Propagule Availability Research Articles

Grassland restoration on former croplands in Europe: an assessment of applicability of techniques and costs

Grasslands used to be vital landscape elements throughout Europe. Nowadays, the area of grasslands is dramatically reduced, especially in industrial countries. Grassland restoration is widely applied to increase the naturalness of the landscape and preserve biodiversity. We reviewed the most frequently used restoration techniques (spontaneous succession, sowing seed mixtures, transfer of plant material, topsoil removal and transfer) and techniques used to improve species richness (planting, grazing and mowing) to recover natural-like grasslands from ex-arable lands. We focus on the usefulness of methods in restoring biodiversity, their practical feasibility and costs. We conclude that the success of each technique depends on the site conditions, history, availability of propagules and/or donor sites, and on the budget and time available for restoration. Spontaneous succession can be an option for restoration when no rapid result is expected, and is likely to lead to the target in areas with high availability of propagules. Sowing low-diversity seed mixtures is recommended when we aim at to create basic grassland vegetation in large areas and/or in a short time. The compilation of high-diversity seed mixtures for large sites is rather difficult and expensive; thus, it may be applied rather on smaller areas. We recommend combining the two kinds of seed sowing methods by sowing low-diversity mixtures in a large area and high-diversity mixtures in small blocks to create species-rich source patches for the spontaneous colonization of nearby areas. When proper local hay sources are available, the restoration with plant material transfer can be a fast and effective method for restoration.

Restoration of Threatened Arable Weed Communities in Abandoned Mountainous Crop Fields

Arable fields maintaining species‐rich weed communities have almost disappeared from European mountain areas due to their abandonment or replacement by extensive grazing systems during the last decades. The restoration of arable weed communities have become an important issue in these habitats which have a large potential to contain rare and threatened (red‐list) species. In a site in the Swiss Alps, formerly managed for crop production, we investigated in experimental plowed plots the 1‐year effects of fertilizing, sowing a mixture of arable weed species and sowing a crop species (triticale) on the natural regeneration (species richness and cover) of arable weed and non‐weed species. The high number of newly appeared arable weed species, of which four were red‐list species, indicated that propagule availability was still important after about 50 years of arable field abandonment. Plowed plots which did not receive any treatment and those with fertilization alone favored the regeneration of both wild weed and non‐weed species. The crop species limited, but did not inhibit, the recovery success of weed and non‐weed species and this negative effect tended to be higher under fertilization. Sowing of arable weed seeds decreased the natural regeneration of both weeds and non‐weeds in the absence of triticale. Our results show that plowing is a promising method to successfully restore the weed community in abandoned arable mountain areas. Moreover, the sowing of weed species in combination with a crop species might produce sustainable food and restore and maintain the diversity of threatened arable weeds at the same time.

Consequences of propagule dispersal and river fragmentation for riparian plant community diversity and turnover

The spatial distribution and temporal availability of propagules fundamentally constrain plant community development. This study experimentally tested several hypotheses about the relative roles of wind and water dispersal in colonization and development of riparian communities along rivers. Through controlling the source of propagules (dispersed by wind, water, or both) reaching newly created, bare river margin sites, we isolated the relative roles of dispersal and other factors in plant community development over five years. Replicated treatments were established at 12 sites spanning 400 km along two adjacent rivers in northern Sweden, one fragmented by a series of dams, the other free‐flowing. Bare river margins receiving only water‐dispersed propagules had significantly higher species richness compared to plots receiving only wind‐dispersed propagules during the initial two years of colonization. Species richness increased annually throughout the study along tranquil and turbulent reaches of the free‐flowing river but reached an asymptote at comparatively low richness after a single year on the impounded river. Propagule source strongly influenced species richness during the initial establishment along both rivers, with richness being significantly higher in plots receiving water‐dispersed seeds. This strong treatment effect continued to be important through time along the regulated river but diminished in importance along the free‐flowing river where other factors such as soil moisture, light availability, and exposure of sites to fluvial disturbance overshadowed the influence of dispersal pathway in mediating species richness. This suggests that hydrochory (plant dispersal by water) may be more important for maintenance of diversity in regulated systems where long‐distance dispersal is absent or negligible, but that the rich local propagule source along free‐flowing rivers supports high species richness. The number of unique species was higher in water‐dispersal plots along both the regulated and free‐flowing rivers. This result suggests that hydrochory may contribute to temporal variability of sites, may enhance richness over time, and may have an important role in meta‐population and meta‐community dynamics of plant communities through long‐distance (and local) dispersal and chance colonization. Our findings provide experimental evidence that water dispersal of plant propagules influences colonization dynamics and is important for long‐term community development in riparian zones.

Constraints on Salt Marsh Development Following Managed Coastal Realignment: Dispersal Limitation or Environmental Tolerance?

Salt marshes restored through managed coastal realignment (MR) often develop slowly and show persistent differences in vegetation from natural marshes. Development might be constrained by the availability of propagules or poor suitability of the abiotic environment for their establishment. To distinguish between these factors, we compared vegetation colonization and environmental conditions at a salt marsh created by MR at Brancaster, Norfolk, UK, with five reference marshes, varying in age from 30 to circa 6,000 years. After 5 years, plant communities of the MR site remained different from those in mature reference marshes. In contrast, the communities of the youngest reference marsh were not significantly different from mature reference marshes. At the MR site, abundance of perennial and later‐successional species was low and large areas remained unvegetated. These differences are unlikely to be due to dispersal limitation, because 76% of the species from the local species pool colonized the site within 5 years. Although the annuals Salicornia europaea and Suaeda maritima were abundant by year 2, they were not ubiquitous until the end of the study. Tidal elevations of the MR site were suitable for vegetation development, but soil redox potentials were lower than that at the reference sites. Reducing conditions in the MR site appear to be the major cause of vegetation differences from the reference marshes, as they are associated with an abundance of bare ground and a small range of vegetation clusters. Measures to avoid low sediment redox potentials may have a great benefit in some salt marsh restoration projects.

Herb establishment in a young Pseudotsuga menziesii forest: a seven-year seed addition experiment1

The abundance of old-growth associated herbs in young Pseudotsuga menziesii forests is limited in part by the low availability of propagules of these species, but it remains unclear whether the slow post-logging development of old-growth species in younger forests also reflects a lack of microhabitats suitable for seed germination. This study attempts to fill an almost total lack of knowledge about germination of understory species in second-growth Douglas-fir forests, and in young coniferous forests in general, while providing a preliminary exploration of the relative importance of these two limiting factors. The ability of six common forest herbs to germinate in a 40-year-old Pseudotsuga menziesii stand was tested, and survival and growth were monitored for seven years. Five of the six species germinated well (8–50% germination), while one open-site species failed to germinate. After seven years, survival of four old-growth associated species was ∼10% (of the original seed number), while only 1% of seeds of a disturbance-adapted species were still present as seedlings. For all species, seedlings grew slowly and did not reach reproductive size, forming a seedling bank analogous to that of shade-tolerant tree seedlings. All old-growth associated species showed higher overall seedling numbers or greater survival in gaps than in non-gap microsites, and two species showed higher survival in the more mesic lower part of the watershed. Naturally occurring seedlings were rare and included only easily dispersed species that are forest generalists or characteristic of mature, but not of old-growth forests. I conclude that germination of late-seral herb species in young Pseudotsuga menziesii forests is not strictly microsite limited, but that these species are instead limited by a combination of low propagule availability and low resource levels with concomitant slow growth rates.

Recent literature on lichens—218

] Bielczyk, U., M. Jedrzejczyk-Korycinska & J. Kiszka. 2009. Lichens of abandoned zinc-lead mines. Acta Mycologica 44(2): 139–149. [Many species (76) reported from this mining area in south-central Poland (Silesian-Krakow Upland). Most common genera are Cladonia (18 species) and Verrucaria (8 species). A few rare or endangered taxa were also found. In English with a Polish summary.] Bjork, C. R. 2010. Additions to the lichen flora of Washington State, United States. I. Evansia 27(1): 18–20. [Report of 13 taxa new to the state.] ———, T. Goward & T. Spribille. 2009. New records and range extensions of rare lichens from waterfalls and sprayzones in inland British Columbia, Canada. Evansia 26(4): 219–224. [Notes on several species, including Psoroglaena stigonemoides reported new to North America.] Boonpragob, K. 2010. A tribute to Natsurang Homchantara. Lichenologist 42(2): 129–130. [Brief note on the life and work of Thailand’s ‘‘... first trained lichen taxonomist.’’] Bowker, M. A., J. Belnap & D. W. Davidson. 2010. Microclimate and propagule availability are equally important for rehabilitation of dryland N-fixing lichens. Restoration Ecology 18(1): 30–33. [‘‘It is possible that attempts to improve N cycling via re-establishment of Collema might be best served by developing economical means of simulating moister, cooler microhabitats, e.g., sloping soil or creating partial shade, which would favor the establishment of naturally dispersed propagules, rather than introducing propagules.’’] ———, F. T. Maestre & C. Escolar. 2010. Biological crusts as a model system for examining the biodiversity-ecosystem function relationship in soils. Soil Biology and Biochemistry 42(3): 405–417. [Study of data from the USA and Spain. ‘‘Functional group richness did not seem to adequately capture biodiversity-function relationships, suggesting that bryophyte and lichen components of BSC [biological soil crusts] may exhibit low redundancy.’’] Brodo, I. M. & J. P. Bennett. 2009. John Walter Thomson, 1913–2009. The Bryologist 112(4): 860–868. [Tribute to John Thomson including a list of his publications and new lichens species he named.] Buckley, L. G. & R. T. McCrea. 2009. The sodium hypochlorite solution for the removal of lichen from vertebrate track surfaces. Ichnos: An International Journal for Plant and Animal Traces 16(3): 230–234. [‘‘This paper describes a technique using a sodium hypochlorite solution (bleach) to remove lichen from a track surface containing numerous avian prints and small theropod prints with skin impressions.’’] Egan: Recent literature on lichens 691

Fire does not facilitate invasion by alien annual grasses in an infertile Australian agricultural landscape

Plant invasions are a significant threat to fragmented native plant communities in many agricultural regions. Fire potentially facilitates invasions, but in landscapes historically subject to recurrent fires, exclusion of fire is also likely to result in loss of biodiversity. We investigated the relationship between fire, fragmentation and alien plant invasion in mallee communities of the Western Australian wheatbelt. We hypothesized that invasion is limited by lack of propagules and the low soil nutrient levels of this old, infertile landscape, but that fire and/or fragmentation disrupt these limits. We tested the effects of three factors on establishment and abundance of alien annuals: ± fire, ± post-fire seeding with the locally invasive Avena barbata (propagule availability) and three landscape contexts. The three landscape contexts, exploring site limitations, were reserve interiors, perimeter edges adjacent to agricultural land and internal reserve roadside edges. Our first hypothesis was supported: Avena establishment was consistently greater in seeded plots, but away from perimeter edges, growth was poor. Our second hypothesis was supported only for perimeter edges: neither fire nor fragmentation by interior roads enhanced invasive plant establishment or biomass. At perimeter edges, invasive plant biomass was significantly greater. This was associated with higher propagule availability and elevated soil nutrient levels but was not enhanced by fire. We conclude that fire is unlikely to promote invasion by alien annuals in low-nutrient ecosystems such as mallee, hence is a viable disturbance strategy for biodiversity conservation away from nutrient-enriched edges.

Arbuscular mycorrhizal propagules in soils from a tropical forest and an abandoned cornfield in Quintana Roo, Mexico: visual comparison of most-probable-number estimates

The present study was aimed at comparing the number of arbuscular mycorrhizal fungi (AMF) propagules found in soil from a mature tropical forest and that found in an abandoned cornfield in Noh-Bec Quintana Roo, Mexico, during three seasons. Agricultural practices can dramatically reduce the availability and viability of AMF propagules, and in this way delay the regeneration of tropical forests in abandoned agricultural areas. In addition, rainfall seasonality, which characterizes deciduous tropical forests, may strongly influence AMF propagules density. To compare AMF propagule numbers between sites and seasons (summer rainy, winter rainy and dry season), a "most probable number" (MPN) bioassay was conducted under greenhouse conditions employing Sorgum vulgare L. as host plant. Results showed an average value of 3.5 ± 0.41 propagules in 50 ml of soil for the mature forest while the abandoned cornfield had 15.4 ± 5.03 propagules in 50 ml of soil. Likelihood analysis showed no statistical differences in MPN of propagules between seasons within each site, or between sites, except for the summer rainy season for which soil from the abandoned cornfield had eight times as many propagules compared to soil from the mature forest site for this season. Propagules of arbuscular mycorrhizal fungi remained viable throughout the sampling seasons at both sites. Abandoned areas resulting from traditional slash and burn agriculture practices involving maize did not show a lower number of AMF propagules, which should allow the establishment of mycotrophic plants thus maintaining the AMF inoculum potential in these soils.

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.

BIODIVERSITY RESEARCH: Population expansion in an invasive grass, Microstegium vimineum: a test of the channelled diffusion model

AbstractAim The greatest biodiversity impact of non‐native plant species is caused by rapid expansion of colonist populations. Unfortunately, invasion has rarely been documented in real time at a population scale, and demographic mechanisms of invasion remain unclear. Our goal is to describe real‐time expansion of populations, using channelled diffusion as a null model.Location The study examined three populations of the invasive annual grass Microstegium vimineum in mature second‐growth forests of south‐eastern Ohio and nearby West Virginia, USA.Methods Distributions were recorded in belt transects perpendicular to population edges over a period of 3 years. A second group of belt transects documented spread along five types of potential movement corridor. Observed changes in distribution were compared with predictions from a diffusion model. A seed‐sowing experiment tested seed availability, microsite quality and proximity to potential movement corridors as factors controlling population spread.Results Population boundaries showed little change over the study period. Colonization was limited by propagule availability over distances as little as 0.25 m, and to a lesser extent by litter cover. Populations did not advance along several potential movement corridors including unpaved roads, off‐road vehicle trails and footpaths. Advance was observed along deer trails and stream courses but did not conform to the wave‐form distribution predicted by diffusion theory. During the study, seeds were moved out of experimental plots by sheet flow and minor flooding events along small streams.Main conclusion At a population level, invasion is driven by processes that are episodic in time and non‐random in space – probably a common condition in non‐native plant species. Spatially realistic models are likely to be more useful than diffusive models in managing invasions at these scales.

Vegetative reproduction facilitates early expansion of Phyla canescens in a semi-arid floodplain

Phyla canescens (Verbenaceae, hereafter lippia) is an invasive perennial forb from South America that is now widespread in Australia, threatening the biological values of internationally significant wetlands (Ramsar listed) and the productivity of the grazing industry (Crawford 2008; Earl 2003). Under certain conditions, lippia can displace native herbaceous vegetation and form mono-specific stands, but little is known about the process of colonization and early invasion. Lippia has a dual reproductive strategy, recruiting via seedlings and vegetative fragments in periods following inundation (Macdonald 2008). Determining the relative contribution of vegetative and sexual reproduction for lippia across environmental gradients is important for management action—if spread is primarily by vegetative fragments then control efforts should be focussed on managing vectors (e.g. movement of stock and vehicles), whereas if spread is mostly by seeds then management should focus on reducing the conditions favourable to seed production and seedling establishment. The predominant reproductive mode also has implications for the selection of biocontrol agents. A comparison of lippia in Australia with native range populations found that Australian populations had evolved to invest more energy in sexual reproduction than in the native range (Xu et al. 2009). Genetic analysis found that the importance of vegetative and sexual reproduction is highly variable among populations in Australia (Fatemi and Gross, unpublished data). In some cases, populations exhibited extremely low diversity, with single clones covering distances of up to 100 m and in others, genetic diversity was relatively high with samples 20 m apart representing genetically distinct individuals (Fatemi and Gross, unpublished data). It remains unknown what factors influence the mode of reproduction in different populations in Australia but it is likely that the type of flood event and propagule availability are important. Previous field studies have found that seedling recruitment was the predominant reproductive mode (approx. 70%) following winter flooding, in single site studies (Macdonald 2008; Price et al. 2010a). We examine the density of recruits, dispersal distance and direction from adult plants, and mode of reproduction in a semi-arid floodplain. The study area J. N. Price M. J. Macdonald C. L. Gross I. H. Simpson Ecosystem Management, University of New England, Armidale, NSW 2351, Australia

Aquatic pollution increases the relative success of invasive species

Although individual ecosystems vary greatly in the degree to which they have been invaded by exotic species, it has remained difficult to isolate mechanisms influencing invader success. One largely anecdotal observation is that polluted or degraded areas will accumulate more invaders than less-impacted sites. However, the role of abiotic factors alone in influencing invisibility has been difficult to isolate, often because the supply of potential invaders is confounded with conditions thought to increase vulnerability to invasion. Here, we conducted a field experiment to test how the assemblages of exotic versus native marine invertebrates changed during community assembly under different exposure levels of a common pollutant, copper. The experiment was conducted by deploying fouling panels in a Randomized Block Design in San Francisco Bay. Panels were periodically removed, placed into buckets with differing copper concentrations, and returned to the field after 3 days. This design allowed propagule availability to the plates to be statistically independent of short-term copper exposure. The results demonstrate that copper caused significant differences in community structure. Average native species richness was significantly affected by copper exposure, but average exotic richness was not. The total native species pool within treatments exhibited a greater than 40% decline within increasing copper, while the exotic species pool did not change significantly. These results confirm that anthropogenic alteration of abiotic factors influences invader success, indicating that management strategies to reduce invader impacts should include both efforts to improve environmental conditions as well as reduce invader supply.

Soil Seed Bank and Standing Vegetation of Abandoned Croplands on Chinese Loess Plateau: Implications for Restoration

Vegetation succession depends on the availability of suitable propagules in the soils, and an understanding of soil seed banks is important for effective vegetation restoration of abandoned croplands. Aims of this study were to identify characteristics of soil seed bank, relationship with standing vegetation, and potential significance of soil seed bank for vegetation restoration in abandoned croplands on the hilly-gullied Loess Plateau. Results of field survey, germination, and correspondence analysis showed that density of soil seed bank ranged from almost 900 through almost 6470 seeds m−2 at 0–5 cm depth and almost 120 through almost 2470 seeds m−2 at 5–10 cm depth, with species richness of 7–14. Early successional species dominated soil seed bank, while later successional species occurred only at low densities. Similarity between soil seed bank and standing vegetation was low (with an average of 0.261). Most important variables contributing to the variation of the standing vegetaion included soil water, extractable P, soil seed bank density, and aspect. Soil seed bank alone explained 32% of vegetation community variation. Potential for vegetation restoration from soil seed banks is limited, and it is recommended to transplant some later successional species into abandoned croplands to accelerate succession.

Environmental determinism versus biotic stochasticity in the appearance of plant species in salt-marsh succession

Background and aims It is generally accepted that in terrestrial ecosystems the occurrence and abundance of plant species in late successional stages can be predicted accurately from prevailing soil conditions, whereas in early succession their presence is much more influenced by chance events (e.g. propagule availability). Late successional vegetation stages would therefore be deterministically structured, while early succession would be dominated by more stochastic features. To test this hypothesis in salt marsh conditions, we compared the effect of abiotic environmental factors on vegetation composition and probability of occurrence of individual species in two adjacent salt marshes, differing in age (i.e. successional stage). Material and methods In 2002, a new salt marsh was created on substrate devoid of plant diaspores in the nature reserve The Uzermonding (Nieuwpoort, Belgium). From 2002 onwards, primary colonization started on that sterile substrate by hydrochoric seed dispersal, induced by tidal water currents from an adjacent 5 ha relic of the old salt marsh. In 2005, three years after the start of the colonization process of the new salt marsh, vegetation and three abiotic environmental factors (soil texture, salinity and elevation) were recorded in a set of 155 releves on the new and old salt marsh. Key results In contrast to the general observation in other terrestrial ecosystems, the vegetation composition of the early successional stage of the new salt marsh appeared to be at least as much determined by the combined effect of the measured abiotic factors as that of the old salt marsh. As revealed by logistic regression the presence/absence of perennial species as well as annual species oldie young salt marsh could be well predicted by the measured abiotic variables. For the old salt marsh this also held for the perennial species, but not for the annual species. The stochastic appearance of gaps in the perennial vegetation cover appeared to be important for the establishment of annuals in the older salt marsh. Conclusion In the case of salt marsh succession, the generally accepted hypothesis of early successional stochasticity dominance versus late successional environmental determinism must be rejected.

Prospects for fen meadow restoration on severely degraded fens

The majority of fens in Europe have been transformed for agricultural purposes and have disappeared or become degraded. Fen meadows that developed under low-intensity management of fens also have become degraded. In this paper, we consider the available restoration methods, biotic constraints for restoration and new prospects and approaches for the restoration of severely degraded fens. Due to irreversible changes in landscape settings, hydrology, soil and trophic conditions, a full restoration to natural mires is unlikely. Yet, an improvement of the ecosystem functions and revival of biodiversity in degraded fens is possible. A restoration of semi-natural meadows is one of the alternative targets. Important for restoration efforts to succeed are a sufficient reduction of nutrient levels and preventing acidification. In general, a combination of topsoil removal and seed transfer is an effective measure for fen meadow restoration, provided that groundwater seepage can be re-established. There are also several biotic limitations to fen meadow restoration, due to limited propagule availability of target species and the legacy of the former vegetation in form of its soil seed bank and high seed production by unwanted species. Under the present environmental conditions, the re-development of fen meadows on degraded fens will result in species compositions different from those observed in the past and such restoration may require considerable time and effort.

Field evidence of dispersal of branchiopods, ostracods and bryozoans by teal (Anas crecca) in the Camargue (southern France)

Invertebrate propagules may survive internal and external transports by waterbirds, thus facilitating their dispersal between aquatic habitats. However, field data on such transport remain limited, especially for exozoochory. We quantified and compared the rates of internal and external invertebrate transports simultaneously in a wintering population of teal (Anas crecca) in the Camargue (southern France). We inspected lower gut (rectum) contents of birds that had been shot (N = 366) and washed birds that had been live-trapped (N = 68) during the winters 2006–2007 and 2007–2008. At least one propagule was recorded in 2.5% and 10.3% of internal and external samples, respectively. Cladoceran, ostracod, plumatellid bryozoan and anostracan propagules were all recorded in both internal and external samples. Hatching confirmed their viability, except for anostracan eggs. At least three cladoceran species and two ostracod species were recorded, none of which had previously been shown to be dispersed by birds. Amongst external samples, we recorded a significant seasonal trend in 1 year with most propagules recorded before December, keeping with a seasonal decline in the availability of propagules on the water surface. This study provides evidence that teal may be important vectors of invertebrate dispersal both within the Camargue and along migratory flyways.

Frankia and Alnus rubra Canopy Roots: An Assessment of Genetic Diversity, Propagule Availability, and Effects on Soil Nitrogen

The ecological importance of microbial symbioses in terrestrial soils is widely recognized, but their role in soils that accumulate in forest canopies is almost entirely unknown. To address this gap, this study investigated the Frankia-Alnus rubra symbiosis in canopy and forest floor roots at Olympic National Park, WA, USA. Sixteen mature A. rubra trees were surveyed and Frankia genetic diversity in canopy and forest floor nodules was assessed with sequence-based nifH analyses. A seedling bioassay experiment was conducted to determine Frankia propagule availability in canopy and forest floor soils. Total soil nitrogen from both environments was also quantified. Nodules were present in the canopies of nine of the 16 trees sampled. Across the study area, Frankia canopy and forest floor assemblages were similar, with both habitats containing the same two genotypes. The composition of forest floor and canopy genotypes on the same tree was not always identical, however, suggesting that dispersal was not a strictly local phenomenon. Frankia seedling colonization was similar in canopy soils regardless of the presence of nodules as well as in forest floor soils, indicating that dispersal was not likely to be a major limiting factor. The total soil nitrogen of canopy soils was higher than that of forest floor soils, but the presence of Frankia nodules in canopy soils did not significantly alter soil nitrogen levels. Overall, this study indicates that the Frankia-A. rubra symbiosis is similar in canopy and forest floor environments. Because canopy roots are exposed to different environmental conditions within very small spatial areas and because those areas can be easily manipulated (e.g., fertilizer or watering treatments), they present microbial ecologists with a unique arena to examine root-microbe interactions.

Microclimate and Propagule Availability are Equally Important for Rehabilitation of Dryland N-Fixing Lichens

In some arid regions, rehabilitation of whole system N-fixation may be strongly facilitated by the recovery of populations of the lichen genus Collema. Identification of the limits to recovery of Collema in apparently suitable habitat should inform selection of rehabilitation techniques. We simultaneously tested the relative importance of three hypothetical limits to Collema recovery: active erosion, resource limitation, and propagule scarcity. We found that in our experimental system, active erosion had no effect on short-term establishment of Collema, whereas propagule addition did enhance recovery and microhabitat (a resource availability gradient) also exerted a strong influence. It is possible that attempts to improve N cycling via re-establishment of Collema might be best served by developing economical means of simulating moister, cooler microhabitats, e.g., sloping soil or creating partial shade, which would favor the establishment of naturally dispersed propagules, rather than introducing propagules.

How does surrounding vegetation affect the course of succession: A five‐year container experiment

AbstractQuestion: How does location and time of insertion affect the course of succession in experimental containers?Location: Benešov nad Lipou, Českomoravská vrchovina (Czech‐Moravian uplands), Czech RepublicMethods: We designed a 5‐year container experiment in which plant succession started from scratch. Soil conditions were constant and all containers were filled with homogeneous substrate containing no propagules. We placed the containers in two contrasting habitats (meadow and floodplain) under identical climatic conditions but differing in surrounding vegetations and hence seed input. New containers were installed (and hence succession started) in two subsequent years, twice in each year (spring and autumn). We assume that the individual dates would lead to differences in propagule input and weather conditions.Results: Although both year and season of succession initiation considerably affected the initial species composition, we observed a pronounced convergence within the set of containers located in each habitat. However, the similarity of containers initiated at the same time but located in different habitats decreased over the course of succession. Final composition of the meadow and floodplain containers was therefore mostly determined by permanent seed input from their nearby neighborhood.Conclusions: This study demonstrated that propagule availability is an important determinant of the course of succession, and that differential seed input leads to different pathways of succession, even when all other environmental conditions are equal.

PLANT DISTRIBUTION IN FRESHWATER LAKESHORE: RELATIVE IMPORTANCE OF SPECIES POOL LIMITATION VS. NICHE LIMITATION

Aims Much research has focused on the effects of competition and stress tolerance on plant zonation in wetlands, but few studies have explored the role of propagule availability. This study examined the relative importance of niche limitation and species pool limitation in structuring lakeshore plant communities. Methods We sampled the soil seed banks from the hygrophyte, emergent-plant and submerged-plant zones in a subtropical lakeshore marsh, Longgan Lake, China and used the seedling germination method to examine the density and species composition of seed banks. To assess the effects of species pool and water depth on community establishment, we exposed seed bank samples to three water-level treatments (depths of 0, 25 and 50 cm) and, 45 and 90 days later, compared the species richness and functional groups of the communities that established. Important findings Species of the seed banks were grouped according to their ability to tolerate or respond to water-depth gradients. The established vegetations varied significantly among sites and treatments. For all three sites, mud-flat and emergent communities established at the 0 water-level treatment, while submerged communities established at the 25 and 50 cm water-level treatments, show-ing that the species belonging to different functional group have different tolerances to water depths and that water depth (niche limitation) was important in shaping plant zonation in freshwater marsh. Only the simplest community developed when the seed bank of the emergent-plant zone was exposed to the 25 and 50 cm water-level treatments, as well as when the seed bank of submerged-plant zone was exposed to the 0 water-level treatment. This shows that community species richness depends primarily on the composition and viability of the seeds in the soil seed bank. These findings suggest that the structure and distribution pattern of the wetland community were determined by both niche limitation and species pool limitation. Their relative importance may depend on the stability of the water regime.