Bret E. Olson

Effects of livestock grazing on rangeland grasshopper (Orthoptera: Acrididae) abundance

Livestock may impact habitat quality for grasshoppers by reducing food availability and by altering microclimate and potential oviposition sites. A 5-year study was conducted to create consistent grazing impacts on replicated plots and measure their effects on plant cover, microclimate, and grasshopper abundance. Cattle were used to produce two levels of grazing intensity that were compared to ungrazed controls. Differences in plant cover were greatest immediately after grazing each summer, grasshopper microhabitats tending to be shadier, cooler, less windy, and more humid in the ungrazed plots. The grasshopper assemblage included five of the worst pest grasshopper species in North America: Ageneotettix deorum, Aulocara elliotti, Melanoplus sanguinipes, M. packardii, and Camnula pellucida. Most species had greater abundance on ungrazed pastures, particularly during the 4–6 weeks after grazing each year. However, A. elliotti was often more abundant in heavily grazed areas early in the year when early instars were present and in late summer when adults were predominant. There was no strong evidence that the effect of grazing on grasshopper abundance increased over the 5-year study. At this time, all changes in grasshopper numbers cannot be directly attributed to particular habitat characteristics that changed after grazing, but the results suggest that grazing management could be used to reduce pest grasshopper densities.

Effect of weed seed rate and grass defoliation level on diffuse knapweed.

Diffuse knapweed (Centaurea diffusa Lam.), an invasive weed, has reduced forage production and biodiversity, and increased soil erosion on over a million hectares of rangeland in the western United States. This study evaluated the effects of a single grass defoliation on establishment of diffuse knapweed seeded at 2 rates into a bluebunch wheatgrass (Pseudoroegneria spicata [Pursh.] Scribn and Smith)/needle-and-thread (Stipa comata Trin. &Rupr.) community and a crested wheatgrass (Agropyron cristatum (L.) Gaertn.) community. Six defoliation levels (0, 20, 40, 60, 80, 100%) and 2 seeding rates (3,000, 6,000 diffuse knapweed seeds) were applied to 1 m2 plots in a randomized-complete-block design (n=4). Diffuse knapweed was seeded in the fall of 1992, and grasses were defoliated on 28 April 1993. The number of flowering culms and weed seedlings were counted in September 1993. Densities of diffuse knapweed seedlings, juveniles, and adults, as well as plant standing crop, were determined in May 1994. Seed rate had minimal effect on diffuse knapweed density. By May 1994, densities of diffuse knapweed were about 20 and 30 plants m-2 on undefoliated bluebunch wheatgrass and crested wheatgrass plots, respectively, indicating that defoliation is not required for this noxious weed to become established. Higher levels of grass defoliation (>60%), especially of bluebunch wheatgrass, enhanced diffuse knapweed establishment, indicating that moderate (<60%) defoliation would not necessarily accelerate invasion by diffuse knapweed.

Effects of invasive forb litter on seed germination, seedling growth and survival

Summary Two Eurasian forbs, Euphorbia esula L. and Centaurea maculosa Lam., continue to spread in the northwestern United States despite extensive and expensive control efforts. We investigated whether litter from these forbs and associated soils suppress germination and growth of two native perennial grasses (Pseudoroegneria spicata [Scribn. & Smith A. Love], Pascopyrum smithii [Rydb.] A. Love), which may partly explain the success of these invasive forbs. Seed germination was unaffected, but roots were shorter at higher leachate concentrations. The physical presence of litter (Euphorbia, Centaurea, none) did not affect seed germination or number of leaves of seedlings of the four species, but it affected seedling heights; the tallest seedlings were those covered with Centaurea litter; the shortest were those without litter. As a mechanical barrier, litter type did not affect survival or number of leaves of pregerminated seeds, but litter inhibited shoot growth. Seed germination of the four species on soils from infested or noninfested areas differed; Euphorbia germination, albeit low ( Die Ausbreitung zweier Unkrauter eurasiatischer Herkunft (Euphorbia esula L. und Centaurea maculosa Lam.) in den nordwestlichen USA konnte trotz umfangreicher Gegenmasnahmen bisher nicht aufgehalten werden. Wir untersuchten, ob diese Krauter durch Streuproduktion oder andere Bodenveranderungen eine negative Wirkung auf Keimfahigkeit und Wachstum zweier heimischer mehrjahriger Graser (Pseudoroegneria spicata [Scribn. & Smith A. Love], Pascopyrum smithii [Rydb.] A. Love) ausuben, was zum Teil den Erfolg dieser Unkrauter erklaren konnte. Saataufgang war nicht beeintrachtigt, aber Wurzeln waren kurzer und wiesen hohere Exsudatkonzentrationen auf. Die pure Anwesenheit von Euphorbia- oder Centaurea streu hatte keinen Effekt auf Auflauf oder Blattanzahl der Samlinge aller vier Arten, sowohl auf Boden aus verunkrauteten Arealen als auch auf vorher unkrautfreien Boden. Streu beeinflusste aber die Samlingshohe: die hochsten Samlinge traten in Verbindung mit Centaurea Streu auf, die niedrigsten unter streulosen Bedingungen. Die Streuart hatten keinen Einfluss auf Uberlebensfahigkeit oder Blattzahl vorgekeimter Samen, jedoch beeintrachtigte das Vorhandensein von Streu die Sprosslange. Die Keimfahigkeit der vier Arten hing davon ab, ob die Boden aus verunkrauteten oder unkrautfreien Gebieten stammten. Die Euphorbia-Keimrate, obwohl generell niedrig (

Spotted knapweed seed viability after passing through sheep and mule deer.

Spotted knapweed (Centaurea maculosa Lam.), an introduced perennial plant, has invaded large areas of rangeland in the northwestern United States. Grazing animals may disseminate the weed by transporting seeds in their digestive system and depositing them in their feces. In this study percent viability and emergence of spotted knapweed seeds that passed through mule deer (Odocoileus hemionus hemionus) and sheep (Ovis aries) were determined. Percent viability included seeds that germinated and seeds that tested positive with tetrazolium. In the first trial, we pulse dosed 3 mule deer and 4 ewes with 5,000 spotted knapweed seeds each. Seed recovered from manure collected daily for 10 (days after dosing was tested for percent viability. We recovered 11% of the knapweed seeds from the 3 mule deer, and 4% from the sheep. Based on high variability in (0 to 26%) percent viability of recovered seed, we thought that our drying the manure at a 50 degrees C may have killed some of the spotted knapweed embryos. To determine if drying at 50 degrees C affected viability, we pulse dosed 41 rams with 5,000 spotted knapweed seeds each in a second trial. One subsample of manure was washed the same day to recover seeds and then dried at 35 degrees C, a second subsample was dried at d 50 degrees C, washed, and then dried at 35 degrees C. We recovered 17% of the spotted knapweed seeds from the 4 rams. No viable seeds were recovered from manure heated at 50 degrees C, and no viable seeds were recovered more than 2 days after dosing. Percent viability of seeds recovered from manure dried at 35 degrees C ranged from 0 to 22%. In both trials, percent viability of recovered seeds was lower compared with seeds that did not pass through animals. Sheep and mule deer can ingest, transport, and disseminate viable seeds of spotted knapweed in their feces.

Effects of sheep grazing on a spotted knapweed-infested Idaho fescue community.

Spotted knapweed (Centaurea Maculosa Lam.), a Eurasian perennial forb, is replacing many native perennial grasses, such as Idaho fescue (Festuca idahoensis Elmer.), in foothills of the Northern Rocky Mountain region. Our objective was to determine if 3 summers of repeated sheep grazing would reduce spotted knapweed without impacting the dominant, associated native perennial grass. Each summer, small pastures were grazed for 1-7 days in mid-June, mid-July, and early September. Areas repeatedly grazed by sheep had lower densities of seedlings, rosettes, and mature spotted knapweed plants than ungrazed areas. In addition, the proportion of young plants in the population was less in grazed than ungrazed areas. Basal areas of spotted knapweed plants were greater in grazed (8.2 cm2) than ungrazed areas (4.0 cm2). There were fewer spotted knapweed seeds in soil samples from grazed areas (12 seeds m-2) than from ungrazed (49 seeds m-2). Idaho fescue plant density increased 40% in grazed areas from 1991 to 1994, but leaves and flower stems on these plants were 38% and 17% shorter, respectively, than in ungrazed areas. By 1994, frequency of Kentucky bluegrass (Poa pratensis L.) was 35% greater in grazed than ungrazed areas. Grazing did not alter the amount of litter; however the amount of bare soil increased from 2.2 to 5.6% in grazed areas, while it decreased from 4 to 1% in ungrazed areas. Three summers of repeated sheep grazing negatively impacted spotted knapweed, but minimally affected the native grass community. A long term commitment to repeated sheep grazing may slow the rate of increase of spotted knapweed in native plant communities.

ADVANTAGES IN WATER RELATIONS CONTRIBUTE TO GREATER PHOTOSYNTHESIS IN CENTAUREA MACULOSA COMPARED WITH ESTABLISHED GRASSES

Semiarid steppe communities in North America appear particularly vulnerable to persistent infestations by exotic, taprooted forbs, such as European spotted knapweed (Centaurea maculosa). We determined whether species differences in ecophysiological response to water availability could help link traits of Centaurea with invasibility of steppe communities. Plant‐soil water relations and photosynthesis were measured under three water levels in a greenhouse and at two sites over two years in the field for Centaurea and dominant rangeland species of southwestern Montana: Pseudoregneria spicata, Pascopyron smithii, and Bromus inermis. Centaurea had greater and more seasonally persistent photosynthesis than the other species under field conditions but not in the greenhouse, where water availability was similar for the species. Centaurea had no greater water use efficiency, except under unusually dry conditions, but maintained greater water potentials despite greater transpiration than the grasses. Changes in soil water indicated uptake from deeper and wetter soils in Centaurea than in grasses. Greater photosynthesis in Centaurea compared with grasses may result from uptake of deeper soil water and corresponding drought avoidance. Interspecific differences in resource use may therefore contribute to the success of Centaurea, and Centaureas ecological requirement for water matches an available resource niche in the communities we examined.

Effects of the Invasive Forb Centaurea maculosa on grassland Carbon and Nitrogen Pools in Montana, USA

Invasions by exotic forbs are changing large areas of North American grasslands, but their biogeochemical impacts are not well characterized. Additionally, although many invasive plants may alter biogeochemistry, an invasive species’ effects have rarely been evaluated across physically diverse sites. We sampled nine sites containing the perennial Eurasian forb Centaurea maculosa to determine if this invasive species alters soil C and N pools in native grasslands in Montana, USA. We sampled surface soil in adjacent microsites with C. maculosa and native grasses and analyzed soil C and N pools with slow to rapid turnover. None of the pools evaluated in the laboratory showed significant differences between C. maculosa and grass microsites when analyzed across all sites. Some differences were found at individual sites, but they were infrequent and inconsistent: Four sites had no differences, four had differences in one or two pools with intermediate (particulate organic matter C or N) or rapid turnover rates (potentially mineralizable N), and just one site had differences encompassing pools with rapid, intermediate, and slow (total C and N, silt-and-clay-associated N) turnover rates. Where they differed, pools were usually smaller under C. maculosa plants than under native grasses, but the opposite was found at one site. In situ N availability, estimated using ion exchange resins, was significantly lower under C. maculosa than under grasses at one of three sites sampled. Results indicate that C. maculosa may sometimes reduce soil C and N pools, including those related to N availability, but they argue against generalizing about the impacts of C. maculosa in grasslands.

Traits of the invasive Centaurea maculosa and two native grasses: effect of N supply

The Eurasian forb Centaurea maculosa (Lam.; spotted knapweed) has invaded millions of hectares of semi-arid grasslands in western North America. It readily colonizes disturbed areas, but also invades pristine grasslands. C. maculosas success could be attributed to greater use, or more efficient use, of available soil nitrogen (N). Soil N often limits growth on semi-arid grasslands. Greater or more efficient use of soil N by C. maculosa, if this occurred, may inhibit establishment, survival, or reproduction of native grasses. In a glasshouse, C. maculosa and two native grasses, Pseudoroegneria spicata [Scribn. and Smith] A. Love and Pascopyrum smithii [Rybd.] A. Love, were grown in mixed- and monoculture for 8 weeks to determine growth response to two soil N supplies, which mimicked low and high N mineralization rates in semi-arid grasslands. At the end of the 8 weeks, plants were exposed to 15N-labeled nitrate for 24 h, and harvested to compare uptake of NO3−. C. maculosas growth response to N indicated that it was more competitive for N than the tussock grass P. spicata, but less competitive than the rhizomatous grass P. smithii. C. maculosa used nitrogen less efficiently than both of these native grasses. C. maculosa roots took up more 15N per unit root mass than the grasses, but acquired less N than P. smithii because P. smithii had greater root mass than C. maculosa. Total biomass and 15N uptake of C. maculosa varied depending on which species it was growing with. C. maculosas success cannot be explained wholly by greater or more efficient use of soil N than that of the native grasses with which it competes.

Recovery of leafy spurge seed from sheep

Sheep are often used to graze North American rangelands infested with leafy spurge (Euphorbia esula L.), a long-lived perennial forb from Eurasia. Our objective was to determine if sheep grazing infested rangelands disperse leafy spurge seed by transport in their fleece or by depositing seeds in their feces. Twenty-four yearling Targhee ewes grazed a 2.4 ha native bunchgrass range site infested with leafy spurge from late-May through mid-August of 1993 and 1994. Six of the 24 ewes were shorn in October 1993. To recover leafy spurge seeds from those fleeces, we used a standard method to test wool for vegetable matter. On average, 38 seeds were recovered per fleece. During these summers, 6 small groups (n = 4 sheep per group) each grazed 3 separate paddocks. We estimated the density of leafy spurge seed before the groups were moved into 1 of 3 paddocks. After the sheep were moved into a paddock (day 0), we collected fresh feces from each group on or about day 4, 10, and 14. Feces were then washed over sieves to recover leafy spurge seeds. All seeds were tested for germinability and viability. The number of viable seeds excreted daily per ewe was estimated. In 1993, 1,796 +/- 405 (S.E.) leafy spurge seeds m-2 were produced in the field, whereas in 1994, 399 +/- 63 (S.E.) leafy spurge seeds m-2 were produced. The summer of 1994 was much drier than the summer of 1993. We estimated that 41 to 144 leafy spurge seeds were excreted daily per animal in mid-July 1993. Viability of seeds in the feces averaged 5%, whereas viability of seeds collected from seed stalks was 42%. We estimated that the ewes excreted from 2 to 41 leafy spurge seeds daily at the peak in mid-July 1994. Viability of seeds excreted during 1994 averaged 24%, whereas viability of seeds collected from seed stalks was 68%. Sheep can pick up leafy spurge seed in their fleece, and will consume and pass viable seed. However, viability of seed recovered from feces was highly variable and almost always lower than seed collected in the field. Despite reduced seed numbers and viability, sheep have the potential to spread leafy spurge and should be managed accordingly.

Effect of previous experience on sheep grazing leafy spurge

Herbivores develop preferences for certain plant species in several ways. One way is to learn from role models, another is through postingestive feedback. In this study we determined (1) whether yearling sheep exposed to leafy spurge (Euphorbia esula L.) as lambs graze it more readily than yearlings that were not exposed to it as lambs, and (2) whether this difference, if present, persists through the grazing season. On a leafy spurge-infested, Idaho fescue (Festuca idahoensis Elmer) range site, we compared the grazing behavior of naive groups of Targhee yearlings with grazing behavior of experienced groups for 3 years. Using focal animals, we assessed diet selectivity by counting bites and by determining time spent grazing different forage types. During the grazing season, nutritive value (crude protein, neutral detergent fiber, in vitro dry matter digestibility) of leafy spurge was always higher than nutritive value of Idaho fescue. Experienced yearlings spent more than four times as much time grazing leafy spurge in early summer compared with naive yearlings (averaged across 1992 and 1993, experienced 13.6% vs. naive 2.9%), whereas in mid- (39% vs. 31%) and late summer (37% vs. 38%) both groups spent similar amounts of time grazing leafy spurge. Neither group readily grazed the plant in early summer. Experienced yearlings had higher bite rates of leafy spurge than naive yearlings during the early summer sampling periods in 1992 and 1993 (averaged across 1992 and 1993, 15.9 vs. 5.0 bites min−1) but their bite rates were similar by mid-summer (22.7 vs. 20.6 bites min−1). In 1994, we observed their grazing behavior every 5 days for 35 days. Initially, naive yearlings spent less time grazing leafy spurge than experienced yearlings, but were spending similar amounts of time grazing leafy spurge by day 25. Previous experience with leafy spurge, compared with no experience, resulted in a slight but short-lived advantage in early summer use of the species. Within 3–4 weeks, sheep that had never previously been exposed to leafy spurge readily grazed the plant.