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.     

Control of Exotic Annual Grasses to Restore Native Forbs in Abandoned Agricultural Land

Exotic annual grasses are a major challenge to successful restoration in temperate and Mediterranean climates. Experiments to restore abandoned agricultural fields from exotic grassland to coastal sage scrub habitat were conducted over two years in southern California, U.S.A. Grass control methods were tested in 5 m² plots using soil and vegetation treatments seeded with a mix of natives. The treatments compared grass‐specific herbicide, mowing, and black plastic winter solarization with disking and a control. In year two, herbicide and mowing treatments were repeated on the first‐year plots, plus new control and solarization plots were added. Treatments were evaluated using percent cover, richness and biomass of native and exotic plants. Disking alone reduced exotic grasses, but solarization was the most effective control in both years even without soil sterilization, and produced the highest cover of natives. Native richness was greatest in solarization and herbicide plots. Herbicide application reduced exotics and increased natives more than disking or mowing, but produced higher exotic forb biomass than solarization in the second year. Mowing reduced grass biomass and cover in both years, but did not improve native establishment more than disking. Solarization was the most effective restoration method, but grass‐specific herbicide may be a valuable addition or alternative. Solarization using black plastic could improve restoration in regions with cool, wet summers or winter growing seasons by managing exotic seedbanks prior to seeding. While solarization may be impractical at very large scales, it will be useful for rapid establishment of annual assemblages on small scales.     

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.     

Restoration of Native Perennials in a California Annual Grassland after Prescribed Spring Burning and Solarization

Grasslands dominated by exotic annual grasses have replaced native perennial vegetation types in vast areas of California. Prescribed spring fires can cause a temporary replacement of exotic annual grasses by native and non‐native forbs, but generally do not lead to recovery of native perennials, especially where these have been entirely displaced for many years. Successful reintroduction of perennial species after fire depends on establishment in the postfire environment. We studied the effects of vegetation changes after an April fire on competition for soil moisture, a key factor in exotic annual grass dominance. As an alternative to fire, solarization effectively kills seeds of most plant species but with a high labor investment per area. We compared the burn to solarization in a study of establishment and growth of seeds and transplants of the native perennial grass Purple needlegrass (Nassella pulchra) and coastal sage species California sagebrush (Artemisia californica). After the fire, initial seed bank and seedling densities and regular percent cover and soil moisture (0–20 cm) data were collected in burned and unburned areas. Burned areas had 96% fewer viable seeds of the dominant annual grass, Ripgut brome (Bromus diandrus), leading to replacement by forbs from the seed bank, especially non‐native Black mustard (Brassica nigra). In the early growing season, B. diandrus dominating unburned areas consistently depleted soil moisture to a greater extent between rains than forbs in burned areas. However, B. diandrus senesced early, leaving more moisture available in unburned areas after late‐season rains. Nassella pulchra and A. californica established better on plots treated with fire and/or solarization than on untreated plots. We conclude that both spring burns and solarization can produce conditions where native perennials can establish in annual grasslands. However, the relative contribution of these treatments to restoration appears to depend on the native species being reintroduced, and the long‐term success of these initial restoration experiments remains to be determined.     

Differential responses to nitrogen fertilization in native shrubs and exotic annuals common to mediterranean coastal sage scrub of California

This study examined the growth responses of exotic annuals and native shrubs to elevated N levels to test the hypothesis that increased N availability favors nitrophilous annuals over the slower-growing shrubs. The vegetation structure of the coastal sage scrub ecosystems in southern California is shifting from shrubland to annual grasslands. Over the last 30 years large tracts of wildlands, particularly those adjacent to urban centers, have lost significant native shrub cover, which has been replaced by exotic annuals native to the Mediterranean Basin. During this same time, air pollution has led to increased terrestrial eutrophication by atmospheric deposition. Changes in vegetation are often the result of changes in resource availability. The results of our experiments showed the three native shrubs tested to be more nitrophilous than the three annuals tested, which contrasts with most models of perennial species' adaptation to stressful environments. Under greenhouse conditions the annual grasses exhibited yield depression at the highest N treatments of 80 g g^–1 in soil. The three shrub species evaluated continued to increase shoot biomass at 80 g g^–1 N in soil. The grasses also exhibited increased tissue N concentrations with increased soil N in contrast with the shrubs where there was little difference in tissue N concentrations with increasing availability. Although the differential yield responses to elevated N do not explain the success of the annual vegetation in replacing shrubs, the inability of the shrubs to regulate growth under elevated N levels may explain the poor survival of mature individuals.     

Identifying Native Vegetation for Reducing Exotic Species during the Restoration of Desert Ecosystems

There is currently much interest in restoration ecology in identifying native vegetation that can decrease the invasibility by exotic species of environments undergoing restoration. However, uncertainty remains about restoration's ability to limit exotic species, particularly in deserts where facilitative interactions between plants are prevalent. Using candidate native species for restoration in the Mojave Desert of the southwestern U.S.A., we experimentally assembled a range of plant communities from early successional forbs to late‐successional shrubs and assessed which vegetation types reduced the establishment of the priority invasive annuals Bromus rubens (red brome) and Schismus spp. (Mediterranean grass) in control and N‐enriched soils. Compared to early successional grass and shrub and late‐successional shrub communities, an early forb community best resisted invasion, reducing exotic species biomass by 88% (N added) and 97% (no N added) relative to controls (no native plants). In native species monocultures, Sphaeralcea ambigua (desert globemallow), an early successional forb, was the least invasible, reducing exotic biomass by 91%. However, the least‐invaded vegetation types did not reduce soil N or P relative to other vegetation types nor was native plant cover linked to invasibility, suggesting that other traits influenced native‐exotic species interactions. This study provides experimental field evidence that native vegetation types exist that may reduce exotic grass establishment in the Mojave Desert, and that these candidates for restoration are not necessarily late‐successional communities. More generally, results indicate the importance of careful native species selection when exotic species invasions must be constrained for restoration to be successful.     

Creosote growth rate and reproduction increase in postfire environments

Human activities are changing patterns of ecological disturbance globally. In North American deserts, wildfire is increasing in size and frequency due to fuel characteristics of invasive annual grasses. Fire reduces the abundance and cover of native vegetation in desert ecosystems. In this study, we sought to characterize stem growth and reproductive output of a dominant native shrub in the Mojave Desert, creosote bush (Larrea tridentata (DC.) Coville) following wildfires that occurred in 2005. We sampled 55 shrubs along burned and unburned transects 12 years after the fires (2017) and quantified age, stem diameter, stem number, radial and vertical growth rates, and fruit production for each shrub. The shrubs on the burn transects were most likely postfire resprouts based on stem age while stems from unburn transects dated from before the fire. Stem and vertical growth rates for shrubs on burned transects were 2.6 and 1.7 times higher than that observed for shrubs on unburned transects. Fruit production of shrubs along burned transects was 4.7‐fold more than shrubs along paired unburned transects. Growth rates and fruit production of shrubs in burned areas did not differ with increasing distance from the burn perimeter. Positive growth and reproduction responses of creosote following wildfires could be critical for soil stabilization and re‐establishment of native plant communities in this desert system. Additional research is needed to assess if repeat fires that are characteristic of invasive grass‐fire cycles may limit these benefits.     

Grass invasion causes rapid increases in ecosystem carbon and nitrogen storage in a semiarid shrubland

Accurately predicting terrestrial carbon (C) and nitrogen (N) storage requires understanding how plant invasions alter cycling and storage. A common, highly successful type of plant invasion occurs when the invasive species is of a distinctly different functional type than the native dominant plant, such as shrub encroachment throughout the western United States and annual grass invasions in Mediterranean shrublands, as studied here. Such invasions can dramatically transform landscapes and have large potential to alter C and N cycling by influencing storage in multiple pools. We used a manipulation of non‐native annual grass litter within a shrub‐dominated habitat in southern California (coastal sage scrub, CSS) to study how grass invasion alters ecosystem C and N storage. We added, removed, or left unchanged grass litter in areas of high and low invasion, then followed soil and vegetation changes. Grass litter greatly increased C and N storage in soil, aboveground native and non‐native biomass. Aboveground litter storage increased due to the greater inputs and slower decomposition of grass litter relative to shrub litter; shading by grass litter further reduced decomposition of both non‐native and native litter, which may be due to reduced photodegradation. Soil C and N pools in areas of high litter increased ～20% relative to low litter areas in the two years following manipulation and were generally sinks for C and N, while areas with low litter were sources. We synthesize our results into a C cycle of invaded and uninvaded areas of CSS and link changes in storage to increases in the soil fungi : bacteria ratio, increased plant inputs, and decreased litter loss. Overall, we show that grasses, especially through their litter, control important abiotic and biotic mechanisms governing C and N storage, with widespread implications for C sequestration and N storage in semiarid systems undergoing grass or shrub invasions.     

Postwildfire seeding to restore native vegetation and limit exotic annuals: an evaluation in juniper‐dominated sagebrush steppe

Reestablishment of perennial vegetation is often needed after wildfires to limit exotic species and restore ecosystem services. However, there is a growing body of evidence that questions if seeding after wildfires increases perennial vegetation and reduces exotic plants. The concern that seeding may not meet restoration goals is even more prevalent when native perennial vegetation is seeded after fire. We evaluated vegetation cover and density responses to broadcast seeding native perennial grasses and mountain big sagebrush (Artemisia tridentata Nutt. spp. vaseyana [Rydb.] Beetle) after wildfires in the western United States in six juniper (Juniperus occidentalis ssp. occidentalis Hook)‐dominated mountain big sagebrush communities for 3 years postfire. Seeding native perennial species compared to not seeding increased perennial grass and sagebrush cover and density. Perennial grass cover was 4.3 times greater in seeded compared to nonseeded areas. Sagebrush cover averaged 24 and less than 0.1% in seeded and nonseeded areas at the conclusion of the study, respectively. Seeding perennial species reduced exotic annual grass and annual forb cover and density. Exotic annual grass cover was 8.6 times greater in nonseeded compared to seeded areas 3 years postfire. Exotic annual grass cover increased over time in nonseeded areas but decreased in seeded areas by the third‐year postfire. Seeded areas were perennial‐dominated and nonseeded areas were annual‐dominated at the end of the study. Establishing perennial vegetation may be critical after wildfires in juniper‐dominated sagebrush steppe to prevent the development of annual‐dominated communities. Postwildfire seeding increased perennial vegetation and reduced exotic plants and justifies its use.     

Strengthening Invasion Filters to Reassemble Native Plant Communities: Soil Resources and Phenological Overlap

Preventing invasion by exotic species is one of the key goals of restoration, and community assembly theory provides testable predictions about native community attributes that will best resist invasion. For instance, resource availability and biotic interactions may represent “filters” that limit the success of potential invaders. Communities are predicted to resist invasion when they contain native species that are functionally similar to potential invaders; where phenology may be a key functional trait. Nutrient reduction is another common strategy for reducing invasion following native species restoration, because soil nitrogen (N) enrichment often facilitates invasion. Here, we focus on restoring the herbaceous community associated with coastal sage scrub vegetation in Southern California; these communities are often highly invaded, especially by exotic annual grasses that are notoriously challenging for restoration. We created experimental plant communities composed of the same 20 native species, but manipulated functional group abundance (according to growth form, phenology, and N‐fixation capacity) and soil N availability. We fertilized to increase N, and added carbon to reduce N via microbial N immobilization. We found that N reduction decreased exotic cover, and the most successful seed mix for reducing exotic abundance varied depending on the invader functional type. For instance, exotic annual grasses were least abundant when the native community was dominated by early active forbs, which matched the phenology of the exotic annual grasses. Our findings show that nutrient availability and the timing of biotic interactions are key filters that can be manipulated in restoration to prevent invasion and maximize native species recovery.     

Using consecutive prescribed fires to reduce shrub encroachment in grassland by increasing shrub mortality

Woody plant encroachment into open grasslands occurs worldwide and causes multiple ecological and management impacts. Prescribed fire could be used to conserve grassland habitat but often has limited efficacy because many woody plants resprout after fire and rapidly reestablish abundance. If fire‐induced mortality could be increased, prescribed fire would be a more effective management tool. In California's central coast, shrub encroachment, especially of Baccharis pilularis (coyote brush), is converting coastal prairie into shrub‐dominated communities, with a consequent loss of native herbaceous species and open grassland habitat. B. pilularis has not been successfully controlled with single prescribed fire events because the shrub resprouts and reestablishes cover within a few years. We investigated whether two consecutive annual burns would control B. pilularis by killing resprouting shrubs, without reducing native herbaceous species or encouraging invasive plants. As expected, resprouting did occur; however, 2 years after the second burn, B. pilularis cover on burned plots was only 41% of the cover on unburned plots. Mortality of B. pilularis more than doubled following the second burn, likely maintaining a reduction in B. pilularis cover for longer than a single burn would have. Three native coastal prairie perennial grasses did not appear to be adversely affected by the two burns, nor did the burns result in increased cover of invasive species. Managers wanting to restore coastal prairie following B. pilularis encroachment should consider two consecutive annual burns, especially if moderate fire intensity is achievable.     

Influence of fire and mechanical sagebrush reduction treatments on restoration seedings in Utah,United States

Overabundance of woody plants in semiarid ecosystems can degrade understory herbaceous vegetation and often requires shrub reduction and seeding to recover ecosystem services. We used meta‐analysis techniques to assess the effects of fire and mechanical shrub reduction over two post‐treatment timeframes (1–4 and 5–10 years) on changes in cover and frequency of 15 seeded species at 63 restoration sites with high potential for recovery. Compared to mechanical treatments, fire resulted in greater increases in seeded species. Native shrubs did not increase, and forbs generally declined over time; however, large increases in perennial grasses were observed, suggesting that seeding efforts contributed to enhanced understory herbaceous conditions. We found greater increases in a few non‐native species than native species across all treatments, suggesting the possibility that interference among seeded species may have influenced results of this regional assessment. Differences among treatments and species were likely driven by seedbed conditions, which should be carefully considered in restoration planning. Site characteristics also dictated seeded species responses: while forbs showed greater increases in cover over the long term at higher elevation sites considered to be more resilient to disturbance, surprisingly, shrubs and grasses had greater increases in cover and frequency at lower elevation sites where resilience is typically much lower. Further research is needed to understand the causes of forb mortality over time, and to decipher how greater increases of non‐native relative to native seeded species will influence species diversity and successional trajectories of restoration sites.     

Impacts of invasive annuals on soil carbon and nitrogen storage in southern California depend on the identity of the invader

Non‐native plant invasions can alter nutrient cycling processes and contribute to global climate change. In southern California, California sage scrub (hereafter sage scrub), a native shrub‐dominated habitat type in lowland areas, has decreased to <10% of its original distribution. Postdisturbance type‐conversion to non‐native annual grassland, and increasingly to mustard‐dominated invasive forbland, is a key contributor to sage scrub loss. To better understand how type‐conversion by common invasive annuals impacts carbon (C) and nitrogen (N) storage in surface soils, we examined how the identity of the invader (non‐native grasses, Bromus spp.; and non‐native forbs, Brassica nigra), microbial concentrations, and soil properties interact to influence soil nutrient storage in adjacent native and invasive habitat types at nine sites along a coast to inland gradient. We found that the impact of type‐conversion on nutrient storage was contingent upon the invasive plant type. Sage scrub soils stored more C and N than non‐native grasslands, whereas non‐native forblands had nutrient storage similar to or higher than sage scrub. We calculate that >940 t C km^?2 and >60 t N km^?2 are lost when sage scrub converts to grass‐dominated habitat, demonstrating that grass invasions are significant regional contributors to greenhouse gas emissions. We found that sites with greater total C and N storage were associated with high cation exchange capacities and bacterial concentrations. Non‐native grassland habitat type was a predictor of lower total C, and soil pH, which was greatest in invasive habitats, was a predictor of lower total N. We demonstrate that modeling regional nutrient storage requires accurate classification of habitat type and fine‐scale quantification of cation exchange capacity, pH, and bacterial abundance. Our results provide evidence that efforts to restore and conserve sage scrub enhance nutrient storage, a key ecosystem service reducing atmospheric CO₂ concentrations.     

Composition of soil seed banks in southern California coastal sage scrub and adjacent exotic grassland

Soil seed banks are important to many plant communities and are recognized as an important component of management plans. Understanding seed bank composition and density is especially important when communities have been invaded by exotic species and must be managed to promote desirable species. We examined germinable soil seed banks in southern California coastal sage scrub (CSS) that is heavily invaded by exotic grasses and in adjacent exotic grassland. Soils from both communities had similar seed banks, dominated by high densities of exotic grass and forb species. Up to 4,000 exotic grass seeds and at least 400 exotic forb seeds/m² were found in most soils, regardless of aboveground vegetation type. Native forbs averaged 400 seeds/m² in grass-dominated areas and about 800 in shrub-dominated soils. Shrub seed density was <1 and <10 seeds/m² in grass- and shrub-dominated areas, respectively, indicating that the shrub seed bank is not persistent compared to annuals. We also compared pre- and post-burn soil seed banks from one location that burned in October 2003. Late-season burning in both grass- and CSS-dominated areas disproportionately reduced exotic grass seed densities relative to native seed densities. The similarity of the seed banks in adjacent grass and shrub communities suggests that without intervention, areas currently dominated by CSS may become more similar to grass-dominated areas in terms of aboveground vegetation. In such areas, the first growing season following a wildfire is a window of opportunity for increasing native diversity at a time when density of exotic grass seeds is low. At time of research, Robert D. Cox was graduate student.     

Shrubs as ecosystem engineers in a coastal dune: influences on plant populations,communities and ecosystems

Question: How do two shrubs with contrasting life‐history characteristics influence abundance of dominant plant taxa, species richness and aboveground biomass of grasses and forbs, litter accumulation, nitrogen pools and mineralization rates? How are these shrubs – and thus their effects on populations, communities and ecosystems – distributed spatially across the landscape? Location: Coastal hind‐dune system, Bodega Head, northern California. Methods: In each of 4 years, we compared vegetation, leaf litter and soil nitrogen under canopies of two native shrubs –Ericameria ericoides and the nitrogen‐fixing Lupinus chamissonis– with those in adjacent open dunes. Results: At the population level, density and cover of the native forb Claytonia perfoliata and the exotic grass Bromus diandrus were higher under shrubs than in shrub‐free areas, whereas they were lower under shrubs for the exotic grass Vulpia bromoides. In contrast, cover of three native moss species was highest under Ericameria and equally low under Lupinus and shrub‐free areas. At community level, species richness and aboveground biomass of herbaceous dicots was lower beneath shrubs, whereas no pattern emerged for grasses. At ecosystem level, areas beneath shrubs accumulated more leaf litter and had larger pools of soil ammonium and nitrate. Rates of nitrate mineralization were higher under Lupinus, followed by Ericameria and then open dune. At landscape level, the two shrubs – and their distinctive vegetation and soils – frequently had uniform spatial distributions, and the distance separating neighbouring shrubs increased as their combined sizes increased. Conclusions: Collectively, these data suggest that both shrubs serve as ecosystem engineers in this coastal dune, having influences at multiple levels of biological organization. Our data also suggest that intraspecific competition influenced the spatial distributions of these shrubs and thus altered the distribution of their effects throughout the landscape.     

Restoration of a Restinga Sandy Coastal Plain in Brazil: Survival and Growth of Planted Woody Species

In this article we report the results of an experiment introducing 17 native shrub and tree species into a Brazilian restinga (i.e., coastal sandy plain vegetation). Restingas have been affected by human impact for about 8,000 years, and human occupation for housing, tourism, and land speculation has recently increased in such a way that there is a need for conservation of remnant patches and restoration of degraded areas throughout the coast to protect biodiversity. Our study site is a remnant located in Rio de Janeiro, the second largest city in the country, and has been subjected in the past to deforestation, man‐made fire, and sand extraction. Although trees and shrubs predominantly compose natural restinga vegetation, local vegetation after impact was replaced by an exotic grass cover, which meant a drastic reduction in species richness. Thus, in this experiment we removed the grass cover, introduced shrub and tree species, and monitored survival and growth of 20 plants per species for 2 years. Despite the adversities imposed by the nutrient‐poor sandy soil, 70% of the species showed high survival percentage and considerable growth. This report on restoration initiatives in the restingas points out the viability of shrub and tree plantation following exotic grass removal as a strategy to restore Brazilian coastal vegetation.     

Promoting Native Vegetation and Diversity in Exotic Annual Grass Infestations

Exotic plant invasions are especially problematic because reestablishment of native perennial vegetation is rarely successful. It may be more appropriate to treat exotic plant infestations that still have some remaining native vegetation. We evaluated this restoration strategy by measuring the effects of spring burning, fall burning, fall applied imazapic, spring burning with fall applied imazapic, and fall burning with fall applied imazapic on the exotic annual grass, medusahead (Taeniatherum caput‐medusae (L.) Nevski), and native vegetation at six sites in Oregon for 2 years post‐treatment. Medusahead infestations included in this study had some residual native perennial bunchgrasses and forbs. Burning followed by imazapic application provided the best control of medusahead and resulted in the greatest increases in native perennial vegetation. However, imazapic application decreased native annual forb cover the first year post‐treatment and density the first and second year post‐treatment. The spring burn followed by imazapic application produced an almost 2‐fold increase in plant species diversity compared to the control. The fall burn followed by imazapic application also increased diversity compared to the control. Results of this study indicate that native plants can be promoted in medusahead invasions; however, responses vary by plant functional group and treatment. Our results compared to previous research suggest that restoration of plant communities invaded by exotic annual grass may be more successful if efforts focus on areas with some residual native perennial vegetation. Thus, invasive plant infestations with some native vegetation remaining should receive priority for restoration efforts over near monocultures of invasive plant species.     

Invasive grass litter facilitates native shrubs through abiotic effects

Questions: Plant invasions are considered one of the top threats to the biodiversity of native taxa, but clearly documenting the causal links between invasions and the decline of native species remains a major challenge of invasion biology. Most studies have focused on impacts of invaders' living biomass, rather than on mechanisms mediated by litter. However, invasive plant litter, which is often of a very different type and quantity than a system's native plant litter, can have multiple important effects on ecosystem processes – such as nitrogen cycling and soil microclimate – that may influence native plants. Location: We studied effects of litter of invasive grass species that are widespread throughout western North America on native shrubs in southern California's semi‐arid habitat of coastal sage scrub. Methods: We combined a 3‐year field manipulation of non‐native litter with structural equation modeling to understand interacting effects on non‐native grasses, native shrubs, soil nitrogen (available and total), and soil moisture. Results: Litter addition facilitated non‐native grass growth, revealing a positive feedback likely to enhance invasion success. Contrary to a major paradigm of invasion biology – that competition with invasive plant species causes declines of native plants – we found that litter also facilitated growth of the native dominant shrub, a result supported by observational trends. Structural equation models indicated that enhanced soil moisture mediated the positive effects of litter on shrub growth. Conclusions: We demonstrate that invasive plants, via their litter, can facilitate dominant native plants by altering soil moisture. Our results highlight that understanding the impacts and mechanisms of plant invasions may be enhanced by considering the role of invasive plant litter on native plants and ecosystem properties.     

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.     

Post-pastoral changes in composition and guilds in a semi-arid conservation area, Central Otago, New Zealand

Changes in the vegetation of Flat Top Hill, a highly modified conservation area in semi;arid Central Otago, New Zealand, are described four years after the cessation of sheep and rabbit grazing. Unusually moist weather conditions coincide with the four-year period of change in response to the cessation of grazing. Between 1993 and 1997, the average richness and diversity (H') of species increased, and the average proportion of native species decreased significantly. The vegetation was significantly richer in exotic annual and perennial grass species, exotic perennial forbs, exotic woody species and native tussock grasses in 1997 than in 1993. Eight response guilds of species are identified. Most "remnant" native shrubs and forbs were stable, in that they remained restricted to local refugia and showed little change in local frequency. However, taller native grass species increased, some locally, and others over wide environmental ranges. Rare native annual forbs and several native perennial species from "induced" xeric communities decreased, and this may be a consequence of competition from exotic perennial grasses in the absence of grazing. The invasive exotic herb Sedum acre decreased in abundance between 1993 and 1997, but several other prominent exotic species increased substantially in range and local frequency over a wide range of sites. Exotic woody species, and dense, sward-forming grasses are identified as potential threats to native vegetation recovery.