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Alpine Phytogeography Across the Great Basin W Great Basin Naturalist Memoirs Volume 2 Intermountain Biogeography: A Symposium Article 7 3-1-1978 Alpine phytogeography across the Great Basin W. D. Billings Department of Botany, Duke University Durham, North Carolina 27706 Follow this and additional works at: https://scholarsarchive.byu.edu/gbnm Recommended Citation Billings, W. D. (1978) "Alpine phytogeography across the Great Basin," Great Basin Naturalist Memoirs: Vol. 2 , Article 7. Available at: https://scholarsarchive.byu.edu/gbnm/vol2/iss1/7 This Article is brought to you for free and open access by the Western North American Naturalist Publications at BYU ScholarsArchive. It has been accepted for inclusion in Great Basin Naturalist Memoirs by an authorized editor of BYU ScholarsArchive. For more information, please contact [email protected], [email protected]. ALPINE PHYTOGEOGRAPHY ACROSS THE GREAT RASIN W. D. Billings' Abstract.—Alpine vegetations and floras are compared in two transects across the Intermountain Region. The first extends from the Beartooth Mountains in the central Rocky Mountains to the central Sierra Nevada some 1200 km to the southwest. It includes six mountain ranges. The second transect crosses the Mojave Desert from Olancha Peak in the southern Sierra Nevada to Charleston Peak and thence to San Francisco Peaks in northern Arizona. The largest numbers of arctic-alpine species are in the Beartooth and Ruby mountains, indicating migra- tions of these species along the Rocky Mountain cordillera. The lowest numbers of arctic-alpine species are in the central and western Great Basin and in the Sierra Nevada. Sdrensen's Index of Floristic Similarity was calcu- lated for all possible pairs of the nine alpine areas. There is little correlation of floristic similarity with alpine proximity across the Intermountain Region. Rather, any such correlation seems to be in a north-south direction; increase to- this is stronger in the eastern part of the region. Insularity and uniqueness of alpine floras seem to ward the western part of the basin. This is probably due to evolution of alpine endemics from preadapted low- land taxa. The middle-latitude mountains of North floor has been uplifted in the Pleistocene. America north of Mexico, for simplicity and Orogenic activity continues at present. Even convenience, may be grouped into four though the Sierran batholith is rather old, large systems. With few exceptions, the the present Sierra Nevada is primarily a mountains in these systems trend north to product of uplift during the Pliocene and south, a fact of considerable importance in Pleistocene (Axelrod 1962 and pers. comm. the phytogeography of arctic and alpine 1973; Rateman and Wahrhaftig 1966). Fossil plants. The four systems are the Appala- lobed oak leaves at 2850 m in the lower al- chians in the eastern part of the continent, pine zone on Elephant's Rack, south of Car- the Rocky Mountains, the Cascades-Sierra son Pass, lend additional evidence of recent Nevada, and last, but not least, the Great Sierran uplift. The high volcanoes of the Rasin ranges. The latter three systems domi- Cascades are also of similarly recent age. nate the western third of the continent. mountain ranges, in their In general, the Alpine Islands in the Great Rasin present forms, are younger the closer they are to the Pacific Coast. The Appalachians There are some 200 individual mountain constitute a very old mountain complex dat- ranges within the Great Rasin. Most of ing from Permian and Triassic times. Much these trend in a general north to south di- of the large Rocky Mountain system origi- rection and are separated by broad desert nated in the Laramide Revolution in late valleys. In the days when mountain ranges Cretaceous and early Paleocene. The oldest were shown on maps by hachures, Dutton basin ranges also rose during the Laramide described the pattern as similar to an "army Orogeny, but most of these ranges, particu- of caterpillars marching to Mexico" (Morri- larly in the west, have been upthrust during son 1965). These basin ranges, with eleva- a period of time from the Oligocene to the tions which vary from about 1800 m to Pleistocene. Additionally, the whole basin over 4300 m, are by no means alike either Department of Botany, Duke University, Durham, North Carolina 27706. 105 106 GREAT BASIN NATURALIST MEMOIRS No. 2 geologically or botanically. Holmgren (1972) the desert sea eventually diminishes until it divided the region, on the bases of floristics is blocked by the jumbled mountain masses and geology, into four main divisions made of British Columbia which connect the up of 16 sections. The mountain ranges coastal mountains and the Rockies. within any one section have certain charac- Johnson (1975) used the lower edge of teristics in common but also there are some forest-woodland as the perimeter of his is- rather remarkable ecological differences be- lands. This is a real biological boundary. On tween mountains within the same section. the other hand, Brown (1971), Harper et al. Holmgren notes that there is greater varia- (this symposium), and I have defined the tion in floristic composition of the alpine lower boundaries of the montane islands zone across the Great Basin from one peak rather arbitrarily. Harper et al. and Brown to another than occurs in any other zone. I have used an elevation of 7500 ft (2286 m) agree. while I have used 9000 ft (2743 m) as an For many years, biogeographers working approximation of the extreme lower eleva- in mountain areas have compared isolated tion of alpine plants (Fig. 1). This latter fig- mountain ranges and summits to islands ure is a somewhat liberal estimate of the (Wallace 1880, Willis 1922). With increased lower limits of alpine islands in the Inter- interest in island biogeography (MacArthur mountain Region, but some alpine sites do and Wilson 1967), several important papers exist this low, particularly in cirques. Tim- have appeared which provide quantitative berline is usually higher than this and is of- information on the geographical relation- ten very ragged at its upper limits. Upper ships among the biota on isolated montane timberline is frequently used as the bound- "islands." Notable among these are those of ary between subalpine and alpine vegeta- Hedberg (1970) on the Afroalpine floras, F. tion in North America. However, on most Vuilleumier (1970) on paramo avifaunas of mountains of the earth it is not a particu- the northern Andes, B. S. Vuilleumier (1971) larly good boundary, and it is not a good and Simpson (1974) on paramo floras, boundary on most American mountains ei- Brown (1971) on mountaintop mammals in ther, including those of the Great Basin. the Great Basin, and, recently, Johnson Timberlines almost always exist at much (1975) on bird species on montane islands in higher elevations than the lowest patches of the Great Basin. MacArthur (1972) also car- alpine vegetation. This is often true around ried his theory over to montane islands in glacial valleys, both those with and those the Appalachians in his study of the distri- without glaciers at the present time. For ex- butions of thrush species. It is notable that ample, timberline around the Athabaska two of these papers (Brown's and Johnson's) Glacier in the Canadian Rockies is fully 675 are concerned with the islandlike distribu- m above the terminus of the glacier, where tion of vertebrates in the Great Basin. Nor in the morainal gravels there are a number have plant geographers ignored the island of arctic-alpine plant species but no trees. nature of the Great Basin montane islands The reasons for this ubiquitous phenomenon (P.V. Wells, pers. comm. July 2, 1968, and, are rather simple: those factors which de- of course, Harper et al. in this symposium). fine the lower limits of alpine species are Both Brown and Johnson have viewed the not necessarily those which limit the up- Great Basin desert "sea" and its montane is- ward distribution of trees. lands as being bordered on the east and As Figure 1 indicates, even conservative west by large cordilleras, the Rocky Moun- alpine islands in the Great Basin are much tains and Sierra Nevada, which they desig- smaller and more isolated from each other nate as "mainlands" or "continents." The than the mountain ranges themselves. But desert sea is open to the south as far as the these alpine islands have been both larger real sea off Mexico. However, to the north, and smaller in the past than they are at 1978 INTERMOUNTAIN BIOGEOGRAPHY: A SYMPOSIUM 107 120°W 46"N- : j r ° c,?c ftMi BJ V, , ^ \i V 18W represent Fig. I. Map of the Intermountain Region showing, in black, areas above 2750 m in elevation. These areas which are wholly or partially "alpine" at the present time. The dashed line at the 1850 m contour is an approximate estimate of what could have been the lower limit of alpine vegetation at full-glacial. Alpine regions used in the northern transect are indicated from left to right by letters: P, Piute Pass (Sierra Nevada); W, Pelli- sier Flats (White Mts.); T, Toiyabe Mts.; R, Ruby Mts.; DC, Deep Creek Mts.; B, Beartooth Mts. Those in the southern transect are: O, Olancha Pk and two nearby peaks in the southern Sierra Nevada; C, Spring Mts. (Char- leston Peak); and SF, San Francisco Peaks. 108 GREAT BASIN NATURALIST MEMOIRS No. 2 present. The areas of such islands during tation shadow. There is a trend toward glacial and hypsithermal times have had a more winter snow and summer rain in a great deal to do with their present floristics. traverse across the basin in an easterly di- For example, Simpson (1974) showed that rection toward the Rocky Mountains. The the larger sizes of Andean paramos as they summer rains in the east are usually in the existed at full glacial are more highly and form of freshening thundershowers, which, I significantly correlated with numbers of believe, have much to do with the survival plant species per paramo than are the so far south of populations of certain arctic present sizes of each paramo.
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