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1. Plant roots mediate the impacts of environmental change on ecosystems, yet knowledge of root responses to environmental change is limited because few experiments evaluate multiple environmental factors and their interactions. Inferences about root functions are also limited because root length dynamics are rarely measured. 2. Using a 5-year experiment in a mixed-grass prairie, we report the responses of root biomass, length and life span to elevated carbon dioxide (CO₂), warming, elevated CO₂ and warming combined, and irrigation. Root biomass was quantified using soil cores and root length dynamics were assessed using minirhizotrons. By comparing root dynamics with published results for soil resources and aboveground productivity, we provide mechanistic insights into how climate change might impact grassland ecosystems. 3. In the upper soil layer, 0-15 cm depth, both irrigation and elevated CO₂ alone increased total root length by twofold, but irrigation decreased root biomass and elevated CO₂ had only small positive effects on root biomass. The large positive effects of irrigation and elevated CO₂ alone on total root length were due to increases in both root length production and root life span. The increased total root length and life span under irrigation and elevated CO₂ coincided with apparent shifts from water limitation of plant growth to nitrogen limitation. Warming alone had minimal effects on root biomass, length and life span in this shallow soil layer. Warming and elevated CO₂ combined increased root biomass and total root length by c. 25%, but total root length in this treatment was lower than expected if the effects of CO₂ and warming alone were additive. Treatment effects on total root length and root life span varied with soil depth and root diameter. 4. Synthesis. Sub-additive effects of CO₂ and warming suggest studies of elevated CO₂ alone might overestimate the future capacity of grassland root systems to acquire resources. In this mixed-grass prairie, elevated CO₂ with warming stimulated total root length and root life span in deeper soils, likely enhancing plant access to more stable pools of growth-limiting resources, including water and phosphorus. Thus, these root responses help explain previous observations of higher, and more stable, above-ground productivity in these projected climate conditions.

Potential natural vegetation (PNV) remains an important concept in ecology by providing the basis for comparing current or future conditions and trends in vegetation with historic conditions. However, nonstationary climate and land‐use‐driven deviations from baseline conditions challenge the validity of PNV at local scales or for locations that are especially sensitive to environmental changes, such as ecotonal boundaries between ecosystem types. Our goal was to identify the historic boundaries between major grassland types in the Central Great Plains (CGP) of North America (tallgrass prairie and northern and southern mixed‐grass prairies) by examining the environmental drivers of primary production prior to, during, and after the decades‐long drought in the 1930s. We sought to explore differences among grassland types in resistance and resilience to drought, and to evaluate legacy effects resulting from spatially explicit processes during drought. We used county‐level cropland production data from 1926 to 1948 as a proxy for native grass production data. We regressed production data against a suite of models containing variables associated with climate, soils, land use, and drought indices prior to, during, and after drought. We used a dimension reduction approach to identify the locations of grassland boundaries in multivariate space defined by each model. Our results show that climate and soil factors predicted grass production prior to the drought, yet additional land use and erosion factors were needed to explain production in both drought and post‐drought. Grassland responses were most variable along ecotonal boundaries. Tallgrass prairie exhibited stronger production responses to precipitation and temperature compared with northern and southern mixed‐grass prairies during all periods. Tallgrass prairie counties along the ecotone with mixed‐grass prairies were more negatively impacted by soil loss from abandoned cropland during the drought compared with other tallgrass prairie counties. As the intensity and frequency of drought are predicted to increase, landscape‐scale variables and spatially explicit processes that govern novel production responses will need to be accounted for in explaining heterogeneity in grassland responses.

Fire refugia and patchiness are important to the persistence of fire-sensitive species and may facilitate biodiversity conservation in fire-dependent landscapes. Playing the role of ecosystem engineers, large herbivores alter vegetation structure and can reduce wildfire risk. However, herbivore effects on the spatial variability of fire and the persistence of fire-sensitive species are not clear. To examine the hypothesis that large herbivores support the persistence of firesensitive species through the creation of fire refugia in fire-prone landscapes, we examined the response of a fire-sensitive plant, Wyoming big sagebrush (Artemisia tridentata ssp. wyomingensis [Beetle & Young]) to fire and grazing in the fire-dependent mixed-grass prairie of the northern Great Plains. We carried out a controlled burn in 2010 within pre-established exclosures that allowed differential access to wild and domestic herbivores and no record of fire in the previous 75 years due to fire suppression efforts. The experiment was set up with a split-plot design to also examine potential changes in plots that were not burned. Canopy cover of big sagebrush was recorded before the burn in 2010 and again in 2011 with percent area burned recorded within 1-month post-fire in the burned plots. Percentage area burned was the greatest in ungulate exclosures (92% ± 2%) and the least in open areas (55% ± 21%), suggesting that large herbivores influenced fire behavior (e.g., reducing fire intensity and rate of spread) and are likely to increase fire patchiness through their alterations to the fuel bed. Regression analysis indicated that the proportion of sagebrush cover lost was significantly correlated with the proportion of area burned (𝑅² = 0.76, 𝑝 = 0.05). No differences in the non-burn plots were observed among grazing treatments or among years. Altogether, this illustrates the potential importance of large herbivores in creating biotic-driven fire refugia for fire-sensitive species to survive within the flammable fuel matrix of fire-dependent grassland ecosystems such as the mixed-grass prairie. Our findings also attest to the resiliency of the northern Great Plains to fire and herbivory and underscore the value of managing grasslands for heterogeneity with spatial and temporal variations in these historic disturbances.

Livestock grazing is a predominant land use worldwide and can influence wildlife populations by altering grassland composition, structure, and productivity. Conceptually, rest-rotation livestock grazing could increase pasture-level heterogeneity that would allow wildlife to balance the need for resources with the risk of predation. Prairie-grouse (Tympanuchus spp.) are recognized as important indicator species for grassland ecosystems, so identifying management approaches suitable for prairie-grouse could have implications for other species. We monitored radio-collared female sharp-tailed grouse (T. phasianellus) to evaluate the effects of 3 systems of livestock grazing management on the breeding season survival and habitat-associated mortality risk of adult grouse in the northern mixed-grass prairie in eastern Montana and western North Dakota, USA, during 2016–2018. Cumulative breeding season survival was 0.65 ± 0.04 (SE) and annual survival varied from 0.28 ± 0.04 to 0.50 ± 0.05. Grazing management did not have a meaningful influence on any aspect of the cumulative breeding season survival of adult female sharp-tailed grouse, although the seasonal timing of peaks in mortality risk differed among systems. A 10% increase in cropland increased mortality risk of adult female sharp-tailed grouse by a factor of 1.27. Overall, our results suggest that strategies that preserve economically viable ranching systems in unfragmented grasslands may have greater benefits for sharp-tailed grouse survival than prescriptive livestock grazing systems.

Factors associated with the nest survival of mixed-grass prairie passerines are not well known, especially in the context of contemporary grassland management. We documented the nest survival of clay-colored sparrows (Spizella pallida), savannah sparrows (Passerculus sandwichensis), and bobolinks (Dolichonyx oryzivorus) in managed prairie in northwestern North Dakota, USA. We used logistic exposure models and an information-theoretic framework to estimate nest survival and evaluate support for mechanisms (grazing, temporal factors, nest parasitism, nest-site vegetation, and nest-patch factors) relevant to nest survival. Survival for the entire nesting interval (23–28 days) was low for clay-colored sparrow (18.2%), savannah sparrow (15.5%), and bobolink (3.5%). We found support for a cubic effect of nest age; survival of savannah and clay-colored sparrow nests was greatest during mid-incubation and least during the mid-nestling period. Parasitized clay-colored sparrow and bobolink nests had greater survival rates than nonparasitized nests. Nest survival of clay-colored sparrows increased with increasing vegetation height and density. For savannah sparrows, nest survival was lower when cattle were present than when cattle were absent. Characteristics of the nest patch did not have strong effects based on model coefficients and confidence intervals, though they appeared in many of the most supported models. Positive effects of vegetation height and density on nest survival of clay-colored sparrows and negative effects of cattle presence on nest survival of savannah sparrows suggest some detrimental effects of grazing. However, the need to restore and maintain intact prairies likely warrants the continuation of cattle grazing on conservation lands.

Grazing intensity elicits changes in the composition of plant functional groups in both shortgrass steppe (SGS) and northern mixed‐grass prairie (NMP) in North America. How these grazing intensity‐induced changes control aboveground net primary production (ANPP) responses to precipitation remains a central open question, especially in light of predicted climate changes. Here, we evaluated effects of four levels (none, light, moderate, and heavy) of long‐term (>30 yr) grazing intensity in SGS and NMP on: (1) ANPP; (2) precipitation‐use efficiency (PUE, ANPP : precipitation); and (3) precipitation marginal response (PMR; slope of a linear regression model between ANPP and precipitation). We advance prior work by examining: (1) the consequences of a range of grazing intensities (more grazed vs. ungrazed); and (2) how grazing‐induced changes in ANPP and PUE are related both to shifts in functional group composition and physiological responses within each functional group. Spring (April–June) precipitation, the primary determinant of ANPP, was only 12% higher in NMP than in SGS, yet ANPP and PUE were 25% higher. Doubling grazing intensity in SGS and nearly doubling it in NMP reduced ANPP and PUE by only 24% and 33%, respectively. Increased grazing intensity reduced C₃ graminoid biomass and increased C₄ grass biomass in both grasslands. Functional group shifts affected PUE through biomass reductions, as PUE was positively associated with the relative abundance of C₃ species and negatively with C₄ species across both grasslands. At the community level, PMR was similar between grasslands and unaffected by grazing intensity. However, PMR of C₃ graminoids in SGS was eightfold higher in the ungrazed treatment than under any grazed level. In NMP, PMR of C₃ graminoids was only reduced under heavy grazing intensity. Knowing the ecological consequences of grazing intensity provides valuable information for mitigation and adaptation strategies in response to predicted climate change. For example, moderate grazing (the recommended rate) in SGS would sequester the same amount of aboveground carbon as light grazing because ANPP was nearly the same. In contrast, reductions in grazing intensity in NMP from moderate to light intensity would increase the amount of aboveground carbon sequestrated by 25% because of increased ANPP.

Prescribed fire is an important, ecology-driven tool for restoration of grassland systems. However, prescribed fire remains controversial for some grassland managers because of reported reductions in bird use of recently burned grasslands. Few studies have evaluated effects of fire on grassland bird populations in the northern mixed-grass prairie region. Fewer studies yet have examined the influence of fire on nest density or survival. In our review, we found no studies that simultaneously examined effects of fire on duck and passerine nesting. During 1998–2003, we examined effects of prescribed fire on the density of upland-nesting ducks and passerines nesting in north-central North Dakota, USA. Apparent nest densities of gadwall (Anas strepera), mallard (A. platyrhynchos), and all duck species combined, were influenced by fire history of study units, although the degree of influence was not compelling. Fire history was not related to nest densities of blue-winged teal (A. discors), northern shoveler (A. clypeata), or northern pintail (A. acuta); however, apparent nest densities in relation to the number of postfire growing seasons exhibited a strikingly similar pattern among all duck species. When compared to ducks, fire history strongly influenced apparent nest densities of clay-colored sparrow (Spizella pallida), Savannah sparrow (Passerculus sandwichensis), and bobolink (Dolichonyx oryzivorus). For most species examined, apparent nest densities were lowest in recently burned units, increased during the second postfire growing season, and stabilized or, in some cases, decreased thereafter. Prescribed fire is critical for restoring the ecology of northernmixed-grass prairies and our findings indicate that reductions in nest densities are limited mostly to the first growing season after fire. Our results support the premise that upland-nesting ducks and several grassland passerine species are adapted to periodic fires occurring at a frequency similar to that of pre-Euro-American settlement of the region.

A popular approach for conserving multiple organisms in a given region is to identify one or more umbrella species. Identifying and managing for species that have a high degree of overlap in their habitat requirements with a broader suite of organisms is expected to enhance the effectiveness of conservation actions when resources are limited. The greater sage-grouse (Centrocercus urophasianus) has received considerable attention as a potential umbrella species for shrub-steppe flora and fauna in the core of its range. However, the extent to which conserving sage-grouse habitat benefits mixed-grass prairie songbirds in the northern portion of its range is unclear. We compared abundance of grassland songbirds within grasslands designated as sage-grouse critical habitat by the Canadian federal government, and within a 1.6-km buffer of adjacent habitat in southwestern Saskatchewan, Canada in 2016 and 2017. We also quantified relationships between bird abundance and gradients of vegetation structure. Lark bunting (Calamospiza melanocorys) abundance was 3 times greater within sage-grouse critical habitat, whereas chestnut-collared longspur (Calcarius ornatus), Baird’s sparrow (Centronyx bairdii), and Sprague’s pipit (Anthus spragueii) abundance was 1.6–1.8 times greater outside of these areas. The abundance of grassland specialists was negatively associated with total shrub and silver sagebrush (Artemisia cana) cover, both of which characterize sage-grouse critical habitat in Canada at both landscape and local scales. Furthermore, characteristics of vegetation structure associated with grassland generalists overlapped with characteristics of vegetation structure that defined greater sage-grouse critical habitat. Conservation of greater sage-grouse critical habitat in Canada is likely to have a positive influence on shrub-dependent or shrub-tolerant species, such as lark bunting. The conservation of open, native grasslands within greater sage-grouse critical habitat could benefit grassland specialists, however, any changes in the current management that increases the cover of exotic species, silver sagebrush, and other shrubs will negatively affect grassland specialists. We recommend that protection and management activities be targeted towards greater sage-grouse, Sprague’s pipit, and chestnut-collared longspur to maximize the conservation of mixed-grass prairie songbirds and minimize the number of focal umbrella species required.

Differences in the direction and degree to which invasive alien and native plants are influenced by mycorrhizal associations could indicate a general mechanism of plant invasion, but whether or not such differences exist is unclear. Here, we tested whether mycorrhizal responsiveness varies by plant invasive status while controlling for phylogenetic relatedness among plants with two large grassland datasets. Mycorrhizal responsiveness was measured for 68 taxa from the Northern Plains, and data for 95 taxa from the Central Plains were included. Nineteen percent of taxa from the Northern Plains had greater total biomass with mycorrhizas while 61% of taxa from the Central Plains responded positively. For the Northern Plains taxa, measurable effects often depended on the response variable (i.e., total biomass, shoot biomass, and root mass ratio) suggesting varied resource allocation strategies when roots are colonized by arbuscular mycorrhizal fungi. In both datasets, invasive status was nonrandomly distributed on the phylogeny. Invasive taxa were mainly from two clades, that is, Poaceae and Asteraceae families. In contrast, mycorrhizal responsiveness was randomly distributed over the phylogeny for taxa from the Northern Plains, but nonrandomly distributed for taxa from the Central Plains. After controlling for phylogenetic similarity, we found no evidence that invasive taxa responded differently to mycorrhizas than other taxa. Although it is possible that mycorrhizal responsiveness contributes to invasiveness in particular species, we find no evidence that invasiveness in general is associated with the degree of mycorrhizal responsiveness. However, mycorrhizal responsiveness among species grown under common conditions was highly variable, and more work is needed to determine the causes of this variation. After controlling for phylogenetic similarity, we found no evidence that invasive taxa responded differently to mycorrhizas than other taxa. Although it is possible that mycorrhizal responsiveness contributes to invasiveness in particular species, weak mycorrhizal responsiveness may not be a general mechanism of plant invasion.

PREMISE OF THE STUDY: Tiller recruitment from the belowground bud bank of caespitose grasses influences their ability to monopolize local resources and, hence, their genet fitness. Differences in bud production and outgrowth among tiller types within a genet and among species may explain co-occurrence of caespitose grasses. This study aimed to characterize genet bud-bank and tiller production and dynamics in two co-occurring species and compare their vegetative reproductive strategies. METHODS: Bud-bank and tiller dynamics of Hesperostipa cornata and Nassella viridula, dominant C₃ caespitose grasses in the northern mixed-grass prairie of North America, were assessed throughout an annual cycle. KEY RESULTS: The two species showed similar strategies, maintaining polycyclic tillers and thus creating mixed-age genet bud banks comprising multiple bud cohorts produced in different years. Vegetative tillers produced the majority of buds, whereas flowering tillers contributed little to the bud bank. Buds lived for at least 2 yr and were maintained in multiple developmental stages throughout the year. Because bud longevity rarely exceeded tiller longevity, tiller longevity drove turnover within the bud bank. Tiller population dynamics, more than bud production per tiller, determined the differential contribution of tiller types to the bud bank. Nassella viridula had higher bud production per tiller, a consistent annual tiller recruitment density, and greater longevity of buds on senesced and flowering tillers than H. comata. CONCLUSIONS: Co-occurring C₃ caespitose grasses had similar bud-bank and tiller dynamics contributing to genet persistence but differed in bud characteristics that could affect genet longevity and species coexistence.