NATURE OF THE PLANT COMMUNITY. V. VARIATION WITHIN THE TRUE PRAIRIE COMMUNITY‐TYPE

MCMILLAN, C. (U. Texas, Austin.) Nature of the plant community. V. Variation within the true prairie community-type. Amer. Jour. Bot. 46(6): 418-424. Illus. 1959.-Population samples of grass species were transplanted to Lincoln, Nebraska, from two grassland communities within the general distribution of the true prairie community-type. These represented extensive grassland areas near Watertown, South Dakota, and Manhattan, Kansas. True prairie relicts near Colorado Springs, Colorado and in the Black Hills of southwestern South Dakota were also studied in the transplant garden. Only in one species, Stipa spartea Trin., were the transplanted populations essentially similar in behavior. In 8 of the species, Koeleria cristata (L.) Pers., Bouteloua gracilis (H.B.K.) Lag., B. curtipendula (Michx.) Torr., Schizachyrium scoparium Nash, Andropogon gerardi Vitman, Panicum virgatum L., Sorghastrum nutans (L.) Nash, and Sporobolus heterolepis (A. Gray) A. Gray, the Manhattan populations were the latest flowering. In Elymus canadensis L., the Manhattan population was the earliest flowering. The abstraction of two communities into a true prairie type of community with other communities of similar species-populations is a convenient method for discussing distributional phenomena. However, extreme caution must be used in generalizing about characteristics other than distributional. If the 10 species in the present study were designated by letters, the Watertown and Manhattan communities could be compared ecologically as a b' c d e f g h i j and a b c' d' e' f' g' h' i' j'. The use of taxonomic relationship in vegetational studies is convenient for the reduction of sample size, but the vital aspect of studying variation within the species is in the determination of harmony between vegetation and its habitat. The relict communities are in adjustment with their habitats and represent the results of natural selection in favoring early-flowering variants within a number of different species. The key to distribution of the true prairie vegetation, widespread or relict, lies in the harmony between a habitat variable and a vegetation variable. Through natural selection, each stand of true prairie may be fundamentally different from any other stand. THE EXTENT of a regional grassland climax, as interpreted by Clements (1936), was based primarily upon 2 criteria. The first, the life form of the dorninants, the perennial grasses, provided the visible unity of the climax. The second, involving the widespread occurrence of certain genera and the geographic repetition of certain combinations of species and genera, was drawn from distributional data. Both of these criteria, one, physiognomic, and the other, distributional, can be used in delineating the present extent of a regional grassland vegetation in North America. Clements (1936) recognized that the climate and the grassland climax were not "matched by a similar uniformity," although this seems to have been overlooked by a number of his critics (Malin, 1956; Whittaker, 1957, among others). Clements noted that the' average period without killing frost is about 9 mo. in the south "but in the Canadian provinces it is less than 3, while the mean annual temperatures are 700 and 33?F., respectively." Furthermore, he noted that the maximum rainfall at the extrenies of the climatic pattern not infrequently amounted to 3 and 4 times that of the minimum. He stated that the difference in climate from Texas 'Received for publication January 30, 1959. This study was aided by funds from National Science Foundation Grant G-3809 and the Nebraska Agricultural Experiment Station. to Manitoba and Saskatchewan was particularly striking because only 1 association (mixed prairie) was involved. The difference in climate within the area of the true prairie type of community is equally noticeable. Although Clements devoted little writing to the apparent disharmony of vegetation and climate, he suggested in 1928 (p. 128) that "while there is every certainty theoretically that the responsive unit, the formation," (the climax) "is in harmony with the causal unit, the habitat, our present knowledge is inadequate to prove this." There can be only speculation on the reasoning that Clements may have used in resolving this apparent lack of correlation between uniformity of vegetation and non-uniformity in the climate. Clements and Shelford (1939) suggest that the chief explanation of this seeming anomaly is to be found for plants in the evasion of temperature extremes; however, this apparently has primary reference to the life form, the visible unity of the grassland. The results of his many transplant studies (Clements et al., 1950), indicating that highly plastic behavior is a common characteristic of plants, may have provided Clements with the basis for his resolution of the distributional non-correlation. The present study has the goal of furthering our understanding of the nature of grassland vegetation, particularly that with the composition referable to