistoric fluctuation in fire regime at one of the sites. Pollen evidence suggests that the fluctuations in fire regime may be due to changes in hydrology. Declining groundwater levels during the Medieval Warm Period allowed the replacement of substantial areas of shrub birch (Betula glandulosa) with the less fire-prone aspen (Populus tremuloides) causing a decline in fire frequency and/or severity, while increasing carbon storage on the landscape. This is counter to the intuitive increase in fire activity with warmer and drier climate. Canadian national parks are currently managed under a ‘natural processes and conditions’ paradigm; the changes in conditions and consequent changes in processes evident due to the relatively minor climatic fluctuations of the Little Ice Age and Medieval Warm Period shown here suggest a need to revisit this paradigm in consideration of future anthropogenic climate change. © 2000 Elsevier Science B.V. All rights reserved. Keywords: aspen parkland; boreal forest; charcoal; fire history; Little Ice Age; Medieval Warm Period

1. Introduction The Aspen Parkland, which forms the southern tree-line in the western interior of Canada, has received little study from paleoecologists compared with the northern tree-line. Most of the published paleoecological studies in or near this ecotone (Ritchie, 1964; Ritchie and Lichti-Federovich, 1968; Mott, 1973; Vance et al., 1983; McAndrews, * Corresponding author. Tel.: +1-780-435-7300; fax: +1780-435-7359. E-mail address: [email protected] (I.D. Campbell )

1988; Campbell et al., 1994b) assessed millennialscale changes in vegetation and climate, and do not have adequate temporal resolution or dating control to extract shorter time-scale signals in the late Holocene. One exception, Strong (1977), showed a strong impact of EuroCanadian anthropogenesis, but the dating control was provided only by the pollen itself. While possible global warming may be expected to cause forest expansion into the tundra, it may also be expected to cause forest retreat at the more heavily populated and economically important southern margin. In many sites, this retreat may take the form of

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failure of regeneration following disturbance (Hogg and Schwarz, 1997); a simultaneous study of disturbance and vegetation history is therefore desirable. By detailed examination of the paleoecological record of this ecotone it may be possible to establish its sensitivity to climatic change at a century or even decadal scale. Mott (1973) examined the palynology of four sites forming a north–south transect through Saskatchewan, at a 100–200 year sampling interval, spanning the entire postglacial record at each site. He showed that the conifer Picea (spruce) has been present in the southern boreal region of Saskatchewan since between 8,000 and 11,000 14C years BP. There was a period of very dry climate (the hypsithermal, or early–mid Holocene warm period) initiated ca 10,000 14C years BP, which caused grassland to invade the parkland, and parkland to invade the southern boreal. This warm/dry interval ended ca 6000 14C years BP, allowing the southwards re-expansion of the forest and parkland. Very little change is evident in the last several thousand years of record. Vance et al. (1983) studied the pollen records of three sites in the transitional forest of central Alberta, including one site in Elk Island National Park ( EINP). This site began accumulating sediments after 4000 14C years BP, with an initial pollen assemblage most strongly resembling those of modern dry grassland environments. After 2800 14C years BP, increasing moisture was indicated by an increase in arboreal pollen taxa, dominantly Betula. Although the temporal resolution of this diagram was reasonably high (apparently ca 50– 100 years/sample), the dating control was weak in the last 1500 years. Another Alberta site, further north in the boreal forest, shows significant changes in the vegetation during the hypsithermal (Lichti-Federovich, 1970). Other sites in central Alberta demonstrate that the hypsithermal interval brought significant reductions in groundwater levels, possibly by as much as 15 m (Schweger and Hickman, 1989). There have, however, been substantial climatic fluctuations in the last 1500 years, including the Medieval Warm Period (MWP) and the Little Ice Age (LIA). The MWP was an apparently global period of generally warmer climate spanning

roughly the period AD 750–1250, in many areas accompanied by increased aridity (Lamb, 1965; Campbell, 1998), although in some areas possibly moister (Mohammed et al., 1995). The LIA was a similarly global period of generally cooler and in most areas moister climate, starting ca AD 1450 and ending ca AD 1850 (LeRoy Ladurie, 1971; Grove, 1988; Mikami, 1992), although some areas experienced greater drought (Mohammed et al., 1995). Fluctuations are recognized within both periods, which were not uniformly, but only generally, warmer or cooler than present. Both periods have been recognized in proxy climate records from the Canadian prairies (e.g. Vance et al., 1992; Campbell, 1998). Also, recent human activities have affected the vegetation of the region, generally increasing the density of Populus in the Aspen Parkland (Strong, 1977); a regional increase in Populus pollen has been noted between 1880 and 1900 AD (McAndrews, 1988; Campbell et al., 1994b). Here, we examine the pollen and charcoal records of three sites in EINP, Alberta, Canada. The sites span the southern boreal forest/Aspen Parkland ecotone, and are separated by only 17 km (from the furthest north to the furthest south). The purpose of this study is threefold: first, to determine the impact of land use changes and park management on the vegetation and fire regime since park formation in 1906; secondly, to determine the short-term (decade- to century-scale) responsiveness of the natural vegetation and fire regime, and hence landscape carbon storage, to climate change. 1.1. Study area The EINP covers 194 km2 on the Beaver Hills in east-central Alberta ( Fig. 1). The terrain is hummocky moraine, formed during the retreat of the Late Wisconsinan glaciation 15,000–20,000 calendar years ago (Campbell and Campbell, 1997). The moraine is some 30–60 m above the surrounding plain, and local relief is up to 30 m. The EINP was established as a game preserve in 1906, and has since been expanded several times. EINP is entirely fenced to prevent the escape of elk and bison onto the highway traversing the

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Fig. 1. The EINP lies at the boundary between the boreal forest and the Aspen Parkland. Sites discussed in the text: S=South Pond, B=Birch Island Pond, 5=Pen 5 Pond, P=Pine Lake (Campbell et al., 1994a,b), E=Elk Island Pond ( Vance et al., 1983), L=Lofty Lake (Lichti-Federovich, 1972). Vegetation boundaries after EcoRegions Working Group (1989). Park map shows known fires in the park, 1922–1987 [modified from Deering (1987)]. Years are provided for the fires >10 ha. Dots represent fires <1 ha; squares represent fires between 1–10 ha.

park and into neighboring agricultural areas, and hosts over 300,000 visitors each year ( Kraulis and McNamee, 1994). EINP is in the Aspen Parkland region, which has a summer wet continental climate, with long cold winters and short warm summers. Total precipitation exceeds 400 mm in most years, and the frost-free period is generally longer than 80 days ( Environment Canada, 1982a,b,c). Hogg (1994) and Campbell et al. (1994a) show the importance of climatic moisture balance for vegetation zonation in this region; EINP lies roughly at the zero point for the Hogg (1994) climatic moisture index, indicating that, in most years, precipitation is roughly balanced by potential evapotranspiration. This balance corresponds with the southern margin of the boreal forest (Hogg, 1994) and with a

boundary between stable and unstable lake levels (Campbell et al., 1994a). EINP has three large lakes and a large number of small ponds, together covering over 20% of the park’s area. Many of these are maintained by beaver dams, although most would still exist as shallower ponds in the absence of beaver. Historic aerial photographs show a high level of variability in the extent of the ponds through time; in wet periods, many of the ponds become interconnected, whereas many dry out completely in dry periods. There are few permanent streams in the park. The vegetation of EINP today varies strongly with topography, and also with latitude. The Beaver Hills have been classified as an outlier of the southern boreal forest (Rowe, 1972); however,

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others (e.g. Blyth and Hudson, 1987) have classified it as Aspen Parkland. The modal upland vegetation is closed-canopy Populus (Populus tremuloides) forest with a dense beaked hazel (Corylus cornuta) understory. Hilltops, south-facing slopes, and recently burned areas often have dry meadows, and ephemeral ponds become wet meadows in the dry season. Particularly in the north end of EINP, low wet areas are often colonized by black spruce (Picea mariana) and tamarack (Larix laricina), while north-facing slopes often have white spruce (Picea glauca). Fens with Sphagnum are more abundant in the north end of the park. P. glauca also forms nearly pure stands on some islands in the large lakes and in other protected areas. Emergent flats around ponds often support shrub birch (Betula glandulifera), and shallow areas in the ponds typically support cattails (Typha latifolia). Paper birch (Betula payrifera) also occurs scattered throughout the park, but does not form large pure stands. This vegetation is typical of those portions of the Aspen Parkland that have not been cleared for agriculture. The Populus-dominated forest grades northwards into the southern boreal forest, which is Populus dominated but with abundant conifers, and grades southwards into the grassland, where stands of Populus are largely restricted to northfacing slopes, and conifers occur only in rare sheltered microhabitats. Today, EINP is managed principally for the large herbivores, but with an increasing emphasis on preserving the ‘natural’ ecology of the area. To this end, EINP started a program of prescribed fire in 1979, in order to emulate the presumed more frequent prehistoric fire regime (Deering, 1987). Also, there are annual, occasionally large culls of the large herbivore herds, required by both the absence of wolves or other large predators and the inability of the herds to migrate seasonally.

2. Methods Three sites were selected to form a north–south transect through the park. Sites were selected for size (all less than 2 ha), and for stable water levels as seen in the available aerial photography. All

three sites have ~2 m of water at the deepest point, and all have relatively flat bottoms. None has permanent inflowing or outflowing streams. Each site was cored in winter 1995 using a freezing sampler (Shapiro, 1958). One slab from each freezing sampler was removed and sent frozen to Flett Research in Winnipeg for 210Pb analysis. A shell sample from Pen 5 Pond was radiocarbon dated by AMS. For all sites, dates were interpolated and extrapolated from these age estimates using fitted power functions, which allow for sediment compaction (Campbell, 1994). Charcoal was analyzed by nitric acid assay using a modification of the method developed by Winkler (1985). This method yields the total elemental carbon content of the sample by first removing unreduced carbon with fuming nitric acid, then removing the charcoal by dry ashing at 500°C. The method was slightly modified to include repeated hot nitric acid digestions alternating with centrifuging and decanting, until the nitric acid supernatant was clear and not discolored, and in no other way showed signs of further reaction with the sediment. Because this wet/dry digest method includes all sizes of charcoal particles, it is believed to include charcoal from fires as much as several hundred kilometers upwind of the sampling site, and may thus yield a more regional (from several hundred square kilometers to subcontinental depending on the site) fire activity signal rather than a site-specific local signal (MacDonald et al., 1991). If the fine charcoal particles behave similarly to pollen, however, then the small size of the lakes used here should ensure a dominantly local signal from the nitric acid assay. This method has drawn criticism for the possible loss of water from clays and for lack of repeatability (MacDonald et al, 1991; Novakov et al., 1997). It has also been shown that the assay values are inflated by the adsorption of nitric acid on the charcoal particles (Laird and Campbell, 2000). Despite these drawbacks, it has recently been demonstrated that, when suitably modified, this technique is reliable for determining trends in charcoal content, if not absolute abundances (Laird and Campbell, 2000). The advantages of the method are that it is inexpensive and rapid, allowing a large number of samples to be analyzed

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for a minimal unit effort. Nitric acid assay was done at 1 cm intervals in all cores. Selected samples were prepared for pollen analysis using standard techniques (Faegri et al., 1989), including the use of Lycopodium tablets as an exotic marker for absolute abundance determinations (Maher, 1981), and mounted in glycerine jelly. Pollen sums were at least 250 grains (excluding Lycopodium); a count of 200 grains has been shown to be suitable for routine pollen analyses (Maher, 1972). Charcoal particles larger than 25 mm in long axis were also counted on the pollen slides. Geochemistry was conducted using an ICP– AES analysis of mild hydrochloric acid extracts of

the bulk sediment, which yields the exchangeable and readily soluble ions without significantly leaching the silicates (Malo, 1977). Loss on ignition (LOI ) was conducted according to Dean (1974), to assess the organic carbon, carbonate, and ash (mainly silicate) components. Geochemistry and LOI were done at the same 1 cm intervals as the nitric acid assay.

3. Results The sediments retrieved at all three sites were a dark brown diatomaceous gyttja with low carbon-

Table 1 210Pb results Site

Interval (cm)

Dry mass in interval (g)

Years

Est. date AD at interval bottom

0.132 0.184 0.14 0.18 0.23 0.363 0.548 0.584 3.60 3.95

4.52 6.31 4.80 6.17 7.88 12.4 18.8 20.0 123 135

1990 1984 1979 1973 1965 1953 1934 1914 1791 1655

top

bottom

South Pond

0 2 4 6 8 10 13 17 21 48.5

2 4 6 8 10 13 17 21 48.5 75

Birch Island Pond

0 2 4 6 8 10 13 17 21 61

2 4 6 8 10 13 17 21 61 100

0.049 0.057 0.065 0.075 0.081 0.137 0.190 0.144 1.48 3.48

1.59 1.84 2.10 2.43 2.62 12.3 17.0 12.9 132 311

1993 1992 1989 1987 1984 1972 1955 1942 1810 1499

Pen 5 Pond

0 2 4 6 8 10 13 17 21 46

2 4 6 8 10 13 17 21 46 70

0.056 0.086 0.116 0.142 0.16 0.294 0.496 0.576 3.40 3.12

2.84 4.35 5.87 7.19 8.10 14.9 25.1 29.2 172 158

1992 1988 1982 1975 1967 1952 1927 1897 1725 1567

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ate content. There was no visible stratigraphy except in the South Pond core, where there were two pale layers between 60 and 50 cm depth. 3.1. South Pond The 210Pb results from South Pond suggest either some degree of sediment mixing, or else significant changes in the sedimentation rate in recent decades (R. Flett, written personal communication, 1995). In either case, the dating control of this site must be considered poor. All that can be confidently said is that the upper few centimeters are likely to represent recent decades; estimated ages for South Pond presented in Table 1 and in the figures must be treated with caution. The LOI curve ( Fig. 2) shows two strong carbonate peaks between 60 and 50 cm, preceded by

a weaker peak in organic matter; there is also an historic increase in both carbonates and organic matter at the expense of ash (principally silicate minerals) in the upper few centimeters. The bottom of the core has more ash than organic matter, but this reverses just prior to the carbonate peaks. The charcoal:pollen ratio shows a peak at about 35 cm, and generally shows lower values in the upper portion of the core compared with the lower portion. The nitric acid assay charcoal curve shows a strong decline in the upper few centimeters of sediment. The rapid changes in charcoal abundance argue against sediment mixing, and suggest instead that the difficulty with the 210Pb dating was due to a change in sedimentation rate and possibly in the 210Pb-shed (and hence probably also the charcoal-shed) of the pond. The nitric acid assay results are roughly congruent with the

Fig. 2. South Pond charcoal and LOI. See the text for cautions regarding dating control.

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Fig. 3. South Pond percentage pollen diagram. The gray shading indicates a 10× exaggeration. The pollen sum is all taxa up to and including Epilobium. See the text for cautions regarding dating control.

Fig. 4. South Pond geochemistry. See the text for cautions regarding dating control.

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pollen slide charcoal counts, although individual peaks cannot be matched, partly due to the difference in sampling intervals. The pollen diagram ( Fig. 3) shows a decrease in the abundance of grass pollen between 50 and 40 cm depth. Other taxa indicative of open grassland also decline, including long-spine Compositae and Artemisia. Populus, Corylus, and Ambrosia increase above 40 cm. Rosa increases in the upper 15 cm. The geochemistry (Fig. 4) shows sharp declines in Al, As, Cu, Fe, and Pb between 50 and 40 cm depth. Ca, Cu, K, Mg, Mn, Na, P, Zn, Ti, and particularly Pb show strong peaks in the upper few centimeters. 3.2. Birch Island Pond The 210Pb results from Birch Island Pond ( Table 1) do not show the same ambiguities as the

results from South Pond. The AD 1900 level is estimated to occur at about 35 cm depth. The LOI curve from Birch Island Pond ( Fig. 5) shows a strong increase in organic matter at the expense of ash in the uppermost 2 cm, but this is preceded by a long-term increase in organic matter at the expense of ash starting at 70–60 cm depth. At around 70 cm depth there is a broad peak in ash content and a corresponding low in the organic content. There is also a slight decrease in the carbonate content above 55 cm. The charcoal:pollen ratio shows generally higher values in the lower half of the core, and is broadly consistent with the nitric acid assay results, which also show a decline starting above 40 cm; this depth was chosen for the placement of a local zone boundary for discussion purposes. The upper half of the pollen diagram (Fig. 6) shows increases in grasses, sedges, Cheno-

Fig. 5. Birch Island Pond charcoal and LOI. Local zone boundaries, subjectively placed based on the nitric acid assay curve, are provided to facilitate discussion.

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Fig. 6. Birch Island Pond percentage pollen diagram. The gray shading indicates a 10× exaggeration. The pollen sum is all taxa up to and including fenestrate Compositae.

Fig. 7. Birch Island Pond geochemistry. The anomalous values for Cu, Ni, and Pb at level 49 may indicate contamination in the laboratory.

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podiaceae, the tree Populus (aspen), the shrubs Salix (willow), Corylus (hazel ), and Alnus (alder), and a decline in Betula [birch, which is common in both tree (Betula papyrifera) and shrub (Betula glandulosa) form in this region]. The diameters of the Betula pollen grains (mostly less than 23 mm) suggest this is mainly shrub Betula rather than tree Betula (Ritchie, 1972). The geochemistry (Fig. 7) shows a broad peak of Cu, Pb, and Ti between 50 and 80 cm depth, with As increasing sharply at 75 cm. There are also near-surface peaks in K, Mn, Na, P, and Pb starting between 10 and 15 cm. 3.3. Pen 5 Pond The 210Pb results from Pen 5 Pond, which is a longer core than the others ( Table 1), show no significant change in sedimentation rate, and suggest a dry sediment accumulation rate of ca

0.02 g cm−2 year−1. The 1900 level is estimated to occur at 20 cm depth. An AMS radiocarbon date on shell fragments at a depth of 101 cm yields a calibrated date between 211 and 344 AD [CAMS-33996: 1780±50 14C years BP, calibrated using Stuiver and Reimer (1993), using the 97% probability at one standard deviation]. Unlike the other two sites, the LOI curve ( Fig. 8) shows no strong increase in organic matter content in the upper few centimeters, although there is an increase in organic content at the expense of ash at ca 50 cm. The charcoal:pollen ratio shows two intervals of increased charcoal; one from ca 100 to ca 80 cm, and the second from ca 60 to ca 40 cm. The nitric acid assay curve shows two major periods of deviation from an otherwise steady charcoal content. The first occurs between 85 and 65 cm depth, and the second starts at 20 cm depth and continues to the top of the

Fig. 8. Pen 5 Pond charcoal and LOI. Local zone boundaries, determined from the nitric acid assay curve, are provided for discussion.

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core. These periods of deviation are markedly similar to each other, in that each comprises strong swings between the ‘normal’ charcoal content and a much lower charcoal content. These horizons do not show as particularly sandy intervals, so it is unlikely that the low charcoal content is due to dilution with other sediments; further, the dating does not suggest significant changes in sedimentation rate. These periods also broadly correspond with the lows in the charcoal:pollen curve, albeit with an apparent slight offset. For discussion purposes, local zone boundaries have been drawn following the nitric acid assay curve. The pollen diagram ( Fig. 9) also shows interesting changes at these same levels, including slight increases in Populus and Chenopodiaceae (mainly Salsola kali in local zone 4) pollen. Local zone 4 also shows a decrease in Betula pollen, again

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mainly shrub Betula, and Picea, as well as an increase in Typha. The low in the charcoal:pollen ratio from 80 to 60 cm corresponds with a low in pine pollen and an increase in Betula pollen, while the low in the nitric acid assay in local zone 2 corresponds with the early part of this Betula pollen increase and the preceding low in Betula pollen. The geochemistry (Fig. 10) shows three small peaks of Al and Fe at 30–40 cm, ca 70 cm, and 110–120 cm, as well as a broad peak in Ca at 110–80 cm. Cu, Fe, Ni, Pb, and Ti decline sharply at 15 cm, in local zone 4.

4. Discussion As the three sites do not show the same pollen and charcoal signals, it can be assumed that the

Fig. 9. Pen 5 Pond pollen diagram. The gray shading indicates a 10× exaggeration. (A) Percentage diagram; the pollen sum is all taxa up to and including Primula. (B) Influx diagram.

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Fig. 10. Pen 5 Pond geochemistry.

charcoal and pollen of each site represents mainly the area immediately around each site, and not the Beaver Hills as a whole. There is a trend from north to south along the transect towards decreasing Pinus pollen percentages, from a time-averaged mean of ~35% at Birch Island Pond to ~30% in Pen 5 Pond and ~25% in South Pond. As there is very little Pinus in EINP, this undoubtedly reflects the increasing distance from the boreal forest margin. All three sites show historic increases in Populus pollen and declines in charcoal. The increases in Pb, Populus, and Chenopodiaceae at the tops of all cores lend some confidence to the recent dating. The sequence near the bottom of the South Pond core (organic matter peak, followed by carbonates, followed by rapid decline of metals and grass pollen) may suggest a wet soil or ephemeral pond being covered first by a shallow marl pond then by deeper water. If this is the case, then the change in sedimentary environment would make any further interpretation of the pollen, the charcoal, or even the 210Pb below 50 cm depth at this site potentially misleading, as both the sedimentation rate and taphonomic processes were likely

very different. This proposed increase in water level may relate to the early 1960s construction of the fence line and culvert at one end of the pond (Blyth and Hudson, 1987), and other anthropogenic disturbances to the hydrological balance of the site. The Populus rise and 210Pb results, however, imply that the 50 cm level is much older than this, closer to AD 1850 or 1900. 4.1. Recent (local zone 4) changes Birch Island Pond may have been affected by the construction of the highway along its east shore in the 1920s, although there is no clear evidence of any impact in either the geochemistry or other available data. There is an extensive Typha fringe between the road and the open water, which may have protected the pond from the worst impacts of road construction. At all three sites, the declines in charcoal abundance appear to start early in the 20th century. This decline is readily explained by active fire suppression in the park combined with what may be thought of as unintentional fire suppression due to agricultural activity around the park, which

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would have reduced the incidence of fires burning into the park from the surrounding area. Of the two, the unintentional fire suppression is likely to have been most effective, as shown further east in Prince Alberta National Park, Saskatchewan ( Weir, 1996). No site shows any evidence of the recent prescribed fires (fires set and controlled by the park wardens for landscape management purposes), which started in 1979 but have mainly been less than 1 ha in area (Deering, 1987). Since 1979, there has been one small burn (<5 ha, 1985) near Birch Island Pond, and two larger burns (>200 ha in 1982 and ~175 ha in 1983) east of South Pond (Fig. 1). Starting in 1987, prescribed burning increased, with more than 100 ha burned in most years, and over 2000 ha burned in 11 separate fires in 1991. These burns do not show as separate events in the charcoal records of these sites; this is probably due in part to sediment mixing, which smooths the charcoal signal over a variable number of years, and in part due to the fact that the burns were small and did not reach the shorelines of the ponds. Pen 5 Pond provides the longest, most well dated, and arguably most interesting record from these three sites. The recent change in fire activity indicated by the sudden fluctuations of total charcoal abundance ( local zone 4) apparently started early in the 20th century, and is consistent with the declines seen at the other two sites. This parkwide decline in both the nitric acid assay and the charcoal:pollen ratio is most parsimoniously explained as due to a combination of active fire suppression and the settlement and farming of land around the park in the early 20th century, which would have reduced the incidence of fires burning into the park from outside. The only recorded historic burns in the area around Pen 5 Pond are a large wildfire in 1937, which burned nearly 400 ha south of Pen 5 Pond ( Fig. 1; Deering, 1987), and a small prescribed burn in 1992 immediately west of Pen 5 Pond (A. Dickinson, oral personal communication, 1995). Historical documents indicate that there was abundant fire in the Canadian prairies in general during the 1790s, 1812–13, and 1895, with the Beaver Hills — including the area now occupied by EINP — specifically

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mentioned as burning in this last period (Blyth and Hudson, 1987). A similar historic change in fire regime at Pine Lake, further south in the Aspen Parkland, has recently been interpreted as the result of increasing Populus abundance in what was previously a predominantly grassland area (Campbell et al., 1994a,b, 2000). An increase in Populus is believed to reduce fire frequency and/or severity due to a combination of Populus’s moisture storage capacity, which is greater than that of a grassland, its thick, dense bark, which is more difficult to ignite than the barks of many boreal tree species, its architecture, which does not provide ladder fuels for ground fire to reach the canopy, its litter, which decomposes more rapidly than does conifer litter, and the moister microclimate under the Populus canopy. Since all three sites in the park also show a historic increase in Populus pollen (strongest in South Pond ), this same explanation may account for some of the change in the park’s fire regime, although the active and passive (by land clearance) fire suppression may account for most of the historic change. 4.2. Prehistoric (local zone 2) changes The prehistoric period of fluctuating charcoal deposition (inferred from nitric acid assays) at Pen 5 ( local zone 2; ca 1200–800 AD) is substantially more intriguing, as it predates any possible settlement-related disturbances to the natural ecology. This prehistoric fluctuation in fire activity occurs at the same time as an increase in Populus pollen and a decrease and start of a subsequent rise in shrub Betula pollen. The corresponding minimum in the charcoal:pollen ratio appears somewhat lagged in relation to the minimum in the nitric acid assay results. The charcoal:pollen minimum matches well with a peak in Betula pollen and a minimum in pine pollen, but not particularly with other pollen taxa. There are several possible explanations for this apparent lag. It could be that the local area surrounding the pond responded slightly later than did the general region, and the lag is due to the different catchments for the different size fractions of charcoal. It is also possible that the coarser

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charcoal fragments took longer to reach the lake. The terrain around Pen 5 Pond is a very low relief hummocky moraine, with abundant sedge meadows. The relief is low enough that the actual watershed boundaries of the pond cannot be defined precisely. Furthermore, there are no permanent streams either entering or draining the pond. In such a terrain, surface runoff is not likely to be very effective in transporting particulates into the pond. If the bulk of the >25 mm charcoal fragments were brought to the pond not by direct aerial deposition but by this slower runoff process, then a considerable time might have elapsed between a change in fire regime and its reflection in the coarse charcoal fraction in the pond sediments. However, the finer fractions included in the nitric acid assay would be more readily transported, even in the relatively ineffectual surface runoff, as well as by direct aerial deposition, and so would not show such a topographically induced lag. Finally, there might be a significant difference in the charcoal produced by different fuel types, plant parts, and charring temperatures, affecting the settling time of the charcoal (Nichols et al., 2000; Scott et al., 2000). Indeed, we would expect grass fires to produce generally finer charcoal particles than would forest fires; shrub fires might produce intermediate-sized fragments. Differences in fuel types through time may therefore explain this lag. In general, the Pen 5 Pond record shows a very strong opposing relationship between Betula pollen and the charcoal:pollen ratio. It could be that there is competition between Betula and Populus on the landscape; because Betula is a more abundant pollen producer than Populus, increases in Betula at the expense of Populus would reduce the charcoal:pollen ratio without necessarily reflecting any change in fire regime. This possibility can be examined through the pollen influx diagram (Fig. 9b); although the reduction in charcoal influx at this time is less pronounced than the reduction in the charcoal:pollen ratio, there is still a decrease, indicating that the increased pollen productivity of a Betula-dominated landscape over a Populusdominated landscape is not the full explanation. If, however, we examine the influx diagram for possible changes in fuel type, we see that local

zones 2 and 4 both have slightly elevated Populus influx, suggesting more Populus on the landscape, while local zones 1 and 3 have elevated Betula influx, indicating more shrubs. Fire in a Populus stand mainly burns the grass and shrub understory; in a shrub Betula stand, however, the shrubs themselves are the principal fuel. Shrub Betula is highly flammable and is fire adapted (de Groot and Wein, 2000). The charcoal:pollen ratio increases with shrub Betula, which may not so much indicate a change in fire activity as a change in fuel and, therefore, the particle size of the charcoal produced. The nitric assay results, on the other hand, are not sensitive to particle size, only to total biomass converted to charcoal; therefore, they may reflect more faithfully the biomass burning through time. If, therefore, we focus mainly on the nitric acid assay results, an explanation must be found for the correspondence of low fire activity with low Betula pollen abundance and high Populus pollen abundance in both local zones 2 and 4. Local zone 2 occurred approximately from 800– 1200 AD; this was the time of the MWP in Europe, and was also a warm/dry period in Alberta ( Vance et al., 1992; Campbell and Campbell, 1997; Campbell, 1998). Although this warm dry period has been shown to have had effects on groundwater levels and shoreline/aquatic plants in Alberta, it has not yet been shown to have affected the upland vegetation. The MWP likely caused a drop in groundwater levels throughout the Beaver Hills. The groundwater level can be seen to be highly variable today; historic aerial photography shows a widely varying number of ponds depending on the season and year of the photography. If groundwater levels, and hence pond levels, were to drop significantly today, a great many ponds would become wet sedge meadows. The pollen diagram from Pen 5 does not suggest a wet sedge meadow at any time in the nearly 2000 years of record from the site, nor is there geochemical evidence of a dry period such as at South Pond. The present climate, however, is warmer and drier than that of either the LIA (which followed the MWP) or the early Neoglacial (preceding the MWP); we must, therefore, consider the present

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pond levels to be more representative of the MWP than of the time before or after. In the case of water levels higher than they are today, we would expect to see submergence of the wet fringes of many of the ponds, and connections would form between many of the ponds. Additionally, many of the wet sedge meadows might become shallow ponds. Many of the ponds in the park today, including Pen 5 Pond, are bordered by flats of varying widths, which often support Typha or shrub Betula. If water levels were raised, these flats would be submerged, and those supporting shrub Betula would be likely to support Typha instead, whereas those supporting Typha might no longer support any emergent vascular plants. If water levels were high enough, much of the flat land between ponds might become moist enough to support shrub Betula. At Pen 5 Pond, the following scenario may have occurred: prior to the MWP, the relatively cool/moist climate of the early Neoglacial allowed water levels high enough to support shrub Betula on much of the flat land around Pen 5 Pond. As water levels dropped with the MWP, this shrub Betula was replaced with Populus, and the pondedge flats became shallow enough to support Typha. The replacement of shrub Betula with Populus would have reduced the frequency and severity of fires on the upland around the site, producing the observed fluctuations in charcoal input to the pond. After the MWP, increasing water levels would have restored the area to its previous state. Since the present climate is more similar to that of the MWP than that of either the LIA or the early Neoglacial, we must re-examine the assumption of fire suppression, either actively by park managers or passively by agriculture around the park, as the principle cause of the fluctuating charcoal input to the pond in local zone 4. Pen 5 Pond today has a Typha fringe and is surrounded on the upland by Populus, as it would have been during the MWP. The scenario that is proposed here to have caused the change in fire regime during the MWP is likely playing out again today, in addition to the intentional and unintentional anthropogenic fire suppression that is not fully

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counterbalanced by the limited prescribed burning program. What of the other sites in the park? If a natural, climatically driven mechanism can account for the historic change in fire regime in the middle of the park, it may also be at least partly responsible for the similar historic changes elsewhere in the Aspen Parkland. However, Pen 5 Pond is located in an unusually low relief area of the park; Birch Island Pond and South Pond are both in areas of much stronger relief, where changes in water level would have to be much stronger to produce the same effects. Indeed, it is doubtful whether the upland around Birch Island Pond could ever have supported an abundance of shrub Betula, as it is a gravelly, well-drained topographic high. The pollen diagram from the park in Vance et al. (1983) does show, interestingly, the first increase in Populus at about the time of the MWP, suggesting that the impact of the MWP inferred at Pen 5 Pond may have been more widespread within the park; a similar, stronger increase in Populus pollen occurs in the Vance et al. (1983) site at the very top of the diagram, in our local zone 4. As with our data, the diagram of Vance et al. (1983) suggests a competitive relationship between Betula and Populus over the last 1500 years. Elsewhere in western Canada, it has been proposed that Populus abundance increased historically due to the elimination of bison (Campbell et al., 1994a,b). This may explain the historic increase in Populus pollen at Birch Island Pond and particularly at South Pond, and may explain the greater strength of the historic Populus rise compared with the MWP Populus rise at Pen 5 Pond. As would be expected in this ecotonal region, the strongest historic Populus rise is at the southernmost site, and the strength of the historic Populus rise decreases northwards. Aerial photography indicates an increase in Populus cover during the historic period (Blyth and Hudson, 1987). The northern end of the park was slightly more than 20% grassland and less than 20% dense Populus cover in 1924, but in 1983 supported a negligible grassland area and over 50% dense Populus forest. Similarly, the central and southern ends of the park have changed from 50% and 40% grassland respectively and negligible amounts of dense

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Populus in 1924 to negligible amounts of grassland cover and over 40% respectively and over 60% dense Populus cover in 1983 (Fig. 11; Blyth and Hudson, 1987). Park records further suggest an increase in Populus cover following each major cull of the ungulate herds (Blyth and Hudson, 1987).

5. Conclusions The fire regime of EINP, which straddles the ecotone between the Boreal Forest and the Aspen Parkland, has changed historically and prehistorically. Whereas the prehistoric changes may reflect

Fig. 11. Changes in the vegetation in the park since 1924 [modified from Blyth and Hudson (1987)]. Vertical axes represent percent cover in the park.

changing groundwater levels and hence vegetation during the MWP, the historic changes are more complex and may reflect the combined actions of three separate factors: the recent warming, which would tend to have the same effect as did the MWP; the extirpation of bison, which would tend to increase the abundance of Populus on the landscape and hence modify the fuel distribution; and intentional and unintentional fire suppression during the last ~100 years. There is no evidence in the sedimentary records shown here for the recent introduction of controlled burning in the park. Similarly, there is little evidence of the logging of Pinus and Picea, which is known to have occurred early in the 20th century, with the possible exception of the weak decline in Picea pollen in local zone 4 at Pen 5 Pond. It is likely that these activities are, and were, at too small a scale to show in the sedimentary records of these sites; this implies that those changes that are seen in these records were of substantially greater magnitude or frequency than the prescribed burning and the logging. Warmer and drier climate is often assumed to translate into increased fire activity. Campbell and Flannigan (2000) have shown this assumption to be flawed in situations where the warmer and drier climate leads to a less fire-prone vegetation, such as replacement of conifers with Populus. This study in EINP provides a further example of this effect. Although the situation at Pen 5 Pond, which makes this inverse relationship between fire activity and drought possible, is arguably not sufficiently widespread to alter significantly the fire regime at a regional to subcontinental scale, it is also not likely to be unique to this site. Further, it is probably not the only type of condition in the western boreal that can have this effect. We must, therefore, think not so much in terms of a regional fire regime, as in terms of a fire regime mosaic, much like the forest mosaic itself. Any large and extensive change in vegetation and fire regime has consequences for landscape carbon storage. Over a period of decades, carbon storage may increase with a decrease in disturbance frequency ( Kurz and Apps, 1994), as older vegetation generally contains more carbon. At EINP, this increase in carbon storage with landscape

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aging was accompanied by a replacement of grassland and shrubland by Populus forest, which would further increase carbon storage. The total biomass carbon storage in EINP is likely to be greater today than at any time since the MWP, several hundred years ago. If this is typical of the Aspen Parkland region, then this large area (over 1500 km2; Bird, 1961) may have been a significant carbon sink over the course of the 20th century. This study has implications for park management. The current paradigm in Canadian National Parks management is to attempt to maintain park ecosystems in as ‘natural’ a state as is consistent with public access, enjoyment, and safety. The difficulty emerges with what is considered to be ‘natural’. Here we have evidence that the fire regime and vegetation have both changed through time in the absence of human interference. The introduction of vigorous prescribed burning in the park might return the park’s fire regime to a state more similar to that which occurred during the LIA, but this will be limited by the need to prevent fire escapes into the area outside the park. It will also tend to reduce landscape carbon storage. With future climate expected to reach and exceed the warmth and dryness of the MWP in the next few decades (IPCC, 1996), the park’s hydrological state can be expected to change, and this will inevitably have consequences for the vegetation and the natural fire regime. Further, any largescale changes in the vegetation may affect local hydrology, causing a feedback loop to develop. Park managers must decide which, if any, ‘natural’ state they should — or perhaps practically can — attempt to emulate.

Acknowledgements We thank Anne Dickinson, Charles Blyth, Dennis Madsen, Brent McDougall, and the rest of the present and former staff at EINP. Brian Haskell conducted the 14C dating, and Bob Flett conducted the 210Pb dating. Thanks also to Thierry Varem-Sanders and Lana Laird for field and laboratory assistance. Mike Flannigan, Bill de Groot, Lana Laird, John Lowe, Dennis Madsen and an anonymous reviewer provided useful comments on the manuscript.

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