Effects of Species Richness on Resident and Target Species Components in a Prairie Restoration

The ecological role of biodiversity in achieving successful restoration has been little explored in restoration ecology. We tested the prediction that we are more likely to create persistent, species‐rich plant communities by increasing the number of species sown, and, to some degree, by varying functional group representation, in experimental prairie plantings. There were 12 treatments consisting of 1‐, 2‐, 3‐, 4‐, 8‐, 12‐, and 16‐species mixtures of native perennials representing four functional groups (C₄ grasses, C₃ grasses, nitrogen‐fixing species, and late‐flowering composites) that predominate within Central Plains tallgrass prairies. In 2000, species were seeded into square plots (6 × 6 m), with five replicates per treatment, on former agricultural land. Annually, we measured total species richness and evenness, target species richness and cover, and richness and cover of resident species (i.e., those emerging from the seed bank). Both target species richness and rate of establishment of target communities were highest in the most species‐rich mixtures, but there was no additional benefit for treatments that contained more than eight species. Richness of resident species did not vary with target species richness; however, cover by resident species was lower in the higher target species treatments. Our results, indicating that establishment of species‐rich prairie mimics can be enhanced by starting with larger numbers of species at the outset, have implications for grassland restoration in which community biodiversity creation and maintenance are key goals.     

Restoration of a Landfill Site in Berlin, Germany by Spontaneous and Directed Succession

We report on spontaneous and directed succession on a dry sandy landfill site of low fertility at Berlin‐Malchow, Germany. Changes in species composition and cover were followed on unmown and mown permanent plots of 2 × 2 m size through 5 years of vegetation development. Species richness on unmown plots was relatively constant during the time of observation, with 20 to 25 species per 4 m². Total cover of unmown plots continuously increased from approximately 10% in the first year to 80% in the fifth year. There are no clearly discernible sequential successional stages until present. The species composition includes species of all life forms, which colonized the site immediately after the initiation of the succession process representing the initial floristic composition type of vegetation development. However, perennial grasses and herbs gradually increased in cover up to approximately 40%. Woody plants were also present from the first year of succession and increased up to more than 20% cover in the fifth year, forming a shrub layer (>0.5 m) after the second year. Mowing significantly increased species richness, which was evident from the third year onward. This effect was mainly due to the reduction of the tall perennial grass Calamagrostis epigejos. Solidago canadensis and woody species were also significantly affected (lower cover and height), whereas short perennial herbs like Plantago lanceolata and Trifolium repens benefited from mowing.     

Restoration Potential of Bighorn Sheep in a Prairie Region

Efforts to recover Rocky Mountain bighorn sheep (Ovis canadensis canadensis) throughout western North America have had limited success with the majority of current populations remaining in small and isolated areas on a fraction of their historical range. Prairie environments with rugged topography throughout the Northern Great Plains ecoregion were historically occupied by relatively robust bighorn sheep populations. We predicted there is likely unrealized potential habitat for restoring bighorn sheep to these areas; however, relatively little attention has been devoted to identifying habitat in unoccupied prairie regions. We used global positioning system (GPS)-collar data collected from 43 female bighorn sheep in 2 populations located in the eastern Montana, USA, portion of the Northern Great Plains during 2014–2018 to estimate a population-level annual resource selection model and identify the important factors affecting bighorn sheep resource selection. We extrapolated model predictions across eastern Montana's prairie region and identified potential habitat to understand restoration potential and assist with future translocations of bighorn sheep. Resource selection of bighorn sheep was most strongly associated with terrain slope and ruggedness, tree canopy cover, and a normalized difference vegetation index metric. Within currently unoccupied areas of the historical range, the model extrapolation predicted 7,211 km² of habitat, with most owned and managed by private landowners (44%), Bureau of Land Management (33%), and the United States Fish and Wildlife Service (15%). Our results provide an empirical evaluation of landscape covariates influencing resource selection of bighorn sheep occupying prairie environments and provide a habitat model that may be generalizable to other areas in the Northern Great Plains ecoregion. We demonstrate substantial potential for restoration opportunities of bighorn sheep in the Northern Great Plains ecoregion. Broad restoration of bighorn sheep across the ecoregion would likely require strong collaboration among and between public resource managers, private landowners, and livestock producers given the heterogeneous land ownership patterns, management strategies, and domestic sheep distributions. © 2020 The Wildlife Society.     

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.     

Effects of Soil Carbon Amendment on Nitrogen Availability and Plant Growth in an Experimental Tallgrass Prairie Restoration

Restoration of tallgrass prairie on former agricultural land is often impeded by failure to establish a diverse native species assemblage and by difficulties with nonprairie, exotic species. High levels of available soil nitrogen (N) on such sites may favor fast‐growing exotics at the expense of more slowly growing prairie species characteristic of low‐N soils. We tested whether reducing N availability through soil carbon (C) amendments could be a useful tool in facilitating successful tallgrass prairie restoration. We added 6 kg/m² hardwood sawdust to experimental plots on an abandoned agricultural field in the Sandusky Plains of central Ohio, United States, increasing soil C by 67% in the upper 15 cm. This C amendment caused a 94% reduction in annual net N mineralization and a 27% increase in soil moisture but had no effect on total N or pH. Overall, plant mass after one growing season was reduced by 64% on amended compared with unamended soil, but this effect was less for prairie forbs (?34%) than for prairie grasses (?67%) or exotics (?62%). After the second growing season, only exotics responded significantly to the soil C amendment, with a 40% reduction in mass. The N concentration of green‐leaf tissue and of senescent leaf litter was also reduced by the soil C treatment in most cases. We conclude that soil C amendment imparts several immediate benefits for tallgrass prairie restoration––notably reduced N availability, slower plant growth, and lower competition from exotic species.     

Effects of Prescribed Fire on Vegetation and Passerine Birds in Northern Mixed-Grass Prairie

ABSTRACT Prescribed fire is used widely to manage grasslands on National Wildlife Refuges and other public lands in the northern Great Plains, but its effects on habitat use or production of wildlife in the region are poorly understood. During 1998–2003, we used point counts to examine effects of prescribed fire on vegetation and passerines in a mixed-grass prairie complex in north-central North Dakota, USA (n = 7 units, each 40–70 ha). Vegetation structure and, to a lesser extent, plant community composition varied with year of study (likely related to changes in annual precipitation) and with number of growing seasons since fire. Fire altered plant structure, especially the amount of residual vegetation, which in turn influenced bird species richness and abundance. The number of indicated pairs for sedge wren (Cistothorus platensis), clay-colored sparrow (Spizella pallida), Le Conte's sparrow (Ammodramus leconteii), Savannah sparrow (Passerculus sandwichensis), and bobolink (Dolichonyx oryzivorus) were lowest during the first postfire growing season but generally increased and stabilized within 2–3 postfire growing seasons. Our results support the premise that grassland passerines are well-adapted to frequent, periodic fires, generally corresponding to those occurring prior to Euro-American settlement of the region. Prescribed fire is important for reducing tree and shrub invasion, restoring biological integrity of plant communities, and maintaining or enhancing populations of grassland-dependent bird species. Managers in the northern mixed-grass prairie region should not be overly concerned about reductions in bird abundances that are limited mostly to the first growing season after fire.     

Oligochaete respiration as a measure of sediment toxicity in Puget Sound,Washington

Respiration rate measurements were conducted with the marine oligochaete Monopylephorus cuticulatus Baker and Brinkhurst to determine the sublethal toxicity of sediments collected from Puget Sound, Washington. Worms were exposed to elutriates prepared from centrifuged sediment slurries. Standard respiration rates were measured at high dissolved oxygen levels for each sample tested and were compared with control and other test results. A total of 97 sediment samples were tested; 40 samples demonstrated significant respiration effects (elevation or depression). Comparison with results of other tests conducted at the same stations or geographic locations (genotoxicity to fish cells, lethality to sensitive species, reproductive impairment tests) indicated very good agreement on broad scale toxicity patterns at different geographical areas, and that respiration measurements effectively determined sublethal toxicity of sediments. This study represents the first application of respiration measurements as a sublethal toxicity test for field-collected sediments.     

Legacy effects of afforestation on prairie plant and seed bank communities in a northern Canadian prairie

Afforestation resulting from fire suppression, modified grazing, plantation establishment and climate change poses a threat to northern prairie ecosystems. Trees alter the composition and function of plant and soil fauna communities and can compromise the restoration of afforested prairies. To evaluate the hypothesis that legacies of afforestation persist in restored prairie communities and decrease the potential for restoration, we examined the composition, structure, and diversity of plant and seed bank communities along a 20 year chronosequence of plantation tree removal from a northern fescue prairie in Riding Mountain National Park, Manitoba, Canada. Tree removal increased the abundance of weedy species in the plant and seed bank communities of restored prairies and plant diversity peaked and declined over the 20 year period of passive restoration. As a result, time since tree removal and the encroachment of invasive species were key in explaining the composition of restored prairie communities. Low correlation between the species composition of plant and seed bank communities, including the complete absence of Festuca hallii in restored treatments, demonstrated that legacies of afforestation compromised the potential of seed banks to facilitate prairie restoration. We conclude that tree removal alone is insufficient for the restoration of northern fescue prairies and that, in the absence of active management, the persistence of low-diversity plant and seed bank communities constitutes an important legacy of afforestation and an important barrier to future restoration.     

Effects of Patch-Burn Management on Dickcissel Nest Success in a Tallgrass Prairie

Abstract Grassland birds have declined more than any other North American habitat-associated bird community. Because most species of grassland birds evolved within heterogeneous landscapes created by the interaction of fire and grazing, traditional rangeland management that promotes homogeneity, including annual dormant-season burning combined with early-intensive grazing, might be partly responsible for these declines, especially in some regions of the Great Plains, USA. Recently, an alternative grassland management practice known as patch-burning has been promoted as a means of restoring heterogeneity to grasslands by mimicking the grazing-fire interaction that once occurred on the prairie before European settlement. From 2003 to 2004, we examined effects of patch-burning and traditional management (annual burning followed by early-intensive grazing) on the reproductive success of dickcissels (Spiza americana) in tallgrass prairie in Oklahoma. We monitored 296 dickcissel nests and found that dickcissel nesting phenology differed between traditional and patch-burned pastures. Specifically, dickcissels tended to initiate their nests later in the traditional pasture. Mean number of eggs laid and fledglings produced were similar between the treatments, but nest densities were higher in traditional pastures. Predation was the predominant cause of nest failure and was higher in traditional pastures than in patch-burned pastures. Brown-headed cowbird (Molothrus ater) parasitism was higher in traditional pastures than in patch-burned pastures. Overall, dickcissel nest success was higher in patch-burned pastures than in traditional pastures. The positive response of dickcissel nest success to patch-burn management provides further evidence that this practice can be a useful tool for grassland bird conservation. By creating a mosaic of different stature vegetation, patch-burn management enhances productivity of grassland bird species by providing a refuge area in the unburned patches that affords dickcissels and other nesting grassland birds some protection from the direct (e.g., trampling) and indirect (e.g., cowbird parasitism and predation) effects of grazing, which are not available under traditional management. Patch-burn management should be encouraged as a conservation strategy for grassland birds throughout the Great Plains.     

Interim Responses of Floodplain Wetland Vegetation to Phase I of the Kissimmee River Restoration Project: Comparisons of Vegetation Maps from Five Periods in the River's History

Phase I of the Kissimmee River Restoration Project (KRRP) reestablished intermittent inundation of the river's floodplain by backfilling 12 km of the C‐38 flood control canal in 2001. We compared floodplain vegetation maps based on 2003 and 2008 aerial imagery (2 and 7 years following completion of Phase I, respectively) to vegetation maps from 1954 (pre‐channelization), 1974 (3 years after channelization), and 1996 (25 years after channelization) to evaluate broad‐scale vegetation responses to Phase I restoration. Results indicate that the extent of wetland plant communities expanded rapidly, more than doubling in area within 2 years after completion of Phase I, and that by 2008 wetlands had nearly recovered to pre‐channelization levels. However, full reestablishment of the pre‐channelization wetland mosaic has not yet occurred. Prior to channelization, much of the floodplain was dominated by a broadleaf marsh (BLM) community associated with extended, deep annual flooding, while shorter‐hydroperiod communities dominated the floodplain in 2003 and 2008. Prior to restoration construction, the reestablishment of BLM was predicted to be slow because suitable hydrology is dependent on project components that will not be in place until all restoration components are completed (projected for 2019). Hydrologic data indicate that the duration and variability of floodplain inundation have not yet achieved restoration targets over the entire Phase I study area. Other factors affecting vegetation responses are likely involved, including the age and viability of soil seed banks, the rarity of relict propagule sources following the channelized period, and competition from an invasive wetland shrub species.     

Prairie plant phenology driven more by temperature than moisture in climate manipulations across a latitudinal gradient in the Pacific Northwest,USA

Plant phenology will likely shift with climate change, but how temperature and/or moisture regimes will control phenological responses is not well understood. This is particularly true in Mediterranean climate ecosystems where the warmest temperatures and greatest moisture availability are seasonally asynchronous. We examined plant phenological responses at both the population and community levels to four climate treatments (control, warming, drought, and warming plus additional precipitation) embedded within three prairies across a 520 km latitudinal Mediterranean climate gradient within the Pacific Northwest, USA. At the population level, we monitored flowering and abundances in spring 2017 of eight range‐restricted focal species planted both within and north of their current ranges. At the community level, we used normalized difference vegetation index (NDVI) measured from fall 2016 to summer 2018 to estimate peak live biomass, senescence, seasonal patterns, and growing season length. We found that warming exerted a stronger control than our moisture manipulations on phenology at both the population and community levels. Warming advanced flowering regardless of whether a species was within or beyond its current range. Importantly, many of our focal species had low abundances, particularly in the south, suggesting that establishment, in addition to phenological shifts, may be a strong constraint on their future viability. At the community level, warming advanced the date of peak biomass regardless of site or year. The date of senescence advanced regardless of year for the southern and central sites but only in 2018 for the northern site. Growing season length contracted due to warming at the southern and central sites (~3 weeks) but was unaffected at the northern site. Our results emphasize that future temperature changes may exert strong influence on the timing of a variety of plant phenological events, especially those events that occur when temperature is most limiting, even in seasonally water‐limited Mediterranean ecosystems.     

Vascular Epiphytes on Isolated Pasture Trees Along a Rainfall Gradient in the Lowlands of Panama

Secondary habitats are increasing in importance in tropical countries due to ongoing destruction of pristine vegetation. In spite of the magnitude of current changes, our understanding of their effects on nontrees (e.g., nonvascular or vascular epiphytes) is still very patchy, particularly in lowland habitats. Here, we report a study with isolated pasture trees in southwest Panama. The >800 studied trees, which belonged to >100 different species, harbored almost 27,000 epiphytes of 83 species. Orchidaceae was the most species‐rich family, with almost 60 percent of all species, while Bromeliaceae were most abundant. A rainfall gradient in the study region from ca 1000 to >3000 mm explained more of the variation in species abundance and richness than host characteristics (e.g., species identity, tree size). The unexpectedly large number of epiphytes in these pastures still represents a substantial change relative to a natural setting, which is suggested by a comparison with a forest inventory under similar climatic conditions. In pastures, species richness was lower as deduced from individual‐based rarefaction curves, a larger proportion of species and individuals showed crassulacean acid metabolism, and the relationship of epiphyte abundance/species richness and tree diameter was much less steep. Even the already reduced diversity, however, may be only transient in secondary habitats—the long‐term persistence of epiphyte populations in pastures is an open question and has to be addressed by repeated monitoring to fully evaluate the significance of pasture trees for the conservation of vascular epiphytes in tropical lowlands.     

Influence of Aboveground Biomass Removal on Nitrogen Mineralization in a Restored Tallgrass Prairie

Prescribed burning in prairies influences soil nitrogen (N), which is the primary nutrient that limits plant growth and is an important factor in plant competition and diversity. The primary objective of the experiment described here was to better understand the changes in net N mineralization that occur after a fire. We compared soil properties after a fire with those following vegetation removal by mowing and raking in a restored prairie in southeastern Minnesota. The treatments occurred in the spring of two consecutive years. Calcium oxide, burnt lime, was added to some of the raked plots in the first year to mimic the deposition of basic cations during a fire, which cause an increase in soil pH. Aboveground biomass removal by raking or by burning had similar effects on soil moisture, temperature, and inorganic N. The removal treatments caused warmer and drier soil than in the untreated plots. The change in net N mineralization after raking was unaffected by the addition of lime. In the first year, with low rainfall, removal caused net N mineralization rates similar to those in the untreated controls, but during the second year, with heavy rainfall, net N mineralization rates were significantly higher after removal. We predict that if water is sufficient, increased soil temperature after biomass removal will increase soil microbial activity and net N mineralization, but during drought, water will limit microbial activity. Furthermore, depending on soil N concentrations, which are very high at this study site, altered soil microbial activity will have variable effects on net N mineralization.     

Habitat Selection by Prairie Dogs in a Disturbed Landscape at the Edge of Their Geographic Range

ABSTRACT Although habitat attributes of black-tailed prairie dog (Cynomys ludovicianus) colonies have been described for central and northern portions of the species' geographic range, little is known about these associations at the southern edge of this species' distribution. Because high-quality habitats are expected to be scarcer at the edge of the species' geographic range, different patterns of habitat selection might emerge in these populations. We analyzed habitat selection by black-tailed prairie dogs in a human-disturbed mosaic of desert grasslands and shrublands in northwestern Chihuahua, Mexico. We contrasted 151 used and 133 unused habitat units producing 11 case-control logistic regression models to explain site occupancy by prairie dogs with different combinations of environmental variables. Prairie dogs from Chihuahua occupy sites similar in most respects to sites in more northern regions, although these prairie dogs appear to be more tolerant of increased shrub density and reduced herbage cover. We found that site occupancy was best modeled by positive effects of soil moisture level, cover of forbs, cover of unpalatable vegetation, cover of bare ground, and amount of prairie-dog colony area within 1 km and by the inverse of altitude, shrub density, herbage height, and amount of hostile habitat within 1 km. The 2 most significant variables were herbage height and shrub density, which might reflect the prominent role that visibility plays in habitat selection by prairie dogs. In contrast, we found weak evidence that human features have significant impacts on site occupancy by prairie dogs. Our results support the prediction that environmental conditions of sites used by prairie dogs in edge regions partially differ from those observed in more northern latitudes. We suggest that reserve managers focus conservation efforts on areas with short vegetation, low density of shrubs, and high herbage cover, conditions that could be promoted by controlled burns, herbage mowing, and mechanical removal of shrubs.     

Modeling the Impact of Exotic Annual Brome Grasses on soil Organic Carbon Storage in a Northern Mixed-Grass Prairie

Annual brome grasses, Bromus japonicus and B. tectorum, are common invaders of northern mixed-grass prairie, and have been shown to alter the structure and function of prairie ecosystems, including plant biomass production and litter decomposition. To build on previous findings, our objective was to model the impact of annual brome grasses on soil organic carbon storage as a step towards forecasting ecological change. Specifically, we measured differences in carbon storage between patches dominated by annual bromes and perennial grasses, in addition to evaluating key plant functional characteristics that impact carbon storage. Using the CENTURY model, we simulated high- and low-brome vegetation based on differences in functional characteristics, allowing us to extrapolate the findings from the field study across a broader time scale. We sampled a prairie site in 1996 and 1997 to quantify differences between the high- and low-brome cover plots. High-brome plots averaged 40% brome cover, while the low-brome plots averaged 1% brome cover. We found differences in functional attributes for growth characteristics and litter quality, as well as minor differences in edaphic variables between the plots. Based on field measurements, more soil organic carbon was stored under high-brome vegetation than low-brome, but the differences were not statistically significant. Results from model simulations were consistent with field measurements, and suggested that this prairie ecosystem was not significantly impacted by the functional differences between high- and low-brome vegetation for the first 50 years after the brome invasion under historical management and climate. However, the model results also showed that the differences in soil organic carbon storage continue to diverge after 50 years and consequently could be significant in the future.     

Successional Position of Dry Andean Dwarf Forest Species as a Basis for Restoration Trials

The successional affinity of nine woody species was inferred from the structure, diversity and disturbance history of the vegetation where these occurred. This was done in order to obtain a basis for a restoration experiment, currently in execution, in the dry Andean dwarf forest zone on the edge of the High Plain of Bogotá (Colombia), at 2600–2950 m.a.s.l. We laid out 101 relevees in grassland and shrubland types in different stages of recovery, and in relatively little disturbed endemic Condalia thomasiana dwarf forest. The disturbance history of sites over the last ~60 years was inferred from aerial photograph series (1941–1991). CCA and logistic regression were applied to relate species composition to diversity, environment and disturbance history. All species showed a preference for certain structural groups. Also, a clear relation between species occurrence and vegetation diversity was found. Baccharis macrantha, and Dalea coerulea appeared relatively tolerant to grazing, while the remaining seven species reacted negatively. Soil clay content, base availability and organic carbon content was also an important factor for occurrence of each species. Invasion of grasslands by woody species is pioneered by Baccharis macrantha and followed by Dodonaea viscosa. Dalea coerulea was predominantly found on truncated clayey soils, which will probably not support Condalia dwarf forest. The hypothesized classification of the nine planted species to either pioneers or late-successional was fine-tuned. This exploratory study will be of use in the set-up of future succession-based restoration experiments, and for converting exotics afforestations to natural vegetation.     

An Investigation into the Effect of Soil and Vegetation on the Successful Creation of a Hay Meadow on a Clay‐Capped Landfill

This paper investigates the effect that manipulation of soil and vegetation conditions has on plant community development during attempts to create neutral hay meadow communities on a clay‐capped landfill in Somerset, United Kingdom. The objectives are (1) to determine the effect of manipulation of soil and vegetation on the development of the target plant community, (2) to identify whether these treatments had an effect on edaphic factors (physical and chemical properties, earthworm populations), and (3) to establish which, if any, of these edaphic parameters are underlying factors in determining the vascular plant community composition. In 2001 a commercial hay meadow seed mix was sown on three substrate treatments: (i) bare clay, (ii) a mixture of topsoil and compost ameliorant in equal proportions, and (iii) over naturally colonized vegetation. Plant community development and edaphic factors were monitored between 2001 and 2007. Although initially the presence of ameliorant promoted germination and growth of seeded species, after 2004 nonseeded competitive grasses dominated the sward. Where ameliorant was removed the target community continued to develop proving this to be the most successful treatment. This was found to be a due to suppression of competition from Elytrigia repens (Common couch), which grew most abundantly on soils with high organic matter and high soil water levels within the soil. Contrary to previous research, Trifolium repens (white clover) was not found to reduce forb diversity in this study. No evidence was found of earthworm abundance affecting plant community development.     

Nurse Trees as a Forest Restoration Tool for Mixed Plantations: Effects on Competing Vegetation and Performance in Target Tree Species

Multi‐species mixed plantations can be designed to meet social, economic, and environmental objectives during forest restoration. This paper reports results from an experiment in southern Sweden concerning the influence of three different fast growing nurse tree species on the cover of herbaceous vegetation and on the performance of several target tree species. After 10 years, the nurse trees had reduced the competing herbaceous vegetation but the effect was weak and it may take more than a decade to achieve effective vegetation control. The nurse tree species Betula pendula and Larix x eurolepis did improve stem form in some target tree species, but had a minor effect on survival and growth. The open conditions before crown closure of nurse trees strongly influence seedling performance and so delayed planting of target tree species may provide a means to avoid those conditions. Survival and growth differed greatly among the tree species. Besides the two nurse tree species mentioned above, high survival was found in Picea abies and Quercus robur and intermediate survival in Fagus sylvatica, Tilia cordata, and in the N‐fixing nurse tree Alnus glutinosa. Survival was low in the target tree species Fraxinus excelsior L. and Prunus avium. For restoration practitioners, our results illustrate the potential of using nurse trees for rapidly building a new forest structure and simultaneously increase productivity, which might be a cost‐effective strategy for forest restoration.     

Emergence of the Introduced Grass Agropyron cristatum and the Native Grass Bouteloua gracilis in a Mixed-grass Prairie Restoration

Abstract Prairie restoration at the northern edge of the Great Plains can be frustrated by previously established non‐native perennial grasses. We compared the emergence of a widely introduced grass, Agropyron cristatum, and a common native grass, Bouteloua gracilis, in a 4‐year‐old field experiment in which the Agropyron‐dominated vegetation had either been left intact or treated annually with herbicide. This was done at two levels of water supply, reflecting conditions expected in wet and dry years, to examine the effects of among‐year variability in precipitation. Water addition significantly increased the emergence of both surface‐sown and buried (1 cm deep) seeds. Herbicide treatment of neighbors did not increase the emergence of experimentally added seeds. Emergence was much greater for buried (80%) than surface‐sown seeds (20%). Significantly more Bouteloua than Agropyron germinated from experimentally buried seeds. Whereas only a single seedling of Bouteloua emerged from the existing seed bank, the mean density of Agropyron seedlings emerging from the seed bank was 930/m² (range, 0 to 6,455/m²). Surprisingly, the emergence of Agropyron from the seed bank was not decreased by 4 years of herbicide treatment, possibly because herbicide may release Agropyron from intraspecific competition and allow increased seed production to compensate for decreased plant abundance. In summary, we found few differences between Agropyron and Bouteloua in spring and summer emergence at high or low water availability. The persistence of Agropyron stands despite repeated herbicide application may be partly due to increased seed production.