Strengthening Invasion Filters to Reassemble Native Plant Communities: Soil Resources and Phenological Overlap

Preventing invasion by exotic species is one of the key goals of restoration, and community assembly theory provides testable predictions about native community attributes that will best resist invasion. For instance, resource availability and biotic interactions may represent “filters” that limit the success of potential invaders. Communities are predicted to resist invasion when they contain native species that are functionally similar to potential invaders; where phenology may be a key functional trait. Nutrient reduction is another common strategy for reducing invasion following native species restoration, because soil nitrogen (N) enrichment often facilitates invasion. Here, we focus on restoring the herbaceous community associated with coastal sage scrub vegetation in Southern California; these communities are often highly invaded, especially by exotic annual grasses that are notoriously challenging for restoration. We created experimental plant communities composed of the same 20 native species, but manipulated functional group abundance (according to growth form, phenology, and N‐fixation capacity) and soil N availability. We fertilized to increase N, and added carbon to reduce N via microbial N immobilization. We found that N reduction decreased exotic cover, and the most successful seed mix for reducing exotic abundance varied depending on the invader functional type. For instance, exotic annual grasses were least abundant when the native community was dominated by early active forbs, which matched the phenology of the exotic annual grasses. Our findings show that nutrient availability and the timing of biotic interactions are key filters that can be manipulated in restoration to prevent invasion and maximize native species recovery.     

The effect of soil nitrogen on competition between native and exotic perennial grasses from northern coastal California

The invasion of European perennial grasses represents a new threat to the native coastal prairie of northern California. Many coastal prairie sites also experience anthropogenic nitrogen (N) deposition or increased N availability as a result of invasion by N-fixing shrubs. We tested the hypothesis that greater seedling competitive ability and greater responsiveness to high N availability of exotic perennial grasses facilitates their invasion in coastal prairie. We evaluated pairwise competitive responses and effects, and the occurrence of asymmetrical competition, among three common native perennial grasses (Agrostis oregonensis, Festuca rubra, and Nassella pulchra) and three exotic perennial grasses (Holcus lanatus, Phalaris aquatica, and Festuca arundinacea), at two levels of soil N. We also compared the root and shoot biomass and response to fertilization of singly-grown plants, so we could evaluate how performance in competition related to innate plant traits. Competitive effects and responses were negatively correlated and in general varied continuously across native and exotic species. Two exceptions were the exotic species Holcus, which had large effects on neighbors and small responses to them, and competed asymmetrically with all other species in the experiment, and the native grass Nassella, which had strong responses to but little effect on neighbors, and was out-competed by all but one other species in the experiment. High allocation to roots and high early relative growth rate appear to explain Holcus’s competitive dominance, but its shoot biomass when grown alone was not significantly greater than those of the species it out-competed. Competitive dynamics were unaffected by fertilization. Therefore, we conclude that seedling competitive ability alone does not explain the increasing dominance of exotic perennial grasses in California coastal prairie. Furthermore, since native and exotic species responded individualistically, grouping species as ‘natives’ and ‘exotics’ obscured underlying variation within the two categories. Finally, elevated soil N does not appear to influence competition among the native and exotic perennial grasses studied, so reducing soil N pools may not be a critical step for the restoration of California coastal prairie.     

Native species richness buffers invader impact in undisturbed but not disturbed grassland assemblages

Many systems are prone to both exotic plant invasion and frequent natural disturbances. Native species richness can buffer the effects of invasion or disturbance when imposed in isolation, but it is largely unknown whether richness provides substantial resistance against invader impact in the face of disturbance. We experimentally examined how disturbance (drought/burning) influenced the impact of three exotic invaders (Centaurea stoebe, Linaria dalmatica, or Potentilla recta) on native abundance across a gradient of species richness, using previously constructed grassland assemblages. We found that invaders had higher cover in experimentally disturbed plots than in undisturbed plots across all levels of native species richness. Although exotic species varied in cover, all three invaders had significant impacts on native cover in disturbed plots. Regardless of disturbance, however, invader cover diminished with increasing richness. Invader impacts on native cover also diminished at higher richness levels, but only in undisturbed plots. In disturbed plots, invaders strongly impacted native cover across all richness levels, as disturbance favoured invaders over native species. By examining these ecological processes concurrently, we found that disturbance exacerbated invader impacts on native abundance. Although diversity provided a buffering effect against invader impact without disturbance, the combination of invasion and disturbance markedly depressed native abundance, even in high richness assemblages.     

Exotic grasses and feces deposition by an exotic herbivore combine to reduce the relative abundance of native forbs

Increased resource availability can facilitate establishment of exotic plant species, especially when coincident with propagule supply. Following establishment, increased resource availability may also facilitate the spread of exotic plant species if it enhances their competitive abilities relative to native species. Exotic Canada geese (Branta canadensis) introduce both exotic grass seed and nutrients to an endangered plant community on the Gulf Islands of southwestern British Columbia, Canada. I used greenhouse experiments to assess the competitive advantage of the exotic grasses relative to native and exotic forbs in this community and to test the impacts of nutrient addition from goose feces on competitive outcomes. I grew experimental communities varying in their proportion of forbs versus exotic grasses, and added goose feces as a nutrient source. I found that both native and exotic forbs produced significantly more biomass in competition with conspecifics than in competition with the grasses, and that the proportional abundance of two out of three native forbs was lowest in the combined presence of exotic grasses and nutrient addition. In a second experiment, I found that in monoculture all species of forbs and grasses showed equal growth responses to nutrients. The exotic species did not convert additional nutrients into additional biomass at a higher rate, but did germinate earlier and grow larger than the native species regardless of nutrient availability. This suggests that the exotic species may have achieved their competitive advantage partly by pre-empting resources in community mixtures. Small and late-germinating native forbs may be particularly vulnerable to competitive suppression from exotic grasses and forbs and may be at an even greater disadvantage if their competitors are benefiting from early access to additional nutrients. In combination, the input of exotic propagules and additional nutrients by nesting geese may compromise efforts to maintain native community composition in this system.     

Fire indirectly benefits fitness in two invasive species

Ecosystems perturbed from their natural disturbance regimes are more vulnerable to establishment and dominance of exotic plant species. Restoration efforts that reintroduce fire have achieved mixed success in reducing the abundance of exotic plants. The responses of many native species to fire are well known; fire-adapted species respond directly (heat and smoke cue germination) and indirectly (post-fire environment benefits seedling survivorship and growth) to fire. However, the direct and indirect effects of fire are unknown for most exotic plant species. We tested the direct and indirect effects of fire on two exotic invaders of Asian origin, Ailanthus altissima and Lonicera maackii, in North American woodlands. To quantify the direct effects of fire, we compared germination rates of seeds exposed to varying levels of heat and smoke in a laboratory and placed at different soil depths during a prescribed fire in the field. We examined the indirect effects of fire by comparing seedling recruitment in burned and unburned woodland plots. Results indicate that neither A. altissima nor L. maackii have germination cues associated with fire. However, both species have greater seedling recruitment in burned as compared to unburned areas in the field. Although seeds of these invasive species are not specifically adapted to fire, they still benefit from post-fire environments and pose a challenge to restoration of fire-maintained ecosystems. Future studies using our approach will allow land managers to better predict how communities will respond to restoration efforts and to understand variability observed in past restoration projects.     

Restoration through reassembly: plant traits and invasion resistance

One of the greatest challenges for ecological restoration is to create or reassemble plant communities that are resistant to invasion by exotic species. We examine how concepts pertaining to the assembly of plant communities can be used to strengthen resistance to invasion in restored communities. Community ecology theory predicts that an invasive species will be unlikely to establish if there is a species with similar traits present in the resident community or if available niches are filled. Therefore, successful restoration efforts should select native species with traits similar to likely invaders and include a diversity of functional traits. The success of trait-based approaches to restoration will depend largely on the diversity of invaders, on the strength of environmental factors and on dispersal dynamics of invasive and native species.     

Establishment of native grasses and their impact on exotic annuals in degraded box gum woodlands

Restoration goals often involve the addition of new species to resident, degraded communities but in box gum woodlands such restoration is often constrained by competition from persistent exotic annuals that control critical ecological processes. Nutrient reduction (via carbon addition) and seed bank depletion are two approaches to reduce competition from exotic annuals but to be effective these treatments must allow establishment of species such as native grasses. This experiment was conducted in two degraded Austrostipa understoreys in the box gum woodlands of south‐east Australia. It compares the effects of carbon addition (sugar), seed depletion (spring burning or spring grazing) and combinations of carbon addition and seed depletion treatments on the establishment of C3 and C4 native grasses, and measured the effects of their establishment on soil nitrate concentration and exotic annuals. Treatments that reduced exotic annual abundance did not increase initial germination of the C4 native grasses, Bothriochloa or Themeda. However, sugar increased seedling survival of Themeda and Bothriochloa and grazing increased seedling survival of Bothriochloa, presumably by reducing effects of exotic annuals. Poa and Rytidosperma (C3 native grasses) failed to establish. Although we were unable to detect any reduction in soil nitrate concentration, swards with successful recruitment of C4 grasses suppressed exotic annuals more than the Austrostipa‐only swards at one site (the other was affected by wildfire). Further, Austrostipa‐Themeda swards were more effective than Austrostipa‐Bothriochloa for suppressing exotics, pointing to a role for both functional and species identity in the degree of resistance conferred.     

Do higher resource capture ability and utilization efficiency facilitate the successful invasion of native plants?

The great damage caused by native invasive species on natural ecosystems is prompting increasing concern worldwide. Many studies have focused on exotic invasive species. In general, exotic invasive plants have higher resource capture ability and utilization capacity, and lower leaf construction cost (CC) compared to noninvasive plants. However, the physiological mechanisms that determine the invasiveness of native plants are poorly understood. We hypothesized that native invaders, like exotic invaders, may have higher resource capture ability and utilization efficiency compared to native noninvaders. To test this hypothesis, ecophysiological traits including light-saturated photosynthetic rate (A_max), specific leaf area (SLA), photosynthetic nitrogen use-efficiency (PNUE), photosynthetic energy-use efficiency (PEUE), and mass-based and area-based leaf construction cost (CC_mass and CC_area) were measured. We compared the above traits between three pairs of native invasive and noninvasive native species, and between three pairs of exotic invasive and noninvasive species in Guangzhou, southern China. Our results showed that the native invaders had higher A_max, SLA, PNUE, PEUE and lower CC_mass, CC_area, compared to native noninvaders and that these traits were also found in the exotic invaders. PNUE and PEUE in the native invaders were 150.3 and 129.0% higher, respectively, than in noninvasive native species, while these same measures in exotic invaders were 43.0 and 94.2% higher, respectively, than in exotic noninvasive species. The results indicated that native invaders have higher resource capture ability and resource utilization efficiency, suggesting that these traits may be a common biological foundation underlying successful invasion by both native and exotic invasives.     

A plant traits approach to managing legacy species during restoration transitions in temperate eucalypt woodlands

Degraded communities often contain a subset of the species that comprised the predisturbance community. These represent an important legacy of the predisturbance state, yet restoration treatments may be detrimental to them. This study examined the potential of leaf traits and life form to predict whether restoration treatments can maintain legacy swards of Austrostipa bigeniculata (hereafter Austrostipa) while controlling exotic annuals in temperate eucalypt woodlands. Treatments included carbon addition to reduce soil nitrate, both with and without burning or pulse grazing to deplete exotic seed pools. We compared leaf traits of Austrostipa with a native grass (Themeda triandra) known to be advantaged, and 8 exotic annual species known to be disadvantaged by these treatments. Leaf traits indicated potentially greater negative impacts of carbon addition on exotic annuals compared to Austrostipa, and on Austrostipa compared to Themeda, suggesting a net restoration benefit. Similarly, burning or pulse grazing is expected to have little negative impact on perennial resprouting grasses (hemicryptophytes; Austrostipa and Themeda) compared with annual exotics (therophytes) with short‐lived seed banks. Treatment responses were largely consistent with predictions: treatments that significantly reduced exotic annuals had no net disadvantage to Austrostipa swards despite significant reductions in Austrostipa seedling growth with carbon addition. Indeed by Year 3, Austrostipa mortality in untreated plots led to 46% lower Austrostipa abundance than in treated plots at one site, potentially due to litter build‐up or other mechanisms. We conclude that plant traits provide a useful framework for designing restoration transitions that retain native legacy species while controlling exotics.     

Evolutionary responses of natives to introduced species: what do introductions tell us about natural communities?

Biological invasions dramatically affect the distribution, abundance and reproduction of many native species. Because of these ecological effects, exotic species can also influence the evolution of natives exposed to novel interactions with invaders. Evolutionary changes in natives in response to selection from exotics are usually overlooked, yet common responses include altered anti-predator defenses, changes in the spectrum of resources and habitats used, and other adaptations that allow native populations to persist in invaded areas. Whether a native population is capable of responding evolutionarily to selection from invaders will depend on the demographic impact of the invader, the genetic architecture and genetic variability of the native population and potentially the history of previous invasions. In some cases, natives will fail to evolve or otherwise adapt, and local or global extinction will result. In other cases, adaptive change in natives may diminish impacts of invaders and potentially promote coexistence between invaders and natives. Here, we review the evidence for evolutionary responses of native species to novel community members. We also discuss how the effects of introduced species may differ from those caused by natural range expansions of native species. Notably, introduced species may come from remote biotas with no previous evolutionary history with the native community. In addition, the rate of addition of introduced species into communities is much greater than all but the most extreme cases of historical biotic exchange. Understanding the evolutionary component of exotic/native species interactions is critical to recognizing the long-term impacts of biological invasions, and to understanding the role of evolutionary processes in the assembly and dynamics of natural communities.     

Differentiation between native and exotic plant species in a dry grassland: realized responses to perturbation,and comparison with fundamental responses

Abstract An area of dry grassland in New Zealand, comprising an equal mixture of native and exotic species, was subject to perturbations of irrigation, fertilization and cessation of grazing. The vegetation response was recorded for 3 years. Total cover, and the contribution of native species to that cover, fluctuated between years even in the control plots. Irrigation increased total cover, but decreased the cover of native species. Fertilization produced the same effects, only less strongly, and also reduced species richness, the loss being in native species. In spite of overall effects of treatments on native and exotic cover, when individual species’ responses to irrigation, fertilization or exclosure were calculated, there was no significant difference between the native and exotic plant guilds. Species differed in their responses, but the native and exotic guilds overlapped. When grouped by morphology, the only significant difference between the responses to perturbation was that forbs and graminoids responded more positively to irrigation than woody and cryptogamic species. The realized responses of the species to the perturbations described here showed little correlation with their physiological responses as determined in previous greenhouse experiments. It is suggested that the realized responses are strongly, and currently unpredictably, influenced by competition from the other species present. Soil nutrients and soil water were both important controls on the community. The relative similarity in the nature of the response to these two factors – nutrients and water – suggests that they affect species in similar ways, possibly because the greater growth rate of the exotic species mediates the short‐term response to both. Grazing has less effect on the current community than either nutrients or water, although it may have been historically important in shaping the species pool. From the poor predictability of field responses from morphological guilds or from ecophysiological responses, it is suggested that the ‘functional types’ approach, although conceptually attractive, lacks experimental support in these grasslands. It is concluded that the exotic species have invaded by being pre‐adapted to the environment with the same environmental responses as the natives, but with the advantage of generally higher growth rates.     

Native versus exotic community patterns across three scales: Roles of competition,environment and incomplete invasion

Three fundamental, interrelated questions in invasion ecology are: (1) to what extent do exotic species outcompete natives; (2) are native and exotic communities functionally similar or different; and (3) are differences in biogeographic patterns in native and exotic communities due to incomplete invasions among exotics? These questions are analogous to general questions in community ecology regarding the relative roles of competition, environmental response and dispersal limitation in community assembly. We addressed each of these questions for plant communities in discrete meadow patches, using analyses at three scales ranging from the landscape to microsites. A weak positive relationship between native and exotic species richness in microsites, and a predominance of positive correlations in abundance among native and exotic species pairs suggest that competition has been less important than other factors in determining native versus exotic abundance and community composition. In contrast, models of species richness and community compositional change across scales suggest native versus exotic community patterns are largely determined by a mix of scale-dependent concordant (shared positive or negative) and discordant relationships with environmental variables. In addition, detailed analyses of species-area and species-abundance relationships suggest ongoing expansion of exotic species populations, indicating that the assembly of the exotic community is in its early stages. Thus, while competition does not appear to strongly affect native versus exotic abundances and compositions at present, it may intensify in the future. Our results indicate that synoptic patterns in native versus exotic richness that have been previously attributed to a single cause may in fact be due to a complex mix of concordant and discordant responses to environmental factors across scales. They also suggest that conservation efforts aimed at promoting natives and reducing exotics should focus on the factors and scales for which such a response (i.e., promotion of high native and low exotic richness) can be expected.     

Can Carbon Addition Increase Competitiveness of Native Grasses? A Case Study from California

There is growing interest in the addition of carbon (C) as sucrose or sawdust to the soil as a tool to reduce plant‐available nitrogen (N) and alter competitive interactions among species. The hypothesis that C addition changes N availability and thereby changes competitive dynamics between natives and exotics was tested in a California grassland that had experienced N enrichment. Sawdust (1.2 kg/m) was added to plots containing various combinations of three native perennial bunchgrasses, exotic perennial grasses, and exotic annual grasses. Sawdust addition resulted in higher microbial biomass N, lower rates of net N mineralization and net nitrification, and higher concentrations of extractable soil ammonium in the soil. In the first year sawdust addition decreased the degree to which exotic annuals competitively suppressed the seedlings of Nassella pulchra and, to a lesser extent, Festuca rubra, both native grasses. However there was no evidence of reduced growth of exotic grasses in sawdust‐amended plots. Sawdust addition did not influence interactions between the natives and exotic perennial grasses. In the second year, however, sawdust addition did not affect the interactions between the natives and either group of exotic grasses. In fact, the native perennial grasses that survived the first year of competition with annual grasses significantly reduced the aboveground productivity of annual grasses even without sawdust addition. These results suggest that the addition of sawdust as a tool in the restoration of native species in our system provided no significant benefit to natives over a 2‐year period.     

How Does Restoration of Native Canopy Affect Understory Vegetation Composition? Evidence from Riparian Communities of the Hunter Valley Australia

Restoration of native vegetation often focuses on the canopy layer species, with the assumption that regeneration of the understory elements will occur as a consequence. The goal of this study was to assess the influence of canopy restoration on the composition and abundance of understory plant species assemblages along riparian margins in the Hunter Valley, NSW, Australia. We compared the floristic composition (richness, abundance, and diversity) of understory species between nonrevegetated (open) and canopy revegetated plots across five sites. A number of other factors that may also influence understory vegetation, including soil nutrients, proximity to main channel, and light availability, were also measured. We found that sites where the canopy had been restored had lower exotic species richness and abundance, as well as higher native species cover, but not native species richness, compared with open sites. Multivariate analysis of plots based on plant community composition showed that revegetated sites were associated with lower total species diversity, light availability, and exotic cover. This study has found that the restoration of the canopy layer does result in lower exotic species richness and cover, and higher native species cover and diversity in the understory, a desirable restoration outcome. Our results provide evidence that restoration of native canopy species may facilitate restoration of native understory species; however, other interventions to increase native species richness of the understory should also be considered as part of management practice.     

Plant responses to livestock grazing frequency in an Australian temperate grassland

Livestock grazing is often thought to enhance native plant species co-existence in remnant grasslands but may also favour exotic invaders. Recommendations for appropriate grazing strategies are needed, for which an understanding of the response of plant species is necessary. We explored the response of plant species and plant functional groups to grazing in temperate grassland of the Monaro Tablelands of south-east Australia by comparing species abundance in adjacent areas that differed in livestock grazing regime (minimal, infrequent and frequent). We also examined whether species with similar responses to grazing share certain traits and consider whether these traits might provide a useful method of assessing grazing impact. At the scale measured (0.25 m²), an infrequent grazing regime maximised plant species co-existence in these grasslands due to widespread invasion by exotic plant species at infrequent grazing intensity. Many native species declined in abundance when grazing frequency increased from minimal to infrequent. Annuals invaded under infrequent grazing while perennials declined most strongly under high frequency grazing. Low levels of grazing apparently reduce cover and create sites suitable for seed recruitment whereas more frequent grazing reduces the persistence of perennials. While there was a tendency for native species to be more susceptible to grazing impact than exotics, plant traits, in particular longevity (perennial, annual) provided a better prediction of the response of plants to grazing. Although a few native plant species persisted at high grazing frequency, even infrequent livestock grazing may not be appropriate for the conservation of many native perennial grassland species. Targeted reductions in grazing frequency may be necessary to enable the long-term coexistence of grazing susceptible species.     

Identifying Native Vegetation for Reducing Exotic Species during the Restoration of Desert Ecosystems

There is currently much interest in restoration ecology in identifying native vegetation that can decrease the invasibility by exotic species of environments undergoing restoration. However, uncertainty remains about restoration's ability to limit exotic species, particularly in deserts where facilitative interactions between plants are prevalent. Using candidate native species for restoration in the Mojave Desert of the southwestern U.S.A., we experimentally assembled a range of plant communities from early successional forbs to late‐successional shrubs and assessed which vegetation types reduced the establishment of the priority invasive annuals Bromus rubens (red brome) and Schismus spp. (Mediterranean grass) in control and N‐enriched soils. Compared to early successional grass and shrub and late‐successional shrub communities, an early forb community best resisted invasion, reducing exotic species biomass by 88% (N added) and 97% (no N added) relative to controls (no native plants). In native species monocultures, Sphaeralcea ambigua (desert globemallow), an early successional forb, was the least invasible, reducing exotic biomass by 91%. However, the least‐invaded vegetation types did not reduce soil N or P relative to other vegetation types nor was native plant cover linked to invasibility, suggesting that other traits influenced native‐exotic species interactions. This study provides experimental field evidence that native vegetation types exist that may reduce exotic grass establishment in the Mojave Desert, and that these candidates for restoration are not necessarily late‐successional communities. More generally, results indicate the importance of careful native species selection when exotic species invasions must be constrained for restoration to be successful.     

Native Cuscuta campestris restrains exotic Mikania micrantha and enhances soil resources beneficial to natives in the invaded communities

Nutrients in exotic species and invaded communities play a key role in determining the dynamics of invaders and the invasibility of a receipt community. This study focused on the effects of the native holoparasite Cuscuta campestris (for short Cuscuta) on nutrients in the exotic invasive Mikania micrantha (for short Mikania) and stands invaded by Mikania. We conducted a set of field investigations on Mikania with Cuscuta parasitism for 1–4 years, and measured soil properties, community composition, and the growth and nutrient content of Mikania and Cuscuta in two types of sub-communities (i.e. with Mikania only, or with Mikania and Cuscuta). Cuscuta dramatically reduced the cover, biomass, and nutrients (i.e. N, P, and K content) of Mikania, significantly enhanced soil water, pH and nutrient content (i.e. organic matter, total N and P, available P and K), and greatly increased the cover and species richness of native plants. In addition, N and K of Cuscuta were positively correlated with N of Mikania, which was negatively associated with soil total N, available P and K. These findings suggest that Cuscuta may be an effective measure against Mikania and be beneficial to the restoration of invaded communities.     

The paradox of invasion in birds: competitive superiority or ecological opportunism?

Why can alien species succeed in environments to which they have had no opportunity to adapt and even become more abundant than many native species? Ecological theory suggests two main possible answers for this paradox: competitive superiority of exotic species over native species and opportunistic use of ecological opportunities derived from human activities. We tested these hypotheses in birds combining field observations and experiments along gradients of urbanization in New South Wales (Australia). Five exotic species attained densities in the study area comparable to those of the most abundant native species, and hence provided a case for the invasion paradox. The success of these alien birds was not primarily associated with a competitive superiority over native species: the most successful invaders were smaller and less aggressive than their main native competitors, and were generally excluded from artificially created food patches where competition was high. More importantly, exotic birds were primarily restricted to urban environments, where the diversity and abundance of native species were low. This finding agrees with previous studies and indicates that exotic and native species rarely interact in nature. Observations and experiments in the field revealed that the few native species that exploit the most urbanized environments tended to be opportunistic foragers, adaptations that should facilitate survival in places where disturbances by humans are frequent and natural vegetation has been replaced by man-made structures. Successful invaders also shared these features, suggesting that their success is not a paradox but can be explained by their capacity to exploit ecological opportunities that most native species rarely use.     

Root plasticity of native and invasive Great Basin species in response to soil nitrogen heterogeneity

Soil nutrients are heterogeneously distributed in natural systems. While many species respond to this heterogeneity through root system plasticity, little is known about how the magnitude of these responses may vary between native and invasive species. We quantified root morphological and physiological plasticity of co-occurring native and invasive Great Basin species in response to soil nitrogen heterogeneity and determined if trade-offs exist between these foraging responses and species relative growth rate or root system biomass. The nine study species included three perennial bunchgrasses, three perennial forbs, and three invasive perennial forbs. The plants were grown in large pots outdoors. Once a week for 4 weeks equal amounts of ¹⁵NH₄ ¹⁵NO₃ were distributed in the soil either evenly through the soil profile, in four patches, or in two patches. All species acquired more N in patches compared to when N was applied evenly through the soil profile. None of the species increased root length density in enriched patches compared to control patches but all species increased root N uptake rate in enriched patches. There was a positive relationship between N uptake rate, relative growth rate, and root system biomass. Path analysis indicated that these positive interrelationships among traits could provide one explanation of how invasive forbs were able to capture 2 and 15-fold more N from enriched patches compared to the native grasses and forbs, respectively. Results from this pot study suggest that plant traits related to nutrient capture in heterogeneous soil environments may be positively correlated which could potentially promote size-asymmetric competition belowground and facilitate the spread of invasive species. However, field experiments with plants in different neighbor environments ultimately are needed to determine if these positive relationships among traits influence competitive ability and invader success.     

外来入侵植物的根系觅养行为研究进展

土壤养分分布具有高度空间异质性, 植物的根系觅养行为是其对土壤养分异质性的一种适应。不同植物为了适应养分异质性会产生不同的根系觅养行为, 通过调整自身的根系觅养范围、觅养精度和觅养速度来更好地吸收利用土壤中的养分。外来植物与本地植物的竞争是决定其成功入侵的重要因素, 土壤养分等环境因素会影响它们之间的竞争关系。近年来, 外来入侵植物的觅养行为逐渐受到人们的关注, 关于入侵植物根系觅养行为的研究成果陆续出现: (1)总体来看, 外来入侵植物具有较强的根系觅养能力, 但根系觅养范围与觅养精度之间的权衡关系还不确定; (2)营养异质性会影响入侵植物与本地植物之间的竞争, 反过来, 二者之间的竞争也会影响根系觅养行为对营养异质性的响应; (3)丛枝菌根真菌(arbuscular mycorrhizal fungi, AMF)能够提高入侵植物的根系觅养能力, 外来植物入侵能够改变入侵植物对AMF的偏好性, 形成AMF对入侵的正反馈作用, 而本地植物与AMF的相互作用也会影响入侵植物的竞争力。未来还应加强营养异质环境下种间竞争和AMF共生对入侵植物根系觅养行为的影响机制研究, 以及全球变化背景下入侵植物根系觅养行为的变化与机制方面的研究, 可以更深入地认识外来植物的觅养行为在其成功入侵中的作用, 并为利用营养调控来防控入侵植物提供理论依据。