California native and exotic perennial grasses differ in their response to soil nitrogen,exotic annual grass density,and order of emergence

Early emergence of plant seedlings can offer strong competitive advantages over later-germinating neighbors through the preemption of limiting resources. This phenomenon may have contributed to the persistent dominance of European annual grasses over native perennial grasses in California grasslands, since the former species typically germinate earlier in the growing season than the latter and grow rapidly after establishing. Recently, European perennial grasses have been spreading into both non-native annual and native perennial coastal grass stands in California. These exotic perennials appear to be less affected by the priority effects arising from earlier germination by European annual grasses. In addition, these species interactions in California grasslands may be mediated by increasing anthropogenic or natural soil nitrogen inputs. We conducted a greenhouse experiment to test the effects of order of emergence and annual grass seedling density on native and exotic perennial grass seedling performance across different levels of nitrogen availability. We manipulated the order of emergence and density of an exotic annual grass (Bromus diandrus) grown with either Nassella pulchra (native perennial grass), Festuca rubra (native perennial grass), or Holcus lanatus (exotic perennial grass), with and without added nitrogen. Earlier B. diandrus emergence and higher B. diandrus density resulted in greater reduction in the aboveground productivity of the perennial grasses. However, B. diandrus suppressed both native perennials to a greater extent than it did H. lanatus. Nitrogen addition had no effect on the productivity of native perennials, but greatly increased the growth of the exotic perennial H. lanatus, grown with B. diandrus. These results suggest that the order of emergence of exotic annual versus native perennial grass seedlings could play an important role in the continued dominance of exotic annual grasses in California. The expansion of the exotic perennial grass H. lanatus in coastal California may be linked to its higher tolerance of earlier-emerging annual grasses and its ability to access soil resources amidst high densities of annual grasses.     

Can Carbon Addition Increase Competitiveness of Native Grasses? A Case Study from California

There is growing interest in the addition of carbon (C) as sucrose or sawdust to the soil as a tool to reduce plant‐available nitrogen (N) and alter competitive interactions among species. The hypothesis that C addition changes N availability and thereby changes competitive dynamics between natives and exotics was tested in a California grassland that had experienced N enrichment. Sawdust (1.2 kg/m) was added to plots containing various combinations of three native perennial bunchgrasses, exotic perennial grasses, and exotic annual grasses. Sawdust addition resulted in higher microbial biomass N, lower rates of net N mineralization and net nitrification, and higher concentrations of extractable soil ammonium in the soil. In the first year sawdust addition decreased the degree to which exotic annuals competitively suppressed the seedlings of Nassella pulchra and, to a lesser extent, Festuca rubra, both native grasses. However there was no evidence of reduced growth of exotic grasses in sawdust‐amended plots. Sawdust addition did not influence interactions between the natives and exotic perennial grasses. In the second year, however, sawdust addition did not affect the interactions between the natives and either group of exotic grasses. In fact, the native perennial grasses that survived the first year of competition with annual grasses significantly reduced the aboveground productivity of annual grasses even without sawdust addition. These results suggest that the addition of sawdust as a tool in the restoration of native species in our system provided no significant benefit to natives over a 2‐year period.     

Restoring Native Perennial Grasses to Rural Roadsides in the Sacramento Valley of California: Establishment and Evaluation

Along rural roadsides of the Sacramento Valley of California, we seeded native and non-native perennial grasses to gauge their potential value in roadside vegetation management programs. In trial I (polycultures), three seeded complexes and a control (resident vegetation only) were tested. Each seeded plant complex included a different mix of perennial grasses seeded into each of several roadside topographic zones. The seeded levels of plant complex were: native perennial grasses 1 (8 species); native perennial grasses 2 (13 species); and non-native perennial grasses (3 species). In trial II, plots were seeded to monocultural plots of 15 accessions of native Californian and three cultivars of non-native perennial grasses. Plots in both trials were seeded during January 1992 and evaluated for three successive years.     

Effects of exotic annual grass litter and local environmental gradients on annual plant community structure

Exotic annual grasses have been introduced into many semi-arid ecosystems worldwide, often to the detriment of native plant communities. The accumulation of litter from these grasses (i.e. residual dry biomass) has been demonstrated to negatively impact native plant communities and promote positive feedbacks to exotic grass persistence. More targeted experiments are needed, however, to determine the relative impact of exotic grass litter on plant community structure across local environmental gradients. We experimentally added exotic grass litter to annual forb-dominated open woodland communities positioned along natural canopy cover gradients in southwest Western Australia. These communities are an important component of this region’s plant biodiversity hotspot and are documented to be under threat from exotic annual grasses. After a one-year treatment period, we measured the effects of exotic grass litter, soil properties, and canopy cover on native and exotic species richness and abundance, as well as common species’ biomass and abundances. Plant community structure was more strongly influenced by soil properties and canopy cover than by grass litter. Total plant abundances per plot, however, were significantly lower in litter addition plots than control plots, a trend driven by native species. Exotic grass litter was also associated with lower abundances of one very common native species: Waitzia acuminata. Our results suggest that exotic grass litter limits the establishment of some native species in this system. Over multiple years, these subtle impacts may contribute substantially to the successful advancement of exotic species into this system, particularly in certain microenvironments.     

Old-field successional dynamics on the Inland Pampa,Argentina

Abstract. The first 10 yr of old-field successional dynamics on the Argentine Inland Pampa were studied on a series of adjacent plots established consecutively between 1978 and 1989. We examined differences in species abundance patterns among plots in order to detect the spatial and temporal variability of succession. Perennial grasses steadily increased in cover and replaced the dominant annual species after 5 yr. Pioneer dicots persisted in older seral stages with 20 — 23 species/plot. Overall, exotic species (mostly the grasses Lolium multiflorum and Cynodon dactylon) contributed much to the plant cover in these communities. Native grasses comprised 45 % of total cover at years 7 — 10 of succession, but occurred with less than 7 species/plot. Substantial variation was found in the successional pathway, which reflected the particular sequence from annual forbs to short-lived and perennial grasses in the various plots. The course of succession was apparently influenced by a 2-yr period of unusually high rainfall. Deyeuxia viridiflavescens, a native perennial grass virtually absent before the wet period, spread over the study area and dominated seral communities for 3 yr, irrespective of plot age. Climatic conditions thus affected the successional turnover of life forms by increasing the rate of colonization by perennial grasses. We further point out the constraints imposed on secondary succession by the life histories of ‘available’ species.     

Manipulating disturbance regimes and seeding to restore mesic Mediterranean grasslands

Question: Can managing disturbance regimes alone or in combination with seeding native species serve to shift the balance from exotic towards native species? Location: Central coast of California, USA. Methods: We measured vegetation composition for 10 yr in a manipulative experiment replicated at three sites. Treatments included no disturbance, grazing and clipping at three frequencies with and without litter removal. We seeded eight native species into clipped plots and compared cover in comparable plots with no seeding. Results: Regardless of frequency, clipping generally shifted community dominance from exotic annual grasses to exotic annual forbs, rather than consistently favoring native species. At one site, perennial grass cover decreased in no‐disturbance plots, but only after 4 yr. Litter removal had minimal impact on litter depth and plant community composition. Grazing had a highly variable effect on the abundance of different plant guilds across sites and years. Seeding increased abundance of only two of eight native species. Conclusions: Managing disturbance regimes alone is insufficient to restore native species guilds in highly‐invaded grasslands and seeding native species has highly variable success.     

Reducing Competitive Suppression of a Rare Annual Forb by Restoring Native California Perennial Grasslands

Populations of the rare annual forb Amsinckia grandiflora may be declining because of competitive suppression by exotic annual grasses, and may perform better in a matrix of native perennial bunchgrasses. We conducted a field competition experiment in which Amsinckia seedlings were transplanted into forty 0.64‐m² experimental plots of exotic annual grassland or restored perennial grassland. The perennial grassland plots were restored using mature 3 cm‐diameter plants of the native perennial bunchgrass Poa secunda planted in three densities. The exotic annual grassland plots were established in four densities through manual removal of existing plants. Both grass types reduced soil water potential with increasing biomass, but this reduction was not significantly different between grass types. Both grass types significantly reduced the production of Amsinckia inflorescences. At low and intermediate densities (dry biomass per unit area of 20–80 g/m²), the exotic annual grasses reduced Amsinckia inflorescence number to a greater extent than did Poa, although at high densities (>90 g/m²) both grass types reduced the number of Amsinckia inflorescences to the same extent. The response of Amsinckia inflorescence number to Poa biomass was linear, whereas the same response to the annual grass biomass is logarithmic, and appeared to be related to graminoid cover. This may be because of the different growth forms exhibited by the two grass types. Results of this research suggest that restored native perennial grasslands at intermediate densities have a high habitat value for the potential establishment of the native annual A. grandiflora.     

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.     

The effect of soil nitrogen on competition between native and exotic perennial grasses from northern coastal California

The invasion of European perennial grasses represents a new threat to the native coastal prairie of northern California. Many coastal prairie sites also experience anthropogenic nitrogen (N) deposition or increased N availability as a result of invasion by N-fixing shrubs. We tested the hypothesis that greater seedling competitive ability and greater responsiveness to high N availability of exotic perennial grasses facilitates their invasion in coastal prairie. We evaluated pairwise competitive responses and effects, and the occurrence of asymmetrical competition, among three common native perennial grasses (Agrostis oregonensis, Festuca rubra, and Nassella pulchra) and three exotic perennial grasses (Holcus lanatus, Phalaris aquatica, and Festuca arundinacea), at two levels of soil N. We also compared the root and shoot biomass and response to fertilization of singly-grown plants, so we could evaluate how performance in competition related to innate plant traits. Competitive effects and responses were negatively correlated and in general varied continuously across native and exotic species. Two exceptions were the exotic species Holcus, which had large effects on neighbors and small responses to them, and competed asymmetrically with all other species in the experiment, and the native grass Nassella, which had strong responses to but little effect on neighbors, and was out-competed by all but one other species in the experiment. High allocation to roots and high early relative growth rate appear to explain Holcus’s competitive dominance, but its shoot biomass when grown alone was not significantly greater than those of the species it out-competed. Competitive dynamics were unaffected by fertilization. Therefore, we conclude that seedling competitive ability alone does not explain the increasing dominance of exotic perennial grasses in California coastal prairie. Furthermore, since native and exotic species responded individualistically, grouping species as ‘natives’ and ‘exotics’ obscured underlying variation within the two categories. Finally, elevated soil N does not appear to influence competition among the native and exotic perennial grasses studied, so reducing soil N pools may not be a critical step for the restoration of California coastal prairie.     

The grass may not always be greener: projected reductions in climatic suitability for exotic grasses under future climates in Australia

Climate change presents a new challenge for the management of invasive exotic species that threaten both biodiversity and agricultural productivity. The invasion of exotic perennial grasses throughout the globe is particularly problematic given their impacts on a broad range of native plant communities and livelihoods. As the climate continues to change, pre-emptive long-term management strategies for exotic grasses will become increasingly important. Using species distribution modelling we investigated potential changes to the location of climatically suitable habitat for some exotic perennial grass species currently in Australia, under a range of future climate scenarios for the decade centred around 2050. We focus on eleven species shortlisted or declared as the Weeds of National Significance or Alert List species in Australia, which have also become successful invaders in other parts of the world. Our results indicate that the extent of climatically suitable habitat available for all of the exotic grasses modelled is projected to decrease under climate scenarios for 2050. This reduction is most severe for the three species of Needle Grass (genus Nassella) that currently have infestations in the south-east of the continent. Combined with information on other aspects of establishment risk (e.g. demographic rates, human-use, propagule pressure), predictions of reduced climatic suitability provide justification for re-assessing which weeds are prioritised for intensive management as the climate changes.     

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.     

Resistance to Centaurea solstitialis invasion from annual and perennial grasses in California and Argentina

A common explanation for Centaurea solstitialis invasion in California is that it occupies an ??empty niche?? created by the replacement of native perennial grasses by exotic annual grasses and concomitant increases in soil water availability. This hypothesis, however, cannot explain C. solstitialis invasion into perennial-dominated grasslands of central Argentina. We assessed invasibility of annual versus perennial grass communities in these regions through parallel field experiments where we created grass plots and, after one year of establishment, measured effects on water and light, and added C. solstitialis seeds in two successive trials. Additionally, we removed vegetation around naturally occurring C. solstitialis in both regions, and examined the performance of Californian and Argentinean C. solstitialis individuals when growing under common conditions simulating climate in California and Argentina. In California, both grass types offered high resistance to C. solstitialis invasion, water was generally greater under perennials than annuals, and light was similarly low beneath both types. In Argentina, invasibility was generally greater in annual than perennial plots, water was similar between groups, and light was much greater beneath annuals. Removal experiments showed that competition from annual grasses in California and perennial grasses in Argentina greatly reduce C. solstitialis performance. Additionally, Californian and Argentinean individuals did not exhibit genetic differentiation in studied traits. Our results suggest that dominant plant functional groups in both California and Argentina offer substantial resistance to C. solstitialis invasion. The success of this species might be tightly linked to a remarkable ability to take advantage of disturbance in both regions.     

Grazing in remnant woodland vegetation: changes in species composition and life form groups

Abstract. Grazing by domestic livestock in native woodlands can have major effects on ecosystem functioning by the removal of plant species that form important functional groups. This paper documents the changes in floristics in a large group of remnants of native woodland left after agricultural clearing in southwestern Australia. Species richness and diversity were significantly reduced in remnants and the proportion of exotic species increased. Detrended Correspondence Analysis (DCA) was used to identify floristic and environmental patterns among plots and identified two distinct groups based on grazing intensity. This indicated that the significance of the relationship between grazing effects and DCA floristic axes was greater than edaphic characteristics that normally influence floristic patterns. Floristic characteristics of sites that were influencing the position of plots on the ordination diagram included proportion of exotic species and proportion of native perennial shrubs and herbs. Numbers of species of native shrubs and perennial herbs were significantly reduced in grazed plots and numbers of exotic annual grasses and herbs were significantly higher. Other life form groups such as native perennial grasses and geophytes were not significantly affected by grazing. Reproductive strategies of perennial species showed a significant decrease in numbers of resprouters and a significant increase in numbers of facultative seeder/sprouters. Exclosure plots showed increases in number and cover of perennial shrubs and herbs after three years whereas number and cover of exotic species did not change. Time series DCA showed that the floristic composition of exclosure plots in grazed sites became closer to that of the ungrazed sites.     

Mechanisms of resistance to invasion in a California grassland: the roles of competitor identity, resource availability, and environmental gradients

Resistance to the invasion of exotic plants may sometimes result from the strong effects of a relatively small number of resident species. Understanding the mechanisms by which such species resist invasion could provide important insights for the management of invaded ecosystems. Furthermore, the individualistic responses of community members to resource availability and environmental gradients could drive spatial variation in resistance at the local to landscape scales. We tested the resistance of monoculture plots of three native perennial grasses from the California coastal prairie to the invasion of the European perennial grass Holcus lanatus. We also used a watering treatment that increased early summer water availability and a natural elevational gradient in resource availability and soil texture to evaluate how resident identity interacted with abiotic resistance to affect Holcus establishment. Two native species, Festuca rubra and Calamagrostis nutkaensis , exhibited strong resistance, correlated with their negative effects on light availability. A third native grass, Bromus carinatus var. maritimus , had either no effect or a weakly facultative effect on Holcus performance relative to bare plots. Water addition did not alter the resistance of these species, but the elevation gradient did. Holcus invasion increased with improving abiotic conditions towards the slope bottom in bare and Bromus plots, but invasion decreased towards the bottom in Calamagrostis plots, where better conditions favored competitive residents. These results support the idea that resistance to invasion can sometimes be due to a subset of native species, and that the resistance provided by even a single species is likely to vary across the landscape. Identifying the mechanisms by which species resist invasion could facilitate the selection of management strategies that at best increase, or at worst do not decrease, natural resistance.     

The invasive grass,Melinis minutiflora,inhibits tree regeneration in a Neotropical savanna

Abstract Exotic grasses are becoming increasingly abundant in Neotropical savannas, with Melinis minutiflora Beauv. being particularly invasive. To better understand the consequences for the native flora, we performed a field study to test the effect of this species on the establishment, survival and growth of seedlings of seven tree species native to the savannas and forests of the Cerrado region of Brazil. Seeds of the tree species were sown in 40 study plots, of which 20 were sites dominated by M. minutiflora, and 20 were dominated by native grasses. The exotic grass had no discernable effect on initial seedling emergence, as defined by the number of seedlings present at the end of the first growing season. Subsequent seedling survival in plots dominated by M. minutiflora was less than half that of plots dominated by native species. Consequently, at the end of the third growing season, invaded plots had only 44% as many seedlings as plots with native grasses. Above‐ground grass biomass of invaded plots was more than twice that of uninvaded plots, while seedling survival was negatively correlated with grass biomass, suggesting that competition for light may explain the low seedling survival where M. minutiflora is dominant. Soils of invaded plots had higher mean Ca, Mg and Zn, but these variables did not account for the higher grass biomass or the lower seedling survival in invaded plots. The results indicate that this exotic grass is having substantial effects on the dynamics of the tree community, with likely consequences for ecosystem structure and function.     

Invasion by Non-Native Annual Grasses: The Importance of Species Biomass, Composition, and Time Among California Native Grasses of the Central Valley

The Central Valley of California is noted for its dearth of remnant native grass populations and for low native grass seedling establishment within grasslands now dominated by non‐native annual species. In contrast, remnant populations are common along the coast, and studies have shown an ability for seedlings and adults to compete with non‐native annual grasses. The invasibility of well‐established populations of native grasses in the Central Valley remains unclear. The objectives of this study were to compare the invasibility of native grasses differing in density and species composition and, given the species in this study, to assess the ability of mixes with greater species richness to resist invasion relative to their abilities in monoculture. In the Sacramento Valley of California, six species of native grasses were planted at three densities in monospecific and mixed‐species plots. Percent cover of native perennial and non‐native annual grasses was measured in years 2 and 3, and biomass was sampled in year 5. Native grass biomass and, to a lesser extent, species composition were important in explaining variation in non‐native grass invasibility in the fifth year. Species‐rich treatments did not experience less invasion than would be expected by the proportional invasibility of each species in monoculture. However, invasibility of plots consisting of slower growing, shorter statured species decreased over time, suggesting a successional benefit to diverse communities. This study demonstrates that established stands of native grasses in the Sacramento Valley can resist invasion by non‐native annual grasses and that stand biomass is a particularly important factor in determining invasibility.     

Do Tree Canopies Enhance Perennial Grass Restoration in California Oak Savannas?

Scattered trees in grass‐dominated ecosystems often act as islands of fertility with important influences on community structure. Despite the potential for these islands to be useful in restoring degraded rangelands, they can also serve as sites for the establishment of fast growing non‐native species. In California oak savannas, native perennial grasses are rare beneath isolated oaks and non‐native annual grasses dominate. To understand the mechanisms generating this pattern, and the potential for restoration of native grasses under oaks, we asked: what are the effects of the tree understory environment, the abundance of a dominant non‐native annual grass (Bromus diandrus), and soils beneath the trees on survival, growth, and reproduction of native perennial grass seedlings? We found oak canopies had a strong positive effect on survival of Stipa pulchra and Poa secunda. Growth and reproduction was enhanced by the canopy for Poa but negatively impacted for Stipa. We also found that Bromus suppressed growth and reproduction in Stipa and Poa, although less so for Stipa. These results suggest the oak understory may enhance survival of restored native perennial grass seedlings. The presence of exotic grasses can also suppress growth of native grasses, although only weakly for Stipa. The current limitation of native grasses to outside the canopy edge is potentially the result of interference from annual grasses under oaks, especially for short‐statured grasses like Poa. Therefore, control of non‐native annual grasses under tree canopies will enhance the establishment of S. pulchra and P. secunda when planted in California oak savannas.     

Grazing and productivity alter individual grass size dynamics in semi-arid woodlands

The spatial arrangement of perennial vegetation is critical for ecosystem function in drylands. While much is known about how vegetation patches respond to grazing and abiotic conditions, the size dynamics of individual plants is mostly limited to theoretical studies. We measured the size distribution (mean, variance, skewness) and density of individual grasses, and grass species composition at 451 sites spanning a range of grazing intensities across three broad vegetation communities in semi-arid eastern Australia. We assessed the relative role of grazing by livestock (cattle and sheep), native (kangaroos) and introduced (rabbits) free ranging herbivores, and several environmental measures (productivity, diversity, composition and groundstorey plant cover) on the size distribution and density of individual grasses. We found mean grass size and density were more sensitive to shifts in grazing intensity and environmental conditions than size variance or the frequency of the smallest individuals (skewness), and shifts were mostly driven by site productivity and cattle and kangaroo grazing. Sheep grazing only reduced mean grass size, and rabbit grazing had no consistent effects. Importantly, we found that site productivity and species composition altered the impacts of grazing on grass density and size distribution. For example, increasing cattle grazing led to larger grasses in low productivity sites. It also led to larger, denser, more variable-sized grasses among grass species from sites with finer soil texture. Increasing kangaroo grazing led to smaller, denser individuals among grass species from sites with coarse soil texture. At high diversity sites kangaroo grazing led to denser, more homogenised grass sizes with a lower frequency of small individuals. Understanding the in situ response of individual plant sizes gives us insights into the processes driving shifts in perennial vegetation patchiness, improving our ability to predict how the spatial arrangement of ecosystems might change under global change scenarios.     

Direct and indirect effects of exotic annual grasses on species composition of a South Australian grassland

Abstract Invasion by Mediterranean annual grasses, such as Avena L. spp. and Bronms L. spp, is one of the major threats to temperate perennial grassland. This study investigated the effects of annual grasses and their litter on the species composition of a grassland near Burra, South Australia. The placement of annual grass litter on soil samples in the glasshouse decreased the establishment or growth of several exotic annual dicots. In the field the addition of annual grass litter slightly decreased the frequency of Danthonia Lam. & DC. tussocks. Furthermore, litter strongly reduced the species richness from 13 species in plots with no litter to nine species in plots with the highest litter level, mainly by decreasing the frequency of common exotic dicots. Native dicot frequency similarly appeared to be decreased by litter addition. In addition to the negative effects of their litter, annual grasses also directly competed with perennial grasses. The magnitude of the competitive effect varied systematically along a slope, suggesting that other factors such as soil properties may control competitive inter actions. The biomass of annual grasses also tended to increase with the addition of their own litter. This combination of positive and negative feedback mechanisms suggests that brief periods favourable for annual grasses, either through management changes or environmental conditions, can lead to persistent changes in the species composition of the system.