Indian Ricegrass Research Articles

A Systematic Review of Wild Burro Grazing Effects on Mojave Desert Vegetation, USA

Wild burros (Equus asinus), protected by the 1971 Wild Free-Roaming Horse and Burro Act on some federal lands but exotic animals many ecologists and resource mangers view as damaging to native ecosystems, represent one of the most contentious environmental management problems in American Southwest arid lands. This review synthesizes the scattered literature about burro effects on plant communities of the Mojave Desert, a center of burro management contentions. I classified 24 documents meeting selection criteria for this review into five categories of research: (i) diet analyses directly determining which plant species burros consume, (ii) utilization studies of individual species, (iii) control-impact comparisons, (iv) exclosure studies, and (v) forage analyses examining chemical characteristics of forage plants. Ten diet studies recorded 175 total species that burros consumed. However, these studies and two exclosure studies suggested that burros preferentially eat graminoid and forb groups over shrubs. One study in Death Valley National Park, for example, found that Achnatherum hymenoides (Indian ricegrass) was 11 times more abundant in burro diets than expected based on its availability. Utilization studies revealed that burros also exhibit preferences within the shrub group. Eighty-three percent of reviewed documents were produced in a 12-year period, from 1972 to 1983, with the most recent document produced in 1988. Because burros remain abundant on many federal lands and grazing may interact with other management concerns (e.g., desert wildfires fueled by exotic grasses), rejuvenating grazing research to better understand both past and present burro effects could help guide revegetation and grazing management scenarios.

Aboveground decomposition in arid environments: Results of a long-term study in central New Mexico

The literature on aboveground litter decomposition in arid environments, which is dominated by short-term studies (one- to three-year duration), shows no consistent relationship between mass loss rates and factors controlling decomposition in mesic environments. For ten years, we repeated a decomposition study of five species in four arid to semi-arid biomes in central New Mexico. Annual precipitation during the study ranged from 60% to 150% of the long-term mean. Mass remaining after one year of decomposition varied widely between species: Bouteloua eriopoda (47–85%), Bouteloua gracilis (31–87%), Juniperus monosperma (62–85%), Larrea tridentata (14–67%), and Oryzopsis hymenoides (22–88%). No consistent results were found relating percent mass change during several intra-annual intervals with initial litter C:N or current, cumulative or antecedent precipitation for any species at any site. The relative rates of decomposition were L. tridentata⪢ O. hymenoides> B. eriopoda= J. monosperma> B. gracilis. Except for percent lignin, litter quality measures were not good predictors of relative mass loss rates among species. Although a few significant relationships between decomposition and precipitation and litter quality were found, our study results were overwhelmingly unsupportive of these factors having major impacts on litter decomposition processes in central New Mexico.

The ratio of germanium to silicon in plant phytoliths: quantification of biological discrimination under controlled experimental conditions

Slight differences in the chemical behavior of germanium (Ge) and silicon (Si) during soil weathering enable Ge/Si ratios to be used as a tracer of Si pathways. Mineral weathering and biogenic silicon cycling are the primary modifiers of Ge/Si ratios, but knowledge of the biogenic cycling component is based on relatively few studies. We conducted two sets of greenhouse experiments in order to better quantify the range and variability in Ge discrimination by plants. Graminoid species commonly found in North American grassland systems, Agropyron smithii, Schizachyrium scoparium, and Andropogon gerardii were grown under controlled hydroponic environmental conditions. Silicon leaf contents were positively correlated with solution Si and ambient temperature but not with nutrient solution pH, electrical conductivity, or species. The Ge/Si ratio incorporated into phytoliths shows a distribution coefficient [(Ge/Si)phytolith/(Ge/Si)solution] of about 0.2 and is remarkably invariant between species, photosynthetic pathway, and solution temperature. Ge seems to be discriminated against during the uptake and translocation of Si to the opal deposition sites by about a factor of five. In the second experiment, a wider range of graminoid species (Agropyron smithii, Bouteloua gracilis, Buchloe dactyloides, Oryzopsis hymenoides, Schizachyrium scoparium and Andropogon gerardii) were grown in two different soil mediums. Plant phytoliths showed a distribution factor of about 0.4 for field grown grasses, and 0.6 for potting soil grown grasses with no clear trends among the species. Evidence of the direction and degree of biological Ge discrimination during plant uptake provides a geochemical finger print for plants and improves the utility of Ge/Si ratios in studies of terrestrial weathering and links between Si cycles in terrestrial and marine systems.

POLYMORPHIC INDIAN RICEGRASS POPULATIONS RESULT FROM OVERLAPPING WAVES OF MIGRATION

Abstract Indian ricegrass (Achnatherum hymenoides [Roem. & Schult.] Barkworth) is a rangeland species native to western North America with populations that may exhibit distinct seed-size morphs borne on separate plants. These morphs are often associated with very different levels of seed dormancy. We compared RAPD profiles of the relatively infrequent and highly dormant jumbo seed morph to its more germinable small or large (non-jumbo) companion seed morphs collected from 9 sites where they were sympatric. Jumbo seed accessions from 9 additional sites that did not exhibit seed polymorphism were included to better sample this seed morph across the species' range. Sympatric seed morphs were consistently distinguishable by their RAPD profiles, and sympatric seed morphs were no more likely to cluster together than nonsympatric seed morphs. The ability of sympatric seed morphs to maintain their genetic identity is compatible with the hypothesis of a predominately autogamous breeding system. Furthermore, these ...

Heterogeneity of soil nutrients and subsurface biota in a dryland ecosystem

Dryland ecosystems have long been considered to have a highly heterogeneous distribution of nutrients and soil biota, with greater concentrations of both in soils under plants relative to interspace soils. We examined the distribution of soil resources in two plant communities (dominated by either the shrub Coleogyne ramosissima or the grass Stipa hymenoides) at two locations. Interspace soils were covered either by early successional biological soil crusts (BSCs) or by later successional BSCs (dominated by nitrogen (N)-fixing cyanobacteria and lichens). For each of the 8 plant type×crust type×locations, we sampled the stem, dripline, and 3 interspace distances around each of 3 plants. Soil analyses revealed that only available potassium ( K av) and ammonium concentrations were consistently greater under plants (7 of 8 sites and 6 of 8 sites, respectively). Nitrate and iron (Fe) were greater under plants at 4 sites, while all other nutrients were greater under plants at less than 50% of the sites. In contrast, calcium, copper, clay, phosphorus (P), and zinc were often greater in the interspace than under the plants. Soil microbial biomass was always greater under the plant compared to the interspace. The community composition of N-fixing bacteria was highly variable, with no distinguishable patterns among microsites. Bacterivorous nematodes and rotifers were consistently more abundant under plants (8 and 7 sites, respectively), and fungivorous and omnivorous nematodes were greater under plants at 5 of the 8 sites. Abundance of other soil biota was greater under plants at less than 50% of the sites, but highly correlated with the availability of N, P, K av, and Fe. Unlike other ecosystems, the soil biota was only infrequently correlated with organic matter. Lack of plant-driven heterogeneity in soils of this ecosystem is likely due to (1) interspace soils covered with BSCs, (2) little incorporation of above-ground plant litter into soils, and/or (3) root deployment patterns.

Do soil characteristics or microhabitat determine field emergence and success of Bromus tectorum?

In southeastern Utah, Bromus tectorum occurs where Hilaria jamesii is dominant and rarely where Stipa hymenoides/ S. comata dominate. To determine whether this distribution is due to soil characteristics or microhabitat, we transplanted H. jamesii soil to a Stipa site and vice versa during a severe drought (2001) and a wetter year (2002). Additionally, we planted B. tectorum under H. jamesii and Stipa canopies, with or without H. jamesii litter, and with or without herbivory. Bromus tectorum emergence and biomass in reciprocal transplants were similar at both sites; there were no site differences for all microhabitat treatments. Being under a plant canopy increased emergence in 2001 and decreased survival during 2002. Herbivory decreased emergence in 2001 and decreased survival during 2002. Litter increased emergence only under the canopy in 2001 but did not affect survival in 2002. Survival in 2001 was so low that biomass was unattainable; no microhabitat treatments affected biomass in 2002. We found that soil characteristics and microhabitat affected B. tectorum similarly in H. jamesii and Stipa patches, suggesting that these factors do not explain the association between B. tectorum and H. jamesii. However, these relationships may change during wet years when B. tectorum invasions most often occur.

Effects of water additions, chemical amendments, and plants on in situ measures of nutrient bioavailability in calcareous soils of southeastern Utah, USA

We used ion-exchange resin bags to investigate effects of water additions, chemical amendments, and plant presence on in situ measures of nutrient bioavailability in conjunction with a study examining soil controls of ecosystem invasion by the exotic annual grass Bromus tectorum L. At five dryland sites in southeastern Utah, USA, resin bags were buried in experimental plots randomly assigned to combinations of two watering treatments (wet and dry), four chemical-amendment treatments (KCl, MgO, CaO, and no amendment), and four plant treatments (B. tectorum alone, the perennial bunchgrass Stipa hymenoides R. & S. alone, B. tectorum and S. hymenoides together, and no plants). Resin bags were initially buried in September 1997; replaced in January, April, and June 1998; and removed at the end of the study in October 1998. When averaged across watering treatments, plots receiving KCl applications had lower resin-bag NO3− than plots receiving no chemical amendments during three of four measurement periods—probably due to NO3− displacement from resin bags by Cl− ions. During the January–April period, KCl application in wet plots (but not dry plots) decreased resin-bag NH4+ and increased resin-bag NO3−. This interaction effect likely resulted from displacement of NH4+ from resins by K+ ions, followed by nitrification and enhanced NO3− capture by resin bags. In plots not receiving KCl applications, resin-bag NH4+ was higher in wet plots than in dry plots during the same period. During the January–April period, resin-bag measures for carbonate-related ions HPO42−, Ca2+, and Mn2+ tended to be greater in the presence of B. tectorum than in the absence of B. tectorum. This trend was evident only in wet plots where B. tectorum densities were much higher than in dry plots. We attribute this pattern to the mobilization of carbonate-associated ions by root exudates of B. tectorum. These findings indicate the importance of considering potential indirect effects of soil amendments performed in␣conjunction with resource-limitation studies, and they suggest the need for further research concerning nutrient acquisition mechanisms of B.␣tectorum.

English

In the late 1980s, two coal gasification concepts were tested at a 15- acre site in the Hanna Basin of south-central Wyoming. Both tests were conducted in the Hanna No. 1 Coal Seam, which is about 30 feet thick and 365 feet below surface at this site. Approximately 4500 tons and 11,300 tons of coal were burned in the two tests, respectively. The Land Quality Division (LQD) of the Wyoming Department of Environmental Quality (WDEQ) issued Research and Development License No. 15 (RD15) to Stearns Catalytic Corporation (now Washington Group International, Inc.) for the tests. Per the license requirements, ground water restoration and surface reclamation were necessary after the tests. Because of the efficiency of the burns and the effectiveness of venting and flushing the burn cavities after each test, ground water restoration was completed in 1989. Once ground water restoration was deemed complete, the 30+ wells were abandoned and surface reclamation began. The reported seed mix included Wheatgrass (western, thickspike, and slender), Indian Ricegrass, Fourwing Saltbush, and Big Sagebrush. In addition, Big Sagebrush seedlings were also planted. Surface reclamation was completed in 1993, with limited repair work (primarily weed control) in subsequent years. The LQD released the reclamation bond and terminated the license in late 2005. Even though this site was tiny in comparison with most in situ noncoal mines and traditional coal mines, it provides a microcosm of reclamation issues, including: drought impacts; selection of areas for evaluation of reclaimed vegetation; and comparison of species planted with species expressed. It also provides information specific to reclamation issues at in situ mining projects, such as extent of ground water contamination, measurement of revegetation success at sites with multiple, relatively small areas of disturbance, and soil sampling for water treatment impacts. The site was one of the first evaluated for bond release under Category 5 of the LQD Coal Rules.

English

Boron toxicity studies have been carried out since the 1920's. Almost without exception, these investigations emphasized agricultural crops. The results of these studies should not be extrapolated to arid land minesoils without confirmation from research in semiarid regions using native plant species. This solution culture study revealed a wide range of boron tolerance in common western range species. Two saltbushes and greasewood revealed tolerances for very high boron concentrations in the root media. Two wheatgrasses revealed lower but relatively high boron tolerances. Big sagebrush, Rubber rabbitbrush, Indian ricegrass, and Silky lupine revealed low tolerance for this element.

Arbuscular Mycorrhizal Assemblages in Native Plant Roots Change in the Presence of Invasive Exotic Grasses

Plant invasions have the potential to significantly alter soil microbial communities, given their often considerable aboveground effects. We examined how plant invasions altered the arbuscular mycorrhizal fungi of native plant roots in a grassland site in California and one in Utah. In the California site, we used experimentally created plant communities composed of exotic (Avena barbata, Bromus hordeaceus) and native (Nassella pulchra, Lupinus bicolor) monocultures and mixtures. In the Utah semi-arid grassland, we took advantage of invasion by Bromus tectorum into long-term plots dominated by either of two native grasses, Hilaria jamesii or Stipa hymenoides. Arbuscular mycorrhizal fungi colonizing roots were characterized with PCR amplification of the ITS region, cloning, and sequencing. We saw a significant effect of the presence of exotic grasses on the diversity of mycorrhizal fungi colonizing native plant roots. In the three native grasses, richness of mycorrhizal fungi decreased; in the native forb at the California site, the number of fungal RFLP patterns increased in the presence of exotics. The exotic grasses also caused the composition of the mycorrhizal community in native roots to shift dramatically both in California, with turnover of Glomus spp., and Utah, with replacement of Glomus spp. by apparently non-mycorrhizal fungi. Invading plants may be able to influence the network of mycorrhizal fungi in soil that is available to natives through either earlier root activity or differential carbon provision compared to natives. Alteration of the soil microbial community by plant invasion can provide a mechanism for both successful invasion and the resulting effects of invaders on the ecosystem.

Direct Effects of Soil Amendments on Field Emergence and Growth of the Invasive Annual Grass Bromus tectorum L. and the Native Perennial Grass Hilaria jamesii (Torr.) Benth

Bromus tectorum L. is a non-native, annual grass that has invaded western North America. In SE Utah, B. tectorum generally occurs in grasslands dominated by the native perennial grass, Hilaria jamesii (Torr.) Benth. and rarely where the natives Stipa hymenoides Roem. and Schult. and S. comata Trin. & Rupr. are dominant. This patchy invasion is likely due to differences in soil chemistry. Previous laboratory experiments investigated using soil amendments that would allow B. tectorum to germinate but would reduce B. tectorum emergence without affecting H. jamesii. For this study we selected the most successful treatments (CaCl2, MgCl2, NaCl and zeolite) from a previous laboratory study and applied them in the field in two different years at B. tectorum-dominated field sites. All amendments except the lowest level of CaCl2 and zeolite negatively affected B. tectorum emergence and/or biomass. No amendments negatively affected the biomass of H. jamesii but NaCl reduced emergence. Amendment effectiveness depended on year of application and the length of time since application. The medium concentration of zeolite had the strongest negative effect on B. tectorum with little effect on H. jamesii. We conducted a laboratory experiment to determine why zeolite was effective and found it released large amounts of Na+, adsorbed Ca2+, and increased Zn2+, Fe2+, Mn2+, Cu2+, exchangeable Mg2+, exchangeable K, and NH4+ in the soil. Our results suggest several possible amendments to control B. tectorum. However, variability in effectiveness due to abiotic factors such as precipitation and soil type must be accounted for when establishing management plans.

Effects of Nitrogen Deposition on an Arid Grassland in the Colorado Plateau Cold Desert

Historically, ecosystems in the southwestern United States derived much of their nitrogen (N) from N-fixation in biological soil crusts. Today, these regions have highly reduced crust cover, and atmospheric deposition may be the dominant source of N. This study investigates the effects of increased nitrogen deposition on nitrogen uptake, photosynthesis, and growth of the two main forage grasses on the Colorado Plateau, galleta (Hilaria jamesii [Torr.] Benth.) and Indian ricegrass (Oryzopsis hymenoides, [Roemer J.S. Schultes] Ricker ex Piper). Plots were fertilized for 2 years with 0, 10, 20, and 40 kg nitrogen ha-1 annually, up to 43 the estimated current annual deposition rate, in 2 applications per year (spring and summer). Half-plots were fertilized with either (NH4)2SO4 in KCl solution or with KNO3 solution to determine possible differences in the effects of NH4+ and NO3 in this system. Neither grass increased leaf photosynthesis or tiller size due to supplemental N. Galleta also did not increase tiller density, while estimated live tiller density in Indian ricegrass increased up to 50% in the second year. Nitrogen applications accelerated the onset of water stress in both species presumably through stimulating ecosystem transpiration. Nitrogen form did not significantly affect any aspect of grass physiological performance or growth. However, leaf nitrogen in NH4+/-fertilized plants was significantly more isotopically enriched than in NO3-fertilized plants, suggesting that both species incorporated NH4-N only after it had been enriched by soil turnover. Seedlings of Russian Thistle (Salsola iberica, Sennen Pau), a noxious annual invasive weed on western rangelands, grew rapidly in the first summer on plots with 40 kg nitrogen ha-1 per annum, and more so on plots fertilized with NO3 than with NH4+. The study suggests that changes in the timing and amount of nitrogen input may alter community composition through facilitating the invasions of summer-active noxious weeds.

Fire Rehabilitation Using Native and Introduced Species: A Landscape Trial

Following the 1999 Railroad Fire in Tintic Valley, Utah, we initiated a large-scale fire rehabilitation study comparing a predominately introduced species seed mix used by the US Department of Interior-Bureau of Land Management (BLM), a mix of native and introduced species provided by the US Department of Agriculture-Agricultural Research Service (ARS), and 2 native seed mixes (high and low diversity). Mixes were seeded with a rangeland drill on the big sagebrush (Artemisia tridentata var. wyomingensis [Beetle A. Young] Welsh) study area whereas the pinyon-juniper (Pinus edulis Engelm.-Juniperus osteosperma [Torr.] Little) woodland study area was aerially seeded followed by 1-way chaining. On drill-seeded plots and by the third year after seeding the native high-diversity mix (16.4 kg pure live seed [PLS] ha-1) had the highest seeded species cover (11.5%) and density (14 plants m-2). Both the BLM (9.3 kg PLS ha-1) and ARS (9.1 kg PLS ha-1) seed mixes had higher seeded species cover (BLM = 8.5%, ARS = 8.2%) and density (BLM = 8.4 and ARS = 7.2 plants m-2) than plots seeded to the low-diversity native mix (8 kg PLS ha-1, cover = 3.8%, density = 3.6 plants m-2). Indian ricegrass (Achnatherum hymenoides [Roemer and J. A. Schultes] Barkworth ‘Nezpar’) in the native high-diversity mix was especially successful on the sandy soils of the drill site, whereas seeds of other species may have been buried too deep for optimum emergence. Aerially-seeded and chained plots had similar and successful seeded species frequency, cover, and density (third-year average = 10.6% cover, 17.2 plants m-2) among all species mixes. All seeded plots had lower cover of annual species than unseeded plots, indicating that revegetation is necessary to reduce weed invasion following catastrophic wildfire in big sagebrush communities lacking residual perennial understory vegetation.

Effects of Nitrogen Deposition on an Arid Grassland in the Colorado Plateau Cold Desert

Abstract Historically, ecosystems in the southwestern United States derived much of their nitrogen (N) from N-fixation in biological soil crusts. Today, these regions have highly reduced crust cover, and atmospheric deposition may be the dominant source of N. This study investigates the effects of increased nitrogen deposition on nitrogen uptake, photosynthesis, and growth of the two main forage grasses on the Colorado Plateau, galleta ( Hilaria jamesii [Torr.] Benth.) and Indian ricegrass ( Oryzopsis hymenoides , [Roemer & J.S. Schultes] Ricker ex Piper). Plots were fertilized for 2 years with 0, 10, 20, and 40 kg nitrogen ha −1 annually, up to 4× the estimated current annual deposition rate, in 2 applications per year (spring and summer). Half-plots were fertilized with either (NH 4 ) 2 SO 4 in KCl solution or with KNO 3 solution to determine possible differences in the effects of NH 4 + and NO 3 − in this system. Neither grass increased leaf photosynthesis or tiller size due to supplemental N. Galleta also did not increase tiller density, while estimated live tiller density in Indian ricegrass increased up to 50% in the second year. Nitrogen applications accelerated the onset of water stress in both species presumably through stimulating ecosystem transpiration. Nitrogen form did not significantly affect any aspect of grass physiological performance or growth. However, leaf nitrogen in NH 4 + -fertilized plants was significantly more isotopically enriched than in NO 3 − -fertilized plants, suggesting that both species incorporated NH 4 -N only after it had been enriched by soil turnover. Seedlings of Russian Thistle ( Salsola iberica , Sennen & Pau), a noxious annual invasive weed on western rangelands, grew rapidly in the first summer on plots with 40 kg nitrogen ha −1 per annum, and more so on plots fertilized with NO 3 − than with NH 4 + . The study suggests that changes in the timing and amount of nitrogen input may alter community composition through facilitating the invasions of summer-active noxious weeds.

Plant Responses to an Edaphic Gradient across an Active Sand Dune/Desert Boundary in the Great Basin Desert

In arid ecosystems, variation in precipitation causes broad‐scale spatial heterogeneity in soil moisture, but differences in soil texture, development, and plant cover can also create substantial local soil moisture heterogeneity. The boundary between inland desert sand dunes and adjacent desert habitats exhibits abrupt changes in soil and vegetation characteristics that may be associated with differences in plant‐available water and nutrients. We hypothesized that differences in plant water status between habitats would mirror changes in soil texture and vegetation cover. Because the dunes have higher water availability, we predicted that plants on dunes would have higher plant water potential (Ψ) and be less water use efficient than plants off dunes. We tested these predictions for three species—Psoralidium lanceolatum (a C3 perennial N‐fixing subshrub), Stipa hymenoides (a C3 perennial bunchgrass), and Salsola iberica (a C4 annual)—on actively moving desert dunes and adjacent stabilized dunes at Little...

Playing Softball With the Rancher

Introduction How many times does a rancher who grazes livestock on public lands hear they can use 50% of the key herbaceous forage species? Yet, the rancher does not fully understand, or lacks familiarity with, how that would be measured or determined by the specific public land management agency. As a result, the rancher may use the grazing allotment based on the agency’s authorized period of use. At the end of the grazing season the rancher finds that there is 1 or 2 inches average stubble height remaining on the key herbaceous species and the measured utilization is 70% after the livestock are removed. The rancher is then in a position of exceeding the use level objectives. This is, in effect, a communication gap that often results in resource objectives not being fully realized in public land management. To help the rancher and the range conservationist better manage the resource and reduce the time necessary to do so, the following softball stubble height method is suggested to monitor the forage utilization. Looking at Figure 1, of the 5 Indian ricegrass plant heights, where would you graze to achieve to 50% use by weight? Using the Interagency Height–Weight method (US Department of the Interior–Bureau of Land Management 1996), if the ungrazed Indian ricegrass is 16 inches tall, then when there are 5.5 inches remaining, there is a 50% utilization level by weight. How often do we leave that much stubble height?

Summer and winter drought in a cold desert ecosystem (Colorado Plateau) part I: effects on soil water and plant water uptake

We investigated the effects of winter and summer drought on plants of the Colorado Plateau in western North America. This winter-cold, summer-hot desert region receives both winter and summer precipitation. Droughts were imposed for two consecutive years using rainout shelters. Here, we examine drought effects on the hydrologic interactions between plants and soil. We chose three perennial species for this study, representing different rooting patterns and responsiveness to precipitation pulses: Oryzopsis hymenoides, a perennial bunch grass with shallow roots; Gutierrezia sarothrae, a subshrub with dimorphic roots; and Ceratoides lanata, a predominantly deep-rooted woody shrub. Drought effects on plant water status were qualitatively similar among species, despite morphological differences. Summer drought affected the water status of all species more negatively than winter drought. Isotopic analysis of stem water revealed that all three species took up deeper soil water under drought conditions and shallow soil water after a large rainfall event in summer. Thus all three species appeared to use the same water sources most of the time. However, after a particularly dry summer, only the deepest-rooted species continued to take up soil water, while the more shallow-rooted species were either dead or dormant. Our study suggests therefore that increased occurrence of summer drought could favor the most deep-rooted species in ecosystem.

Summer and winter drought in a cold desert ecosystem (Colorado Plateau) part II: effects on plant carbon assimilation and growth

We investigated the effects of winter and summer drought on a shrub/grass community of the Colorado Plateau in western North America, a winter-cold, summer-hot desert that receives both winter and summer precipitation. Summer, winter and yearlong drought treatments were imposed for 2 consecutive years using rainout shelters. We chose three perennial species for this study, representing different rooting patterns and responsiveness to precipitation pulses: Oryzopsis hymenoides, a perennial bunch grass with shallow roots; Gutierrezia sarothrae, a subshrub with dimorphic roots; and Ceratoides lanata, a predominantly deep-rooted woody shrub. Growth for all three species was far more sensitive to winter than to summer drought. The primary reason was that plants did not grow in summer and also did not appear to use summer-assimilated carbon to support growth in the following spring. We hypothesize that the relative scarcity and uncertainty of summer rain on the Colorado Plateau prevents most species from evolving adaptations that would improve their use of summer rain. Together with the results of the companion paper, which focused on plant water relations, we conclude that variation in fall to spring precipitation would have strong effects on primary productivity, and could cause reversible fluctuations in community composition, while increased variation in summer precipitation, through causing high rates of mortality among shallow-rooted species in dry years, has the potential to cause lasting and perhaps irreversible community change, especially if coinciding with the invasion of western landscapes by cheatgrass, tumble weed and other grazing tolerant exotics.

Dominant cold desert plants do not partition warm season precipitation by event size.

We conducted experiments to examine the quantitative relationships between rainfall event size and rainwater uptake and use by four common native plant species of the Colorado Plateau, including two perennial grasses, Hilaria jamesii (C(4)) and Oryzopsis hymenoides (C(3)), and two shrubs, Ceratoides lanata (C(3)), and Gutierrezia sarothrae (C(3)). Specifically, we tested the hypothesis that grasses use small rainfall events more efficiently than shrubs and lose this advantage when events are large. Rainfall events between 2 and 20 mm were simulated in spring and summer by applying pulses of deuterium-labeled irrigation water. Afterwards, pulse water fractions in stems and the rates of leaf gas exchange were monitored for 9 days. Cumulative pulse water uptake over this interval (estimated by integrating the product of pulse fraction in stem water and daytime transpiration rate over time) was approximately linearly related to the amount of pulse water added to the ground in all four species. Across species, consistently more pulse water was taken up in summer than in spring. Relative to their leaf areas, the two grass species took up more pulse water than the two shrub species, across all event sizes and in both seasons, thus refuting the initial hypothesis. In spring, pulse water uptake did not significantly increase photosynthetic rates and in summer, pulse water uptake had similar, but relatively small effects on the photosynthetic rates of the three C(3) plants, and a larger effect on the C(4) plant H. jamesii. Based on these data, we introduce an alternative hypothesis for the responses of plant functional types to rainfall events of different sizes, building on cost-benefit considerations for active physiological responses to sudden, unpredictable changes in water availability.

Effect of American bison (Bison bison L.) on the recovery and germinability of seeds of range forage species

Abstract The effect of passage through the digestive tract of Bison bison on the recovery and germinability of undamaged passed seeds of Pseudoroegneria spicata, Elymus cinereus, Oryzopsis hymenoides, Stipa comata, Balsamorhiza sagittata and Sphaeralcea coccinea was tested. Recovery of undamaged passed seeds peaked 2 days after ingestion and then decreased to the lowest level on day 5 of sample collection for all species used, but the pattern of seed passage through time significantly differed among species. Recovery of passed seeds was greatest for round‐shaped, hard seed‐coated O. hymenoides and lowest for largest seed‐sized B. sagittata. The digestive tract of B. bison did not break seed dormancy of O. hymenoides, S. comata, S. coccinea or B. sagittata. Germination percentage of P. spicata and E. cinereus seeds decreased compared with unpassed seeds of the same species.