Propagule input,transport and deposition in riparian environments: the importance of connectivity for diversity

Question: How important is hydrochory for dispersing propagules along riverbanks and to what extent do the quantity and species composition of deposited propagules reflect the riparian vegetation, represent “new” species that are not present in the vegetation, and vary with river flow and season? Location: River Frome, Dorset, UK. Methods: Over 13 consecutive 6‐week time periods, during which river water levels were continuously monitored, aerial inputs of propagules to riverbanks were sampled using funnels, hydrochorous propagule transport was sampled using drift nets, and deposition across the riverbanks was sampled using astroturf mats. A survey of the riparian vegetation enabled comparison between samples and the standing vegetation, so that “new” species could be identified. Differences in propagule abundance and diversity between sampling methods, time periods and locations were tested using Mann‐Whitney U‐tests and Kruskall‐Wallis ANOVA. DCA established contrasts in the floristic composition of all deposited propagules and “new” propagules between different sample types, time periods and locations. Results: Aerial seed fall generated few propagules of low species richness. Hydrochory introduced large numbers of propagules and new species, resulting in high propagule deposition on the riverbank. The number and diversity of deposited propagules was governed by seasonal patterns of seed release and the hydrological regime. Propagule deposition was significantly greater on the most frequently inundated parts of the riverbank and autumn floods were particularly important for transporting “new” species to the study site and for remobilizing previously released propagules. Conclusions: The abundance and diversity of propagules deposited along riverbanks is dependent upon high river flows, which facilitates connectivity between the channel and the riparian zone.     

Urban riparian systems function as corridors for both native and invasive plant species

Riparian areas are often the only green areas left in urban and suburban landscapes, providing opportunities for conservation and connectivity of both aquatic and terrestrial organisms. While city planners and land managers often tout the importance of riparian networks for these uses, it is not well established if urban riparian plant communities are actually functioning as connected assemblages. Furthermore, urban riparian zones are well known to be highly invaded by non-native plant species and may be functioning to increase the spread of non-native species across the landscape. Here we examine connectivity of plant assemblages in riparian networks within an extensively urbanized landscape. We sampled riparian plant communities at 13 sites along three second-order streams of the Rahway River watershed, New Jersey. We also characterized propagule dispersal at each site by sampling litter packs on the river banks five times between March–October 2011 and identifying germinants from litter packs after cold stratification. Species turnover of both riparian and litter vegetation was more strongly associated with flow distance, particularly for native species, indicating that riverine systems are important for promoting connectivity of native plant assemblages in urban landscapes. However, non-native germinants significantly dominated propagule dispersal along the stream reaches, particularly early in the growing season, suggesting spread utilizing the river system and preemption may be an important mechanism for invasion success in this system. Our data show that management of invasive species should be planned and implemented at the watershed scale to reduce spread via the river system.     

Restoration of invaded Cape Floristic Region riparian systems leads to a recovery in foliage-active arthropod alpha- and beta-diversity

The Cape Floristic Region of South Africa is a global biodiversity hotspot threatened by invasive alien plants (IAPs). We assessed the effect of plant invasions, and their subsequent clearing, on riparian arthropod diversity. Foliage-active arthropod communities were collected from two native and one invasive alien tree species. Alpha- and beta-diversity of their associated arthropod communities were compared between near pristine, Acacia-invaded and restored sites. Arthropod alpha-diversity at near pristine sites was higher than at restored sites, and was lowest at invaded sites. This was true for most arthropod taxonomic groups associated with all native tree species and suggests a general trend towards recovery in arthropod alpha-diversity after IAP removal. Overall, arthropod species turnover among sites was significantly influenced by plant invasions with communities at near pristine sites having higher turnover than those at restored and invaded sites. This pattern was not evident at the level of individual tree species. Although arthropod community composition was significantly influenced by plant invasions, only a few significant differences in arthropod community composition could be detected between restored and near pristine sites for all tree species and arthropod taxonomic groups. Assemblage composition on each tree species generally differed between sites with similar degrees of plant invasion indicating a strong turnover of arthropod communities across the landscape. Results further suggest that both arthropod alpha- and beta-diversity can recover after IAP removal, given sufficient time, but catchment signatures must be acknowledged when monitoring restoration recovery.     

Stream invertebrate diversity reduces with invasion of river banks by non‐native plants

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The distribution and habitat associations of non‐native plant species in urban riparian habitats

Questions: 1. What are the distribution and habitat associations of non‐native (neophyte) species in riparian zones? 2. Are there significant differences, in terms of plant species diversity, composition, habitat condition and species attributes, between plant communities where non‐natives are present or abundant and those where non‐natives are absent or infrequent? 3. Are the observed differences generic to non‐natives or do individual non‐native species differ in their vegetation associations? Location: West Midlands Conurbation (WMC), UK. Methods: 56 sites were located randomly on four rivers across the WMC. Ten 2 m × 2 m quadrats were placed within 15 m of the river to sample vegetation within the floodplain at each site. All vascular plants were recorded along with site information such as surrounding land use and habitat types. Results: Non‐native species were found in many vegetation types and on all rivers in the WMC. There were higher numbers of non‐natives on more degraded, human‐modified rivers. More non‐native species were found in woodland, scrub and tall herb habitats than in grasslands. We distinguish two types of communities with non‐natives. In communities colonized following disturbance, in comparison to quadrats containing no non‐native species, those with non‐natives had higher species diversity and more forbs, annuals and shortlived monocarpic perennials. Native species in quadrats containing non‐natives were characteristic of conditions of higher fertility and pH, had a larger specific leaf area and were less stress tolerant or competitive. In later successional communities dominated by particular non‐natives, native diversity declined with increasing cover of non‐natives. Associated native species were characteristic of low light conditions. Conclusions: Communities containing non‐natives can be associated with particular types of native species. Extrinsic factors (disturbance, eutrophication) affected both native and non‐native species. In disturbed riparian habitats the key determinant of diversity is dominance by competitive invasive species regardless of their native or non‐native origin.     

Impacts of the invader giant reed (<Emphasis Type="Italic">Arundo donax</Emphasis>) on riparian habitats and ground arthropod communities

Riparian areas have experienced long-term anthropogenic impacts including the effects of plant introductions. In this study, 27 plots were surveyed across three Mediterranean rivers in north-eastern Spain to explore the effects of the invader giant reed (Arundo donax) on riparian habitat features and the diversity, trophic structure, body size, and abundances of epigeal and hypogeal arthropods in riparian areas. Using pitfall traps and Berlese funnels, this study detected a significant increase in collembola abundance and a decrease in the abundance, body size and diversity of macro-arthropods at order and family levels in invaded plots compared to native stands. Invaded and un-invaded areas also differed in the taxonomical structure of arthropod assemblies but not in trophic guild proportions. However, the fact that arthropods were smaller in A. donax soils, together with the absence of particular taxa within each trophic guild or even an entire trophic group (parasitoids), suggests that food-web alterations in invaded areas cannot be discarded. Habitat features also differed between invaded and un-invaded areas with the poorest herbaceous understory and the largest leaf litter deposition and soil carbon stock observed in A. donax plots. The type of vegetation in riparian areas followed by the total native plant species richness were identified as major causal factors to changes in the abundance, diversity and composition of macro-arthropods. However, our analyses also showed that some alterations related to A. donax invasion were inconsistent across rivers, suggesting that A. donax effects may be context dependent. In conclusion, this study highlights an impoverishment of native flora and arthropod fauna in A. donax soils, and suggests major changes in riparian food webs if A. donax displaces native riparian vegetation.     

Alien grass disrupts reproduction and post-settlement recruitment of co-occurring native vegetation: a mechanism for diversity decline in invaded forest?

Invasive plants significantly threaten native plant biodiversity, yet the mechanisms by which they drive species losses and maintain their own dominance are poorly known. We examined the effects of alien grass invasion (Stenotaphrum secundatum) on (1) abundance and frequency of occurrence, (2) reproductive effort (flowering) and output (fruit production) and (3) soil seed banks for three focal native plants that are characteristic of endangered coastal forest of south-eastern Australia. First, we sampled and compared the foliage cover abundance and frequency (proportion of sites occupied) of the focal natives across invaded and non-invaded (reference) sites (n = 20). We then intensively sampled reproductive effort and output (range of 5–9 sites per species), and density of propagules within the soil (using a standard glasshouse ‘emergence’ method; n = 26) for each species. Invasion was associated with reduced population sizes of all species within the standing vegetation but did not affect population frequency (i.e. proportion of sites where each species was present). Reproductive effort and output were about 75 % lower at invaded than native sites for all species. However, invasion had no effect on propagule densities of the focal natives within the seed bank, despite the substantial reduction in their reproduction. This indicates that the ultimate driver of population declines across invaded landscapes is post-settlement recruitment limitation from the seed bank (e.g. low rates of germination and seedling survival) rather than a reduction in the arrival and storage of propagules at invaded sites. Removal of Stenotaphrum alone might thus be sufficient to stimulate the recovery of native populations from the seed bank.     

Digging deep for diversity: riparian seed bank abundance and species richness in relation to burial depth

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Impacts of Myriophyllum aquaticum invasion in a Mediterranean wetland on plant and macro-arthropod communities

The invasion by alien macrophytes in aquatic ecosystems may produce a strong alteration of the native aquatic vegetation leading to heavy impacts for both plant and faunal native diversity. Myriophyllum aquaticum is an aquatic plant native of Southern America, invasive in several part of the world. We studied the effects of M. aquaticum invasion on plant and macro-arthropod communities in the canals around a protected wetland in the Mediterranean basin. We sampled plant and macro-arthropod communities in 10 transects in invaded and non-invaded tracts of the canals. We assessed the differences in plant and macro-arthropod species richness, diversity, taxonomic diversity and species composition between invaded and non-invaded habitats by means of univariate and multivariate analyses. Our study shows a significant loss of plant diversity between non-invaded to invaded sites, leading to communities numerically and taxonomically impoverished and highly divergent in the species composition. We also detected significant differences in arthropod species composition between invaded and non-invaded transects. Some taxa such as mosquitoes and malacostraca were more frequent in the M. aquaticum-dominated stands. Furthermore, the study shows a positive relation between invaded habitats and juvenile individuals of the invasive alien crayfish Procambarus clarkii.     

Impacts of an invasive willow (Salix × rubens) on riparian bird assemblages in south‐eastern Australia

We explored how a woody plant invader affected riparian bird assemblages. We surveyed 15 200‐m‐long transects in riparian zones in a much‐changed landscape of eastern Victoria, Australia. Abundance, species‐richness, foraging‐guild richness and composition of birds were compared in transects in three habitat types: (i) riparian zones dominated by the invasive willow Salix × rubens; (ii) riparian zones lined with native woody species; and (iii) riparian zones cleared of almost all woody vegetation. We also measured abundance and richness of arthropods and habitat structure to explore further the effects of food resources and habitat on the avifauna. We observed 67 bird species from 14 foraging guilds. Native riparian transects had more birds, bird species and foraging guilds than willow‐invaded or cleared transects. Habitat complexity increased from cleared to willow‐invaded to native riparian transects, as did abundance of native and woodland‐dependent birds. Native shrub and trees species had more foliage and branch‐associated arthropods than did willows, consistent with a greater abundance and variety of foraging guilds of birds dependent on this resource. Willow spread into cleared areas is unlikely to facilitate greatly native bird abundance and diversity even though habitat complexity is increased. Willow invasion into the native riparian zone, by decreasing food resources and altering habitat, is likely to reduce native bird biodiversity and further disrupt connectivity of the riparian zone.     

Do riparian plant community characteristics differ between <Emphasis Type="Italic">Tamarix</Emphasis> (L.) invaded and non-invaded sites on the upper Verde River,Arizona?

Invasion by Tamarix (L.) can severely alter riparian areas of the western U.S., which are globally rare ecosystems. The upper Verde River, Arizona, is a relatively free-flowing river and has abundant native riparian vegetation. Tamarix is present on the upper Verde but is a minor component of the vegetation (8% of stems). This study sought to determine whether riparian vegetation characteristics differed between sites where Tamarix was present and sites where Tamarix was absent during the invasion of the upper Verde. We hypothesized that herbaceous understory and woody plant communities would differ between Tamarix present and absent sites. Our hypothesis was generally confirmed, the two types of sites were different. Tamarix present sites had greater abundance of all vegetation, native understory species, graminoids, and native trees, and a positive association with perennial native wetland plant species. Tamarix absent sites had greater abundance of exotic plants and upland adapted plants and an association with greater abiotic cover and litter. These results are contrary to other reports of Tamarix association with depauperate riparian plant communities, and suggest that Tamarix invasion of a watershed with a relatively natural flow regime and a robust native plant community follows similar establishment patterns as the native riparian plant community.     

Impact of three aquatic invasive species on native plants and macroinvertebrates in temperate ponds

Biological plant invasions pose a serious threat to native biodiversity and have received much attention, especially in terrestrial habitats. In freshwater ecosystems impacts of invasive plant species are less studied. We hypothesized an impact on organisms from the water column and from the sediment. We then assessed the impact of three aquatic invasive species on the plants and macroinvertebrates: Hydrocotyle ranunculoides, Ludwigia grandiflora and Myriophyllum aquaticum. Our research on 32 ponds in Belgium indicated that the reduction in the native plant species richness was a common pattern to invasion. However, the magnitude of impacts were species specific. A strong negative relationship to invasive species cover was found, with submerged vegetation the most vulnerable to the invasion. Invertebrate richness, diversity and abundance were measured in sediments of invaded and uninvaded ponds along a gradient of H. ranunculoides, L. grandiflora, and M. aquaticum species cover. We found a strong negative relationship between invasive species cover and invertebrate abundance, probably due to unsuitable conditions of the detritus for invertebrate colonization. Taxonomic compositions of aquatic invertebrate assemblages in invaded ponds differed from uninvaded ponds. Sensitive benthos, such as mayflies were completely absent in invaded ponds. The introduction of H. ranunculoides, L. grandiflora, and M. aquaticum in Belgian ponds has caused significant ecological alterations in the aquatic vegetation and the detritus community of ponds.     

Fluvially-deposited large wood and riparian plant diversity

Out-of-channel wood jams centred by fluvially-deposited large wood represent an important habitat for riparian plant species along large rivers dominated by exposed riverine sediments. Such wood jams often display distinct physical features associated with the jams, such as areas of scour immediately upstream, plumes of fine sediment downstream and abundant organic material deposited throughout the jam. This paper examines the relationship between physical characteristics of young wood jams (1–3 years since deposition) and riparian plant diversity along the braided River Tagliamento in Northeast Italy. Species richness of riparian plants was significantly positively correlated to the depth of scour features and amount of fine sediment around the wood jams, which facilitate improved access to the water table and increase available water in the sediments, respectively. Species diversity was significantly positively correlated to scour pool depth only. Scour depth was in turn influenced by wood jam size, indicating that larger jams may indirectly increase riparian plant diversity. Management and restoration of riparian zones containing exposed riverine sediments should allow for the formation of wood jams and their associated features where possible, in order to potentially increase localised biodiversity.     

Importance of low-flow and high-flow characteristics to restoration of riparian vegetation along rivers in arid south-western United States

1. Riparian vegetation in dry regions is influenced by low‐flow and high‐flow components of the surface and groundwater flow regimes. The duration of no‐flow periods in the surface stream controls vegetation structure along the low‐flow channel, while depth, magnitude and rate of groundwater decline influence phreatophytic vegetation in the floodplain. Flood flows influence vegetation along channels and floodplains by increasing water availability and by creating ecosystem disturbance. 2. On reference rivers in Arizona's Sonoran Desert region, the combination of perennial stream flows, shallow groundwater in the riparian (stream) aquifer, and frequent flooding results in high plant species diversity and landscape heterogeneity and an abundance of pioneer wetland plant species in the floodplain. Vegetation changes on hydrologically altered river reaches are varied, given the great extent of flow regime changes ranging from stream and aquifer dewatering on reaches affected by stream diversion and groundwater pumping to altered timing, frequency, and magnitude of flood flows on reaches downstream of flow‐regulating dams. 3. As stream flows become more intermittent, diversity and cover of herbaceous species along the low‐flow channel decline. As groundwater deepens, diversity of riparian plant species (particularly perennial species) and landscape patches are reduced and species composition in the floodplain shifts from wetland pioneer trees (Populus, Salix) to more drought‐tolerant shrub species including Tamarix (introduced) and Bebbia. 4. On impounded rivers, changes in flood timing can simplify landscape patch structure and shift species composition from mixed forests composed of Populus and Salix, which have narrow regeneration windows, to the more reproductively opportunistic Tamarix. If flows are not diverted, suppression of flooding can result in increased density of riparian vegetation, leading in some cases to very high abundance of Tamarix patches. Coarsening of sediments in river reaches below dams, associated with sediment retention in reservoirs, contributes to reduced cover and richness of herbaceous vegetation by reducing water and nutrient‐holding capacity of soils. 5. These changes have implications for river restoration. They suggest that patch diversity, riparian plant species diversity, and abundance of flood‐dependent wetland tree species such as Populus and Salix can be increased by restoring fluvial dynamics on flood‐suppressed rivers and by increasing water availability in rivers subject to water diversion or withdrawal. On impounded rivers, restoration of plant species diversity also may hinge on restoration of sediment transport. 6. Determining the causes of vegetation change is critical for determining riparian restoration strategies. Of the many riparian restoration efforts underway in south‐western United States, some focus on re‐establishing hydrogeomorphic processes by restoring appropriate flows of surface water, groundwater and sediment, while many others focus on manipulating vegetation structure by planting trees (e.g. Populus) or removing trees (e.g. Tamarix). The latter approaches, in and of themselves, may not yield desired restoration outcomes if the tree species are indicators, rather than prime causes, of underlying changes in the physical environment.     

Riparian zones as havens for exotic plant species in the central grasslands

In the Central Grasslands of the United States, we hypothesized that riparian zones high in soil fertility would contain more exotic plant species than upland areas of low soil fertility. Our alternate hypothesis was that riparian zones high in native plant species richness and cover would monopolize available resources and resist invasion by exotic species. We gathered nested-scale vegetation data from 40 1 m²subplots (nested in four 1000 m² plots) in both riparian and upland sites at four study areas in Colorado, Wyoming, and South Dakota (a total of 320 1 m² subplots and 32 1000 m² plots). At the 1 m² scale, mean foliar cover of native species was significantly greater (P<0.001) in riparian zones (36.3% ± 1.7%) compared to upland sites (28.7% ± 1.5%), but at this small scale there were no consistent patterns of native and exotic species richness among the four management areas. Mean exotic species cover was slightly higher in upland sites compared to riparian sites (9.0% ± 3.8% versus 8.2% ± 3.0% cover). However, mean exotic species richness and cover were greater in the riparian zones than upland sites in three of four management areas. At the 1000 m² scale, mean exotic species richness was also significantly greater (P<0.05) in riparian zones (7.8 ± 1.0 species) compared to upland sites (4.8 ± 1.0 species) despite the heavy invasion of one upland site. For all 32 plots combined, 21% of the variance in exotic species richness was explained by positive relationships with soil % silt (t =1.7, P=0.09) and total foliar cover (t = 2.4, P=0.02). Likewise, 26% of the variance in exotic species cover (log₁₀ cover) was explained by positive relationships with soil % silt (t =2.3, P=0.03) and total plant species richness (t = 2.5, P=0.02). At landscape scales (four 1000 m² plots per type combined), total foliar cover was significantly and positively correlated with exotic species richness (r=0.73, P<0.05) and cover (r=0.74, P<0.05). Exotic species cover (log₁₀ cover) was positively correlated with log₁₀% N in the soil (r=0.61, P=0.11) at landscape scales. On average, we found that 85% (±5%) of the total number of exotic species in the sampling plots of a given management area could be found in riparian zones, while only 50% (±8%) were found in upland plots. We conclude that: (1) species-rich and productive riparian zones are particularly invasible in grassland ecosystems; and (2) riparian zones may act as havens, corridors, and sources of exotic plant invasions for upland sites and pose a significant challenge to land managers and conservation biologists.     

Invasive moss alters patterns in life-history traits and functional diversity of spiders and carabids

Invasive plants can modify terrestrial habitats and affect the natural faunal composition. In acidic coastal dunes the invasive moss Campylopus introflexus can form dense carpets that largely replace native vegetation. As shown in a previous study, moss invasion affects habitat structure and ground-dwelling arthropod diversity. We suggested that including the functional diversity concept in the analysis of moss invasion impacts may offer further insights. We used pitfall trap data to compare trait composition and functional diversity of spiders and carabids in (a) invaded, moss-rich (C. introflexus) and (b) native, lichen-rich (Cladonia spp.) acidic coastal dunes. Moss invasion induced shifts in the trait values body size and feeding preference (carabids) and hunting mode (spiders): Species were smaller in native sites, and the percentages of web-building spiders and phytophagous beetles were reduced in invaded sites. Furthermore, moss invasion led to a more heterogeneous trait composition for spiders, and changed functional diversity of both arthropod groups, although with the opposite effects: While spiders were functionally more diverse in invaded sites, moss invasion reduced carabid beetles’ functional diversity. We also observed changes in the relationship between species richness and functional diversity that indicate a high functional similarity for spiders but a lower one for carabid beetles in native grey dunes. C. introflexus invasion not only alters the arthropod diversity and assemblage structure of endangered acidic coastal dunes but also interferes at a functional level. These results provide further insight into the way plant invasions might alter the structure and function of ecosystems.     

Experimental evidence for a delayed response of the above-ground vegetation and the seed bank to the invasion of an annual exotic plant in deciduous forests

Invasions by alien plants significantly affect native biodiversity and ecosystem functioning. We conducted a 5-year field experiment to investigate potential effects of the annual invasive plant Impatiens glandulifera on both the native above-ground vegetation and the soil seed bank in a deciduous forest in Switzerland. Eight years after the establishment of I. glandulifera, we set up plots in patches invaded by the alien plant, in plots from which the invasive plant had been manually removed and in plots which were not yet colonized by the invasive plant. We examined plant species richness, diversity and plant species composition in the above-ground vegetation and soil seed bank in all plots one year and five years after the initiation of the experiment. The 36 plots (3 plot types × 6 replicates × 2 sites) were equally distributed over two forest sites. Neither the native above-ground vegetation nor the soil seed bank was influenced by the presence of I. glandulifera one year after the start of the field experiment. After five years, however, plant species richness of both the above-ground vegetation and the soil seed bank was reduced by 25% and 30%, respectively, in plots invaded by the alien plant compared to plots from which I. glandulifera had been removed or uninvaded plots. Furthermore, plots invaded by the alien plant had a lower total seedling density (reduction by 60%) and an altered plant species composition in the soil seed bank compared to control plots. Our field experiment indicates that negative effects of the annual invasive plant on the native above-ground vegetation and soil seed bank of deciduous forests become visible with a delay of several years.     

Development of Vegetation and Aquatic Habitat in Restored Riparian Sites of California's North Coast Rangelands

The preponderance of short‐term objectives and lack of systematic monitoring of restoration projects limits opportunities to learn from past experience and improve future restoration efforts. We conducted a retrospective, cross‐sectional survey of 89 riparian revegetation sites and 13 nonrestored sites. We evaluated 36 restoration metrics at each site and used project age (0–39 years) to quantify plant community and aquatic habitat trajectories with a maximum likelihood model selection approach to compare linear and polynomial relationships. We found significant correlations with project age for 16 of 21 riparian vegetation, and 11 of 15 aquatic habitat attributes. Our results indicated improvements in multiple ecosystem services and watershed functions such as diversity, sedimentation, carbon sequestration, and available habitat. Ten riparian vegetation metrics, including native tree and exotic shrub density, increased nonlinearly with project age, while litter and native shrub density increased linearly. Species richness and cover of annual plants declined over time. Improvements in aquatic habitat metrics, such as increasing pool depth and decreasing bankfull width‐to‐depth ratio, indicated potentially improved anadromous fish habitats at restored sites. We hypothesize that certain instream metrics did not improve because of spatial and/or temporal limitations of riparian vegetation to affect aquatic habitat. Restoration managers should be prepared to maintain or enhance understory diversity by controlling exotic shrubs or planting shade‐tolerant native species as much as 10 years after revegetation.     

Impact of invasions by alien plants on soil seed bank communities: Emerging patterns

Much of our current understanding of the impact of invasive species on plant communities is based on patterns occurring in the above-ground vegetation, while only few studies have examined changes in soil seed banks associated with plant invasions, despite their important role as determinants of vegetation dynamics. Here, we reviewed the literature on the impact of plant invasions on the seed bank and we provide a quantitative synthesis using a meta-analysis approach. Specifically, (1) we quantified the impact of 18 invasive alien plants on (i) species richness and (ii) density of the seed banks of invaded communities, based on 58 pair-wise invaded-uninvaded comparisons (cases); we identified (2) the invasive taxa that are responsible for the largest changes in the seed bank; and (3) the habitats where substantial changes occur. Our study showed three major findings: (1) species richness (68% of cases) and density (58% of cases) were significantly lower in native seed banks invaded by alien plants; (2) species richness and density of native and alien species were remarkably lower in seed banks invaded by large, perennial herbs compared to uninvaded sites; and (3) invaded seed banks were often associated with a larger richness and/or abundance of alien species. This study indicates a need for additional seed bank data in invasion ecology to characterize species-specific and habitat-specific impacts of plant invasions, and to determine whether changes in the seed banks of native and alien species are a symptom of environmental degradation prior to a plant invasion or whether they are its direct result. The findings of this study help improve our capacity to predict the long-term implications of plant invasions, including limitations in the recruitment of native species from the seed bank and the potential for secondary invasions by seeds of other alien species.     

Are boreal ecosystems susceptible to alien plant invasion? Evidence from protected areas

Although biological invasion by alien species is a major contributor to loss of indigenous biological diversity, few studies have examined the susceptibility of the boreal biome to invasion. Based on studies of other ecosystems, we hypothesized that alien plants will be restricted to disturbed areas near human activity and will not be found in natural areas of boreal ecosystems in Gros Morne National Park (Canada), a protected area experiencing a wide range of disturbance regimes. The distribution of alien plants in the region was evaluated using surveys, and study sites were established in naturally and anthropogenically disturbed habitats that had been invaded. Within study sites, randomization tests evaluated the importance of disturbance to alien plant invasion by examining changes in environmental conditions and species abundance within various disturbance regimes, while the importance of site characteristics limiting the distribution of alien plants were examined using Canonical Correspondence Analysis. Consistent with studies in a variety of biomes, areas of high disturbance and human activity had the greatest abundance of resources and the highest percentage of alien species. However, contrary to our hypothesis, natural areas of boreal ecosystems were found susceptible to alien plant invasion. Vegetation types vulnerable to invasion include forests, riparian areas, fens, and alpine meadows. Natural disturbance occurring in these vegetation types caused increases in bare ground and/or light availability facilitating alien plant invasion. Although high soil pH was associated with alien plants in these areas, disturbance was not found to cause changes in soil pH, suggesting susceptibility to invasion is pre-determined by bedrock geology or other factors influencing soil pH. Moose (Alces alces), a non-native herbivore, acts as the primary conduit for alien plant invasion in GMNP by dispersing propagules and creating or prolonging disturbance by trampling and browsing vegetation. The recurrent nature of disturbance within the boreal biome and its interaction with site conditions and herbivores enables alien plants to persist away from areas of high human activity. Managers of natural lands should monitor such interactions to decrease the invasion potential of alien plants.