Spatial patterns of atmospheric pollen dispersal in the Colorado Rocky Mountains, USA

Review of Palaeobotany and Palynology, 74 (1992): 293-313 Elsevier Science Publishers B.V., Amsterdam

293

Spatial patterns of atmospheric pollen dispersal in the Colorado Rocky Mountains, USA Patricia L. Fall Department of Geography, Arizona State University, Tempe, A Z 85287, USA (Received June 20, 1991; revised and accepted May 13, 1992)

ABSTRACT Fall, P.L., 1992. Spatial patterns of atmospheric pollen dispersal in the Colorado Rocky Mountains, USA. Rev. Palaeobot. Palynol., 74:293-313. Atmospheric pollen dispersal in the southern Rocky Mountains was sampled with 18 Tauber traps located between 2470 and 3780 m elevation. A three year study provided data on both pollen frequencies and pollen accumulation (grains cm- 2 yr- !) for six distinct vegetation types. Pollen spectra from the shrub steppe below the forest belt is characterized by Artemisia, Gramineae, Compositae, and other non-arboreal pollen taxa. These pollen typc~ are readily carried upslope to higher elevations. Pollen from sub-alpine conifers, Picea and Abies, is not transported far by the wind, and indicates the presence of nearby host trees. Pinus pollen, readily distributed by the wind, is found in all traps, but only exceeds 20% where the trees are present, and in tundra vegetation. While pollen from Pseudotsuga and Populus trees occurs in low frequencies and low influx rates, it is relatively abundant in montane forest stands dominated by these trees. Although pollen grains are transported both upslope and downslope, most pollen transport is upslope. At least 50% of the pollen deposited in alpine tundra, 20% in the sub-alpine forest, and 7% in the montane forest, comes from plants growing at lower elevations. Pollen spectra deposited in steppe vegetation are the least distorted by pollen from plants growing in other vegetation types, and characterize the broad valleys of sagebrush steppe that makes up the regional vegetation. Spectra in alpine tundra are dominated by pollen from plants growing at lower elevations and at long distances from the collection site. In a comparison of paired collectors, Tauber traps with aerodynamically designed lids were more effective in open vegetation where they collected substantially more pollen than traps without the lids. Tauber lids appear to have less effect on pollen accumulation rates in forested stands, probably due to the filtering of pollen by trees. Annual pollen accumulation rates in west-central Colorado range between 1000 and 5000 grains cm-2 yr-l. Mean rates of modern pollen influx (in grains c m - ' yr -!) are 1100 in alpine tundra, 2700 in the sub-alpine forest, 3400 in montane forest types, and 2200 in the shrub steppe. These pollen accumulation values reported for the mountains of Colorado are comparable to pollen accumulation rates from other localities in the western United States.

Introduction Interpretations of fossil plant assemblages from fossil pollen spectra often depend upon analogies based on correlation of modern pollen with modern vegetation. Not only are pollen frequencies useful in this respect, but pollen influx or pollen accumulation rates may be helpful in the interpretation of fossil pollen spectra. Atmospheric Correspondence to: Dr. P.L. Fall, Department of Geography, Arizona State University, Tempe, Arizona 85287, USA. Fax: 602 965 8313. 0034-6667/92/$05.00

samplers can provide data on modern pollen influx, information that cannot be obtained from moss polsters, surface sediment or lake samples due to uncertainty regarding the amount of time these samples represent. Pollen influx data also can provide additional information regarding the structure and density of a vegetation community. This is especially crucial for the recognition of tundra vegetation in which local pollen production can be greatly overshadowed by pollen from plants growing far from the sample site. The overrepresentation of pollen from other vegetation types is a problem that becomes even more acute

© 1992 - - Elsevier Science Publishers B.V. All rights reserved.

294

in alpine settings (Spear, 1989), where more productive and broader expanses of forests and steppes may lie within a few hundred meters. Few studies in the montane western United States have utilized both modern pollen frequencies and pollen influx data to characterize vegetation communities (exceptions: Solomon and Silkworth, 1986; Hall, 1990). Explanatory models have been developed for the mechanisms of pollen dispersal (e.g. Tauber, 1965). Tauber's model, developed for pollen dispersal in the forests of Europe, proposes that the greatest amount of pollen deposited in a basin comes from pollen transported below the canopy within the trunk space (Tauber, 1965, 1967, 1974). Given the aerodynamic potential of pollen, areas with denser vegetation will provide more filtering than more open areas. Interestingly, a comparison of airborne pollen data with those from surface lake muds shows that the greatest proportion of pollen entering a depositional basin is not via the atmosphere, but rather from introduction by streams (Peck, 1973; Bonny, 1976; Tauber, 1977; Pennington, 1979). Sources of uncertainties regarding pollen accumulation in lake sediments have been discussed (e.g. Davis et al., 1973; Webb et al., 1978; Howe and Webb, 1983), and problems associated with variations in lake size, lake depth, and source areas have been outlined (e.g. Pennington, 1973, 1979; Lehman, 1975; Bonny, 1976, 1978; Jacobson and Bradshaw, 1981). With these uncertainties in mind, perhaps, atmospheric traps may provide information on airborne input to depositional basins which can be helpful in understanding the relative proportions of atmospheric pollen and fluvial pollen input. By comparing atmospheric pollen accumulation in Tauber traps with annual pollen accumulation rates in the upper sediments in lakes and bogs a relative proportion of the amount of pollen entering the basin from the atmosphere and that entering by water (streams and slope wash) can be deduced (Fall, 1992). Pollen spectra from Tauber traps should be similar to other collectors - - moss polsters and bogs - - where virtually all of the pollen input is airborne. In addition, these data from atmospheric collectors will provide pollen influx data for modern vegetation types that can

P.L. FALL

be used as analogs for past vegetation and pollen relationships. Other studies have evaluated theoretical models of pollen dispersal, representation, and source areas (Jacobson and Bradshaw, 1981; Prentice, 1985; Jackson, 1990). These discussions primarily pertain to pollen transport and deposition in forested vegetation. Even very small openings (<0.5 ha) within forests record substantial amounts of regional pollen introduced from above the canopy (Jackson, 1990). Pollen source areas in open vegetation have rarely been addressed and will be more difficult to pinpoinL Tundra vegetation can be difficult to recognize by relative pollen frequencies alone. Relative frequencies of pollen taxa can be misleading in tundra environments due to the effects of long-distance transport of pollen to high latitudes from distant source areas (e.g. in the Arctic: Ritchie and LichtiFederovich, 1967; Kappen and Straka, 1988; and in the Antarctic: Van der Knapp, 1990). Because tundra communities produce relatively little pollen compared with other vegetation types, their pollen spectra can be readily obscured by pollen from long-distance sources. The importance of pollen influx data for the recognition of tundra environments has been demonstrated empirically for both the fossil record (e.g. Davis 1967; Pennington and Bonny, 1970; Maxwell and Davis, 1972) and for modern studies in tundra environments (e.g. Ritchie and Lichti-Federovich, 1967; Bourgeois et al., 1985; Johansen and Hafsten, 1988). Palynological recognition of alpine tundra where vegetation types that produce disproportionate amounts of pollen occur in close proximity, separated only by altitude, may be even more difficult than the recognition of arctic tundra. Several studies have utilized Tauber's model and his aerodynamically designed pollen collectors in the lowland forests of Europe (e.g. Andersen, 1970, 1974; Berglund, 1973; Krzywinski, 1977; Hicks and Hyv/irinen, 1986). Markgraf (1980) employed Tauber traps in her study of pollen transport in the Swiss Alps and found that a trap located just above the mountain-pine/spruce timberline contained somewhat less pollen (4480 grains cm -z yr-1) than traps located in the lower elevation forests where pollen accumulation ranged

ATMOSPHERIC POLLEN DISPERSAL IN THE COLORADO ROCKY MOUNTAINS. USA

between 5000 and 20 000 grains cm- '~ yr -~. Thus, Markgraf's study suggests that alpine tundra may be delineated by slightly lower pollen accumulation values than those recorded in lower elevation vegetation types. In general, the nature of pollen dispersal in mountains is complex. Montane regions may provide extreme situations in which pollen from a variety of source areas is readily mixed due to the close juxtaposition of several vegetation types. Most notably, alpine tundra vegetation may be difficult to distinguish palynologically for a number of reasons: (1) the low stature of tundra plants, (2) sterility of many of the plants, (3) their short flowering season, (4) low pollen production due to an abundance of insect pollinated taxa, and (5) the significant introduction of pollen from longdistance sources. Deposition of pollen in treeless vegetation is further amplified by the smaller local pollen source area at high elevations compared to the much larger areal extents of the lower elevation vegetation types (Gaudreau et al., 1989; Spear, 1989; Jackson and Whitehead, 199 I). Because of these difficulties an understanding of modern pollen deposition in alpine regions becomes vital for the interpretation of fossil pollen spectra. I present results of a three-year study of pollen accumulation in 18 Tauber traps located in the mountains of central Colorado to help clarify processes of pollen dispersal, transportation, and deposition in a variety of vegetation types in this mountainous setting. The objectives of this analysis are to characterize the modern vegetation according to both modern pollen frequency and influx data, to determine whether pollen spectra in atmospheric samplers represents the vegetation adequately, and to elucidate atmospheric dispersal of pollen in a montane region where distinct vegetation types lie in close proximity. As a means of clarifying alpine pollen dispersal and sedimentation I test several related hypotheses: (I) Trees in forested vegetation types filter pollen from other vegetation types. Thus, pollen spectra from forests will represent local vegetation components better than will pollen spectra from open areas that lack this filtering. (2) Although pollen spectra from alpine tundra will include pollen from tundra plants and pollen

295

from plants growing at lower elevations, it will be possible to recognize tundra by its relatively low annual pollen accumulation rates and by indicator taxa. (3) Pollen spectra from atmospheric samplers provide adequate analogs for modern vegetation types in montane regions. (4) Pollen is collected at higher rates in traps with the aerodynamically designed Tauber lid, reflecting the additional component of pollen transported horizontally by the wind. This effect will be enhanced in open vegetation due to filtering of pollen by trees in forests.

Geographical setting The study area lies on the western slope of the southern Rocky Mountains in central Colorado near the town of Crested Butte (38°52'30"N and 106°59'W) (Fig. I). Montane topography creates a diverse array of microclimates. The climatic zones roughly correspond to altitude; temperature decreases and precipitation increases with elevation. The Rocky Mountains display a strong precipitation gradient; the west slope receives much more precipitation than the east slope. During the winter months, Pacific air masses bring relatively moist storms into Colorado which, due to orographic effects, release most of their precipitation on the west slope. A secondary precipitation peak occurs in the summer, when subtropical air masses bring moisture from the Gulf of Mexico and the southern Pacific Ocean by way of the Gulf of California (Hales, 1974). Annual temperatures at Crested Butte, Colorado (2700m elevation) average 1.8°C, and can vary from a maximum of 33°C to a minimum of -41°C (Siemer, 1977). Altitudinal temperature and precipitation gradients, create four major plant zones in the central Rockies: (l) a-shrub steppe or grassland zone below the lower tree limit, (2) a montane zone comprised of drought resistant trees, (3) a subalpine zone, the major forest belt in the Rocky Mountains, and (4) an alpine zone above upper treeline (Marr, 1961). Within the study area, the mountains rise from a treeless shrub steppe that covers the intermontane valleys (Fig. 2). Big sagebrush (Artemisia tri-

296

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dentata) is the dominant shrub. Other common shrubs include rabbitbrush (Chrysothamnus viscidiflorus and C. parryi) and antelopebrush (Purshia tridentata). Bunchgrass (Festuca thurberi) communities form a secondary dominant below the forest border. Montane conifers, restricted to rocky outcrops, are found within the big sagebrush steppe and grasslands. Junipers (Juniperus osteosperma and J. scopulorurn)and an occasional douglas fir (Pseudotsuga menziesii) are found in these limited habitats. Ponderosa pine (Pinus ponderosa) is found in only two small stands that total approximately 6 hectares, and pinyon pine (Pinus edulis), a common montane conifer in other parts of Colorado, does not grow in the study area. In central Colorado big sagebrush vegetation is terminated by aspen (Populus tremuloides) at the lower forest border between approximately 2800 and 2900 m elevation. In this region, the montane forest zone consists primarily of aspen. Small stands of lodgepole pine (Pinus contorta) and

douglas fir (Pseudotsuga menziesii) occupy suitable, but limited substrates. The main forest type, the sub-alpine forest, which ranges from 3000 m to upper treeline at 3500 m elevation, is dominated by Engelmann spruce (Picea engelmannii) and subalpine fir (Abies lasiocarpa). Aspen and bunchgrass are successional communities. Exposed bedrock, talus, and rock glaciers cover much of the area. above treeline on the highest peaks in the region (up to 4300 m). Alpine vegetation forms a diverse mosaic of herb meadows with grasses, sedges, forbs, and low perennial mat and cushion herbs typical of alpine fell fields. Methods

The modern pollen rain was sampled with pollen collectors modeled after those designed and tested by Henrik Tauber (1965, 1967, 1974, 1977). Tauber's pollen traps were designed aerodynamically to capture airborne particles carried horizontally by

ATMOSPHERIC POLLEN DISPERSALIN THE COLORADO ROCKY MOUNTAINS, USA

297 1e

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Alkali 107°05. W

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Lily Lake

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Basin .13

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8,11 - 1 3 Alkali Creek 9,1 0

East River

14 1 5-1 8

Jack's Cutoff Taylor River

'N

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Fig. 2. Vegetation map of the study area near Crested Butte, Colorado (after Keammerer and Stoecker, 1980). The locations of the Tauber traps are shown as numbers 1-18. Descriptions of each sample locality are provided in Table I.

298

the wind. Lids for the Tauber traps, each with a 5 cm diameter opening following Tauber's specifications, were modelled after a trap designed at the University of Berne (Markgraf, 1980). Replication of this design facilitates comparison of these data with other airborne pollen studies (Hicks and Hyv/irinen, 1986). The base of each trap was a plastic jar (10 cm high and 10 cm in diameter) that could be changed easily in the field. Each trap received 20 ml of glycerine and 3% phenol to prevent sample desiccation, and bacterial and fungal growth. Twenty-four Tauber traps were located between 2470 and 3780 m elevation in lower elevation steppe, montane and sub-alpine forests, and alpine tundra above upper treeline. The traps were established in late May/early June 1980, and were collected at intervals varying from three months to over one year, until August 1983 when they were retrieved. The forests traps were located in dead trees, on tree stumps, or on fence posts. These traps ranged in height from 1 to 3 m above the ground in order to prevent their burial by snow. In tundra and steppe vegetation traps were placed in rock piles, generally between 0 to 0.5 m above the ground. A vegetation map of the study area (see Fig. 2) follows Keammerer and Stoecker's (1980) map. Plant lists made at each Tauber trap locality follow the nomenclature of Harrington (1954) and Weber (1976). Of the original 24 samplers placed in the field, 18 provided at least one full year of pollen accumulation, and are used in this study (Table I). (Thirteen samplers survived the entire three year experiment). The data provided by these traps estimate the annual pollen accumulation in each vegetation type in central Colorado. After collection from the field, each Tauber jar was rinsed thoroughly, and its contents were sieved through !.5 mm and 2001~m mesh screens to remove plant fragments and insects, then centrifuged to concentrate the pollea. Each sample received five to fifteen tablets of Lycopodium spores. (Five tablets contain 60,383 + 1892 spores or approximately 12,100 per tablet - - University of Lund, 1984, Batch No. 414831). The samples were treated with 4~]% HF and acetolysis mixture (Faegri and Iversen, 1989). Absolute pollen influx

i,.L. FALL

is estimated by calculating the total number of pollen grains per unit time per unit area (in this case, 19.6 cm 2, based on a 5 cm diameter opening in the trap lid). Pollen residues were placed in either silicone oil or glycerine, mounted on microscope slides, and then counted at 450 to 950 x. Generally, only one slide or a portion of one slide provided 500 or more pollen grains. Published keys and the reference collection at the University of Arizona aided the identification of pollen taxa. Pollen from trees, shrubs, herbs, and indeterminant (corroded, degraded, crumpled, concealed, and broken pollen grains) and unknown pollen types are included within the terrestrial pollen sum (Appendix I). lndeterminant and unknown pollen taxa are assumed to have been produced by terrestrial plants, and thus, are included in the pollen sum. Palynomorphs from pteridophytes, Cyperaceae, riparian shrubs, aquatics,, algae, and fungi are excluded from the terrestrial pollen sum. These taxa are excluded because they represent locally abundant types. Percentages of these types are calculated relative to the terrestrial pollen sum (Appendix 2).

Atmospheric pollen accumulation Pollen percentage data

In general, pollen spectra from atmospheric collectors vary according to elevation, and correspond to the surrounding vegetation (Figs.3 and 4). The most abundant pollen taxon is Artemisia, which shows a pronounced elevational trend. Artemisia pollen percentages vary between 40 and 60% in Artemisia steppe, from 12 to 35% in forest stands within steppe vegetation, from 6 to 12% in forests, and drops to 11% above treeline. Pollen from shrub and herb taxa are well-represented in the collectors. Conifer pollen frequencies are lower than expected. Abundances of Pinus pollen are localized, amounting to 64% in one trap in the Pinus contorta forest (No. 10), but Pinus pollen is relatively sparse in most traps. Frequencies of Pinus pollen average 22% in alpine tundra, 9% in the sub-alpine forest, 41% in Pinus contorta stands, 11% in the montane forests without Pinus contorta,

299

ATMOSPHERIC POLLEN DISPERSAL IN THE COLORADO ROCKY MOUNTAINS. USA

TABLE I Description of Tauber Trap localities Sample No.

Tauber trap

Elevation

Vegetation type

Location

Height above ground

Mt. Emmons 1

3760 m

Alpine tundra

S-facing slope; rock pile on ground

0m

Comments: Grasses and forbs include Festuca brachyphyila, Poa fendleriana, Calamagrostis purpurescens, and Carex bre~,ipes. Perennial herbs include Geum turbinatum, Achillea lanulosa, Artemisia scopulorum, Potentiila spp., Trifolium spp., and Selaginella densa. Dwarf Picea engelmannii, Abies lasiocarpa, and Salix spp. grow within 100 m of trap. Copley Lake 1

3250 m

Sub-alpine forest

S side of lake; tree stump

3m

Comments: Trap in dense Picea engelmannii-Abies lasiocarpa forest. Shrubs include Juniperus communis, Lonicera involucrata, Ribes spp., Rosa woodsii, Sambucus racemosa, Pachystima myrsinites, and Vaccinium myrtiilus. Sedges and forbs include Carex spp., Arnica cordifolia, and Pedicularis racemosa. Copley Lake 2

3250 m

Sub-alpine forest

N edge of meadow; dead tree

3m

Comments: Trap at edge of meadow with Bromus sp., Carex spp., Deschampsia caespitosa, Festuca spp., and Juncus sp. Forest species same as for CL I. 4

Lake Irwin 2

3150 m

Sub-alpine forest

NE edge of lake; tree stump

2m

Comments: Picea engelmannii and Abies lasiocarpa forest surrounds lake, Salix spp. along shore. Lily Lake 1

3150 m

Sub-alpine forest

E edge of lake; fence post

2m

Comments: Picea engelmannii and Abies lasiocarpa forest surrounds lake (see vegetation description for CL l). Trap near outlet in open grass and sedge meadow with Salix spp. thicket around western and southern edge of lake. Ironbog 1

2930 m

Mixed conifer forest

SE edge of basin; tree stump

2m

Comments: Pinus contorta forest with Picea engelmannii, Abies lasiocarpa, and a few Pseudotsuga men:iesii. A stand of Populus tremuloides lies 150 m upslope from bog. Shrubs include Betula glandulosa, Vaccinium myrtillus, V. caespitosum, Lonicera involucrata, Pachystima myrsinites, Ribes montigenum, and Salix spp. Sphagnum moss covers wet areas on the bog around open water. Care.,: spp., Bromus sp., forbs, Deschampsia caespitosa, and Eriophorum angustifolium grow in the open meadow. Drosera rotundifolia, a rare species with disjunct distribution in N W Montana, grows in iron-rich bog. Ironbog 2

2930 m

Mixed conifer forest

N side of bog; dead lodgepole pine tree

2.5 m

Comments: T,ap in forest at edge of bog. Same vegetation as described for IB 1. Alkali Creek 3

2830 m

Mixed conifer forest

N-facing slope; tree stump

2m

Comments: Trap on north slope of Flat Top Mountain at the lower elevationai limit of mixed conifer stands above Alkali Basin. Pseudmsuga menziesii, Picea engelmannii, Abies lasiocarpa, and Populus tremuloides trees surround trap. Shrub species include Comus stolonifera, Ribes spp., and Thalictrum fendleriana. P. tremuioides and Artemisia tridentata dominate at lower elevation. 9

East River 2

2770 m

Douglas fir ~,brest

W-facing talus slope; dead aspen tree

2m

Comments: Pseudotsuga menziesii, and Populus tremuloides forest surrounds trap locality on talus 200 m above the East River. Pinus contorta stand is within 100 m of trap. Shrub taxa are Juniperus communis, Amelanchier alnifolia, Symphoricarpos oreophilus, and Arctostaphylos uva-ursi. 10

East River 1

2740 m

Lodgepole pine forest

W-facing talus slope; log

1m

Comments: Trap is in Pinus contorta stand within Pseudotsuga menziesii forest. Populus tremuloides and shrub taxa, Amelanchier alnifolia, Symphoricarpos oreophilus, Juniperus communis, Berberis repens, and Artemisia tridentata grow in the forest.

P.L. FALL

300 TABLE I (continued) Sample

Tauber trap

Elevation

Vegetation type

Location

Height above ground

Alkali Creek 2

2740 m

Aspen forest

N-facing slope; dead aspen tree

2m

No.

Comments: Dense Populus tremuioides stand on slope above Alkali Basin. Undergrowth includes Acer glabrum, Rosa woodsii, Symphoricarpos oreophilus, Pedicularis bracteosa, Thalictrum fendleriana, and Valeriana acutiloba. Artemisia tridentata steppe within 100 m. 12

Alkali Creek 4

2720 m

Sagebrush steppe

Flats; fence post

1.5 m

Comments: Artemisia tridentata steppe. Trap was located 100 m NE of Alkali Basin core (Markgraf and Scott, 1981) on modern surface of Pleistocene lake beds. Other shrubs and herbs include Chrysothamnus viscidiflorus, Festuca spp., Poa spp., Stipa sp., and Carex spp. Salix spp. thicket grows in creek bed. 13

Alkali Creek 1

2710 m

Steppe/mixed conifer

N-facing slope; aspen tree

2m

Comments: Mixed conifer stand on talus in Artemisia steppe 50 m above Alkali Creek. Populus tremuloides, Pseudotsuga menziesii, and Picea pungens are trees present. Shrubs include Ribes spp., Sambucus sp., Rosa woodsii, Juniperus communis, Symphoricarpos oreophillus, and Artemisia tridentata. 14

Jack's Cut-off 2

2680 m

Sagebrush steppe

Flats; rock pile on ground

0m

Comments: Open Artemisia tridentata steppe. Festuca spp., Poa spp., Achillea lanulosa, Purshia tridentata, Chrysothamnus viscidiflorus, and Gutierrezia sarothrae surround trap. Stands of Populus tremuloides and Pseudotsuga menziesii grow approximately 1000 m from trap. 15

Taylor River 1

2550 m

Steppe/Pond. Pine

Outcrop above river; rockpile

0.5 m

Comments: TR ! and TR 2 are located in one of two small stands (6 ha) of Pinus ponderosa and Juniperus osteosperma growing on a basalt substrate approximately 100 m above the confluence of the Taylor and East rivers. Other species include Ribes montigenum, Purshia tridentata, Berberis repens, Selaginella densa, and several species of grass. Conifers grow within Artemisia steppe. 16

Taylor River 2

2550 m

Steppe/Pond. Pine

Rock outcrop above river

0.5 m

Comments: Trap is in rocky basalt outcrop at top of cliff, vegetation is same as for TR !. 17

Taylor River 3

2470 m

Steppe/Juniper

SE-facing slope; dead juniper tree

2m

Comments: TR 3 and TR 4 are on talus slope 50 m above the Gunnison River. Surrounding vegetation is Artemisia steppe, vegetation near trap is Juniperus osteosperma, Pseudotsuga menziesii, Amelanchier sp., Chrysothamnus sp.. Yucca angustissima, Gutierrezia sarothrae and Festuca sp. Populus angustifolia, P. balsamifera, Picea pungens, Salix spp. and AInus tenuifolia grow along the river banks. 18

Taylor River 4

2470 m

Steppe/.[aiilv,.~

SE-facing slope; dead juniper tree

2m

Comments: Vegetation is the same as for TR 3

only 9% in Pinus ponderosa stands (traps Nos. 15 and 16), and 9% in Artemisia steppe. Thus, except above treeline, Pinus pollen makes up only about 10% of the regional pollen rain. Picea and Abies pollen grains may be carried by wind currents less efficiently than are Pinus pollen grains. Even within the sub-alpine forest, Picea averages only a little over 6%, while Abies measures about 2%. Both

of these sub-alpine trees are slightly more productive in mixed conifer forest stands where the trees are also present. Besides Pinus, the three most common pollen taxa in all traps are Artemisia, Juniperus, and Gramineae. These three types are produced by several species each, and, therefore, are not ecologically diagnostic of any vegetation type. For

ATMOSPHERICPOLLENDISPERSALIN THE COLORADOROCKYMOUNTAINS.USA

301

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% Fig. 3. Atmospheric pollen frequencies from tree and shrub taxa. The combined values for three years of pollen data from 18 Tauber collectors are plotted by elevation. Pollen taxa included in "other shrubs" are listed in Appendix 1.

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Fig. 4. Atmospheric pollen frequencies from herbaceous and aquatic taxa. The combined values for three years of pollen data from 18 Tauber collectors are plotted by elevation. Percentages of SelaginelTa densa, Cyperaceae, Alnus, Betula, Salix, and "'Aquatics" are calculated outside the terrestrial pollen sum. Pollen taxa included in "other herbs," "other," and "aquatics" are listed in Appendix !.

example, Juniperus pollen is produced by two lower elevatioil trees, Juniperus osteosperma and J. scopulorum, and by the montane and sub-alpine understory shrub, Juniperus communis. Juniperus communis also is a common prostrate shrub near and above treeline. Similarly, Artemisia pollen can

be attributed to several species, most notably Artemisia tridentata, the dominant steppe shrub, as well as the alpine species Artemisia scopulorum. Plants in the Gramineae family are abundant both in tundra and steppe vegetation, and throughout the forests and meadows in the mountains. While

302

these three pollen taxa, Artemisia, Juniperus, and Gramineae, may be ecologically undiagnostic, they, along with Tubuliflorae pollen, are the most common types collected not only in traps in big sagebrush steppe, but by atmospheric samplers within the forest in the winter, and interestingly in late-glacial deposits in this area (Fall, 1988). Thus, these taxa not only reflect the regional vegetation of big sagebrush steppe and grasslands that today covers the lower mountain slopes and intermontane valleys of central Colorado, but also provide an analog for the late-glacial steppe vegetation. Herbaceous pollen is common in the Tauber traps. In particular, the Ranunculaceae and Rosaceae families are well represented. Rosaceae pollen (I 1%) collected in alpine tundra (No. 1), identified as cf. Geum, and probably is produced by Geum turbinatum, a common alpine herb. Ranunculaceae pollen, averages 9% in the sub-alpine forest, and comes primarily from Caltha, Ranunculus, and Thalictrum. Pollen from other herbaceous taxa, generally amounting to less than 1% each of the total pollen sum, are combined in the "other herb" category (see Appendix i). Unusually high percentages of herbaceous or shrub pollen are included in the "other" pollen category. Types included in this category contribute between 2 and 6% of the pollen sum, usually in a single pollen type per trap. These types include Campanula, Caryophyllaceae, Cruciferae, Labitae, Liguliflorae, Liliaceae, Lonicera, Papilionaceae, Phlox, Polemonium, cf. Rosaceae, and UmbeUiferae. The relatively high frequencies of these insect pollinated taxa are most likely due to the introduction of insects along with their pollen load into the traps. However, because pollen spectra from moss polsters in this same region also contain localized peaks for many of these same pollen taxa (Fall, 1992), this "other" pollen category was included in the terrestrial pollen sum. Sample No. 12 (AKC 4) contains anomalously high frequencies and influx of cf. Ranunculaceae (> 1000% calculated outside the terrestrial pollen sum, and > 14000 grains cm -2 yr- 1). Sample No. 13 (AKC 1) has anomalous values for cf. Ranunculaceae and Cruciferae (>300% and 101%, 13000 and 1450 grains cm -2 yr- 1, respec-

P.L. FALL

tively). Pollen identified as cf. Ranunculaceae and Cruciferae presumably were added to these samples by insects, and, therefore, are not included in the terrestrial pollen sum. All pollen deposited in the Tauber collectors, except for apparent additions of large amounts of pollen by insects, is considered part of the atmospheric pollen rain. However, frequencies of several additional sparse pollen types are calculated outside the terrestrial pollen sum to facilitate comparisons with other types of surface samples (moss polsters, lake muds and bog samples). These types include Selaginella densa, Cyperaceae, Alnus, Betula, Salix, and "other aquatics." Occasional pollen grains of Typha latifolia, PotamogetonTriglochin-type, and Typha-Sparganium-type, as well as Isob'tes spores and the algae Pediastrum and Botryococcus were identified. A few Arceuthobium pollen grains and Pteridophyta spores also are found, as are spores of the dung fungus Sporomiella (< 5 cells in each of seven traps).

Pollen influx data Pollen influx data from Tauber traps in Colorado (Figs. 5 and 6) reflect trends similar to those for the frequency data discussed above. Relatively large amounts of pollen from arboreal taxa from traps within closed forest stands reflect primarily local production. Pseudotsuga and Populus pollen show high influx values in traps located in stands of these trees. Populus pollen reaches a maximum of 430 grains cm- 2 yr- 1 in a dense Populus tremuIoides forest (No. I1). Similarly, Pseudotsuga has an influx of 320 grains cm -2 yr-1 in the Pseudotsuga menziesii forest (No. 9). Pollen influx is low for Picea and Abies in the atmospheric traps. Picea pollen influx averages 160 grains cm- 2 yr-1 in the sub-alpine forest, and 260 grains cm-2 yrin the mixed conifer forest surrounding the Keystone Ironbog. Abies pollen influx is also greatest in mixed conifer stands (60 grains cm- 2 yr- ~), and about 50 grains cm- 2 yr- ~ in the sub-alpine forest. Again Picea and Abies trees are more productive at their lower elevational limits. Pinus pollen influx is relatively high only in the three traps within Pinus contorta stands, IB 1 (No. 6), IB 2 (No. 7), and ER 2 (No. 9). These

ATMOSPHERIC POLLEN DISPERSAL IN THE COLORADO ROCKY MOUNTAINS. USA

303

/ ,oooK

......II.....

.-.3500

E P

a~

o o~ ~3000 UJ

Alpine Tundra

Subalpine Forest Montane Forest

.......

Steppe 2500 -~--H 0 400

100

0

1 1 i i i i i i 800 1600

-FI 0 400

-T'T-1

0 400

-T-I 0 400

I I I I 1

0 400 1000 20

60

100

0

800

1600

200

50

Grains (cm "2 yr "1)

Fig. 5. Average atmospheric pollen influx (grains cm -2 yr-l) from tree and shrub taxa. Three years of pollen influx from 18 Tauber traps are plotted by elevation. Pollen taxa included in "other shrubs" are listed in Appendix I.

/ y

,b"

40O0 -

~


t

3500 -

5"

,+'

o-~

~,~-

@

2-

I

Alpine Tundra

SuI~piae

c

-5

.9 m 3000

t

UJ

........... ?

2500

~ .....

i ....... i ...........

i ......

iiiiiiiil-....

=

Forest Montane Forest

_--~5

Steppe

i

0 400

200

-I-I 0 400

7 200

-F-] 0 400

II11111~"]

0 400

0

800

1600

0 400

200

0 400

200

50

100

200

50

Grains (cm "a yr "1)

Fig. 6. Average'atmospheric pollen influx (grains cm -2 yr-l) from herbaceous and aquatic taxa. Three years of pollen influx from 18 Tauber traps are plotted by elevation. Pollen taxa included in "'other herbs," "'other,"and "aquatics" are listed in Appendix I. values range from 736 to 1420 grains c m - 2 yr-~. In all other traps Pinus pollen influx values are consistently low, averaging approximately 200 grains cm -2 yr-~, even in traps situated in small stands (6 ha) of Pinus ponderosa surrounded by big sagebrush steppe (Nos. 15 and 16). Thus, Pinus pollen appears to be relatively evenly distributed

at all elevations within the mountains of west central Colorado. Paleoecologicai inferences based on Pinus pollen distribution in other regions of Colorado or the Rocky Mountains, where many species of Pinus occupy wide elevational and ecological ranges, should not be based upon these relatively low Pinus pollen influx values.

304

Artemisia pollen accumulates at rates of 100 to 400 grains cm-2 yr-1 in the sub-alpine forest, and 120 grains cm-2 yr-1 in alpine tundra. Below the lower forest border Artemisia pollen influx ranges from 200 to 1600 grains cm-2 yr-1, averaging 740 grains cm- 2 yr- 1. Pollen influx from taxa growing at elevations below the traps remains fairly consistent in all atmospheric samples. Average influx rates (in grains cm- 2 yr- 1) are: Quercus (75), Ephedra (8), Sarcobatus (19), Cercocarpus-type (21), Chenopodiineae (75), and Ambrosia-type (62). The total influx of these types is 260 grains cm-2 yr-1. This value, when combined with the "regional" Pinus pollen influx, results in an average pollen influx from regional plants to all traps at all elevations of at least 500 grains cm -z yr -~. This value increases with elevation as more regional taxa contribute pollen to local deposition. Annual pollen accumulation rates

Forests provide the highest mean annual pollen accumulation values in Colorado. Montane mixed conifer forests, averaging 3385 grains cm -2 yr-1, have the highest pollen influx, while sub-alpine conifer forests average 2723 grains cm-2 yr-1. A Tauber trap located above treeline in alpine tundra, MtE l (No. I), collected 1092 grains cm-2 yr-1, and traps located below the lower forest border in Artemisia steppe averaged 2229 grains cm -2 yr- ~. These data from Colorado can be compared with modern pollen influx values for the Canadian arctic where pollen accumulation rates are generally less than 800 grains cm -2 yr-1 in tundra vegetation, approximately I 100 grains cm - 2 yrin forest-tundra vegetation, and about 7000 grains cm-2 yr- t in the boreal forest (Ritchie and Lichti-Federovich, 1967). Thus, pollen influx in alpine tundra in Colorado is comparable with pollen accumulation rates at the forest-tundra border in the arctic; the elevationally restricted Rocky Mountain forests produce about one third to one half the pollen of the geographically broader boreal forest. The few other studies of atmospheric pollen influx in the intermontane west provide results comparable to those presented here (Table II). An

P.L. FALL

average pollen influx of 4018 grains cm-2 yr-1 is recorded from five snowbanks in the sub-alpine forests of the Albion Mountains, southern Idaho (Davis, 1984). Based on a study of two Tauber traps, one in southern Wyoming (Ice Slough) and one in southern Idaho (Grays Lake), Beiswenger (1987) reported an average pollen accumulation of 1023 grains c m - ' yr -1 for Artemisia steppe in the northern Great Basin. In a six year study of pollen influx, Solomon and Silkworth (1986) found the average pollen accumulation on the east slope of the Sierra Nevada to be 4137 grains cm- 2 yr- 1 in the upper montane Pinus-Abies forest and 3337 grains cm -z yr -1 in the Pinus edulis woodland. One Tauber trap in Artemisia steppe collected an average of 5420 grains cm -z yr-1 over a 5 year period. Pollen accumulation below the montane forest types averaged 2140 grains cm - 2 yr- ~ (Solomon and Silkworth, 1986). The authors included only the major anemophilous taxa, and did not provide total pollen accumulation values. Thus, annual pollen accumulation values in their study region may be somewhat higher than the numbers reported here. In general, the pollen data provided by Solomon and Silkworth's (1986) study are comparable to those for montane Pinus forests and Artemisia steppe in Colorado (see Table II). However, these two data sets may not be as similar as they appear. In the southern Sierra Nevada, where several species of Pinus dominate the montane forests, Solomon and Silkworth (1986) recorded Pinus pollen deposition between 2000 and 3000 (mean = 2472) grains cm- 2 yr- ~ in the forests. Pinus pollen accumulation is 3295 grains cm -2 yr -1 in Artemisia steppe just below the montane forest, and remains approximately 800 grains cm -z yr -1 at lower elevations even where pine trees are absent (Solomon and Silkworth, 1986). These values for Pinus pollen are about four times those found in central Colorado where Picea and Abies dominate the forests. The Crested Butte study area provides a unique ecological setting because Pinus contorta is the only abundant Pinus species in the area, and its distribution is restricted by the predominantly shaie substrates. The small number of pine species and the limited extent of pine forests in the Crested Butte

ATMOSPHERICPOLLENDISPERSALIN THE COLORADOROCKYMOUNTAINS.USA

305

TABLE !I Mean annual pollen accumulation (pollen grain cm -2 yr -!) for Tauber trap localities in the western United States; each locality (n) may represent several years of pollen influx

Vegetation type

Location

Mean (n)

Range

Combined Mean

-

Range

Alpine tundra

CO t

1092

(1)

1092

Sub-alpineforest

CO t ID z NM a

2723 4018 9125

(4) (5) (6)

2147-2934 1613-8138 7000-13,250

5720

1613-13,250

Montane forest

CO t CA 4 NM a'6

3385 (6) 3660 (5) 10,325 (10)

2216-4903 3025-5077 5500-15,250

6760

2216-15,250

Sagebrush steppe

ID/WY s CO t CA 4

! 023 2229 5420

(2) (7) (i)

710- ! 651 858-4484 -

2310

710-5420

Grassland

NMa'7

3517 (15)

2000-6750

3520

2000-6750

Data sources and notes: tThis study; ZDavis, 1983; 3Hall, 1990(digitized from fig. 15);4Solomon and Silkworth, 1986; 5Beiswenger, 1987; 6Meanand range are calculated for the montane forest in New Mexicoexcludingone anomalously high valueof approximately 65,000 (59,600 pine) pollen influxin 1982for sample 33 (Hall 1990,fig. 15); 7Mean and range are calculated for grassland vegetation on the High Plains excluding one sample from near the Rita Blanca Creek, Texas which yielded an influx of approximately 24,000 pollen grains in 1982 and approximately 14,000 in 1981 (Hall, 1990, fig. 15).

area provide fossil pollen records which are not dominated by pine pollen (Fall, 1988). Accounting for the differences in Pinus pollen production between the Colorado Rocky Mountains and the Sierra Nevada the data are very consistent between these mountain regions. In an extensive study along the east slope of the southern Rocky Mountains from the Sangre de Cristo Mountains in northern New Mexico onto the plains of west Texas, Hall (1990) found atmospheric pollen influx to be somewhat higher than the values reported above for the intermontane west. Hall's study emphasized variability in pollen influx over two years, 1981 and 1982. When pollen influx values for the two years are combined, they demonstrate that the annual pollen infliJx in the spruce-fir forest is approximately 9000 grains cm- z yr- 1, influx in the pine-scrub oak woodland is about 10,000 grains cm-2 yr- ~ (excluding sample, 33 in which pollen influx was 65,000 in one year), and pollen accu~nulation is approximately 3500 grains cm-2 yr-1 in the grassland (excluding

one site along Rita Blanca Creek where pollen influx averaged 19,000 grains cm -2 yr-1 for the two years) (Hall, 1990; see Table 11). These values, particularly those for the forests, are about two to three times higher than their counterparts in west-central Colorado. One possible explanation for this discrepancy is that the forests of the Sangre de Cristo Mountains produce much more pine pollen. The influx of Pinus is approximately 2000 grains c m - Z y r -1 in the spruce-fir forest, 7000 grains cm -2 yr -1 in the pine-scrub oak woodland, and 1000 grains c m - 2 y r -~ on the High Plains (Hall, 1990, fig. 15). In contrast, the extremely limited distribution of pine trees near Crested Butte, Colorado produces pine pollen at an average rate of only 200 grains cm-2 yr-1 in vegetation types without pine trees and approximately 1000 grains cm-2 yr-1 in lodgepole pine stands. Besides this discrepancy in the amount of Pinus pollen collected in the two regions, the data from Colorado and New Mexico are comparable.

306

Mean annual pollen influx in the montane west averages between approximately 1000 to 7000 grains cm -2 yr-1 (as indicated by the combined mean for each vegetation type in Table II). Pollen accumulates most slowly in open vegetation, especially alpine tundra, and, to a lesser extent, shrub steppe and grassland. The highest pollen influx values come from forested vegetation, particularly where pine trees dominate. The highest mean annual pollen influx values are in the montane and sub-alpine forests of New Mexico, and to a lesser extent, in the Great Basin, whe:~ pine species dominate. The lowest forest pollen accumulation values come from central Colorado where pine is largely absent and where spruce and fir dominate.

Long-distance pollen transport The Tauber trap data demonstrate that a substantial amount of pollen is contributed to upland sites by species growing at lower elevations in Colorado. Pollen from plants that grow below 2700 m elevation is contributed by Quercus, Ephedra, Sarcobatus, Cercocarpus-type (which includes pollen from Cercocarpus, Purshia, and Cowania), and Ambrosia-type. These pollen taxa average about 260 grains cm- 2 yr- 1 in all traps, and when combined with arboreal pollen, which is also transported upslope, make up approximately 50% (540 grains cm -z yr -1) of the annual pollen influx in alpine tundra. Pollen from lower elevation taxa amounts to about 20% of the annual pollen rain (600 grains cm -2 yr -1) in the sub-alpine forest, 7% (250 grains cm-2 yr- 1) in the montane forest, and only 4% (about 70 grainscm-2 yr -1) in the shrub steppe. These are minimum values because much of the Chenopodiineae, Juniperus, Artemisia, Tubuliflorae, and Gramineae pollen probably is also from plants growing at elevations below the location of the traps. Small quantities of pollen clearly were transported downslope from higher elevations. Pollen taxa which contribute pollen to lower elevations are Picea, Abies, Pseudotsuga, Pinus, and Populus. If we assume that the pollen from these taxa originated in the montane and sub-alpine forests, then about 2.8% of the pollen in the steppe and

P.L. FALL

1.4% of the pollen in montane forests is carried downslope. However, trees of these same genera (e.g. Picea pungens, Pseudotsuga menziesii, Populus angustifolia, and P. balsamifera) also grow at lower elevations in riparian habitats. Pinus pollen collected in steppe vegetation, which presumably originated in Pinus contorta stands, is carried to lower elevations. Most atmospheric transport in the mountains near Crested Butte involves pollen from lower elevation vegetation which is transported upslope. Pollen spectra in steppe vegetation along the mountain flanks are the least affected by this phenomenon, and, therefore, can be used to document the "regional pollen rain". The lower elevations of Colorado (below 2100 m) and the intermontane valleys (up to 3000 m elevation along the Gunnison River Valley; see Fig. 1) are dominated by Artemisia steppe vegetation and grasslands. The broad areal extent and domination of these vegetation types contributes large amounts of pollen to the mountains of central Colorado. The forest belt near Crested Butte falls between 3000 and 3500 m elevation and occupies significantly less area than the steppe both in terms of elevational range and areal extent due to the geometry of the mountains. The open steppe vegetation produces the majority of the regional or above canopy portion of the pollen rain (sensu Jacobson and Bradshaw, 1981). The forests have larger components of the local and extra-local (Jacobson and Bradshaw, 1981) proportions of the total pollen. A greater amount of the pollen within the forests comes from the trunk space or gravity components and is deposited within the forest. The influx of pollen from lower elevation taxa becomes more of a problem with increased elevation. This is most apparent in alpine tundra where at least one half of the pollen accumulation in Tauber traps is from plants growing as much as 1500 m lower in elevation. Pollen spectra in alpine tundra reflect to a greater extent the regional r~ollen component (with a pollen sourc., radius t¥om 500 m to 50 km) than the local or extra-local pollen. Thus, pollen spectra from high elevation sites may provide good records of regional pollen and could be used to track the br,~adest changes in regional vegetation.

307

ATMOSPHERIC POLLEN DISPERSAL IN THE COLORADO ROCKY MOUNTAINS. USA

E~ficiency of Tauber traps: comparison of samplers with and without Tauber lids

Three pairs of traps, in which collectors with Tauber lids were paired with jars capped by flat lids, illustrate the relative efficiency of the aerodynamically designed Tauber lid. Both types of collectors had openings of the same size (50mm diameter). One pair of traps, IB I (No. 6) and IB 2 (No. 7), was located in the mixed conifer forest surrounding the Keystone lronbog. Two other pairs, TR I (No. 15)and TR 2 (No. 16), and TR 3 (No. 17) and TR 4 (No. 18), were located in conifer stands within the Artemisia steppe. In general, the paired traps had very similar relative pollen percentages. Only Picea pollen was collected consistently in greater abundance in the traps with the Tauber lids. Interestingly, all of the jars without the Tauber lids had higher frequencies of Artemisia, Populus, Quercus, and Gramineae pollen, and two jars had higher frequencies of Juniperus. This suggests that, perhaps, the design of the Tauber lid has little effect on the capture of these grains. However, annual influx data demonstrate the greater efficiency of traps with Tauber lids (Table III). In all three paired samples the traps with Tauber lids collected more pollen than did the jars without Tauber lids. The importance of the Tauber lid in aiding pollen capture is least apparent for the paired collectors within the forest, IB l and IB 2. This may be due to greater filtering of pollen and lower wind speeds in the forest. The efficiency of the Tauber traps is demonstrated most clearly in open vegetation where wind speeds are higher and filtering by vegetation is not as important. In a similar study of paired atmospheric collec-

tors, Bonny and Allen (1983) found collectors with Tauber lids to be much more efficient than traps without Tauber lids. Their study was designed to test whether pollen splashed by rain from the collar of Tauber lids added significantly to the traps as had been proposed by Krzywinski (1977). Contrary to expectations, they showed that traps in the open collected more pollen than traps in the forest, where significant rainsplash had been predicted. In closed forest vegetation their jar samplers collected only 67% of the amount collected in samplers with Tauber lids. Over an open lake, the efficiency of the jars without Tauber lids ranged from 33% (5 m offshore) to 20% (15-50m from shore) (Bonny and Allen, 1983). Their data compare well with pollen efficiency found in collectors in Colorado (Table III). In the Rocky Mountain forests the efficiency of sample jars without Tauber lids was 70% of those with Tauber lids. Jars in open steppe vegetation collected only 39% to 23% of the pollen which accumulated in the traps with Tauber lids. These results suggest that collectors with Tauber lids may be more effective in collecting localized pollen, particularly conifer pollen. Atmospheric pollen accumulation in winter

Pollen accumulation between October and May in the mountains of western Colorado averages approximately 18% of the annual pollen influx in the sub-alpine forest, 18% in the montane forest, and 14% in the shrub steppe. Although the higher elevations mountains are snow-covered, plants at lower elevations can flower during this period, and may contribute some of the pollen collected in traps during the winter. Populus and many of the conifers flower in late spring, Juniperus flowers in winter, and plants in the Compositae (Tubuliflorae,

TABLE I!I Comparison of influx values from atmospheric collectors with and without Tauber lids Traps with Tauber lids

Jars without Tauber lids

Efficiency of jars without Tauber lids

IB l= 4393 grainscm-2 yrTR 2 = 4484 grainscm- 2 y r TR 3=2216 grainscm-2 yr-

IB 2 = 3028grainscm- 2 yrTR i= 1031grainscm-2 Yr-m TR 4=858 grainscm-2 yr- '

70% 23% 39%

308

Ambrosia-type, and Artemisia), Chenopodiineae, and Gramineae families flower in late summer and into autumn. However, most of this is pollen that has been reworked in the atmosphere during the winter months. The persistence of deteriorated pollen in all traps suggests that pollen is reworked throughout the year. The highest frequencies of deteriorated pollen are in traps located near the ground. A collector in alpine tundra, MtE 1 (No. I), contains 5% deteriorated pollen grains. Three traps in Artemisia steppe average 7.5%, with one trap documenting up 14% deteriorated pollen (see Fig.4). Deteriorated pollen grains in all other traps average 2-3%. Saltation of previously deposited pollen may explain the high frequencies in collectors near the ground surface. These data show that pollen grains are carried by the wind at least 0.5 m above the ground before they are redeposited. Although the relative frequencies of deteriorated pollen are higher in traps located in open vegetation pollen accumulation rates for deteriorated pollen are comparable because less pollen is deposited in tundra and steppe vegetation than in forested vegetation. Average accumulation rates of deteriorated pollen (in grains cm-2yr -1) are: alpine tundra: 56, sub-alpine forest: 80, montane forests: 72, and big sagebrush steppe: 80. Contrary to expectations, the highest amount of deteriorated pollen (136 grains cm -z yr-1) was collected in a trap in the sub-alpine forest (CL 1; No. 2); the lowest (19 grains cm -z yr-1) was recovered in a trap in steppe/mixed conifer forest (AKC I; No. 13). Thus, there is little difference between the amounts of deteriorated pollen deposited in each vegetation type. Deteriorated pollen is assumed to represent a portion of the regional pollen component, averaging 76 grains cm -2 y r - 1 that is dispersed rather evenly to all elevations. Hicks and Hyv~irinen (1986) suggested that trap height may influence the composition of pollen spectra. They found that traps located at ground level will contain both local and regional pollen components, where traps positioned above 1 m height collect more regional pollen. Al~o trap height may in particular influence the amount of pollen (or influx) in each trap. They found that while relative pollen frequencies, particularly of tree pollen, did not vary appreciably, influx was

P.L. FALL

effected by trap height. Traps located at ground height had higher influx of both arboreal and nonarboreal pollen (Hicks and Hyv~irinen, 1986). These observations may hold for traps in forested vegetation, but the influences of trap height may be greatly overshadowed by the large representation of regional pollen in traps situated in open vegetation regardless of the height of the trap. In Colorado, the traps located in open vegetation, tundra and steppe, were, for the most part located on the ground or approximately 0.5 m above the ground surface. These traps consistently contained proportionally more regional than local pollen. Whereas, traps within the forests were located between 1-3 m above the ground surface, and these traps collected significantly more of the local, particularly arboreal, pollen. Contrary to Hi~zks and Hyv~irinen's (1986) predictions, significantly higher pollen accumulation rates were found in traps within the forest vegetation, where traps were located at least 1 m above the ground surface. Thus, trap height seems not be a dominant factor influencing the amount of pollen collected in a trap when compared to the differences between traps placed in open versus forest vegetation types. Two collectors, CL l (No. 2) and CL 2 (No. 3), at Copley Lake (3250 m) in the sub-alpine forest measured pollen accumulation between October 12, 1980 and December 13, 1980, a period during which sub-alpine and alpine plants were not flowering. Pollen accumulation during these two months amounted to 3.3% of the annual pollen influx, and pollen from sub-alpine conifers was absent. Average pollen frequencies in these two traps are: approximately 28% Artemisia, 20% Tubuliflorae, 12% Gramineae, 12% Juniperus, 6% Ambrosia-type, 5% Chenopodiineae, 2% Pinus and 4% deteriorated pollen. All of the pollen collected near this sub-alpine lake in winter is produced by plants growing at lower elevations. Interestingly, these winter pollen spectra very strongly resemble modern spectra from Artemisia steppe, as well as fossil pollen spectra from lateglacial deposits (Fall, 1988). Thus, there are at least small amounts of pollen in the atmosphere year-round. Further, the shorter the flowering season of high elevations plants, the more likely it is that local pollen accumulation will be masked by pollen from lower elevation taxa.

ATMOSPHERIC POLLEN DISPERSAL IN THE COLORADO ROCKY MOUNTAINS. USA

Discussion

This study of atmospheric pollen dispersal in the mountains of Colorado shows that pollen spectra become increasingly distorted at higher elevations. At least half of the pollen deposition ifi alpine tundra comes from taxa growing at lower elevations. This effect may be enhanced by winter pollen depos':tion from lower elevation taxa. A longer growing season for plants at lower elevations allows proportionally more pollen from these plants to be concentrated in alpine pollen spectra. The main pollen taxa in the atmosphere during winter are Artemisia, Tubliflorae, Gramineae, Juniperus, Ambrosia-type, Chenopodiineae, and Pinus. These pollen taxa are abundant in alpine tundra, and many are transported by wind particularly well (e.g. Artemisia, Juniperus, Gramineae, and Pinus). Pollen spectra from steppe vegetation at the base of the mountains characterize the regional pollen rain most accurately. Pollen spectra in the forest belts provide the most localized vegetation record, but pollen from lower elevation taxa still amounts to approximately 7% in Pinus contorta, and 20% in Picea-Abies forest stands. The first hypothesis, that trees help to filter pollen in forested regions, appears to be supported. Pollen spectra from Tauber traps located within forest stands provide the most localized records of vegetation, particularly of forest tree taxa. The forests in the Crested Butte area are depauperate in arboreal species. Only five species of trees dominate: Picea engelmannii, Abies lasiocarpa, Populus tremuloides, Pseudotsuga menziesii, and Pinus contorta. Picea and Abies co-dominate in the sub-alpine forest, the latter three species dominate local forest stands almost exclusively. Tauber traps located within closed canopy forests or in small openings within the forest (0.5-5 ha) contained high relative frequencies and total accumulations of pollen grains from the surrounding forest species. None of the pollen from forest trees is carried very far or in great quantity beyond the extent of the forest stand, with the exception of Pinus pollen produced by Pinus contorta trees. In spite of the fact that pollen spectra provide a clear record of local vegetation, there remains a substantial long-distance or regional pollen component in

309

these traps. Approximately 20-45% of the pollen deposition in forest stands comes from the regional pollen rain with an inferred source radius from approximately 100 m to 50 km. The second supposition, that alpine tundra should be recognizable by its pollen spectrum, is less readily supported. Based on pollen frequency data alone it would be difficult to recognize alpine tundra vegetation. Although the trap on Mt. Emmons (No. l; 3760 m) in alpine tundra had high percentages of non-arboreal pollen taxa, most of these types are not diagnostic of tundra (e.g. Juniperus, A rtemisia, Tubuliflorae, Gramineae, and Cyperaceae). Furthermore, Pinus pollen occurs at a higher frequency (22%) than in any other trap, except those in Pinus contorta stands. No specific tundra indicator taxa were identified, although this trap did have a higher proportion of Rosaceae pollen, including Geum and Dryas pollen, than the other traps. In sum, this Tauber trap provides pollen frequency data that are the least representative of local and extra-local vegetation. At least half of the pollen comes from plants growing at least 1000 m below the trap site, and it would be difficult, based on pollen frequency data alone, to conclude that these data indicate alpine tundra vegetation. A more hopeful conclusion can be reached by looking at pollen accumulation in alpine tundra. Alpine tundra has the lowest average pollen influx of all vegetation types in the mountains of Colorado, thereby corroborating Markgraf's (1980) conclusions regarding pollen accumulation rates in the Swiss Aips. However, three Tauber traps in the shrub steppe had comparable or lower pollen accumulation rates (Nos. 14, 15, and 18). Although the total average pollen influx in alpine tundra is only slightly lower than in other vegetation types, influx of key taxa is appreciably lower than in other vegetation types (see Figs.5 and 6). Pinus pollen influx is no higher than in other vegetation types where pine trees are not present. Similarly, influx values of the main pollen taxa - - Juniperus, Artemisia, Gramineae, and Tubuliflorae B are significantly lower than in other vegetation types. Thus, the influx (grains cm -2 yr-1) of abundant pollen taxa like Artemisia ( l 17) and Juniperus (144) are relatively low in alpine tundra, even though

310

their relative frequencies are quite high (11% and 13%, respectively). Thirdly, in most cases, pollen collected by the Tauber traps provides a record of the surrounding vegetation that compares well with pollen spectra from moss polsters collected at the same sites (Fall, 1992). Exceptions to this correspondence include greater concentrations of conifer pollen in moss polsters than in the Tauber traps. Picea and Abies pollen, in particular, are not deposited in large numbers in the traps, even those in the sub-alpine forest. This result agrees well with Jackson's (1990) findings from the Adirondack Mountains regarding pollen source areas. Jackson demonstrated that Picea pollen is generally under-represented in small basin pollen assemblages, and that little Picea pollen comes from > 100 m source area. Whereas, Pinus pollen is always well represented in all small basins even where the ~:rees are absent; Pinus has a source area > 1000m (Jackson, 1990). Pinus pollen frequencies in Colorado average 10% (range is 6-22%) in all traps where the trees are absent; source areas range up to 50 km. Picea pollen frequencies > 5%, however, are only found within 100 m of their source trees. Another exception to the similarities between pollen spectra in moss polsters and in atmospheric traps is that insect pollinated species are overrepresented in the Tauber traps. No doubt some of the pollen in these collectors is deposited by insects, the remains of which were found in all traps. In most cases it is very apparent which taxa have been introduced by insects, and these types can be excluded from the pollen sum. Modern pollen spectra from Tauber traps may provide analogs for both modern pollen spectra in moss polsters and surface sediments, and for fossil pollen spectra from small bogs where virtually all of the pollen is introduced from the atmosphere. Tauber traps also allow calculation of the annual pollen influx, which cannot be obtained from other types of surface samples which lack temporal control. Pollen influx data provide additional information concerning the structure Of vegetatiop, which is particularly helpful for the recognition of tundra. Alpine tundra may be characterized by relatively high frequencies of pollen from taxa which dominate much lower elevation vegetation

P.L. FALL

types (e.g. Pinus, Artemisia, Juniperus, and Gramineae). However, pollen accumulation values for these same taxa are no higher than in other vegetation types in which these taxa are not dominant. Also, atmospheric pollen accumulation rates from Tauber traps may be used to estimate the relative proportions of atmospheric pollen and pollen brought into a basin by either slope wash or by streams (Fall, 1992). Thus, modern pollen accumulation rates are invaluable for the comparison of pollen influx in various environments, and as analogs for the interpretation of fossil pollen spectra. Lastly, the lid of the Tauber trap helps to collect pollen, particularly in open vegetation. In comparing paired collectors in forested and in open vegetation, one with a Tauber lid and one without, the traps in the open demonstrate the greater efficiency of the aerodynamically designed Tauber lid. While the relative frequencies for pollen types were very similar, the traps without Tauber lids collected only 23-39% of the pollen found in the traps with lids. On a related note, a forest trap without a Tauber lid collected 70% of the pollen accumulated in its paired trap, demonstrating the significant filtering effect of forest vegetation on pollen transported in the trunk space. Conclusions

Atmospheric pollen dispersal, transport, and deposition in the mountains of west central Colorado were investigated using modern pollen spectra collected by atmospheric samplers. Atmospheric samplers provide pollen records that can be used to characterize the main vegetation types. Pollen spectra from Tauber traps placed within forest stands reflect the local or extra-local vegetation. The highest pollen frequencies and accumulation rates for arboreal taxa are found in traps within forest stands dominated by local tree species. Pollen representation of open vegetation is more difficult. Pollen spectra from Tauber traps located below the lower forest border characterize these broad open valleys of big sagebrush steppe and grasslands, and are dominated by pollen from Artemisia, Gramineae, Tubuliflorae, Juniperus, and other non-arboreal taxa.

ATMOSPHERICPOLLEN DISPERSALIN THE COLORADO ROCKY MOUNTAINS.USA

Pollen spectra from alpine tundra also very strongly resembles pollen frequency data from the steppe. Pollen spectra in alpine tundra are characterized more by pollen from regional sources than by local or extra-local pollen types. However, there are higher amounts of local pollen from plants in the Rosaceae family. Pollen accumulation rates may be helpful for the recognition of alpine vegetation which has slightly lower values than are found in the big sagebrush steppe. Atmospheric pollen spectra from the mountains of central Colorado display a strong regional pollen component. This effect becomes exaggerated at higher elevations. Pollen accumulation in the steppes and grasslands at the base of the mountains characterizes the regional pollen rain. Although the forest stands have the most localized collection of pollen they still reflect a relatively large regional pollen component that comes from a source area with a radius of 100 m to 50 km. Except for Pinus pollen, which has a very large source area, pollen from arboreal taxa (Picea, Abies, Pseudotsuga, and Populus) have very localized source areas, and are generally not dispersed in large numbers very far beyond their host trees. At least 50% of the atmospheric pollen collected in alpine tundra comes from plants growing up to 1500 m lower in elevation and from a source area with a radius between 500 m and 50 kin. Modern pollen spectra from atmospheric collectors provide detailed information on spatial distribution patterns of pollen in the mountains in Colorado. These data offer useful analogs for modern pollen spectra in moss polsters and for fossil pollen spectra in depositional environments where most or all of the pollen comes from the atmosphere. Modern pollen accumulation rates from Tauber traps may also help to sort out the relative amounts of atmospheric pollen from the introduction of pollen by fluvial sources. Annual pollen accumulation rates in the mountains of Colorado range between 1000 and 5000 grains cm -2 yr -1. Pollen influx values in the mountains of Colorado are comparable to or slightly lower than the modern accumulation rates reported by the few other studies of pollen influx in the intermontane west. Reported pollen influx rates throughout the western United States are highest in forests with abundant species of pine,

31 !

and in areas with the broadest distributions of pine forests. Because pine trees are very restricted, the mean annual pollen influx in the Crested Butte study area is at the lower end of the range for both sub-alpine and montane forests. Pollen accumulation rates in open vegetation, the sagebrush steppe and alpine tundra, are about one-half to one-fourth of those found in forested vegetation.

Acknowledgements Funding was provided by AMAX, Inc. through C.S. Robinson and Associates as part of a grant to V. Markgraf and myself to study the paleoecology of the environment near Cr¢.sted Butte, Colorado. S.E. Falconer, V. Markgraf, J.P. Bradbury, P. Dea, and W.A. Watts assisted in the location and retrieval of the traps. Unpublished maps and descriptions of the modern vegetation by StoeckerKeammerer and Associates of Boulder, Colorado were invaluable in producing Fig.2, and for interpretating the distribution of the vegetation types in the study area. AMAX, Inc. also provided a four-wheel drive truck in the summer, and two snowmobiles and a Thiocol snow tractor to collect the traps in the winter. This research has benefited by discussions with V. Markgraf, M.K. O'Rourke, R.S. Thompson, and O.K. Davis. I thank S.E. Falconer for editorial suggestions, and B. Trapido and C. Sternberg for drafting Figs. I and 2, respectively. T. Webb III and M. Hjelmroos offered critical reviews and editorial comments which greatly improved this paper. Appendix 1 Pollen taxa identified in Tauber collectors other than those plotted in Figs. 3 and 4. Sample numbers follow taxa. (Descriptions of sample localities in Table I.) Other trees Cao'a I 7 Eleagnus ! 6, 16 Juglans 5 UImus ! 3-7, 9, 12, 14, 15, 17, 18 Other shrubs Ceanothus 2, 4, 5, 7-9, 12 13, 15-17 Ericaceae undiff. 3, 5 Lonicera 3, 5, 10, 12, i 3 Potentilla 2-4, 7, 11, 16, 17 Ribes montigenum-type 9

312

P.L. FALL

Ribes undiff. 12, ! 3 Vaccinium-type 6, 17

Typha latifolia !-4, 6, 7, 9 Typha/Sparganium 2, 7, I I

Other herbs Bistorta-type !, 3, 5, 6, 8, 14, 18 Caltha-type 3-6 Campanula 4-6 Cannabis ~ 4, 6, 7, 16, 17 Caryophyllaceae undiff, l, 3, 5, 6, 8, 12-14, 17, 18 Castilleja 2, 3, 5, 6, 15, 16 Cerealia 8- I !. 18 Cruciferae I-3, 5-9, 11-14, 16-18 Eriogonum 2, 3, 6-8, 14, ! 6, 17 Euphorbia 16 Galium 6, 7 cf. Geum 3, 5 Labiatae 2-8, 10, 13 Liguliflorae 4-6, 10, ! 2, 13, 15, 17, 18 Liliaceae undiff. 2, 4-7, 9, 14, 17, 18 Linum 3 Leguminosae 2-18 Mimmulus glutattus 3 Onagraceae cf. Epilobium-type 5 Opuntia-type 18 Pedicularis contorta-type 3, 5, 6, 11, ! 2 Pedicularis groenlandica-type 2, 5, ! ! Phlox 16, 17 Plantago !, 2, 5-10, 12, 15-17 Plantago lanceolata 2-7, l I. 13, 16, 17 Polemonium 3, 5, 7 Polygonum aviculare-type 3, 4, 16 P. californica-type 14 P. cristatum-type 6, 7 Portulaca 6 Primula 6, 15 Rosa-type 2, 3, 5-8, l l, 13, 16, 17 Rumex acetose/la-type 3-9, 15- ! 7 Sa.,cifraga 6, 7 Salsola i-7, 9, 10, 12-18 cf. Solanum 14 Sphaeralcea 2 Thalictrum-type 2-7, 9, I l - 13 Umbeiliferae I- ! 8 Urtica 2, 4-6, 9- ! 3, i 5 Valeriana dioica-type 2, 3 Valeriana officinalis-type 4 Valeriana undiff. 3, 4, 6, 13, 17 cf. Veratrum 6

Other palynomorphs Arceuthobium 2, 6, 7, 9, 12 Fungal spores undiff. 7, ! 5, 16 Sporomiella l, 7, 10, 14, 15, 17, 18

Pteridophytes L ycopodium 4 Monolete undiff. 7, 16 Selagineila densa 2, 6, 8, !!, 15-17 Selagineila undiff. 6, 14 Trilete undiff. 2, 3, 6, 10, 13, 16 Aquatics or bog indicators Botryococcus 3 iso~tes 2, 3, 5 cf. Nymphaea 6 Pediastrum undiff. ! 1 Potamogeton/Triglochin 2, 6, 10, 13

I Introduced taxa. Appendix 2

Pollen sums (based on the total pollen from terrestrial plants) and annual pollen influx (grains cm- 2 yr- I) for Tauber traps. Sample No.

Pollen sum

Pollen influx

I 2 3 4 5 6 7 8 9 10 !1 12 13 14 15 16 17 18

588 864 1537 790 1040 2483 1838 989 1625 398 853 1073 927 655 1613 1646 2135 1399

1092 3172 2147 2861 2711 4393 3028 3262 2241 2483 4903 4453 1434 !124 1031 4484 2216 858

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ATMOSPHERIC POLLEN DISPERSAL IN THE COLORADO ROCKY MOUNTAINS. USA

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