A correlation between photosynthetic temperature adaptation and seasonal phenology patterns in the shortgrass prairie

Summary The temperatures at which chlorophyll fluorescence yield is substantially increased and the temperatures at which the quantum yield for CO₂ uptake is irreversibly inhibited were measured for three shortgrass prairie species. The experimental taxa include, a cool season species (Agropyron smithii), a warm season species (Bouteloua gracilis), and a species which grows throughout the cool and warm seasons (Carex stenophylla). Agropyron smithii exhibited lower high temperature damage thresholds (43°C in cool grown plants, 46°C in warm grown plants), relative to the other two species. Bouteloua gracilis exhibited the highest tolerance to high temperature, with threshold values being 44–49°C for cool grown plants and 53–55°C for warm grown plants. Carex stenophylla exhibited threshold values which were intermediate to the other two species (43–47°C for cool grown plants, and 51–53°C for warm grown plants). Seasonal patterns in the fluorescence rise temperatures of field grown plants indicated acclimation to increased temperatures in all three species. The results demonstrate a correlation between the high temperature thresholds for damage to the photosynthetic apparatus, and in situ seasonal phenology patterns for the three species.     

Root Biomass of Individual Species, and Root Size Characteristics After Five Years of CO2 Enrichment on Native Shortgrass Steppe

Information from field studies investigating the responses of roots to increasing atmospheric CO₂ is limited and somewhat inconsistent, due partly to the difficulty in studying root systems in situ. In this report, we present standing root biomass of species and root length and diameter after five years of CO₂ enrichment (∽720 μmol mol⁻¹) in large (16 m² ground area) open-top chambers placed over a native shortgrass steppe in Colorado, USA. Total root biomass in 100 cm long×20 cm wide×75 cm depth soil monoliths and root biomass of the three dominant grass species of the site were not significantly affected by elevated CO₂. Root biomass of Stipa comata in the 0–20 cm soil depth was nearly 100% greater in elevated vs. ambient CO₂ chambers, but this was not statistically significant (P=0.14). However, there was a significant 37% increase in fine root length under elevated CO₂ in the 0–10 cm soil depth layer. Other reports from this study suggest that the increase in fine roots is primarily from improved seedling recruitment of S. comata under elevated CO₂. Few treatment differences in root length or diameter were detected in lower 10 cm depth increments, to 80 cm. These results reflect the root status integrated over two wet, two dry and one normal precipitation years and approximately one complete cycle of root turn-over on the shortgrass steppe. We conclude that increasing atmospheric CO₂ will have only small effects on standing root biomass and root length and diameter of most shortgrasss steppe species. However, the potential increased competitive ability of Stipa comata, a low forage quality species, could alter the ecosystem from the current dominant, high forage quality species, Bouteloua gracilis. B. gracilis is very well adapted to the frequent droughts of the shortgrass steppe. Increased competitive ability of less desirable plant species under increasing atmospheric CO₂ will have large implications for long-term sustainability of grassland ecosystems.     

Geographic patterns of simulated establishment of two Bouteloua species: implications for distributions of dominants and ecotones

Abstract. Our overall objective was to use a soil water model to predict spatial patterns in germination and establishment of two important perennial C₄-bunchgrasses across the North American shortgrass steppe and desert grassland regions. We also predicted changes in establishment patterns under climate change scenarios. Bouteloua gracilis dominates the shortgrass steppe from northeastern Colorado to southeastern New Mexico. Bouteloua eriopoda dominates desert grasslands in central and southern New Mexico. Germination and establishment for each species were predicted at 16 sites along the gradient using a daily time step, multi-layer soil water model (SOILWAT) to determine the percentage of years that temperature and soil water criteria for germination and establishment were met. Percentage of years with predicted establishment decreased from north to south for B. gracilis, but increased from north to south for B. eriopoda, comparable to observed dominance patterns. The 95 % confidence interval around the point at which simulated establishment were equal for the two species was near the location of the shortgrass steppe-desert grassland ecotone where both species are abundant. The intersection in percentage of years with establishment for the two species was predicted to move further north when climate was scaled using three Global Circulation Models (GCMs), indicating a possible northward expansion of B. eriopoda. Our results suggest that recruitment by seed may be an important process in determining, at least in part, the geographic distribution of these two species. Changes in climate that affect establishment constraints could result in shifts of species dominance that may or may not be accompanied by changes in species composition.     

Effects of ungulates and prairie dogs on seed banks and vegetation in a North American mixed-grass prairie

The relationship between vegetation cover and soil seed banks was studied in five different ungulate herbivore-prairie dog treatment combinations at three northern mixed-grass prairie sites in Badlands National Park, South Dakota. There were distinct differences in both the seed bank composition and the aboveground vegetation between the off-prairie dog colony treatments and the on-colony treatments. The three on-colony treatments were similar to each other at all three sites with vegetation dominated by the forbs Dyssodia papposa, Hedeoma spp., Sphaeralcea coccinea, Conyza canadensis, and Plantago patagonica and seed banks dominated by the forbs Verbena bracteata and Dyssodia papposa. The two off-colony treatments were also similar to each other at all three sites. Vegetation at these sites was dominated by the grasses Pascopyrum smithii, Bromus tectorum and Bouteloua gracilis and the seed banks were dominated by several grasses including Bromus tectorum, Monroa squarrosa, Panicum capillare, Sporobolus cryptandra and Stipa viridula. A total of 146 seedlings representing 21 species germinated and emerged from off-colony treatments while 3069 seedlings comprising 33 species germinated from on-colony treatments. Fifteen of the forty species found in soil seed banks were not present in the vegetation, and 57 of the 82 species represented in the vegetation were not found in the seed banks. Few dominant species typical of mixed-grass prairie vegetation germinated and emerged from seed banks collected from prairie dog colony treatments suggesting that removal of prairie dogs will not result in the rapid reestablishment of representative mixed-grass prairie unless steps are taken to restore the soil seed bank.     

Extirpation or Coexistence? Management of a Persistent Introduced Grass in a Prairie Restoration

Abstract Introduced perennial grasses are one of the greatest constraints to prairie restoration. Herbicides suppress but do not eliminate introduced grasses, so we explored the interaction of herbicide with two additional controls: heavy clipping (to simulate grazing) and competition from native species. A 50‐year‐old stand of the introduced perennial grass Agropyron cristatum (crested wheatgrass) in the northern Great Plains was seeded with native grasses and treated with herbicide annually for 7 years in a factorial experiment. Clipping was applied as a subplot treatment in the final 3 years. Both herbicide and clipping significantly reduced the cover of A. cristatum, but clipping produced an immediate and consistent decrease, whereas herbicide control varied among years. The cover of A. cristatum decreased significantly with increasing cover of a seeded native grass, Bouteloua gracilis (blue grama), suggesting that both top‐down (i.e., grazing) and bottom‐up (i.e., resource competition) strategies can contribute to A. cristatum control. No treatment had any effect on the seed bank of A. cristatum. Even in the most effective control treatments, A. cristatum persisted at low amounts (approximately 5% cover) throughout the experiment. The cover of B. gracilis increased significantly with seed addition and herbicide, and, after 7 years, was similar to that in undisturbed prairie. The total cover of native species increased significantly with clipping and herbicide, and species richness was significantly higher in plots receiving herbicide. Clipping season had no effect on any variable. In summary, no method extirpated A. cristatum, but clipping reduced its cover by 90% and doubled the cover of native species. Extirpation might not be a realistic goal, but relatively simple management allowed coexistence of native species.     

Semiarid grassland responses to short-term variation in water availability

Standing crop and species composition in semiarid grassland are linked to long-term patterns of water availability, but grasslands are characterized by large single-season variability in rainfall. We tested whether a single season of altered water availability influenced the proportions of grasses and shrubs in a semiarid grassland near the northern edge of the North American Great Plains. We studied stands of the clonal shrub snowberry (Symphoricarpos occidentalis) and adjacent grassland dominated by the native grasses Stipa spartea and Bouteloua gracilis. Rain was excluded and water supplied in amounts corresponding to years of low, medium, and high rainfall, producing a 2 − 4-fold range in monthly precipitation among water supply treatments. There were ten replicate plots of each water treatment in both snowberry stands and grassland. Grass standing crop increased significantly with water availability in grassland but not inside snowberry stands. Total standing crop and shrub stem density increased significantly with water supply, averaged across both communities. In contrast, water had no effect on shrub standing crop or light penetration. In summary, our finding that water has significant effects on a subset of components of grassland vegetation is consistent with long-term, correlational studies, but we also found that a single season of altered water supply had no effect on other important aspects of the ecosystem. This revised version was published online in June 2006 with corrections to the Cover Date.     

Plant biodiversity on shortgrass steppe after 55 years of zero,light, moderate,or heavy cattle grazing

Shortgrass steppe rangeland near Nunn, Colorado, USA, has been lightly,moderately, or heavily grazed by cattle, or protected from grazing inexclosures, for 55 years. Plant species biodiversity and evenness were greatestin lightly- and moderately-grazed pastures. Both pastures weredominated by the warm-season shortgrass Boutelouagracilis, but the cool-season midgrasses Pascopyrumsmithii and Stipa comata contributedsignificantly to biomass production on the lightly-grazed pasture, asthey did in the exclosures. Diversity was least in the exclosures, which werestrongly dominated by the cactus Opuntia polyacantha.Buchloë dactyloides, another warm-seasonshortgrass, and Bouteloua gracilis were co-dominantsunder heavy grazing, and diversity was intermediate. Plant community structureand diversity were controlled by selective grazing by cattle and soildisturbance by cattle and rodents. Shortgrass steppe moderately or heavilygrazed by cattle was similar to and probably as sustainable as steppe grazed formillenia by bison and other wild ungulates.     

Longevity and turnover of roots in the shortgrass steppe: influence of diameter and depth

We used minirhizotrons to determine patterns of root longevity andturnover for the perennial bunchgrass Bouteloua gracilisinthe shortgrass steppe of eastern Colorado, USA. We hypothesized that rootlongevity would be partially controlled by root diameter, following previouslyobserved patterns in woody plants. In addition, we hypothesized that rootturnover would be greatest in surface soil horizons and decrease with depth dueto variation in soil moisture availability and temperature. Root longevity wascorrelated with root diameter. Median life span of roots > 0.4mm was approximately 320 days, while roots < 0.2mmhad a median life span of 180 days. There was approximately a 6%decreasein the likelihood of mortality with a 0.10-mm increase inroot diameter, controlling for the effect of depth in the soil profile. Rootlength production and mortality were highest in the upper20 cm of the soil profile and decreased with depth.However,because root length density also decreased with depth, there were nosignificantdifferences in turnover rate of root length among sampling intervals. Turnoverwas approximately 0.86 yr^–1 based on root length production,while turnover was 0.35 yr^–1 using root length mortality as ameasurement of flux. The imbalance between turnover estimates may be aconsequence of the time the minirhizotrons were in place prior to imaging or mayresult from our lack of over-winter measures of mortality. Our worksuggests that Bouteloua gracilis roots have complex lifehistory strategies, similar to woody species. Some portion of the root systemishighly ephemeral, while slightly larger roots persist much longer. Thesedifferences have implications for belowground carbon and nitrogen cycles in theshortgrass steppe.     

Distribution,species diversity and life-form spectra of plant communities along an altitudinal gradient in the northern slopes of Qilianshan Mountains,Gansu, China

We studied the distribution pattern, species diversity and life-formspectra of plant communities along an altitudinal gradient in the mid-sectionofthe northern slopes of Qilianshan Mountains by means of multivariate analyses.Two data sets (167 species × 75 plots, 10 environmental variables ×75 plots), originated from the fieldworks in 1998–1999, were subjected toTWINSPAN and DCCA, resulting in 8 major plant communities: 1)Asterothamnus centraliasiaticus–Halogetonarachnoideus desert grassland on azonal substrates from 1450 to 1600m and 2) zonal Reaumuria soogorica desertgrassland on gravels from 1470 to 1900 m; 3) Stipaprzewalskii–Stipa purpurea montane grassland from 2200 to 2900m; 4) Polygonum viviparum alpine grasslandfrom 2900 to 3700 m; 5) Caraganastenophylla–Ajaniafruticulosa dry-warm shrubland from 2350 to 2800 m; 6)Sabina przewalskii mid-wet warm forest from 2700 to 3300m; 7) Picea crassifolia cold coniferousforestfrom 2450 to 3200 m; 8) Caragana jubatawet-cold alpine shrubland from 3100 to 3700 m. Species diversityand species richness of both grasslands and forests peaked at the intermediateportion of the elevational gradient. Evenness might be strongly influenced byeither biotic or abiotic factors at a local scale, while seems quiteindependentof an elevational gradient at landscape scales. Beta-diversity decreased from1500 to 3700 m, indicating that species turnover declined withincreased elevation. Both richness of life-form and total species richness in agiven altitudinal belt (gamma-diversity) peaked at intermediate elevations,while relative species richness of different life-form varied differently alongthe altitudinal gradient.     

NATURE OF THE PLANT COMMUNITY. VI. TEXAS GRASSLAND COMMUNITIES UNDER TRANSPLANTED CONDITIONS

Mc Millan , Calvin . (U. Texas, Austin.) Nature of the plant community. VI. Texas grassland communities under transplanted conditions. Amer. Jour. Bot. 48(9): 778–785. Illus. 1961.—Clones of 7 grass taxa, Bouteloua gracilis (H. B. K.) Lag., B. eriopoda (Torr.) Torr., B. curtipendula (Michx.) Torr., Panicum virgatum L., the Andropogon scoparius Michx. complex, the Andropogon gerardi Vitman-hallii Hack. complex, and Sorghastrum nutans (L.) Nash, were transplanted from throughout their distribution in Texas and studied in an experimental garden at Austin. Restricted to western Texas and Panhandle areas, Bouteloua gracilis and B. eriopoda contained similar early-flowering clones throughout their distribution. Less restricted to western sites, B. curtipendula contained later-flowering types from eastern and central areas. In the 4 remaining, widespread taxa, early-flowering potential characterized clones from western sites. These 4 widespread taxa contain the latest-flowering clones from the coast of southern Texas. Clones of Stipa leucotricha Trin. and Rupr. from a broad area in Texas lacked a flowering gradient. Grassland communities of western Texas and the Panhandle, attuned to short growing seasons and low rainfall, were composed of opportunists, the Bouteloua species, and early-flowering variants within the widespread species. Communities of central Texas in habitats of highly unpredictable moisture pattern and a relatively long growing season contained later-flowering variants. Coastal communities attuned to a long growing season contained the latest-flowering variants.     

Short-term effects of fire frequency on vegetation composition and biomass in mixed prairie in south-western Manitoba

The net effects of one, two, and three spring burns in consecutive yearson the aboveground biomass, species composition, and soil variables wereassessed in two different mixed-grass prairie sites in south-western Manitoba.Precipitation in the first year was greater than the 30-year average but lowerduring the next three years. The first site (Area 6) was characterized byBouteloua gracilis, Stipa spartea,Selaginella densa and lichens, while the second, somewhatdrier site (Area 10) was dominated by B. gracilis andCarex spp. Each burn treatment was applied to 15 plots inarandom block design. Vegetation and soil data collected following the thirdburnare presented. In general, the effects of repeated burning were more pronouncedin the drier Area 10, where litter was significantly reduced with eachadditional burn. In Area 6, litter was significantly reduced only after threeyears of burning. In both sites, the cumulative effect of fire had nosignificant effect on total standing crop, even after three consecutive yearsofburning. In the drier Area 10, however, the biomass of several componentsshifted such that B. gracilis biomass increasedsignificantly and forb biomass decreased significantly after three burns. Plantcover was also affected more in the drier Area 10. The cover of B.gracilis, the dominant C₄ grass, significantly increasedand Carex spp. cover decreased after two burns in thedriersite. In Area 6, the most significant effect of fire was a reduction inSelaginella and lichen cover. Bare ground increased inbothsites as the number of burns increased. There was little change in soilnutrients with burning, but soil moisture somewhat decreased and surfacetemperature significantly increased after three burns.     

Effects of nitrogen availability on competition between Bromus tectorum and Bouteloua gracilis

Exotic plant invasions are a serious concern for land managers and conservationists. There is evidence that increased nitrogen availability favors exotic species and decreased nitrogen availability favors non-weedy native species. This study was conducted to test the effect of nitrogen availability on competition between two grass species with contrasting life histories, cheatgrass (Bromus tectorum), a North American exotic, and blue grama (Bouteloua gracilis), a North American native. We investigated the effects of nitrogen availability and competition on aboveground biomass, belowground biomass, height, and % nitrogen tissue concentrations by growing the two species in the greenhouse under five levels of nitrogen and six levels of competition. Nitrogen availability affected competition between Bromus tectorum and Bouteloua gracilis. At the lowest level of N availability, neither species was affected by competition. As N availability increased, aboveground biomass gain of Bromus was more negatively affected by intraspecific competition relative to interspecific competition while the opposite occurred for Bouteloua. At the competition level at which each species gained the most aboveground biomass, Bromus had a linear response to increasing N availability while the response of Bouteloua was asymptotic. Our results do provide some support for the theory that fast growing exotic species have a rapid response to nutrient enrichment while native non-weedy species do not, and that low N levels can reduce competitive pressure from the exotic on the native.     

Climatic variation and simulated patterns in seedling establishment of two dominant grasses at a semi‐arid‐arid grassland ecotone

Abstract. The objective of this study was to predict the effects of climatic variation at multiple temporal frequencies on seedling establishment by two congeneric C₄ perennial grasses (Bouteloua gracilis and B. eriopoda) at the ecotone between shortgrass steppe grassland and Chihuahuan desert grassland in central New Mexico, USA. The approach was to use a daily time‐step simulation model to determine the occurrence of a recruitment event in each year based upon the amount and timing of soil water required for establishment. Historical weather data were used to predict effects of seasonal and inter‐decadal variation in climate on establishment. A sensitivity analysis was used to predict effects of directional climate change on establishment. Bouteloua gracilis had a broad pattern of simulated establishment from May through September that included periods with high year‐to‐year variation in precipitation. B. eriopoda establishment events occurred primarily in July when precipitation amounts were most reliable. Climatic conditions from 1949 through 1968 were more favorable for B. eriopoda establishment compared to the cooler, wetter conditions from 1969 through 1988 that favored B. gracilis. Establishment of B. eriopoda was lowest in El Niño years whereas B. gracilis establishment was highest in La Niña years. Establishment of B. gracilis was most sensitive to temperature when precipitation was higher than current amounts. The greatest response to temperature by B. eriopoda for all precipitation amounts occurred at cooler temperatures than found currently. These results indicate that climatic variation at multiple frequencies has differential effects on seedling establishment for these two perennial grasses, and may account at least in part for patterns in dominance at this biome transition zone.     

Seedling Survival from Locally and Commercially Obtained Seeds on Two Semiarid Sites

Local populations of plants are likely to be better adapted to a site than populations from elsewhere. Thus, local seeds should yield higher survival in restoration attempts than commercial seed stocks. We compared seedling survival from locally and commercially obtained seeds of seven species, Pseudoroegneria spicata (bluebunch wheatgrass), Elymus elymoides (squirreltail), Pascopyrum smithii (western wheatgrass), Stipa hymenoides (Indian ricegrass), Stipa comata (needle‐and‐thread), Chrysothamnus nauseosus (rubber rabbitbrush) and Ephedra nevadensis (Mormon tea) over three years on two sites in Utah (Dugway and Tintic) that were dominated by the introduced annual Bromus tectorum (cheatgrass). At the Dugway site we included burned and unburned seedbed treatments. For all species at Dugway, seedling survival to the first summer was higher on burned plots where B. tectorum densities were greatly reduced. First‐year seedling survival was 20–30% for most species on the Dugway burned plots and at Tintic. At the drier Dugway site, only S. hymenoides and Ephedra had substantial third‐year survival. Elymus and Pascopyrum survived to the third year only at the moister Tintic site. Survival to the third year was less than 3% for all species except S. comata (6% survival), and densities were low (0.2–1.0 plants/m²). However, third‐year plants were well established and the grasses flowered. Pseudoroegneria and Ephedra at Dugway (on burned plots) and S. comata and Elymus at Tintic had higher first‐year survival or higher survival based on survival curves from local than from commercial seeds. However, final survival was never significantly higher, although such a trend was suggested. Seed dormancy traits could also provide advantages to local populations, and we observed differences in dormancy between local and commercial S. comata and S. hymenoides seeds that may be an example.     

Genetic variability of Bouteloua gracilis populations differing in forage production at the southernmost part of the North American Graminetum

Bouteloua gracilis (blue grama grass) native populations have been shown to be highly variable, however the genetic basis of this variability has not been well established. Determining the extent of genetic variability within and among plant populations have important repercussions for the management and conservation of species, and in particular for those subjected to intensive use such as forage plants. Using RAPD, this study was undertaken to investigate the genetic variability of four B. gracilis native populations developed in three grasslands and one shrubland at the southernmost part of the North American Graminetum in México. Significant differences in grass aboveground production were found among the study sites, while considerable genetic variation within each of the four blue grama populations evaluated was detected. The molecular analysis, based on 55 individuals, revealed a total of 108 scorable repeatable bands, with 99 of them being polymorphic (overall polymorphism= 91.7%). Within every population each individual was genetically distinct and no population-specific bands (fixed marker differences) were identified. Pair-wise Φ _ST comparisons indicated that the four blue grama populations examined were significantly different in their genetic constitution (P<0.001). AMOVA revealed that most of the genetic variation detected in Bouteloua gracilis was explained by intra- (88.53%), rather than by inter-population (11.47%) differences. UPGMA based on the Φ _ST values indicated that the blue grama population collected from the shrubland displayed the RAPD profiles that most differed among the study sites. Possible causes of these results could reside on intensive grazing reducing, and proper management conserving, the forage production and genetic diversity of blue grama native populations. Our results are consistent with previous studies analyzing population genetic variation in outcrossing grasses and, in particular, with ecological and cytological evidence for a high genetic variability in native populations of B. gracilis. The implications of our findings and prospective studies to be undertaken using molecular tools in the study of blue grama biology and ecology are discussed. This revised version was published online in August 2006 with corrections to the Cover Date.     

Distribution of plant species at a biome transition zone in New Mexico

Question: Is there a difference in plant species and life form composition between two major patch types at a biome transition zone? Are subordinate species associated with different patch types at the shortgrass steppe — Chihuahuan desert grassland transition zone? Is this association related to differences in soil texture between patch types and the geographic range of associated species? Location: central New Mexico, USA. Methods: Patches dominated by either Bouteloua gracilis, the dominant species in the shortgrass steppe, or Bouteloua eriopoda, dominant species in the Chihuahuan desert grasslands, were sampled for the occurrence of subordinate species and soil texture within a 1500‐ha transitional mosaic of patches. Results: Of the 52 subordinate species analysed, 16 species were associated with B. gracilis‐dominated patches and 12 species with B. eriopoda‐dominated patches. Patches dominated by B. gracilis were richer in annual grasses and forbs, whereas patches dominated by B. eriopoda contained more perennials forbs and shrubs. Soils of B. gracilis‐dominated patches had higher clay and lower rock contents compared with soils of B. eriopoda‐dominated patches. Differences in species characteristics of the dominant species as well as differences in soil texture between patch types contribute to patch‐scale variation in composition. The association of species to patch types was not related to their geographic range and occurrence in the adjacent biomes. Conclusions: Patch types at this biome transition zone have characteristic life‐form and species composition, but species are associated to patch types due to local constraints, independently from their affinity to the adjacent biomes.     

The quantum yield for CO2 uptake in C3 and C4 grasses

The quantum yield for CO₂ uptake was measured in C₃ and C₄ monocot species from several different grassland habitats. When the quantum yield was measured in the presence of 21% O₂ and 340 cm³ m^-3 CO₂, values were very similar in C₃ monocots, C₃ dicots, and C₄ monocots (0.045–0.056 mole CO₂ · mole^-1 quanta absorbed). In the presence of 2% O₂ and 800 cm³ m^-3 CO₂, enhancements of the quantum yield values occurred for the C₃ plants (both monocots and dicots), but not for C₄ monocots. A dependence of the quantum yield on leaf temperature was observed in the C₃ grass, Agropyron smithii, but not in the C₄ grass, Bouteloua gracilis, in 21% O₂ and 340 cm³ m^-3 CO₂. At leaf temperatures between 22–25°C the quantum yield values were approximately equal in the two species.     

The impact of an invading alien grass (Agropyron cristatum) on species turnover in native prairie

Abstract. Alien invasions typically reduce species richness of habitats, but few studies have examined their effects on species turnover, the difference in species composition between localities. Agropyron cristatum (L.) Gaertn. (crested wheat grass) has been planted on 6–10 million ha of North American prairie, and is invading native prairie. We studied the invasion of A. cristatum into native prairie by measuring species composition along a gradient from maximum to minimum A. cristatum abundance. As A. cristatum increased, the abundance of most common native species decreased, but one appeared to be unaffected (Bouteloua gracilis (H.B.K.) Lag.), and another (Poa sandbergii Vasey) increased. The effect of A. cristatum on species turnover was investigated by examining species–area curves for areas from 0.5 m² to 8.0 m². Species diversity was reduced by 35% at high A. cristatum abundances at all areas. A. cristatum reduced the intercept of the species–area curve, but not the slope, suggesting that A. cristatum affected species turnover proportionally in all areas and habitats. This unusual result may indicate a homogeneous environment where species are distributed randomly. A. cristatum produced almost twice as many seeds as all native grasses combined. The number of seeds collected of native grasses and A. cristatum was highly correlated with the number of seed heads immediately nearby, but not with transect position. This suggests most seeds were dispersed over distances less than 5 m. In sum, the invasion of native prairie by A. cristatum might be related to high rates of seed production, and has the effect of decreasing species turnover by reducing the intercept of the species–area curve.     

Fungal endophytes intrinsically associated with micropropagated plants regenerated from native <Emphasis Type="Italic">Bouteloua eriopoda</Emphasis> Torr. and <Emphasis Type="Italic">Atriplex canescens</Emphasis> (Pursh) Nutt.

Summary Black grama (Bouteloua eriopoda) and fourwing saltbush (Atriplex canescens) are important grass and shrub species in arid rangelands of the northern Chihuahuan Desert. They are naturally colonized by dark septate endophytic fungi that cannot be eliminated by seed disinfestation. Plants were regenerated from both species and appeared to be fungus-free in axenic cultures. Analysis of callus and regenerated plants of both species using dual staining with light and scanning electron microscopy revealed fungal endophytes intrinsically associated with cells, roots and leaves of regenerated plants that are also associated with native plants. Fungal layers and biofilms prevent direct exposure of callus, root or leaf tissues to the external environment. Micropropagation is a valuable tool for identifying key fungal endophytes that enhance drought tolerance in native desert plants.     

Plant population and community responses to removal of dominant species in the shortgrass steppe

Question: What are the plant population‐ and community‐level effects of removal of dominant plant species in the shortgrass steppe? Location: The Shortgrass Steppe Long‐Term Ecological Research site in northern Colorado, USA. Methods: We annually measured plant cover and density by species for 10 years after a one‐time aboveground removal of the dominant perennial grass, Bouteloua gracilis. Removal and control plots (3 m × 3 m) were within grazed and ungrazed locations to assess the influence of grazing on recovery dynamics. Our analyses examined plant species, functional type, and community responses to removal, paying special attention to the dynamics of subdominant and rare species. Results: Basal cover of B. gracilis increased by an average of 1% per year, but there was significantly less plant cover in treatment compared to control plots for 5 years following removal. In contrast to the lower cover in treatment plots, the plant density (number of plants m^?2) of certain subdominant perennial grasses, herbaceous perennial and annual forbs, a dwarf shrub, and cactus increased after removal of the dominant species, with no major change in species richness (number of species per 1 m × 1 m) or diversity. Subdominant species were more similar between years than rare species, but dominant removal resulted in significantly lower similarity of the subdominant species in the short term and increased the similarity of rare species in the long term. Conclusions: Removal of B. gracilis, the dominant perennial grass in the shortgrass steppe, increased the absolute density of subdominant plants, but caused little compensation of plant cover by other plants in the community and changes in species diversity.