Proceedings of the Iowa Academy of Science

Volume 92 Number Article 13

1985

The Loess Hills of Western Iowa: a Problem in Phytogeography

Jean M. Novacek Iowa State University

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Recommended Citation Novacek, Jean M. (1985) "The Loess Hills of Western Iowa: a Problem in Phytogeography," Proceedings of the Iowa Academy of Science, 92(5), 213-219. Available at: https://scholarworks.uni.edu/pias/vol92/iss5/13

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Proc. Iowa Acad. Sci. 92(5):213-219, 1985

The Loess Hills of Western Iowa: a Problem in Phytogeography

JEAN M. NOVACEK

Botany Department, Iowa State University, Ames, Iowa 50011

Based upon.personal observati~ns ?ver a four year peri~, a preliminary overview of the phytogeographical significance of the prairies of the Loess Hills ofwestern Iowa 1s discussed, concentratmg upon a few representative taxa. A number ofGreat Plains vascular plant species reach. the easternmost edge of their ranges in the Loess Hills. Climatic changes during the Holocene Period, especially during the Hyps1thermal Interval, have had great effects upon the vegetation of the Great Plains and upon the xerophytic components of the Loess Hills. In addition, recent extended drought episodes have affected the flora. It is postulated that the Missouri River system has served, and may still serve, as a major migratory route for many of the Great Plains species found in the Loess Hills. INDEX DESCRIPTORS: Iowa vascular flora, Iowa Loess Hills prairie, Great Plains flora, phytogeography.

Mimicking the dry conditions of the Great Plains, the Loess Hills state: "The existing flora of any region may be looked upon as the of western Iowa (Fig. 1) support a xerophytic prairie flora including survivors of an army of migrants which ... either drifted into the many Great Plains vascular plant species which reach the easternmost region from the outside or were born in it from a migrant parentage. limits of their ranges on these dry bluffs. The Loess Hills support the Their survival and distribution in the region are ... ecologically last relatively contiguous area of mixed-grass prairie remaining in determined." Iowa. The upland loess prairies are similar to the mixed-grass prairies Extensive migration of plants proceeding at fairly rapid rates occurs of the Great Plains more than 70 miles co the west. Weaver and along routes with nearly continuous suitable habitats, for example, Bruner ( 1954) placed the transition of call-grass prairie to mixed-grass upland bluffs along rivers for xerophytic species (Gleason, 1922). In prairie near 98°W longitude. The Loess Hills are located along 96°W addition co exposure to dry westerly winds and intense solar radiation, longitude ( ± 0. 5°), and the mixed-grass prairies here represent a slope and aspect are important factors influencing the vegetation of narrow penisular range extension into an area which otherwise the Loess Hills. Adams (1905) pointed out that topographic condi­ typically supports tall-grass prairie. See Novacek et al. (this issue) for tions have significant influence upon habitats. When xerophytic further discussion of the vegetation of the Loess Hills landform. elements invade an area with a greater moisture regime, he noted chat The typical upland loess prairie is dominated by rwo warm-season they occupy the dry sites which are most similar to their original grasses, Andropogon scoparius Michx. (little bluescem) and Bouteloua habitats, forming "islands" of xerophytes surrounded by more meso­ curtipendula (Michx.) Torr. (side-oats grama). The short-grass compo­ phytic plane associations. nent is represented by B. hirsuta Lag. (hairy grama) most commonly I speculate chat many of the Great Plains species found in the Loess and by B. gracilis (H.B.K.) Griffiths (blue grama) not so commonly, Hills probably initially entered northwest Iowa through Plymouth with reports of Buchloe dactyloides (Nutt.) Engelm. (buffalograss) County from Nebraska and the high plains of the Dakotas. It is my (Pammel, 1901; Thome, 1956; Goodnight, 1964; Iffrig, 1980). opinion chat the Missouri River drainage system has served as a major Iffrig (1980) pointed out that the loess bluffs also support the only migratory route for many of these planes. It is well known that birds known mixed-grass prairie community in the state of Missouri, with are a primary disseminating factor for many plants and that the the dominant grasses being Bouteloua hirsuta, B. curtipendula, Buchloe Missouri River system is a major avian flyway. The river itself may also dactyloides and Andropogon scoparius. aid in plant migration either by purely mechanical means or by With the westerly summer winds constantly sweeping chem, the offering suitable habitat on its sandbars and on the upland bluffs southwest slopes are directly exposed to the hot mid-day sun. This, which line the floodplain. coupled with the capacity of loess soil to dry out quickly, creates a As an example, I postulate that these factors may have played a role truly xerophytic environment. In general, the southwest slopes in the migration of Shepherdia argentea (Pursh) Nutt. (Elaeagnaceae), support the prairie vegetation, with the woody species growing in buffalo berry, a medium-sized shrub quite common in the northern protected places, such as draws, lower slopes and north- and east­ Great Plains. Nearby in South Dakota, dense populations line the facing slopes. bluffs and sandy areas along the Missouri River. Its bright red fruit This paper presents a preliminary overview of the phytogeographic­ attracts birds which dine upon the berries and, in turn, disseminate al aspects of the Loess Hills prairies, concentrating upon a few the seeds. The first report of S. argentea in Iowa was recorded by representative taxa. The ideas discussed herein result from personal Hitchcock ( 1889), who collected a specimen in 1888 from a Missouri observations made during a four-year floristic study of the region, River sandbar near Sioux City in Woodbury County. Hitchcock which included previous thesis work (Novacek Bates, 1983). The speculates that the plant "wandered down the river from the north­ nomenclature and distributional maps are based upon the Atlas of the west where it abounds." Pammel ( 1895) also refers co it as a "waif from Flora of the Great Plains (Barkley, 1977). The distribution maps are the northwest." representations of the general area occupied by each species east of the S. argentea is listed in Iowa as endangered (Roosa and Eilers, 1978). Rocky Mountains within the Great Plains of the United States. The The largest population in Iowa is located in Plymouth County on high scale for each map is 1 inch to 240 miles (25 mm. to 386 km.). loess prairie. South of there in Woodbury County, there are a few individuals on upland prairie in Stone Seate Park, in addition co a DISCUSSION scattered few on old dunes near an oxbow lake on the Missouri River floodplain. These are not the only sites, however, as there is a small Plant Migration population in Emmet County seemingly disjunct from the main In order to better understand plant geography, the nature of plane population (Wolden, 1932; pees. observ.). S. argentea may have migration must be considered. McLean and lvimey-Cook (1973) migrated from South Dakota through Minnesota along the appropri-

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214 PROC. IOWA ACAD. SCI. 92(1985)

ate river bluff habitat of the West Des Moines River in order to reach migration began. He also pointed out that migration is demonstrated Emmet County. mainly by the role the species plays in succession. When at or near the As mentioned previously, many Great Plains species probably edge of its range the species participates in succession as an invader, it initially entered Iowa through Plymouth County. Extensive field is advancing. However, if it is unable ro withstand competition with work (Novacek et al., this issue) has revealed that Plymouth County members of a different flora or if members of its own floristic contains the greatest number of Great Plains species. Adams ( 1905) component are regularly succeeded by others of a different flora, it is emphasized that "invading elements tend to enter a region not only at retreating. a definite place, but also tend to remain in definite habitat associations and conditions" after having entered. He noted that this habitat Climate individuality causes the isolation of the invading elements and Climate is an important consideration when attempting to examine provides an "index to their direction of origin." He further contended the flora of a region, especially in the Great Plains. Davis (1983) that habitat studies reveal important insights into the geographic pointed out that species ranges are continually expanding and con­ origin of migrating elements and the necessary conditions for their tracting in response to minor changes in overall climate. She noted survival, which both determine the routes of dispersal. that current geographic distributions have been recently attained and According to Gleason ( 1922), when entire floras migrate together, that they will change again as modifications in climate occur. Climatic the most rapidly advancing species arrive first and the others arrive at changes during the Holocene Period have had great effects upon the varying degrees behind. The present migrational status of a plant, he vegetation_ of the Great Plains and, in my opinion, the Loess Hills. noted, depends upon its migration rate and the elapsed time since According to all available data (Transeau, 1935; Sears, 1942;

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Fig. I. Shaded area indicates location of the Loess J:Iills. Numbers refer to counties: South Dakota: Union County (1); Iowa: Plymouth County (2)· W<><><;Ibury County (3); Monona County (4); Harrison County (5); Pottawattamie County (6); Mills County (7)· Fremont County (8)· Missouri: Atchison County (9); Holt County (10). Scale is 1 inch to 65 miles (25 mm. to 105 km.). ' ' · https://scholarworks.uni.edu/pias/vol92/iss5/13 2 Novacek: The Loess Hills of Western Iowa: a Problem in Phytogeography

PHYTOGEOGRAPHY OF LOESS HILLS 215

McComb and Loomis, 1944; Borchert, 1950; Deevey and Flint, of. drought in ~h~ 193~'s the mid-grass prairies were replaced by 1957; Wright, 1976; King and Allen, 1977; Baker and Van Zant, m1xed-g.rass prame~,. whICh became dominated by short-grasses, and 19~0; Wendland, 1980; King, 1981; Ahlbrandt et al., 1983; the species composltlon of the short-grass prairies was modified. He Gnmm, 1983), the climatic fluctuations of the Holocene were no.red ~hat thes~ changes were so extensive that they resulted in the episodi~ in nature and, thus, have had an episodic effect upon the migration of mixed-grass prairie about 100 to 150 miles (161-242 vegetation. km.) eastward in the central Great Plains. Weaver (1943) indicated By far the most important climatic change during the Holocene that mixed-grass prairie replaced tall-grass prairie along a front more was the Hypsithermal Interval, sometimes referred to as the xero­ than 1000 miles .in length. This is quite significant when considering thermic, which has been variously dated. Recent work in the that such a massive movement of floras occurred during such a short Nebraska Sandhills (Bradbury, 1980; Ahlbrandt et al., 1983) has period of time. In 1954 Weaver and Bruner observed that the recovery provided dates for climatic changes during the Hypsithermal which I of tall-grass prairie after this drought was quite slow, noting that even believe could have directly influenced the vegetation of the Loess after 12 years mixed-grass prairie still dominated over two-thirds of Hills. Ahlbrandt et al. (1983) found at least 3 periods of extremely dry the land it had invaded during the 7 years of drought. conditions: 7500-4000 YBP (years before present), 3000-1500 YBP, Ahlbrandt eta!. (1983) stated that during the last 800 to 500 years and 600-250 YBP. The period with the most severe aridity ranges drought conditions persisted long enough in parts of the Nebraska between 8500-5050 YBP (Wendland, 1980), 7300-6300 YBP Sandhills that the vegetative cover was effectively reduced such that (Baker and Van Zant, 1980), and 7500-4000 YBP (Ahlbrandt et al., aeolian movement of sand occurred a number of times. Dendro­ 1983). This period was dry enough to cause dwarfing in bison, chronological data (Weakly, 1943) indicate that at least 13 droughts migration of human cultures, and the large scale retreat of coniferous lasting 5 or more years have occurred in western Nebraska during the and deciduous woodlands with the advance of the prairie flora past 400 years. Six of these lasted more than 10 years and 2 continued (Ahlbrandt et al., 1983 ). According to Wendland ( 1980), the for over 20 years. The average period between major drought episodes temperatures during this extremely dry interval were as much as 2 was 20. 58 years. The most extensive drought according to Weakly's degrees Celsius higher than at present, and the precipitation amounts data lasted for 26 years and occurred between 1539 and 1564 A.D. were about 10 percent below that of today. This has been attributed to The data from Weakly's study correlate well with the data of Ahl­ persistent, almost year-round westerlies blowing eastward from the brandt et al. hot, dry continental interior (Borchert, 1950). More recently, there have been dry decades occurring in 1927- According to Borchert ( 1950), the Great Plains and the prairie 1936, 1933-1942, 1947-1956 and 1953-1962 according to Barry peninsula region receives their greatest precipitation during the (1983). In addition, Tomanek and Hulett (1970) pointed out the months of April through June. This results from a shift from occurrence of droughts of more than three years duration in 1868- continental airflow to tropical Atlantic airflow from the Gulf of 1873, 1879-1884, 1910-1914, 1933-1939and 1952-1956. Certain­ Mexico. During summers of major drought the normal southerly ly during these times there must have been shifting of floristic ele­ winds from the Gulf fail and the majority of the airflow comes from ments in the Great Plains region. the arid west. The stronger these westerly winds, the more extensive the drought-stricken area is east of the Rocky Mountains. Borchert Great Plains Species in the L:>ess Hills characterizes these drought episodes as periods of abnormally high The state of a flora is a dynamic; there are continual adjustments, summer temperatures accompanied by below-average cloud cover and migrations in and out by various species, possible species extinctions rainfall, in addition to above-average frequency of hot continental and species differentiation, and relicts left behind by a previous flora. winds. He concluded that the post-glacial eastward expansion of According to Gleason ( 1922), relicts are individual species or vegeta­ prairie was a result of long-term increase and dominance of mean tion types which are surrounded by an advancing flora and left isolated westerly circulation and also noted that the current forest invasion of from the main body, and relicts are common wherever migrations are the easternmost prairies is due to a long period of decrease in westerlies relatively recent. An example of a relict in the Loess Hills, in my to their current level. opinion, is Sphaeralcea coccinea (Pursh) Rydb. (Malvaceae), cowboy's Bernabo and Webb ( 1977) have suggested that the position, height delight (Fig. 2). There are two populations, one located in Plymouth and albedo of the Laurentide ice sheet created boundry conditions County and the other in Harrison County, separated by a distance of which directly influenced the general atmospheric circulation during approximately 80 miles ( 129 km.). The Plymouth County population the early Holocene. They stated that in summer a sharp thermal is disjunct from the main body of S. coccinea in South Dakota and gradient would have existed near the ice margin due to differences in Nebraska by about 100 miles (161 km.) to the east. Kearney (1935) the albedo of the ice sheet and the land to the south. As the ice sheet noted that this extremely xerophytic species is widely distributed, receded to the north, this thermal gradient also shifted, thus affecting extending farther eastward than any other North American Sphaeral­ the location of storm tracks during summer months. The period of cea, but is more common west of the 98th meridian. Coupland ( 1958) progressively drier climate due to these boundry effects according to pointed out that during the 1930's drought, this plant was the only their data was from 10,000 to 7000 YBP. They also stated that the generally distributed forb besides cactus (Opuntia spp.) to increase in changes in atmospheric circulation were less dramatic once the ice mixed-grass prairie, presumedly due to vegetative reproduction, early sheet retreated north of Hudson Bay approximately 7000 YBP. Cooler spring growth and the habit of semi-dormancy during drought. After and wetter conditions followed causing the retreat of the prairies and this drought it decreased in numbers nearly to its pre-drought allowing the advancement of mesic forests. abundance. It is unknown whether S. coccinea in the Loess Hills is a Borchert ( 1950) noted that vegetational gradients coincide with relict from the Hypsithermal or from a more recent extended drought. climatic gradients. If the pattern of climatic gradients changes, so also As Gleason (1922) pointed out, some rapidly migrating species does the vegetational pattern. This idea has been more recently may currently be at their maximum potential range, awaiting some emphasized by Davis (1983). The Hypsithermal Interval was a period environmental change to initiate movement once again. One such of long-term changes which exerted extensive effects upon the vegeta­ species in the Loess Hills might be Dalea enneandra Nutt. (Fabaceae), tion of the Great Plains. However, comparatively short-term droughts nine-anther dalea. Its distribution pattern at the eastern edge of its have also been important events influencing the vegetation (Tomanek range is classic (Fig. 3). This species occupies a narrow zone in all Loess and Hulett, 1970). Coupland (1958) observed that during the decade Hills counties right up to the terminus of the loess landform in Missouri (Bush, 1895; Goodnight, 1964; Iffrig, 1980). These plants

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216 PROC. IOWA ACAD. SCI. 92(1985)

which demonstrates its invasional capacity by its ability to colonize roadcuts and eroded areas (Wemple, 1970; pers. observ.). The migrational status of some Great Plains species in the Loess Hills is questionable as to whether they are retreating or advancing. An example is Mentzelia decapetala (Pursh) Urban and Gilg. (Loasaceae), sand lily. In Iowa its distribution is limited to till or shale outcrops or to steep, near-vertical faces ofloess on the extreme western bluffs in Plymouth, Woodbury, Harrison and Pottawattamie counties (Fig. 5). Its occurrence is not continuous, the distances between stands being variable; the greatest separation is between the Wood­ bury and Harrison county sites, approximately 70 miles (113 km.). This species is weakly perennial and typically occupies disturbed sites or barren ground (Darlington, 1934; Hill, 1975); as a result, if the appropriate habitat is available, Af. decapetala may yet advance in the Loess Hills. Two westerly, morphologically similar, but taxonomically distinct, species of Astragalus are present on upland loess prairies: A. missourien­ sis Nutt., Missouri milkvetch, and A. lotiflorus Hook., lotus-flowered milkvetch (Fabaceae). A. missouriensis, a threatened species in Iowa (Roosa and Eilers, 1978), is locally abundant in western Plymouth County, but becomes less common southward, with its southern terminus just north of Holly Springs in Woodbury County (Fig.6). Thus, it barely occupies a narrow area in Iowa approximately 50 miles (81 km.) in length. Apparently, this distribution has not changed MO greatly since the first documented collections were made (Fox, 1945; Grant, 1953; Thorne, 1956; Carter, 1960; Novacek Bates, 1983). One possible reason it has not advanced might be because the distance between its preferred habitat is perhaps too great for effective dissemi­ nation. In this species, the seeds are shed quite close to the parent plant (Barneby, 1964); in addition, the areas between likely habitats

Fig. 2. Distribution of Sphaeralcea coccinea,- Loess Hills outliers indi­ cated by asterisks.

are connected to the main population farther west via the appropriate river bluff habitat along the Missouri River. This constricted peninsu­ lar distribution in the Loess Hills is completely surrounded by more mesic tall-grass prairies and woodlands, environments inhospitable to this xerophytic species. A similar pattern exists for Yucca glauca Nutt. (Liliaceae), yucca, albeit not as dramatic as that of Dalea enneandra. In the Loess Hills, yucca is confined mainly to west-facing slopes on upland prairies (Fig. 4). It is quite common in western Kansas and Nebraska and the Dakotas, but it constricts its range in northeastern Nebraska and southeastern South Dakota, confined to the upland prairies along the Missouri River bluffs; from there it is found only on the loess bluff prairies of western Iowa and northwestern Missouri. Pammel ( 1901) mentioned that yucca was a rare plant in the Loess Hills, but more common northward near Sioux City. This statement infers that, at that time, yucca was uncommon in the southern Loess Hills. Personal observations indicate that yucca does decrease in frequency in the southern part. It is further interesting to note that yucca is currently listed on Missouri's rare and endangered species list (Nordstrom et al., 1977). This distributional pattern throws some credibility upon the idea of a northwest entry for migrating Great Plains taxa. The most valuable evidence for migration in progress is the successional behavior of a species at the edge of its range. According to MO Gleason ( 1922), if the species acts as an invader, it is advancing. In my opinion, Petalostemon occidentale (Gray) Fern. (Fabaceae), western prairie clover, is expanding its range within the Loess Hills. This species is the xerophytic counterpart ro P. candidum Michx., white prairie clover, and is ubiquitous throughout its range in the Great Plains. (See lsely and Welsh, 1960.) According to Wemple (1970), P. occidentale is "adventive to the east in particularly xeric environ­ Fig. 3. Distribution of Dalea enneandra, exhibiting an extremely ments, e.g. the Loess Hills of western Iowa." It is an aggressive plant narrow distribution toward the eastern part of its range. https://scholarworks.uni.edu/pias/vol92/iss5/13 4 Novacek: The Loess Hills of Western Iowa: a Problem in Phytogeography

PHYTOGEOGRAPHY OF LOESS HILLS 217

have been greatly modified by human activities, possibly preventing its further migration. On the other hand, A. lotiflorus is found the entire length of the Loess Hills (Fig. 7). This plant is similar to A. missouriensis in its preference for a fairly open microhabitat (Barneby, 1964), but it is unknown why it has migrated farther than A. missouriensis. A. lotiflorus may have migrated prior to disruption of its required habitat and possibly long before A. missouriensis, which may represent a relatively "recent" entry into Iowa. Current research (Novacek, unpublished data) has revealed, however, the presence of A. lotiflorus on sand dune areas along the Missouri River. Perhaps this is an answer to this distributional problem. According to Barneby (1964), the majority of Astragalus species exist in fine-tuned balance with the environment, their distributional patterns dependent upon complex ecological and genetic factors. He has indicated that the largest number of species thrive only in harsh situations without a continuous groundcover, or they are confined to soils literally hostile to other plant life. From turn-of-the-century records and accounts, the prairies of the Loess Hills at that time had a greater amount of bare interstitial soil, and there were far fewer trees and shrubs than at the present time (Hitchcock, 1889; Bush, 1895; Pammel, 1895; Shimek, 1909, 1915). Since that time, fires have been suppressed, allowing a greater litter accumulation on the prairies and the advancement of woody species. It remains to be seen whether either species of Astragalus will maintain their numbers or decline in MO the Loess Hills. If conditions are favorable to migration along the Missouri River bluffs, then are there any plants which "should" be in the Loess Hills or are on their way? One possibility is Astragalus racemosus Pursh, creamy poisonvetch. According to Barneby (1964), A. racemosus is

Fig. 5. Distribution of Mentzelia decapetala; it must be noted that its range is not nearly as continuous as the map indicates.

quite common on dry, rugged and eroded terrain along rivers, and its primary natural habitat is on open prairie, particularly around calcareous or shaley outcrops. The nearest population to western Iowa is just 70 miles (113 km.) west along the Missouri River in Nebraska and South Dakota, where it is quite common (Van Bruggen, 1976) (Fig. 8). Possibly with the correct environmental impetus, this species will eventually reach the Loess Hills.

CONCLUSIONS While circumstantial evidence may seem to indicate that, since the Hypsithermal Interval, the Great Plains floral component in the Loess Hills of western Iowa is retreating, I believe that there may be a few exceptions to the case: plants which are either currently advancing, such as Petalostemon occidentale and possibly Mentzelia decapetala, or at least appear to be at their maximum potenial range, such as Dalea enneandra and Yucll:l glauca. There may be a number of relict species from the Hypsithermal Interval, but certainly a good candidate, in my opinion, is Sphaeralcea coccinea. I do not feel, however, that all of the Great Plains components in the Loess Hills are retreating Hypsither­ mal relicts. If the xerophytic Great Plains species expanded eastward beyond the Loess Hills and northwest Iowa and are now retreating, why is there no evidence of these plants remaining eastward in Iowa or westward in eastern Nebraska? Surely there are spots of suitable habitat scattered here and there in both of these areas; in which case, there should be a few relict colonies hanging on in favorable sites. Since there appear not to be, however, I am led to the conclusion that Fig. 4. Distribution of Yucca glauca, which may be currently at its the migratory route for many of the Great Plains taxa present in the maximum range potential. Loess Hills today has been the Missouri River system, either the bluffs

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218 PROC. IOWA ACAD. SCI. 92(1985)

MO

Fig. 6. Distribution ofAstragalus missouriensis, found growing in only Fig. 7. Distribution of Astragalus lotiflorus, found on most loess two Iowa counties, Plymouth and Woodbury. prairies, but not abundantly. lining river or the dry sandy habitats on the floodplain. I also speculate reconstruction in northwestern Iowa. p. 123-138. In D. C. Anderson and that there is a good possibility that some of the actual migration H. A. Semken, Jr. The Cherokee Excavations: Holocene Ecology and Human Adaptations in Northwestern Iowa. Academic Press, New York. occurred relatively recently, that is, at least within the last 3000 years. BARKLEY, T. M., ed. 1977. Adas of the Flora of the Great Plains. Iowa Work is currently underway more thoroughly investigating the State University Press, Ames. Missouri River as a migratory pathway for species of the northern BARNEBY, R. C. 1964. Adas of North American Astragalus. Mem. N.Y. Great Plains and the Nebraska Sandhills. Bot. Gard. 13:1-1188. BARRY, R. G. 1983. Climatic environment of the Great Plains, past and ACKNOWLEDGEMENTS present. Trans. Nebr. Acad. Sci. 11:45-55. BERNABO, J. C. and T. WEBB III. 1977. Changing patterns in the I would like to thank Dr. Theodore Van Bruggen, University of Holocene pollen record of northeastern North America: a mapped sum­ South Dakota, for his guidance and encouragement throughout my mary. Quaternary Research 8:64-96. undergraduate years; by his boundless enthusiasm for floristics and BORCHERT, J. R. 1950. The climate of the central North American the Great Plains, he set an example to which I can only hope to aspire. grassland. Ann. Assoc. Amer. Geog. 40(1):1-39. I also thank Dr. Dean M. Roosa, Iowa Preserves Board, who has BRADBURY, J. P. 1980. Late Quartnary vegetation history of the central provided many valuable suggestions and insights for my work in the Great Plains and its relationship to eolian processes in the Nebraska Sand Loess Hills. Thanks also goes to Bill Pusateri, Donald Farrar, Jack Hills. U. S. Geo!. Surv. Prof. Paper 1120. BUSH, B. F. 1895. Notes on the mound flora of Atchison County, Missouri. White, and my parents, Donald and Beverly,. for their support. Part of Ann. Rpt. Mo. Bot. Gard. 6:121-134. the research for this project was supported by contract work undertak­ CARTER, J. L. 1960. The flora of northwestern Iowa. Ph.D. dissertation, en for the Iowa Natural Areas Inventory, and by funds provided by the University of Iowa, Iowa City. University of South Dakota Honors Program. COUPLAND, R. T. 1958. The effects of fluctuations in weather upon the grasslands of the Great Plains. Bot. Review 24(5):273-317. DARLINGTON, J. 1934. A monograph of the genus Mentze/ia. Ann. Mo. REFERENCES Bot. Gard. 21:103-226. DAVIS, M. B. 1983. Quaternary history ofdeciduous forests of eastern North ADAMS, C. C. 1905. The postglacial dispersal of the North American biota. America and Europe. Ann. Mo. Bot. Gard. 70:550-563. Biol. Bull. 9:53-71. DEEVEY, E. S., and R. F. FLINT. 1957. Postglacial Hypsithermal Interval. AHLBRANDT, T. S.,J. B. SWINEHART, andD. G. MARONEY. 1983. Science 125(3240): 182-184. The dynamic Holocene dune fields of the Great Plains and Rocky FOX, W. B. 1945. The Leguminosae in Iowa. Amer. Mid. Nat. 34:207- Mountain basins, U.S.A. p. 379-406. In M. E. Brookfield and T. S. 230. Ahlbtandt. Eolian Sediments and Processes. Elsevier Science Publishers GLEASON, H. A. 1922. The vegetational history of the Middle West. Ann. B.V., Amsterdam. Assoc. Amer. Geog. 12:39-85. BAKER, R. G., and K. L. VAN ZANT. 1980. Holocene vegetational GOODNIGHT, L. M. 1964. Flora of Holt County, Missouri. M. S. thesis, https://scholarworks.uni.edu/pias/vol92/iss5/13 6 Novacek: The Loess Hills of Western Iowa: a Problem in Phytogeography

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