Impact of early root competition on fitness components of four semiarid species

Summary Plant demographic and root exclusion approaches were used to examine the influence of roots of adult Artemisia tridentata, Agropyron desertorum, and Agropyron spicatum individuals on seedling survival of four C₃ semiarid species, three perennials, Ar. tridentata, Ag. desertorum, Ag. spicatum, and an annual, Bromus tectorum. Furthermore, height of Ar. tridentata seedlings and seed production of B. tectorum were assessed. The probability of a seedling being alive significantly depended on the seedling species, the neighboring adult species, and on the depth to which root competition was excluded. As seedlings, survival of Agropyron species did not differ, whereas survival of Ar. tridentata seedlings was higher than Ag. desertorum and was similar to Ag. spicatum. Bromus tectorum maintained significantly higher survival rates than perennial seedlings. Established individuals of Ar. tridentata reduced seedling survival more than established individuals of either Agropyron species. Seedling survival significantly increased with greater depth of root exclusion for the perennials but did not significantly affect seedling survival of B. tectorum. Height of Ar. tridentata seedlings and seed production of B. tectorum significantly increased with depth of root exclusion. Seed production of B. tectorum was highest when competing with Ag. desertorum and was lowest with Ar. tridentata. Root competition decreased the seed population of B. tectorum in the next generation even though it had no impact on survival. Competition in the upper soil horizon occurs between seedlings and established adults early in the growing season and potentially restricts root growth of seedlings. In arid and semiarid ecosystems, soil moisture is depleted from the upper horizons first, resulting in the death of seedlings that do not have access to moisture.     

The effect of soil compaction on germination and early growth of Eucalyptus albens and an exotic annual grass

Most agricultural land has been compacted to some degree by heavy machinery or livestock trampling. This legacy is expected to influence the success of tree seedling recruits in farmland areas where natural regeneration is being encouraged. We investigated the impact of soil compaction on seedlings of a woodland eucalypt (Eucalyptus albens) and an annual grass competitor (Vulpia myuros) in a laboratory experiment. Replicate soil cores were created at five bulk density levels; 1.0, 1.1, 1.2, 1.3 or 1.4 Mg m^?3 with a soil water content of 20%. The depth of root penetration declined linearly with increasing bulk density, resulting in a decrease in root depth of around 75% in the most compacted soil compared with the least compacted soil for both species. Shoot length and primary root length did not vary between soil bulk density levels for either species, but seedlings responded to increasing levels of compaction with oblique (non‐vertical) root growth. Results suggest that young seedlings of both E. albens and V. myuros will be more susceptible to surface drying in compacted than uncompacted soils and therefore face a greater risk of desiccation during the critical months following germination. Any competitive advantage that V. myuros may have over E. albens is not evident in differential response to soil compaction.     

Effects of resource availability and propagule supply on native species recruitment in sagebrush ecosystems invaded by Bromus tectorum

Resource availability and propagule supply are major factors influencing establishment and persistence of both native and invasive species. Increased soil nitrogen (N) availability and high propagule inputs contribute to the ability of annual invasive grasses to dominate disturbed ecosystems. Nitrogen reduction through carbon (C) additions can potentially immobilize soil N and reduce the competitiveness of annual invasive grasses. Native perennial species are more tolerant of resource limiting conditions and may benefit if N reduction decreases the competitive advantage of annual invaders and if sufficient propagules are available for their establishment. Bromus tectorum, an exotic annual grass in the sagebrush steppe of western North America, is rapidly displacing native plant species and causing widespread changes in ecosystem processes. We tested whether nitrogen reduction would negatively affect B. tectorum while creating an opportunity for establishment of native perennial species. A C source, sucrose, was added to the soil, and then plots were seeded with different densities of both B. tectorum (0, 150, 300, 600, and 1,200 viable seeds m⁻²) and native species (0, 150, 300, and 600 viable seeds m⁻²). Adding sucrose had short-term (1 year) negative effects on available nitrogen and B. tectorum density, biomass and seed numbers, but did not increase establishment of native species. Increasing propagule availability increased both B. tectorum and native species establishment. Effects of B. tectorum on native species were density dependent and native establishment increased as B. tectorum propagule availability decreased. Survival of native seedlings was low indicating that recruitment is governed by the seedling stage.     

The population biology of Bromus tectorum in forests: effect of disturbance,grazing, and litter on seedling establishment and reproduction

Summary The effect of tree canopy, understory, herbivores, and litter depth on seedling establishment, survival, and reproduction of the alien grass, Bromus tectorum (cheatgrass), was examined in a series of experiments in four forest habitat types in western North America. Higher recruitment, survival, and reproduction on clearcuts, which would be expected if the overstory alone is limiting the distribution of cheatgrass in forests, were not observed. Removing the understory in an otherwise undisturbed Pinus ponderosa forest did, however, increase the emergence of B. tectorum, but plants in these experimentally-created openings were more vulnerable to grazing by small mammals. In contrast, removing the sparse understory in an Abies forest neither enhanced recruitment nor increased the incidence of grazing of B. tectorum seedlings. Regardless of the forest habitat, most grazed plants died before maturity; even fewer grazed plants produced seeds. Litter depth influenced both recruitment and biomass production: both the rate of germination and the size of resultant seedlings were lower on thick litter (6 cm) compared to results on thin litter (1.5 cm). In the more open Pinus ponderosa and Pseudotsuga menziesii forests, cheatgrass colonization may often occur in openings in the understory alone. Colonization in the more shady A. grandis and Thuja plicata forests is unlikely, however, unless the opening extends through both the understory and the overstory. As a result, cheatgrass is unlikely to increase in any of these forests unless the scale and incidence of disturbance increases substantially.     

Effects of climate and snow depth on Bromus tectorum population dynamics at high elevation

Invasive plants are thought to be especially capable of range shifts or expansion in response to climate change due to high dispersal and colonization abilities. Although highly invasive throughout the Intermountain West, the presence and impact of the grass Bromus tectorum has been limited at higher elevations in the eastern Sierra Nevada, potentially due to extreme wintertime conditions. However, climate models project an upward elevational shift of climate regimes in the Sierra Nevada that could favor B. tectorum expansion. This research specifically examined the effects of experimental snow depth manipulations and interannual climate variability over 5 years on B. tectorum populations at high elevation (2,175 m). Experimentally-increased snow depth had an effect on phenology and biomass, but no effect on individual fecundity. Instead an experimentally-increased snowpack inhibited population growth in 1 year by reducing seedling emergence and early survival. A similar negative effect of increased snow was observed 2 years later. However, a strong negative effect on B. tectorum was also associated with a naturally low-snow winter, when seedling emergence was reduced by 86%. Across 5 years, winters with greater snow cover and a slower accumulation of degree-days coincided with higher B. tectorum seedling density and population growth. Thus, we observed negative effects associated with both experimentally-increased and naturally-decreased snowpacks. It is likely that the effect of snow at high elevation is nonlinear and differs from lower elevations where wintertime germination can be favorable. Additionally, we observed a doubling of population size in 1 year, which is alarming at this elevation.     

Contrasting effects of resource availability and plant mortality on plant community invasion by Bromus tectorum L.

The positive effect of disturbance on plant community invasibility is one of the more consistent results in invasion ecology. It is generally attributed to a coincident increase in available resources (due to the disturbance) that allows non-resident plant species to establish (Davis MA, Grime JP Thompson K, J Ecol 88:528–534, 2000). However, most research addressing this issue has been in artificial or highly modified plant communities. Our goal in this study was to investigate the interactive effects of resource availability and plant mortality disturbance on the invasion of natural plant communities. We conducted a series of experiments that examined the response of Bromus tectorum L., a highly invasive annual grass, to experimentally created gradients of resource availability [nitrogen (N) and water] and resident plant species mortality. We found that B. tectorum biomass was co-limited by N and water. Biomass at the end of the growing season was a saturating function (i.e., increased to a maximum) of water, which determined maximum biomass, and N, which determined the rate at which maximum biomass was attained. Despite that fact that plant mortality increased N availability, it had a negative impact on invasion success. Plant mortality also decreased foliar cover, standing dead biomass, and soil cover by litter. In harsh environments, removing foliar and soil cover may increase germination and seedling stress by increasing soil temperatures and water loss. Across all treatments, B. tectorum success decreased with decreasing foliar cover and standing dead biomass. This, in combination with the strong limitation of B. tectorum biomass by water in this experiment, suggests that our plant mortality disturbance removed soil cover that may have otherwise aided B. tectorum invasion into this semi-arid plant community by reducing water stress.     

The role of successional stage and small‐scale disturbance for establishment of pioneer grass Corynephorus canescens

Question: Which mechanisms promote the maintenance of the protected pioneer grass Corynephorus canescens in a mosaic landscape? Which are the interactive effects of small‐scale disturbances, successional stage and year‐to‐year variation on early establishment probabilities of C. canescens? Location: Brandenburg, NE Germany. Methods: We measured emergence and survival rates over 3 yr in a sowing‐experiment conducted in three successional stages (C. canescens‐dominated site, ruderal forb site and pioneer forest) under two different regimes of mechanical ground disturbance (disturbed versus undisturbed control). Results: Overall, disturbance led to higher emergence in a humid year and to lower emergence in a very dry year. Apparently, when soil moisture was sufficient, the main factor limiting C. canescens' establishment was competition, while in the dry year, water became the limiting factor. Survival rates were not affected by disturbance. In humid years, C. canescens emerged in higher numbers in open successional stages while in the dry year, emergence rates were higher in late stages, suggesting an important role of late successional stages for the persistence of C. canescens. Conclusions: Our results suggest that small‐scale disturbances can promote germination of C. canescens. However, disturbances should be carefully planned. The optimal strategy for promoting C. canescens is to apply disturbances just before seed dispersal and not during dry years. At the landscape scale, a mosaic of different vegetation types is beneficial for the protected pioneer grass as facilitation by late‐successional species may be an important mechanism for the persistence of C. canescens, especially in dry years.     

The role of successional stage,vegetation type and soil disturbance in the invasion of the alien grass Cortaderia selloana

Question : This paper studies the establishment and performance of Cortaderia selloana (Pampas grass), an alien South American ornamental species that is invading many parts of the world. We asked whether (1) early successional stages were the most susceptible to C. selloana invasion; (2) soil microdisturbances increased invasion at any point of succession, and (3) C. selloana invasion of later successional stages was modulated by vegetation type Location : Delta del Llobregat (Catalonia, NE Spain). Methods : We monitored survival and growth of transplanted C. selloana seedlings in disturbed and non‐disturbed plots throughout a successional gradient with an age range of < 1 to > 10 years in different vegetation types and within the area of influence of coexisting species with similar growth form. Results : Although seedling survival was extremely low in all treatments, our results revealed that early successional stages were not the most easily invaded since we found no significant differences in the percentage survival of C. selloana along the successional gradient. However, survival and seedling bio‐mass were enhanced by soil disturbance at any seral stage. This result suggested that inhibition ruled C. selloana invasion. Invasibility neither depended on the invaded vegetation type nor on the co‐existing species with similar growth form. Finally, C. selloana invasion was not enhanced by decreasing competition with Phragmites australis, a native coexisting species because survival rates after a year were not significantly different. However, Phragmites increased C. selloana leaf length probably due to shading. Conclusions : C. selloana recruitment appears to be positively affected by soil disturbance but it is independent of successional stage or vegetation type.     

Temporal patterns of forest seedling emergence across different disturbance histories

1. Forest ecosystems experience a myriad of natural and anthropogenic disturbances that shape ecological communities. Seedling emergence is a critical, preliminary stage in the recovery of forests post​ disturbance and is triggered by a series of abiotic and biotic changes. However, the long‐term influence of different disturbance histories on patterns of seedling emergence is poorly understood.
2. Here, we address this research gap by using an 11‐year dataset gathered between 2009 and 2020 to quantify the influence of different histories of natural (wildfire) and anthropogenic (clearcut and postfire salvage logging) disturbances on emerging seedlings in early‐successional Mountain Ash forests in southeastern Australia. We also describe patterns of seedling emergence across older successional forests varying in stand age (stands that regenerated in <1900s, 1939, 1970–90, and 2007–11).
3. Seedling emergence was highest in the first three years post disturbance. Stand age and disturbance history significantly influenced the composition and abundance of plant seedlings. Specifically, in salvage‐logged forests, plant seedlings were the most different from similarly aged forests with other disturbance histories. For instance, relative to clearcut and unlogged, burnt forests of the same age, salvage logging had the lowest overall richness, the lowest counts of Acacia seedlings, and an absence of common species including Acacia obliquinervia, Acacia frigescens, Cassinia arcuealta, Olearia argophylla, Pimelea axiflora, Polyscias sambucifolia, and Prosanthera melissifolia over the survey period.
4. Synthesis: Our findings provide important new insights into the influence of different disturbance histories on regenerating forests and can help predict plant community responses to future disturbances, which may influence forest recovery under altered disturbance regimes.

     

Germination and seedling establishment of two annual grasses on lichen-dominated biological soil crusts

Biological soil crusts dominated by lichens are common components of shrub-steppe ecosystems in northwestern US. We conducted growth chamber experiments to investigate the effects of these crusts on seed germination and initial seedling establishment of two annual grasses; the highly invasive exotic Bromus tectorum L. and the native Vulpia microstachys Nutt. We recorded germination time courses on bare soil and two types of biological soil crusts; one composed predominantly of the lichen Diploschistes muscorum (Scop.) R. Sant. (lichen crust) and the other comprised of an assortment of lichens and mosses (mixed crust). Final germination on the lichen crust for both grass species was about a third of that on the bare soil surface. Mean germination time (MGT) was 3–4 days longer on the lichen crust compared with the bare soil. In contrast, there was no difference in germination percentage or MGT between the mixed crust and bare soil, and results were similar for both grass species. For both species, root penetration of germinating seeds on the lichen crust was lower than on the bare soil or mixed crust surfaces. The combined effects of the lichen crust on germination and root penetration resulted in an overall reduction in seedling establishment of 78% for V. microstachys and 85% for B. tectorum relative to the bare soil treatment. Our results clearly demonstrate that lichen-dominated biological soil crust can inhibit germination and root penetration, but the extent of these effects depends on the composition of the crust. Responsible Editor: Tibor Kalapos     

Facilitation and interference of seedling establishment by a native legume before and after wildfire

In semi-arid ecosystems, heterogeneous resources can lead to variable seedling recruitment. Existing vegetation can influence seedling establishment by modifying the resource and physical environment. We asked how a native legume, Lupinus argenteus, modifies microenvironments in unburned and burned sagebrush steppe, and if L. argenteus presence facilitates seedling establishment of native species and the non-native annual grass, Bromus tectorum. Field treatments examined mechanisms by which L. argenteus likely influences establishment: (1) live L. argenteus; (2) dead L. argenteus; (3) no L. argenteus; (4) no L. argenteus with L. argenteus litter; (5) no L. argenteus with inert litter; and (6) mock L. argenteus. Response variables included soil nitrogen, moisture, temperature, solar radiation, and seedling establishment of the natives Elymus multisetus and Eriogonum umbellatum, and non-native B. tectorum. In both unburned and burned communities, there was higher spring soil moisture, increased shade and reduced maximum temperatures under L. argenteus canopies. Adult L. argenteus resulted in greater amounts of soil nitrogen (N) only in burned sagebrush steppe, but L. argenteus litter increased soil N under both unburned and burned conditions. Although L. argenteus negatively affected emergence and survival of B. tectorum overall, its presence increased B. tectorum biomass and reproduction in unburned plots. However, L. argenteus had positive facilitative effects on size and survival of E. multisetus in both unburned and burned plots. Our study indicates that L. argenteus can facilitate seedling establishment in semi-arid systems, but net effects depend on the species examined, traits measured, and level of abiotic stress.     

Direct Effects of Soil Amendments on Field Emergence and Growth of the Invasive Annual Grass Bromus tectorum L. and the Native Perennial Grass Hilaria jamesii (Torr.) Benth

Bromus tectorum L. is a non-native, annual grass that has invaded western North America. In SE Utah, B. tectorum generally occurs in grasslands dominated by the native perennial grass, Hilaria jamesii (Torr.) Benth. and rarely where the natives Stipa hymenoides Roem. and Schult. and S. comata Trin. & Rupr. are dominant. This patchy invasion is likely due to differences in soil chemistry. Previous laboratory experiments investigated using soil amendments that would allow B. tectorum to germinate but would reduce B. tectorum emergence without affecting H. jamesii. For this study we selected the most successful treatments (CaCl₂, MgCl₂, NaCl and zeolite) from a previous laboratory study and applied them in the field in two different years at B. tectorum-dominated field sites. All amendments except the lowest level of CaCl₂ and zeolite negatively affected B. tectorum emergence and/or biomass. No amendments negatively affected the biomass of H. jamesii but NaCl reduced emergence. Amendment effectiveness depended on year of application and the length of time since application. The medium concentration of zeolite had the strongest negative effect on B. tectorum with little effect on H. jamesii. We conducted a laboratory experiment to determine why zeolite was effective and found it released large amounts of Na⁺, adsorbed Ca²⁺, and increased Zn²⁺, Fe²⁺, Mn²⁺, Cu²⁺, exchangeable Mg²⁺, exchangeable K, and NH₄⁺ in the soil. Our results suggest several possible amendments to control B. tectorum. However, variability in effectiveness due to abiotic factors such as precipitation and soil type must be accounted for when establishing management plans.     

Effects of Small Rodent and Large Mammal Exclusion on Seedling Recruitment in Costa Rica1

We examined whether the experimental exclusion of large mammalian and small rodent seed predators had differing effects on seedling recruitment under natural seed rain conditions. In both primary and late‐successional secondary forested areas, exclosure experiments using natural seed densities were designed to assess seedling recruitment. To assess the differences in seedling recruitment, we monitored three exclosure treatments (1.2 m radius/1.5 m height) in two forest types (primary vs. late‐successional secondary forest): (1) fenced exclosures that excluded large mammals; (2) fenced exclosures that excluded both large and small mammals; and (3) open controls. Within each exclosure treatment, we marked and identified all seedlings at the beginning of the experiment (February 2001), followed the marked seedlings' fate for a year, and then marked and identified all new seedlings after a year. Two preliminary findings were generated from these data: for some tree species, small rodents and large mammals have differential effects on seedling recruitment, and the effect of excluding mammals did not differ with habitat type (primary vs. late‐successional secondary forest). These preliminary results highlight the need to examine further how the effects of small rodent and large mammal exclusion may affect species‐specific seed predation and seedling recruitment in a variety of habitat/land use types (e.g., primary forest, late‐successional forest, and early‐successional forest).     

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.     

Nitrogen acquisition by annual and perennial grass seedlings: testing the roles of performance and plasticity to explain plant invasion

Differences in resource acquisition between native and exotic plants is one hypothesis to explain invasive plant success. Mechanisms include greater resource acquisition rates and greater plasticity in resource acquisition by invasive exotic species compared to non-invasive natives. We assess the support for these mechanisms by comparing nitrate acquisition and growth of invasive annual and perennial grass seedlings in western North America. Two invasive exotic grasses (Bromus tectorum and Taeniatherum caput-medusae) and three perennial native and exotic grasses (Pseudoroegneria spicata, Elymus elymoides, and Agropyron cristatum) were grown at various temperatures typical of autumn and springtime when resource are abundant and dominance is determined by rapid growth and acquisition of resources. Bromus tectorum and perennial grasses had similar rates of nitrate acquisition at low temperature, but acquisition by B. tectorum significantly exceeded perennial grasses at higher temperature. Consequently, B. tectorum had the highest acquisition plasticity, showcasing its ability to take advantage of transient warm periods in autumn and spring. Nitrate acquisition by perennial grasses was limited either by root production or rate of acquisition per unit root mass, suggesting a trade-off between nutrient acquisition and allocation of growth to structural tissues. Our results indicate the importance of plasticity in resource acquisition when temperatures are warm such as following autumn emergence by B. tectorum. Highly flexible and opportunistic nitrate acquisition appears to be a mechanism whereby invasive annual grasses exploit soil nitrogen that perennials cannot use.     

The Mark of Zorro: Effects of the Exotic Annual Grass Vulpia myuros on California Native Perennial Grasses

Native perennial grasses were once common in California prairies that are now dominated by annual grasses introduced from Europe. Competition from exotics may be a principal impediment to reestablishment of native perennial grasses. Introduced annual grasses, such as Vulpia myuros (zorro fescue), are often included with native perennial species in revegetation seed mixtures used in California. To examine the potential suppressive effect of this graminoid, we evaluated the growth and performance of a mixture of California native perennial grasses and resident weeds when grown with varying densities of V. myuros. The annual fescue exhibited a strongly plastic growth response to plant density, producing similar amounts of above‐ground biomass at all seeding densities. Perennial grass seedling survival and above‐ ground biomass decreased and individuals became thinner (i.e., reduced weight‐to‐height ratio) with increasing V. myuros seeding density. V. myuros also significantly suppressed above‐ground biomass and densities of weeds and had a more negative effect on weed densities than on native perennial grass densities. Biomass of native grasses and weeds was not differentially affected by increasing densities of V. myuros. Overall, because V. myuros significantly reduced the survival and performance of the mixture of native perennial grasses and this effect increased with increasing V. myuros density, we conclude that including this exotic annual in native seed mixtures is counterproductive to restoration efforts.     

Environmentally‐contingent behaviour of invasive plants as drivers or passengers

Invasive plants are often regarded as drivers that actively reduce diversity and alter ecosystem processes such as succession. Alternatively, invaders may be passengers that simply colonize openings produced by anthropogenic disturbance and are present only temporarily. Here we test whether the behaviour of invasive species as drivers or passengers is contingent on disturbance and nutrient availability. We created twelve experimental environments (three levels of annual disturbance × four levels of nitrogen availability) for 18 years in a grassland at the northern edge of the North American Great Plains. Out of 19 invasive species initially present, two perennials (Bromus inermis, a grass, and Cirsium arvense, a forb) acted as drivers, maintaining or increasing dominance, maintaining low species richness, and forming an invader‐ dominated successional sequence. Behaviour as drivers was environmentally contingent: Bromus behaved as a driver only in less disturbed environments, and the tendency of Cirsium to behave as a driver increased significantly with both disturbance and nitrogen availability. Most invasive species (90%), however, consistently behaved as passengers, disappearing or becoming rare. The importance of disturbance and fertility for starting invasions is well‐known, but our study shows that these factors also contribute to the behaviour of some invaders as drivers. The emergence of drivers and invader‐dominated successional sequences suggests that, as rates of invasion, disturbance and eutrophication continue to increase with human activity, invasive species that act as drivers may form low‐diversity communities that persist for decades.     

Reduced soil compaction enhances establishment of non-native plant species

Many studies have shown that soil disturbance facilitates establishment of invasive, non-native plant species, and a number of mechanisms have been isolated that contribute to the process. To our knowledge no studies have isolated the role of altered soil compaction, a likely correlate of many types of soil disturbance, in facilitating invasion. To address this, we measured the response of seeded non-native and native plant species to four levels of soil compaction in mesocosms placed in an abandoned agricultural field in the Methow Valley, Washington, USA. Soil compaction levels reflected the range of resistance to penetration (0.1–3.0 kg cm⁻²) measured on disturbed soils throughout the study system prior to the experiment. Percent cover of non-native species, namely Bromus tectorum and Centaurea diffusa, decreased by 34% from the least to the most compacted treatments, whereas percent cover of native species, mostly Pseudoroegneria spicata and Lupinus spp., did not respond to compaction treatments. Experimental results were supported by a survey of soil penetration resistance and percent cover by species in 18 abandoned agricultural fields. Percent cover of B. tectorum was negatively related to soil compaction levels, whereas none of the native species showed any response to soil compaction. These results highlight a potentially important, though overlooked, aspect of soil disturbance that may contribute to subsequent non-native plant establishment.     

Soil disturbance,vegetation cover and the establishment of the exotic shrub <Emphasis Type="Italic">Pyracantha coccinea</Emphasis> in southern France

We evaluate the mechanisms that determine the establishment of the non-indigenous shrub Pyracantha coccinea (Rosaceae) in the Montpellier region of southern France. P. coccinea establishes in abandoned agricultural fields in this region; yet, despite its high propagule pressure, it has not become a widespread invasive. We hypothesized that the disturbance conditions prevailing in abandoned agricultural fields right after abandonment may enhance the emergence, survival and growth of P. coccinea, but that shortly after abandonment colonizing vegetation prevents further establishment of this species. We conducted a field experiment to evaluate this hypothesis, studying the response of seedling emergence and growth of P. coccinea to soil and vegetation disturbance. Our results show that both lack of vegetation cover and soil disturbance promote the emergence of seedlings of P. coccinea. Thus, the disturbance conditions prevailing in abandoned agricultural fields seem crucial to allow establishment of this species. However, other factors such as lack of summer dormancy and seed predation might explain why this species has not become a widespread invasive.     

Linking seed dispersal,germination and seedling recruitment in the invasive species <Emphasis Type="Italic">Berberis darwinii</Emphasis> (Darwin’s barberry)

Seedling recruitment is a multi-phased process involving seed production, dispersal, germination, seedling establishment and subsequent survival. Understanding the factors that determine success at each stage of this process is of particular interest to scientists and managers seeking to understand how invasive species spread and persist, and identify critical stages for management. To understand the factors and processes influencing recruitment of the invasive species Berberis darwinii Hook. (Darwin’s barberry), temporal and spatial patterns of seed dispersal, germination and seedling establishment were examined. Seed dispersal from a large source population was measured over two fruiting seasons, and subsequent patterns of seedling emergence and survival within each cohort were measured. Seed longevity was tested under both natural and artificial conditions. Seeds were widely dispersed by birds, up to 450 m from the source population. Dispersal was essential to seedling establishment, as few seedlings survived beneath the parent canopy. Seeds were relatively short-lived in the soil under both field and glasshouse conditions, with few surviving for more than 1 year. Patterns of newly emerged seedlings largely reflected patterns of seed rain, but seedling survival was significantly affected by distance from source population, seedling density and light environment. These results suggest that recruitment of B. darwinii is dependent on dispersal of seeds to favourable microsites. Management priorities should include the removal of fruiting plants, and seedling control in highlight areas.