Showing papers on "Native plant published in 2020"

Biotic resistance to invasion is ubiquitous across ecosystems of the United States.

Abstract: The biotic resistance hypothesis predicts that diverse native communities are more resistant to invasion. However, past studies vary in their support for this hypothesis due to an apparent contradiction between experimental studies, which support biotic resistance, and observational studies, which find that native and non-native species richness are positively related at broad scales (small-scale studies are more variable). Here, we present a novel analysis of the biotic resistance hypothesis using 24 456 observations of plant richness spanning four community types and seven ecoregions of the United States. Non-native plant occurrence was negatively related to native plant richness across all community types and ecoregions, although the strength of biotic resistance varied across different ecological, anthropogenic and climatic contexts. Our results strongly support the biotic resistance hypothesis, thus reconciling differences between experimental and observational studies and providing evidence for the shared benefits between invasive species management and native biodiversity conservation.

The biogeography of South African terrestrial plant invasions.

Abstract: Thousands of plant species have been introduced, intentionally and accidentally, to South Africa from many parts of the world. Alien plants are now conspicuous features of many South African landscapes and hundreds of species have naturalised (i.e. reproduce regularly without human intervention), many of which are also invasive (i.e. have spread over long distances). There is no comprehensive inventory of alien, naturalised, and invasive plants for South Africa, but 327 plant taxa, most of which are invasive, are listed in national legislation. We collated records of 759 plant taxa in 126 families and 418 genera that have naturalised in natural and semi-natural ecosystems. Over half of these naturalised taxa are trees or shrubs, just under a tenth are in the families Fabaceae (73 taxa) and Asteraceae (64); genera with the most species are Eucalyptus, Acacia, and Opuntia. The southern African Plant Invaders Atlas (SAPIA) provides the best data for assessing the extent of invasions at the national scale. SAPIA data show that naturalised plants occur in 83% of quarter-degree grid cells in the country. While SAPIA data highlight general distribution patterns (high alien plant species richness in areas with high native plant species richness and around the main human settlements), an accurate, repeatable method for estimating the area invaded by plants is lacking. Introductions and dissemination of alien plants over more than three centuries, and invasions over at least 120 years (and especially in the last 50 years) have shaped the distribution of alien plants in South Africa. Distribution patterns of naturalised and invasive plants define four ecologically-meaningful clusters or “alien plant species assemblage zones”, each with signature alien plant taxa for which trait-environment interactions can be postulated as strong determinants of success. Some widespread invasive taxa occur in high frequencies across multiple zones; these taxa occur mainly in riparian zones and other azonal habitats, or depend on human-mediated disturbance, which weakens or overcomes the factors that determine specificity to any biogeographical region.

Exotic garden plants partly substitute for native plants as resources for pollinators when native plants become seasonally scarce

Abstract: Urban green spaces such as gardens often consist of native and exotic plant species, which provide pollen and nectar for flower-visiting insects. Although some exotic plants are readily visited by pollinators, it is unknown if and at which time of the season exotic garden plants may supplement or substitute for flower resources provided by native plants. To investigate if seasonal changes in flower availability from native vs. exotic plants affect flower visits, diversity and particularly plant-pollinator interaction networks, we studied flower-visiting insects over a whole growing season in 20 urban residential gardens in Germany. Over the course of the season, visits to native plants decreased, the proportion of flower visits to exotics increased, and flower-visitor species richness decreased. Yet, the decline in flower-visitor richness over the season was slowed in gardens with a relatively higher proportion of flowering exotic plants. This compensation was more positively linked to the proportion of exotic plant species than to the proportion of exotic flower cover. Plant-pollinator interaction networks were moderately specialized. Interactions were more complex in high summer, but interaction diversity, linkage density, and specialisation were not influenced by the proportion of exotic species. Thus, later in the season when few native plants flowered, exotic garden plants partly substituted for native flower resources without apparent influence on plant-pollinator network structure. Late-flowering garden plants support pollinator diversity in cities. If appropriately managed, and risk of naturalisation is minimized, late-flowering exotic plants may provide floral resources to support native pollinators when native plants are scarce.

Urbanization drives an early spring for plants but not for pollinators

Abstract: Urbanization is one of the major threats to wild plants and pollinators, and its global increase demands a better understanding of the mechanism driving its negative impact. Urban warming and altered local environmental conditions have the potential to affect the timing of flowering and of pollinator activity. While previous evidence has shown that plant phenology tends to advance in urban areas, little is known about its effects on pollinator phenology. In this study we simultaneously assessed the response of the timing of flowering of native plants and of the flight period of wild pollinators to increased urbanization. We collected data from 12 sites along an urbanization gradient in northern France, a region under strong anthropogenic pressure. Overall, we recorded more than 70 plant species, and we sampled more than 4300 wild bees and hoverflies belonging to 154 species. Plant flowering showed a strong response to urbanization at the community level with a striking advancement of the flowering peak in sites at high urbanization. On the contrary, pollinator communities did not show any clear shift of their flight phenology along the gradient, neither regarding abundance nor diversity. Our results indicate that phenologies of plant and pollinator communities can respond differently along the same urbanization gradient. These asymmetric responses can drive modifications in the structure of plant–pollinator networks, and potentially negatively affect the fitness of both mutualistic partners.

Climate change and invasion may synergistically affect native plant reproduction.

Abstract: Global change drivers can interact in synergistic ways, yet the interactive effect of global change drivers, such as climatic warming and species invasions, on plant pollination are poorly represented in experimental studies. We paired manipulative experiments to probe two mechanistic pathways through which plant invasion and warming may alter phenology and reproduction of native plant species. In the first, we tested how experimental warming (+1.7°C) modulated flowering phenology and how this affected flowering overlap between a native plant (Dracophyllum subulatum) and an invasive plant (Calluna vulgaris L.). In the second experiment, we explored how variation in the ratio of native to invasive flowers, and the overall quantity of resources in a floral patch, affected the reproduction of the native species. We hypothesized that the flowering overlap of native and invasive plants would be altered by warming, given that invading plants typically exhibit greater phenological plasticity than native plants. Further, we hypothesized that pollination of native plant flowers would decrease in floral patches dominated by invasive plant flowers, but that this effect would depend on total floral density in the patch. As predicted, the invasive plant had a stronger phenological response to experimental warming than the native plant, resulting in increased flowering overlap between the native the invasive plants. There was a four-fold increase in the number of native flowers co-flowering with high densities of invasive flowers suggesting native plant competition for pollinators with invasive plants under a warmed climate. In the second experiment, we found depressed seed masses of the native species in high density floral patches that were dominated by invasive flowers relative to high density floral patches dominated by native flowers. At low floral densities, seed mass of native plants was unaffected by invasion. Together, these results demonstrate that by increasing their phenological overlap, warming may enhance the magnitude of existing competition for pollination exerted by an invasive plant on a native plant, particularly in plant patches with high floral density. Our results illustrate a novel pathway through which global change drivers can operate synergistically to alter an important ecosystem service: pollination.

Responses of Invasive and Native Populations of the Seagrass Halophila stipulacea to Simulated Climate Change

Abstract: Climate change fuels invasions of plant species and displacement of local plants. Little is known about the ecophysiological adaptation of the invasive species, and their ability to cope with the changing conditions in their new habitat. Halophila stipulacea, a tropical seagrass native to the Gulf of Aqaba (GoA; northern Red Sea), became a Lessepsian migrant spreading within the eastern Mediterranean where it could potentially outcompete local species. We analyzed temperature records in the last 35 years and show that water temperature has increased faster in the eastern Mediterranean Sea compared to GoA, suggesting that H. stipulacea’s invasive success is associated with adaptation to thermal warming. Furthermore, we compared the responses of native (Eilat, Israel) and invasive (Limassol, Cyprus) H. stipulacea plants to current (26oC) and predicted thermal maxima (29℃ and 32℃) in a controlled experimental microcosm. Morphological and photo-physiological results showed negative effects of heat stress on the native plants while un-affected/ or even enhanced performance in their invasive counterparts. Gene expression, studied for the 1st time in H. stipulacea, pointed to differences in the molecular responses of two populations to thermal stress. Results predict that sea warming will cause vast reductions in H. stipulacea meadows growing in the GoA while sea warming will facilitate H. stipulacea’s spread within the Mediterranean Sea.

Are native and non-native pollinator friendly plants equally valuable for native wild bee communities?

Abstract: Bees rely on floral pollen and nectar for food. Therefore, pollinator friendly plantings are often used to enrich habitats in bee conservation efforts. As part of these plantings, non-native plants may provide valuable floral resources, but their effects on native bee communities have not been assessed in direct comparison with native pollinator friendly plantings. In this study, we performed a common garden experiment by seeding mixes of 20 native and 20 non-native pollinator friendly plant species at separate neighboring plots at three sites in Maryland, USA, and recorded flower visitors for 2 years. A total of 3,744 bees (120 species) were collected. Bee abundance and species richness were either similar across plant types (midseason and for abundance also late season) or lower at native than at non-native plots (early season and for richness also late season). The overall bee community composition differed significantly between native and non-native plots, with 11 and 23 bee species being found exclusively at one plot type or the other, respectively. Additionally, some species were more abundant at native plant plots, while others were more abundant at non-natives. Native plants hosted more specialized plant-bee visitation networks than non-native plants. Three species out of the five most abundant bee species were more specialized when foraging on native plants than on non-native plants. Overall, visitation networks were more specialized in the early season than in late seasons. Our findings suggest that non-native plants can benefit native pollinators, but may alter foraging patterns, bee community assemblage, and bee-plant network structures.

Honey bees and wild pollinators differ in their preference for and use of introduced floral resources

Abstract: Introduced plants may be important foraging resources for honey bees and wild pollinators, but how often and why pollinators visit introduced plants across an entire plant community is not well understood Understanding the importance of introduced plants for pollinators could help guide management of these plants and conservation of pollinator habitat We assessed how floral abundance and pollinator preference influence pollinator visitation rate and diversity on 30 introduced versus 24 native plants in central New York Honey bees visited introduced and native plants at similar rates regardless of floral abundance In contrast, as floral abundance increased, wild pollinator visitation rate decreased more strongly for introduced plants than native plants Introduced plants as a group and native plants as a group did not differ in bee diversity or preference, but honey bees and wild pollinators preferred different plant species As a case study, we then focused on knapweed (Centaurea spp), an introduced plant that was the most preferred plant by honey bees, and that beekeepers value as a late-summer foraging resource We compared the extent to which honey bees versus wild pollinators visited knapweed relative to coflowering plants, and we quantified knapweed pollen and nectar collection by honey bees across 22 New York apiaries Honey bees visited knapweed more frequently than coflowering plants and at a similar rate as all wild pollinators combined All apiaries contained knapweed pollen in nectar, 86% of apiaries contained knapweed pollen in bee bread, and knapweed was sometimes a main pollen or nectar source for honey bees in late summer Our results suggest that because of diverging responses to floral abundance and preferences for different plants, honey bees and wild pollinators differ in their use of introduced plants Depending on the plant and its abundance, removing an introduced plant may impact honey bees more than wild pollinators

Co-introduction of native mycorrhizal fungi and plant seeds accelerates restoration of post-mining landscapes

Abstract: Grasslands are among the most threatened terrestrial biomes, and habitat conservation alone will be insufficient to meet biodiversity goals. While restoration of indigenous grasslands is a priority, conflict with economic objectives means that incorporation of alternative habitats is necessary to offset grassland loss. With up to 800,000 km2 of land affected by mining globally, there is an opportunity to create additional grassland habitat in post-mining landscapes. We aimed to assess whether co-introduction of native arbuscular mycorrhizal (AM) fungi and plants is an efficient means of initializing species-rich vegetation recovery in barren post-mining landscapes. We established an experiment in three post-mining areas in Estonia, where we seeded plots with native plant seeds and inoculated them with trap-cultured native AM fungi from a similar habitat. We measured the abundance and composition of soil AM fungal and above-ground plant communities in two consecutive years using releves, high-throughput sequencing and fatty acid profiling. Our results demonstrate that co-introduction of native plants and AM fungi is an effective way to establish species-rich vegetation in post-mining areas. Co-introduction of symbiotic partners resulted in higher richness, diversity and abundance of plants and AM fungi than when either partner was introduced individually. However, the plant and AM fungal communities in sown and inoculated plots were not distinct from those in uninoculated treatments; they rather formed a subset of all taxa present on the sites but exhibited higher diversity than in uninoculated plots. Synthesis and applications. This study shows that managing the below-ground microbiome is an essential part of vegetation restoration. The availability of symbiotic partners can be considered a key aspect determining the diversity of restored vegetation. Targeted inoculations with native and habitat-specific native arbuscular mycorrhizal fungi could therefore increase restoration success. (Less)

Drought exacerbates negative consequences of high‐intensity cattle grazing in a semiarid grassland

Abstract: Grasslands managed for grazing are the largest land-use category globally, with a significant proportion of these grasslands occurring in semiarid and arid regions. In such dryland systems, the effect of grazing on native plant diversity has been equivocal, some studies suggesting that grazing reduces native plant diversity, others that grazing increases or has little impact on diversity. One impediment toward generalizing grazing effects on diversity in this region is that high levels of interannual variation in precipitation may obfuscate vegetative response patterns. By analyzing a long-term data set collected over a 20-yr period in a semiarid grassland, we explicitly evaluated the role of climate in regulating the effect of cattle grazing on plant communities, finding that climate interacted with grazing intensity to shape grassland communities. Community composition of plots that were intensively grazed varied considerably in response to climatic variation and native species richness was low relative to ungrazed and moderately grazed plots. Following a severe drought in 2002, exotic species richness rapidly increased in the high-intensity grazing plots. While this pattern was mirrored in the other treatments, exotic species richness increased to a greater extent and was slower to return to pre-drought levels in the high-intensity grazing plots. Overall, moderate grazing, even compared to grazing cessation, stabilized grassland communities through time, increased resilience to drought, and maintained the highest levels of native plant diversity and lowest levels of exotic diversity. These findings suggest that grazing, at moderate levels, may support grassland resilience to climate change in semiarid regions. However, grazing that exceeds tolerances, particularly in combination with extreme climatic events, like drought, can alter plant composition over relatively long timescales and possibly increase invasibility by nonnative species.

Traditional Agroforestry Systems and Conservation of Native Plant Diversity of Seasonally Dry Tropical Forests

Abstract: Traditional agroforestry systems (TAFS), which integrate crops with wildlife, are important reservoirs of human culture and technical experiences with a high capacity for biodiversity conservation. Our study aimed to evaluate the capacity of TAFS to conserve the floristic diversity of tropical dry forests (TDF) in the Tehuacan-Cuicatlan Valley, Mexico. We compared TAFS and TDF by measuring their forest cover, floristic composition, and structure, in addition to documenting the motivations of people to maintain native vegetation in their agricultural fields. We conducted a restricted randomized sampling of perennial plant species, including nine sites of TAFS and nine of TDF to determine the alpha, beta, and gamma diversity. Furthermore, we conducted semi-structured interviews with peasants who managed the agricultural plots we studied. We also performed workshops with people of the communities where surveys were performed. Our findings show that TAFS can maintain, on average, 68% of the species (95% of them native to the region) and 53% of the abundance of individuals occurring in the adjacent TDF. TAFS harbour 30% (39 species) of plants endemic to Mexico. Total species richness of TDF and TAFS were similar, as well as the effective number of species or communities estimated for the alpha, beta, and gamma diversity, but differed in the abundance of individuals. The high species turnover recorded in TDF (72%) and TAFS (74%) has profound implications for conservation, suggesting that it would be necessary to maintain several sites in order to conserve the regional diversity of native vegetation. Material, non-material, and regulatory contributions were reported to be the reason that peasants take into account maintaining natural vegetation. TAFS associated with TDF in the region (also called “Apancles”) contain an important richness, diversity, and endemism of components of natural ecosystems, as well as provide multiple socio-ecological contributions. These systems could represent a viable alternative to reconcile biological conservation with social well-being.

Plant Diversity Along the Urban–Rural Gradient and Its Relationship with Urbanization Degree in Shanghai, China

Abstract: Urbanization is one of the major causes for plant diversity loss at the local and regional scale. However, how plant species distribute along the urban–rural gradient and what the relationship between urbanization degree and plant diversity is, is not very clear. In this paper, 134 sample sites along two 18 km width transects that run across the urban center of Shanghai were investigated. We quantified the spatial patterns of plant diversity along the urban–rural gradient and measured the relationship between plant diversity and urbanization degree, which was calculated using a land use land cover map derived from high spatial resolution aerial photos. We recorded 526 vascular plant species in 134 plots, 57.8% of which are exotic plant species. Six spatial distribution patterns of species richness were identified for different plant taxa along the rural to urban gradient. The native plant species richness showed no significant relationship to urbanization degree. The richness of the all plants, woody plants and perennial herbs presented significant positive relationship with urbanization degree, while the richness of annual herbs, Shannon-Wiener diversity and Heip evenness all exhibited a negative relationship to urbanization degree. Urbanization could significantly influence plant diversity in Shanghai. Our findings can provide insights to understand the mechanism of urbanization effects on plant diversity, as well as plant diversity conservation in urban areas.

Traditional medicinal plants in South Tyrol (northern Italy, southern Alps): biodiversity and use

Abstract: Worldwide mountain regions are recognized as hotspots of ethnopharmacologically relevant species diversity. In South Tyrol (Southern Alps, Italy), and due to the region’s high plant diversity and isolated population, a unique traditional botanical knowledge of medicinal plants has flourished, which traces its history back to prehistoric times. However, changes in rural life and culture may threaten this unique biodiversity and cultural heritage. Our study aims to collect and analyze information on native plants used in traditional folk medicine, focusing on the preservation of botanical and cultural diversity. Data were collected through a review of published material that documents traditionally used medicinal plants of South Tyrol in order to capture the total diversity of plants and their usage. We evaluated different parameters, comprising the ethnobotanicity index (EI), ethnophytonomic index (EPI), relative frequency of citation (RFC), red list status, and regional legislation with regard to the plant species. A total of 276 species, including 3 mushrooms and 3 lichens, were identified. These belonged to 72 families, most frequently to the Asteraceae, Rosaceae, and Lamiaceae. The most frequently cited species were Hypericum perforatum L., Urtica dioica L., and Plantago lanceolata L. According to 12 ICPC-2 disease categories, the most frequently treated human health symptoms were from the digestive and respiratory systems as well as the skin. A total of 27 species were listed as endangered, of which 16 are not protected and two are now already extinct. Among the 59 predominantly alpine species, 11 species are restricted to the high altitudes of the Alps and may be threatened by global warming. Our research revealed that the ethnobotanical richness of South Tyrol is among the highest in Italy and throughout the Alps. Nevertheless, it is evident that biodiversity and traditional knowledge have been heavily eroded. Furthermore, we point out particularly sensitive species that should be reconsidered for stronger protections in legal regulations.

Nutrients mitigate the impacts of extreme drought on plant invasions.

Abstract: Extreme climate events, such as drought, are becoming increasingly important drivers of plant community change, yet little is known about their impacts on invasive plants. Further, drought impacts may be altered by other anthropogenic stressors, such as eutrophication. We found drought dramatically reduced density of invasive Lepidium latifolium in salt marshes, and this die-back was mitigated by nutrient addition. In a 3-yr field experiment (2014-2016) conducted during an unprecedented drought (2012-2015), we tracked the effects of drought and nutrient addition on the plant community. We conducted this research at four salt marshes across a salinity gradient in the San Francisco Bay, California, USA. We manipulated paired native and invaded plots, one-half of which were treated monthly with N and P for 1.5 yr during the most intense period of the drought and one subsequent wet winter. In addition, we monitored unmanipulated L. latifolium-invaded transects within our freshest and most saline sites throughout the three years of our manipulative experiment and one additional wet winter. We documented a dramatic die-back of invasive L. latifolium during extreme drought, with reductions in stem density (52-100%) and height (17-47%) that were more severe at low salinity sites than high salinity sites. We found nutrient application lessened the effect of drought on L. latifolium stem density, but not height. In native plots, extreme drought reduced native plant cover (4-24%), but nutrient addition mitigated this impact. Interestingly, native plants in invaded plots did not suffer reductions in cover due to drought, perhaps because they were simultaneously benefiting from the die-back of the invader. Our results show drought negatively impacted both native and invasive plants and this impact was stronger on the invader, which experienced persistent declines two years after the end of the drought. However, by mitigating the effect of drought on invasive plants, nutrient addition potentially erased the advantage drought provided native plants over invasive plants under ambient nutrient conditions.

Seed Networks for Upscaling Forest Landscape Restoration: Is It Possible to Expand Native Plant Sources in Brazil?

Abstract: In this paper, we explore how diverse community networks in Brazil have locally advanced seed production and institutional systems to enhance a restoration economy. By focusing on the experiences of the six major native seed suppliers in Amazonia, the Cerrado, and the Atlantic Forest, we estimate the capacity to scale-up community-based systems to meet a large-scale restoration target as a rural development strategy. Over one decade, 1016 collectors traded 416.91 tonnes of native seeds representing, on average, 31.41 kilos yearly and USD 256.5 as household income. Based on this well documented empirical evidence, we estimate that Brazil’s restoration goal would require from 3.6 to 15.6 thousand tonnes of native seeds depending on the share of each restoration method adopted with potential work opportunities for 13.2 to 57.1 thousand collectors yearly and total income from USD 34 to 146 million. We argue that community networks represent feasible arrangements for increasing the availability of plant material sources which provide high socio-economic benefits. For scaling up native seed sources, we suggest the following key strategies: (i) government incentives and subsidies; (ii) enforcement of ecosystem restoration; (iii) community participation; (iv) adaptation of the seed regulations; (v) technological development; and (vi) seed market diversification.

Native plants for greening Mediterranean agroecosystems

Abstract: In the upcoming United Nations Decade on Ecosystem Restoration, a global challenge for scientists and practitioners will be to develop a well-functioning seed production sector on the basis of a sound species-selection process1. To balance crop production with biodiversity functions in Mediterranean woody crops, agroecological practices2 suggest the need to move towards the establishment of herbaceous ground covers3–5. However, establishing such plants requires a supply of suitable native seeds, which is currently unavailable. Here, we present a comprehensive process for selecting regionally adapted species that also emphasizes considerations for seed production6. Using olive groves as a target system, we found that research on ground covers for regenerative agriculture has largely overlooked native species at the expense of commercial and ill-suited varieties. Our assessment of native annuals showed that 85% of the grasses and forbs evaluated exhibit a suite of ecological and production traits that can be tailored to meet the requirements of farmers, seed producers and environmental agencies. These findings suggest that many native species are neglected in agronomic research, despite being potentially suitable for ground covers and for supporting a nature-based solution7 in restoration practice. The framework used here may be applied in other agroecosystems to follow global greening initiatives and to support native seed production to scale up restoration8–10. Restoring ecosystems depends on the cultivation of native plants to balance biodiversity with agricultural needs. This Letter finds that despite being overlooked for commercial varieties, 85% of Mediterranean native annuals could be used in olive groves for nature-based agronomy.

Native plant abundance, diversity, and richness increases in prairie restoration with field inoculation density of native mycorrhizal amendments

Abstract: Ecological restoration efforts can increase the diversity and function of degraded areas. However, current restoration practices cannot typically reestablish the full diversity and species composition of remnant plant communities. Restoration quality can be improved by reintroducing key organisms from the native plant microbiome. In particular, root symbionts called arbuscular mycorrhizal (AM) fungi are critical in shaping grassland communities, but are sensitive to anthropogenic disturbance, which may pose a problem for grassland restoration. Studies of mycorrhizal amendments include inoculation densities of 2–10,000 kg of inocula per hectare. These studies report variable results that may depend on inocula volume, composition, or nativeness. Here we test eight different densities of native AM fungal amendment, ranging from 0 to 8,192 kg/ha in a newly installed prairie restoration. We found that native plant establishment benefited from native mycorrhizal inocula, resulting in improvements in native plant abundance, richness, and community diversity. Moreover, the application of very low densities of native mycorrhizal inocula, as suggested on commercial mycorrhizal products, were ineffective, and higher concentrations were required to benefit native plant abundance and community diversity. These data suggest that higher densities of mycorrhizal amendment or perhaps alternative distribution methods may be required to maximize benefits of native mycorrhizal amendments in restoration practices.

Land-use change and propagule pressure promote plant invasions in tropical rainforest remnants

Abstract: Intact tropical rainforests are considered robust to plant invasions. However, land-use change alters the structure and species composition of native forest, opening up tropical landscapes to invasion. Yet, the relative roles of key drivers on tropical forest invasions remain little investigated. We examine factors affecting plant invasion of rainforest remnants in oil-palm dominated landscapes in Sabah, Malaysian Borneo. We hypothesized that invasion is greater in highly fragmented landscapes, and in disturbed forests with lower native plant diversity (cf. old-growth rainforests). Native and exotic plants were surveyed in 47 plots at 17 forest sites, spanning gradients in landscape-scale fragmentation and local forest disturbance. Using partial least squares path-modelling, we examined correlations between invasion, fragmentation, forest disturbance, propagule pressure, soil characteristics and native plant community. We recorded 6999 individuals from 329 genera in total, including eight exotic species (0–51% of individuals/plot, median = 1.4%) representing shrubs, forbs, graminoids and climbers. The best model (R2 = 0.343) revealed that invasion was correlated with disturbance and propagule pressure (high prevalence of exotic species in plantation matrix), the latter being driven by greater fragmentation of the landscape. Our models revealed a significant negative correlation between invasion and native tree seedlings and sapling community diversity. Increasing landscape fragmentation promotes exotic plant invasion in remnant tropical forests, especially if local disturbance is high. The association between exotic species invasion and young native tree community may have impacts for regeneration given that fragmentation is predicted to increase and so plant invasion may become more prevalent.

Repeated, high-severity wildfire catalyzes invasion of non-native plant species in forests of the Klamath Mountains, northern California, USA

Abstract: Non-native plant invasions are often catalyzed by wildfires and may alter the structure, function, and composition of ecosystems. Of particular concern are non-native annual grasses that promote increased fire frequency and compete with native vegetation for early season moisture. While these effects are well studied in non-forested ecosystems, relatively few studies document invasions of non-native annual grasses in forested ecosystems. We document non-native plant invasion following repeated high-severity wildfire in mixed-conifer/hardwood forests dominated by Douglas-fir (Pseudotsuga menziesii) in the Klamath Mountains of northern California. Despite great concern regarding the loss of conifer dominance and transitions to non-forested states related to recent and projected increases in wildfire activity, there is little known about invasions of non-native plant species following fire in this region. Non-native plant species capable of long-distance dispersal had an average total cover of 26% and were ubiquitous across a systematic, gridded sample of twenty 800 m2 plots. Non-native forbs included multiple species of common post-disturbance invaders (e.g. Cirsium vulgare, Senecio sylvaticus, Lactuca serriola). Non-native annual grasses comprised more of the vegetative cover than non-native forbs (17% vs. 9%), and were dominated by two species, Aira caryophyllea and Vulpia myuros, which reached as high as 80% cover. Our findings indicate that non-native annual grasses are a novel stressor that may accelerate projected loss of conifer dominance in the Klamath Mountains by facilitating fire spread and increasing competition for early season soil moisture. Additional monitoring will be crucial to understanding the impacts of non-native plants as fire activity in this region continues to increase.

Soil resource heterogeneity competitively favors an invasive clonal plant over a native one.

Abstract: Soil resource heterogeneity can affect plant growth and competitive ability. However, little is known about how soil resource heterogeneity affects competitive interactions between invasive and native plants. We conducted an experiment with an invasive clonal plant Alternanthera philoxeroides and a coexisting native one Alternanthera sessilis. The experiment was a randomized design with three factors, i.e. two species (A. philoxeroides and A. sessilis), two interspecific competition treatments (with and without) and five soil treatments (three homogeneous treatments and two small-scale heterogeneous treatments consisting of two patches of 10 cm × 15 cm and with different initial planting positions). Irrespective of competition, increasing soil resource availability increased the growth of A. philoxeroides. Increasing soil resource availability also increased the growth of A. sessilis without competition, but had no impact with competition. Irrespective of competition, soil resource heterogeneity increased biomass and ramet production of A. philoxeroides, and such effects were independent of initial planting position. For A. sessilis, however, soil resource heterogeneity only increased ramet production when the initial plant was grown in the low-resource patch without competition. Our results suggest that both high soil resource availability and small-scale soil resource heterogeneity can increase the relative competitive ability of the invasive plant A. philoxeroides when grown with its native congener A. sessilis. These findings may partly explain the invasion success of this clonal species in area with high soil resource availability and heterogeneity caused by e.g. nitrogen deposition, fertilization and disturbance.

Phytophthora introductions in restoration areas: Responding to protect California native flora from human-assisted pathogen spread

Abstract: Over the past several years, plantings of California native plant nursery stock in restoration areas have become recognized as a pathway for invasive species introductions, in particular Phytophthora pathogens, including first in the U.S. detections (Phytophthora tentaculata, Phytophthora quercina), new taxa, new hybrid species, and dozens of other soilborne species. Restoration plantings may be conducted in high-value and limited habitats to sustain or re-establish rare plant populations. Once established, Phytophthora pathogens infest the site and are very difficult to eradicate or manage—they degrade the natural resources the plantings were intended to enhance. To respond to unintended Phytophthora introductions, vegetation ecologists took a variety of measures to prevent pathogen introduction and spread, including treating infested areas by solarization, suspending plantings, switching to direct seeding, applying stringent phytosanitation requirements on contracted nursery stock, and building their own nursery for clean plant production. These individual or collective actions, loosely coordinated by the Phytophthoras in Native Habitats Work Group ensued as demands intensified for protection from the inadvertent purchase of infected plants from commercial native plant nurseries. Regulation and management of the dozens of Phytophthora species and scores of plant hosts present a challenge to the state, county, and federal agriculture officials and to the ornamental and restoration nursery industries. To rebuild confidence in the health of restoration nursery stock and prevent further Phytophthora introductions, a voluntary, statewide accreditation pilot project is underway which, upon completion of validation, is planned for statewide implementation.

Effects of native and exotic congeners on diversity of invertebrate natural enemies, available spider biomass, and pest control services in residential landscapes

Abstract: Exotic plants are common in urban landscapes and are often planted by landscape managers in an effort to reduce herbivory damage and improve landscape aesthetics. However, exotic plants may be less palatable to many native insects and reduce herbivore biomass that may fuel higher trophic levels. Furthermore, a loss of herbivores in exotic ornamental landscapes may reduce top-down control by natural enemies. In this study, we compare herbivory in native and exotic congener ornamental landscapes. We also explore if caterpillar abundance, natural enemy abundance, diversity, community composition, spider biomass, and egg predation differ between the two landscape types. We predicted that herbivory, as well as natural enemy abundance and predation would be greater in native landscapes. Although we found that leaf area lost to herbivory was greater in native plots in one of the collection years, this relationship was weak. Natural enemy diversity differed between landscape types, but depended on plant genus. The relationship between plant origin and natural enemy diversity was also weak. Caterpillar abundance, natural enemy community composition, spider biomass, and predation services did not differ between treatments. Overall, our results suggest that ornamental landscapes planted in native plants may not differ from landscapes planted in exotic congeners in the pest management and conservation services they provide, particularly with regard to invertebrate natural enemies. However, our findings cannot be used to make more general claims about plant origin, especially with regard to native plants and non-congeners, as we only compared ornamental landscapes with native plants and their exotic congeners in this study. We conclude that for optimizing natural enemy diversity and biomass on city landscapes, plant choice and other plant traits may be as important as plant origin to consider. Our work demonstrates that both native and exotic congener ornamental landscapes provide valuable ecosystem services and will help guide landscape design that serves both the people and wildlife that use them.