Modern pollen rain across the Wyoming basins and the northern Great Plains (U.S.A.)

Review of Palaeobotany and Palynology Elsevier Publishing Company, Amsterdam - Printed in The Netherlands

M O D E R N P O L L E N RAIN ACROSS T H E W Y O M I N G BASINS A N D T H E N O R T H E R N G R E A T PLAINS (U.S.A.) J. H. McANDREWS AND H. E. WRIGHT JR. Royal Ontario Museum, Toronto, Ont. (Canada) Limnological Research Center, University of Minnesota, Minneapolis, Minn. (U.S.A.) (Received July 7, 1969)

SUMMARY

Pollen analyses were made of samples of dry organic detritus (duff), or "moss" polsters in two 1000-km transects across the northern Great Plains from the Rocky Mountains to the extensively cultivated area east of the Missouri River. They show that pollen of Picea, Abies, and Pseudotsuga travels only a few tens of kilometers beyond its source areas in the western mountains, but that pollen of Pinus travels in quantity far to the east, with values as high as 20 ~ at a distance exceeding 300 km from the nearest pine forest. Deciduous-tree pollen is largely confined to ~ites near local woodlands along river valleys in the east. Artemisia pollen is found in high percentages in the intermountain basins of Wyoming, where sagebrush dominates, In the Plains, east of the area of sagebrush, other species of Artemisia contribute equally high values of pollen. Ambrosia pollen is largely confined to the Dakota plains. Chenopod pollen is abundant in some of the lower, drier Wyoming basins, but it is also common in the eastern part of the transects where agricultural disturbance has favored the growth of weeds. The regional variations in pollen percentages of surface samples across the Great Plains strengthen the basis for paleoecological interpretation of stratigraphic pollen sequences, especially for sites located in regions where the foreststeppe border or the forest-prairie border may have moved or where the dominance of Artemisia, Ambrosia or other major non-arboreal pollen producers may have changed. INTRODUCTION It is well known that the fall-out of pollen grains on a lake bottom does not provide an exact representation of the vegetation of the surrounding area, because of differential production and dispersal of pollen from different plant species, Rev. Palaeobotan. PalynoL, 9 (1969) 17-43

17

among other factors. Knowledge of the modern pollen rain in different regions of known vegetation, however, provides a basis for the ecological interpretation of the fossil pollen assemblages found in lake sediments (WRIGHT,1967). Although Late Quaternary climatic changes undoubtedly involved some recombinations of plant species into different associations, application of surface-sample data to fossil assemblages provides a first approximation in the interpretation of vegetation history. In the forested areas of North America several surface-sample transects have been made to determine the variations and the trends in the pollen rain across major vegetation regions (PoTZGERand COURTEMANCHE,1956; KING and KAPP, 1963; LICHTI-FEDOROVICHand RITCHIE, 1965; MCANDREWS,1966; etc.), and these have been summarized and applied to interpretation of Late Glacial and Postglacial pollen sequences (DAVIS, 1967; WRIGHT, 1968a). Only a few surface-sample analyses are available for treeless regions, however. Here the problems are more difficult, because the vegetation is often severely disturbed, and because its pollen production is supposedly low compared to forested regions. The one extensive survey that was made, however--from the Mesopotamian piedmont across the Zagros Mountains to the Iranian Plateau--furnished enough information to permit elaboration of the climatic interpretation of a Late Quaternary pollen diagTam (WRIGHTet al., 1967). In the prairie and shrub steppe of the U.S.A., knowledge of the modern pollen rain is useful in tracing the course of Postglacial migration of the prairieforest border in the area of the prairie peninsula (WRIGHT, 1968b) and possibly also in interpreting the Late Glacial herb pollen zone in the Minnesota area, which might indicate prairie vegetation rather than tundra (CUSHING, 1965). Closer to the Rocky Mountains, it is useful in understanding the importance of longdistance transport of pollen from forest trees out into the shrub steppe, so that past movements of tree line can be inferred from pollen stratigraphy in mountain areas. Previous surface-sample pollen analyses in the northern Great Plains have been limited to sixteen published by KAPP (1965) for a transect from Colorado to southwestern Minnesota, and four by WATTS and WRIGHT (1966) for western Nebraska and adjacent Wyoming. Other studies in the West have been made in western Texas (HAFSTEN, 1964), New Mexico (BENT and WRIGHT, 1963), Arizona (MARTIN, 1963; SCHOENWETTER and EDDY, 1964), Colorado (MAHER, 1963), and California (ADAM, 1967), where the problems are somewhat different from those in the central and northern Great Plains. Our paper reports the results of analyses of samples collected along two 1000-km transects from the Rocky Mountains across the Great Plains. The southern transect of 28 localities, collected in August 1961, extends from the Green River Basin of western Wyoming across four mountain ranges (Wind River, Owl Creek, and Bighorn Mountains and the Black Hills), and thence across the Great Plains to eastern South Dakota ]8

Rev. Palaeobotan. Palynol., 9 (1969) 17~,3

(Table I). The northern transect of 41 localities, collected in June 1967, extends from the northern Rocky Mountains at Glacier National Park eastward along the Canadian border across the plains of Montana to central North Dakota (Table II). The vegetation in these regions has been moderately disturbed by grazing, but extensive destruction of the vegetation by cultivation is mainly confined to eastern South Dakota and parts of eastern Montana and North Dakota. METHODS

In the southern transect, all except two samples consisted of dry organic detritus (duff) collected from beside utility poles, rocks, shrubs or other obstacles not subject to easy rainwash. Samples 28t and 32t consisted of the largely algal muck that had accumulated in windmill cattle tanks. In the northern transect the samples consisted of six pinches of Selaginella densa from a 10-m diameter area, but where Selaginella was absent pinches of Phlox hoodii were substituted. The Selaginella-Phlox samples better reflect the regional pollen rain than the detritus samples because the detritus may contain flower parts and cause local overrepresentation in the pollen spectrum. The samples were soaked in K O H and washed through a screen to eliminate the many coarse fragments. The residue was treated with HF, HNO3, and a solution of 9 parts conc. H2SO4 with 1 part acetic anhydride, stained with safranin, and mounted in silicone oil, the treatment customarily used for sediment samples. Preservation of pollen grains was good, apparently because the pollen grains in most samples had remained dry (or wet in the case of the cattle-tank samples) rather than subject to the intermittent wetting with oxygenated water that causes corrosion of pollen grains in soils. The pollen diagrams are based on counts usually of 300-350 grains. Selaginella spores in Selaginella samples and Phlox pollen in Phlox samples were not counted. All pollen types are included in the base for calculation of percentage. Types not shown on the diagram are listed by percentage in Table III and Table IV. VEGETATION

Forest The decidious forest, east of the northern Great Plains (Fig. 1), is dominated by various species of Quercus (oak), Ulmus (elm), Fraxinus (ash), Populus (popla0, Acer (maple), and Betula (birch) (McANDREWS, 1966). Deciduous forest stands occur westward into the prairie not only as riparian woodlands, i.e., along the Missouri River, but also along scarps (WELLS, 1965). Deciduous forest does not occur at the western margin of the shrub steppe, although groves of Populus tremuloides (aspen) occur between the conifer forest and steppe in Montana (LYNCH, 1955). Rev. Palaeobotan.PalynoL, 9 (1969) 17-43

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The western coniferous forest, in areas generally with 16-25 inches annual precipitation, is dominated by Pinus ponderosa (ponderosa pine), P. contorta (lodgepole pine), Pseudotsuga menziesii (Douglas-fir), Picea engelmanni (Engelmann spruce), and Populus tremuloides and includes also Abies lasiocarpa (subalpine fir), A. concolor (white fir), Tsuga heterophylla (western hemlock), Pinus flexilis (limber pine), P. albicaulis (whitebark pine), Larix lyallii (western larch), and Betula papyrifera (paper birch). At lower elevations and along scarps eastward in the steppe occur open stands of Pinus ponderosa, P. flexilis, and Juniperus scopulorum (juniper).

Steppe The shrub steppe is largely confined to arid intermontane areas of western Wyoming and southwestern Montana having 6-12 inches mean annual precipitation. Chenopod steppe is dominated by open stands of low shrubs such as Atriplex confert(folia (shadscale) and Sarcobatus vermiculatus (greasewood). The less arid sagebrush steppe is dominated by Artemisia tridentata (sagebrush) and grasses such as Agropyron (wheatgrass) and Stipa comata (needle-and-thread grass). Other components include Atriplex confertifolia, Sarcobatus vermiculatus, Artemisia cana (white sage), and A. frigida (pasture sage). A prairie or grass steppe occupies the bulk of the northern Great Plains (WEAVER and ALBERTSON, 1956). In response to increasing aridity westward, the vegetation becomes shorter and less dense. The true prairie (16-24 inches mean annual precipitation) that extends westward from the deciduous forest into the eastern Dakotas is now largely cultivated but was formerly dominated by tall grasses such as the Andropogon (bluestem) and many forbs, including Artemisia spp. Today the abundant weedy flora of this area includes Ambrosia artemisiifolia (common ragweed) and chenopods such as Chenopodium album (lambsquarters), Kochia scoporia (Mexican fireweed), and), and Amaranthus retroflexus (rough pigweed). West of the true prairie lies the partly cultivated mixed-grass prairie (12-16 inches mean annual precipitation), dominated by various combinations of midgrasses, such as Stipa spartea (needlegrass) and Agropyron spp. (wheatgrass), and shortgrasses such as Bouteloua spp., and including Carex spp. (sedges). Artemisia cana is common throughout. In the eastern part of the mixed prairie in the Dakotas, midgrasses predominate, and Artemisia ludoviciana (mugwort sage) is common along with the chenopods Eurotia lanata and Chenopodium leptophyllum. In this area Ambrosia artemisiifolia is essentially absent, but the perennial ragweed Ambrosia psilostachya occurs locally. Chenopodium album, Salsola kali (Russian thistle), and Amaranthus albus (common amaranth) are the usual field weeds. The western part of the mixed prairie is similar except that the short grasses are more important, Artemisia cana is prominent, and Ambrosia is absent. The fescue prairie (14-16 inches mean annual precipitation) occurs in western Rev. Palaeobotan.Palynol., 9 (1969) 17-43

31

Montana along the higher elevations adjacent to the Rocky Mountains. It is dominated by midgrasses such as Festuca spp. (fescue) and Agropyron spicatum (blue-bunch wheatgrass). In addition, Artemisia ludoviciana and A. frigida are common. Selaginella densa and Phlox hoodii occur as common although inconspicuous components of the fescue and mixed prairie (VAN DYNE and VOGEL, 1967). The fescue prairie is largely grazed. The steppe dominants, namely grasses (Poaceae), sages (Artemisia), and chenopods (Atriplex, Sarcobatus, Chenopodium, and Eurotia), all produce abundant pollen easily dispersed by the wind (WODEHOUSE,1945), so we should expect them to be well-represented in the pollen rain. However, the abundance of weedy chenopods sensu lato (Salsola, Kochia, and Amaranthus) in cultivated areas poses a problem in evaluating the pollen rain of the native steppe vegetation. SOUTHERN TRANSECT

The southern transect (Fig.2), which crosses several major mountain ranges and basins, provides the opportunity to test the relation of the regional pollen rain to the regional vegetation for individual topographic units, and to judge the importance of long-distance transport of certain pollen types. Starting in the Green River Basin west of the Wind River Mountains, sample $6 is dominated by Artemisia and chenopods (including especially Sarcobatus), for it was taken near the center of the basin, where sagebrush of the pediments and coarse alluvial fans grades into the chenopod steppe of lower elevations. The pine pollen value is very low: the locality is about 40 km from the pines on the mountain flank. Samples $7 and $8 came from higher on the piedmont, where Artemisia tridentata dominates. Close to the $8 locality were outlying trees of Pinusflexilis on the west side of South Pass. The pine pollen values for these two samples were about 15 ~. Sample $9 from the east side of the South Pass was in an area of sagebrush and grasses at the edge of woodlands of Pinus contorta. Accordingly, the grass (20 ~) and pine (30 ~) pollen values are higher than in preceding samples, and chenopods lower. In the Wind River Basin east of the mountains, sample S10 shows grass percentages reduced and chenopods higher. The 25 ~ pine pollen was blown from the mountains 35 km to the west. Just south of Wind River Canyon through the low Owl Creek Mountains, the very high chenopod pollen percentages of sample S 11 represent chenopod steppe, which has an area apparently too small to have been mapped by KUCHLER(1964). Within the Owl Creek Mountains themselves (sample Sl2), juniper is abundant, and Juniperus pollen reaches almost 70 ~. Pine is not present in the Owl Creek Mountains, so pine pollen is only l0 ~ in samples S11 and S12. 32

Rev. Palaeobotan.Palynol., 9 (1969) 17--43

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Toward the center of the Big Horn Basin near Worland, where sagebrush gives way to chenopod steppe, two samples (S13 and S14) show high pollen values of chenopods (including Sarcobatus), Artemisia, and grasses. In sample S13 the pine pollen value (24 ~o) is higher than in sample S 14 (4 ~o) despite greater distance from the forest. Perhaps this reflects stronger westerly winds. In the Big Horn Mountains, sample S15 on the west side is dominated by Artemisia pollen, but Pinus, Juniperus, Picea, and Abies pollen grains reflect the presence of conifers in the area. The pine here is scattered Pinus flexilis. Deeper in the mountains (sample S16 and S17) dense stands of Pinus contorta bring the pine pollen value to more than 60 ~, the relatively abundant grass pollen grains (17 ~) result from large park-like openings in the conifer forest. Chenopod percentages are very small in the Big Horn Mountains samples. At the east base of the mountains (sample S18), 5 km beyond the tree line, the pine pollen value is down to 25 ~, other conifer pollen is absent, and grasses jump to almost 50 ~o. In the Powder River Basin (samples S19 and $20), in the heart of the sagebrush steppe, Artemisia-pollen values are 50 ~, whereas pine drops to 10 ~o or less. In the Black Hills, samples $21 and $22 at the edge of the open Pinus ponderosa parkland have only about 20 ~ pine pollen, and Artemisia and grasses still dominate. But in the heart of the forest (sample $23) the pine value is almost 70 ~. The lack of sagebrush openings in these mountains (compared to the Big Horn Mountains) is reflected in the very small percentage of Artemisia pollen in this sample. Between the Black Hills and the Missouri River (samples $24-$30), pollen of Artemisia, grasses, and chenopods occur in variable quantity, probably depending in large part on local conditions. Most of the area is utilized for grazing rather than for cultivation. Artemisia frigida and A. ludoviciana are common instead of A. tridentata, but they apparently produce enough pollen so that the Artemisia values exceed 45 ~ in three of the samples. Ambrosia pollen is consistently present in all samples, at values reaching 6 ~, whereas to the west it was essentially absent. Pine pollen values reach almost 20 ~ within 125 km east of the Black Hills, but farther east are 3 ~ or less. At one of the localities in this group, two samples were analyzed, one ($28d) consisting of the usual plant duff, the other ($28t) of organic ooze from the bottom of a windmill cattle tank. Windmill tanks are customarily surrounded by solid stands of weedy chenopod species, because of trampling by cattle, so the chenopod pollen value in sample $28t is extremely high (70 ~). This situation indicates that cattle-tank samples are not the most satisfactory to determine the modern pollen rain; chenopods cannot be excluded from the pollen sum, because an unknown portion represents regional rather than local chenopod pollen. East of the Missouri River (samples $31-33), most of the land is cultivated. Percentages of Artemisia and Pinus pollen are very low, whereas chenopods, grasses, and Ambrosia dominate. Sample $32 was double, and again the cattle-tank sample ($32t) had much more chenopod pollen than the duff sample ($32d). 37

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NORTHERN TRANSECT

The northern transect (Fig.3) provides a clearer reflection of broad regional vegetation units than the southern transect, because it has fewer interruptions by separate mountain ranges. The first four samples in the transect came from the generally forested mountains of the Glacier National Park area, although the sample sites were in local treeless openings. In the six samples, Pinus-pollen values average 6 0 ~ , Picea 5 ~ , Abies 2.7~o, and Pseudotsuga (or Larix) 0.6~. Also characteristic is Alnus pollen, for the shrub A. rugosa is common along mountain streams. Among the herbs, grass pollen is common (20 ~), but Artemisia is less than 5 ~ , and Ambrosia and chenopods are essentially absent. In the fescue prairie of the foothills (samples N 5 - N l l ) and the mixed Bouteloua-Agropyron prairie to the east (samples N12-N16), pine-pollen values are only slightly lower than in the mountains (55 ~), because of easy wind dispersal from the forest, and the percentages of other conifers and of alder are also almost as high. Grass percentages are similar to those in the mountains. Artemisia, however, is distinctly higher (10 ~). Although the area is north of the major range of A. tridentata, the rather similar shrub A. cana is common, as is the herbaceous A.frigida. Chenopod pollen rises to 15 ~ in samples N14-N16. In the Sweetgrass Hills (samples N17-N19) the pine-pollen values are lower (30 ~o), even though the hills have an open forest of Pinusponderosa above 5,000 ft. Spruce pollen reaches 2 ~o even though spruce does not occur in the hills. Grasses are variable in value (7-47 ~ ) but average about 25 ~. Artemisia continues to rise from the west, averaging about 30 ~. Chenopod values are low. In the mixed prairie of the Montana plains east of the Sweetgrass Hills (samples N20--N36) pine-pollen values average 25 ~--still relatively high because of long-distance transport. Grasses are generally higher than westward (20 ~o), as is also Artemisia (25 ~o). Chenopods reach their highest values on the transect, averaging 20 ~ . Ambrosia is slightly more common than westward. After a gap of 250 km, the samples of the eastern part of the mixed prairie in western Dakota (N37-N41) show lower pine-pollen values (10 ~o)--the distance to the nearest pine forests in the Black Hills and Minnesota is now 400 km. Quercus, Ulmus, and Fraxinus pollen are generally represented, presumably coming from deciduous woodlands along stream-valley slopes and bottoms. Grass pollen is less common than westward (15 ~), and Artemisia is higher (40 %)--A. frigida and A. ludoviciana are common in this area. Sample 40, for example, from a heavily grazed pasture where A..frigida was abundant, has an Artemisia pollen value of 60 ~o. Ambrosia and Iva xanthifolia pollen are especially distinctive of this group of samples, for the plants are fairly common in the eastern part of the mixed prairie, although Ambrosia is still more common eastward. Fig.3. Percentages of main pollen types for northern transect of surface samples. See Fig. I, for location of samples. Analyses by J. H. McAndrews, made in 1968.

Rev. Palaeobotan. Palynol., 9 (1969) 17--43

39

CONCLUSIONS

These transects of pollen surface samples from the Rocky Mountains across the Great Plains provide a general picture of the regional pollen rain in the shrub and grass steppe of interior North America, as it is affected by the proximity to major mountain ranges and by the west-to-east gradient in vegetation and land use. Pollen of Picea, Abies, and Pseudotsuga type are largely confined to the mountains and adjacent foothills. Pinus pollen, which exceeds 70 ~o in some mountain areas, contributes 5-25 ~ of the pollen rain on the plains within 125 km of pine forest in the southern transect. In the northern transect, however, values of 20 ~o are sustained as far as 400 km from the nearest pine forests. Deciduous-tree pollen is largely restricted to Quercus, Ulmus, and Fraxinus in the east near the deciduous woodlands of river valleys. Pollen of steppe plants dominates in the basins and plains. Artemisia is common in the sagebrush steppes of the Wyoming basins, except in the lower parts of the large basins, where chenopod pollen is more conspicuous. East of the mountains in both South Dakota and Montana, Artemisia pollen is abundant because of species of Artemisia other than sagebrush. Here the local abundance of chenopod pollen probably reflects land disturbance by cultivation. Sarcobatus pollen is common only west of the Black Hills in the southern transect, whereas Ambrosia is largely confined to the area east of the Black Hills on the southern transect and to comparable longitudes on the northern transect. Pollen of grasses is generally higher east of the Black Hills than in the western basins, although it is also fairly high in the Big Horn Mountains, where grassy openings are widespread. Comparisons of the two transects with surface samples southward in the central Great Plains are shown in Table V, along with others from non-forested regions. Pinus pollen carries farther out into the plains in the northern transect than farther south, partly because of a larger source in the Montana mountains to the west. The isolated Wyoming ranges are separated by broad basins, and the total area covered by dense forest is much smaller. But in the Colorado transect of KAPP (1965, table III), pine pollen values adjacent to the forested Front Range reduce abruptly eastward to 5 ~. Perhaps an additional factor in pine dispersal is the relatively greater westerly air flow across the Montana Rockies than in Colorado. Artemisia is a dominant pollen type in the Wyoming basins and mountains because of sagebrush (A. tridentata), but the Montana mountains and plains are north of the range of this species. On the other hand, it is replaced eastward on the plains by other Artemisia species that apparently produce as much pollen. On the Central Plains KAPP (1965) included Artemisia in his Compositae group, but along his transect this group has values generally comparable to those of Artemisia in our transects. Ambrosia pollen appears in significant amounts only in samples east of the 40

Rev. Palaeobotan.Palynol., 9 (1969) 17-43

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Black Hills longitude (104 °) on both transects and southward in the Central Plains. Increasing values southward reflect the fact that ragweed plants are more common to the south, being limited northward by a combination of day length and shorter growing season. Chenopods are more common in the southern transect. They are abundant in the arid Wyoming basins in quantities comparable to those of the chenopod steppe of New Mexico and the plateau steppe of Iran. In the less arid portions of the plains, chenopods have variable values related to the availability of weedy habitats caused by agriculture and grazing practices. Sarcobatus is essentially confined to the Wyoming basins. Grass is represented throughout the central and northern plains by about 15 ~ of the pollen rain. This modest amount is surprisingly low, considering that grass is dominant over much of the steppe. The two surface-sample transects reported here provide basic information useful in the interpretation of pollen sequences of Late Quaternary deposits. The presence of 20 ~ or more pine pollen in an herbaceous pollen assemblage does not necessarily mean pine-covered mountains within 400 km of the site, but the presence of more than 2 or 3 ~ pollen of spruce, fir, or Douglas-fir implies that these conifers were abundant within 20 km of the site. High Artemisia percentages need not be attributed to sagebrush, but they do indicate lower precipitation than do high values of Ambrosia. Because a percentage figure is determined by the total of the other components of the pollen rain as well as by the absolute amount of the individual type in question, the comments here referring to percentage values for a particular pollen type cannot be applied quantitatively in interpreting past vegetation from stratigraphic pollen diagrams without a great deal of caution. Nonetheless, the general geographical trends seen from these transects of pollen surface samples provide the best available control on vegetational reconstructions, which otherwise can be worked out only from speculation. REFERENCES

ADAM,D. P., 1967. Late Pleistocene and Recent palynology in the central Sierra Nevada, California. In: E. J. CUSHINGand H. E. WRIGHTJR. (Editors), Quaternary Paleoecology. Yale Univ. Press, New Haven, Conn., pp.275-301. BENt, A. M. and WRIGHTJR., H. E., 1963. Pollen analyses of surface materials and lake sediments from the Chuska Mountains, New Mexico. GeoL Soc. Am. Bull., 74: 491-500. CUSHING,E. J., 1965. Problems in the Quaternary phytogeography of the Great Lakes region. In: H. E. WRIGHTJR. and D. G. FREY(Editors), The Quaternary of the United States. Princeton Univ. Press, Princeton, N.J., pp.403-416. DAvis, M. B., 1967. Late Glacial climate in northern United States: a comparison of New England and the Great Lakes region. In" E. J. CUSHINGand H. E. WRIGHTJR. (Editors), Quaternary Paleoecology. Yale Univ. Press, New Haven, Conn., pp.ll-43. HAFSTEN,U., 1964. A standard pollen diagram for the southern High Plains, U.S.A., covering

42

Rev. Palaeobotan. Palynol., 9 (1969) 17-43

the period back to the Early Wisconsin Glaciation. Congr. Intern. Quaternary Assoc., 6th, Warsaw, 1961, Rept., 5(2): 407~[20. KAPP, R. O., 1965. Illinoian and Sangamon vegetation in southwestern Kansas and adjacent Oklahoma. Univ. Mich. Museum Paleontol. Contrib., 19(14): 167-255. KING, J. E. and KAPP, R. O., 1963. Modern pollen rain studies in eastern Ontario. Can. J. Botany, 41: 243-252. K~2CHLER, A. W., 1964. Potential natural vegetation of the conterminous United States. Am. Geograph. Soc., Spec. Publ., 3 6 : 3 8 pp. L1CHTI-FEDOROVICH, S. and RITCHIE, J. C., 1965. Contemporary pollen in central Canada, 2. The forest-grassland transition in Manitoba. Pollen Spores, 7: 63-88. LYNCH, D., 1955. Ecology of the aspen groveland in Glacier County, Montana. Ecol. Monographs, 25: 321-344. MAHER JR., L. J., 1963. Pollen analyses of surface materials from the San Juan Mountains, Colorado. Geol. Soc. Am. Bull., 74: 1485-1504. MARTIN, P. S., 1963. The last 10,000 years. A fossil pollen record of the American Southwest. Univ. Arizona, Tucson, Ariz., 87 pp. MCANDREWS, J. H., 1966. Postglacial history of prairie, savanna, and forest in northwestern Minnesota. Torrey Bot. Club. Mere., 22(2): 72 pp. POTZGER, J. E. and COURTEMANCHE, A., 1956. A series of bogs across Quebec from the St. Lawrence Valley to James Bay. Can. J. Botany, 34: 473-500. SCHOENWETTER, J. and EDDY, F. W., 1964. Alluvial and Palynological Reconstruction of Environments, Navajo Reservoir District. Museum New Mexico, Santa Fe, N.M., 155 pp. VAN DYNE, I. M. and VOGEL, E. I., 1967. Relation of Selaginella densa to site, grazing, and climate. Ecology, 48:438 444. WATTS, W. A. and WRIGHT JR., H. E., 1966. Late Wisconsin pollen and seed analysis from the Nebraska sandhills. Ecology, 47: 202-210. WEAVER, J. E. and ALBERTSON, F. W., 1956. Grasslands of the Great Plains. Johnsen, Lincoln, Nebr., 395 pp. WELLS, P. V., 1965. Scarp woodlands, transported grassland soils, and concept of grassland climate in the Great Plains region. Science, 148: 246-249. WODEHOUSE, R. P., 1945. Hayfever Plants. Chronica Botanica Co., Waltham, Mass., 245 pp. WRIGHT JR., H. E., 1967. The use of surface samples in Quaternary pollen analysis. Rev. Palaeobotan. Palynol., 2: 321-330. WRIGHT JR., H. E., 1968a. The roles of pine and spruce in the forest history of Minnesota and adjacent areas. Ecology, 49: 937-955. WRIGHT JR., H. E., 1968b. History of the prairie peninsula. Univ. Illinois Coll. Agr., Spec. Publ., 14: 78-88. WRIGHT JR., H. E., MCANDREWS, J. H. and VAN ZEIST, W., 1967. Modern pollen rain in western Iran, and its relation to plant geography and Quaternary vegetational history. J. Ecol., 55:415~,43.

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43