Shortgrass Steppe Research Articles

Patch Burn Grazing Management in a Semiarid Grassland: Consequences for Pronghorn, Plains Pricklypear, and Wind Erosion

Management strategies that allow for spatiotemporal interactions between fire and herbivores can potentially achieve multiple management goals related to livestock production and wildlife conservation, but little is known about such interactions in semiarid grasslands where fire has traditionally been viewed as having few management applications. We studied patch burn grazing management in the shortgrass steppe of northeastern Colorado, comparing unburned pastures to pastures where 25% of the area was burned in October or November each year over 4years. Our objective was to examine the interactive effects of patch burns and the subsequent response by pronghorn (Antilocapra americana) on plains pricklypear (Opuntia polyacantha) and wind erosion rates. We monitored abundance of plains pricklypear and wind erosion rates throughout the experiment and quantified seasonal pronghorn densities and postburn damage to plains pricklypear cladodes during the latter 2years of the study. Pronghorn density was 26 times greater in winter and 7 times greater in spring on patch burns compared with unburned pastures. By late winter, densities of bitten or uprooted plains pricklypear cladodes were five times greater on patch burns compared with unburned pastures. Patch burns, as well as the subsequent response of pronghorn, reduced plains pricklypear density by 54–71% during the first year after the burns, and density remained suppressed for up to 6years after burns. Wind erosion rates on patch burns were greater compared with unburned pastures but were two orders of magnitude lower than rates measured on fallow croplands in the region. Autumn patch burns can be a valuable means to suppress plains pricklypear and thereby increase grass available for livestock consumption in the shortgrass steppe. These outcomes can be achieved without increasing wind erosion in a manner that threatens long-term soil sustainability and without negative consequences for livestock weight gains.

Testing for Thresholds in a Semiarid Grassland: The Influence of Prairie Dogs and Plague

State-and-transition models for semiarid grasslands in the North American Great Plains suggest that the presence of herbivorous black-tailed prairie dogs (Cynomys ludovicianus) on a site 1) creates a vegetation state characterized by increased dominance of annual forbs and unpalatable bunchgrasses and increased bare soil exposure and 2) requires long-term (> 40 yr) prairie dog removal to transition back to a vegetation state dominated by palatable perennial grasses. Here, we examine 1) how the recent history of prairie dog occupancy on a site (1–10 yr) influences the magnitude of prairie dog effects on vegetation composition and 2) how occupancy history affects vegetation dynamics following extirpation of prairie dogs. We used a natural experiment in the shortgrass steppe of northeastern Colorado, USA, where prairie dogs were extirpated from multiple sites during an outbreak of epizootic plague. On sites occupied by prairie dogs for 1–4 yr prior to extirpation, plant cover and composition recovered to conditions similar to unoccupied sites within a single growing season. Larger reductions in perennial C4 grasses occurred on sites occupied for the prior 7–10 yr compared to sites with shorter occupancy histories (< 6 yr). On sites occupied for the prior 7–10 yr, C4 perennial grasses recovered after 5 yr following prairie dog extirpation; in addition, C3 perennial graminoids and forbs remained more abundant (compared to sites with no history of prairie dogs) throughout the 5-yr period. Our findings showcase that prior site occupancy (up to 10 yr) by prairie dogs did not induce irreversible shifts in vegetation state in this semiarid grassland. Rather, vegetation changes induced by prairie dogs represent primarily a phase shift in landscapes where prairie dog populations are regulated by epizootic plague.

Characteristics of Burns Conducted under Modified Prescriptions to Mitigate Limited Fuels in a Semi-Arid Grassland

AbstractIn semi-arid grasslands of the North American Great Plains, fire has traditionally been viewed as having few management applications, and quantitative measurements of fire behavior in the low fuel loads characteristic of this region are lacking. More recently, land managers have recognized potential applications of prescribed fire to control undesirable plant species and to manage habitat for wildlife in this region. Working in the shortgrass steppe of northeastern Colorado over a 7-year period, we quantified peak temperatures, heating duration, and heat dosage produced near ground level during prescribed burns conducted under a wide range of fuel loads and weather conditions. We use an information theoretic approach to develop models that predict peak temperature and heat dosage as a function of weather parameters and fuel loads. Under the weather conditions that we examined, successful burns (&gt;80 % of target area burnt) occurred with fuel loads varying from 350 kg ha−1 to 1175 kg ha−1, while burns with fuel loads &lt;350 kg ha−1 generally failed to spread and burned less than 60 % of target areas. Peak temperatures, heat duration, and heat dosage during shortgrass burns: 1) were lower than reported for mixed grass prairies, 2) increased linearly with increasing fuel loads, and 3) were secondarily influenced by wind speed, ambient air temperature, and relative humidity. Compared to desert grassland, heat doses near the ground surface were similar, but peak temperatures were lower and heat duration longer in shortgrass steppe burns. Our findings provide quantitative predictions for heat production from fires in shortgrass steppe near the ground surface, where most plant meristems are located. Based on these relationships, we provide suggestions for burn prescriptions to achieve goals such as reducing abundance of undesirable plant species and providing habitat for native grassland birds.

Community responses of arthropods to a range of traditional and manipulated grazing in shortgrass steppe.

Responses of plants to grazing are better understood, and more predictable, than those of consumers in North American grasslands. In 2003, we began a large-scale, replicated experiment that examined the effects of grazing on three important arthropod groups-beetles, spiders, and grasshoppers-in shortgrass steppe of north-central Colorado. We investigated whether modifications of the intensity and seasonality of livestock grazing alter the structure and diversity of macroarthropod communities compared with traditional grazing practices. Treatments represented a gradient of grazing intensity by cattle and native herbivores: long-term grazing exclosures; moderate summer grazing (the traditional regime); intensive spring grazing; intensive summer grazing; and moderately summer-grazed pastures also inhabited by black-tailed prairie dogs (Cynomys ludovicianus Ord). Beetles and spiders were the most common groups captured, comprising 60% and 21%, respectively, of 4,378 total pitfall captures. Grasshopper counts were generally low, with 3,799 individuals observed and densities <4 m(-2). Two years after treatments were applied, vegetation structure differed among grazing treatments, responding not only to long-term grazing conditions, but also to the short-term, more-intensive grazing manipulations. In response, arthropods were, in general, relatively insensitive to these grazing-induced structural changes. However, species-level analyses of one group (Tenebrionidae) revealed both positive and negative effects of grazing treatments on beetle richness and activity-density. Importantly, these responses to grazing were more pronounced in a year when spring-summer rainfall was low, suggesting that both grazing and precipitation-which together may create the greatest heterogeneity in vegetation structure-are drivers of consumer responses in this system.

Mountain plover nest survival in relation to prairie dog and fire dynamics in shortgrass steppe

ABSTRACTDisturbed xeric grasslands with short, sparse vegetation provide breeding habitat for mountain plovers (Charadrius montanus) across the western Great Plains. Maintaining local disturbance regimes through prairie dog conservation and prescribed fire may contribute to the sustainability of recently declining mountain plover populations, but these management approaches can be controversial. We estimated habitat‐specific mountain plover densities and nest survival rates on black‐tailed prairie dog (Cynomys ludovicianus) colonies and burns in the shortgrass steppe of northeastern Colorado. Mountain plover densities were similar on prairie dog colonies (5.9 birds/km2; 95% CI = 4.7–7.4) and sites burned during the preceding dormant season (6.7 birds/km2; 95% CI = 4.6–9.6), whereas the 29‐day nest survival rate was greater on prairie dog colonies (0.81 in 2011 and 0.39 in 2012) compared to the burned sites (0.64 in 2011 and 0.17 in 2012). Reduced nest survival in 2012 compared to 2011 was associated with higher maximum daily temperatures in 2012, consistent with a previous weather‐based model of mountain plover nest survival in the southern Great Plains. Measurements of mountain plover density relative to time since disturbance showed that removal of prairie dog disturbance by sylvatic plague reduced mountain plover density by 70% relative to active prairie dog colonies after 1 year. Plover densities declined at a similar rate (by 78%) at burned sites between the first and second post‐burn growing season. Results indicate that black‐tailed prairie dog colonies are a particularly important nesting habitat for mountain plovers in the southern Great Plains. In addition, findings suggest that prescribed burning can be a valuable means to create nesting habitat in landscapes where other types of disturbances (such as prairie dog colonies) are limited in distribution and size. Published 2014. This article is a U.S. Government work and is in the public domain in the USA.

Prescribed fire, soil inorganic nitrogen dynamics, and plant responses in a semiarid grassland

In arid and semiarid ecosystems, fire can potentially affect ecosystem dynamics through changes in soil moisture, temperature, and nitrogen cycling, as well as through direct effects on plant meristem mortality. We examined effects of annual and triennial prescribed fires conducted in early spring on soil moisture, temperature, and N, plant growth, and plant N content in semiarid shortgrass steppe. Annual burning increased soil inorganic N availability throughout the growing season, which was associated with increased soil temperature and a reduction in aboveground N in C3 plants. Furthermore, the increase in soil inorganic N pools with annual burning was modest and did not facilitate success of ruderal species. Negative fire effects on C3 plant production could be due to increased soil temperature, reduced soil moisture, or direct negative effects on C3 plant meristems, although fuel loads and fire temperatures were low relative to other grasslands. Triennial burning had intermediate effects on N availability and C3 plant production compared to annual burning and unburned controls. Results show that prescribed burns can be used in the management of this semiarid grassland without facilitating annual plant invasion, but excessively frequent burning can reduce production of C3 plants.

Geographic variation in growth and phenology of two dominant central US grasses: consequences for climate change

Aims The rate of climate change may exceed many plant species' migration rates, particularly for long-lived perennial species that dominate most ecosystems. If bioclimatic envelopes shift more rapidly than dominant species can migrate, individuals located peripheral to biomes or in adjacent biomes may become a significant source of traits for future dominant populations (DPs). Thus, traits of individuals from peripheral populations (PPs) may affect future ecosystem functioning more than those of today's DPs. Methods We assessed key traits of individuals collected from populations that currently dominate two central US grasslands, the shortgrass steppe (Bouteloua gracilis) and the tallgrass prairie (Andropogon gerardii). We compared these to individuals from PPs in a reciprocal-transplant common garden experiment with gardens at the Shortgrass Steppe Long Term Ecological Research site in Colorado and the Konza Prairie Biological Station Long Term Ecological Research site in Kansas. DPs and PPs were subjected to high and reduced water availability in common gardens located in each biome. Traits measured included the following: individual plant biomass, reproductive allocation, specific leaf area (SLA) and plant-water relations. We focused on the climate-change relevant comparisons of traits from PPs versus DPs expressed under the climate of DPs. Important Findings PPs of B. gracilis differed from DPs primarily in phenological traits. Under a semiarid shortgrass steppe climate, PPs initiated flowering later in the season, produced fewer reproductive tillers and were more sensitive to water stress. Biomass differences between populations were minimal. For A. gerardii, biomass in PPs was 50% lower than in DPs under the mesic tallgrass prairie climate and reproductive tillers were considerably smaller, despite higher SLA in PPs. Biomass of PPs was less sensitive to water stress, however. From these results, we conclude that key traits of PPs differed from DPs in both grassland types, but potential effects on reproductive phenology were greater for the bioclimatic shift in which a mesic biome becomes arid, whereas aboveground productivity may be affected more when a semiarid biome becomes more mesic.

Plant phenotypic functional composition effects on soil processes in a semiarid grassland

Our knowledge of plant functional group effects on ecosystem processes is relatively well established, but we know much less on how changes in plant phenotypic composition affect ecosystem functioning (i.e., phenotypic functional composition). Understanding phenotypic functional composition (PFC) is relevant in plant communities strongly dominated by a few keystone species, since alteration of phenotypic composition of these species might be a mechanisms by which land management practices such as grazing impact on ecosystem functioning. Here, we report results from a field experiment where we manipulated PFC of Bouteloua gracilis (Kunth) Lag. ex Griffiths, a keystone species in the semiarid shortgrass steppe. B. gracilis' PFC was altered by using plant genetic lines which expressed consistently either high or low leaf tissue lignin content (LC), a plant trait known to affect soil biogeochemical processes. High-LC lines came from an area subjected to low grazing intensities, whereas low-LC lines came from an area historically overgrazed. Additionally, those plant genetic lines expressing high LC also expressed high dry matter content and vice versa. We established experimental plots with High (only high-LC plants), Low (only low-LC plants), and mixed (both high- and low- LC plants, BHL) LC genetic lines; and measured selected soil processes after the application of a small water pulse. We hypothesised that those soil processes related to microbial activity and nitrogen cycling would be higher in Low plots in comparison with High and BHL plots. Phenotypic functional composition did not affect most of our soil processes including ammonium and nitrate concentrations (inorganic N), microbial activity, potential ammonium mineralization, and microbial profiles of substrate utilization). Nonetheless, we observed a general response to the water pulse so that soil inorganic N increased, but soil water content and soil microbial activity decreased 48 h after the pulse application. This general response suggests that fractions of the soil microbial community with different soil moisture optima mineralise N-rich substrates. Overall, lack of response to plant phenotypic functional composition suggests that grazing effects on soil biogeochemical processes in the shortgrass steppe are not directly mediated through how grazing affects the phenotypic functional composition of B. gracilis.

Bison Selection of Prairie Dog Colonies on Shortgrass Steppe

American bison (Bison bison L.) preferentially use black-tailed prairie dog (Cynomys ludovicianus L.) colonies over uncolonized range in the mixed-grass prairie of North America. To assess bison use of prairie dog colonies in a different ecosystem, the shortgrass steppe, this study was conducted at the Vermejo Park Ranch, New Mexico. Driving surveys were conducted in summer 2007 to determine the number of bison on and off prairie dog colonies in 2 pastures. Prairie dogs occupied 25.5% and 48.5% of the 2 pastures surveyed. Bison of both sexes used prairie dog colonies more than expected compared to a scenario of bison using colonies based on colony availability (P < 0.001). With the exception of bulls in one pasture, bison observed off-colony were more likely to be grazing than bison observed oncolony (P < 0.04). When selection of prairie dog colonies was assessed for only those bison observed grazing, bulls in both pastures and cows in one pasture used prairie dog colonies more than expected based on availability (P ≤ 0.001). Forage quality was superior on prairie dog colonies, with crude protein higher (P < 0.001) and acid detergent fiber lower (P < 0.001) in blue grama (Bouteloua gracilis [Willd. ex Kunth] Lag. ex Griffiths) collected on-colony than offcolony. Further studies under a variety of conditions are still needed, but selection for prairie dog colonies by bison at Vermejo, combined with findings from previous studies, suggests that during the growing season, bison and cattle (Bos taurus L.) might select prairie dog colonies over uncolonized range in both the northern and southern extents of North American grasslands.

Cover, density, and demographics of shortgrass steppe plants mapped 1997–2010 in permanent grazed and ungrazed quadrats

AbstractThis data set was created to map and analyze demographic rates of many common plant species in the shortgrass steppe of North America under grazed and ungrazed conditions. Twenty‐four permanent 1‐m2 quadrats were established as part of the Shortgrass Steppe Long Term Ecological Research project in six grazed and ungrazed study sites on the Central Plains Experimental Range in Nunn, Colorado, USA. We used a pantograph to record the location and shape of individual plants annually from 1997 to 2010. These data make it possible to determine the fate of individual plants using a Geographic Information System and spatial analysis techniques. The data provide unique opportunities to test the interactive effects of grazing and climate variables on demographic rates, plant–plant interactions, and population and community dynamics. We provide the following data and data formats: (1) the digitized maps in shapefile format; (2) a tabular representation of centroid or point location (x, y coordinates), and basal cover (m2) for plants mapped as polygons; (3) quadrat information including location (longitude and latitude), elevation, and livestock grazing treatment and history; (4) detailed information on stocking rates during the period of quadrat sampling; (5) daily temperature and precipitation values; (6) a species list including the total records for each species, plant growth forms, and longevity; and (7) an inventory of the years each quadrat was sampled.

Contrasting Effects of Precipitation Manipulations on Production in Two Sites within the Central Grassland Region, USA

In grassland ecosystems, where soil water most frequently controls ecosystem processes, expected changes in precipitation and temperature may have dramatic effects on ecosystem dynamics. Previous observational studies have reported that aboveground net primary production (ANPP) in grasslands is very sensitive to changes in precipitation. Yet, we lack experimentally based evidence to support these observations. Further, most of the studies have focused solely on ANPP, neglecting belowground production (BNPP). This is an important gap in our knowledge, as BNPP represents 50% or more of total net primary production (NPP) in grasslands. Here, we present results from a 3-year water manipulation experiment (2008–2010) at two sites in the central grassland region of North America, USA. We were successful in changing the soil water content in our treatments, but these changes resulted in different, but significant responses in ANPP and BNPP at our two sites. At the shortgrass steppe, we found that neither NPP nor ANPP were sensitive to treatment precipitation, and although we found BNPP was sensitive to changes in treatment precipitation, the direction of the response varied between years. In contrast, ANPP was very sensitive to treatment precipitation on the mixed-grass prairie, whereas BNPP was insensitive. Based on our finding that two grassland ecosystems showed dramatically different above and belowground production responses to soil water manipulations, we cannot assume that predicted changes in climate will cause similar above- and belowground production responses. Second, our results demonstrated that sites within the same region may differ markedly in the sensitivity of ANPP to changes in growing season precipitation.

Disproportionate effects of non‐colonial small herbivores on structure and diversity of grassland dominated by large herbivores

The response of semiarid grasslands to small, non‐colonial herbivores has received little attention, focusing primarily on the effects of granivore assemblages on annual plant communities. We studied the long‐term effects of both small and large herbivores on vegetation structure and species diversity of shortgrass steppe, a perennial semiarid grassland considered marginal habitat for small mammalian herbivores. We hypothesized that 1) large generalist herbivores would affect more abundant species and proportions of litter‐bare ground‐vegetation cover through non‐selective herbivory, 2) small herbivores would affect less common species through selective but limited consumption, and 3) herbivore effects on plant richness would increase with increasing aboveground net primary production (ANPP). Plant community composition was assessed over a 14‐year period in pastures grazed at moderate intensities by cattle and in exclosures for large (cattle) and large‐plus‐small herbivores (additional exclusion of rabbits and rodents). Exclusion of large herbivores affected litter and bare ground and basal cover of abundant, common and uncommon species. Additional exclusion of small herbivores did not affect uncommon components of the plant community, but had indirect effects on abundant species, decreased the cover of the dominant grass Bouteloua gracilis and total vegetation, and increased litter and species diversity. There was no relationship between ANPP and the intensity of effects of either herbivore body size on richness. Exclusion of herbivores of both body sizes had complementary and additive effects which promoted changes in vegetation composition and physiognomy that were linked to increased abundance of tall and decreased abundance of short species. Our findings show that small mammalian herbivores had disproportionately large effects on plant communities relative to their small consumption of biomass. Even in small‐seeded perennial grasslands with a long history of intensive grazing by large herbivores, non‐colonial small mammalian herbivores should be recognized as an important driver of grassland structure and diversity.

Impact of labile and recalcitrant carbon treatments on available nitrogen and plant communities in a semiarid ecosystem

In a 10-year study, we assessed the influence of five carbon (C) treatments on the labile C and nitrogen (N) pools of historically N-enriched plots on the Shortgrass Steppe Long Term Ecological Research site located in northeastern Colorado. For eight years, we applied sawdust, sugar, industrial lignin, sawdust + sugar, and lignin + sugar to plots that had received N and water additions in the early 1970s. Previous work showed that past water and N additions altered plant species composition and enhanced rates of nutrient cycling; these effects were still apparent 25 years later. We hypothesized that labile C amendments would stimulate microbial activity and suppress rates of N mineralization, whereas complex forms of carbon (sawdust and lignin) could enhance humification and lead to longer-term reductions in N availability. Results indicated that, of the five carbon treatments, sugar, sawdust, and sawdust + sugar suppressed N availability, with sawdust + sugar being the most effective treatment to reduce N availability. The year after treatments stopped, N availability remained less in the sawdust + sugar treatment plots than in the high-N control plots. Three years after treatments ended, reductions in N availability were smaller (40-60%). Our results suggest that highly labile forms of carbon generate strong short-term N sinks, but these effects dissipate within one year of application, and that more recalcitrant forms reduce N longer. Sawdust + sugar was the most effective treatment to decrease exotic species canopy cover and increase native species density over the long term. Labile carbon had neither short- nor long-term effects on exotic species. Even though the organic amendments did not contribute to recovery of the dominant native species Bouteloua gracilis, they were effective in increasing another native species, Carex eleocharis. These results indicate that organic amendments may be a useful tool for restoring some native species in the shortgrass steppe, though not all.

Grazing effects on aboveground primary production and root biomass of early-seral, mid-seral, and undisturbed semiarid grassland

Annual/perennial and tall/short plant species differentially dominate early to late successional shortgrass steppe communities. Plant species can have different ratios of above-/below-ground biomass distributions and this can be modified by precipitation and grazing. We compared grazing effects on aboveground production and root biomass in early- and mid-seral fields and undisturbed shortgrass steppe. Production averaged across four years and grazed and ungrazed treatments were 246, 134, and 102 g m−2 yr−1 for the early-, mid-seral, and native sites, respectively, while root biomass averaged 358, 560, and 981 g m−2, respectively. Early- and mid-seral communities provided complimentary forage supplies but at the cost of root biomass. Grazing increased, decreased, or had no effect on aboveground production in early-, mid-seral, and native communities, and had no effect on roots in any. Grazing had some negative effects on early spring forage species, but not in the annual dominated early-seral community. Dominant species increased with grazing in native communities with a long evolutionary history of grazing by large herbivores, but had no effects on the same species in mid-seral communities. Effects of grazing in native communities in a region cannot necessarily be used to predict effects at other seral stages.

Erratum to: Carbon and Nitrogen Decoupling Under an 11-Year Drought in the Shortgrass Steppe

-N) captured on ion-exchange membrane probes in the 10th year of a long-term drought manipulation in the shortgrass steppe (2008), when rainfall the plots received varied (shown by open dots)and the first year of recovery (2010), when all plots received the same amount of rainfall. Bars show lg-N accumulated per10 cm probe per day in each 2-month period of burial, whereas inset shows lg-N per cm rainfall during 2008, whentreatments received unequal rainfall. Where significant differences among treatments occurred, significant pairwise dif-ferences (P < 0.05) are indicated by letters. Ratios of nitrate to ammonium were consistent across treatments within anygiven time point, so are not shown separately.

Grazing effects on plant community succession of early‐ and mid‐seral seeded grassland compared to shortgrass steppe

AbstractQuestionsGrazing may speed or slow secondary succession, and the direction may depend on seral stage and relative tolerance of native perennial grasses compared with annual invasive species. How does grazing affect succession where undisturbed communities have a long evolutionary history of grazing by native herbivores and are tolerant to livestock grazing?LocationShortgrass steppe, North American Great Plains, Colorado (40°49′N, 104°46′W), USA.MethodsUngrazed and grazed early‐seral (4–6 yr) and mid‐seral (18–20 yr) seeded fields (Conservation Reserve Program) and traditionally grazed native steppe were compared for effects on plant composition in relation to changes expected from regional succession models.ResultsRecovery towards undisturbed native shortgrass steppe for early‐ and mid‐seral communities, respectively, was 19% and 36% for total vegetation cover, 5% and 21% for planted native species, 10% and 88% for non‐planted native perennial grasses, only 0.2% and 13% for short grasses, and overall dissimilarity in community species compositions was 97% and 68%. In general, grazing effects were neutral or most often not significant in all years and/or were small in overall community magnitude. The early‐seral community displayed more changes indicative of a slowing of succession with grazing (total vegetative and grass basal cover) rather than reducing invasive species (species targeted by timing of grazing), although drought had limited the establishment of grazing‐tolerant short grasses. The mid‐seral community showed more changes consistent with advancing successional recovery with grazing (overall community dissimilarity, forbs, planted native perennial grasses, tall grasses and target species). However, non‐planted native perennial grasses responded negatively to grazing in the mid‐seral community and positively in native shortgrass steppe where outside seed would originate.ConclusionsGrazing effects on particular functional groups and species were not the same across seral stages, were mixed in terms of speeding or slowing succession, and were generally not large at the community level. Evolutionary history of grazing may serve as a general guide but decisions on whether to graze successional grasslands may best be made after assessing whether tolerant perennial short grass species are significant components. Monitoring may then be necessary to determine species responses in particular community matrixes and effects on subsequent immigration of non‐seeded native perennial species.

Edge effect of a pine plantation reduces dry grassland invertebrate species richness

Natural steppes in European agricultural landscapes are characterized by high biotic richness but are subject to fragmentation and associated edge effects. Edge effects on species richness were investigated at an ecotone from a pine plantation to a short-grass steppe in Eastern Austria for eleven invertebrate taxa differentiated into habitat guilds based on known live-history strategies of individual species (grassland species, forest species, generalist species), including Red-Listed and non-threatened grassland species. The large size of the studied grassland site provided an opportunity to test edge effects in the absence of confounding factors and to a gradient length of 208 m into the grassland habitat along a clear-cut border to a pine plantation. All sampling was done by pitfall trapping. Species richness of habitat guilds, but not total richness, was effectively explained by biotic variables reflecting the influence of shading in particular (i.e. soil temperature sums). Total species richness showed a bimodal response pattern, with increases towards the habitat edge and interior grassland habitat. Habitat guilds showed diverging responses to distances from the edge, but no saturation in species richness, with a continuum of edge effects across the entire distance of the grassland samples. Our findings contrast those of previous investigations based on samples taken from smaller patches and across shorter distances from the edge. Methodological and conservation implications are discussed.

Allergen of the Month—Fringed Sagebrush

Artemisia species are important aeroallergen sources in both the Eastern and Western hemispheres. 1 Lewis W.H. Vinay P. Zenger V.E. Airborne and Allergenic Pollen of North America. Johns Hopkins University Press, Baltimore1983 Google Scholar , 2 Bassett I.J. Crompton C.W. Parmalee J.A. An atlas of airborne pollen grains and common fungus spores of Canada. Printing and Publishing, Supply and Services Canada, Hull, Quebec1978 Google Scholar , 3 Wodehouse R.P. Hayfever Plants. 2nd Ed. Hafner, New York1971 Google Scholar At least 19 native and introduced species are found within the United States. [1] Lewis W.H. Vinay P. Zenger V.E. Airborne and Allergenic Pollen of North America. Johns Hopkins University Press, Baltimore1983 Google Scholar Fringed sagebrush, A. frigida, is native to North America, ranging from the Yukon to New Brunswick, and southward through the western states into Mexico, where it is known as estafiata. 1 Lewis W.H. Vinay P. Zenger V.E. Airborne and Allergenic Pollen of North America. Johns Hopkins University Press, Baltimore1983 Google Scholar , 2 Bassett I.J. Crompton C.W. Parmalee J.A. An atlas of airborne pollen grains and common fungus spores of Canada. Printing and Publishing, Supply and Services Canada, Hull, Quebec1978 Google Scholar , 4 Dayton W.A. Range Plant Handbook. Dover Publications, Inc., New York1988 Google Scholar It is also found throughout the Siberian steppes, and is considered native to Eurasia as well, being the most widespread of all Artemisia species. 4 Dayton W.A. Range Plant Handbook. Dover Publications, Inc., New York1988 Google Scholar , 5 Morgan J.A. Milchunas D.G. LeCain D.R. West M. Mosier A.R. Carbon dioxide enrichment alters plant community structure and accelerates shrub growth in the shortgrass steppe. PNAS. 2007; 104: 14724-14729 Crossref PubMed Scopus (166) Google Scholar The species name derives from the cold bleak regions of eastern Siberia, where it was first described. [4] Dayton W.A. Range Plant Handbook. Dover Publications, Inc., New York1988 Google Scholar A. frigida will grow on a variety of sites, to over 11,000 feet (3353 m). Fringed sagebrush is exceedingly responsive to increased CO2 associated with climate change: a doubling of the ambient CO2 results in a 40-fold increase in above ground biomass and a 20-fold increase in ground cover from the shrub. [5] Morgan J.A. Milchunas D.G. LeCain D.R. West M. Mosier A.R. Carbon dioxide enrichment alters plant community structure and accelerates shrub growth in the shortgrass steppe. PNAS. 2007; 104: 14724-14729 Crossref PubMed Scopus (166) Google Scholar Spreading of A. frigida is also potentiated by having allelopathic effects, and inhibition of grass root and shoot growth has been shown. [6] Li X.F. Wang J. Xu W.B. Wang K. Allelopathic effects of Artemisia frigida on three Poaceae plants seed germination and seedling growth. Ying Yong Sheng Tai Xue Bao. 2010; 21: 1702-1708 PubMed Google Scholar Essential oil of A. frigida is fragrant, with high content of camphor and cineol, and has antiseptic and antioxidant properties. 4 Dayton W.A. Range Plant Handbook. Dover Publications, Inc., New York1988 Google Scholar , 7 Lopes-Lutz D. Alviano D.S. Alviano C.S. Kolodziejczyk P.P. Screening of chemical composition, antimicrobial and antioxidant activities of Artemisia essential oils. Phytochemistry. 2008; 69: 1732-1738 Crossref PubMed Scopus (427) Google Scholar

Carbon and Nitrogen Decoupling Under an 11-Year Drought in the Shortgrass Steppe

The frequency and magnitude of drought is expected to increase in the US Great Plains under future climate regimes. Although semiarid systems are considered highly resistant to water limitation, novel drought events could alter linkages among biogeochemical processes, and result in new feedbacks that influence the timescale of ecosystem recovery. We examined changes in carbon and nitrogen cycling in the last 2 years of an 11-year drought manipulation in the shortgrass steppe, and under the first 2 years of recovery from drought. We measured plant production, plant tissue chemistry, soil trace gas flux, and soil inorganic nitrogen dynamics to test the extent that this magnitude of drought altered carbon and nitrogen fluxes and how these changes affected post-drought dynamics. We found that soil inorganic nitrogen was up to five times higher under severe drought than under control conditions, but that this nitrogen may not have been accessible to plants and microbial communities during drought due to diffusion limitations. Drought plots had higher N2O flux when they received equal rainfall pulses, showing that this accumulated N may be vulnerable to loss. In addition, plants in drought plots had higher tissue nitrogen for 2 years following drought. These results show that decadal-length droughts that may occur under future precipitation regimes are likely to alter ecosystem properties through interactions among precipitation, vegetation, and N cycling. Shifts in plant N, vulnerability of nitrogen to loss, and rainfall use efficiency that we observed are likely to affect the recovery time of semiarid systems subject to droughts of this magnitude.

Ecosystem engineering varies spatially: a test of the vegetation modification paradigm for prairie dogs

Colonial, burrowing herbivores can be engineers of grassland and shrubland ecosystems worldwide. Spatial variation in landscapes suggests caution when extrapolating single‐place studies of single species, but lack of data and the need to generalize often leads to ‘model system’ thinking and application of results beyond appropriate statistical inference. Generalizations about the engineering effects of prairie dogs (Cynomys sp.) developed largely from intensive study at a single complex of black‐tailed prairie dogs C. ludovicianus in northern mixed prairie, but have been extrapolated to other ecoregions and prairie dog species in North America, and other colonial, burrowing herbivores. We tested the paradigm that prairie dogs decrease vegetation volume and the cover of grasses and tall shrubs, and increase bare ground and forb cover. We sampled vegetation on and off 279 colonies at 13 complexes of 3 prairie dog species widely distributed across 5 ecoregions in North America. The paradigm was generally supported at 7 black‐tailed prairie dog complexes in northern mixed prairie, where vegetation volume, grass cover, and tall shrub cover were lower, and bare ground and forb cover were higher, on colonies than at paired off‐colony sites. Outside the northern mixed prairie, all 3 prairie dog species consistently reduced vegetation volume, but their effects on cover of plant functional groups varied with prairie dog species and the grazing tolerance of dominant perennial grasses. White‐tailed prairie dogs C. leucurus in sagebrush steppe did not reduce shrub cover, whereas black‐tailed prairie dogs suppressed shrub cover at all complexes with tall shrubs in the surrounding habitat matrix. Black‐tailed prairie dogs in shortgrass steppe and Gunnison's prairie dogs C. gunnisoni in Colorado Plateau grassland both had relatively minor effects on grass cover, which may reflect the dominance of grazing‐tolerant shortgrasses at both complexes. Variation in modification of vegetation structure may be understood in terms of the responses of different dominant perennial grasses to intense defoliation and differences in foraging behavior among prairie dog species. Spatial variation in the engineering role of prairie dogs suggests spatial variation in their keystone role, and spatial variation in the roles of other ecosystem engineers. Thus, ecosystem engineering can have a spatial component not evident from single‐place studies.