Palaeoecology of postglacial treeline shifts in the northern Cascade Mountains, Canada

ELSEVIER

Palaeogeography, Palaeoclimatology, Palaeoecology 141 (1998) 123–138

Palaeoecology of postglacial treeline shifts in the northern Cascade Mountains, Canada Marlow G. Pellatt a,Ł , Michael J. Smith a , Rolf W. Mathewes a , Ian R. Walker a,b a

Department of Biological Sciences and Institute for Quaternary Research, Simon Fraser University, 8888 University Drive, Burnaby, B.C. V5A 1S6, Canada b Department of Biology, Okanagan University College, 3333 College Way, Kelowna, B.C. V1V 1V7, Canada Received 22 July 1997; accepted 12 January 1998

Abstract Postglacial changes in vegetation and chironomid communities at a subalpine lake in the Engelmann Spruce–Subalpine Fir zone in the northern Cascade Mountains, British Columbia, indicate patterns of treeline and climate fluctuation during the Holocene. In late-glacial sediments of Cabin Lake, pollen assemblages representative of alpine vegetation and cold-stenothermous chironomids indicate cold conditions prior to the Holocene. In the early Holocene (10,090 to 7000 14 C yr BP) co-occurrence of spruce–fir parkland and a warm-adapted chironomid community indicates a warm and probably dry climate. In the mid-Holocene, inferred forest closure suggests that precipitation increased, and a mixture of warm- and cold-adapted chironomids indicates temperatures warmer than present, but cooler than in the early Holocene. This period between 7000 and 3200 14 C yr BP represents a transitional climate in which temperature gradually declined, culminating in cool neoglacial conditions. This transitional interval may correspond with the ‘mesothermic period’ proposed for lowland sites in southern British Columbia. Palaeobotanical evidence suggests that moist subalpine forest began to establish around 4800 14 C yr BP with minimum temperatures and maximum precipitation between 2435 and ca. 1700 14 C yr BP, corresponding with neoglacial advances throughout the northern Cordillera. A cool late Holocene (3200 14 C yr BP to present) is also supported by a further decline in warm-adapted chironomids. Comparisons with other study sites in the Pacific Northwest reveal that regional climatic changes were a major factor in driving biotic changes in this area.  1998 Elsevier Science B.V. All rights reserved. Keywords: palaeoclimate; pollen analysis; chironomids; treeline; Cascade Mountains; British Columbia; vegetation history

1. Introduction The postglacial vegetation and climate history of the southern interior of British Columbia has been examined at only a few locations (Alley, 1976; Mathewes and King, 1989; Hebda, 1995), all at relatively low elevations. Palaeoecological studies in north-central Washington (Mack et al., 1978a,b, Ł Fax:

C1 (604) 291-3496; E-mail: [email protected]

1979; Mehringer, 1985), reflect a similar lack of high elevation data. We present here a study of treeline and vegetation shifts in the subalpine Engelmann Spruce–Subalpine Fir zone (ESSF), as an aid in the understanding of Holocene vegetation dynamics and palaeoclimatic history in the southern interior. Our study also allows comparisons with better-studied coastal localities. Palaeoecological studies of subalpine treeline shifts on the Queen Charlotte Islands revealed three

0031-0182/98/$19.00  1998 Elsevier Science B.V. All rights reserved. PII S 0 0 3 1 - 0 1 8 2 ( 9 8 ) 0 0 0 1 4 - 5

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phases of vegetation and inferred climate change over the last 10,000 radiocarbon years (Pellatt and Mathewes, 1994, 1997; Pellatt, 1996). The early Holocene (10,000 to ca. 6000 14 C yr BP) on the Queen Charlotte Islands was warmer and drier than present, sustaining tree species such as western hemlock (Tsuga heterophylla) and Sitka spruce (Picea sitchensis) that are now characteristic of lowland temperate forests. This warm=dry period is recorded at lowland study sites throughout British Columbia, southeast Alaska and northern Washington (Warner, 1984; Mathewes, 1985; Mehringer, 1985; Barnosky et al., 1987; Quickfall, 1987; Fedje, 1993; Hebda, 1995; Mann and Hamilton, 1995; Hansen and Engstrom, 1996). Subalpine conditions became established on the Queen Charlotte Islands after 6000 14 C yr BP as the Pacific Northwest coast became cooler and wetter. Modern vegetation and climate became established on the Queen Charlotte Islands, coastal British Columbia, northern Washington, and southeast Alaska by ca. 3500 14 C yr BP (Fig. 1). While coastal localities were experiencing a cooling trend in the middle Holocene, it appears that warm temperatures were prolonged in the interior of British Columbia. Hebda (1995) suggests that mid-Holocene (7000 to 4500 14 C yr BP) temperatures were similar to the early Holocene, with modern levels of precipitation (Fig. 1). He calls this warm=moist climatic phase the mesothermic period. Temperatures apparently reached modern levels in the southern interior of British Columbia and northeast Washington around 4500 to 2500 14 C yr BP (Alley, 1976; Mack et al., 1978a,b; Mathewes and King, 1989; Hebda, 1995). This late Holocene climatic deterioration correlates with neoglacial advances in the Coast and Rocky Mountains (Porter and Denton, 1967; Ryder and Thomson, 1986; Clague, 1989; Luckman et al., 1993; Mann and Hamilton, 1995). Subalpine treelines are climatically sensitive tension zones, and are therefore ideal locations for reconstructing palaeoclimatic regimes (Clague and Mathewes, 1989; Pellatt and Mathewes, 1994). In this study we employ palynological analyses, as well as chironomid head capsules from a sediment core from Cabin Lake, British Columbia (121º13.20 W, 49º58.40 N), to reconstruct the Holocene history of vegetation and climate in the Engelmann Spruce–Subalpine Fir biogeoclimatic zone. These

analyses are supported by pollen ratio analysis, tephrochronology, AMS radiometric dating, loss on ignition, and statistical zonation, and allow us to compare the timing of climatic changes between high and low elevations in the interior, and between high elevation sites on the coast and in the interior.

2. Study area 2.1. Physiography Cabin Lake is located at 1850 m asl on Stoyoma Mountain (2283 m asl; 121º130 W, 49º590 N) in the southwestern interior of British Columbia (Fig. 2) at the northern limit of the Cascade Mountains. The Cascade Mountains of British Columbia merge into the Kamloops Plateau to the east, and are separated from the Coast Mountains to the west by the Fraser River (Holland, 1976). The eastern margin of the Cascade Mountains is a transition zone where summit elevation and dissection progressively decrease toward the Kamloops Plateau (Holland, 1976). The peaks of the Hozameen Range of the Cascade Mountains are characterized by high, serrated ridges that show the effects of intense alpine glaciation (Holland, 1976). Cirque basins are common on northand northeast-facing slopes of peaks and ridges. At lower elevations, between 1830 and 2135 m, there are rounded ridges and dome-shaped mountains which were over-ridden by ice at the maximum of the Cordilleran ice sheet (Holland, 1976). 2.2. Vegetation and climate Stoyoma Mountain is located in the central dry climate region of the Kamloops Forest Region (Lloyd et al., 1990). The ESSF zone is the uppermost forested zone in the southern three-quarters of interior British Columbia (Coupe, 1983; Meidinger and Pojar, 1991). The continental climate is relatively cold, moist, and snowy, with a short growing season, and long, cold winters. Mean annual temperature is 2 to 2ºC with 5 to 7 months below 0ºC and only 2 months or less above 10ºC (Coupe, 1983; Meidinger and Pojar, 1991). Precipitation ranges from 400 mm in the drier portions to 2200 mm in the wetter areas. As much as 70% of the precipitation falls as snow.

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Fig. 1. Comparison of climate change at selected sites along the Pacific Northwest coast and in the southern interior of British Columbia. IDF D Interior Douglas Fir Zone.

Engelmann spruce (Picea engelmannii) and subalpine fir (Abies lasiocarpa) are the climax trees of the ESSF. Engelmann spruce is longer-lived and thus often dominates the canopy of mature stands with subalpine fir in the understory. Subalpine fir becomes dominant at the higher elevations of the ESSF and in wetter areas (Meidinger and Pojar, 1991). Whitebark pine (Pinus albicaulis) appears in drier parts of the ESSF and mountain hemlock (Tsuga mertensiana) may occur in the western portion of

the ESSF near the Mountain Hemlock zone. The Cabin Lake study site is located in the dry, cold Engelmann Spruce–Subalpine Fir subzone variant 2 (ESSFdc2) (Lloyd et al., 1990). Typical trees and shrubs encountered are subalpine fir, Engelmann spruce, whitebark pine, black huckleberry (Vaccinium membranaceum), and white-flowered rhododendron (Rhododendron albiflorum). Typical dwarf shrubs, herbs and mosses include grouseberry (Vaccinium scoparium), five-leaved bramble (Rubus pe-

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Fig. 2. Map of British Columbia indicating the location of the Cabin Lake study site.

datus), mountain arnica (Arnica latifolia), Sitka valerian (Valeriana sitchensis), and red-stemmed feathermoss (Pleurozium schreberi) (Lloyd et al., 1990). 2.3. Study site: Cabin Lake (Figs. 2 and 3) is located at 1850 m asl. It is about 4 ha in area with a maximum water depth of 4.2 m. A small intermittent inlet stream runs into the north end of the lake. It carried water in

July but was dry in late August of 1995. An overflow outlet drains the south end of the lake when water levels permit. A well developed forest surrounds Cabin Lake on the east, south, and part of the west slopes. A fire has burned much of the slope north of the lake leaving open meadow vegetation. Some of the common trees and shrubs surrounding Cabin Lake are Engelmann spruce (the dominant tree species), subalpine fir, white-flowered rhododendron, Luetkea pectinata,

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Fig. 3. (a) Photograph of Cabin Lake showing ESSF forest surrounding the lake. (b) Photograph of Stoyoma Mountain showing the transition from ESSF forest to alpine tundra.

Vaccinium membranaceum, Vaccinium scoparium, Phyllodoce empetriformis, and Cassiope mertensiana. Some of the common herbs include Valeriana sitchensis, Veronica cf. wormskjoldii, Castilleja miniata, Saxifraga ferruginea, Senecio triangularis, Leptarrhena pyrolifolia, Lupinus arcticus, Caltha leptosepala, Arnica cf. cordifolia, Anemone occidentalis, Caltha biflora, Pyrola sp., Carex sp., and Eriophorum sp.

3. Methods 3.1. Pollen analysis A 399-cm sediment core was recovered near the centre of Cabin Lake at a water depth of 4.2 m.

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Subsamples of 1 ml were removed at 5 cm intervals, except in the basal 91 cm of clay and silt where 2 cm intervals were processed for pollen. Volumes were determined by displacement in water, using a 10 ml graduated cylinder. A known concentration of marker spores (11; 300 š 400 Lycopodium) was added to the subsamples before processing. The protocols for pollen extraction follow Berglund and Ralska-Jasiewiczowa (1986). Identifications of pollen and spores were aided by published keys (McAndrews et al., 1973; Fægri and Iversen, 1989; Moore et al., 1991) and the Simon Fraser University modern reference collection. Routine counting of palynomorphs was carried out at 500ð magnification and critical identifications were made under oil immersion at 1200ð. The basic pollen sum (between 500 and 875 grains) used for percentage calculation includes all terrestrial pollen. Raw data were converted into percentages using TILIA 2.0 (Grimm, 1993). TILIAGRAPH 1.25 was used to generate the pollen diagrams (Figs. 4 and 5) which were subdivided into local pollen zones using constrained cluster analysis. The computer program used for statistical zonation was CONISS (Grimm, 1987). Plant taxa used to generate the zonation dendrogram included trees and shrubs with values of at least 2% in two intervals. Mount Mazama (6730 14 C yr BP) and Bridge River (2435 14 C yr BP) tephras are present in the Cabin Lake sediment core. These well-dated tephras (Clague et al., 1995; Hallett et al., 1997) provide chronological control, along with 14 C dating. Tephras were identified by Gerald Osborn and Glen DePaoli at the University of Calgary using microprobe analysis. In order to supplement the age control provided by the tephras, three AMS radiocarbon ages were also determined (Table 1). One age is based on dating of a pollen concentrate, prepared by the method of Brown (1994). Radiocarbon ages between dated levels were interpolated using polynomial regression analysis (Grimm, 1993). It is well known that Diploxylon pine (Pinus contorta type) pollen is greatly overrepresented in pollen assemblages from the ESSF (Hebda, 1995; Pellatt, 1996; Pellatt et al., 1997). This pollen is largely of regional and extra-local origin and does not represent the local vegetation at Cabin Lake. In order to increase the resolution of local pollen types, a pollen diagram with lodgepole pine removed

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Fig. 4. Percentage pollen diagram for Cabin Lake, with 10ð exaggeration curves (stippled) to highlight abundances of infrequent pollen types. AMS dates are shown on the left. Pollen concentrations are ð100. Zones were derived by stratigraphically constrained cluster analysis (CONISS).

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Fig. 5. Percentage pollen diagram with lodgepole pine removed for Cabin Lake, with 10ð exaggeration curves (stippled) to highlight abundances of infrequent pollen types. AMS dates are shown on the left. Pollen concentrations are ð100. Zones were derived by stratigraphically constrained cluster analysis (CONISS).

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Table 1 AMS radiocarbon dates for the Cabin Lake sediment core 14 C

Sample identification

Sample description

Lab No.

Age,

yr BP

CLJL94-5-12, 304 cm CLJL94-5, 324 cm CLJL94-5, 368–370 cm

carbonized wood sample pollen leaf fragment

TO-5205 CAMS-29826 CAMS-29829

8; 910 š 120 10; 090 š 70 9; 860 š 60

Errors presented at š1s.

Fig. 6. Spruce=Diploxylon pine percent pollen ratios for Cabin Lake.

Fig. 7. Spruce=Diploxylon pine percent pollen ratios for surface sediment samples at selected elevations in the ESSF zone.

from the pollen sum was also prepared (Fig. 5). Spruce=Diploxylon pine percent ratios (spruce=pine ratios) were calculated (Fig. 6) to assist with interpretation of changes. These ratios are compared with the spruce=pine ratios (Fig. 7) calculated for modern lake surface sediment samples (Pellatt, 1996; Pellatt et al., 1997). 3.2. Chironomid analysis Cabin Lake sediments were mostly subsampled every 15 cm, with higher resolution sampling in regions of expected faunal change (i.e., suspected

late-glacial basal clay and basal clay=gyttja interface). Subsamples normally consisted of 0.5 ml or 1 ml of sediment, but up to 10 ml of sediment was necessary in some intervals to obtain sufficient numbers (at least 30) of chironomid head capsules for analysis. Isolation of chironomid head capsules, Chaoborus mandibles, and ceratopogonid head capsules followed the procedures outlined by Walker (1987). Remains were identified at 100–400ð magnification. Identifications were based on descriptions and keys by Oliver and Roussel (1983); Wiederholm (1983) and Walker (1988). Whole head capsules and fragments containing greater than half of the mentum were counted as one head capsule. Fragments that were exactly half of a head capsule were counted as one half, and fragments that consisted of less than half of the mentum were not counted. Most identifications were made at the generic level, although a few species identifications were possible. Broader taxonomic categories were necessary where genera could not be determined (i.e., Tanytarsina, Cricotopus=Orthocladius, Corynoneura=Thienemanniella). Data were compiled on a spreadsheet using TILIA 2.0 (Grimm, 1993), and chironomid percentage diagrams were produced using TILIAGRAPH 1.25 (Grimm, 1991) (Fig. 8). A constrained sum-of-squares cluster analysis (CONISS) was done to examine major changes in chironomid communities (Grimm, 1987). 3.3. Loss on ignition 1 ml of sediment was sampled at 5 cm intervals to determine organic content through loss on ignition. Protocol for the procedure was taken from Hakanson and Jansson (1983) in which dried sediment was combusted at 550ºC for 1 h. Percent LOI results are shown in Fig. 4 along with the pollen data.

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Fig. 8. Chironomid head capsule percentage diagram for Cabin Lake. AMS dates are shown on the left. Zones were derived by stratigraphically constrained cluster analysis (CONISS).

4. Results and discussion 4.1. Pollen assemblage zones at Cabin Lake The Cabin Lake pollen assemblages are shown in Fig. 4. A pollen diagram with Diploxylon pine removed from the pollen sum is shown in Fig. 5. Spruce=pine ratios are plotted in Fig. 6. 4.1.1. Zone CP-1 (Pinus cf. contorta – Picea – Poaceae – Artemisia, 399–312 cm, late-glacial, >10,090 š 70 14 C yr BP) In Zone CP-1, lodgepole pine type (Diploxylon pine) pollen percentages exceed 80%, the highest levels recorded in the core. Spruce pollen is also an important component of the assemblage, peaking at 350 cm along with Sitka alder and an abundance of herb pollen. Herbs such as grass (Poaceae), sedge (Cyperaceae), Artemisia, Caryophyllaceae, Epilobium, Asteraceae, Selaginella densa type, and Botrychium reach their highest levels here, and indicate an open subalpine=alpine vegetation cover. The sediments in this zone are composed of clay

with sand=silt bands (Figs. 4 and 5). Total pollen concentration was extremely low throughout, and dramatically increases at the clay=gyttja interface (324 cm; 10; 090 š 70 14 C yr BP). An AMS date of 9860 š 60 was determined from a leaf fragment between 368 and 370 cm. Variability in 14 C production, commonly referred to as a 14 C plateau, causes multiple calibrated ages for 14 C over certain time periods (Stuiver et al., 1991; Bartlein et al., 1995). Therefore, this 9860 š 60 14 C yr BP age is not considered significantly different than the 10; 090 š 70 14 C yr BP age obtained at 324 cm, at the clay=gyttja interface (Table 1). Low pollen concentration in conjunction with low spruce=pine ratios indicates an open vegetation cover and=or rapid sedimentation. Pine and spruce are the dominant pollen types, even though this zone probably represents an alpine tundra-like environment with only scattered spruce krummholz. Macrofossils obtained from silt bands in this zone include Elaeagnaceae trichomes, dwarf willow (Salix) leaf fragments and buds, a cf. Dryas leaf fragment, and a Phyllodoce needle. These macrofossils are indicative

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of a continental alpine=subalpine plant community. Thus late-glacial pollen and plant macrofossils suggest cold conditions at Cabin Lake. 4.1.2. Zone CP-2 (Pinus cf. contorta – Picea – Alnus viridis, 312–270 cm, 10,090 š 70 to ca. 7000 14 C yr BP) In Zone CP-2, lodgepole pine type pollen decreases relative to CP-1, but remains at over 40% of the pollen sum. Sitka alder type pollen achieves its highest levels in the core. Spruce and Abies pollen increase throughout this zone and whitebark pine type (Haploxylon pine) pollen is fairly high. Artemisia and Poaceae are the dominant herbs. Valeriana sitchensis and Liliaceae enter the pollen record in this zone. Total pollen concentration is at its highest here, attaining values of over 80,000 grains per ml. AMS radiocarbon dates of 10; 090 š 70 were obtained on a pollen concentrate from 324 cm and, 8910 š 120 14 C yr BP on a piece of wood at 304 cm (see Table 1). Relatively high levels of spruce and Abies pollen suggest that trees typical of the ESSF zone were present during this early Holocene period. The high values of lodgepole pine (¾40 to 50%) and Sitka alder pollen are most likely of regional and extra-local origin and suggest open conditions at Cabin Lake. High levels of spruce and Abies, in conjunction with initially high levels of Sitka alder, suggest that an environment with no modern analogue existed in the early Holocene. The absence of cool or moist indicators such as Ericales (heaths and Empetrum) and the significant presence of shade-intolerant taxa like whitebark pine type, Poaceae and Artemisia suggest a dry climate with open growing conditions. Relatively high spruce (probably Picea engelmannii) and Haploxylon pine (probably whitebark pine) pollen values, and low Abies pollen values (probably subalpine fir) suggest warmer=drier conditions than present. This interpretation is consistent with the early Holocene xerothermic period noted in coastal British Columbia (Mathewes, 1973; Mathewes and Heusser, 1981; Hebda, 1995). 4.1.3. Zone CP-3 (Pinus cf. contorta – Picea – Abies, 270–190 cm, ca. 7000–4800 14 C yr BP) In Zone CP-3, levels of lodgepole pine type and spruce pollen remain relatively constant. Abies

pollen increases relative to the previous zone whereas whitebark pine (Haploxylon) type and Sitka alder decrease. Aquatic Isoetes microspores appear for the first time. This zone appears to represent a period of vegetational and climatic transition at Cabin Lake. Abies (probably subalpine fir) becomes the dominant tree around Cabin Lake. Because Abies is under-represented in modern pollen assemblages (Dunwiddie, 1987; Hebda and Allen, 1993), 20% abundance suggests that Abies dominated the surrounding forest (Hebda and Allen, 1993). High Abies values and other subalpine indicators of moisture, such as Cyperaceae and Ericales, indicate that conditions were wetter than inferred in either zones CP-1 or CP-2. High pollen concentrations (Figs. 4 and 5) in conjunction with high organic content (% LOI) and low spruce=pine ratios (Fig. 6) indicate that climate was warmer than present. The increases in these subalpine taxa indicate that temperature was beginning to decrease during this zone. This zone corresponds with the mesothermic period observed in the southern interior of coastal British Columbia (Hebda, 1995). 4.1.4. Zone CP-4a (Pinus cf. contorta – Picea – Tsuga heterophylla – Cyperaceae – Ericales, 190–95 cm, 4800–2435 14 C yr BP) In Zone CP-4a lodgepole pine type pollen decreases to about 40% of the pollen sum. Spruce pollen increases and Abies decreases from CP-3 but remains as an important component of the pollen sum. Cupressaceae, Ericales, Rosaceae, and Cyperaceae all increase in abundance. Caltha biflora and Ranunculus enter the pollen record and Isoetes remains important. Regional transport of western hemlock pollen (Tsuga heterophylla) increases. The increased values of Ericales, Caltha biflora, Ranunculus type, Rosaceae, and Cyperaceae pollen indicate that typical subalpine vegetation had become established. A significant decrease in pollen concentration occurs — lower concentrations were only recorded in zone CP-1 (late-glacial). Values of regionally transported western hemlock pollen attain their highest values in this zone, suggesting a decrease in local pollen productivity, decreased temperature, and increased precipitation at lower elevations, as observed in the Fraser Canyon (Mathewes

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and King, 1989). It appears that climate was cooler than in zones CP-2 and CP-3, with increased importance of Engelmann spruce in the surrounding forest. 4.1.5. Zone CP-4b (Pinus cf. contorta – Picea – Abies, 95–65 cm, 2435 to ca. 1700 14 C yr BP) In CP-4b Abies, lodgepole pine type, Poaceae and Cyperaceae pollen increase whereas spruce pollen decreases. Subalpine=alpine herbs remain diverse and pollen concentrations remain low. Bridge River tephra (2435 14 C yr BP) occurs at the base of this zone. Increases in Abies and Cyperaceae indicate that conditions may have been wetter than those observed in CP-4a, but remaining cool. This increased wetness, corresponding with cool=wet conditions after deposition of the Bridge River tephra (ca. 2435 14 C yr BP), may be the same climatic factor that promoted glacial advances in the northern Cascade Mountains (moraines of the Burroughs Mountain Stade 2050 14 C yr BP), and the Canadian Rocky Mountains (Porter and Denton, 1967; Luckman et al., 1993), as well as cool=moist conditions in the Interior Douglas Fir (IDF) zone of the Fraser Canyon (Mathewes and King, 1989). Cool=moist conditions in CP-4b correspond with the development of modern forests in the Fraser Canyon (Mathewes and King, 1989), and in the interior Pacific Northwest, U.S.A. (Mehringer, 1985). At the same time pollen ratio analysis suggests that local spruce production was low, indicating that the environment was likely more open than in zone CP-4a (Fig. 6). 4.1.6. Zone CP-5 (Pinus cf. contorta – Picea – Pinus cf. albicaulis, 65–0 cm, ca. 1700 14 C yr BP to present) In Zone CP-5 whitebark pine type, spruce, and Isoetes pollen and spores increase. Abies, Sitka alder, and Ericales decrease. Pollen concentration increases to levels not seen since CP-3 and CP-2. Diversity of subalpine=alpine herbs decreases. An increase in spruce=pine ratio values is observed (Fig. 6). This zone represents modern conditions at Cabin Lake. High values of whitebark pine and Engelmann spruce indicate that conditions are drier than in Zones 4a and 4b. This relatively drier, possibly

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warmer phase appears to correspond with conditions in the interior Pacific Northwest, U.S.A. (Mehringer, 1985), but still represents cooler conditions than observed in the early Holocene xerothermic and mesothermic periods. 4.2. Chironomid assemblage zones at Cabin Lake The chironomid fossil record of Cabin Lake has been divided into four zones representative of the main changes that occur in chironomid community composition (Fig. 8). Zone 1 (399–330 cm) represents Late-Pleistocene sediments before ca. 10,000 yr BP at the clay=gyttja transition. Zone 2 (330– 265 cm) encompasses the early Holocene, between 10,000 yr BP and ca. 7200 14 C yr BP. Zone 3 (265– 112 cm) spans the mid-Holocene between ca. 7200 and ca. 3200 14 C yr BP. The most recent sediments make up Zone 4 (112–0 cm), representing ca. 3200 14 C yr BP to present. 4.2.1. Zone CC-1 (399–330 cm, late-glacial, >10,090š70 14 C yr BP) The late-glacial assemblage primarily consists of the widespread Tanytarsina group (up to 54%), and cold-stenotherms typical of oligotrophic (Paracladius, Parakiefferiella nigra, Mesocricotopus, Stictochironomus, Protanypus, and Heterotrissocladius), and mesotrophic (Sergentia) waters. The predatory Procladius also makes up a significant proportion of the late-glacial assemblage (up to 33%), and remains relatively abundant throughout the Holocene. The late-glacial chironomid assemblage at Cabin Lake is very similar to the ‘late-glacial Heterotrissocladius fauna’, coined by Walker and Mathewes (1987a) for coastal British Columbia sites, and is also found in New Brunswick (Levesque et al., 1993) and Germany (Hofmann, 1983). This assemblage consists of typical cold-stenothermous taxa whose distributions are primarily restricted to cold oligotrophic arctic and alpine waters, or the deep, cold profundal regions of large, thermally stratified temperate lakes (Walker, 1987, 1990; Walker and Mathewes, 1987a, 1989a). The Cabin Lake sediments show no indication of thermal stratification since this relatively shallow lake was formed, and so it is reasonable to assume that there was no deep, cold habitat isolated from the influences of climate at the surface waters.

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Thus, the cold-stenothermous assemblage suggests that the late-glacial water temperature, and presumably air temperature, was relatively cold. This is consistent with conclusions based on pollen and chironomid inferences on the British Columbia coast (Mathewes, 1973; Mathewes and Heusser, 1981; Walker and Mathewes, 1987a, 1989a). 4.2.2. Zone CC-2 (330–265 cm, 10,090 š 70 to ca. 7200 14 C yr BP) The beginning of the Holocene is characterized by the sudden disappearance of most cold-stenotherms, with only Heterotrissocladius and Sergentia persisting as small proportions of the faunal assemblage. In conjunction with this trend, significant increases in the warm-adapted chironomids occur. Most notably, Chironomus, Stempellinella=Zavrelia, Pagastiella, and Microtendipes, along with Chaoborus, rapidly become major faunal elements in the early Holocene. The eurythermic Tanytarsina and Procladius approach their lowest levels in this zone, which may be a result of less production in these groups or simply because of a greater prevalence of other taxa. This zone also shows an increase in proportions of the rheophilous Corynoneura=Thienemanniella group, and the eurythermic Psectrocladius and Corynocera. The rapid decline in cold-stenothermous taxa, and subsequent dominance of typical warm-water taxa strongly suggest significant climatic warming. Although not as stenothermic as the cold-water taxa, the warm-adapted chironomids in this assemblage are most often found in warm, temperate waters (Walker, 1987; Walker and Mathewes, 1987a,b, 1989a,b; Walker et al., 1997). In the relatively uncommon instances where a few of these taxa occur in arctic or alpine sites, they are restricted to small shallow ponds which attain high summer water temperatures (Walker and MacDonald, 1995). This warm early Holocene period at Cabin Lake confirms the widespread extent of the early Holocene xerothermic interval as described by Mathewes and Heusser (1981), Mathewes (1985); Hebda (1995) and Elias (1996). 4.2.3. Zone CC-3 (265–112 cm, ca. 7200 to ca. 3200 14 C yr BP) Just prior to Mazama ash deposition, a major shift in fauna occurs. Although this mid-Holocene zone

still supports a large group of warm-adapted chironomids, a notable reduction in Stempellinella, Zavrelia and Pagastiella is seen, and Sergentia, a cold-water genus, reaches significant levels (up to 27%). Heterotrissocladius also increases to levels comparable to its late-glacial abundance. Chironomus remains a dominant genus through this zone (up to 27%), with Microtendipes consistently making up approximately 5% of the community. Other important warmwater taxa comprising this zone are Dicrotendipes and Parakiefferiella cf. bathophila, with Chaoborus mandibles reaching their greatest abundance. Tanytarsina (37%) and Procladius (34%) again attain significant relative abundances in the mid-Holocene, and various rheophilous taxa continue to be consistently represented in small proportions. The decline in some warm-water chironomids (most notably Stempellinella, Zavrelia and Pagastiella) and the significance of the cold-stenothermic Sergentia suggest that this period was characterized by a cooling trend. Although Sergentia is indicative of cold waters (Walker et al., 1997), the persistence of a relatively diverse and abundant warm-water assemblage reveals that water temperature had cooled in comparison to the early Holocene, but was still warmer than present. In addition Sergentia is commonly found in waters with moderate oxygen depletion, often in shallow lakes that freeze to the bottom in winter, as Cabin Lake may have in the mid-Holocene. Increased precipitation may have played a role in winter anoxia. Greater snowpack on the lake would lead to longer winter conditions, as the time required for the snow to melt would be extended and anoxic conditions would persist for a longer period. This transitional cooling trend throughout the mid-Holocene at Cabin Lake supports Hebda’s (1995) designation of a mesothermic period in the southern interior of B.C. 4.2.4. Zone CC-4 (112–0 cm, 3200 14 C yr BP to present) The most recent, late-Holocene sediments reveal a dramatic decrease in the relative contribution of warm-water taxa as a group, with many genera becoming locally extirpated and others reaching their minimum Holocene values. Sergentia remains relatively abundant, with Heterotrissocladius re-attaining its typical late-glacial abundance at the begin-

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ning and end of this interval. Possibly significant is the reappearance of Paracladius, a strongly coldstenothermous taxon (Walker et al., 1997), in the late-Holocene fossil assemblage. Procladius remains a dominant faunal element, whereas Tanytarsina reaches its maximum abundance (61%), surpassing its late-glacial abundance. Rheophilous taxa continue to be deposited into the lake, although a less diverse group is evident in the late-Holocene. The persistence of Sergentia and Heterotrissocladius, and further reduction in the warm-water chironomid assemblage in the late Holocene indicate further cooling. This is consistent with observed cooling in the southern interior of British Columbia (Mathewes and King, 1989) and corresponds to the timing of neoglacial advances in the northern Cascades (Porter and Denton, 1967).

5. Conclusions Four main periods of climate are documented since deglaciation at high elevations in the Canadian Cascade Mountains. These periods are: (1) a late-glacial cold period (>10,090 14 C yr BP); (2) an early Holocene warm, dry period (10,090 to 7000 14 C yr BP); (3) a mid-Holocene period of climatic transition beginning with a warm, moist phase from 7000 to 4800 14 C yr BP and then cooling between 4800 to 2435 14 C yr BP and; (4) modern neoglacial (cool=moist) conditions (2435 14 C yr BP to present). The oldest palaeobotanical evidence from Cabin Lake indicates continental, alpine tundra conditions in the late-glacial. Elaeagnaceae trichomes (probably Shepherdia canadensis), cf. Dryas leaf fragments, dwarf willow remains and a Phyllodoce needle attest to a cold continental climate during this time. Abundant Shepherdia pollen was reported by Clague et al. (1995) from early Holocene peat above the present treeline in the Coast Mountains. Chironomid communities in Cabin Lake during the late-glacial consist of typical cold-adapted species. Early Holocene conditions at Cabin Lake may well represent a vegetation assemblage that has no modern analogue. High values of lodgepole pine type, spruce, and Sitka alder type pollen indicate open parkland conditions. This spruce–fir parkland

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environment suggests that dry conditions hindered the development of closed forests. This data, in conjunction with a warm-adapted chironomid assemblage indicates that the early Holocene was warm and dry at Cabin Lake and corresponds with the early Holocene xerothermic period. After ca. 7000 14 C yr BP, percentages of spruce and Abies pollen increase. This palaeobotanical evidence implies that ESSF forests developed as precipitation increased toward the end of the early Holocene xerothermic period (Mathewes and King, 1989; Hebda, 1995). Chironomid evidence suggests a slight cooling and possibly increased precipitation during the mid-Holocene. The changes in pollen and chironomid assemblages between ca. 7200 and ca. 7000 14 C yr BP appear to follow a notable cooling event detected from Greenland, Antarctica, and Africa around 7500 14 C yr BP (Alley et al., 1997; Stager and Mayewski, 1997). This cooling event was approximately half the amplitude of the Younger Dryas and may well have had a global expression (Alley et al., 1997). Hebda (1995) suggests that a climatic period termed the ‘mesothermic’ be incorporated into the climatic history of British Columbia. He suggests that this period may be an extension of the Hypsithermal, extending the timing of Holocene maximum warmth to around 4000 14 C yr BP. This would incorporate the timing of the Hypsithermal in most of Canada (Anderson et al., 1989). Palaeobotanical evidence at Cabin Lake indicates that early Holocene warmth extended into the mid-Holocene, thus supporting Hebda’s hypothesis. Warm, moist conditions, relative to present climate, led to the development of ESSF forest at Cabin Lake. A modern chironomid community became established at Cabin Lake around 3200 14 C yr BP whereas modern pollen assemblages appear at Cabin Lake after deposition of the Bridge River tephra (2435 14 C yr BP). A period of increased moisture falls between 2435 and ca. 1700 14 C yr BP, corresponding with neoglacial advances throughout the Canadian Cordillera. Although the pattern of change between pollen and chironomid assemblages at Cabin Lake is similar, it is important to note that there is a definite lag in the response time between chironomids and vegetation (Fig. 9). Time lags between plant and insect migration and establishment have been ob-

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RWM and IRW. We would also like to thank Markus Heinrichs, Doug Cheetham, and Andre´ Tessier for their help in the field. Our appreciation is extended to S.A. Elias and an anonymous reviewer for comments that improved the manuscript.

References

Fig. 9. Comparison between pollen and chironomid assemblage zones for Cabin Lake.

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Acknowledgements We would like to thank the Natural Sciences and Engineering Research Council (NSERC) of Canada for their support in funding this project via grants to

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