Native Plant Recovery Research Articles

Multiple stressors prevent gains in native plant diversity following invasive species removal

AbstractLand managers spend substantial resources managing invasive species to mitigate their negative ecological and economic impacts. However, invasive species management is seldom coupled with empirical assessments of management outcomes or the ecological impacts of the target species. Additionally, the study of invasive species has, until recently, neglected to acknowledge co‐stressors on native communities, which often are the drivers of ecological degradation. We assessed the effects of the invasive plant Vincetoxicum rossicum and its removal on understory plant communities, as well as whether these effects were mediated by deer and invasive earthworms. We used paired fenced and open plots with V. rossicum and an open plot without V. rossicum at each of the three forested sites in New York State Parks from 2017 to 2020. Following a nested design, we located paired sets of open and fenced plots in areas where V. rossicum was experimentally removed and in areas where it was unmanaged. After three years of V. rossicum removal, V. rossicum significantly declined. However, the ultimate goal of management was native plant recovery. Contrary to this aim, native plant diversity and cover increased if V. rossicum was left unmanaged but did not change with V. rossicum removal. Thus, we provide strong evidence that reducing target species abundance does not always translate into native plant recovery. This disparity may be because deer and invasive earthworms are stronger drivers of understory plant communities than V. rossicum, as native plant cover and diversity were lower the more earthworms were in a plot and invasive plant species declined after fencing to exclude deer. Management that prioritizes reducing deer and earthworm impacts over invasive plant removal is therefore worth exploring, especially when coupled with continued monitoring to assess if these actions result in the desired management outcomes. Overall, our findings underscore the need to collect data to test management assumptions: reducing the abundance of an invader may not always result in native plant recovery, especially if other stressors are the ultimate drivers of change in invaded communities.

Landscape and site factors drive invasive Phragmites management and native plant recovery across Chesapeake Bay wetlands

AbstractSuccessful invasive plant management—where invaders are sufficiently reduced and diverse native plant communities recover—remains an elusive goal for land managers. The site‐ and landscape‐scale drivers of variable management outcomes and vegetation recovery are poorly understood due to a lack of rigorous experiments that characterize longer term vegetation trends across contexts. We present the results of a five‐year experiment across eight subestuaries of Chesapeake Bay, representing a gradient of watersheds with differing dominant land‐use types and anthropogenic impacts, to evaluate invasive and native plant response to herbicide management. The focal invader, Phragmites australis (common reed), is one of the most aggressive and pervasive invasive plants in North American wetlands. We found that with multiyear herbicide treatments, it was possible to greatly reduce Phragmites across an array of subestuaries while increasing the cover and quality of native plant communities. Yet, by the end of the study, plant community composition in all Phragmites‐managed sites remained distinct from, even if composition was shifting toward, reference sites. There was also large inter‐site variation in the vegetation responses related to site environmental conditions and subestuary vegetation conditions. We uncovered specific aspects of the surrounding landscape that were linked to improved vegetation recovery—the species richness and conservation value of nearby wetlands. Results from this five‐year experiment conducted at multiple sites in Chesapeake Bay inform what is possible for management, particularly in more degraded landscapes and sites where setting realistic expectations and pragmatic goals will be essential. Assessing environmental and vegetation conditions of the site and surrounding landscape prior to commencing invasive species management is critical to predict the time and effort required to achieve restoration goals.

Short‐term effects of the control of the invasive plant Asclepias syriaca: Secondary invasion of other neophytes instead of recovery of the native species

AbstractQuestionThis study aimed to assess the impact of the invasion of Asclepias syriaca, a perennial non‐native herbaceous species, on basic soil attributes and vegetation composition, and to study the effect of mechanical control, namely the cutting of Asclepias, on target and other plant species.LocationSandy region of Kiskunság National Park, Hungary, Central Europe.MethodsIn ten old‐fields, four 4 m × 4 m plots were established (n = 40), of which three were invaded by Asclepias, and one was uninvaded. The invaded plots were treated as: (1) all Asclepias shoots removed, (2) half of the Asclepias shoots removed, or (3) untreated. The treatment was conducted twice a year in summer during the period 2019–2021. Before the first treatment, soil and vegetation were sampled. Afterwards, vegetation monitoring was performed twice a year: the cover of each vascular plant species and the number of Asclepias shoots were recorded in each plot.ResultThere were no differences in the studied soil attributes between the uninvaded and invaded plots. However, there were differences in vegetation composition, namely, the cover of sand grassland specialists was higher in uninvaded plots. Short‐term cutting negatively affected Asclepias after two years. The cover of specialists did not change in response to treatments, but the cover of other neophyte plant species increased.ConclusionsBased on our results, the invasion of Asclepias changes the vegetation composition, but not the soil. Although short‐term mowing can reduce the cover of Asclepias, but the grassland specialist plants do not regenerate; instead, secondary invasion occurs. We conclude that more time or additional treatment is required for native plant recovery.

Tipping the balance: The role of seed density, abiotic filters, and priority effects in seed-based wetland restoration.

Sowing native seeds is a common approach to reintroduce native plants to degraded systems. However, this method is often overlooked in wetland restoration despite the immense global loss of diverse native wetland vegetation. Developing guiding principles for seed-based wetland restoration is critical to maximize native plant recovery, particularly in previously invaded wetlands. Doing so requires a comprehensive understanding of how restoration manipulations, and their interactions, influence wetland plant community assembly. With a focus on the invader Phragmites australis, we established a series of mesocosm experiments to assess how native sowing density, invader propagule pressure, abiotic filters (water and nutrients), and native sowing timing (i.e., priority effects) interact to influence plant community cover and biomass in wetland habitats. Increasing the density of native seeds yielded higher native cover and biomass, but P. australis suppression with increasing sowing densities was minimal. Rather, community outcomes were largely driven by invader propagule pressure: P. australis densities of ≤500 seeds/m2 maintained high native cover and biomass. Low-water conditions increased the susceptibility of P. australis to dominance by native competitors. Early sowing of native seeds showed a large and significant benefit to native cover and biomass, regardless of native sowing density, suggesting that priority effects can be an effective restoration manipulation to enhance native plant establishment. Given the urgent wetland restoration need combined with the limited studies on seed-based wetland restoration, these findings provide guidance on restoration manipulations that are grounded in ecological theory to improve seed-based wetland restoration outcomes.

Plant community context controls short‐ versus medium‐term effects of pre‐emergent herbicides on target and non‐target species after fire

AbstractQuestionsSelective herbicide application is a common restoration strategy to control exotic invaders that interfere with native plant recovery after wildfire. Whether spraying with pre‐emergent or bio‐herbicides releases native plants from competition with exotics (“spray‐and‐release” strategy) and makes communities resistant to reinvasion by exotic annual grasses (e.g. cheatgrass, medusahead), without risks to non‐target native plants or secondary invasion, is a major question for land managers of semiarid plant communities.LocationSagebrush steppe of southwest Idaho, USA.MethodsWe applied chemical herbicides (imazapic, rimsulfuron) and weed‐suppressive bacteria (Pseudomonas fluorescens strains MB906 and D7) to three sagebrush steppe communities after fire. We measured plant cover prior to burning and for four years (five for exotic annual grasses) post treatment.ResultsBoth chemical herbicides significantly reduced exotic annual grass cover in all communities in the first post‐spraying year, but rimsulfuron plots were re‐invaded after 1–2 years, while imazapic plots continued to resist reinvasion four and even five years post spraying, well after the herbicide should have degraded. We did not detect any increase in native perennial grass cover with either herbicide, and herbicides had both positive and negative effects on individual bunchgrass basal diameter, depending on species and plant community. Rimsulfuron was more damaging than imazapic to shallow‐rooted perennial bunchgrasses. Moss and lichen cover, key components of soil integrity, increased with chemical herbicide treatments in some communities. Both herbicides increased secondary invaders (exotic forbs or grasses), which varied by plant community and herbicide. Weed‐suppressive bacteria treatments had no significant effects on cover of any functional group.ConclusionsWhile short‐term effects of chemical herbicides that target exotic annual grasses were relatively consistent and predictable, longer‐term effects were specific to the herbicide and plant community. The “spray‐and‐release” strategy may confer resistance to re‐invasion by exotic annual grasses if herbicides prevent re‐invasion for an extended period.

Do common assumptions about the wetland seed bank following invasive plant removal hold true? Divergent outcomes following multi‐year Phragmites australis management

AbstractQuestionLimited funds and compressed timelines frequently translate into a reliance on seed banks for native plant recovery following invasive plant management. This approach assumes: (a) baseline seed bank communities are sufficient for native plant recovery regardless of site environmental conditions; (b) different management actions variably impact native and invader seed banks; (c) management actions reduce invader seed banks; while (d) native seed banks do not decline following management; and (e) returning above‐ground vegetation reflects the seed bank. Do these assumptions hold true in the context of invasive Phragmites australis management?LocationGreat Salt Lake wetlands, Utah, USA.MethodsAcross six sites, we applied six Phragmites management treatments (solarization, or glyphosate or imazapyr herbicide in the summer or fall with mowing) for three years, and monitored two more. Each year, we assessed the above‐ground vegetation and seed bank using the seedling emergence method.ResultsSite environmental conditions drove baseline seed bank communities: seed banks differed substantially across sites with varying anthropogenic degradation as less disturbed sites had greater seed bank richness. Different management actions had similar impacts on native and invader seed banks. Phragmites seed density was reduced after herbicide treatments while native seed bank communities were unaffected. Above‐ground vegetation generally reflected seed bank communities. However, native graminoids were present in seed banks but surprisingly rare above‐ground.ConclusionCommonly held assumptions about invasive plant management and native recovery from the seed bank did not all hold true. Native seed banks were particularly insufficient for recovery at sites with greater degradation, suggesting managers should prioritize management at less degraded sites or consider revegetation in highly degraded sites. Despite reductions in Phragmites seed bank density, Phragmites propagule pressure remained high, requiring a long‐term management commitment. Often seed banks are insufficient and revegetation of desired above‐ground species is necessary to meet restoration goals.

Interaction of restored hydrological connectivity and herbicide suppresses dominance of a floodplain invasive species

Invasive species present one of the largest threats to the recruitment and persistence of native plant communities. Land managers generally apply two approaches to help control invasive species and restore native plant communities: (1) improve ecosystem processes that preferentially support native species (e.g. reintroduce disturbance regimes) or (2) directly treat non‐native species (e.g. herbicide use). Few utilize both. Over nearly a decade, we tracked the establishment and population growth of Lepidium latifolium (perennial pepperweed) on a hydrologically restored floodplain and adjacent grassland sites, concurrently with a controlled herbicide application experiment. We found that perennial pepperweed stem counts were lower in years with longer duration flood events (p < 0.0001) and after herbicide application (p < 0.0005). In floodplain areas, native species increased in the years following herbicide treatments or after wet water years, while grassland sites were reinvaded by other non‐native species. Our results suggest that long inundation periods can effectively control perennial pepperweed over large areas when used in concert with herbicide control during dry years or in drier areas. Given the natural variability in inundation periods, our study highlights the need for longer‐term (>3–5 years) studies to evaluate the effectiveness of invasive species control programs. Overall, our study emphasized the need for further integration of traditional weed control approaches with restored ecological function to effectively control invasive plants and stimulate native plant recovery.

Invasive Phragmites australis management outcomes and native plant recovery are context dependent.

The outcomes of invasive plant removal efforts are influenced by management decisions, but are also contingent on the uncontrolled spatial and temporal context of management areas. Phragmites australis is an aggressive invader that is intensively managed in wetlands across North America. Treatment options have been understudied, and the ecological contingencies of management outcomes are poorly understood. We implemented a 5‐year, multi‐site experiment to evaluate six Phragmites management treatments that varied timing (summer or fall) and types of herbicide (glyphosate or imazapyr) along with mowing, plus a nonherbicide solarization treatment. We evaluated treatments for their influence on Phragmites and native plant cover and Phragmites inflorescence production. We assessed plant community trajectories and outcomes in the context of environmental factors. The summer mow, fall glyphosate spray treatment resulted in low Phragmites cover, high inflorescence reduction, and provided the best conditions for native plant recruitment. However, returning plant communities did not resemble reference sites, which were dominated by ecologically important perennial graminoids. Native plant recovery following initial Phragmites treatments was likely limited by the dense litter that resulted from mowing. After 5 years, Phragmites mortality and native plant recovery were highly variable across sites as driven by hydrology. Plots with higher soil moisture had greater reduction in Phragmites cover and more robust recruitment of natives compared with low moisture plots. This moisture effect may limit management options in semiarid regions vulnerable to water scarcity. We demonstrate the importance of replicating invasive species management experiments across sites so the contingencies of successes and failures can be better understood.

Invasive species removal increases species and phylogenetic diversity of wetland plant communities.

Plant invasions result in biodiversity losses and altered ecological functions, though quantifying loss of multiple ecosystem functions presents a research challenge. Plant phylogenetic diversity correlates with a range of ecosystem functions and can be used as a proxy for ecosystem multifunctionality. Laurentian Great Lakes coastal wetlands are ideal systems for testing invasive species management effects because they support diverse biological communities, provide numerous ecosystem services, and are increasingly dominated by invasive macrophytes. Invasive cattails are among the most widespread and abundant of these taxa. We conducted a three‐year study in two Great Lakes wetlands, testing the effects of a gradient of cattail removal intensities (mowing, harvest, complete biomass removal) within two vegetation zones (emergent marsh and wet meadow) on plant taxonomic and phylogenetic diversity. To evaluate native plant recovery potential, we paired this with a seed bank emergence study that quantified diversity metrics in each zone under experimentally manipulated hydroperiods. Pretreatment, we found that wetland zones had distinct plant community composition. Wet meadow seed banks had greater taxonomic and phylogenetic diversity than emergent marsh seed banks, and high‐water treatments tended to inhibit diversity by reducing germination. Aboveground harvesting of cattails and their litter increased phylogenetic diversity and species richness in both zones, more than doubling richness compared to unmanipulated controls. In the wet meadow, harvesting shifted the community toward an early successional state, favoring seed bank germination from early seral species, whereas emergent marsh complete removal treatments shifted the community toward an aquatic condition, favoring floating‐leaved plants. Removing cattails and their litter increased taxonomic and phylogenetic diversity across water levels, a key environmental gradient, thereby potentially increasing the multifunctionality of these ecosystems. Killing invasive wetland macrophytes but leaving their biomass in situ does not address their underlying mechanism of dominance and is less effective than more intensive treatments that also remove their litter.

Abiotic and Landscape Factors Constrain Restoration Outcomes Across Spatial Scales of a Widespread Invasive Plant.

The natural recolonization of native plant communities following invasive species management is notoriously challenging to predict, since outcomes can be contingent on a variety of factors including management decisions, abiotic factors, and landscape setting. The spatial scale at which the treatment is applied can also impact management outcomes, potentially influencing plant assembly processes and treatment success. Understanding the relative importance of each of these factors for plant community assembly can help managers prioritize patches where specific treatments are likely to be most successful. Here, using effects size analyses, we evaluate plant community responses following four invasive Phragmites australis management treatments (1: fall glyphosate herbicide spray, 2: summer glyphosate herbicide spray, 3: summer imazapyr herbicide spray, 4: untreated control) applied at two patch scales (12,000 m2 and 1,000 m2) and monitored for 5 years. Using variation partitioning, we then evaluated the independent and shared influence of patch scale, treatment type, abiotic factors, and landscape factors on plant community outcomes following herbicide treatments. We found that Phragmites reinvaded more quickly in large patches, particularly following summer herbicide treatments, while native plant cover and richness increased at a greater magnitude in small patches than large. Patch scale, in combination with abiotic and landscape factors, was the most important driver for most plant responses. Compared with the small plots, large patches commonly had deeper and more prolonged flooding, and were in areas with greater hydrologic disturbance in the landscape, factors associated with reduced native plant recruitment and greater Phragmites cover. Small patches were associated with less flooding and landscape disturbance, and more native plants in the surrounding landscape than large patches, factors which promoted higher native plant conservation values and greater native plant cover and richness. Herbicide type and timing accounted for very little of the variation in native plant recovery, emphasizing the greater importance of patch selection for better management outcomes. To maximize the success of treatment programs, practitioners should first manage Phragmites patches adjacent to native plant species and in areas with minimal hydrologic disturbance.

Native Plant Recovery following Three Years of Common Reed (Phragmites australis) Control

AbstractProjects that aim to control invasive species often assume that a reduction of the target species will increase native species abundance. However, reports of the responses of native species following exotic species control are relatively rare. We assessed the recovery of the native community in five tidal wetland locations in which we attempted to eradicate the invasive common reed [Phragmites australis (Cav.) Trin. ex Steud.]. We tested whether 3 yr of treatment were able to eradicate Phragmites and promote recovery of the native plant community. After 3 yr of treatment, Phragmites density declined sharply in all treated stands, though it was not eradicated in any of them. Native plant cover increased significantly in treated areas, and community composition, particularly in smaller stands, converged toward that of uninvaded habitat. Thus, even within the relatively short timescale of the treatments and monitoring, significant progress was made toward achieving the goals of controlling Phragmites infestations and promoting native biodiversity. There was a trend toward greater promise for success in smaller stands than larger stands, as has been observed in other studies. A greater emphasis on monitoring whole-community responses to exotic plant control, across a range of conditions, would enhance our ability to plan and design successful management strategies.

Rainfall and removal method influence eradication success for Lantana camara

The success of invasive species eradication depends on a variety of factors, including those that initially facilitated the invasion, as well as removal and post-removal protocols. Two factors that appear to influence invasion by, and eradication of, the Neotropical shrub Lantana camara (L.), in southern Indian deciduous forests, are rainfall and removal method. However, their role in influencing eradication success is yet to be quantified, and remains unclear. We conducted an experiment to clarify how rainfall (high vs. low) and removal method (cutting vs. uprooting Lantana) influence re-invasion by Lantana, and native plant recovery. Rainfall influenced both eradication effort and outcomes—drier forest had lower starting levels of invader biomass, requiring less initial eradication effort, as well as lower subsequent Lantana re-invasion (from seed and rootstock) whereas wetter forest typically had greater starting levels of invader biomass, requiring considerably greater initial eradication effort, and greater Lantana re-invasion. However, wetter forest also showed greater native tree and forb recovery. Therefore, the availability of funds, local environmental gradients, and restoration priorities should inform the selection of restoration sites. With regard to removal method, uprooting combined with weeding of germinating Lantana, particularly after the rainy season, minimized overall re-invasion. Therefore, uprooting, followed by regular weeding of germinating Lantana and secondary invaders, is crucial to long-term Lantana eradication success.

Can aerial herbicide application control Grey Willow (Salix cinerea L.) and stimulate native plant recovery in New Zealand wetlands?

SummaryGrey Willow (Salix cinerea L.) poses a significant threat to wetland ecosystems in New Zealand. To manage the ecological impacts and to control further spread, cost‐effective large‐scale control methods are needed. We investigated the response of Grey Willow and dominant wetland plant groups to the aerial boom‐spray application of glyphosate at 9 L/ha and triclopyr (amine) at 18 L/ha at three New Zealand wetlands. We found glyphosate substantially reduced the dominance of tall (>2 m) Grey Willow with commensurate increases in the dominance of most native plant groups. Triclopyr (amine) application resulted in poor Grey Willow control, was not associated with increased native plant group dominance, and some native plant groups declined where triclopyr (amine) was applied. We conclude that the aerial application of glyphosate is an effective large‐scale Grey Willow control tool and could be used to initiate the restoration of native plant communities in wetlands dominated by Grey Willow. But, evidence of Grey Willow recovery after control suggests that increases in native plant dominance will be reversed as Grey Willow re‐establish. Further research is needed to determine how to maintain and enhance native plant dominance after control, and to determine how to manage Grey Willow in fen areas where the Grey Willow canopy is discontinuous and nontarget herbicide impacts can occur. The aerial boom‐spray application of triclopyr (amine) for large‐scale Grey Willow control should be discontinued as it does not provide effective control and results in negative ecological outcomes.

DoesEucalyptus grandisinvasion and removal affect soils and vegetation in the Eastern Cape Province, South Africa?

Many invasive alien plants drive changes in native community composition, structure and diversity. They alter soil nutrient regimes of native communities and affect native plant recovery outcomes following their removal. We assessed whether Eucalyptus grandis invasion and removal alters the soil physico-chemical properties and native vegetation recovery in the Eastern Cape Province, South Africa. We collected samples from topsoil in E. grandis invaded sites (canopy cover > 75%), cleared sites (eight years ago) and native sites (canopy cover > 80%) and quantified soil moisture, concentrations of soil macro elements (N, C and P), pH and exchangeable cations (K, Ca, Mg, Na) as well as measured soil water repellency using the Water Droplet Penetration Time and infiltration. We conducted vegetation surveys in plots measuring 10 × 10 m. Invasion by E. grandis had varying effects on soil physico-chemical properties, causing increase in soil pH and P, while decreasing total N and C. The removal of E. grandis also showed varying effects on soil physico-chemical properties, but seems to have further triggered the loss of some soil nutrients (especially soil P). Soil water repellency (a measure of soil compaction) has improved in cleared sites to non-repellent soils compared to repellent soils in invaded site. Eucalyptus grandis reduced species richness of the invaded sites. The presence of native species on cleared sites indicates a positive trajectory towards vegetation recovery. We conclude that E. grandis invasion and removal trigger varying effects on soil properties (both increases and decreases). For soil and vegetation restoration of cleared sites to be effective, active restoration techniques such as soil transfer, nutrient manipulation and native plant seeding should be considered.

Exotic Slugs Pose a Previously Unrecognized Threat to the Herbaceous Layer in a Midwestern Woodland

Developing effective restoration strategies requires first identifying the underlying factors limiting native plant recovery. The slug Deroceras reticulatum is an important herbivore in Europe, a global agricultural pest, and is introduced and abundant throughout eastern North America, but little information is available on the effect of this exotic herbivore on the forest herbaceous layer. Here, we test the palatability of 12 forest herbs to the introduced slug D. reticulatum and use field surveys to determine the degree to which slugs are damaging plants in the field. In laboratory feeding trials, slugs readily consumed most plants, but avoided the grass Elymus virginicus, the invasive forb Alliaria petiolata (garlic mustard), and thicker leaved plants. In the field, we documented significant slug damage, with close to 50% or more of plant leaves damaged by slugs on five of the six native species tested. Slug damage in the field was predicted by laboratory‐determined acceptability, but was significantly greater on short‐statured rosette species than on erect plants for a given acceptability value. Our results identify introduced slugs as an important, but overlooked obstacle to forest herb restoration and potential drivers of larger scale understory compositional change. The relaxed herbivore pressure on A. petiolata, relative to native competitors, suggests that invasive plant removal alone may not result in the recovery of native flora. Rather, restoration of unpalatable native species should accompany invasive plant control in slug invaded areas. Erect forbs, thick‐leaved plants, and graminoids should have the greatest success where introduced slugs are abundant.

Processes Limiting Native Shrub Recovery in Exotic Grasslands after Non‐Native Herbivore Removal

Although non‐native herbivores are well known to increase the prevalence of exotic species in plant communities, herbivore removal is only sometimes sufficient for native plant recovery. Better predicting the influence of grazer removal, and identifying effective restoration approaches, requires a mechanistic understanding of the processes limiting native plant recovery in post‐grazing landscapes. We evaluated the factors limiting the recovery of native shrubs in exotic annual grasslands after a century of sheep grazing on Santa Cruz Island, CA. To evaluate the life stage transitions most limiting the recovery of the two dominant native shrubs (Eriogonum arborescens and Artemisia californica), we quantified seed rain and censused establishment and growth rates over 5 years that varied widely in rainfall. In addition, we conducted competitor removal experiments to evaluate the influence of exotic annual grasses on shrub establishment. We found that seed rain is unlikely to limit native recovery; shrub recovery was most limited at the seedling establishment stage. For Eriogonum, the wet El Niño year increased establishment by almost an order of magnitude over average rainfall years, while grass competition reduced growth and survival by two thirds. Artemisia failed to recruit in any year of the study. Our results suggest that management efforts seeking to increase shrub recovery need to overcome barriers at the establishment life stage, but once such limitations are overcome, adult shrubs will persist with exotic grass competitors. Our work shows how understanding the factors limiting native recovery can inform the restoration of post‐grazing landscapes.

Post‐Fire Control of Invasive Plants Promotes Native Recovery in a Burned Desert Shrubland

Invasive annual grasses have become increasingly important components of desert vegetation in North America. They are especially problematic because they increase the extent, severity, and frequency of fire in desert shrublands that normally experience fire very rarely, or not at all. After fire, invasive grasses and forbs are often dominant and restoration methods are required to promote native plant recovery. Three treatments to control invasive annual grasses and forbs were implemented in the first 3 years following a fire in creosote bush scrub vegetation. Treatments included early season mechanical removal (raking) of all annuals, grass‐specific herbicide (Fusilade II), and Fusilade II plus hand pulling of exotic forbs. In the first year, all treatments reduced invasive annual grass abundance by about half but had little effect on native annuals. Treatment effectiveness was minimal in the first year due to low and irregular distribution of rainfall. In the second year, insufficient rainfall prevented the germination of any annual plants and no treatments were applied. In the third year, precipitation onset occurred later in the season and was above average. Although the raking treatment performed poorly, treatments utilizing Fusilade II nearly eliminated invasive grasses and forbs, achieved native annual dominance, and increased native perennial abundance. These results indicate that in the absence of invasive grasses and forbs, the native annual community can be resilient to fire disturbance and native perennials can recover. The results also suggest that burned creosote bush shrublands can be managed after fire to decrease the chance of invasive plant–fire feedback.

Mulching effects on vegetation recovery following high severity wildfire in north-central Washington State, USA

Straw mulch application after high severity wildfire has gained favor in recent years due to its efficacy in reducing soil erosion hazards. However, possible collateral effects of mulching on post-fire vegetation recovery have received relatively little study. We assessed mulching effects on plant cover and species richness, tree seedling establishment, and exotic species densities in the second year following the 2006 Tripod Wildfire in north-central Washington State, USA, by observing vegetation responses to spatial variability in mulch cover and depth. Mulch cover averaged about 35%, with a median depth of 0.5cm. Vegetation recovery was generally slow, with median plant cover of only 10%. Tree seedling densities were low and spatially variable. Vegetative cover, species richness, and seedling densities all declined with increasing elevation. Mulch cover was positively associated with plant cover, plant species richness, and conifer seedling densities when second year mulch cover did not exceed 40%. Only when mulch cover exceeded 70% did mulching begin to negatively affect vegetation recovery relative to areas with no mulch. Vegetation responses to mulch depth were minimal at depths under 3cm, but quite strong when mulch depth exceeded 5cm. Exotic plant frequency and density were positively associated with mulch cover, but exotic plant cover was low on average (<1%). In this study, mulch added significant cover to sites with slow natural recovery of vegetation, thereby likely reducing erosion hazard. Mulching also appears to have facilitated native plant recovery and conifer seedling establishment except at very high application levels, easing management concerns about longer-term impacts of mulching treatments on post-fire vegetation recovery.

Control of Lepidium latifolium (perennial pepperweed) and recovery of native plants in tidal marshes of the San Francisco Estuary

Several management techniques are effective in controlling Lepidium latifolium (perennial pepperweed) in rangelands and hay meadows; however, this invader’s rapid spread into sensitive aquatic habitats throughout the western US calls for alternative control strategies. To evaluate control methods for use in tidal marshes of San Francisco Estuary, we tested chemical, mechanical, and biological methods in field and greenhouse experiments. In a field experiment in three brackish marshes spanning the estuary, application of the herbicide glyphosate to re-growth of L. latifolium following hand-removal reduced L. latifolium cover by an average of 80% after 2 years and led to a 60% increase in native vegetation cover. Glyphosate alone was less effective at reducing L. latifolium cover (20% decrease) and increasing native cover (34% increase). Preliminary tests of a potential biological control, a native parasitic plant, were not successful, thus plots intended for field trials were instead used to test the newly approved herbicide imazapyr, which showed promise in controlling L. latifolium. An additional greenhouse experiment found large reductions in stem lengths with either glyphosate following clipping or imazapyr with or without clipping, all significantly more so than glyphosate alone. We conclude that an integrated management approach of applying glyphosate following mechanical removal can be effective at reducing L. latifolium cover and allowing recovery of native tidal marsh plants, providing a useful solution for controlling smaller, accessible infestations of the invader. Our preliminary tests of imazapyr suggest that it may be very effective at controlling L. latifolium in tidal marshes, although further assessment of non-target effects and native plant recovery are needed to evaluate its relative merit.

Response of Native and Exotic Grasses to Increased Soil Nitrogen and Recovery in a Postfire Environment

Abstract Native plant recovery following wildfires is of great concern to managers because of the potential for increased water run‐off and soil erosion associated with severely burned areas. Although postfire seeding with exotic grasses or cultivars of native grasses (seeded grasses) may mitigate the potential for increased run‐off and erosion, such treatments may also be detrimental to long‐term recovery of other native plant species. The degree to which seeded grasses dominate a site and reduce native plant diversity may be a function of the availability of resources such as nitrogen and light and differing abilities of native and seeded grasses to utilize available resources. We tested the hypothesis that seeded grasses have higher growth rates than native grasses when nitrogen and light availability is high in a greenhouse experiment. To determine how differing resource utilization strategies may affect distribution of native and seeded grasses across a burned landscape, we conducted botanical surveys after a wildfire in northern New Mexico, U.S.A., one and four years after the fire. In the greenhouse study we found seeded grasses to produce significantly more biomass than native grasses when nitrogen and light availability was high. Seeded grasses increased in cover from 1–4 years after the fire only in areas where total soil nitrogen was higher. Increased cover of seeded grasses did not affect recovery of native grasses, but it did lead to reduced native species richness at small scales. The potential negative long‐term consequences of seeding with exotic grasses should be considered in postfire rehabilitation treatments.