Water relations in competitive interactions of Mediterranean grasses and shrubs

In Mediterranean ecosystems, competition between opportunistic grasses and slower-growing woody species may affect the speed and path of ecosystem recovery and the success of restoration plantings after natural or human-induced disturbance. In this experiment, competitive interactions between Mediterranean annual and perennial grass species (Avena fatua and Brachypodium retusum, respectively) and an important Mediterranean shrub (Rosmarinus offlcinalis) were examined under semi-controlled conditions simulating wet and dry Mediterranean rainfall regimes. The identity of the grass competitor and the level of water availability in the plots interacted to produce differing rates of R. offlcinalis growth but similar levels of mortality. In particular, competition with the perennial grass resulted in very low rates of R. offlcinalis growth at both irrigation levels. Measurements of soil water content showed that both grasses reduced soil moisture to low levels, though this effect was temporary in the case of the winter annual grass. Resistance to hydraulic flow in roots was highest in the perennial grass, smaller but of similar magnitude in the shrub, and much lower in the annual grass. Transpirational response to decreasing leaf water potential was a quick, sharp drop in conductance in R. offlcinalis, in contrast to a moderated decline from much lower initial transpiration rates in B. retusum. The annual grass largely maintained both leaf water potential and transpiration through leaf-tip senescence and death. Quantification of the rate of hydric recuperation of leaves after irrigation of drought-stressed plants showed that the perennial grass recovered at a rate four times that of R. offlcinalis, suggesting a strategy for making quick use of rare summer rains that may contribute to its competitive success. The appropriateness of planting or suppressing grasses in restoration of disturbed sites in Mediterranean Spain is discussed.     

Disturbance-mediated competition: the interacting roles of inundation regime and mechanical and herbicidal control in determining native and invasive plant abundance

Disturbance is a key component of many successful plant invasions. However, interactions among natural and anthropogenic disturbances and effects of these interacting disturbances on invasive plants and desired vegetation are rarely examined. We investigated the effect of anthropogenic disturbance (herbicidal and mechanical) along a natural inundation gradient (20–282 days) on the biomass and resource allocation of the invasive wetland plant, alligator weed (Alternanthera philoxeroides), and two co-occurring competitor plants, the introduced grass, kikuyu (Pennisetum clandestinum), and the native grass, couch (Cynodon dactylon), over a 2-year period. In the absence of additional disturbance, kikuyu biomass was negatively affected, alligator weed biomass was positively affected, and couch biomass was not affected by inundation disturbance. In addition, kikuyu was not affected by the selective removal of alligator weed, while couch increased in wetter habitats where kikuyu was absent due to inundation stress. This suggests that kikuyu is a superior competitor in drier habitats and inundation facilitates the invasion of alligator weed, while couch is an inferior competitor to both kikuyu and alligator weed and is therefore suppressed across its entire niche by these two introduced species. Mowing alone had no effect on the biomass of the species, suggesting the plants are equally tolerant of shoot removal. Selective herbicide reduced alligator weed biomass by 97.5% and the combination of selective herbicide and mowing reduced the biomass of alligator weed significantly more than herbicide alone, by 98.6% compared with un-manipulated controls. To predict community change and prevent sequential exotic plant invasions after weed removal, it is necessary to consider the interacting effects of disturbance and the niche space of invasive species in the local propagule pool.     

Vegetation analysis in colonies of an endangered rodent, the Cabrera vole (Microtus cabrerae), in southern Portugal

The Cabrera vole (Microtus cabrerae) is a threatened rodent endemic in the Iberian Peninsula with a patchy distribution and specific microhabitat requirements. This aim of this study was to document the composition of plant communities in habitats of Cabrera vole colonies in southern Portugal. Differences observed in plant species composition were also compared with vegetation structure, taxonomic and life form groups, species and group diversity, disturbance, topography and soil properties. Vegetation was sampled between March and July 2004, in 26 colonies occurring in five geographical areas. Grasses were the most abundant, common and diverse family in the colonies, and the perennial grass Agrostis castellana was present in 92% of colonies, with a mean cover of 16% of the site. Other frequently occurring species were Briza maxima (85%), Vulpia myuros (85%), Gaudinia fragilis (81%), Leontodon spp. (81%), Avena barbata (77%), Bromus hordeaceus (77%) and Tolpis barbata (77%). Colonies were classified in eight vegetation groups that included meadows, tall perennial grasslands, manured meadows with tall sedges, annual grasslands and ruderal and nitrophilous grasslands. Main gradients associated with composition differences were grass richness, annual and perennial grass cover, vegetation structure (herbaceous vegetation height), soil properties (texture and moisture), disturbance (ruderal species) and colony dimensions (area). Results suggest that the Cabrera vole is able to exploit a wide variety of grasslands, with a varying degree of ecological disturbance. Meadows and perennial grassland communities seem to be higher-quality microhabitats for voles.     

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.     

Does disturbance,competition or resource limitation underlie Hieracium lepidulum invasion in New Zealand? Mechanisms of establishment and persistence,and functional differentiation among invasive and native species

The processes underlying plant invasions have been the subject of much ecological research. Understanding mechanisms of plant invasions are difficult to elucidate from observations, yet are crucial for ecological management of invasions. Hieracium lepidulum, an asteraceous invader in New Zealand, is a species for which several explanatory mechanisms can be raised. Alternative mechanisms, including competitive dominance, disturbance of resident vegetation allowing competitive release or nutrient resource limitation reducing competition with the invader are raised to explain invasion. We tested these hypotheses in two field experiments which manipulated competitive, disturbance and nutrient environments in pre‐invasion and post‐invasion vegetation. H. lepidulum and resident responses to environmental treatments were measured to allow interpretation of underlying mechanisms of establishment and persistence. We found that H. lepidulum differed in functional response profile from native species. We also found that other exotic invaders at the sites were functionally different to H. lepidulum in their responses. These data support the hypothesis that different invaders use different invasion mechanisms from one another. These data also suggest that functional differentiation between invaders and native resident vegetation may be an important contributing factor allowing invasion. H. lepidulum appeared to have little direct competitive effect on post‐invasion vegetation, suggesting that competition was not a dominant mechanism maintaining its persistence. There was weak support for disturbance allowing initial establishment of H. lepidulum in pre‐invasion vegetation, but disturbance did not lead to invader dominance. Strong support for nutrient limitation of resident species was provided by the rapid competitive responses with added nutrients despite presence of H. lepidulum. Rapid competitive suppression of H. lepidulum once nutrient limitation was alleviated suggests that nutrient limitation may be an important process allowing the invader to dominate. Possible roles of historical site degradation and/or invader‐induced soil chemical/microbial changes in nutrient availability are discussed.     

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.     

Importance of soil moisture and its interaction with competition and clipping for two montane meadow grasses

Meadow classification studies have demonstrated the importance of water table fluctuation patterns in determining plant community composition in the western United States. However, a mechanism causing an overall increase in Poa pratensis ssp. pratensis populations and local declines in Deschampsia cespitosa populations in western montane meadows over the past century has not been defined. In order to better understand plant species interactions in these often highly grazed systems, we observed aboveground responses of Poa and Deschampsia to changes in species composition, soil moisture gradients, and clipping in the field. As well, we conducted a factorial greenhouse experiment, varying plant density, water availability, and clipping. While Poa is adapted to dry meadows and Deschampsia to wet meadows, their ranges overlap in wet conditions where soil moisture averages 50% in the early growing season. Deschampsia appears to be excluded from dry meadows where Poa is prevalent and soil moisture is closer to 30% water content in the early growing season. Our greenhouse experiment revealed that Deschampsia’s competitive ability decreases, while Poa’s increases, at soil moistures of 19%. However in more mesic conditions (50% soil moisture), each species aboveground biomass, tillering, and inflorescence weight was adherent to soil moisture conditions, and species interactions were less important. Our early growing season clipping treatments significantly reduced biomass of both grasses, but did not appear to favor one species over the other. This work points to the importance of soil water content in determining the performance of each plant species and the level of species interactions in montane meadows.     

Early emergence and resource availability can competitively favour natives over a functionally similar invader

Invasive plant species can form dense populations across large tracts of land. Based on these observations of dominance, invaders are often described as competitively superior, despite little direct evidence of competitive interactions with natives. The few studies that have measured competitive interactions have tended to compare an invader to natives that are unlikely to be strong competitors because they are functionally different. In this study, we measured competitive interactions among an invasive grass and two Australian native grasses that are functionally similar and widely distributed. We conducted a pair-wise glasshouse experiment, where we manipulated both biotic factors (timing of establishment, neighbour identity and density) and abiotic factors (nutrients and timing of water supply). We found that the invader significantly suppressed the performance of the natives; but its suppression ability was contingent on resource levels, with pulsed water/low nutrients or continuous watering reducing its competitive effects. The native grasses were able to suppress the performance of the invader when given a 3-week head-start, suggesting the invader may be incapable of establishing unless it emerges first, including in its own understorey. These findings provide insight for restoration, as the competitive effect of a functionally similar invader may be reduced by altering abiotic and biotic conditions in favour of natives.     

Invasion resistance and persistence: established plants win,even with disturbance and high propagule pressure

Disturbances and propagule pressure are key mechanisms in plant community resistance to invasion, as well as persistence of invasions. Few studies, however, have experimentally tested the interaction of these two mechanisms. We initiated a study in a southwestern ponderosa pine (Pinus ponderosa Laws.)/bunch grass system to determine the susceptibility of remnant native plant communities to cheatgrass (Bromus tectorum L.) invasion, and persistence of cheatgrass in invaded areas. We used a 2 × 2 factorial design consisting of two levels of aboveground biomass removal and two levels of reciprocal seeding. We seeded cheatgrass seeds in native plots and a native seed mixture in cheatgrass plots. Two biomass removal disturbances and sowing seeds over 3 years did not reverse cheatgrass dominance in invaded plots or native grass dominance in non-invaded native plots. Our results suggest that two factors dictated the persistence of the resident communities. First, bottlebrush squirreltail (Elymus elymoides (Raf.) Swezey) was the dominant native herbaceous species on the study site. This species is typically a poor competitor with cheatgrass as a seedling, but is a strong competitor when mature. Second, differences in pretreatment levels of plant-available soil nitrogen and phosphorus may have favored the dominant species in each community. Annual species typically require higher levels of plant-available soil nutrients than perennial plants. This trend was observed in the annual cheatgrass community and perennial native community. Our study shows that established plants and soil properties can buffer the influences of disturbance and elevated propagule pressure on cheatgrass invasion.     

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.     

Disturbance: a double-edged sword for restoration in a changing climate

Ecological restoration often relies on disturbance as a tool for establishing target plant communities, but disturbance can be a double-edged sword, at times initiating invasion and unintended outcomes. Here we test how fire disturbance, designed to enhance restoration seeding success, combines with climate and initial vegetation conditions to shift perennial versus annual grass dominance and overall community diversity in Pacific Northwest grasslands. We seeded both native and introduced perennial grasses and native forbs in paired, replicated burned-unburned plots in three sites along a latitudinal climate gradient from southern Oregon to central-western Washington. Past restoration and climate manipulations at each site had increased the variation of starting conditions between plots. Burning promoted the expansion of extant forbs and perennial grasses across all sites. Burning also enhanced the seeding success of native perennial grass and native forbs at the northern and central site, and the success of introduced perennial grasses across all three sites. Annual grass dominance was driven more by latitude than burning, with annuals maintaining their dominance in the south and perennials in the north. At the same time, unrestored grasslands surrounding all sites remained dominated by perennial grasses, suggesting that initial plot clearing may have allowed for annual grass invasion in the southern site. When paired with disturbance, further warming may increase the risk of annual grass dominance, a potentially persistent state.     

Ecological correlates of invasion by <Emphasis Type="Italic">Arundo donax</Emphasis> in three southern California riparian habitats

Arundo donax L. (Poaceae) is an aggressive invader in California’s riparian habitats. Field experiments were conducted to examine invader and site attributes important in early invasion. One hundred A. donax rhizomes were planted along five transects into each of three southern California riparian habitats. Pre-planting rhizome weight was recorded, along with site variables including percent bare ground, litter depth, PAR, soil moisture, soil temperature, incidence of herbivory, native canopy cover, and plant community richness and diversity. A. donax shoot emergence, survival time, and shoot height were recorded for approximately 10 months. The experiment was repeated over three years in different locations within each site. When years and sites were pooled to reveal large-scale patterns, A. donax performance was explained by rhizome weight, soil moisture, bare ground, soil temperature, and herbivory. When each site was considered singly, A. donax was positively correlated with different variables in each location. Species richness was correlated with A. donax performance in only one site. Our results indicate that A. donax establishment in riparian habitats is promoted by both vegetative reproduction and favorable abiotic environmental factors and relatively unaffected by the composition of the native community. The positive response of A. donax to disturbance (bare ground) and high resource availability (soil moisture), combined with a competitive perennial habit suggest that this species takes advantage of a competitive-ruderal life history. The ability of A. donax to respond to different conditions in each site combined with low genetic and phenotypic variation seen in other studies also suggests that a high degree of environmental tolerance contributes to invasion success.     

Reed Canary Grass (Phalaris arundinacea) as a Biological Model in the Study of Plant Invasions

Invasive species pose a serious threat to native plant communities and are an important contributor to loss of biodiversity. In the case of Phalaris arundinacea, L. (Poaceae), reed canary grass, a cool-season, long-lived perennial plant native to Eurasia and North America, nonnative agronomically important genotypes were introduced to North America for numerous uses such as forage and soil stabilization. Following repeated introductions, reed canary grass became an aggressive invader that takes over natural wet prairies, stream-banks and wetlands. Reed canary grass can outcompete native plant species, resulting in monospecific stands with concomitant loss of plant and insect diversity and ultimately to alteration in ecosystem function. Abiotic factors such as disturbance, changes in hydrological regime, and particularly nutrient runoff to wetlands can enhance reed canary grass establishment and vegetative spread. In addition, the species' capacity for early season growth, rapid vegetative spread, high stem elongation potential, wide physiological tolerance, and high architectural plasticity make the species highly aggressive under a wide range of ecological conditions. The change in life-history and environmental conditions responsible for the enhanced aggressiveness observed between native and invasive genotypes are not yet understood. Hence, reed canary grass provides an ideal study system to test a number of ecological and genetic hypotheses to explain why some plant species become extremely aggressive when transported into a new geographical area. To date, genetic studies have found that invasive populations have high genetic diversity and that genotypes differ in their phenotypic plasticity and response to ecological conditions, which may contribute to their invasion success. Finally comparative studies currently underway on European native and American invasive genotypes of reed canary grass should shed light on the mechanisms responsible for reed canary grass's aggressiveness and should provide an experimental protocol to test ecological and genetic hypotheses about what makes a species invasive.     

Non-native grass invasion alters native plant composition in experimental communities

Invasions of non-native species are considered to have significant impacts on native species, but few studies have quantified the direct effects of invasions on native community structure and composition. Many studies on the effects of invasions fail to distinguish between (1) differential responses of native and non-native species to environmental conditions, and (2) direct impacts of invasions on native communities. In particular, invasions may alter community assembly following disturbance and prevent recolonization of native species. To determine if invasions directly impact native communities, we established 32 experimental plots (27.5 m²) and seeded them with 12 native species. Then, we added seed of a non-native invasive grass (Microstegium vimineum) to half of the plots and compared native plant community responses between control and invaded plots. Invasion reduced native biomass by 46, 64, and 58%, respectively, over three growing seasons. After the second year of the experiment, invaded plots had 43% lower species richness and 38% lower diversity as calculated from the Shannon index. Nonmetric multidimensional scaling ordination showed a significant divergence in composition between invaded and control plots. Further, there was a strong negative relationship between invader and native plant biomass, signifying that native plants are more strongly suppressed in densely invaded areas. Our results show that a non-native invasive plant inhibits native species establishment and growth following disturbance and that native species do not gain competitive dominance after multiple growing seasons. Thus, plant invaders can alter the structure of native plant communities and reduce the success of restoration efforts.     

Frequent fire promotes diversity and cover of biological soil crusts in a derived temperate grassland

The intermediate disturbance hypothesis (IDH) predicts that species diversity is maximized at moderate disturbance levels. This model is often applied to grassy ecosystems, where disturbance can be important for maintaining vascular plant composition and diversity. However, effects of disturbance type and frequency on cover and diversity of non-vascular plants comprising biological soil crusts are poorly known, despite their potentially important role in ecosystem function. We established replicated disturbance regimes of different type (fire vs. mowing) and frequency (2, 4, 8 yearly and unburnt) in a high-quality, representative Themeda australis–Poa sieberiana derived grassland in south-eastern Australia. Effects on soil crust bryophytes and lichens (hereafter cryptogams) were measured after 12 years. Consistent with expectations under IDH, cryptogam richness and abundance declined under no disturbance, likely due to competitive exclusion by vascular plants as well as high soil turnover by soil invertebrates beneath thick grass. Disturbance type was also significant, with burning enhancing richness and abundance more than mowing. Contrary to expectations, however, cryptogam richness increased most dramatically under our most frequent and recent (2 year) burning regime, even when changes in abundance were accounted for by rarefaction analysis. Thus, from the perspective of cryptogams, 2-year burning was not an adequately severe disturbance regime to reduce diversity, highlighting the difficulty associated with expression of disturbance gradients in the application of IDH. Indeed, significant correlations with grassland structure suggest that cryptogam abundance and diversity in this relatively mesic (600 mm annual rainfall) grassland is maximised by frequent fires that reduce vegetation and litter cover, providing light, open areas and stable soil surfaces for colonisation. This contrasts with detrimental effects of 2-year burning on native perennial grasses, indicating that this proliferation of cryptogams has potentially high functional significance for situations where vegetation cover is depleted, particularly for reducing soil erosion.     

Plastic traits of an exotic grass contribute to its abundance but are not always favourable

In herbaceous ecosystems worldwide, biodiversity has been negatively impacted by changed grazing regimes and nutrient enrichment. Altered disturbance regimes are thought to favour invasive species that have a high phenotypic plasticity, although most studies measure plasticity under controlled conditions in the greenhouse and then assume plasticity is an advantage in the field. Here, we compare trait plasticity between three co-occurring, C(4) perennial grass species, an invader Eragrostis curvula, and natives Eragrostis sororia and Aristida personata to grazing and fertilizer in a three-year field trial. We measured abundances and several leaf traits known to correlate with strategies used by plants to fix carbon and acquire resources, i.e. specific leaf area (SLA), leaf dry matter content (LDMC), leaf nutrient concentrations (N, C:N, P), assimilation rates (Amax) and photosynthetic nitrogen use efficiency (PNUE). In the control treatment (grazed only), trait values for SLA, leaf C:N ratios, Amax and PNUE differed significantly between the three grass species. When trait values were compared across treatments, E. curvula showed higher trait plasticity than the native grasses, and this correlated with an increase in abundance across all but the grazed/fertilized treatment. The native grasses showed little trait plasticity in response to the treatments. Aristida personata decreased significantly in the treatments where E. curvula increased, and E. sororia abundance increased possibly due to increased rainfall and not in response to treatments or invader abundance. Overall, we found that plasticity did not favour an increase in abundance of E. curvula under the grazed/fertilized treatment likely because leaf nutrient contents increased and subsequently its' palatability to consumers. E. curvula also displayed a higher resource use efficiency than the native grasses. These findings suggest resource conditions and disturbance regimes can be manipulated to disadvantage the success of even plastic exotic species.     

An invasive grass shows colonization advantages over native grasses under conditions of low resource availability

Increased or fluctuating resources may facilitate opportunities for invasive exotic plants to dominate. This hypothesis does not, however, explain how invasive species succeed in regions characterized by low resource conditions or how these species persist in the lulls between high resource periods. We compare the growth of three co-occurring C₄ perennial bunchgrasses under low resource conditions: an exotic grass, Eragrostis curvula (African lovegrass) and two native grasses, Themeda triandra and Eragrostis sororia. We grew each species over 12?weeks under low nutrients and three low water regimes differentiated by timing: continuous, pulsed, and mixed treatments (switched from continuous to pulsed and back to continuous). Over time, we measured germination rates, time to germination (first and second generations), height, root biomass, vegetative biomass, and reproductive biomass. Contrary to our expectations that the pulsed watering regime would favor the invader, water-supply treatments had little significant effect on plant growth. We did find inherent advantages in a suite of early colonization traits that likely favor African lovegrass over the natives including faster germination speed, earlier flowering times, faster growth rates and from 2?weeks onward it was taller. African lovegrass also showed similar growth allocation strategies to the native grasses in terms of biomass levels belowground, but produced more vegetative biomass than kangaroo grass. Overall our results suggest that even under low resource conditions invasive plant species like African lovegrass can grow similarly to native grasses, and for some key colonization traits, like germination rate, perform better than natives.     

Effects of an African grass invasion on vegetation,soil and interstitial water characteristics in a tropical freshwater marsh in La Mancha,Veracruz (Mexico)

Abstract

The transformation of freshwater wetlands to pastures is a common practice in Mexico. This rapid loss of wetlands contrasts with the scarce information that exists about these ecosystems. To identify the environmental factors that control vegetation structure of a freshwater wetland invaded by the African grass Echinochloa pyramidalis, we characterized the vegetation (species composition, cover and aerial biomass), soil (moisture, redox potential, bulk density and topography) and water (water depth level, electric conductivity and pH) in two seasons of the year (dry and rainy). In addition, we analyzed the soil and water of three vegetation areas in the wetland, one dominated by E. pyramidalis, another by Sagittaria lancifolia and a third by Typha domingensis. The parameters associated with the hydrology of the wetland (water level, soil moisture, redox potential and bulk density) explained the plant species distribution. The invasive grass dominated in the relatively drier areas in the wetland while native species such as S. lancifolia, T. domingensis and Pontederia sagitatta dominated wetter sites. Introduction of E. pyramidalis has caused negative changes in the wetland, in particular a decrease of the diversity of plant species. In addition, we believe that the invader grass, as a C4 species, has more efficient use of water than the native plants, as well as a larger biomass, characteristics that can change the hydrological pattern of this wetland.     

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.