Probabilities of Seedling Recruitment and the Stability of Crested Wheatgrass Stands Jay E. Anderson and Guy M. Marlette ABSTRACT: Crested wheatgrass seedingsinthe grasses of a particular climax are the best adapted western United States have persisted as virtual to its climate and have a distinct advantage in monocultures for over 50 nears followingtheir. terms of competition over the introduced ones.... establishment.Such stability typically is Crested wheatgrass (Agropyron cristatum) has often attributedtosuperior competition by crested been cited as a warrior such a procedure wheatgrass, but this explanation assumes that native (introduction), but all evidence available indicates propagules are available for recruitment.Data qg that it can persist in the face of the competition seedling emergence from undisturbed topsoil samples of the indigenous grasses only when man aids it by show that there is a paucity of native orooagules cultivation or otherwise."The life expectancy of within crested wheatgrass stands.For two stand,, crested wheatgrass seedings is usually placed at 20 that were near monocultures, the probability of a years or less (Bartolome and Heady 1978), although seedling being crested wheatgrass was over 85%. some investigators considerthattobea Recruitment probabilities favor the maintenance of a conservative estimate (Sonnermann et al.1981). monoculture rather than its successional r_ elacemen t. Contrary to these views ofcrested wheatgrass forming unstable.temporary stands,itis now generally recognized that the species spreads ACKNOWLEDGEMENTS: This paper is a contribution from quickly by seed and is capable of invading native, the Idaho National Engineering Laboratory Ecological undisturbed habitat (Hull and Klomp 1967,Hull Studies Program,supported bythe Office of Health 1971).When sown in a mixture with native species, and Environmental Research,U.S.Department of crested wheatgrass frequently becomes the dominant Energy.We thank Nancy Cole, _?avid Humphrey, Amy species in the stand (Heinrichs and Bolton 1950, Marlette, and Diane Pavek for assisting with data Hull 1971,Holechek et al.1981). Several collection and O.D. Markham for his support and investigators considered crested wheatgrass to be suggestions. competitively superior to dominant native species (Hubbard 1957, Eckert et al. 1961, Robertson et al. 1966, Robertson 1972, Holechek et al. 1981).Recent INTRODUCTION studies by Caldwell et al. (1981, 1983) have elucidated some physiological and morphological Crested wheatgrass( Agropyron cristatum (L.) attributesthat contributetothe superior Gaertn. and A.desertorum (Fisch.) Schult.) often competitive ability and greater grazing tolerance of has been seeTed and managed in North America under crested wheatgrassin comparisontobluebunch the assumption that without continued intervention wheatgrass (Pseudoroegneria spicata (Pursh) Love).1 by man the stand will eventually revert to a native climax community (Lewis 1969).Implicit in the Crested wheatgrass seedings inthe western assumption is the concept of only one stable climax United States have resisted invasion by native for any particular site, and that succession will species for as long as 30 to 50 years (Hull and follow a directional, predictable course toward that Klomp 1966, Looman and Heinrichs 1973, Sonnermann et stabilized community.Clements (1935) wrote, "the al. 1981), which simply indicates the length of time for which data are available ratherthanthe JayE. Anderson andGuy M. Marlette are, respectively, Professor, Biology Department, Idaho !Nomenclature followsHitchcock and Cronquist (1973) State University, Pocatello; Chemist, Radiological except for grasses of the Triticeae tribe.For and Environmental Sciences Laboratory, U.S. bluebunch wheatgrass, we have used the taxonomic Department of Energy, Idaho Falls, Idaho. revision proposed by Love (1980); nomenclature for other members of the Triticeae follows Dewey (1983). 97 longevity of the stands. Soils beneath some & Smith) Gould), bluebunch wheatgrass, needle -and- seedings are not converging totheir original thread (Stipa comata Trin. & Rupt.),and Indian condition even after 50 years and may be evolving ricegrass (Oryzopsis hymenoides (R.&S.) Ricker). toward a different equilibrium pointthan undisturbed soils (Dormaar et al. 1980). In its Average annual precipitation for the INEL is native habitat. crested wheatgrass is knows for its about 21cm, with 40% typically falling during ability to establish on disturbed sites and persist April, May and June (Anderson and Holte 1981). Mean in virtual monocultures (Looman and Heinrichs 1973). annual temperature is about 5.5 °C; the frost free Thus, rather than crested wheatgrass eventually period averages 91 days. The topography is flat to being replaced by native species, it may inhibit or gently rolling, with occasional lava outcrops. The even preclude the recovery of native vegetation. It soils are primarily shallow, calcic aridisols of is. therefore. desirable to understand the factors aeolian origin lying over basalt. esonsible for the stabilitof crested wheatgrass stands_ Four crested wheatgrass stands and adjacent native communities were studied during the summers Succession ultimately depends on whether a of 1978 and 1979. Three stands, located on a remote species replaces itself on a given site or is section of the INEL known as Tractor Flat, were replaced by members of other species (Grubb 1977, established following plowing and seeding in the McIntosh 1980). This replacement process isa fall of 1956. They are referred to as TFA, TFB, and function of two factors (Gleason 1926, Poore 1964, TFC. They have been grazed by sheep, primarily in Harper 1977, Whipple 1978): 1) the availability of the spring, since 1959. The fourth stand (PBF), propagules on the site, and 2) whether those located near the INEL Power Burst Facility, was propagules finda suitable environment for seeded in 1960 and has never been grazed by domestic germination, growth, and ultimately, survival. livestock. ThisstandandTFB were near monocultures; the other two areas were selected Dispersal of native propagules into crested because re- establishment of native species wheatgrass seedings is usually assumed to occur. (primarily big sagebrush) within the seeding was Shepherd (1937 cited by Weldon et al.1959) evident. All study areas were selected so that the concluded that "a supply of viable seed is probably main interface between the native community and the never a limiting factor for thé aggression of adjacent seeding was approximately perpendicular to sagebrush." Looman and Heinrichs (1973) surmised the prevailing wind direction. At PBF, TFA, and that "Over the years, a considerable reservoir of TFB, the seedings were downwind from the native seeds of native and introduced species has built up, communities, which was assumed to represent the and the chances that a given crested wheatgrass maximum potential for wind dispersal of propagules plant will be replaced by its own kind after death from the native communities. are lessened with increasing age." Thus,it is typically assumed that the stability of a crested At each study area, crown cover of shrubs and wheatgrass seeding is a function of the superior basal cover sf grasses and forbs were estimated competitive ability of crested wheatgrass which using a 0.5 m sampling frame that was subdivided renders the site unsuitable for the establishment of into dm2 grids (Floyd and Anderson 1982). The frame other species. was systematically placed at 2- to 4 -m intervals along three parallel transects that extended from Competition can be responsible for the stability the native stand into or across the adjacent created of crested wheatgrass stands only if propagules of wheatgrass seeding. Transects were 23 m apart; native species are dispersed into the stands.The their lengths varied from 115 to over 500 m, purpose of this investigation wasto test the depending on the layout of the seeding and adjacent assumption that there is sufficient dispersal of native community.Seed reserves were sampled in native species into crested wheatgrass seedings to March of 1979 by driving cylinders cut from #10 cans result in a store of native propagules available for into the soil to a depth of 6 cm and removing the recruitment.In essence we asked, "If a safe site intact soil core by passing a trowel under the (sensu Harper 1977) becomes available, what is the cylinder. Paired samples from bare and littered probability that it will be colonized by crested areas were taken at systematic intervals along the wheatgrass as opposed to one of its potential vegetation transects, sampling both the crested competitors ?" Our results suggest that there isa wheatgrass stands and the native community. Between paucity ofnative propagules available for 130 and 180 samples from each study area were placed recruitment within crested wheatgrass stands. in a greenhouse or environmental chamber where they were watered regularly with distilled water. Emerging seedlings were identified, counted, and METHODS removed. We refer to these counts as seed reserves, butit should be noted that the data actually This study was conducted on the Idaho National represent seedling emergence from undisturbed Engineering Laboratory (INEL), which occupied some topsoil, except where natural disturbances occurring 2300 km of sagebrush steppe rangeland on the upper in the field were sampled. Further details about Snake River Plain. Vegetation of the area is the study areas and