Plant litter effects on soil nutrient availability and vegetation dynamics: changes that occur when annual grasses invade shrub-steppe communities

Changes in the quantity and quality of plant litter occur in many ecosystems as they are invaded by exotic species, which impact soil nutrient cycling and plant community composition. Such changes in sagebrush-steppe communities are occurring with invasion of annual grasses (AG) into a perennial grass (PG) dominated system. We conducted a 5-year litter manipulation study located in the northern Great Basin, USA. Springtime litter was partially or completely removed in three communities with differing levels of invasion (invaded, mixed, and native) to determine how litter removal and litter biomass affected plant-available soil N and plant community composition. Litter biomass (prior to the removal treatment) was negatively correlated with plant-available N in the invaded community, but was positively correlated in the native community. Plant-available N had greater intra- and inter-annual fluctuations in the invaded compared to the mixed or native communities, but was not generally affected by removal treatments. Litter removal had negative effects on AG cover during a warm/dry year and negative effects on PG cover during a cool/wet year in the mixed community. Overall, the effectiveness of springtime litter manipulations on plant-available N were limited and weather dependent, and only removal treatments >75 % had effects on the plant community. Our study demonstrates how communities invaded by AGs have significantly increased temporal variability in nutrient cycling, which may decrease ecosystem stability. Further, we found that the ecological impacts from litter manipulation on sagebrush communities were dependent on the extent of AG invasion, the timing of removal, and seasonal precipitation.     

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.     

Interannual climate variability mediates changes in carbon and nitrogen pools caused by annual grass invasion in a semiarid shrubland

Exotic plant invasions alter ecosystem properties and threaten ecosystem functions globally. Interannual climate variability (ICV) influences both plant community composition (PCC) and soil properties, and interactions between ICV and PCC may influence nitrogen (N) and carbon (C) pools. We asked how ICV and non-native annual grass invasion covary to influence soil and plant N and C in a semiarid shrubland undergoing widespread ecosystem transformation due to invasions and altered fire regimes. We sampled four progressive stages of annual grass invasion at 20 sites across a large (25,000 km²) landscape for plant community composition, plant tissue N and C, and soil total N and C in 2013 and 2016, which followed 2 years of dry and wet conditions, respectively. Multivariate analyses and ANOVAs showed that in invasion stages where native shrub and perennial grass and forb communities were replaced by annual grass-dominated communities, the ecosystem lost more soil N and C in wet years. Path analysis showed that high water availability led to higher herbaceous cover in all invasion stages. In stages with native shrubs and perennial grasses, higher perennial grass cover was associated with increased soil C and N, while in annual-dominated stages, higher annual grass cover was associated with losses of soil C and N. Also, soil total C and C:N ratios were more homogeneous in annual-dominated invasion stages as indicated by within-site standard deviations. Loss of native shrubs and perennial grasses and forbs coupled with annual grass invasion may lead to long-term declines in soil N and C and hamper restoration efforts. Restoration strategies that use innovative techniques and novel species to address increasing temperatures and ICV and emphasize maintaining plant community structure—shrubs, grasses, and forbs—will allow sagebrush ecosystems to maintain C sequestration, soil fertility, and soil heterogeneity.     

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.     

Mechanisms underlying the impacts of exotic annual grasses in a coastal California meadow

Biological invasions can impact the abundance and diversity of native species, but the specific mechanisms remain poorly discerned. In California grasslands, invasion by European annual grasses has severely reduced the quality of habitat for native forb species. To understand how introduced grasses suppress native and exotic forbs, we examined the response of a Southern California grassland community to factorial removals of live grass and the litter produced in previous seasons. To examine the role that belowground competition for water plays in mediating the impact of grasses, we crossed grass and litter removal treatments with water addition. Our results show that forbs were almost equally suppressed by both competition from live grass and direct interference by litter. Water addition did not ameliorate the effect of grass competition, suggesting that water was not the resource for which plants compete. This evidence is consistent with the susceptibility of forbs to light limitation, especially considering that litter does not consume water or nutrients. Interestingly, despite different histories of co-occurrence with annual grass dominants, native and exotic forbs were comparably suppressed by exotic grasses. Our results indicate that suppression by both live and dead stems underlie the influence of exotic grasses on forb competitors.     

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.     

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.     

Nitrogen limitation, 15N tracer retention, and growth response in intact and Bromus tectorum-invaded Artemisia tridentata ssp. wyomingensis communities

Annual grass invasion into shrub-dominated ecosystems is associated with changes in nutrient cycling that may alter nitrogen (N) limitation and retention. Carbon (C) applications that reduce plant-available N have been suggested to give native perennial vegetation a competitive advantage over exotic annual grasses, but plant community and N retention responses to C addition remain poorly understood in these ecosystems. The main objectives of this study were to (1) evaluate the degree of N limitation of plant biomass in intact versus B. tectorum-invaded sagebrush communities, (2) determine if plant N limitation patterns are reflected in the strength of tracer ¹⁵N retention over two growing seasons, and (3) assess if the strength of plant N limitation predicts the efficacy of carbon additions intended to reduce soil N availability and plant growth. Labile C additions reduced biomass of exotic annual species; however, growth of native A. tridentata shrubs also declined. Exotic annual and native perennial plant communities had divergent responses to added N, with B. tectorum displaying greater ability to use added N to rapidly increase aboveground biomass, and native perennials increasing their tissue N concentration but showing little growth response. Few differences in N pools between the annual and native communities were detected. In contrast to expectations, however, more ¹⁵N was retained over two growing seasons in the invaded annual grass than in the native shrub community. Our data suggest that N cycling in converted exotic annual grasslands of the northern Intermountain West, USA, may retain N more strongly than previously thought.     

Plant community diversity and native plant abundance decline with increasing abundance of an exotic annual grass

Exotic plants are generally considered a serious problem in wildlands around the globe. However, some argue that the impacts of exotic plants have been exaggerated and that biodiversity and other important plant community characteristics are commonly improved with invasion. Thus, disagreement exists among ecologists as to the relationship of exotic plants with biodiversity and native plant communities. A better understanding of the relationships between exotic plants and native plant communities is needed to improve funding allocation and legislation regarding exotic plants, and justify and prioritize invasion management. To evaluate these relationships, 65 shrub–bunchgrass plant communities with varying densities of an exotic annual grass, Taeniatherum caput-medusae (L.) Nevski (medusahead), were sampled across 160,000 ha in southeastern Oregon, United States. Environmental factors were generally not correlated with plant community characteristics when exotic annual grass density was included in models. Plant diversity and species richness were negatively correlated with exotic annual grass density. Exotic annual grass density explained 62% of the variation in plant diversity. All native plant functional groups, except annual forbs, exhibited a negative relationship with T. caput-medusae. The results of this study suggest that T. caput-medusae invasions probably have substantial negative impacts on biodiversity and native plant communities. The strength of the relationships between plant community characteristics and T. caput-medusae density suggests that some exotic plants are a major force of change in plant communities and subsequently threaten ecosystem functions and processes. However, experimental studies are needed to fully substantiate that annual grass invasion is the cause of these observed correlations.     

Linking dominant Hawaiian tree species to understory development in recovering pastures via impacts on soils and litter

Large areas of tropical forest have been cleared and planted with exotic grass species for use as cattle pasture. These often remain persistent grasslands after grazer removal, which is problematic for restoring native forest communities. It is often hoped that remnant and/or planted trees can jump‐start forest succession; however, there is little mechanistic information on how different canopy species affect community trajectories. To investigate this, I surveyed understory communities, exotic grass biomass, standing litter pools, and soil properties under two dominant canopy trees—Metrosideros polymorpha (‘ōhi‘a) and Acacia koa (koa)—in recovering Hawaiian forests. I then used structural equation models (SEMs) to elucidate direct and indirect effects of trees on native understory. Native understory communities developed under ‘ōhi‘a, which had larger standing litter pools, lower soil nitrogen, and lower exotic grass biomass than koa. This pattern was variable, potentially due to historical site differences and/or distance to intact forest. Koa, in contrast, showed little understory development. Instead, data suggest that increased soil nitrogen under koa leads to high grass biomass that stalls native recruitment. SEMs suggested that indirect effects of trees via litter and soils were as or more important than direct effects for determining native cover. It is suggested that diverse plantings which incorporate species that have high carbon to nitrogen ratios may help ameliorate the negative indirect effects of koa on natural understory regeneration.     

The Effects of Exotic Grasses on Litter Decomposition in a Hawaiian Woodland: The Importance of Indirect Effects

Exotic grasses and grass-fueled fires have altered plant species composition in the seasonal submontane woodlands of Hawaii Volcanoes National Park. These changes have altered both structural and functional aspects of the plant community, which could, in turn, have consequences for litter decomposition and nitrogen (N) dynamics. In grass-invaded unburned woodland, grass removal plots within the woodland, and woodland converted to grassland by fire, we compared whole-system fluxes and the contributions of individual species to annual aboveground fine litterfall and litterfall N, and litter mass and net N loss. We assessed the direct contribution of grass biomass to decomposition and N dynamics, and we determined how grasses affected decomposition processes indirectly via effects on native species and alteration of the litter layer microenvironment. Grasses contributed 35% of the total annual aboveground fine litterfall in the invaded woodland. However, total litterfall mass and N were not different between the invaded woodland and the grass removal treatment because of compensation by the native tree Metrosideros polymorpha, which increased litter production by 37% ± 5% when grasses were removed. The 0.3 g N m^–2/y^–1 contained in this production increase was equal to the N contained in grass litter. Litter production and litterfall N was lowest in the grassland due to the loss of native litter inputs. Decomposition of litterfall on an area basis was highest in the grass-invaded woodland. We attributed this effect to increased inherent decomposability of native litter in the presence of grasses because (a) the microenvironment of the three vegetation treatments had little effect on decomposition of common litter types and (b) M. polymorpha litter produced in the invaded woodland decomposed faster than that produced in the grass removal plots due to higher lignin concentrations in the latter than in the former. Area-weighted decomposition was lowest in the grassland due to the absence of native litter inputs. Across all treatments, most litter types immobilized N throughout the incubation, and litter net N loss on an area basis was not different among treatments. Our results support the idea that the effects of a plant species or growth form on decomposition cannot be determined in isolation from the rest of the community or from the direct effects of litter quality and quantity alone. In this dry woodland, exotic grasses significantly altered decomposition processes through indirect effects on the quantity and quality of litter produced by native species.     

Invasion by <Emphasis Type="Italic">Aegilops triuncialis</Emphasis> (Barb Goatgrass) Slows Carbon and Nutrient Cycling in a Serpentine Grassland

Invasive plant species alter plant community composition and ecosystem function. In the United States, California native grasslands have been displaced almost completely by invasive annual grasses, with serpentine grasslands being one of the few remaining refugia for California grasslands. This study examined how the invasive annual grass, Aegilops triuncialis, has altered decomposition processes in a serpentine annual grassland. Our objectives were to (1) assess howA. triuncialis alters primary productivity and litter tissue chemistry, (2) determine whether A. triuncialis litter is more recalcitrant to decomposition than native litter, and (3) evaluate whether differences in the soil microbial community in A. triuncialis-invaded and native-dominated areas result in different decomposition rates of invasive and/or native plant litter. In invaded plant patches, A. triuncialis was approximately 50% of the total plant cover, in contrast to native plant patches in which A. triuncialis was not detected and native plants comprised over 90% of the total plant cover. End-of-season aboveground biomass was 2-fold higher in A. triuncialis dominated plots compared to native plots; however, there was no significant difference in belowground biomass. Both above- and below-ground plant litter from A. triuncialis plots had significantly higher lignin:N and C:N ratios and lower total N, P, and K than litter from native plant plots. Aboveground litter from native plots decomposed more rapidly than litter from A. triuncialis plots, although there was no difference in decomposition of belowground tissues. Soil microbial community composition associated with different soil patch types had no effect on decomposition rates. These data suggest that plant invasion impacts decomposition and nutrient cycling through changes in plant community tissue chemistry and biomass production.     

Conversion of sagebrush shrublands to exotic annual grasslands negatively impacts small mammal communities

Aim The exotic annual cheatgrass (Bromus tectorum) is fast replacing sagebrush (Artemisia tridentata) communities throughout the Great Basin Desert and nearby regions in the Western United States, impacting native plant communities and altering fire regimes, which contributes to the long‐term persistence of this weedy species. The effect of this conversion on native faunal communities remains largely unexamined. We assess the impact of conversion from native perennial to exotic annual plant communities on desert rodent communities. Location Wyoming big sagebrush shrublands and nearby sites previously converted to cheatgrass‐dominated annual grasslands in the Great Basin Desert, Utah, USA. Methods At two sites in Tooele County, Utah, USA, we investigated with Sherman live trapping whether intact sagebrush vegetation and nearby converted Bromus tectorum‐dominated vegetation differed in rodent abundance, diversity and community composition. Results Rodent abundance and species richness were considerably greater in sagebrush plots than in cheatgrass‐dominated plots. Nine species were captured in sagebrush plots; five of these were also trapped in cheatgrass plots, all at lower abundances than in the sagebrush. In contrast, cheatgrass‐dominated plots had no species that were not found in sagebrush. In addition, the site that had been converted to cheatgrass longer had lower abundances of rodents than the site more recently converted to cheatgrass‐dominated plots. Despite large differences in abundances and species richness, Simpson’s D diversity and Shannon‐Wiener diversity and Brillouin evenness indices did not differ between sagebrush and cheatgrass‐dominated plots. Main conclusions This survey of rodent communities in native sagebrush and in converted cheatgrass‐dominated vegetation suggests that the abundances and community composition of rodents may be shifting, potentially at the larger spatial scale of the entire Great Basin, where cheatgrass continues to invade and dominate more landscape at a rapid rate.     

Effects of exotic grass invasion on spatial heterogeneity in the ground-layer of grassy woodlands

We quantified the effects of exotic annual grass invasion on the ground-layer structure of grassy eucalypt woodlands, with the aim of determining if weed invasion decreased gap size and plant basal area leading to reduced spatial heterogeneity. We measured plant density, distance between plants and basal plant area in woodland sites which ranged from zero to 100% exotic plant cover in the ground-layer. The ground-layer in uninvaded woodlands was heterogeneous, with a large variation in basal plant area and distance between plants. Exotic annual grass density was positively correlated with total plant density, whereas native plant density was negatively correlated. Total plant basal area decreased as total plant density increased, with a lower total plant area in exotic dominated transects compared to native dominated. Variation in basal plant area decreased with increasing plant density. Exotic annual grasses were more closely spaced together (smaller gap size) and had a smaller basal area than the native grasses and rushes. There was also less variation in basal area and gap size with individual exotic annual grasses compared to the native grasses. Inter-plant distance was greater for both the native and exotic grasses when they had native grasses neighbouring them instead of exotic grasses. These findings show that woodlands invaded by exotic annual grasses have relatively less spatial heterogeneity in the ground-layer. These results have implications for other aspects of perennial grassy ecosystems invaded by annual grasses, including plant recruitment and restoration strategies.     

A water-centred framework to assess the effects of salinity on the growth and yield of wheat and barley

We conducted a field experiment in two alpine meadows to investigate the short-term effects of nitrogen enrichment and plant litter biomass on plant species richness, the percent cover of functional groups, soil microbial biomass, and enzyme activity in two alpine meadow communities. The addition of nitrogen fertilizer to experimental plots over two growing seasons increased plant production, as indicated by increases in both the living plant biomass and litter biomass in the Kobresia humilis meadow community. In contrast, fertilization had no significant effect on the amounts of living biomass and litter biomass in the K. tibetica meadow. The litter treatment results indicate that litter removal significantly increased the living biomass and decreased the litter biomass in the K. humilis meadow; however, litter-removal and litter-intact treatments had no impact on the amounts of living biomass and litter biomass in the K. tibetica meadow. Litter production depended on the degree of grass cover and was also influenced by nitrogen enrichment. The increase in plant biomass reflects a strong positive effect of nitrogen enrichment and litter removal on grasses in the K. humilis meadow. Neither fertilization nor litter removal had any impact on the grass biomass in the K. tibetica meadow. Sedge biomass was not significantly affected by either nutrient enrichment or litter removal in either alpine meadow community. The plant species richness decreased in the K. humilis meadow following nitrogen addition. In the K. humilis meadow, microbial biomass C increased significantly in response to the nitrogen enrichment and litter removal treatments. Enzyme activities differed depending on the enzyme and the different alpine meadow communities; in general, enzyme activities were higher in the upper soil layers (0–10 cm and 10–20 cm) than in the lower soil layers (20–40 cm). The amounts of living plant biomass and plant litter biomass in response to the different treatments of the two alpine meadow communities affected the soil microbial biomass C, soil organic C, and soil fertility. These results suggest that the original soil conditions, plant community composition, and community productivity are very important in regulating plant community productivity and microbial biomass and activity.     

Logging,exotic plant invasions,and native plant reassembly in a lowland tropical rain forest

Habitat modification and biological invasions are key drivers of global environmental change. However, the extent and impact of exotic plant invasions in modified tropical landscapes remain poorly understood. We examined whether logging drives exotic plant invasions and whether their combined influences alter understory plant community composition in lowland rain forests in Borneo. We tested the relationship between understory communities and local‐ and landscape‐scale logging intensity, using leaf area index (LAI) and aboveground biomass (AGB) data from 192 plots across a logging‐intensity gradient from primary to repeatedly logged forests. Overall, we found relatively low levels of exotic plant invasions, despite an intensive logging history. Exotic species were more speciose, had greater cover, and more biomass in sites with more local‐scale canopy loss. Surprisingly, though, exotic species invasion was not related to either landscape‐scale canopy loss or road configuration. Moreover, logging and invasion did not seem to be acting synergistically on native plant composition, except that seedlings of the canopy‐dominant Dipterocarpaceae family were less abundant in areas with higher exotic plant biomass. Current low levels of invasion, and limited association with native understory community change, suggest there is a window of opportunity to manage invasive impacts. We caution about potential lag effects and the possibly severe negative impacts of exotic plant invasions on the long‐term quality of tropical forest, particularly where agricultural plantations function as permanent seed sources for recurrent dispersal along logging roads. We therefore urge prioritization of strategic management plans to counter the growing threat of exotic plant invasions in modified tropical landscapes.     

Activated Carbon as a Restoration Tool: Potential for Control of Invasive Plants in Abandoned Agricultural Fields

Exotic plants have been found to use allelochemicals, positive plant–soil feedbacks, and high concentrations of soil nutrients to exercise a competitive advantage over native plants. Under laboratory conditions, activated carbon (AC) has shown the potential to reduce these advantages by sequestering organic compounds. It is not known, however, if AC can effectively sequester organics or reduce exotic plant growth under field conditions. On soils dominated by exotic plants, we found that AC additions (1% AC by mass in the top 10 cm of soil) reduced concentrations of extractable organic C and N and induced consistent changes in plant community composition. The cover of two dominant exotics, Bromus tectorum and Centaurea diffusa, decreased on AC plots compared to that on control plots (14–8% and 4–0.1%, respectively), and the cover of native perennial grasses increased on AC plots compared to that on control plots (1.4–3% cover). Despite promising responses to AC by these species, some exotic species responded positively to AC and some native species responded negatively to AC. Consequently, AC addition did not result in native plant communities similar to uninvaded sites, but AC did demonstrate potential as a soil‐based exotic plant control tool, especially for B. tectorum and C. diffusa.     

Simple plant traits explain functional group diversity decline in novel grassland communities of Texas

Previous research has found that plant diversity declines more quickly in exotic than native grassland plots, which offers a model system for testing whether diversity decline is associated with specific plant traits. In a common garden experiment in the Southern Great Plains in central Texas, USA, we studied monocultures and 9-species mixtures of either all exotic or all native grassland species. A total of 36 native and exotic species were paired by phylogeny and functional group. We used community-level measures (relative abundance in mixture) and whole-plant (height, aboveground biomass, and light capture) and leaf-level traits (area, specific leaf area, and C:N ratio) to determine whether trait differences explained native-exotic differences in functional group diversity. Increases in species’ relative abundance in mixture were correlated with high biomass, height, and light capture in both native and exotic communities. However, increasing exotic species were all C₄ grasses, whereas, increasing native species included forb, C₃ grass and C₄ grass species. Exotic C₄ grasses had traits associated with relatively high resource capture: greater leaf area, specific leaf area, height, biomass, and light capture, but similar leaf C:N ratios compared to native C₄ grasses. Leaf C:N was consistently higher for native than exotic C₃ species, implying that resource use efficiency was greater in natives than exotics. Our results suggest that functional diversity will differ between grasslands restored to native assemblages and those dominated by novel collections of exotic species, and that simple plant traits can help to explain diversity decline.     

Exotic grasses and feces deposition by an exotic herbivore combine to reduce the relative abundance of native forbs

Increased resource availability can facilitate establishment of exotic plant species, especially when coincident with propagule supply. Following establishment, increased resource availability may also facilitate the spread of exotic plant species if it enhances their competitive abilities relative to native species. Exotic Canada geese (Branta canadensis) introduce both exotic grass seed and nutrients to an endangered plant community on the Gulf Islands of southwestern British Columbia, Canada. I used greenhouse experiments to assess the competitive advantage of the exotic grasses relative to native and exotic forbs in this community and to test the impacts of nutrient addition from goose feces on competitive outcomes. I grew experimental communities varying in their proportion of forbs versus exotic grasses, and added goose feces as a nutrient source. I found that both native and exotic forbs produced significantly more biomass in competition with conspecifics than in competition with the grasses, and that the proportional abundance of two out of three native forbs was lowest in the combined presence of exotic grasses and nutrient addition. In a second experiment, I found that in monoculture all species of forbs and grasses showed equal growth responses to nutrients. The exotic species did not convert additional nutrients into additional biomass at a higher rate, but did germinate earlier and grow larger than the native species regardless of nutrient availability. This suggests that the exotic species may have achieved their competitive advantage partly by pre-empting resources in community mixtures. Small and late-germinating native forbs may be particularly vulnerable to competitive suppression from exotic grasses and forbs and may be at an even greater disadvantage if their competitors are benefiting from early access to additional nutrients. In combination, the input of exotic propagules and additional nutrients by nesting geese may compromise efforts to maintain native community composition in this system.     

Mechanisms of resistance of Mediterranean annual communities to invasion by Conyza bonariensis: effects of native functional composition

Recent studies have shown that a high species or functional group richness may not always lead to a greater resistance of plant communities to invasion, whereas species and/or functional group composition can more reliably predict invasion resistance. The aim of this study was to understand the mechanisms through which functional group composition can influence the resistance of Mediterranean annual communities to invasion by the exotic Conyza bonariensis . To analyse the effects of functional composition on the performance of individuals introduced as seedlings we first examined the relationships between the demographic and vegetative parameters of C. bonariensis and the biomass achieved by each functional group (grasses, legumes and Asteraceae rosettes) in synthetic communities. As a further step to approach the mechanisms involved in community resistance to invasion, we included in the analyses measurements of functional variables taken within the synthetic communities.
In agreement with earlier results and theory suggesting that high nutrient availability can favour invasions, an abundant legume biomass in communities increased the final biomass and net fecundity of C. bonariensis , due to positive effects on soil nitrate concentration. Survival and establishment of C. bonariensis were mainly favoured by a high biomass of Asteraceae. Additional results from measurements of herbivory suggested that C. bonariensis survival wasn't related to abiotic conditions but may be owed to a protection against herbivores in plots with abundant Asteraceae . Establishment was on the other hand likely to be hindered by the effects of abundant grass and legume foliage on light quality, and therefore easier within an Asteraceae canopy.
We conclude that invasion of Mediterranean old fields by species with biologies similar to C. bonariensis could be limited by favouring communities dominated by annual grasses.