Post‐fire shrub recruitment in a semi‐arid grassland: the role of microsites

Question: (1) Which factors regulate post‐fire recruitment and spread of the shrub Senecio bracteolatus in Patagonian grasslands? (2) What is the role of the grass Stipa speciosa on S. bracteolatus establishment in the post‐fire succession? Location: Northwest Patagonia, Argentina. Methods: We studied the effect of fire on S. bracteolatus recruitment and density by comparing these variables between burned and unburned grasslands. In burned areas, we compared abiotic characteristics and seedling establishment under the canopy of grasses (S. speciosa) and in gaps (inter‐tussock areas). Post‐fire interactions between S. bracteolatus seedlings and S. speciosa were studied using field and greenhouse experiments. Results: Density of S. bracteolatus was higher in burned than in unburned areas. In burned sites, seedlings were more abundant under tussock grasses, whereas juveniles were more abundant in gaps. Tussocks generated more attenuated micro‐environmental conditions than gaps during stressful summers. Gaps were more abundant in burned sites, while “under tussock” microsites were more frequent in unburned sites. In burned areas, tussocks allowed higher establishment of seedlings (facilitation), but gaps allowed more seedling growth and higher persistence of juveniles. Conclusions: Fire promoted S. bracteolatus recruitment in Patagonian grasslands by increasing the availability of favourable gap microsites. Grass protection for shrub seedlings became negative with time, probably due to competition with grasses. Gaps led to better performance and persistence of shrub plants. Six years after fire, higher shrub recruitment and adult density (observed as a trend) in burned grassland provides an opportunity for potential S. bracteolatus invasion.     

Long‐term changes in semi‐arid vegetation: Invasion of an exotic perennial grass has larger effects than rainfall variability

Abstract. Questions: This paper examines the long‐term change in the herbaceous layer of semi‐arid vegetation since grazing ceased. We asked whether (1) there were differences in the temporal trends of abundance among growth forms of plants; (2) season of rainfall affected the growth form response; (3) the presence of an invasive species influenced the abundance and species richness of native plants relative to non‐invaded plots, and (4) abundance of native plants and/or species richness was related to the time it took for an invasive species to invade a plot. Location: Alice Springs, Central Australia. Methods: Long‐term changes in the semi‐arid vegetation of Central Australia were measured over 28 years (1976–2004) to partition the effects of rainfall and an invasive perennial grass. The relative abundance (biomass) of all species was assessed 25 times in each of 24 plots (8 m × 1 m) across two sites that traversed floodplains and adjacent foot slopes. Photo‐points, starting in 1972, were also used to provide a broader overview of a landscape that had been intensively grazed by cattle and rabbits prior to the 1970s. Species’abundance data were amalgamated into growth forms to examine their relationship with environmental variation in space and time. Environmental variables included season and amount of rainfall, fire history, soil variability and the colonization of the plots by the exotic perennial grass Cenchrus ciliaris (Buffel grass). Results: Constrained ordination showed that season of rainfall and landscape variables relating to soil depth strongly influenced vegetation composition when Cenchrus was used as a covariate. When Cenchrus was included in constrained ordination, it was strongly related to the decline of all native growth forms over time. Univariate comparisons of non‐invaded vs impacted plots over time revealed unequivocal evidence that Cenchrus had caused the decline of all native growth form groups and species richness. They also revealed a contrasting response of native plants to season of rainfall, with a strong response of native grasses to summer rainfall and forbs to winter rainfall. In the presence of Cenchrus these responses were strongly attenuated. Discussion: Pronounced changes in the composition of vegetation were interpreted as a response to removal of grazing pressure, fluctuations in rainfall and, most importantly, invasion of an exotic grass. Declines in herbaceous species abundance and richness in the presence of Cenchrus appear to be directly related to competition for resources. Indirect effects may also be causing the declines of some woody species from changed fire regimes as a result of increased fuel loads. We predict that Cenchrus will begin to alter landscape level processes as a result of the direct and indirect effects of Cenchrus on the demography of native plants when there is a switch from resource limited (rainfall) establishment of native plants to seed limited recruitment.     

Sonoran Desert Ecosystem transformation by a C4 grass without the grass/fire cycle

Aim Biological invasions facilitate ecosystem transformation by altering the structure and function, diversity, dominance and disturbance regimes. A classic case is the grass–fire cycle in which grass invasion increases the frequency, scale and/or intensity of wildfires and promotes the continued invasion of invasive grasses. Despite wide acceptance of the grass–fire cycle, questions linger about the relative roles that interspecific plant competition and fire play in ecosystem transformations. Location Sonoran Desert Arizona Upland of the Santa Catalina Mountains, Arizona, USA. Methods We measured species cover, density and saguaro (Carnegiea gigantea) size structure along gradients of Pennisetum ciliare invasion at 10 unburned/ungrazed P. ciliare patches. Regression models quantified differences in diversity, cover and density with respect to P. ciliare cover, and residence time and a Fisher’s exact test detected demographic changes in saguaro populations. Because P. ciliare may have initially invaded locations that were both more invasible and less diverse, we ran analyses with and without the plots in which initial infestations were located. Results Richness and diversity decreased with P. ciliare cover as did cover and density of most dominant species. Richness and diversity declined with increasing time since invasion, suggesting an ongoing transformation. The proportion of old‐to‐young Carnegiea gigantea was significantly lower in plots with dominant P. ciliare cover. Main conclusions Rich desert scrub (15–25 species per plot) was transformed into depauperate grassland (2–5 species per plot) within 20 years following P. ciliare invasion without changes to the fire regime. While the onset of a grass–fire cycle may drive ecosystem change in the later stages and larger scales of grass invasions of arid lands, competition by P. ciliare can drive small‐scale transformations earlier in the invasion. Linking competition‐induced transformation rates with spatially explicit models of spread may be necessary for predicting landscape‐level impacts on ecosystem processes in advance of a grass–fire cycle.     

Barriers to shrub reestablishment following fire in the seasonal submontane zone of Hawai'i

Introduced grass species have invaded extensive areas of Hawaii Volcanoes National Park and increased the size and frequency of fire. Following fire, grass cover is enhanced while native shrub cover is reduced; the reduction in most shrubs persists for at least 20 years even in the absence of fire. Shrub seedlings were planted in burned and unburned plots with and without grass cover. Biomass of 14 month old shrub seedlings was generally highest in recently burned/grass removed plots, intermediate in old burn/grass removed plots, and lowest in unburned/grass removed plots. In contrast, shrub biomass in plots with grass cover was low and did not differ significantly among burn treatments. Light competition is likely to be responsible for differences in shrub growth rates; grass cover reduced light to 1–10% of background levels. In addition, pool sizes of available soil N were highest in recently burned, intermediate in old burn, and lowest in unburned areas.     

Variation in the impact of exotic grasses on native plant composition in relation to fire across an elevation gradient in Hawaii

The impact that an exotic species can have on the composition of the community it enters is a function of its abundance, its particular species traits and characteristics of the recipient community. In this study we examined species composition in 14 sites burned in fires fuelled by non‐indigenous C4 grasses in Hawaii Volcanoes National Park, Hawaii. We considered fire intensity, time since fire, climatic zone of site, unburned grass cover, unburned native cover and identity of the most abundant exotic grass in the adjacent unburned site as potential predictor variables of the impact of fire upon native species. We found that climatic zone was the single best variable for explaining variation in native cover among burned sites and between burned and unburned pairs. Fire in the eastern coastal lowlands had a very small effect on native plant cover and often stimulated native species regeneration, whereas fire in the seasonal submontane zone consistently caused a decline in native species cover and almost no species were fire tolerant. The dominant shrub, Styphelia tameiameia, in particular was fire intolerant. The number of years since fire, fire intensity and native cover in reference sites were not significantly correlated with native species cover in burned sites. The particular species of grass that carried the fire did however, have a significant effect on native species recovery. Where the African grass Melinis minutiflora was a dominant or codominant species, fire impacts were more severe than where it was absent regardless of climate zone. Overall, the impacts of exotic grass‐fuelled fires on native species composition and cover in seasonally dry Hawaiian ecosystems was context specific. This specificity is best explained by differences between the climatic zones in which fire occurred. Elevation was the main physical variable that differed among the climatic zones and it alone could explain a large percentage of the variation in native cover among sites. Rainfall, by contrast, did not vary systematically with elevation. Elevation is associated with differences in composition of the native species assemblages. In the coastal lowlands, the native grass Heteropogon contortus, was largely responsible for positive changes in native cover after fire although other native species also increased. Like the exotic grasses, this species is a perennial C4 grass. It is lacking in the submontane zone and there are no comparable native species there and almost all native species in the submontane zone were reduced by fire. The lack of fire tolerant species in the submontane zone thus clearly contributes to the devastating impact of fire upon native cover there.     

Effects of invasive plants on fire regimes and postfire vegetation diversity in an arid ecosystem

We assessed the impacts of co‐occurring invasive plant species on fire regimes and postfire native communities in the Mojave Desert, western USA. We analyzed the distribution and co‐occurrence patterns of three invasive annual grasses (Bromus rubens, Bromus tectorum, and Schismus spp.) known to alter fuel conditions and community structure, and an invasive forb (Erodium cicutarium) which dominates postfire sites. We developed species distribution models (SDMs) for each of the four taxa and analyzed field plot data to assess the relationship between invasives and fire frequency, years postfire, and the impacts on postfire native herbaceous diversity. Most of the Mojave Desert is highly suitable for at least one of the four invasive species, and 76% of the ecoregion is predicted to have high or very high suitability for the joint occurrence of B. rubens and B. tectorum and 42% high or very high suitability for the joint occurrence of the two Bromus species and E. cicutarium. Analysis of cover from plot data indicated two or more of the species occurred in 77% of the plots, with their cover doubling with each additional species. We found invasive cover in burned plots increased for the first 20 years postfire and recorded two to five times more cover in burned than unburned plots. Analysis also indicated that native species diversity and evenness as negatively associated with higher levels of relative cover of the four invasive taxa. Our findings revealed overlapping distributions of the four invasives; a strong relationship between the invasives and fire frequency; and significant negative impacts of invasives on native herbaceous diversity in the Mojave. This suggests predicting the distributions of co‐occurring invasive species, especially transformer species, will provide a better understanding of where native‐dominated communities are most vulnerable to transformations following fire or other disturbances.     

Restoration of C4 grasses with seasonal fires in a C3/C4 grassland invaded by Prosopis glandulosa,a fire‐resistant shrub

Questions: Can prescribed fire restore C₄ perennial grasses in grassland ecosystems that have become dominated by fire‐resistant C₃ shrubs (Prosopis glandulosa) and C₃ grasses? Do fires in different seasons alter the direction of change in grass composition? Location: Texas, USA. Methods: We quantified short‐ and long‐term (12 yr post‐fire) herbaceous functional group cover and diversity responses to replicated seasonal fire treatments: (1) repeated‐winter fires (three in 5 yr), (2) repeated‐summer fires (two in 3 yr), and (3) alternate‐season fires (two winter and one summer in 4 yr), compared with a no‐fire control. Results: Summer fires were more intense than winter fires, but all fire treatments temporarily decreased Prosopis and C₃ annual grass cover. The alternate‐season fire treatment caused a long‐term increase in C₄ mid‐grass cover and functional group diversity. The repeated‐summer fire treatment increased C₄ short‐grass cover but also caused a long‐term increase in bare ground. The repeated winter fire treatment had no long‐term effects on perennial grass cover. Mesquite post‐fire regrowth had increasingly negative impacts on herbaceous cover in all fire treatments. Conclusions: Summer fire was necessary to shift herbaceous composition toward C₄ mid‐grasses. However, the repeated‐summer fire treatment may have been too extreme and caused post‐fire herbaceous composition to “over‐shift” toward less productive C₄ short‐grasses rather than C₄ mid‐grasses. This study provides some of the first long‐term data showing a possible benefit of mixing seasonal fires (i.e., the alternate‐season fire treatment) in a prescribed burning management plan to restore C₄ mid‐grass cover and enhance overall herbaceous diversity.     

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.     

Vegetation responses to wildfire in native versus exotic Arizona grassland

Abstract. Grass and herb cover, and woody plant densities were measured on 25 native and 25 exotic grassland plots in southeastern Arizona between 1984 and 1990. At least 40 yr previously, the exotic plots had been seeded with two species of lovegrasses (Eragrostis spp.) native to southern Africa. A 1987 wildfire burned 11 native and 11 exotic plots. The fire reduced cover of both native and African grasses for two post-fire growing seasons. Herb cover as a whole increased after the fire for 2 yr, although there were important differences among species. One of two dominant shrubs (Haplopappus tenuisectus) was killed by the fire, while the other (Baccharis pteronioides) was little affected. Mesquite trees (Prosopis juliflora) were killed to the ground by the fire, but 62 of 66 trees had re-sprouted to an average 48% of pre-burn height by 1990. Native and exotic grasses appeared equally tolerant of fire, probably because both evolved in fire-type ecosystems. There was no evidence that fire can be used to permanently restore the diverse native flora to species-poor plantations of the South African exotics.     

Factors affecting seedling recruitment in an invasive grass (Pennisetum setaceum) and a native grass (Heteropogon contortus) in the Hawaiian Islands

In the Hawaiian Islands, native Heteropogon contortus (pili grass) is being replaced by alien grasses, one of which is Pennisetum setaceum (fountain grass). Both grasses depend on seeds for population growth. To help understand factors promoting the spread of the alien and decline of the native, we investigated the effects of physical disturbance, nutrient addition, and seed supplementation on seedling recruitment in experimental field plots. In the first year, our field site experienced an unusual drought, and seedling recruitment was greater for H. contortus than for P. setaceum under all treatments. Disturbance increased recruitment of H. contortus seedlings during some sampling periods. Recruitment was not significantly increased by seed additions for either species despite our finding of only 49 and 4 seeds m^–2 in the seed bank for H. contortus and P. setaceum, respectively. In the first year, most P. setaceum seedlings died between monthly surveys. We resurveyed our field plots in a second, wetter year and found the pattern was reversed: recruitment of P. setaceum seedlings was greater than H. contortus seedlings in most treatments. Greenhouse comparisons of seedling survival under three drought regimes (water every 5,7 and 10 days) revealed that H. contortus seedlings tolerate drought better than P. setaceum seedlings. Seedling recruitment for these species in the leeward Hawaiian Islands appears to be primarily dependent on water availability, with the alien having the advantage in wetter years. Once seedlings of the long-lived alien become established, the alien seems capable of maintaining its dominance over H. contortus, even during periods of drought.     

Mowing Reduces Exotic Annual Grasses but Increases Exotic Forbs in a Semiarid Grassland

Cheatgrass (Bromus tectorum) and other exotic winter‐active plants can be persistent invaders in native grasslands, growing earlier in the spring than native plants and pre‐empting soil resources. Effective management strategies are needed to reduce their abundance while encouraging the reestablishment of desirable native plants. In this 4‐year study, we investigated whether mowing and seeding with native perennial grasses could limit growth of exotic winter‐actives, and benefit growth of native plants in an invaded grassland in Colorado, United States. We established a split‐plot experiment in October 2008 with 3 mowing treatments: control, spring‐mowed, and spring/summer‐mowed (late spring, mid‐summer, and late summer), and 3 within‐plot seeding treatments: control, added B. tectorum seeds, and added native grass seeds. Cover of plant species and aboveground biomass were measured for 3 years. In March and June of 2010, 2011, and March of 2012, B. tectorum and other winter‐annual grasses were half as abundant in both mowing treatments as in control plots; however, cover of non‐native winter‐active forbs increased 2‐fold in spring‐mowed plots and almost 3‐fold in spring/summer‐mowed plots relative to controls. These patterns remained consistent 1 year after termination of treatments. Native cool‐season grasses were most abundant in spring‐mowed plots, and least abundant in control plots. There was higher cover of native warm‐season grasses in spring/summer‐mowed plots than in control plots in July 2011 and 2012. The timing of management can have strong effects on plant community dynamics in grasslands, and this experiment indicates that adaptive management can target the temporal niche of undesirable invasive species.     

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.     

Post‐fire succession and 20th century reduction in fire frequency on xeric southern Appalachian sites

Abstract. We document post‐fire succession on xeric sites in the southern Appalachian Mountains, USA and assess effects of 20th century reduction in fire frequency on vegetation structure and composition. Successional studies over 18 yr on permanent plots that had burned in 1976–1977 indicate that tree mortality and vegetation response varied with fuel load and fire season. In the first three years after fire, hardwood sprouts dominated tree regeneration. On sites where summer and autumn fires reduced litter depth to less than 1 cm, densities of shade‐intolerant Pinus seedlings increased steadily over this period. 4 to 8 yr after fire, large numbers of newly established seedlings and sprouts had grown to 1 – 10 cm DBH. By year 18 growth of these saplings led to canopy closure on most sites. Herbaceous cover and richness peaked in the first decade after fire, then declined. On similar sites that had not burned in more than 50 yr, regeneration of shade‐intolerant Pinus spp. and mean cover and richness of herbs were considerably lower than those observed on recently burned plots. Reconstructions of landscape conditions based on observed post‐fire succession and 20th century changes in fire regime suggest that reductions in fire frequency circa 1940 led to substantial changes in forest structure and decreases in cover and richness of herbaceous species.     

Disturbance,drought and dynamics of desert dune grassland,South Africa

A seven-year study of marked plants and plots in Stipagrostis ciliata (Desf.) de Winter dune grassland, in the arid (<100 mm yr^–1) Bushmanland area of the Northern Cape province of South Africa, was designed to test the hypothesis that establishment of ephemeral plants, and recruitment of perennial grasses was dependent upon disturbances that reduced the density of living perennial grass tussocks. In 1989, eight 4 m² plots were cleared of perennial vegetation by uprooting and removing all plants so as to resemble small-scale disturbances made by burrowing mammals or territorial antelope. The vegetation on the cleared plots and surroundings was monitored until 1996. Initial results supported our hypothesis. In wet years, when ephemeral plants were abundant, their average fresh mass was 2–3 times greater per unit area on the cleared plots than in control plots in adjacent, undisturbed grassland. Many Stipagrostis seedlings established in the cleared plots over the two years following clearing but were rare in adjacent areas among established conspecifics. However, a drought in 1992 (11 mm of rain over 12 months) lead to widespread mortality of the perennial grass, killing 56% (range 22–79%) of established tufts. High densities of Stipagrostis seedlings appeared following the drought-breaking rains in January 1993, both in the disturbed plots and in the surrounding `undisturbed' dune grassland. Ephemeral plants established in large numbers throughout the area during the high rainfall year of 1996 and were generally more numerous in the old disturbances than in control plots. Seven years after clearing the biomass of grass on the cleared plots was approximately 34% of the mass removed from the plots in 1989 whereas in the undisturbed grassland biomass was 66% of 1989 levels. Drought had little long-term effect on community composition, and Stipagrostis ciliata constituted 94–98% of plant community before and after drought. Cleared plots were recolonised by S. ciliata, but the contribution of other grass species increased by 6–9%. Synchronous recruitment following occasional drought-induced mortality can generate even-aged populations of the dominant desert dune grasses.     

Effects of invasion of fire‐free arid shrublands by a fire‐promoting invasive alien grass (Pennisetum setaceum) in South Africa

Arid shrublands in the Karoo (South Africa) seldom accumulate sufficient combustible fuel to support fire. However, as a result of invasion by an alien perennial grass (Pennisetum setaceum), they could become flammable. This paper reports on an experiment to assess the effects of fire following invasion by P. setaceum. We established 10 plots (5 × 10 m) separated by 2.5 m, and added grass fuel to five plots (5 and 10 tons ha^?1 to alternate halves of the plot) leaving the remaining five plots as interspersed controls. Plots with fuel added were burnt, and fire behaviour was measured during the burns. Rates of fire spread were generally low (0.01–0.07 m s^?1) and did not differ significantly between burn treatments. Mean fireline intensities were higher in the high compared with the low fuel treatments (894 and 427 kW m^?1, respectively). We recorded plant species and their cover before and after burning on each of the plots. After 15 months of follow‐up monitoring in the burn plots, only two species, the dwarf shrub (Tripteris sinuata) and the perennial herb (Gazania krebsiana) resprouted. Most individuals of other species were killed and did not reseed during the 15‐month study. The mass of added fuel load (high or low) did not influence vegetation recovery rates after fire. Should future invasions by P. setaceum lead to similar fuel loads in these shrublands, inevitable fires could change the vegetation and may favour spread of the flammable grass. Our results have important implications for predicting the effects of invasive alien plants (especially grasses) on fire‐free ecosystems elsewhere. The predicted impacts of fire may alter species composition, ultimately affecting core natural resources that support the Karoo economy.     

The contribution of grazing to plant diversity in alpine grassland and heath

Abstract Grazing on transplants of a grass, a forb and a tree was examined in low-diversity grassland and more diverse heath in Australia's Snowy Mountains. Transplants were surrounded by 2 mm mesh netting. In one grassland plot, grazers (probably soil invertebrates) attacked 40–90% of tree and forb seedlings but no grass seedlings. In heath, which had about half the grass cover of grassland, grazers consumed grasses but not trees or forbs. The results suggest that grazers can depress diversity in grassland by attacking species other than grass. In heath, they may promote diversity by attacking only grass and releasing other species from competition.     

Size of the local species pool determines invasibility of a C4-dominated grassland

The size of the local species pool (i.e., species surrounding a community capable of dispersal into that community) and other dispersal limitations strongly influence native plant community composition. However, the role that the local species pool plays in determining the invasibility of communities by exotic plants remains to be evaluated. We hypothesized that the richness and abundance of exotic species would be greater in C4‐dominated grassland communities if the local species pool included a larger proportion of exotic species. We also predicted that an increase in the exotic species pool would increase the invasibility of sites thought to be resistant to invasion (annually burned grassland). To test these hypotheses, study plots were established within two long‐term (>20 yr) fire experiments at a tallgrass prairie preserve in NE Kansas (USA). Study plots were surrounded by either a small pool of exotic species (small species pool (SSP) plots; six species) or a larger exotic species pool (large species pool (LSP) plots; 18 species). We found that richness and absolute cover of exotic species was significantly (P<0.001) lower (～70 and 90%, respectively) in annually burned compared to unburned plots, regardless of the size of the exotic species pool. As predicted, exotic species richness was higher (P<0.001) for LSP plots (3.9 per 250 m2) than for SSP plots (0.7 per 250 m2); however, absolute cover was unaffected by the size of the exotic species pool. In the absence of fire, plots with a LSP had four times as many exotic species than SSP plots. An increase in the local exotic species pool also increased the invasibility of annually burned grassland. Indeed, richness of exotic plant species in annually burned LSP plots did not differ from unburned plots with a SSP, indicating that a larger pool of exotic species countered the negative effects of fire. These findings have important implications for predicting how the invasion of plant communities may respond to human‐induced global changes, such as habitat fragmentation. Community characteristics or factors such as frequent fires in grasslands may impart resistance to invasions by exotic species in large, intact ecosystems. However, when a large pool of exotic species is present, frequent fire may not be sufficient to limit the invasions of exotic plants in fragmented landscapes.     

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.     

Effects of fire on the structure and diversity of a Spartina argentinensis tall grassland

Question: What are the effects of fire on the structure and the diversity of a Spartina ar gentinensis tall grassland in the short and medium run. S. argentinensis is the dominant species of tall grasslands on inland marshes of the Chaco‐Pampean region (Argentina), where spontaneous or man‐made fires are very frequent. Location: Federico Wildermuth Reserve (31°57′S; 61°23′W), Province of Santa Fe, Argentina, an area never ploughed that supported cattle until its exclusion in 1988. Methods: Vegetation was surveyed in randomly placed permanent plots using the Braun‐Blanquet cover‐abundance scale. The data were analysed by multivariate methods (PCA and MRPP) for synthesizing information and testing hypotheses. Results: Fire did not have a long‐lasting effect on the tall grassland. There was only a temporarily reduction of cover‐abundance of S. argentinensis which allowed an increase in the number of accompanying species such as Heliotropium curassavicum, Pluchea sagittalis, and Verbena litoralis and of some naturalised or weed species, such as Melilotus alba and Cirsium vulgare, respectively. Fire increased diversity, which remained relatively high for two to three years. Three years after the fire there were no significant differences on the amount of litter accumulated on burned and unburned plots. Conclusion: Fire should be considered an intrinsic part of the dynamics of S. argentinensis tall grassland.     

Effects of Nutrient Manipulations and Grass Removal on Cover,Species Composition,and Invasibility of a Novel Grassland in Colorado

Cover and richness of a 5‐year revegetation effort were studied with ,respect to small‐scale disturbance and nutrient manipulations. The site, originally a relict tallgrass prairie mined for gravel, was replanted to native grasses using a seed mixture of tall‐, mixed‐, and short‐grass species. Following one wet and three relatively dry years, a community emerged, dominated by species common in saline soils not found along the Colorado Front Range. A single species, Alkali sacaton (Sporobolus airoides), composed nearly 50% of relative vegetation cover in control plots exhibiting a negative relationship between cover and richness. Seeded species composed approximately 92% of vegetation cover. The remaining 8% was composed of weeds from nearby areas, seed bank survivors, or mix contaminants. Three years of soil nutrient amendments, which lowered plant‐available nitrogen and phosphorus, significantly increased relative cover of seeded species to 97.5%. Fertilizer additions of phosphate enhanced abundance of introduced annual grasses (Bromus spp.) but did not significantly alter cover in control plots. Unmanipulated 4‐m² plots contained an average of 4.7 planted species and 3.9 nonplanted species during the 5‐year period, whereas plots that received grass herbicide averaged 5.4 nonplanted species. Species richness ranged from an average 6.9 species in low‐nutrient, undisturbed plots to 10.9 species in the relatively high‐nutrient, disturbed plots. The use of stockpiled soils, applied sparingly, in conjunction with a native seed mix containing species uncommon to the preexisting community generated a species‐depauperate, novel plant community that appears resistant to invasion by ruderal species.