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.     

Long‐term experiment manipulating community assembly results in favorable restoration outcomes for invaded prairies

Invasive species are a common problem in restoration projects. Manipulating soil fertility and species arrival order has the potential to lower their abundance and achieve higher abundances of seeded native species. In a 7‐year experiment in Missouri, United States, we tested how nutrient addition and the timing of arrival of the invasive legume Lespedeza cuneata and seeded native prairie grass and forb species influenced overall community composition. Treatments that involved early arrival of seeded forb and grass species and late arrival of L. cuneata were most successful at creating community structure that fulfilled our restoration goals, displaying high abundance of seeded native forb species, low abundances of L. cuneata, and non‐native species. There were few treatment interactions, with the exception that timing seeded native forbs and timing of L. cuneata arrival interactively influenced the abundance of seeded native forbs. This suggests that the individual treatments are supporting the restoration goals, such as creating a community with low abundance of L. cuneate or high abundance of native seeded species, without restricting each other. This study demonstrates the importance of priority effects in disturbed habitats prone to invasion, the lasting effects of initial seeding on long‐term community composition, and the potential for fertilization to positively benefit restoration of degraded grasslands.     

Dominant Grasses Suppress Local Diversity in Restored Tallgrass Prairie

Warm‐season (C₄) grasses commonly dominate tallgrass prairie restorations, often at the expense of subordinate grasses and forbs that contribute most to diversity in this ecosystem. To assess whether the cover and abundance of dominant grass species constrain plant diversity, we removed 0, 50, or 100% of tillers of two dominant species (Andropogon gerardii or Panicum virgatum) in a 7‐year‐old prairie restoration. Removing 100% of the most abundant species, A. gerardii, significantly increased light availability, forb productivity, forb cover, species richness, species evenness, and species diversity. Removal of a less abundant but very common species, P. virgatum, did not significantly affect resource availability or the local plant community. We observed no effect of removal treatments on critical belowground resources, including inorganic soil N or soil moisture. Species richness was inversely correlated with total grass productivity and percent grass cover and positively correlated with light availability at the soil surface. These relationships suggest that differential species richness among removal treatments resulted from treatment induced differences in aboveground resources rather than the belowground resources. Selective removal of the dominant species A. gerardii provided an opportunity for seeded forb species to become established leading to an increase in species richness and diversity. Therefore, management practices that target reductions in cover or biomass of the dominant species may enhance diversity in established and grass‐dominated mesic grassland restorations.     

Invader to invader facilitation: ice plant mats prompt increased densities and grazing impacts of white garden snails in a Pampean coastal dune system

Non-native species can facilitate other non-native species via their effects on the physical environment. Here, I evaluate whether the ice plant, Carpobrotus edulis, prompts increased densities and grazing impacts of the white garden snail, Theba pisana, in coastal dunes of the Argentinean Pampas, where both species are non-native. Snail densities were higher in ice plant patches than in native graminoid patches over four seasonal samplings. Snail activity and grazing impacts on two co-occurring forbs (the native Hydrocotyle bonariensis and the non-native Calystegia soldanella) were also higher within ice plant patches. Experimental ice plant transplant into graminoid-dominated areas led to increased snail densities and grazing impacts, which demonstrates cause-effect relationships linking ice plant cover with increasing snail densities and grazing damage to forbs. The findings here suggest that ice plant mats provide suitable microclimatic conditions for T. pisana, and thereby facilitate greater snail numbers in ice plant patches leading to increased grazing rates on other plants. On that basis, it can be expected that the cumulative effect of both invaders on co-occurring plants would be larger than their individual effects.

     

Reduced soil compaction enhances establishment of non-native plant species

Many studies have shown that soil disturbance facilitates establishment of invasive, non-native plant species, and a number of mechanisms have been isolated that contribute to the process. To our knowledge no studies have isolated the role of altered soil compaction, a likely correlate of many types of soil disturbance, in facilitating invasion. To address this, we measured the response of seeded non-native and native plant species to four levels of soil compaction in mesocosms placed in an abandoned agricultural field in the Methow Valley, Washington, USA. Soil compaction levels reflected the range of resistance to penetration (0.1–3.0 kg cm⁻²) measured on disturbed soils throughout the study system prior to the experiment. Percent cover of non-native species, namely Bromus tectorum and Centaurea diffusa, decreased by 34% from the least to the most compacted treatments, whereas percent cover of native species, mostly Pseudoroegneria spicata and Lupinus spp., did not respond to compaction treatments. Experimental results were supported by a survey of soil penetration resistance and percent cover by species in 18 abandoned agricultural fields. Percent cover of B. tectorum was negatively related to soil compaction levels, whereas none of the native species showed any response to soil compaction. These results highlight a potentially important, though overlooked, aspect of soil disturbance that may contribute to subsequent non-native plant establishment.     

Patterns of invasion of an urban remnant of a species-rich grassland in southeastern Australia by non-native plant species

Abstract. The invasion by non-native plant species of an urban remnant of a species-rich Themeda triandra grassland in southeastern Australia was quantified and related to abiotic influences. Richness and cover of non-native species were highest at the edges of the remnant and declined to relatively uniform levels within the remnant. Native species richness and cover were lowest at the edge adjoining a roadside but then showed little relation to distance from edge. Roadside edge quadrats were floristically distinct from most other quadrats when ordinated by Detrended Correspondence Analysis. Soil phosphorus was significantly higher at the roadside edge but did not vary within the remnant itself. All other abiotic factors measured (NH₄, NO₃, S, pH and % organic carbon) showed little variation across the remnant. Non-native species richness and cover were strongly correlated with soil phosphorus levels. Native species were negatively correlated with soil phosphorus levels. Canonical Correspondence Analysis identified the perennial non-native grasses of high biomass as species most dependent on high soil nutrient levels. Such species may be resource-limited in undisturbed soils. Three classes of non-native plants have invaded this species-rich grassland: (1) generalist species (> 50 % frequency), mostly therophytes with non-specialized habitat or germination requirements; (2) resource-limited species comprising perennial species of high biomass that are dependent on nutrient increases and/or soil disturbances before they can invade the community and; (3) species of intermediate frequency (1–30 %), of low to high biomass potential, that appear to have non-specialized habitat requirements but are currently limited by seed dispersal, seedling establishment or the current site management. Native species richness and cover are most negatively affected by increases in non-native cover. Declines are largely evident once the non-native cover exceeds 40 %. Widespread, generalist non-native species are numerous in intact sites and will have to be considered a permanent part of the flora of remnant grasslands. Management must aim to minimize increases in cover of any non-native species or the disturbances that favour the establishment of competitive non-native grasses if the native grassland flora is to be conserved in small, fragmented remnants.     

Tallgrass prairie restoration: implications of increased atmospheric nitrogen deposition when site preparation minimizes adventive grasses

Site preparation designed to exhaust the soil seedbank of adventive species can improve the success of tallgrass prairie restoration. Despite these efforts, increased rates of atmospheric nitrogen (N) deposition over the next century could potentially promote the growth of nitrophilic, adventive species in tallgrass restoration projects. We used a field experiment to examine how N addition affected species composition and plant productivity over the first 3 years of a tallgrass prairie restoration that was preceded by the planting of glyphosate‐resistant crops and multiple applications of glyphosate to exhaust the pre‐existing seedbank. We predicted that N addition would increase the percent cover of adventive plant species not included in the original seeding. Contrary to our prediction, only the cover of native species increased with N addition; native non‐leguminous forbs increased substantially, with Conyza canadensis (a weedy native species not part of the restoration seed mix) exploiting the combination of high N and bare ground in the first year, and non‐leguminous forbs (in particular Monarda fistulosa) and native C₃ grasses, all of which were seeded, increasing with N addition by the third year. Native legumes was the only functional group that exhibited lower cover in N addition plots than in control plots. There was no significant response by native C₄ grasses to N addition, and adventive grasses remained mostly absent from the plots. Overall, our results suggest that site pre‐treatment with herbicide may continue to be effective in minimizing adventive grasses in restored tallgrass prairie, despite future increases in atmospheric N deposition.     

Effectiveness of repeated autumn and spring fires for understorey restoration in weed‐invaded temperate eucalypt woodlands

Question. Can strategic burning, targeting differing ecological characteristics of native and exotic species, facilitate restoration of native understorey in weed‐invaded temperate grassy eucalypt woodlands? Location. Gippsland Plains, eastern Victoria, Australia. Methods. In a replicated, 5‐year experimental trial, the effects of repeated spring or autumn burning were evaluated for native and exotic plants in a representative, degraded Eucalyptus tereticornis grassy woodland. Treatments aimed to reduce seed banks and modify establishment conditions of exotic annual grasses, and to exhaust vegetative reserves of exotic perennial grasses. Treatments were applied to three grassland patch types, dominated by the native grass Austrodanthonia caespitosa, ubiquitous exotic annuals, or the common exotic perennial grass Paspalum dilatatum. Results. The dominant native grass Austrodanthonia caespitosa and native forbs were resilient to repeated fires, and target exotic annuals and perennials were suppressed differentially by autumn and spring fires. Exotic annuals were also suppressed by drought, reducing the overall treatment effects but indicating important opportunities for restoration. The initially sparse exotic geophyte Romulea rosea increased in cover with fire and the impact of this species on native forbs requires further investigation. There was minimal increase in diversity of subsidiary natives with fire, probably owing to lack of propagules. Conclusions. While fire is often considered to increase ecosystem invasibility, our study showed that strategic use of fire, informed by the relative responses of available native and exotic taxa, is potentially an effective step towards restoration of weed‐invaded temperate eucalypt woodlands.     

Diversity–invasibility relationships across multiple scales in disturbed forest understoreys

Non-native plant species richness may be either negatively or positively correlated with native species due to differences in resource availability, propagule pressure or the scale of vegetation sampling. We investigated the relationships between these factors and both native and non-native plant species at 12 mainland and island forested sites in southeastern Ontario, Canada. In general, the presence of non-native species was limited: <20% of all species at a site were non-native and non-native species cover was <4% m⁻² at 11 of the 12 sites. Non-native species were always positively correlated with native species, regardless of spatial scale and whether islands were sampled. Additionally, islands had a greater abundance of non-native species. Non-native species richness across mainland sites was significantly negatively correlated with mean shape index, a measure of the ratio of forest edge to area, and positively correlated with the mean distance to the nearest forest patch. Other factors associated with disturbance and propagule pressure in northeastern North America forests, including human land use, white-tailed deer populations, understorey light, and soil nitrogen, did not explain non-native richness nor cover better than the null models. Our results suggest that management strategies for controlling non-native plant invasions should aim to reduce the propagule pressure associated with human activities, and maximize the connectivity of forest habitats to benefit more poorly dispersed native species.     

Effects of Established Perennial Grasses on Introduction of Native Forbs in California

Prairie restoration is not complete without the establishment of both grasses and forbs. However, if desirable forbs and grasses are seeded simultaneously, control of broadleaf weeds is problematic. If possible, a two‐step process of introducing forbs after establishing grasses would allow use of broadleaf‐specific herbicides at the critical early stages of grass growth. We conducted experiments to investigate methods for introducing forbs into previously restored native perennial grasslands on rural roadsides in the Sacramento Valley of California. In one experiment, we studied the effects of background vegetation (established perennial grasses or tilled ground) on seven native forb species planted from seed. In a second experiment, we evaluated the effects of background vegetation (existing perennial grasses or tilled ground) and container size (36 ml or 105 ml) with excavation technique (excavation by core removal [core] or by creating an impression [dibble]) on the growth of transplants of the native perennial forbs Asclepias fascicularis and Sisyrinchium bellum. The presence of established perennial grasses reduced the growth of seeded forbs, but did not affect transplants, indicating the vulnerability of seedling forbs to interference. When compared to control plots that had been tilled in the autumn, weed canopy cover was significantly lower in the presence of perennial grasses if seeded with forbs, but not in the presence of perennial grasses alone. Both transplanted species grew better in the large container/core treatment than the small container/dibble treatment; however, existing grasses eliminated these positive effects. Asclepias fascicularis performed better when grown in large containers than in small containers, but its growth was not affected by excavation method; S. bellum performed better when planted with the core method than the dibble method of excavation, but container size made no difference. We attribute differences in the responses of the species to interactions between phenological differences and expansive clay soils that naturally de‐compact upon drying.     

Native vegetation structure,landscape features and climate shape non-native plant richness and cover in New Zealand native shrublands

Aim

Studies investigating the determinants of plant invasions rarely examine multiple factors and often only focus on the role played by native plant species richness. By contrast, we explored how vegetation structure, landscape features and climate shape non-native plant invasions across New Zealand in mānuka and kānuka shrublands.

Location

New Zealand.

Method

We based our analysis on 247 permanent 20 × 20-m plots distributed across New Zealand surveyed between 2009 and 2014. We calculated native plant species richness and cumulative cover at ground, understorey and canopy tiers. We examined non-native species richness and mean species ground cover in relation to vegetation structure (native richness and cumulative cover), landscape features (proportion of adjacent anthropogenic land cover, distance to nearest road or river) and climate. We used generalized additive models (GAM) to assess which variables had greatest importance in determining non-native richness and mean ground cover and whether these variables had a similar effect on native species in the ground tier.

Results

A positive relationship between native and non-native plant species richness was not due to their similar responses to the variables examined in this study. Higher native canopy richness resulted in lower non-native richness and mean ground cover, whereas higher native ground richness was associated with higher native canopy richness. Non-native richness and mean ground cover increased with the proportion of adjacent anthropogenic land cover, whereas for native richness and mean ground cover, this relationship was negative. Non-native richness increased in drier areas, while native richness was more influenced by temperature.

Main Conclusions

Adjacent anthropogenic land cover seems to not only facilitate non-native species arrival by being a source of propagules but also aids their establishment as a result of fragmentation. Our results highlight the importance of examining both cover and richness in different vegetation tiers to better understand non-native plant invasions.     

The roles of exotic grasses and forbs when restoring native species to highly invaded southern California annual grassland

Many semi-arid shrublands in the western US have experienced invasion by a suite of exotic grasses and forbs that have altered community structure and function. The effect of the exotic grasses in this area has been studied, but little is known about how exotic forbs influence the plant community. A 3-year experiment in southern California coastal sage scrub (CSS) now dominated by exotic grasses was done to investigate the influence of both exotic grasses (mainly Bromus spp.) and exotic forbs (mainly Erodium spp.) on a restoration seeding (9 species, including grasses, forbs, and shrubs). Experimental plots were weeded to remove one, both, or neither group of exotic species and seeded at a high rate with a mix of native species. Abundance of all species varied with precipitation levels, but seeded species established best when both groups of exotic species were removed. The removal of exotic grasses resulted in an increase in exotic and native forb cover, while removal of exotic forbs led to an increase in exotic grass cover and, at least in one year, a decrease in native forb cover. In former CSS now converted to exotic annual grassland, a competitive hierarchy between exotic grasses and forbs may prevent native forbs from more fully occupying the habitat when either group of exotics is removed. This apparent competitive hierarchy may interact with yearly variation in precipitation levels to limit restoration seedings of CSS/exotic grassland communities. Therefore, management of CSS and exotic grassland in southern California and similar areas must consider control of both exotic grasses and forbs when restoration is attempted.     

Can Carbon and Phosphorous Amendments Increase Native Forbs in a Restoration Process? A Case Study in the Northern Tall‐grass Prairie (U.S.A.)

In the northern Great Plains (United States), sites with less than 20% of native species are difficult to restore. We have experimented with a restoration method that shows some promise. It consists of systematically installing simulated small‐scale patches (8.0 m² in size) over 25% of an old field and then seeding these patches with native species. The working hypothesis is that these patches will generate a constant source of propagules which in time will lead to increases in native species diversity within the surrounding grass matrix. The objective of this paper was to determine whether soil amendments should be used to facilitate the establishment and persistence of native species (primarily forbs) within these patches. We seeded the patches with a mixture of native grass and forb species and applied four soil treatments: P fertilization, C additions, C + P, and a control (no amendments). Results for the first 5 years were as follows: (1) seeded forb richness was mostly unaffected by soil amendments; (2) seeded and nonseeded forb biomass and density were substantially reduced by C additions, whereas they were unaffected or increased under P additions; (3) both seeded and non‐native grass biomass substantially increased with C additions; and (4) there was an inverse relationship between native seeded forbs and non‐native grass biomass. Our conclusions are that: (1) P amendments are a potential tool for enhancing native seeded forb biomass in simulated small‐scale disturbance patches; and (2) C additions, although enhancing seeded grass biomass do not reduce the biomass of non‐native grasses.     

Effects of resource availability and propagule supply on native species recruitment in sagebrush ecosystems invaded by Bromus tectorum

Resource availability and propagule supply are major factors influencing establishment and persistence of both native and invasive species. Increased soil nitrogen (N) availability and high propagule inputs contribute to the ability of annual invasive grasses to dominate disturbed ecosystems. Nitrogen reduction through carbon (C) additions can potentially immobilize soil N and reduce the competitiveness of annual invasive grasses. Native perennial species are more tolerant of resource limiting conditions and may benefit if N reduction decreases the competitive advantage of annual invaders and if sufficient propagules are available for their establishment. Bromus tectorum, an exotic annual grass in the sagebrush steppe of western North America, is rapidly displacing native plant species and causing widespread changes in ecosystem processes. We tested whether nitrogen reduction would negatively affect B. tectorum while creating an opportunity for establishment of native perennial species. A C source, sucrose, was added to the soil, and then plots were seeded with different densities of both B. tectorum (0, 150, 300, 600, and 1,200 viable seeds m⁻²) and native species (0, 150, 300, and 600 viable seeds m⁻²). Adding sucrose had short-term (1 year) negative effects on available nitrogen and B. tectorum density, biomass and seed numbers, but did not increase establishment of native species. Increasing propagule availability increased both B. tectorum and native species establishment. Effects of B. tectorum on native species were density dependent and native establishment increased as B. tectorum propagule availability decreased. Survival of native seedlings was low indicating that recruitment is governed by the seedling stage.     

Effects of experimental rainfall manipulations on Chihuahuan Desert grassland and shrubland plant communities

Aridland ecosystems are predicted to be responsive to both increases and decreases in precipitation. In addition, chronic droughts may contribute to encroachment of native C₃ shrubs into C₄-dominated grasslands. We conducted a long-term rainfall manipulation experiment in native grassland, shrubland and the grass–shrub ecotone in the northern Chihuahuan Desert, USA. We evaluated the effects of 5 years of experimental drought and 4 years of water addition on plant community structure and dynamics. We assessed the effects of altered rainfall regimes on the abundance of dominant species as well as on species richness and subdominant grasses, forbs and shrubs. Nonmetric multidimensional scaling and MANOVA were used to quantify changes in species composition in response to chronic addition or reduction of rainfall. We found that drought consistently and strongly decreased cover of Bouteloua eriopoda, the dominant C₄ grass in this system, whereas water addition slightly increased cover, with little variation between years. In contrast, neither chronic drought nor increased rainfall had consistent effects on the cover of Larrea tridentata, the dominant C₃ shrub. Species richness declined in shrub-dominated vegetation in response to drought whereas richness increased or was unaffected by water addition or drought in mixed- and grass-dominated vegetation. Cover of subdominant shrubs, grasses and forbs changed significantly over time, primarily in response to interannual rainfall variability more so than to our experimental rainfall treatments. Nevertheless, drought and water addition shifted the species composition of plant communities in all three vegetation types. Overall, we found that B. eriopoda responded strongly to drought and less so to irrigation, whereas L. tridentata showed limited response to either treatment. The strong decline in grass cover and the resistance of shrub cover to rainfall reduction suggest that chronic drought may be a key factor promoting shrub dominance during encroachment into desert grassland.     

Efficacy and non-target effects of herbicides in foothills grassland restoration are short-lived

Questions

Selective herbicides are frequently used in ecological restoration to control invasive non-native forbs and recover plant communities. However, the long-term efficacy of this practice, its non-target effects on native plants, and its role in facilitating secondary invasions are not well understood. Similarly, little is known about the extent to which herbicide drift may affect native plant communities.

Location

Foothills grasslands of Montana, USA.

Methods

We conducted a 6-year experiment to investigate changes in the abundance of a target invasive plant, knapweed (Centaurea stoebe subsp. micranthos) and plant community structure in response to the herbicides Tordon® (picloram) and Milestone® (aminopyralid), applied at a recommended rate and a diluted rate that simulated drift.

Results

Knapweed cover and the richness of native and non-native forb species declined in the first 3 years in response to treatment at recommended rates, but not drift rates. Secondary invasion by non-native monocots was significant but weak. The cover of native forbs and the cover and richness of native monocots did not differ among treatments but changed significantly with the year. Surprisingly, 6 years after treatments, there were no differences among treatments in the cover of the target invasive plant or community structure.

Conclusions

Our results demonstrate that the efficacy and non-target effects of herbicides in grassland restoration can be short-lived and idiosyncratic because of year effects. Restoration of knapweed invasions might require other active interventions, such as seeding or repeated spraying. Our study supports previous calls for long-term monitoring of herbicides application in ecological restoration.     

Traits of the invasive Centaurea maculosa and two native grasses: effect of N supply

The Eurasian forb Centaurea maculosa (Lam.; spotted knapweed) has invaded millions of hectares of semi-arid grasslands in western North America. It readily colonizes disturbed areas, but also invades pristine grasslands. C. maculosa's success could be attributed to greater use, or more efficient use, of available soil nitrogen (N). Soil N often limits growth on semi-arid grasslands. Greater or more efficient use of soil N by C. maculosa, if this occurred, may inhibit establishment, survival, or reproduction of native grasses. In a glasshouse, C. maculosa and two native grasses, Pseudoroegneria spicata [Scribn. and Smith] A. Love and Pascopyrum smithii [Rybd.] A. Love, were grown in mixed- and monoculture for 8 weeks to determine growth response to two soil N supplies, which mimicked low and high N mineralization rates in semi-arid grasslands. At the end of the 8 weeks, plants were exposed to ¹⁵N-labeled nitrate for 24 h, and harvested to compare uptake of NO₃ ^–. C. maculosa's growth response to N indicated that it was more competitive for N than the tussock grass P. spicata, but less competitive than the rhizomatous grass P. smithii. C. maculosa used nitrogen less efficiently than both of these native grasses. C. maculosa roots took up more ¹⁵N per unit root mass than the grasses, but acquired less N than P. smithii because P. smithii had greater root mass than C. maculosa. Total biomass and ¹⁵N uptake of C. maculosa varied depending on which species it was growing with. C. maculosa's success cannot be explained wholly by greater or more efficient use of soil N than that of the native grasses with which it competes.     

Set‐backs in Replacing Phalaris arundinacea Monotypes with Sedge Meadow Vegetation

Reed canary grass (Phalaris arundinacea) invades wetlands, forms monotypes, and resists control efforts, suggesting that strong feedbacks sustain its dominance, as in the alternative states model. In nine field experiments, we tested the hypothesis that applying a graminicide (sethoxydim) for three years would progressively reduce Phalaris abundance, and that seeding sedge meadow species (except grasses) would reestablish native plant dominance. The graminicide prevented Phalaris from flowering, reduced its height by 50% and reduced its cover, often to less than 40%. However, only two of the nine sites showed progressive declines over the three‐year experiment. The first setback was that Phalaris recovered annually in nearly all treatment plots. A second setback was that seeding did not reestablish sedge meadow. In five sites, unseeded plots had similar numbers of native species as those seeded with either forbs, forbs and graminoids, or graminoids. In four formerly agricultural sites, however, non‐native weeds increased in species richness and cover (a third setback). In only one site did the graminicide's effect on Phalaris allow native species to increase in number and cover. But short‐term gains were not long‐lasting. In year four, three sites that developed high native‐species cover were again strongly dominated by Phalaris (a fourth setback). The feedbacks that sustain this invader include resistance to the graminicide aboveground and rapid and robust regrowth from rhizomes and seeds belowground. The weak effect of this graminicide was a surprise; hence, we recommend stronger management actions to control Phalaris.     

Experimental Restoration of Native Vegetation in Indiana Dunes National Lakeshore

We investigated the effects of prescribed fire, herbicide treatment, and sod removal on the eradication of exotic grasses and the establishment of native plant species in 24 experimental restoration plots in three razed residential sites within the boundary of Indiana Dunes National Lakeshore. During 1992–1995, herbicide treatment and sod removal decreased the combined cover of Poa pratensis (Kentucky blue grass) and Agropyron repens (quackgrass) significantly (from 82% to 13%, and 85% to 8%, respectively), whereas fire did not suppress such exotic lawn grasses. In 1993, several opportunistic species, represented by Cyperus spp. (umbrella sedges), Digitaria sanguinalis (crab grass), and Ambrosia artemisiifolia (common ragweed), filled the gaps left by the removal of lawn grasses. For the same period, Detrended Correspondence Analysis revealed a clear vegetation divergence between the control-fire plots and the herbicide-sod removal plots. While Poa pratensis and Agropyron repens continued to dominate the control and fire plots, the planted native species, represented by Schizachyrium scoparium (little blue-stem), Sorghastrum nutans (Indian grass), Rudbeckia hirta (black-eyed Susan), and Monarda punctata (horsemint), began to dominate in the herbicide and sod removal plots from 1994. In both herbicide and sod removal plots, the ground cover of grasses (68%) was much higher than the forbs (10%). The herbicide plots, where exotic species were removed but nitrogen-rich top soils were not removed, showed a higher diversity of planted native species than the sod removal plots (where both exotic species and top soils were removed) and the control-fire plots (where neither was removed). This finding suggests that an optimum but not excessive concentration of soil nitrogen is needed to support a maximum species diversity in such infertile substrate as sandy soil. In addition, the decrease in potassium in all plots, regardless of treatment, suggests that potassium may become a limiting factor for our restored native vegetation.     

Can perennial dominant grass <Emphasis Type="Italic">Miscanthus sinensis</Emphasis> be nurse plant in recovery of degraded hilly land landscape in South China?

Perennial C₄ grasses, especially Miscanthus sinensis, are widely distributed in the degraded lands in South China. We transplanted native and exotic tree seedlings under the canopy of M. sinensis to assess the interaction (competition or facilitation) between dominant grass M. sinensis and tree seedlings. The results of growth, chlorophyll fluorescence, and ultrastructure showed that negative effects may be stronger in perennial dominant grass M. sinensis. Although M. sinensis buffered the air temperature, improved soil structure, and increased soil phosphorus content, these beneficial effects were outweighed by the detrimental effect, especially overshading. To ensure the establishment of target native species in M. sinensis communities in degraded lands of South China, restoration strategies should include removing aboveground vegetation, planting target species seedlings in openings to reduce the effects of canopy shading, and/or selecting competition-tolerant target species. Also, seedlings of exotic species used in restoration engineering cannot be directly planted under the canopy of M. sinensis.