Spontaneous and initiated succession on unvegetated slopes in the abandoned lignite‐mining area of Goitsche,Germany

Abstract. In this study, we examined the colonization of unvegetated, dry slopes in the lignite‐mining area of Goitsche, Germany. The plots, characterized by different habitat conditions, were studied from 1994–2000. The vegetation development on treated plots was compared to untreated plots showing spontaneous succession. For initial treatments we used fresh plant clippings from a species‐rich sandy grassland (Armerion elongatae) mowed at the end of July. Soil seed bank samples, taken at the beginning of the experiments and cultivated for 18 months, confirmed that the vegetation development on the slopes started with primary succession. Because seed rain is considered to be an important factor in primary succession, we also studied the diaspore input during the first year. The hospitable Quaternary substrate of Site I (pH = 4.1–4.7, 92 % sand) supports fast vegetation development on treated and untreated plots. On treated plots, we have identified plant assemblages similar to initial stages of the Armerion elongatae community. On control plots, Coryne‐phorion communities have established spontaneously. At Site II (mixed Quaternary and Tertiary substrate, pH = 3.0–3.5,40 % sand) the total cover and number of psammophytic species was low. Treated plots showed development towards ruderal sandy grassland, but the establishment of Calamagrostis epigejos would eventually lead to monodominant stands. Untreated control plots showed basically the same pattern, however the development was slower. At Site III (pH = 1.8–2.8, 34 % sand), the hostile Tertiary substrate impeded the succession on both treated and untreated plots because low pH and therefore high amounts of Al³⁺ produced elemental toxicity to plants. The application of plant clippings accelerated the vegetation development in sites with a pH exceeding 3. On plots with the initial treatment, vegetation cover and total number of species were higher than on untreated plots. The application of fresh plant clippings from areas with similar habitat conditions appears to be a viable alternative to traditional restoration methods.     

An Evaluation of Reclamation Success on Idaho's Phosphate Mines

To evaluate reclamation success on the Wooley Valley phosphate mine in southeastern Idaho, we compared vegetation structure and soil physical, chemical, and elemental properties of several different reclamation treatments with those of a nearby reference area (a native Artemisia tridentata vaseyana/Festuca idahoensis association) after 14 years. Vegetation data had been collected four years after reclamation, and we were able to compare differences in biomass and species composition between dates on the reclaimed area. Four years after reclamation there were no differences in total biomass between topsoil or spoil or between seed only, seed + mulch, or control treatments on the different soil types. Most treatments were dominated by seeded perennial grasses. Fourteen years after reclamation there were no differences in biomass or cover between spoil and topsoil plots, but on spoil plots the seeded and mulched treatment had higher total biomass and vegetation cover than on control or seed-only treatments. The seeded perennial legume Medicago sativa was codominant with the seeded forage grasses on all of the treatments. High initial fertilization rates probably facilitated the early establishment and dominance of the forage grasses; once nutrient levels, especially nitrogen, began to decline, the legume increased in abundance. Similarity between the reclaimed area and the reference or native area was low. Reclaimed treatments had higher biomass but lower species richness. The topsoil and spoil plots had similar soil texture, bulk density, pH, cation exchange capacity, electrical conductivity, and phosphorus. Differences in organic carbon, total nitrogen, carbon: nitrogen ratios, and available moisture were related more to treatments than to soil type. High biomass and, thus, litter input on the seed + mulch treatment on spoil plots resulted in both higher OC and TN than any on other soil/treatment combination. The reclaimed area had lower OC, TN, and available moisture than did the reference area on all but seed + mulch spoil plots. Bulk density was higher on reclaimed plots. The long-term differences observed between the reclaimed and reference areas parallel those obtained for other western reclamation sites. Although successional trajectories depend on the attribute measured, similarity to native reference areas depends on the initial reclamation methods. We discuss reclamation methods that would increase the structural and functional similarity of reclaimed and reference areas on the Wooley Valley phosphate mine.     

Short‐Term and Long‐Term Effects of Soil Ripping,Seeding, and Fertilization on the Restoration of a Tropical Rangeland

Rangeland degradation is a serious problem in semiarid Africa. Extensive areas of bare, compacted, nutrient‐poor soils limit the productivity and biodiversity of many areas. We conducted a set of restoration experiments in which all eight combinations of soil tilling, fertilization, and seeding with native perennial grasses were carried out in replicated plots. After 6 months, little aboveground biomass was produced in plots without tilling, regardless of seeding or fertilization. Tilling alone tripled plant biomass, mostly of herbaceous forbs and annual grasses. Perennial grasses were essentially limited to plots that were both tilled and seeded. The addition of fertilizer had no significant additional effects. After 7 years, vegetation had declined, but there were still large differences among treatments. After 10 years, one tilled (and seeded) plot had reverted to bare ground, but the other tilled plots still had substantial vegetation. Only one seeded grass (Cenchrus ciliaris) was still a contributor to total cover after 10 years. We suggest that restoration efforts on these soils be directed first to breaking up the surface crust, and second to the addition of desirable seed. A simple ripping trial inspired by this experiment showed considerable promise as a low‐cost restoration technique.     

Field responses to added organic matter, arbuscular mycorrhizal fungi, and fertilizer in reclamation of taconite iron ore tailing

A three season study was conducted to determine the effect of added composted yard waste, arbuscular mycorrhizal (AM) fungi, and fertilizer on plant cover, standing crop biomass, species composition, AM fungal infectivity and spore density in coarse taconite iron ore tailing plots seeded with a mixture of native prairie grasses. Plant cover and biomass, percent seeded species, mycorrhizal infectivity and spore density were greatly increased by additions of composted yard waste. After three seasons, total plant cover was also greater in plots with added fertilizer. Third season plant cover was also greater in plots amended with the higher rate (44.8 Mg ha^{–1) of compost than the moderate rate (22.4 Mg ha-1}). Field inoculation with AM fungi also increased plant cover during the second season and infectivity during the first two seasons. Seeded native species, consisting mostly of the cover species Elymus canadensis, dominated plot vegetation during the second and third seasons. Dispersal of AM fungal propagules into nonmycorrhizal plots occurred rapidly and increased infectivity in compost-amended plots during the third season. In plots with less than 10% plant cover, AM fungal infectivity of inoculated plots was greatly reduced after the second season. The high level of plant cover and the trend of increasing proportion of mycorrhizal-dependent warm-season grasses, along with increases in infectivity, forecast the establishment of a sustainable native grass community that will meet reclamation goals.     

Exotic Grass Competition in Suppressing Native Shrubland Re-establishment

Disturbance of coastal sage scrub in southern California has led to extensive displacement of native shrubs by exotic annual grasses. The initial conversion from shrubland to exotic grassland is typically associated with disturbance caused by intense grazing, high fire frequency, or mechanical vegetation removal. While native shrubs have been shown to recolonize annual grasslands under some conditions, other annual grasslands are persistent and show no evidence of shrub recolonization. This study examined the mechanisms by which annual grasses may exclude native shrubs and persist after release from disturbance. Grass density was manipulated in experimental plots to achieve a series of prescribed densities. Artemisia californica, a dominant native shrub, was seeded or planted into the plots and responses to the grass density treatments were measured over two growing seasons. A. californica germination, first season growth, and survival were all negatively related to the density of neighboring annual grasses. The most probable mechanism underlying the reduction of first season growth and survival was depletion of soil water by the grasses. The effects of the grasses on A. californica were no longer significant in the second season. The results of this study indicate that Mediterranean annual grasses reduce recruitment and can persist by inhibiting post-disturbance establishment of A. californica from seed. Although succession alone may not return disturbed annual grasslands to their former shrubland composition, the results suggest that restoration can be achieved by using container plantings or grass removal followed by seeding.     

Effects of cover soil stockpiling on plant community development following reclamation of oil sands sites in Alberta

Stockpiling of cover soil can influence vegetation development following reclamation. Cover soil, comprising the upper 15–30 cm of the surface material on sites scheduled for mining, is commonly salvaged prior to mining and used directly or stockpiled for various lengths of time until it is needed. Salvaging and stockpiling causes physical, chemical, and biological changes in cover soils. In particular, stockpiling reduces the availability and vigor of vegetative propagules and seed, and can lead to increases in the abundance of some weedy species. This study uses data from monitoring plots to assess how stockpiling of cover soil impacts plant community development on reclaimed oil sands mine sites in northern Alberta. Development of plant communities differed distinctly between directly placed and stockpiled cover soil treatments even 18 years after reclamation. Direct placement of cover soil resulted in higher percent cover, species richness, and diversity. Nonmetric multidimensional scaling and multiresponse permutation procedure revealed compositional differentiation between the treatments. Indicator species analysis showed that direct placement treatment was dominated by perennial species while grasses and annual forb species dominated sites where stockpiled soil was used. Results indicate that stockpiling leads to slower vegetation recovery while direct placement of cover soil supports more rapid succession (from ruderal and annual communities to perennial communities). In addition, direct placement may be less costly than stockpiling. However, scheduling of salvage and placement remains a challenge.     

Effects of Nutrient Manipulations and Grass Removal on Cover,Species Composition,and Invasibility of a Novel Grassland in Colorado

Cover and richness of a 5‐year revegetation effort were studied with ,respect to small‐scale disturbance and nutrient manipulations. The site, originally a relict tallgrass prairie mined for gravel, was replanted to native grasses using a seed mixture of tall‐, mixed‐, and short‐grass species. Following one wet and three relatively dry years, a community emerged, dominated by species common in saline soils not found along the Colorado Front Range. A single species, Alkali sacaton (Sporobolus airoides), composed nearly 50% of relative vegetation cover in control plots exhibiting a negative relationship between cover and richness. Seeded species composed approximately 92% of vegetation cover. The remaining 8% was composed of weeds from nearby areas, seed bank survivors, or mix contaminants. Three years of soil nutrient amendments, which lowered plant‐available nitrogen and phosphorus, significantly increased relative cover of seeded species to 97.5%. Fertilizer additions of phosphate enhanced abundance of introduced annual grasses (Bromus spp.) but did not significantly alter cover in control plots. Unmanipulated 4‐m² plots contained an average of 4.7 planted species and 3.9 nonplanted species during the 5‐year period, whereas plots that received grass herbicide averaged 5.4 nonplanted species. Species richness ranged from an average 6.9 species in low‐nutrient, undisturbed plots to 10.9 species in the relatively high‐nutrient, disturbed plots. The use of stockpiled soils, applied sparingly, in conjunction with a native seed mix containing species uncommon to the preexisting community generated a species‐depauperate, novel plant community that appears resistant to invasion by ruderal species.     

The influence of aspect on the early growth dynamics of hydroseeded species in coal reclamation areas

Question: Does aspect affect hydroseeding success and the development of vegetation during early vegetation establishment on the steep slopes of coal wastes during the reclamation process? Location: Open‐pit coal mine near Villanueva de la Peña, northern Spain. Methods: In the first year after hydroseeding, we monitored the dynamics of hydroseeded species in three permanent plots of 20 m² on north‐ and south‐facing slopes every two months. Soil properties and weather conditions were also monitored. Results: Aspect was related to total plant cover during early revegetation, and south‐facing slopes had the lowest cover. Aspect also influenced the early dynamics of hydroseeded grasses and legumes establishing on these slopes. Grass cover was greater on the north slope throughout the study, but differences in plant cover between north and south slopes appeared later for the legumes. Aspect also affected the relative contribution of both of grasses and legumes to the total plant cover, with grasses dominant on both northern and southern slopes, except during the summer on the southern slope. The species with the greatest difference in cover between the north‐ and south‐facing slopes were Festuca spp., Lolium perenne and Trifolium repens. Conclusion: In coal mine reclamation areas of Mediterranean climates, differences in the development of hydroseeded species depended on the slope of the coal mine reclamation areas, and this information is of importance to managers in selecting species for use in reclamation.     

Management impacts on vegetation dynamics of hyper‐eutrophicated fields at Berlin,Germany

Abstract. This paper reports on vegetation dynamics on terrestrial, temperate grassland sites at the upper range of the productivity scale, i.e. on abandoned sewage fields (fields once used for waste water disposal) at Berlin‐Blankenfelde, Germany. I studied regeneration and the influence of different management practices (removal of top soil and mowing in late summer). Changes in species composition and cover were followed on permanent plots of 2m × 2m size through five years of vegetation development. At the outset of the experiment the abandoned fields were dominated by dense Urtica dioica /Elymus repens stands. Species richness was 7 species/ 4m², and it remained low on unmanaged plots during the time of observation (7.6 species/plot in year 5). Removal of 20 cm of top soil caused a severe decline of Urtica and a large increase in species richness (21 species in year 1 after disturbance). Mowing was slightly higher compared with unmown plots on both initially excavated and unexcavated plots. Total cover was always near 100 % (except immediately after top soil removal). Colonization of bare soil was very rapid and in late summer of the first year after disturbance cover already increased towards 100%. On all plots the vegetation was mostly dominated by perennial herbs and grasses. Winter season gaps are occupied by Galium aparine, a large‐seeded annual scrambling climber. Monocarpic perennials behaved as winter annuals in most cases. Woody species were inhibited by dense above‐ground biomass and litter cover. The paper questions whether succession on abandoned sewage fields may proceed towards a woodland stage and advises how vegetation of such hyper‐eutrophicated sites can be managed towards higher diversity.     

Secondary succession is influenced by belowground insect herbivory on a productive site

We investigated the effects of insect herbivory on a plant community of a productive old-field community by applying foliar and soil insecticides in a full factorial design. During the first 3 years of succession, insecticide treatments had only minor effects on total cover abundance and species richness. However, species ranking within the plant community was strongly affected by soil insecticide but not by foliar insecticide. Creeping thistle, Cirsium arvense , dominated the experimental plots with reduced root herbivory, while square-stemmed willow-herb, Epilobium adnatum , dominated the control and the plots with foliar insecticide. When soil insecticide was applied, cover abundance of monocarpic forbs increased and cover abundance of polycarpic herbs decreased compared to the control. However, this effect was due to a few abundant plant species and is not based on a consistent difference between life history groups. Instead, application of soil insecticide promoted persistence of species that established at the start of succession, and suppressed species that established in the following years. We conclude that below-ground herbivory reduces competitive ability of resident species and, thus, facilitates colonization by late-successional species. Hence, soil insects can exert strong top-down effects on the vegetation of productive sites by affecting dominant plant species and altering competitive balances.     

Droughting a megadrought: Ecological consequences of a decade of experimental drought atop aridification on the Colorado Plateau

Global dryland vegetation communities will likely change as ongoing drought conditions shift regional climates towards a more arid future. Additional aridification of drylands can impact plant and ground cover, biogeochemical cycles, and plant–soil feedbacks, yet how and when these crucial ecosystem components will respond to drought intensification requires further investigation. Using a long-term precipitation reduction experiment (35% reduction) conducted across the Colorado Plateau and spanning 10 years into a 20+ year regional megadrought, we explored how vegetation cover, soil conditions, and growing season nitrogen (N) availability are impacted by drying climate conditions. We observed large declines for all dominant plant functional types (C₃ and C₄ grasses and C₃ and C₄ shrubs) across measurement period, both in the drought treatment and control plots, likely due to ongoing regional megadrought conditions. In experimental drought plots, we observed less plant cover, less biological soil crust cover, warmer and drier soil conditions, and more soil resin-extractable N compared to the control plots. Observed increases in soil N availability were best explained by a negative correlation with plant cover regardless of treatment, suggesting that declines in vegetation N uptake may be driving increases in available soil N. However, in ecosystems experiencing long-term aridification, increased N availability may ultimately result in N losses if soil moisture is consistently too dry to support plant and microbial N immobilization and ecosystem recovery. These results show dramatic, worrisome declines in plant cover with long-term drought. Additionally, this study highlights that more plant cover losses are possible with further drought intensification and underscore that, in addition to large drought effects on aboveground communities, drying trends drive significant changes to critical soil resources such as N availability, all of which could have long-term ecosystem impacts for drylands.     

Effects of reclamation measures on plant colonization on lignite waste in the eastern Pyrenees,Spain

Abstract. This paper deals with vegetation establishment on waste dumps in the montane and submontane belts of the eastern Pyrenees, where submediterranean Quercus humilis forests, Pinus sylvestris forests and mesoxerophilous pastures make up most of the landscape; these are considered as target communities for reclamation. The waste consists of marl and lime regolith, very poor in nutrients and structure (Poch et al. 1989). The dumps were terraced in 1985–1986; some were left for spontaneous re‐colonization and others were reclaimed in various ways. In 1992 we took 36 vegetation samples on the dumps, based on the point quadrat procedure, to evaluate the colonization status and the differences between reclamation techniques. The samples were analysed on the basis of their species composition (ecological groups and multivariate analysis) and structural aspects (coverage, diversity, etc.). Reclamation treatment results varied widely, most of the dumps showing a low degree of naturalness. Colonizing vegetation ranged from a mixture of opportunist and stress‐resistant taxa, forming poorly covered surfaces on the dumps where colonization has been poor, to dense grasslands dominated by one (or a few) introduced competitive grass species, where reclamation procedures had been intensive. Sowing treatments, where pasture species were sown onto dumps, produced intermediate results, as resulting vegetation cover was similar to control plots and naturalness was low, but in a few cases they yielded more interesting swards, fairly dense and diverse, and including high numbers of spontaneous species. Choice of sown species and proximity to undisturbed vegetation accelerate succession.     

Using Precision Prairie Reconstruction to Drive the Native Seeded Species Colonization Process

Restoring forb richness to the northern tallgrass prairie (U.S.A.) is often problematic. A potential solution is the establishment of native forb‐seeded patches that can serve as colonization sites. This study was designed to determine the following: (1) the success at which native forbs sown in small patches can colonize the surrounding vegetation matrix and (2) whether soil amendments (C additions, P fertilization, and seed bank reduction) applied to the seeded sites can facilitate such colonization (patch quality). Colonization was investigated at (1) the immediate surrounding (1 m belt transect, BT) of the seeded sites and (2) in disturbed but not seeded patches located 3 m from the seeded site (out‐plots). Soil amendments did not affect colonization, but native forb density in the BT and out‐plots was correlated to the density of forbs in the seeded sites (r² = 0.3, p < 0.01). Seeded native forb density in the out‐plots was higher than in the adjacent BT (19 vs. 5 plants/m²), suggesting that competition from the matrix vegetation may be more important in forb colonization than distance from the source. Taller forbs with larger seeds showed advantages in colonization success. This study showed that the colonization of matrix vegetation with native forbs from seeded patches is a viable method for prairie reconstruction but requires the availability of disturbed sites. In most prairies, animal‐generated soil disturbances are common. In their absence, disturbances of the vegetation matrix will need to be added to the management plan.     

Recovery of pristine boreal forest floor community after selective removal of understorey, ground and humus layers

In boreal spruce forests that rarely experience extensive disturbances, fine-scale vegetation gaps are important for succession dynamics and species diversity. We examined the community implications of fine-scale gap disturbances by selective removal of vegetation layers in a pristine boreal spruce forest in Northern Finland. The aim was to investigate how the speed of recovery depends on the type of disturbance and the species growth form. We also wanted to know if there appeared changes in species composition after disturbance. Five different treatments were applied in the study: Control, removal of the ground layer (bryophytes and lichens), removal of the understorey layer (dwarf shrubs, herbs and graminoids), removal of both the ground and understorey layers, and complete removal of the vegetation and humus layers above the mineral soil. The vegetation recovery was monitored in terms of cover and species numbers over a 5-year period. Understorey layer cover, composed mainly of clonal dwarf shrubs, recovered completely in 4 years in treatments where the humus layer remained intact, whereas ground layer cover did not reach the control level in plots from where bryophytes and lichens were removed. Recovery was faster in terms of species number than species cover. Bryophytes, graminoids and dominant dwarf shrubs appeared in all disturbed plots quickly after disturbance. Seedlings of trees appeared exclusively in disturbed plots. Graminoids dominated after the removal of humus layer. The results indicate that the regeneration of forest floor after small gap disturbance occurs mainly by re-establishment of the dominant species. Although destruction of the humus layer leaves a long-lasting scar to the forest floor, exposing of mineral soil may enhance the sexual reproduction of dominant species and the colonization of weaker competitors.     

The effects of arbuscular mycorrhizal fungal inoculation at a roadside prairie restoration site

Arbuscular mycorrhizal fungi (AMF) may play an important role in ecological succession, but few studies have documented the effectiveness of mycorrhizal inoculation at restoration/reclamation sites. At a roadside prairie restoration in Shakopee, Minnesota, we compared AMF root colonization and resulting vegetative cover among four inoculation treatments. After 15 mo of growth, we found that AMF colonization was high in all treatments but was significantly higher in treatments that received AMF inoculum propagated from a local prairie site or commercially available inoculum than the uninoculated control. For the prairie inoculum, this increase in colonization occurred whether the inoculum was applied with seeds in furrows or broadcast with seeds on the soil surface. However, increased colonization did not discernibly affect the restored vegetation; neither total vegetative cover nor the proportion "desired" prairie vegetation differed among inoculation treatments. By the end of the third growing season (27 mo after planting) there were no longer differences in AMF colonization among the inoculation treatments nor were there differences in vegetative cover. It is likely that natural recolonization of the plots by remnant AMF populations at the site limited the duration of the inoculation effect. This natural recolonization, in combination with relatively high soil phosphorus levels, likely rendered inoculation unnecessary. In contrast to previous published studies of AMF inoculation in landscape restorations, this study shows that AMF inoculation may not be warranted under some circumstances.     

Sawdust Addition Reduces the Productivity of Nitrogen‐Enriched Mountain Grasslands

Anthropogenic nutrient enrichment of mountain grasslands has boosted grasses and fast‐growing unpalatable plants at the expense of slow‐growing species, resulting in a significant loss in biodiversity. A potential tool to reduce nutrient availability and aboveground productivity without destroying the perennial vegetation is carbon (C) addition. However, little is known about its suitability under severe climatic conditions. Here, we report the results of a 3‐year field study assessing the effects of sawdust addition on soil nutrients, aboveground productivity, and vegetational composition of 10 grazed and ungrazed mountain grasslands. Of particular interest was the effect of C addition on grasses and on the tall unpalatable weed Veratrum album. After 3 years, soil pH, ammonium, and plant‐available phosphorus were not altered by sawdust application, and nitrate concentrations were marginally higher in treatment plots. However, the biomass of grasses and forbs (without V. album) was 20–25% lower in sawdust‐amended plots, whereas the biomass of V. album was marginally higher. Sawdust addition reduced the cover of grasses but did not affect evenness, vegetation diversity, or plant species richness, although species richness generally increased with decreasing biomass at our sites. Our results suggest that sawdust addition is a potent tool to reduce within a relatively short time the aboveground productivity and grass cover in both grazed and ungrazed mountain grasslands as long as they are not dominated by tall unpalatable weeds. The technique has the advantage that it preserves the topsoil and the perennial soil seed bank.     

Yellow Bush Lupine Invasion in Northern California Coastal Dunes I. Ecological Impacts and Manual Restoration Techniques

We studied the ecological effects of the invasion of coastal dunes by Lupinus arboreus (yellow bush lupine), an introduced species, and used the results to develop manual restoration techniques on the North Spit of Humboldt Bay. Vegetation and soil data were collected in five vegetation types representing points along a continuum of bush lupine's invasive influence. We collected data on the number and size of shrubs, vegetation cover, and soil nutrients. One set of plots was subjected to two restoration treatments: removal of lupine shrubs only, or removal of all nonnative vegetation and removal of litter and duff. Treatments were repeated annually for four years, and emerging lupine seedlings were monitored for three years. Prior to treatment, ammonium and nitrate were found to increase along the lupine continuum, but organic matter decreased at the extreme lupine end. Yellow bush lupine was not the most significant variable affecting variation in soil nutrients. After four years, nonnative grasses, including Vulpia bromoides, Holcus lanatus (velvet grass), Bromus spp. (brome), and Aira spp. (European hairgrass), were significantly reduced in those restoration plots from which litter and duff was removed. Native species increased significantly in vegetation types that were less influenced by lupine. By the third year, soil variables differed among vegetation types but not by treatment. Bush lupine seedling emergence was higher, however, in plots receiving the litter and duff removal treatment. Based on these results, we conclude that bush lupine invasion results in both direct soil enrichment and indirect enrichment as a result of the associated encroachment of other nonnative species, particularly grasses. Although treatment did not affect soil nutrients during the period of this study, it did reduce establishment of nonnative grasses and recruitment of new bush lupine seedlings. Restoration should therefore include litter and duff removal. In areas that are heavily influenced by lupine and contain few native propagules, revegetation is also required.     

Plant Community Recovery Following Restoration in Semiarid Grasslands

The Conservation Reserve Program (CRP) is an extensive land use in the United States, which restores cultivated land to perennial vegetation through seeding. Low precipitation and high potential evapotranspiration are major limitations to the establishment and growth of seeded species in semiarid regions. We tested the rate of development of plant functional types across a chronosequence of restored fields using a model of plant succession. We also determined how the seeding of non‐native (introduced) relative to native perennial grasses influenced plant community recovery. In contrast to the native shortgrass steppe (SGS), recently seeded CRP fields had high cover of annuals, forbs, C₃, and introduced species. The seed mix determined which perennial grasses dominated the plant community within 18 years, but slow establishment prolonged early seral stages, allowed for the spread of colonizing perennial grasses, and limited recovery to less than half the canopy cover of undisturbed shortrass steppe. Species density declined in restored fields as seeded perennial grass cover increased and was lower in CRP fields seeded with introduced compared to native perennial grasses. Plant community composition transitioned to C₄ and native species, even if fields were not seeded with these species, and was modified by shifts in the amount and seasonality of precipitation. Thus, in semiarid CRP fields, we found that the potential for recovery depended on time since CRP enrollment, seed mix, and climatic variability. Full recovery, based on similarity to vegetation cover and composition of undisturbed SGS, requires greater than 20 years.     

Current reclamation practices after oil and gas development do not speed up succession or plant community recovery in big sagebrush ecosystems in Wyoming

Reclamation is an application of treatment(s) following disturbance to promote succession and accelerate the return of target conditions. Previous studies have framed reclamation in the context of succession by studying its effectiveness in reestablishing late‐successional plant communities. Reestablishment of plant communities is especially important and challenging in drylands such as shrub steppe ecosystems where succession proceeds slowly. These ecosystems face threats from climate change, invasive species, altered fire regimes, and land‐use change, as well as fossil‐fuel extraction and associated disturbance. As such, the need for effective reclamation after this type of energy development is great. However, past research regarding this type of reclamation has focused on mining rather than oil and gas development. To better understand the effect of reclamation on rates of succession in dryland shrub steppe ecosystems, we sampled oil and gas wellpads and adjacent undisturbed big sagebrush plant communities in Wyoming, U.S.A., and quantified the extent of recovery for forbs, grasses, and shrubs on reclaimed and unreclaimed wellpads relative to undisturbed plant communities. Reclamation increased the recovery rate for early‐successional types, including combined forbs and grasses and perennial grasses, but did not affect recovery rate of late‐successional types, particularly big sagebrush and perennial forbs. Rather, subsequent analyses showed that recovery of late‐successional types was affected by soil texture and time since wellpad abandonment. This is consistent with studies in other ecosystems where reclamation has been implemented, suggesting that reclamation may not help reestablish late‐successional plant communities more quickly than they would reestablish naturally.     

Restoration of Themeda australis swards suppresses soil nitrate and enhances ecological resistance to invasion by exotic annuals

Understanding processes that underlie ecological resistance to weed invasion is critical for sustainable restoration of invaded plant communities. Experimental studies have demonstrated that invasive nitrophilic annuals can be controlled by addition of carbon to reduce soil nitrate concentrations, sometimes leading to enhanced establishment of native plants. However, effects of carbon supplements on soil nitrate are temporary, and the longer-term value of carbon supplementation as a restoration tool is dependent on the resistance of the re-established ecosystem to repeat invasion. We investigated whether re-established swards of the tussock grass Themeda australis (R.Br.) Stapf (a natural understorey dominant in mesic grassy woodlands of SE Australia) could suppress soil nitrate concentrations, and through this or other means, could impart ongoing resistance to exotic invasion in restored woodlands. In a remnant invaded by exotic annuals, we applied three plot treatments (carbon supplements, annual spring burns and untreated control) and two seed treatments (± Themeda seed) in a replicated, factorial design. Within 3 years, successful establishment of Themeda swards on burnt and carbon-supplemented plots was associated with a reduction in soil nitrate to levels comparable with non-invaded, Themeda-dominated reference sites in the region (<3 mg/kg), and significantly reduced exotic cover compared with unseeded plots. By contrast, on plots not seeded with Themeda, soil nitrate increased after cessation of carbon addition and exotic cover returned to levels comparable with untreated control plots, despite a high cover of other native perennial grasses. Few persistent effects of carbon supplements or spring burning on soil nutrients were evident 9–19 months after cessation of these treatments. Results suggest that Themeda is a keystone species that regulates nitrate cycling, thereby imparting ecological resistance to invasion by nitrophilic annuals.