This file was created by scanning the printed publication. Errors identified by the software have been corrected; however, some errors may remain. DISPLACEMENT OF RARE PLANTS BY EXOTIC GRASSES Roger Rosentreter ABSTRACT alyssum and filaree more typically occur on coarse sandy soils; and Russian thistle, kochia, halogeton, tumblemustard, and perfoliate peppergrass are common on alkaline sites. The domination of a site by a particular species is not wholly dependent on soil chemistry, as these species may proliferate rapidly in an area following a soil-disturbing event. The species that stochastically has the greatest abundance of seed present in the seed bank is often the species to dominate. Exotic grasses are displacing both rare and common plant species on western rangelands. Interspecific competition displaces valuable genetic resources in many parts of the world, including the Intermountain shrub-steppe communities. This problem is especially bad in the Intermountain West because of the invasion of exotic species preadapted to the arid conditions. Case studies of several rare plants, including Aase's onion (Allium aaseae Ownbey), Davis peppergrass (Lepidium davisii Rollins), slick-spot peppergrass (L. papilliferum [Henderson] A Nels. & J.F. Macbr), inch-high lupine (Lupinus uncialis S. Wats), gymnosteris (Gymnosteris nudicaulis [Hook. & Arn.] Gray), and an annual buckwheat (Oxytheca dendroides Nutt.), illustrate the challenge these species have in surviving in environments modified by exotics. Even if grazing were properly controlled or regulated, rare plants would still be threatened unless exotic grasses and the resulting increased fire frequency were controlled. Watershed Values Sites converted to annuals have lost genetic, species, and structural diversity. They represent low-quality watersheds with increased susceptibility to soil erosion and are prone to desertification (Buckhouse 1985). Annual vegetation is more susceptible to drought, as its biomass accumulation fluctuates with the available soil moisture more than perennial vegetation does. Stewart and Young (1939) found forage production of grasses varied much less for perennial grasses than for cheatgrass. They demonstrated that perennials produce twice the biomass of cheatgrass in a moist year and 12 times the biomass during a drought. Managing forage for domestic livestock on annual grasslands can be seen as a gamble under such production schedules (Roberts 1990). An error in planning or grazing management can cause habitat degradation during dry years and leave dangerous flash fuels in moist years. Rangeland users need a more dependable source of forage for proper grazing management (Roberts 1990). INTRODUCTION The displacement of rare plants by annual weeds is a major problem on Intermountain rangelands. Cheatgrass <Bromus tectorum L.) is now the dominant species on more than 40 million ha (100 million acres) of the Intermountain West (Mack 1981). Competition between rare plants and annual weeds is both direct and indirect. Indirect impacts caused by annuals include increased fire frequency and the associated conversion of shrub-steppe to nonshrub annual grasslands, and the associated increased plant litter (Billings 1990; Evans and Young 1970) and changes in nutrient availability. Prior to the arrival of white settlers, fire-return intervals in the sagebrush steppe probably varied between 60 and 110 years, but much of the region now burns at intervals of less than 5 years (Whisenant 1990). Fire Frequency Sites dominated by annuals typically have higher fire frequencies than sites where perennials dominate (Pellant Table 1-Common exotic weedy annual species on Intermountain rangelands The Exotic Annuals The most prevalent exotic annual weeds in the Intermountain West are listed in table 1. These and other annuals may occur alone or in combination. Each year, depending on weather conditions, timing of precipitation events, soil texture, soil pH, and stochastic fluctuations in the availability of seed, exotic annual species may rapidly dominate a disturbed site. For example, medusahead wild.rye is more prevalent on heavy clay soils; pale Common name Cheatgrass Medusahead wildrye Filaree Bur buttercup Perfoliate peppergrass Pale alyssum Tumblemustard Tansymustard Halogeton KochIa Russian thistle Paper presented at the Symposium on Ecology, Management, and Restomtion of Intermountain Annual Rangelands, Boise, ID, May 18-22, 1992. Roger Rosentreter is Botanist, Bureau of Land Management, U.S. Department of the Interior, 3380 Americana Terrace, Boise, ID 83706. 170 Scientific name Bromus tectorum L. Elymus caput-medusae L. Erodium cicutarlum (L.) L'Her. Ranunculus testiculatus Crantz Lepidium perfoliatum L. Alyssum alyssoides L. Sisymbrlum altissimum L. Descurainia pinnata (Walt.)Britt Halogeton glomeratus Meyer Kochiaspp. Sa/sola kali L. 1990). Exotic annuals produce a continuous bed of flash fuels that perpetuates an abbreviated fire cycle, a cycle unsuitable for persistence of most native plants (Wright 1985). The resulting loBS of genetic and structural diversity is important to the functional dynamics of the system and as an indicator of ecosystem health. Studies of rare species not only deepen our knowledge of these species, but they also deepen our understanding of common species, offering insightS to questions of fundamental importance in ecology and evolutionary biology. LoBSes in vegetation diversity cause collateral declines in faunal diversity and shorten the time period of active plant growth (Parmenter and MacMahon 1983). It is the reduced period of active growth and the associated reduction in community biomass that causes the decline in many animal populations. For example, loss of shrubs used as winter food and thermal cover for jackrabbits caused a decline in the use of the Snake River Birds of Prey Area by golden eagles (Nydegger and Smith 1986; Steenhof and Kochert 1988). A reduced period of active green vegetation means a reduced period when wildlife and domestic livestock can properly utilize an area. Ecosystem Stability Loss of diversity generally causes ecosystem instability and, in portions of the Intermountain West, increases fire frequencies (Whisenant 1990). In addition to the biological "cost" of increased fire frequencies, the enormous monetary cost of fire in terms of suppression and rehabilitation provides a major incentive to maintain native perennial rangelands. Annual fire suppression costs on Bureau of Land Management-administered lands in the State of Idaho alone averaged $4,339,000 per year between 1987 and 1991. In 1992 the costs increased to $10,253,000 (Bill Mitchell: personal communication). Without a conversion from the annual grassland vegetation back to a perennial type, these costs will recur as areas repeatedly burn, and traditional rehabilitation costs of these sites can be much higher than the appraised value of the land (Roberts 1990). STUDY AREA The results discussed later are based on several studies of rare plant populations in the Snake River Plain and adjacent foothills. The Snake River Plain is of basaltic origin, while the foothills are a mixture of granitic alluvium and fine silts and clays. Elevations range from 671 m (2,200 ft) to 1,676 m (5,500 ft) at midmountain. Soil moisture& are classified as aridic; soil temperatures are claBSified as mesic. Cooler soil sites that are frigid soil temperature regime or cooler are not addressed in this treatment. Landscape Patterns Annual grasslands that burn frequently are highly uniform, interspersed with very few patches of unburned vegetation remaining (Whisenant 1990). Large, species-poor sites have a strong isolating effect on the remaining native vegetation patches. The new fire regime results in areas with poor species richness, low landscape patchiness, and altered successional patterns. Mature sagebrush steppe is eliminated, as are many earlier successional species that occupy a site after minor disturbances within the mature community. Loss of a dominant species such as sagebrush may cause the loss of sympatric species dependent on it, such as twisted moBS (Tortula ruralis [Hedw.] Gaerth.), which grows in the shade of sagebrush. Likewise, spiny hopsage (Grayia spinosa [Hook.] Moq.) becomes established beneath canopies of mature shrubs (Nancy Shaw: personal communication). Some sagebrush grassland species are mutualistic, sharing mycorrhizal associations as well as complementing each other ecologically (Wicklow-Howard 1989). As one species is lost, associated species may also be threatened. ABIOTIC FACTORS AND LANDSCAPE PA'ITERNS Shrubs increase abiotic microsite variability by providing shade, decreasing wind, and capturing snow and rain on a local scale. This is especially important in areas like the Intermountain West where precipitation comes primarily in the winter as snow. Abiotic factors change when a site is converted from shrubs to exotic annual grasslands, and the loss of shrubs is the major factor affecting abiotic changes (Billings 1990). With shrub loss, there is a decline in (1) forage production, (2) soil stability, (3) diversity, (4) consistent annual biomass production, (5) mineral cycling, (6) thermal and escape cover for wildlife, and (7) esthetic values (Parmenter and MacMahon 1983; Rosentreter and Jorgensen 1986). These local influences on the abiotic conditions are reflected in the vegetative cover at every level of scale on the landscape. For example, the composition of the local vegetation is simplified and the degree of landscape patchiness decreases with an increased prevalence of annual grasses, biotic changes which can persist for long periods due to the changes in the abiotic environment (Billings 1990; Whisenant 1990). In northern Nevada, Billings (1990) followed a sagebrush- steppe site for 41 years after it burned. He found that sagebrush did not return to this cheatgrass-dominated site. Shrubs Biological relationships and their structural dependency are critical in the shrub-steppe of the Intermountain West. Shrubs intercept moisture, provide shade, recycle deep soil nutrients, and modify soil-surface temperatures by decreasing wind velocity (Murray 1975; Rosentreter and Jorgensen 1986). These structural features of steppe shrubs are comparable to values attributed to trees in forests. Conversion of shrub-steppe to annual grasslands represents a drastic and detrimental change in a community. 171 In a climate where most of the annual precipitation comes in the form of snow, shrub presence or absence may determine the effective soil moisture throughout the year. At low and mid-elevations, the snow melts periodically, saturating the ground beneath the snowbank. Repeated occurrences of this snow-capturing and melting process contribute to a patchy vegetation pattern. SPREAD OF EXOTICS Initially, annual grasses may occur only where a fire or other major disturbance has taken place, but they may spread to adjacent sites. Exotic, nongrass species increase slowly after the disturbed site is dominated by cheatgrass. After a second fire, other exotics, in addition to cheatgrass, quickly invade the areas adjacent to the original disturbance. I believe this is largely determined by the "stochastic occurrence of seed." STRUCTURAL DIVERSITY The structural diversity of a site is dramatically lowered as the shrub component declines. Shrubs provide important thermal and escape cover for wildlife, just as they provide favorable microsites for establishment of grass and forb seedlings. The structure of the shrub community influences the pattern of snow accumulation and melting process and moderates soil temperatures both beneath individual shrubs and at the stand level. These differences influence animal and plant composition, dispersal, and vigor. Animals dependant on forbs and grasses in a shrub community find green forage for up to a month longer than in an adjacent annual grassland lacking structural diversity (Rosentreter, personal observation). CASE STUDIES Direct Competition Native Annuals-In portions of the Intermountain West cheatgrass has replaced native annuals such as gymnosteris (Gymnosteris nudicaulis [Hook. & Am.] Gray), inch-high lupine (Lupinus uncialis S. Wats.), oxytheca ((hytheca dendroides Nutt.), and langloisia (Langloisia punctata [Torr.] Greene). Most of these native species are now on the Idaho State sensitive plant list (Moseley and Groves 1990). Exotic annuals compete directly with native plants for space, moisture, and light. Gymnosteris is found in basin big sagebrush (Artemisia tridentata Nutt. ssp. tridentata) habitat (DeBolt and Rosentreter 1988). Label data from specimens collected 20 to 40 years ago state that it was "abundant" or "common" (College of Idaho Herbarium, Caldwell, ID [CIC]). Today this plant is rarely seen, with small colonies found only on good-condition rangelands when spring moisture is abundant. I collected extensively in two above-average moisture years in southwestern Idaho and found few populations of this species. The inflorescences of gymnosteris plants collected in Idaho in the last 10 years are predominately yellow or white. Older collections exhibited a wider range of colors at each collection site, including white, yellow, pink, violet, and orange. This decline in color variation may represent a loss of genetic diversity caused by reduced genetic mixing of the remnant populations. The isolated patches of gymnosteris may have become reproductively isolated and may be experiencing genetic bottlenecks (Falk and Holsinger 1991). This decline in genetic diversity may decrease the vigor of individuals and populations. SUCCESSIONAL PATTERNS It appears that the steppe ecosystem, once controlled by classical successional patterns (relay floristics), is now largely dominated by initial floristics due to the introduction of exotic annuals (Billings 1990). In southwestern Idaho, the first exotic seeds to arrive at a site have a great advantage over other species, often dominating the site for long periods of time (Whisenant 1990). This may be referred to as the "stochastic occurrence of seed." While seasonal fluctuations in precipitation or litter may affect the frequency of specific exotics, displaced native perennials typically do not vary seasonally in frequency (Evans and Young 1970). For example, with fall precipitation, cheatgrass grows rapidly and will dominate a site by the following spring. If fall moisture is minimal or lacking, bur buttercup (Ranunculus testiculatus Crantz) frequently dominates. Russian thistle (Salsola kaU L.) emerges in spring and comes to dominate disturbed sites that receive summer precipitation. These exotics take advantage of the full spectrum of the phenologic and temperature niches available in the Intermountain steppe. While exotic annuals exploit soil moisture at different times of the year, they do not utilize the entire soil profile. This may explain why, without fire, grazing, or other major disturbance, shrub-steppe habitats in good condition can exclude most exotic annuals. Most ecologists recognize the importance of competition for space above the soil surface, but in arid regions, interspecific competition is largely played out underground, in the soil profile. Shrubsteppe communities converted to annual grasslands generally lack deeply rooted species to utilize deep soil moisture. Shrubs recycle nutrients and moisture leached deep into the soil profile (Caldwell and Richards 1989; Murray 1975). Without the return of these deep nutrients and moisture, many sites may become increasingly impoverished, arid, and less productive (Billings 1990). Indirect Competition Aase's Ouion-Aase's onion is a rare (Federal category 1 species), small perennial that occurs on coarse, barren, sandy soils on east-, south-, and west-facing slopes in the Boise foothills (DeBolt and Rosentreter 1988). It is often associated with mature bitterbrush (Purshia tridentata [Pursh] DC.) on or near ridgetops. Aase's onion is restricted both ecologically and geographically. Its phenology is very early, with growth beginning in February and flowering in March and April. While its habitat is not optimal for the growth of cheatgrass, as the surrounding vegetation is converted to annuals, cheatgrass can be found invading Aase's onion habitat. As the occurrence of cheatgrass increases in areas adjacent to onion habitat, litter cover increases thereby modifying soil conditions 172 Table 2-Number of Allium umbels and cover class values of exotic annuals per quadrat. Annual cover classes: 1 = 1-5 percent, 2 = 6-25 percent, 3 = 26-50 percent, 4 =51-75 percent Growth of these annuals on coarse, sandy soil and adjacent areas causes an accumulation of litter and a nutrient flush. This litter is damaging to the onion, which is adapted to barren, nutrient-poor sites. Though no onion sites have been totally displaced by exotics to date, the extent of the onion's suitable habitat has decreased in size. Cover class of Mean number of S.d. of Quadrat sample annual s pecies Allium umbels Allium umbels size (N) 1 2 3 4 11 .5 8.4 5.6 4 8.9 5.9 4.9 2 Davis Peppergrass-Davis peppergrass, a Federal candidate 2 species, is a caespitose perennial mustard restricted to hard-bottom playas (fig. 1) that are usually barren of other vegetation (DeBolt and Rosentreter 1988). The playas are poorly drained and often inundated with standing water in early spring. It occurs in eastern Oregon and southwestern Idaho. Although cheatgrass does not grow on the playas with Davis peppergrass, it indirectly affects peppergrass survival. Playas in the Great Basin receive runoff from adjacent areas. When these adjacent areas are converted to exotic annual grasslands, siltation generally increases (DeBolt and Doremus 1989, 1990). Some playas adjacent to annual grasslands can receive over 1.5 em of siltation in the spring following a summer wildfire (Ann DeBolt 1988: personal observation). Excess siltation may smother and kill individual Davis peppergrass plants. A second and more direct threat created by the degraded grasslands is the growth of other annual weeds, .especially Russian thistle (fig. 2). After death, Russian thistle, kochia, and tumblemustard plants dry and break 38 14 12 3 and encouraging the growth of other exotic annuals such as filaree (Erodium cicutarium [L.] L'Her.) and pale alyssum (Alyssum alyssoides L.) (Young and Evans 1979). These exotics spread easily into sandy sites directly competing with the onion (table 2). Table 2 data group cheatgrass, filaree, and pale alyssum together as annuals. Table 2 shows that the number of Allium umbels decreases as the percentage of annuals increases in a qua drat (unpublished data, Boise District, BLM files). Field observations of Aase's onion show an increase in filaree, pale alyssum, and cheatgrass on sites that have burned (table 2), with filaree the best adapted for sand. .. "" .... ~- - • . ... ~~ • Figure 1- Piaya with Davis peppergrass. ~ Figure 2-ln this playa Davis peppergrass died due to the mulching effect of Russian thistle. 173 - CONCLUSIONS free from the ground to blow about as "tumbleweeds," occasionally coming to rest in the playas. Because playas are depreBBions in the landscape, they may collect large numbers of tumbleweeds to depths of a meter or more. Tumbleweeds can act as a mulch, blocking light for extended portions of the year, and sometimes causing extensive peppergrass mortality. Several playas have been completely covered by tumbleweeds (fig. 3). Figure 8 shows data from two playas (number 64 and 75) where populations of peppergrass went from 114 and 65 plants respectively to none in a 2-year period. Six other playas (fig. 3) with Davis peppergrass have greatly decreased in population size due to the mulching effect of tumbleweeds (DeBolt and Doremus 1989, 1990, and unpublished 1992 data). Throughout the Intermountain West, native plants, both rare and common, are reduced in number and genetic diversity where exotic annuals become dominant. This change in community structure is not a temporary situation, but rather a shift in successional patterns to a self-perpetuating annual community. This change in species composition promotes an increased fire frequency that concomitantly reduces structural diversity and patchiness of the landscape. The loss of structural diversity, once provided by shrubs and perennial bunchgrasses, modifies abiotic conditions in the community. The new community of exotic annuals competes both directly and indirectly with native plants. Indirect competition includes the increase in fire frequency, stochastic distribution of exotic seeds, increased litter accumulation, increased siltation, and loss of microphytic plants. Most native species can tolerate moderate amounts of grazing. However, even one season of misuse may cause degradation and subsequently invasion by exotic annuals, thus predisposing the site to fire. After one or more burns, the diverse native plant comm~ty is converted to an impoverished community of exotic annuals. Maintaining healthy native communities, the suppression of fire, and the prevention of rangeland wildfires is needed to retain rare and common native plant communities. SUck-Spot Peppergrass-Slick-spot peppergrass is a small biennial mustard restricted to relatively barren "slick spots" within the Wyoming big sagebrush (Artemisia tridentata Nutt. BBp. wyomingensis Beetle and Young) community type in southern Idaho (DeBolt and Rosentreter 1988). Slick spots are shallow soil sites or tiny playas also referred to as intercoppice areas (Eckert and others 1986). These sites are variable in size, but they are generally small (2-5 mt) and irregular in shape. They are sparsely vegetated by vascular plants, but are often carpeted with nonvascular microphytic plants, also called cryptogamic crusts (Rosentreter 1986). For slick-spot peppergrass, cheatgrass is not a direct, but rather an indirect, competitor. The intercoppice areas are small, and as the adjacent vegetation is converted from shrubs to annuals conditions change~ Fires become more frequent, altering the character of the soil's surface, causing siltation onto the slick spots. The fires and siltation also destroy the crust of microphytic plants. In Montana, Lesica and Shelly (1992) determined thatArabis fecunda Rollins, another rare mustard, was positively 888ociated with the growth of microphytic plants. The cumulative effects of these changes permit invasion of exotic annual plants into the slick spots. · 9 36 13 37 59 64 75 ACKNOWLEDGMENTS I would like to thank Ann DeBolt for her assistance in the field and in the review of this paper. I would also like to thank Drs. 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