Among-population variation in drought responses is consistent across life stages but not between native and non-native ranges.

Understanding how widespread species adapt to variation in abiotic conditions across their ranges is fundamental to ecology. Insight may come from studying how among-population variation (APV) in the common garden corresponds with the environmental conditions of source populations. However, there are no such studies comparing native vs non-native populations across multiple life stages. We examined APV in the performance and functional traits of 59 Conyza canadensis populations, in response to drought, across large aridity gradients in the native (North America) and non-native (Eurasia) ranges in three experiments. Our treatment (dry vs wet) was applied at the recruitment, juvenile, and adult life stages. We found contrasting patterns of APV in drought responses between the two ranges. In the native range, plant performance was less reduced by drought in populations from xeric than mesic habitats, but such relationship was not apparent for non-native populations. These range-specific patterns were consistent across the life stages. The weak adaptive responses of non-native populations indicate that they can become highly abundant even without complete local adaptation to abiotic environments and suggest that long-established invaders may still be evolving to the abiotic environment. These findings may explain lag times in invasions and raise concern about future expansions.

Development and application of a second-generation multilingual tool for invasion risk screening of non-native terrestrial plants.

Under the increasing threat to native ecosystems posed by non-native species invasions, there is an urgent need for decision support tools that can more effectively identify non-native species likely to become invasive. As part of the screening (first step) component in non-native species risk analysis, decision support tools have been developed for aquatic and terrestrial organisms. Amongst these tools is the Weed Risk Assessment (WRA) for screening non-native plants. The WRA has provided the foundations for developing the first-generation WRA-type Invasiveness Screening Kit (ISK) tools applicable to a range of aquatic species, and more recently for the second-generation ISK tools applicable to all aquatic organisms (including plants) and terrestrial animals. Given the most extensive usage of the latter toolkits, this study describes the development and application of the Terrestrial Plant Species Invasiveness Screening Kit (TPS-ISK). As a second-generation ISK tool, the TPS-ISK is a multilingual turnkey application that provides several advantages relative to the WRA: (i) compliance with the minimum standards against which a protocol should be evaluated for invasion process and management approaches; (ii) enhanced questionnaire comprehensiveness including a climate change component; (iii) provision of a level of confidence; (iv) error-free computation of risk scores; (v) multilingual support; (vi) possibility for across-study comparisons of screening outcomes; (vii) a powerful graphical user interface; (viii) seamless software deployment and accessibility with improved data exchange. The TPS-ISK successfully risk-ranked five representative sample species for the main taxonomic groups supported by the tool and ten angiosperms previously screened with the WRA for Turkey. The almost 20-year continuous development and evolution of the ISK tools, as opposed to the WRA, closely meet the increasing demand by scientists and decision-makers for a reliable, comprehensive, updatable and easily deployable decision support tool. For terrestrial plant screening, these requirements are therefore met by the newly developed TPS-ISK.

Trade-offs in non-native plant herbivore defences enhance performance.

Non-native plants are typically released from specialist enemies but continue to be attacked by generalists, albeit at lower intensities. This reduced herbivory may lead to less investment in constitutive defences and greater investment in induced defences, potentially reducing defence costs. We compared herbivory on 27 non-native and 59 native species in the field and conducted bioassays and chemical analyses on 12 pairs of non-native and native congeners. Non-natives suffered less damage and had weaker constitutive defences, but stronger induced defences than natives. For non-natives, the strength of constitutive defences was correlated with the intensity of herbivory experienced, whereas induced defences showed the reverse. Investment in induced defences correlated positively with growth, suggesting a novel mechanism for the evolution of increased competitive ability. To our knowledge, these are the first linkages reported among trade-offs in plant defences related to the intensity of herbivory, allocation to constitutive versus induced defences, and growth.

Litter, Plant Competition, and Ecosystem Dynamics: A Theoretical Perspective.

AbstractCommunity structure depends jointly on species' responses to, and effects on, environmental factors. Many such factors, including detritus, are studied in ecosystem ecology. Detritus in terrestrial ecosystems is dominated by plant litter (nonliving organic material), which, in addition to its role in material cycling, can act as a niche factor modulating interactions among plants. Litter thus links traditional community and ecosystem processes, which are often studied separately. We explore this connection using population dynamics models of two plant species and a litter pool. We first find conditions determining the outcome of interactions between these species, highlighting the role that litter plays and the role of broader ecosystem parameters, such as decomposition rate. Species trade-offs in tolerance to direct competition and litter-based interference competition allow for coexistence, provided the litter-tolerant species produces more litter at the population level; otherwise, priority effects may result. When species coexist, litter-mediated interactions between plants disrupt the traditional relationship between biomass accumulation and decomposition. Increasing decomposition rate may have no effect on standing litter density and, in some cases, may even increase litter load. These results illustrate how ecosystem variables can influence community outcomes that then feed back to influence the ecosystem.

Acquisition and evolution of enhanced mutualism-an underappreciated mechanism for invasive success?

Soil biota can determine plant invasiveness, yet biogeographical comparisons of microbial community composition and function across ranges are rare. We compared interactions between Conyza canadensis, a global plant invader, and arbuscular mycorrhizal (AM) fungi in 17 plant populations in each native and non-native range spanning similar climate and soil fertility gradients. We then grew seedlings in the greenhouse inoculated with AM fungi from the native range. In the field, Conyza plants were larger, more fecund, and associated with a richer community of more closely related AM fungal taxa in the non-native range. Fungal taxa that were more abundant in the non-native range also correlated positively with plant biomass, whereas taxa that were more abundant in the native range appeared parasitic. These patterns persisted when populations from both ranges were grown together in a greenhouse; non-native populations cultured a richer and more diverse AM fungal community and selected AM fungi that appeared to be more mutualistic. Our results provide experimental support for evolution toward enhanced mutualism in non-native ranges. Such novel relationships and the rapid evolution of mutualisms may contribute to the disproportionate abundance and impact of some non-native plant species.

Invasive grass indirectly alters seasonal patterns in seed predation.

Invasive species threaten ecosystems globally, but their impacts can be cryptic when they occur indirectly. Invader phenology can also differ from that of native species, potentially causing seasonality in invader impacts. Yet, it is unclear if invader phenology can drive seasonal patterns in indirect effects. We used a field experiment to test if an invasive grass (Imperata cylindrica) caused seasonal indirect effects by altering rodent foraging and seed predation patterns through time. Using seeds from native longleaf pine (Pinus palustris), we found seed predation was 25% greater, on average, in invaded than control plots, but this effect varied by season. Seed predation was 24-157% greater in invaded plots during spring and autumn months, but invasion had no effect on seed predation in other months. One of the largest effects occurred in October when longleaf pine seeds are dispersed, suggesting potential effects on tree regeneration. Thus, seasonal patterns in indirect effects from invaders may cause underappreciated impacts on ecological communities.

Interacting global change drivers suppress a foundation tree species.

Ecological stress caused by climate change, invasive species and anthropogenic disturbance is driving global environmental change, but how these stressors interact to impact native species are poorly understood. We used a field experiment to test how two stressors (drought and plant invasion by Imperata cylindrica) interacted to determine the effects of a third stressor (fire) on a foundation tree species (Pinus palustris). The invasion combined with prolonged drought resulted in shorter trees than invasion alone. The invasion also resulted in 65% greater fuel loads, four times taller flames, greater maximum temperatures and longer heating duration. Consequently, nearly all tree mortality occurred due to a synergistic interaction between the drought + invasion treatment and fire, where invasion caused taller flames that impacted trees that were shorter due to drought. These findings demonstrate that synergy amongst ecological stressors can dramatically impact native species, with significant implications for forecasting the effects of multiple stressors under global change.

Native perennial and non-native annual grasses shape pathogen community composition and disease severity in a California grassland.

The densities of highly competent plant hosts (i.e. those that are susceptible to and successfully transmit a pathogen) may shape pathogen community composition and disease severity, altering disease risk and impacts. Life history and evolutionary history can influence host competence; longer lived species tend to be better defended than shorter lived species and pathogens adapt to infect species with which they have longer evolutionary histories. It is unclear, however, how the densities of species that differ in competence due to life and evolutionary histories affect plant pathogen community composition and disease severity.We examined foliar fungal pathogens of two host groups in a California grassland: native perennial and non-native annual grasses. We first characterized pathogen community composition and disease severity of the two host groups to approximate differences in competence. We then used observational and manipulated gradients of native perennial and non-native annual grass densities to assess the effects of each host group on pathogen community composition and disease severity in 1-m2 plots.Native perennial and non-native annual grasses hosted distinct pathogen communities but shared generalist pathogens. Native perennial grasses experienced 26% higher disease severity than non-native annuals. Only the observational gradient of native perennial grass density affected disease severity; there were no other significant relationships between host group density and either disease severity or pathogen community composition.Synthesis. The life and evolutionary histories of grasses likely influence their competence for different pathogen species, exemplified by distinct pathogen communities and differences in disease severity. However, there was limited evidence that the density of either host group affected pathogen community composition or disease severity. Therefore, competence for different pathogens likely shapes pathogen community composition and disease severity but may not interact with host density to alter disease risk and impacts at small scales.

Hybrid coffee cultivars may enhance agroecosystem resilience to climate change.

Anthropogenic climate change is predicted to cause shifts in temperature and precipitation patterns that will be detrimental for global agriculture. Developing comprehensive strategies for building climate resilient agroecosystems is critical for maintaining future crop production. Arabica coffee (Coffea arabica) is highly sensitive to the quantity and timing of precipitation, so alterations in precipitation patterns that are predicted under climate change are likely to be a major challenge for maintaining coffee agroecosystems. We assessed cultivar selection as a potential component of more resilient coffee agroecosystems by evaluating water stress responses among five Arabica coffee cultivars (clonal hybrids H10 and H1 and seedling lines Catuai 44, Catuai, and Villa Sarchi) using a precipitation reduction experiment in the highlands of Tarrazú, Costa Rica. During the first harvest (eighteen months after planting), plants under the rainout treatment had 211 % greater total fruit weight and over 50 % greater biomass than under the control treatment, potentially due to protection from unusually high rainfall during this period of our experiment. At the second harvest (30 months after planting), after a year of more typical rainfall, plants under rainout still produced 66 % more fruit by weight than under control. The magnitude of the responses varied among cultivars where, at the first harvest, H10 and H1 had approximately 92 % and 81 % greater fruit production and 18 % and 22 % greater biomass, respectively, and at the second harvest H10 had 60 % more fruit production than the overall average. Thus, our findings suggest that the hybrid lines H10 and H1 are more resilient than the other cultivars to the stress of high soil moisture. Overall, our results indicate that stress due to higher than average rainfall could impair coffee plant growth and production, and that cultivar selection is likely to be an important tool for maintaining the viability of coffee production, and the resilience of global agroecosystems more generally, under climate change.

Emerging fungal pathogen of an invasive grass: Implications for competition with native plant species.

Infectious diseases and invasive species can be strong drivers of biological systems that may interact to shift plant community composition. For example, disease can modify resource competition between invasive and native species. Invasive species tend to interact with a diversity of native species, and it is unclear how native species differ in response to disease-mediated competition with invasive species. Here, we quantified the biomass responses of three native North American grass species (Dichanthelium clandestinum, Elymus virginicus, and Eragrostis spectabilis) to disease-mediated competition with the non-native invasive grass Microstegium vimineum. The foliar fungal pathogen Bipolaris gigantea has recently emerged in Microstegium populations, causing a leaf spot disease that reduces Microstegium biomass and seed production. In a greenhouse experiment, we examined the effects of B. gigantea inoculation on two components of competitive ability for each native species: growth in the absence of competition and biomass responses to increasing densities of Microstegium. Bipolaris gigantea inoculation affected each of the three native species in unique ways, by increasing (Dichanthelium), decreasing (Elymus), or not changing (Eragrostis) their growth in the absence of competition relative to mock inoculation. Bipolaris gigantea inoculation did not, however, affect Microstegium biomass or mediate the effect of Microstegium density on native plant biomass. Thus, B. gigantea had species-specific effects on native plant competition with Microstegium through species-specific biomass responses to B. gigantea inoculation, but not through modified responses to Microstegium density. Our results suggest that disease may uniquely modify competitive interactions between invasive and native plants for different native plant species.

Domestication does not alter invasion risk of a non-native legume.

Most non-native domesticated plants provide benefits without escaping cultivation, but others have become prominent invaders. A better understanding of how domestication might alter plant traits that influence the ability of species to overcome barriers to invasion could improve invasion risk predictions. We explored how variation in foliar chemistry among cultivars of a widespread invader in the U.S. (Lespedeza cuneata) might influence invasion risk through differences in herbivore interactions and a potential tradeoff with competitive ability. In a no-choice feeding bioassay a generalist herbivore performed better when fed cultivars compared to wild genotypes and native congeners, suggesting domesticated L. cuneata may be more vulnerable to herbivory compared to wild genotypes. However, in a factorial greenhouse experiment, with treatments of herbivory and competition, all cultivars had similar seed and biomass production as the wild genotype. Competition with native species reduced productivity of all L. cuneata types, while experimental herbivory alone did not. We also found far less variation in foliar chemistry among L. cuneata types than expected based on domestication history. While our findings from the bioassay suggest potentially lower invasion risk for cultivars if herbivore populations expand more rapidly when feeding on cultivars, we did not find evidence that cultivars had different responses to herbivory or competition with native species in the greenhouse experiment. Altogether, the findings from our multi-pronged approach for evaluating potential factors underlying invasion success of a domesticated species indicate that domestication has not altered invasion risk of this widespread plant invader.

Large-spored Drechslera gigantea is a Bipolaris species causing disease on the invasive grass Microstegium vimineum.

Environmentally damaging invasive plants can also serve as reservoir hosts for agricultural pathogens. Microstegium vimineum is an invasive C4 annual grass that is present throughout the midwestern and eastern United States. It can reach high densities in disturbed areas such as crop-forest interfaces, which creates the potential for pathogen spillover from M. vimineum to agricultural crops and native plants. A previous study that surveyed disease on M. vimineum found a large-spored Bipolaris species that was widespread on M. vimineum and also isolated from co-occurring native grasses. Here, we report that the large-spored fungus isolated from M. vimineum and the native grass Elymus virginicus is Drechslera gigantea, based on comparison with published descriptions of morphological traits, and establish that D. gigantea is a pathogen of M. vimineum and E. virginicus. We review the phylogenetic placement and taxonomic history of D. gigantea and propose that it be reassigned to the genus Bipolaris as Bipolaris gigantea.

Disease in Invasive Plant Populations.

Non-native invasive plants can establish in natural areas, where they can be ecologically damaging and costly to manage. Like cultivated plants, invasive plants can experience a relatively disease-free period upon introduction and accumulate pathogens over time. Diseases of invasive plant populations are infrequently studied compared to diseases of agriculture, forestry, and even native plant populations. We evaluated similarities and differences in the processes that are likely to affect pathogen accumulation and disease in invasive plants compared to cultivated plants, which are the dominant focus of the field of plant pathology. Invasive plants experience more genetic, biotic, and abiotic variation across space and over time than cultivated plants, which is expected to stabilize the ecological and evolutionary dynamics of interactions with pathogens and possibly weaken the efficacy of infectious disease in their control. Although disease is expected to be context dependent, the widespread distribution of invasive plants makes them important pathogen reservoirs. Research on invasive plant diseases can both protect crops and help manage invasive plant populations.

Plant communities mediate the interactive effects of invasion and drought on soil microbial communities.

Soil microbiomes could play a major role in ecosystem responses to escalating anthropogenic global change. However, we currently have a poor understanding of how soil microbes will respond to interacting global change factors and if responses will be mediated by changes in plant community structure. We used a field experiment to assess changes in soil fungal and bacterial communities in response to plant invasion, experimental drought, and their combination. In addition, we evaluated the relative importance of direct versus indirect pathways of invasion and drought through changes in associated plant communities with structural equation models. We found that fungal communities were interactively structured by invasion and drought, where fungal richness was lowest with invasion under ambient conditions but highest with invasion under drought conditions. Bacterial richness was lower under drought but unaffected by invasion. Changes in the plant community, including lower plant richness and higher root biomass, moderated the direct effects of invasion on microbial richness. Fungal and bacterial functional groups, including pathogens, mutualists, and nitrogen metabolizers, were also influenced by plant community changes. In sum, plant communities mediated the effects of interacting global change drivers on soil microbial community structure, with significant potential consequences for community dynamics and ecosystem functions.

Populations of a widespread invader and co-occurring native species vary in phenotypic plasticity.

Phenotypic plasticity can promote plant invasions and enhance impacts on native species, but little is known about variation in plasticity among invader populations compared with native species. Variation in plasticity among invader populations could inform more precise predictions of invader spread and impacts across heterogeneous resource environments. We used a common garden experiment with sun and shade treatments to test for variation in plasticity among 12 populations of an invasive grass (Imperata cylindrica), and to determine whether the invader exhibited greater plasticity than six native species that co-occur in the Southeast USA. Principal component analysis revealed that invader populations from different native ranges consistently varied from each other and native species in traits linked to more favorable phenotypes under resource limitation. Overall, the invader exhibited greater plasticity than native species did, as demonstrated by higher plasticity index values for traits such as plant height, leaf mass ratio, and root : shoot ratio. Variation in phenotypic plasticity among invader populations suggests the potential for evolution of plasticity, and greater plasticity of invader populations than native species may underlie invader dominance. Differences in plasticity among populations appears to play an important role in predictions of the spread and potentially the impacts of invasive species.

Global change stressors alter resources and shift plant interactions from facilitation to competition over time.

Global change stressors such as drought and plant invasion can affect ecosystem structure and function via mediation of resource availability and plant competition outcomes. Yet, it remains uncertain how native plants respond to drought stress that co-occurs with potentially novel resource conditions created by a nonnative invader. Further, there is likely to be temporal variation in competition outcomes between native and nonnative plant species depending on which resources are most limiting at a given time. Interacting stressors coupled with temporal variation make it difficult to predict how global change will impact native plant communities. To address this knowledge gap, we conducted a 5-yr factorial field experiment to quantify how simulated drought, plant invasion (by cogongrass, Imperata cylindrica), and these stressors combined, affected resource availability (soil moisture and light) and competition dynamics between the invader and native longleaf pine (Pinus palustris), a foundation species in southeast U.S. forests. Drought and invasion mediated the survival and performance of pine seedlings in temporally dynamic and unexpected ways. Drought and invasion alone each significantly reduced pine seedling survival. However, when the stressors occurred together, the invader offset drought stress for pine seedlings by maintaining high levels of soil moisture, humidity, and shade compared to uninvaded vegetation. This facilitative effect was pronounced for 2 yr, yet shifted to strong competitive exclusion as the invasion progressed and the limiting resource switched from soil moisture to light. After 3 yr, pine tree survival was low except for pines growing with uninvaded vegetation under ambient precipitation conditions. After 5 yr, pines experiencing a single stressor were taller and had greater height to diameter ratios than pines under no stress or both stressors. This outcome revealed a filtering effect where poorly performing trees were culled under stressful conditions, especially when pines were growing with the invader. Together, these results demonstrate that although drought and invasion suppressed a foundation tree species, the invader temporarily moderated stressful drought conditions, and at least some trees were able to survive despite increasingly strong competition. Such unpredictable effects of interacting global change stressors on native plant species highlight the need for additional long-term studies.

Grass invasion and drought interact to alter the diversity and structure of native plant communities.

Understanding the interactive effects of species invasions and climate change is essential for predicting future shifts in biodiversity. Because multiple stressors can interact in synergistic or antagonistic ways, it is notoriously difficult to anticipate their combined effects on species assemblages. However, some hypotheses predict that plant invasions will become increasingly problematic as climate change improves conditions for invaders or lowers the biotic resistance of native communities. In a 4-yr field experiment, we quantified the individual and interactive effects of invasion by a globally problematic C4 grass, Imperata cylindrica, and chronic simulated drought imposed by rainout shelters on the whole plant communities of regenerating longleaf pine forest. Invasion both inhibited plant colonization and enhanced plot-level extinctions, resulting in a severe (60%) loss of plant diversity across all functional groups, including perennial grasses and forbs, annual forbs, and woody species and dramatic shifts in community composition. Experimental drought reduced diversity by 20%, and caused a shift in the dominant functional groups, but had no significant effect on cover of the invader. The invader partially ameliorated water stress in the drought treatment such that invaded plots had higher soil moisture than uninvaded plots. Consequently, the combined effects of invasion and drought were lower than expected from an additive model of multiple stressors. These findings, which may have broader implications for how other C4 grass invaders will interact with drought to shift native community dynamics, challenge the perception that climate change will exacerbate invasions. In revealing that invasive species pose a major threat to the diversity and structure of native communities despite their moderating effects on abiotic stress, this work also highlights that management of aggressive invaders may be critical to preserving biodiversity regardless of future climate.

Grazing enhances belowground carbon allocation, microbial biomass, and soil carbon in a subtropical grassland.

Despite the large contribution of rangeland and pasture to global soil organic carbon (SOC) stocks, there is considerable uncertainty about the impact of large herbivore grazing on SOC, especially for understudied subtropical grazing lands. It is well known that root system inputs are the source of most grassland SOC, but the impact of grazing on partitioning of carbon allocation to root tissue production compared to fine root exudation is unclear. Given that different forms of root C have differing implications for SOC synthesis and decomposition, this represents a significant gap in knowledge. Root exudates should contribute to SOC primarily after microbial assimilation, and thus promote microbial contributions to SOC based on stabilization of microbial necromass, whereas root litter deposition contributes directly as plant-derived SOC following microbial decomposition. Here, we used in situ isotope pulse-chase methodology paired with plant and soil sampling to link plant carbon allocation patterns with SOC pools in replicated long-term grazing exclosures in subtropical pasture in Florida, USA. We quantified allocation of carbon to root tissue and measured root exudation across grazed and ungrazed plots and quantified lignin phenols to assess the relative contribution of microbial vs. plant products to total SOC. We found that grazing exclusion was associated with dramatically less overall belowground allocation, with lower root biomass, fine root exudates, and microbial biomass. Concurrently, grazed pasture contained greater total SOC, and a larger fraction of SOC that originated from plant tissue deposition, suggesting that higher root litter deposition under grazing promotes greater SOC. We conclude that grazing effects on SOC depend on root system biomass, a pattern that may generalize to other C4-dominated grasslands, especially in the subtropics. Improved understanding of ecological factors underlying root system biomass may be the key to forecasting SOC and optimizing grazing management to enhance SOC accumulation.

Site conditions are more important than abundance for explaining plant invasion impacts on soil nitrogen cycling.

Invasive plant species can alter critical ecosystem processes including nitrogen transformations, but it is often difficult to anticipate where in an invaded landscape, these effects will occur. Our predictive ability lags because we lack a framework for understanding the multiple pathways through which environmental conditions mediate invader impacts. Here, we present a framework using structural equation modeling to evaluate the impact of an invasive grass, Microstegium vimineum (M.v.), on nitrogen cycling based on a series of invaded sites that varied in invader biomass and non-M.v. understory biomass, tree basal area, light availability, and soil conditions. Unlike previous studies, we did not find an overall pattern of elevated nitrate concentrations or higher nitrification rates in M.v.-invaded areas. We found that reference plot conditions mediated differences in mineralization between paired invaded and reference plots at each site through indirect (via M.v. biomass), direct, and interactive pathways; however, the strongest pathways were independent of M.v. biomass. For example, sites with low reference soil nitrate and high non-M.v. understory biomass tended to have faster mineralization at 5-15 cm in invaded plots. These findings suggest that more attention to reference conditions is needed to understand the impact of invasive species on soil nitrogen cycling and other ecosystem processes and that the greatest impacts will not necessarily be where the invader is most abundant.