Shortgrass Community Research Articles

Ecological Engineering by a Mega-Grazer: White Rhino Impacts on a South African Savanna

Herbivory can change the structure and spatial heterogeneity of vegetation. We ask whether all species of grazers in a savanna ecosystem can have this effect or whether megaherbivores (>1000 kg) have a ‘special‘ role that cannot be replicated by other species of grazers. We performed a replicated landscape scale experiment that examined the effects of White Rhino on the grass sward, on other species of grazing mammals and on the movement of fire through the landscape. White Rhino maintained short grass (‘lawn‘) patches in mesic areas (750 mm pa) with increases in grass sward height when they were removed. Other species of grazers were unable to maintain short grass communities when White Rhino were removed. In semi-arid areas (600 mm pa) other, smaller grazers were able to maintain short grass communities in the absence of White Rhino and sward height did not increase. White Rhino removals affected fire by increasing fuel loads and fuel continuity. This resulted in larger, less patchy fires. We propose that the White Rhino acts as an influential ecosystem engineer, creating and maintaining short grass swards, which alter habitat for other grazers and change the fire regime. These results indicate the existence of context-dependent facilitation between White Rhino and other grazers in mesic, but not in semi-arid, savannas. Such top down effects on the ecosystem may have been much more widespread before the extinction of large grazers in the Pleistocene.

Using Weather Data to Explain Herbage Yield on Three Great Plains Plant Communities

Understanding the drivers that account for plant production allows for a better understanding of plant communities and the transitions within ecological sites and can assist managers in making informed decisions about stocking rates and timing of grazing. We compared climatic drivers of herbage production for 3 plant communities of the Clayey ecological site in southwestern South Dakota: the midgrass community dominated by western wheatgrass (Pascopyrum smithii [Rybd.] A. Love); the mixed-grass community codominated by western wheatgrass, blue grama (Bouteloua gracilis [H.B.K.] Lag. Ex Griffiths), and buffalograss (Buchloe dactyloides [Nutt.] Engelm.); and the shortgrass community dominated by blue grama and buffalograss. We used herbage yield and weather data for the period 1945–1960 collected at the South Dakota State University Range and Livestock Research Station near Cottonwood, South Dakota, to develop stepwise regression models for each plant community. Midgrass herbage production was best predicted by current-year spring (April–June) precipitation, number of calendar days until the last spring day with minimum temperature ≤ -1°C, and previous-year spring precipitation (R2 = 0.81). Mixed-grass herbage production was best predicted by current-year spring precipitation and days until the last spring freeze (R2 = 0.69). Shortgrass herbage production was best predicted by current-year spring precipitation (R2 = 0.52). Midgrass plant communities were, overall, 650 kg·ha-1 (SE = 92 kg·ha-1) more productive (P < 0.01) than mixed- or shortgrass plant communities given the same climatic inputs. Our study enables managers to make timely informed decisions regarding stocking rates and timing of grazing on this ecological site in western South Dakota. Entender los factores que participan en la producción vegetal permite una mejor comprensión de las comunidades vegetales y las transiciones dentro de los sitios ecológicos y puede ayudar a los manejadores de pastizales a tomar decisiones fundamentadas respecto a la carga animal y época de apacentamiento. Comparamos factores climáticos que influyen en la producción de forraje de 3 comunidades vegetales del sitio ecológico Arcilloso en el suroeste de South Dakota: la comunidad de zacates medios, dominada por “western wheatgrass” (Pascopyrum smithii [Rybd.] A. Love); la comunidad de zacates mixtos co-dominada por “western wheatgrass,” “blue grama” (Bouteloua gracilis [H.B.K.] Lag. Ex Griffiths), y “buffalograss” (Buchloe dactyloides [Nutt.] Engelm.); y la comunidad de zacates cortos dominada por “blue grama” y “buffalograss.” Utilizamos datos climáticos y de producción de forraje del periodo 1945–1960 colectados en la Estación Experimental de Investigación en Pastizales y Ganadería de la Universidad Estatal de South Dakota ubicada cerca de Cottonwood, South Dakota, para desarrollar modelos de regresión a pasos para cada comunidad vegetal. En la comunidad de zacates medios la mejor predicción de la producción de forraje fue obtenida con la precipitación de primavera (Abril–Junio) del año en curso, el número de días julianos hasta el último día de primavera con temperatura mínima ≤ -1°C y la precipitación en primavera del año previo (R2 = 0.81). En la comunidad de zacates mixtos, la mejor predicción de la producción de forraje se dio con la precipitación en primavera del año en curso y los días hasta la última helada en primavera (R2 = 0.69). En la comunidad de zacates cortos la mejor predicción se obtuvo con la precipitación en primavera del año en curso (R2 = 0.52). Bajo condiciones climáticas iguales, las comunidades de zacates medios, en general, fueron mas productivas (P < 0.01) que las comunidades de zacates mixtos o cortos, superandolas con 650 kg·ha-1 (SE = 92 kg·ha-1). Nuestro estudio permite a los manejadores de pastizales tomar oportunamente decisiones fundamentadas respecto a las cargas animal y épocas de apacentamiento en este sitio ecológico del oeste de South Dakota.

Is vegetation inside Carex sempervirens tussocks highly specific or an image of the surrounding vegetation?

AbstractQuestions: How do species diversity, frequency and composition in tussocks differ from those in similar sized plots outside tussocks? Does the extent of the differences depend on community types or environmental conditions?Location: A sub‐alpine grassland in the Swiss National Park.Methods: In each of the two communities (short grass and tall graminoid) differing in species composition, grazing intensity and soil nutrient availability, relevés were made in 40 pairs of small circular plots, with one plot located inside a randomly selected Carex sempervirens tussock and the other outside.Results: We found 92 vascular species, of which 46 had a frequency higher than 5%. Species richness (S), pooled cover, Shannon's diversity (H) and cumulative species number (CS) were higher outside than inside the C. sempervirens tussocks, but evenness (J) was lower. S, H and CS differed more in the tall graminoid community than in the short grass community. However, dissimilarity between the paired relevés inside and outside tussocks did not differ between the two communities. Of the 46 most frequent species, 12 were statistically more and only one less frequent outside than inside the tussocks. Vegetation inside and outside tussocks could be clearly distinguished in the ordination space.Conclusion: Vegetation inside C. sempervirens tussocks is different from that in the surrounding area and represents an impoverished but homogenized version of the surrounding vegetation. Although tussocks of C. sempervirens were systematically avoided by grazers, there is little evidence that tussocks facilitate the species growing inside them.

Grazing and Ecosystem Carbon Storage in the North American Great Plains

Isotopic signatures of 13 C were used to quantify the relative contributions of C3 and C4 plants to wholeecosystem C storage (soil+plant) in grazed and ungrazed sites at three distinct locations (short-, mid- and tallgrass communities) along an east–west environmental gradient in the North American Great Plains. Functional group composition of plant communities, the source and magnitude of carbon inputs, and total ecosystem carbon storage displayed inconsistent responses to long-term livestock grazing along this gradient. C4 plants [primarily Bouteloua gracilis (H.B.K.) Lag ex Steud.] dominated the long-term grazed site in the shortgrass community, whereas the ungrazed site was co-dominated by C3 and C4 species; functional group composition did not differ between grazed and ungrazed sites in the mid- and tallgrass communities. Above-ground biomass was lower, but the relative proportion of fine root biomass was greater, in grazed compared to ungrazed sites at all three locations. The grazed site of the shortgrass community had 24% more whole-ecosystem carbon storage compared to the ungrazed site (4022 vs. 3236 g C m )2 ). In contrast, grazed sites at the mid- and tallgrass communities had slightly lower (8%) whole-ecosystem carbon storage compared to ungrazed sites (midgrass: 7970 vs. 8683 g C m )2 ; tallgrass: 8273 vs. 8997 g C m )2 ). Differential responses between the shortgrass and the mid- and tallgrass communities with respect to grazing and whole-ecosystem carbon storage are likely a result of: (1) maintenance of larger soil organic carbon (SOC) pools in the mid- and tallgrass communities (7476–8280 g C m )2 ) than the shortgrass community (2517– 3307 g C m )2 ) that could potentially buffer ecosystem carbon fluxes, (2) lower root carbon/soil carbon ratios in the mid- and tallgrass communities (0.06–0.10) compared to the shortgrass community (0.20–0.27) suggesting that variation in root organic matter inputs would have relatively smaller effects on the size of the SOC pool, and (3) the absence of grazing-induced variation in the relative proportion of C3 and C4 functional groups in the mid- and tallgrass communities. We hypothesize that the magnitude and proportion of fine root mass within the upper soil profile is a principal driver mediating the effect of community composition on the biogeochemistry of these grassland ecosystems.

Is vegetation inside Carex sempervirens tussocks highly specific or an image of the surrounding vegetation?

Questions: How do species diversity, frequency and composition in tussocks differ from those in similar sized plots outside tussocks? Does the extent of the differences depend on community types or environmental conditions? Location: A sub-alpine grassland in the Swiss National Park. Methods: In each of the two communities (short grass and tall graminoid) differing in species composition, grazing intensity and soil nutrient availability, releves were made in 40 pairs of small circular plots, with one plot located inside a randomly selected Carex sempervirens tussock and the other outside. Results: We found 92 vascular species, of which 46 had a frequency higher than 5%. Species richness (S), pooled cover, Shannon’s diversity (H) and cumulative species number (CS) were higher outside than inside the C. sempervirens tussocks, but evenness (J) was lower. S, H and CS differed more in the tall graminoid community than in the short grass community. However, dissimilarity between the paired releves inside and outside tussocks did not differ between the two communities. Of the 46 most frequent species, 12 were statistically more and only one less frequent outside than inside the tussocks. Vegetation inside and outside tussocks could be clearly distinguished in the ordination space. Conclusion: Vegetation inside C. sempervirens tussocks is different from that in the surrounding area and represents an impoverished but homogenized version of the surrounding vegetation. Although tussocks of C. sempervirens were systematically avoided by grazers, there is little evidence that tussocks facilitate the species growing inside them.

Dissolved Inorganic Nitrogen Pools and Surface Flux under Different Brackish Marsh Vegetation Types, Common Reed (Phragmites australis) and Salt Hay (Spartina patens)

The current expansion of Phragmites australis into the high marsh shortgrass (Spartina patens, Distichlis spicata) communities of eastern U.S. salt marshes provided an opportunity to identify the influence of vegetation types on pools and fluxes of dissolved inorganic nitrogen (DIN). Two brackish tidal marshes of the National Estuarine Research Reserve system were examined, Piermont Marsh of the Hudson River NERR in New York and Hog Island in the Jacques Coustaeu NERR of New Jersey. Pools of DIN in porewater and rates of DIN surface flux were compared in replicated pairs of recently-expanded P. australis and neighboring S. patens-dominated patches on the high marsh surface. Both marshes generally imported nitrate (NO3−) and exported ammonium (NH4+), such that overall DIN was exported. No differences in surface exchange of NO3− or NH4+ were observed between vegetation types. Depth-averaged porewater NH4+ concentrations over the entire growing season were 56% lower under P. australis than under S. patens (average 1.4 vs. 3.2 mg NH4+ L−1) with the most profound differences in November. Porewater profiles showed an accumulation of NH4+ at depth in S. patens and constant low concentrations in P. australis from the soil surface to 50 cm depth, with no significant differences in porewater salinity. Despite these profound differences in porewater, NH4+ diffusion from soils of P. australis and S. patens were not measurably different, were similar to other published rates, and were well below estimated rates based on passive diffusion alone. Rapid adsorption and uptake by litter and microbes in surface soils of both communities may buffer NH4+ loss to flooding tides in both communities, thereby reducing the impact of P. australis invasion on NH4+ flux to flooding waters.

Species richness, alien species and plant traits in Central Argentine mountain grasslands

Abstract Pattern of native vegetation, distribution of alien species and variation of environmental parameters were studied in mountain grasslands in a lithologically homogeneous Cordoba mountain range in Central Argentina. CCA showed that altitude was the most important factor determining the compositional variation of the vegetation, with soil nutrient status and stoniness as additional factors. Short-grass communities, associated with the driest habitats on plateaus, showed higher small-scale native species richness than wet-turf communities in valleys and tall-grass communities on slopes. Species richness was negatively correlated with soil parameters that indicate nutrient status and water availability. Also, there was a negative correlation between soil Ca- and Mg-content and richness. High native species richness coincided with high alien species richness. When smaller units – community types – were considered, it became evident that within short-grass vegetation, the three most species-rich commun...

Modeling invasive weeds in grasslands: the role of allelopathy in Acroptilon repens invasion

We used an individual plant-based simulation model (ECOTONE) to evaluate the importance of allelopathy and soil texture to the invasion of semiarid grasslands by the non-native perennial C 3 forb Acroptilon repens. We also assessed the sensitivity of model results to the negative effects of allelochemicals on recruitment and growth of perennial grasses. ECOTONE simulates the recruitment, growth, and mortality of individual plants on a small plot (0.12 m 2) through time at an annual time step. A daily time step, multi-layer model of soil water dynamics (SOILWAT) was incorporated into ECOTONE to represent competition for soil water on a finer temporal scale. The model was parameterized for a shortgrass community in eastern Colorado, USA, using data available from the literature. Experimental simulations examined the effects of four soil textures and a range of levels of plant sensitivity to allelochemicals on the aboveground biomass of A. repens and of native perennial grasses. Simulation results showed that A. repens dominated the aboveground biomass on a plot only if native species were affected by allelopathic interactions. At moderate levels of plant sensitivity, A. repens became dominant faster and reached a higher proportion of the total biomass on fine- than on coarse-textured soils. Community composition and rate of A. repens dominance were more affected by the sensitivity of plant growth to allelochemicals than the sensitivity of species recruitment. Allelopathic interactions were an important component of the invasion dynamics of this perennial invasive weed, and further field investigations are warranted.

Response of Individual Bouteloua gracilis (Gramineae) Plants and Tillers to Small Disturbances

We evaluated effects of small disturbances that kill parts of individual plants on plant survival by measuring tiller survival for the perennial bunchgrass, Bouteloua gracilis (H.B.K.) Lag ex Griffiths (blue grama). The importance of soil texture, grazing by cattle, disturbance type and severity were evaluated. Two disturbance types (covering or removing tillers) and three disturbance severities (50, 75 and 90% tiller mortality) were used to represent effects of natural disturbances in shortgrass communities (cattle fecal pats, nest sites of Western harvester ants, burrows of small animals).Tiller survival was not affected by soil texture or grazing intensity, but was affected by disturbance type and severity. Plants that were covered showed a 33% increase in tiller survival for all levels of disturbance severity from August (1991) to June (1992). No net change in tiller number was observed for removed or reference plants. Different responses between disturbance types were likely due to increases in root:shoot ratios of covered plants that increased tiller production as a result of increased soil water acquisition. The number of tillers produced was small, but statistically significant (average = 20 tillers/plant), which shows that B. gracilis plants do not produce independent tillers, but consist of integrated physiological units (IPUs). The lack of plant mortality, even with 90% tiller mortality, indicates that small disturbances must kill entire plants before gaps in resource space are produced to initiate gap dynamics that result in the recovery of an individual B. gracilis plant. Because recovery through seedling establishment by B. gracilis occurs infrequently, the ability of this species to survive after partial plant mortality is important to its continued dominance of shortgrass steppe communities in the presence of these small but frequent disturbances.

Grazing Lawns in Terai Grasslands, Royal Bardia National Park, Nepal1

ABSTRACTWe compared the community structure, nutritive quality, and aboveground biomass of grazing lawns (patches of shortgrass communities) to neighboring grasslands in the Terai of western Nepal. Grazing lawns differed from the adjacent grasslands in species composition and community structure. Species diversity and species richness were higher on grazing lawns (H = 1.60, S = 20.93) than the grasslands (H = 0.97, S = 8.97). Fencing that excluded grazers for 150 days made areas of grazing lawns indistinguishable from neighboring grasslands in terms of plant height and biomass. Growing shoots of forage from grazing lawns had higher digestibility, crude protein, and sodium than forage from the grasslands. Grazing lawns appear to be maintained by continuous grazing and are enriched by deposition of urine, dung, and by certain plant species not found in the the adjacent grasslands.

Grazing Lawns in Terai Grasslands, Royal Bardia National Park, Nepal1

We compared the community structure, nutritive quality, and aboveground biomass of grazing lawns (patches of shortgrass communities) to neighboring grasslands in the Terai of western Nepal. Grazing lawns differed from the adjacent grasslands in species composition and community structure. Species diversity and species richness were higher on grazing lawns (H = 1.60, S = 20.93) than the grasslands (H = 0.97, S = 8.97). Fencing that excluded grazers for 150 days made areas of grazing lawns indistinguishable from neighboring grasslands in terms of plant height and biomass. Growing shoots of forage from grazing lawns had higher digestibility, crude protein, and sodium than forage from the grasslands. Grazing lawns appear to be maintained by continuous grazing and are enriched by deposition of urine, dung, and by certain plant species not found in the the adjacent grasslands.

Effects of Phragmites australis (Common Reed) Invasion on Aboveground Biomass and Soil Properties in Brackish Tidal Marsh of the Mullica River, New Jersey

Phragmites australis (common reed) has been increasing in brackish tidal wetlands of the eastern United States coast over the last century. Whereas several researchers have documented changes in community structure, this research explores the effects of Phragmites expansion on aboveground biomass and soil properties. We used historical aerial photography and a global positioning system (GPS) to identify and age Phragmites patches within a high marsh dominated by shortgrasses (Spartina patens and Distichlis spicata). Plots along transects were established within the vegetation types to represent a gradient of species dominance and a variety of ages of the Phragmites plots. In comparison to neighboring shortgrass communities, Phragmites communities were found to have nearly 10 times the live aboveground biomass. They also had lower soil salinity at the surface, a lower water level, less pronounced microtopographic relief, and higher redox potentials. These soil factors were correlated with the age and biomass of Phragmites communities, were increasingly different with increasing Phragmites dominance along the transects, and were increasingly altered by the ages of Phragmites communities until the factors stabilized in plots of 8 yr to 15 yr of age. We propose that Phragmites expansion plays an important role in altering these soil properties and suggest a variety of mechanisms to explain these alterations.

Is Species Richness Dependent on the Neighbouring Stands? An Analysis of the Community Patterns in Mountain Grasslands of Central Argentina

We studied the spatial pattern of species richness in the mountain grasslands of central Argentina, where three distinct vegetation types occur: shortgrass in shallow soils, tallgrass in deeper soils and wet turf in moist depressions. The shortgrass community had the highest vascular plant species richness on different scales, the maximal number of vascular plant species per m 2 was 89. The richness within the shortgrass vegetation was not related to spatial location or soil depth. Small-scale species richness was significally positively related to the size of the community species pool. Within short grass and tallgrass communities, the floristic similarity between stands was higher when they were located closer to each other. This pattern was interpreted as showing the internal diaspore limitation. The number of species in shortgrass vegetation was significantly positively dependent on the proximity of tallgrass stands. Also, floristic similarity between shortgrass and other communities (tallgrass and wet turt was significantly higher when stands were located closer in space. This pattern was interpreted as showing the external diaspore limitation, called also spatial mass effect - other communities act as seed donors and increase the number of species coexisting in the shortgrass community. The mean height of the predominating species was lower in the shortgrass than in the tallgrass or wet turf communities. In communities with a tall canopy, species composition and richness were not dependent on surrounding communities, which may be explained by the relatively more significant competition for light there. In the shortgrass community, where the canopy is lower, the pattern of species richness was related to the vicinity of potential diaspore sources, which indicates the relative significance of internal and external diaspore limitation. The proportion of exotic species in the community was not related to the richness of native species. The highest proportion of exotics was observed in the wet turf community, which was also the one most heavily grazed.

Soil water dynamics and vegetation patterns in a semiarid grassland

Long-term (1985–1992) dynamics and spatial variations in soil water below the evaporative zone were evaluated for a shortgrass steppe with a low and variable precipitation regime. Each of a sandy loam, clay loam, and two sandy clay loam sites comprised a toposequence with upland, midslope and lowland positions. Soil water was monitored at 15 cm intervals providing estimates covering 22.5 to 97.5 cm depths. Soil water throughout the profile was highest in the clay loam site and lowest in the sandy loam site. However, stored soil water did not vary systematically among slope positions. Total vegetation cover was highest on the lowland in two sites, but was greatest on the midslope position in the other two. Total vegetation cover was greatest on the CL site, which was the wettest in terms of soil water. Soil water depletion was related to the depth-distribution of roots. There was an inverse relationship between aboveground production and soil water content of the 30, 45 and 60 cm layers during the growth period. Root distributions through the profile did not, however, vary with soil texture or with different soil water profiles controlled by texture. The less variable water content of deeper soil layers is a resource which potentially buffers the impact of pronounced variability in precipitation and thus contributes to vegetation stability of the shortgrass community.

Does grazing mediate soil carbon and nitrogen accumulation beneath C4, perennial grasses along an environmental gradient?

An experiment was conducted to evaluate the influence of long-term (>25 yrs) grazing on soil organic carbon (SOC) and total soil nitrogen (N) accumulation beneath individual plants of three perennial grasses along an environmental gradient in the North American Great Plains. The zone of maximum SOC and N accumulation was restricted vertically to the upper soil depth (0-5 cm) and horizontally within the basal area occupied by individual caespitose grasses, which contributed to fine-scale patterning of soil heterogeneity. Long-term grazing mediated SOC and N accumulation in the tall-, mid- and shortgrass communities, but the responses were community specific. SOC and N were lower beneath Schizachyrium scoparium plants in long-term grazed sites of the tall- and midgrass communities, but higher beneath Bouteloua gracilis plants in the long-term grazed site of the shortgrass community. SOC, but not N, was greater in soils beneath compared to between S. scoparium plants in an abandoned field seeded in 1941, indicating that this caespitose grass accumulated SOC more rapidly than N. SOC and N were greater in the 0-5 cm soil depth beneath a caespitose grass (S. scoparium) compared to a rhizomatous grass (Panicum virgatum) in the tallgrass community, with no significant accumulation of either SOC or N beneath P. virgatum plants. Grazing appears to indirectly mediate nutrient accumulation beneath caespitose grasses along the environmental gradient by modifying the size class distribution of plants. Populations with a greater proportion of large plants have a greater potential for biomass incorporation into soils and may more effectively capture redistributed organic matter from between plant locations. Contrasting plant responses to grazing at various locations along the environmental gradient conform to the predictions of the generalized grazing model, as the selection pressures of grazing and aridity may have also influenced the ability of caespitose grasses to accumulate nutrients in soils beneath them by mediating grazing resistance, competitive ability and population structure.

Recovery of Vegetation in a Semiarid Grassland 53 Years after Disturbance

Plowing and subsequent abandonment of semiarid grasslands in the shortgrass steppe region of North America results in both short‐ and long‐term changes in plant community structure. The traditional Clementsian model of succession in which shortgrasses rapidly dominate the vegetation was modified for these grasslands in the 1970s so that it predicted a prolonged stage characterized by the dominance of the bunchgrass Aristida purpurea, followed by a very slow recovery of shortgrasses after large‐scale disturbances. Because neither the Clementsian nor the modified model was supported by results of recent scale‐dependent field experiments and simulation analyses, we designed a study to evaluate recovery of shortgrasses communities on old fields abandoned for 53 yr in northeastern Colorado, USA. Our objectives were: (1) to compare species composition on abandoned fields with that of adjacent, unplowed areas, (2) to compare vegetation on these fields with predictions from the prevailing conceptual models, and (3) to evaluate the relationship between recovery patterns within fields and distance from the source of propagules at the edge of a field. We reached different conclusions based upon the choice of indicator of recovery. For most cases (9 of 13 fields), relative shortgrass cover did not fit predictions of either the Clementsian model or the modified model. High shortgrass cover on two of the remaining fields was similar to that expected by the Clementsian model, and low shortgrass cover on the remaining two fields was similar to that expected by the modified model. Two fields with high shortgrass cover were dominated by Buchloe dactyloides, a species less resistant to drought and grazing than is Bouteloua gracilis, the dominant species in undisturbed communities. Uniformity in cover of other perennial graminoids and density of perennial forbs and annuals on and off fields indicated that these groups had recovered on most fields. However, differences in similarity of species composition on and off fields indicated that none of the fields had recovered. High variability in recovery of vegetation among fields with similar annual climatic variables and soil textures may be attributed to differences in initial conditions management practices through time, fine‐scale climate, and/or other site characteristics that were not measured in this study. We found the perennial bunchgrass Bouteloua gracilis on all fields sampled, and it dominated basal cover on two fields. Four groups of fields were distinguishable based on the relationship between Bouteloua gracilis cover and distance from the edge with unplowed vegetation: (1) fields with uniformly high cover of Bouteloua gracilis; (2) fields with a decrease in cover with distance, and cover dominated by Bouteloua gracilis; (3) fields with a decrease in cover with distance, and cover dominated by Buchloe dactyloides; and (4) fields with uniformly low cover of Bouteloua gracilis and Buchloe dactyloides, and dominated by other perennial graminoids, indicating that a mid‐ to late‐successional stage had been reached. Our results contrast with the conventional view of shortgrasses community response to disturbances, and suggest an alternative view of the recovery process that focuses on interactions between individual plants and their environment to explain recovery patterns that vary in time or space. Accounting for this variability in recovery is critical to the management of these systems, especially under conditions of changing climate and land use.

Influence of Gap Disturbances and Type of Microsites on Seedling Establishment in Bouteloua Gracilis

1 We examined the effects of patch disturbance leading to removal of conspecific neighbours, and of microsite type on the emergence, survival, and performance of seedlings of Bouteloua gracilis at a shortgrass steppe site. Our hypothesis was that the death and/or removal of a full-size B. gracilis plant, would result in increased availability of below-ground resources and thus promote the establishment of conspecific seedlings. 2 In 1990 and 1991, experimental plots were treated to mimic the effects of some common patch disturbance types. Caryopses were then planted into both plant microsites (i.e. located in the hummock formed by adult plants of B. gracilis) and bare soil. Seedlings were watered until they were past the stage of adventitious root development. Soil water contents were recorded during and after the cessation of watering. 3 In 1991, results clearly showed that the effect of removal of established neighbours was significant in favouring seedling establishment. Differences among disturbance types were of lesser magnitude. Seedling responses were associated with soil water relations. Following cessation of watering, control plots with intact canopies showed more rapid depletion of soil water than disturbed plots. 4 The positive effect of bare soil microsites during emergence and early survival was associated with changes in superficial (0-5 cm) soil-water relations. Type of microsite was important for plant responses only during the early stages of seedling development, although soil water differences remained significant after the cessation of watering. 5 These results support the idea that gap-disturbances must occur to achieve the regeneration of the dominant species. The detrimental effect of microsites associated with the plant hummock may affect the pattern of emergence, resulting in a tendency for seedlings to establish in areas that were not occupied by their predecessors. Interference of established plants via below-ground competition and microsite favourability plays a key role in the regeneration of B. gracilis in this shortgrass community.

Spatial Patterns of Roots in a Semiarid Grassland: Abundance of Canopy Openings and Regeneration Gaps

1 Spatial patterns of root biomass and plant cover were quantified in 10 late-successional, shortgrass steppe communities in which a large proportion of the soil is bare and regeneration is frequently limited by soil water. Our main objectives were to evaluate how patterns of root density were associated with previously documented variation in recruitment in canopy openings of different sizes and to estimate the abundance of openings with low root density. 2 Root biomass in the top 30 cm of soil was much lower in openings of all sizes than under plants and declined steeply as opening size increased. Biomass of light-coloured roots (i.e. those presumed to be functional) in centres of 10-, 20and 60-cm openings was 62, 33 and 4%, respectively, of that under plants. 3 Openings more than 5 cm across made up 34% of the surface. Most were small: 86% of openings were 50 cm across), which are known to enhance regeneration, had low root density. Such openings (2% of total) occupied 2% of the area and nearly all were caused by disturbance. However, many openings caused by disturbance were 30-50 cm across, a size range where interference from neighbours changes from strong to weak, and some were even smaller. Most of the area (>99.5%) was within the range of the root system of the dominant grass, Bouteloua gracilis. 5 We infer that most openings large enough to support enhanced recruitment are explored by roots of dominant bunchgrasses and that gap dynamics in shortgrass steppe involves constraints on water use in B. gracilis root systems. Because large openings are rare, variation in below-ground competition in the abundant, smaller openings may be important for regeneration.

Soil water dynamics, transpiration, and water losses in a crested wheatgrass and native shortgrass ecosystem

The status of water in soil and vegetation was monitored in a stand of crested wheatgrass (Agropyron cristatum) and a nearby shortgrass steppe during a growing season. This was done to determine if water use and losses were similar among two very different communities in a similar climate. Precipitation was similar throughout the study period for both the crested wheatgrass and native shortgrass communities. However, the native shortgrass community with greater root biomass had consistently greater soil water depletion in the deeper soil horizons than was found in the crested wheatgrass community. Greater depletion of soil water by native shortgrass species suggests that they might be more competitive than crested wheatgrass in a water-limited environment.

Vegetation Associated with Nest Sites of Western Harvester Ants (Pogonomyrex occidentalis Cresson) in a Semiarid Grassland

-We evaluated the effects of western harvester ants (Pogonomyrex occidentalis Cresson) on short-term revegetation on their nest sites in a semiarid grassland in northeastern Colorado. Storage of germinable seeds in the soil of nest sites, biomass of plants around nest sites, longevity of nest sites and effects of date of nest abandonment were studied. Perennial plants dominated the nest sites 1 yr after abandonment, whereas annuals dominated the seed bank on the nest sites. Significant differences in plant recovery and seed storage occurred on the cone-shaped mound and the surrounding circular disk. Significantly more annuals occurred on the mound than on the disk reflecting the significantly greater number of seeds of annuals stored in the mound compared to the disk. The perennial grass Bouteloua gracilis (H.B.K.) Lag. ex Griffiths, which dominates most shortgrass communities, was not found on abandoned nest sites nor were its seeds stored in the soil of the mound or disk. A colony of Pogonomyrex occidentalis may inhabit a nest site for 29-58 yr.