Competitive interactions between native and invasive exotic plant species are altered under elevated carbon dioxide

Influence of multiple global change drivers on plant invasion: Additive effects are uncommon

Invasive plants threaten biodiversity and cause huge economic losses. It is thought that global change factors (GCFs) associated with climate change (including shifts in temperature, precipitation, nitrogen, and atmospheric CO₂) will amplify their impacts. However, only few studies assessed mixed factors on plant invasion. We collated the literature on plant responses to GCFs to explore independent, combined, and interactive effects on performance and competitiveness of native and invasive plants. From 176 plant species, our results showed that: (1) when native and invasive plants are affected by both independent and multiple GCFs, there is an overall positive effect on plant performance, but a negative effect on plant competitiveness; (2) under increased precipitation or in combination with temperature, most invasive plants gain advantages over natives; and (3) interactions between GCFs on plant performance and competitiveness were mostly synergistic or antagonistic. Our results indicate that native and invasive plants may be affected by independent or combined GCFs, and invasive plants likely gain advantages over native plants. The interactive effects of factors on plants were non-additive, but the advantages of invasive plants may not increase indefinitely. Our findings show that inferring the impacts of climate change on plant invasion from factors individually could be misleading. More mixed factor studies are needed to predict plant invasions under global change.

Effects of elevated CO2 on competition between native and invasive grasses

Elevated atmospheric CO₂ concentration increases the performance of invasive plants relative to natives when grown in monoculture, but it is unclear how that will affect the relative competitive abilities per se of invasive and native grasses grown together. We tested competitive outcomes for four native and four invasive perennial C3 and C4 grasses under ambient (390 ppm) and elevated (700 or 1000 ppm) CO₂ concentrations in the greenhouse with non-limiting water and nutrients. We predicted that elevated CO₂ would increase the competitive suppression of native grasses by invasive grasses. To test this, we determined the relative interaction intensity of biomass allocation for natives grown alone vs. those grown in native-invasive species pairs. We also measured photosynthetic traits that contribute to plant invasiveness and may be affected by elevated CO₂ concentrations for species pairs in mixture to determine native-invasive relative performance. We found no effect of CO₂ for the aboveground biomass and tiller production measures of interaction intensity or for relative performance for most of the measured photosynthetic traits. In competition, the invaders nearly always outperform natives in biomass and tiller production, regardless of CO₂ level. The results suggest that increasing CO₂ concentration alone has little effect on grass competitive outcomes under controlled conditions.

Evaluation of the growth response of arid zone invasive species Salvia verbenaca cultivars to atmospheric carbon dioxide and soil moisture

Although climate change is expected to affect the ecology of many weed species, the nature and scale of these responses is presently not well defined. This presages a suite of potential problems for the agricultural industries. Consequently, we investigated the effects of anticipated climate change on biomass and seed production, for two varieties of wild sage, Salvia verbenaca L. var. verbenaca and Salvia verbenaca var. vernalis Bioss. For the investigation, ambient (400 ppm) and elevated (700 ppm) carbon dioxide conditions, in combination with well-watered (100% field capacity) and drought conditions (60% field capacity), were selected to represent alternative climate scenarios. The alteration in biomass production was represented by a combined measurement of nine variables; plant height, stem diameter, number of leaves, number of branches, leaf area, leaf thickness, shoot biomass, root biomass and dry leaf weight, and fecundity was measured via two variables; 100 seed weight and number of seeds per plant. All biomass measurements were reduced in a drought situation compared with well-watered conditions in ambient carbon dioxide (400 ppm), and each corresponding measurement was greater under elevated carbon dioxide (700 ppm) regardless of water treatment. In contrast, this was not observed for 100 seed weight or number of seeds per plant. Although a similar profile of a reduction in fecundity parameters was observed under drought conditions compared with well-watered conditions in ambient carbon dioxide, there was an increase in seed mass only for var. verbenaca under elevated carbon dioxide in both water treatments. In addition, there was a very small increase in the number of seeds in this species under drought conditions in elevated carbon dioxide, with neither increase in seed mass or seed number being observed in var. vernalis. These results suggest that although future climate change may result in increased competition of both these varieties with desirable plants, their management strategies will need to focus on effects of increased size of the weeds, rather than only attempting to reduce the seed bank holdings.

Exotic Plants Used by the Hmong in Thailand

Exotic species are an integral part of the plants used by many ethnic groups, but they usually receive little attention and have been considered alien to the ethnobotanical data. Here, we analyze the plants used by Thai Hmong refugees that are not native to their current habitats in Thailand. We attempt to understand the sources of this knowledge. Do people maintain the original traditional knowledge related to exotic species when they migrate to a new region, or does new knowledge originate from acculturation? We interviewed 16 specialist Hmong informants in Nan province, Thailand, about their traditional knowledge of 69 exotic species used. Acquisition of this knowledge has a long history; several species are the same as plants used by the Hmong in China and other countries, others are globally useful species which have become part of the pool of species that the Hmong have developed local knowledge about. However, migration also involves the integration of local knowledge from other cultures, and also adapts them to function in urban settings. This includes using closely related exotic taxa that replace some of the species they used in their original homelands. The migrants' traditional knowledge in their new habitats is more complicated and also involves the development of local knowledge that is entirely new.

Growth and competitive interaction between seedlings of an invasive Rumex confertus and of co-occurring two native Rumex species in relation to nutrient availability

Rumex confertus is an alien invasive perennial plant that has increased its range rapidly within central Europe in the last 100 years. This study examined the effects of a commercial fertilizer on the competition between the invasive Rumex confertus and two non-invasive native species R. acetosa or R. conglomeratus in terms of morphological and physiological traits and relative yield. All three Rumex species were grown in the open field with two levels of nutrient availability in field plots. Competition and fertilizer had significant effects on height, relative growth rate (RGR), specific leaf area (SLA) as well as shoot and root biomass of all three species. The fertilized plants had high macronutrient and nitrate contents in leaf tissue. Relative yield of R. confertus was <1, indicating that for this species the effects of interspecific competition were greater than those of intraspecific competition. The results of this experiment indicate that there is interaction between the nutrient status of the soil and the competition between species. Competitive superiority of R. confertus could explain its dominance in grasslands and in disturbed areas, and might explain its great influence on the occurrence of native species because competition intensity was high in fertilized plots.

A greater foraging scale, not a higher foraging precision, may facilitate invasion by exotic plants in nutrient-heterogeneous conditions

BACKGROUND AND AIMS

Soil nutrient heterogeneity has been proposed to influence competitive outcomes among different plant species. Thus, it is crucial to understand the effects of environmental heterogeneity on competition between exotic invasive and native species. However, the effects of soil nutrient heterogeneity on the competition between invasive and native plants have rarely been linked to root foraging behaviour.

METHODS

In this study, a competition experiment was performed with two invasive-native species pairs (BP-VC, Bidens pilosa vs. Vernonia cinerea; MM-PS, Mikania micrantha vs. Paederia scandens) grown under homogeneous and heterogeneous conditions in a common greenhouse environment. Root activity was assessed by determining the amount of strontium (Sr) taken up by the shoot of each species.

KEY RESULTS

The invasive species exhibited a greater foraging scale, whereas the native species exhibited a higher foraging precision. A trade-off between foraging scale and precision was observed within each pair of invasive-native species. Compared with soil homogeneity, soil heterogeneity significantly increased the biomass of the two invasive species, B. pilosa and M. micrantha, under competitive conditions. Within each pair, the invasive species exhibited greater relative competitive ability with respect to shoot mass, and considerably more Sr taken up by the invasive species compared with the native species. The Sr acquisition results indicate that nutrient-poor conditions may facilitate the competitive ability of the native species V. cinerea, whereas M. micrantha may possess a stronger competitive ability regardless of soil nutrient conditions.

CONCLUSION

Soil nutrient heterogeneity has the potential to promote the invasion of these two exotic species due to their larger foraging scale, stronger competitive ability and greater root activity relative to their counterpart native species. The present work highlights the importance of soil heterogeneity in plant invasion, particularly with regards to root foraging traits and competition between invasive and native plants.

Competition between cheatgrass and bluebunch wheatgrass is altered by temperature, resource availability, and atmospheric CO2 concentration

Global change drivers (elevated atmospheric CO2, rising surface temperatures, and changes in resource availability) have significant consequences for global plant communities. In the northern sagebrush steppe of North America, the invasive annual grass Bromus tectorum (cheatgrass) is expected to benefit from projected warmer and drier conditions, as well as increased CO2 and nutrient availability. In growth chambers, we addressed this expectation using two replacement series experiments designed to test competition between B. tectorum and the native perennial bunchgrass Pseudoroegneria spicata. In the first experiment, we tested the effects of elevated temperature, decreased water and increased nutrient availability, on competition between the two species. In the second, we tested the effects of elevated atmospheric CO2 and decreased water availability on the competitive dynamic. In both experiments, under all conditions, P. spicata suppressed B. tectorum, though, in experiment one, warmer and drier conditions and elevated nutrient availability increased B. tectorum's competitiveness. In experiment two, when grown in monoculture, both species responded positively to elevated CO2. However, when grown in competition, elevated CO2 increased P. spicata's suppressive effect, and the combination of dry soil conditions and elevated CO2 enhanced this effect. Our findings demonstrate that B. tectorum competitiveness with P. spicata responds differently to global change drivers; thus, future conditions are unlikely to facilitate B. tectorum invasion into established P. spicata communities of the northern sagebrush steppe. However, disturbance (e.g., fire) to these communities, and the associated increase in soil nutrients, elevates the risk of B. tectorum invasion.

Elevated nitrogen allows the weak invasive plant Galinsoga quadriradiata to become more vigorous with respect to inter-specific competition

Elevated nitrogen associated with global change is believed to promote the invasion of many vigorous exotic plants. However, it is unclear how a weak exotic plant will respond to elevated nitrogen in the future. In this study, the competitive outcome of a weak invasive plant (Galinsoga quadriradiata) and two non-invasive plants was detected. The plants were subjected to 3 types of culture (mixed, monoculture or one-plant), 2 levels of nitrogen (ambient or elevated at a rate of 2 g m^-2 yr^-1) and 2 levels of light (65% shade or full sunlight). The results showed that elevated nitrogen significantly promoted the growth of both the weak invader and the non-invasive plants in one-plant pots; however, growth promotion was not observed for the non-invasive species in the mixed culture pots. The presence of G. quadriradiata significantly inhibited the growth of the non-invasive plants, and a decreased negative species interaction was detected as a result of elevated nitrogen. Our results suggest that competitive interactions between G. quadriradiata and the non-invasive plants were altered by elevated nitrogen. It provides exceptional evidence that an initially weak invasive plant can become an aggressive invader through elevated nitrogen deposition.

Sexual competition affects biomass partitioning, carbon-nutrient balance, Cd allocation and ultrastructure of Populus cathayana females and males exposed to Cd stress

Although increasing attention has been paid to plant adaptation to soil heavy metal contamination, competition and neighbor effects have been largely overlooked, especially in dioecious plants. In this study, we investigated growth as well as biochemical and ultrastructural responses of Populus cathayana Rehder females and males to cadmium (Cd) stress under different sexual competition patterns. The results showed that competition significantly affects biomass partitioning, photosynthetic capacity, leaf and root ultrastructure, Cd accumulation, the contents of polyphenols, and structural and nonstructural carbohydrates. Compared with single-sex cultivation, plants of opposite sexes exposed to sexual competition accumulated more Cd in tissues and their growth was more strongly inhibited, indicating enhanced Cd toxicity under sexual competition. Under intrasexual competition, females showed greater Cd accumulation, more serious damage at the ultrastructural level and greater reduction in physiological activity than under intersexual competition, while males performed better under intrasexual competition than under intersexual competition. Males improved the female microenvironment by greater Cd uptake and lower resource consumption under intersexual competition. These results demonstrate that the sex of neighbor plants and competition affect sexual differences in growth and in key physiological processes under Cd stress. The asymmetry of sexual competition highlighted here might regulate population structure, and spatial segregation and phytoremediation potential of both sexes in P. cathayana growing in heavy metal-contaminated soils.

Cheatgrass is favored by warming but not CO2 enrichment in a semi-arid grassland

Elevated CO2 and warming may alter terrestrial ecosystems by promoting invasive plants with strong community and ecosystem impacts. Invasive plant responses to elevated CO2 and warming are difficult to predict, however, because of the many mechanisms involved, including modification of phenology, physiology, and cycling of nitrogen and water. Understanding the relative and interactive importance of these processes requires multifactor experiments under realistic field conditions. Here, we test how free-air CO2 enrichment (to 600 ppmv) and infrared warming (+1.5 °C day/3 °C night) influence a functionally and phenologically distinct invasive plant in semi-arid mixed-grass prairie. Bromus tectorum (cheatgrass), a fast-growing Eurasian winter annual grass, increases fire frequency and reduces biological diversity across millions of hectares in western North America. Across 2 years, we found that warming more than tripled B. tectorum biomass and seed production, due to a combination of increased recruitment and increased growth. These results were observed with and without competition from native species, under wet and dry conditions (corresponding with tenfold differences in B. tectorum biomass), and despite the fact that warming reduced soil water. In contrast, elevated CO2 had little effect on B. tectorum invasion or soil water, while reducing soil and plant nitrogen (N). We conclude that (1) warming may expand B. tectorum's phenological niche, allowing it to more successfully colonize the extensive, invasion-resistant northern mixed-grass prairie, and (2) in ecosystems where elevated CO2 decreases N availability, CO2 may have limited effects on B. tectorum and other nitrophilic invasive species.

Plant-plant interactions mediate the plastic and genotypic response of Plantago asiatica to CO2: an experiment with plant populations from naturally high CO2 areas

BACKGROUND AND AIMS

The rising atmospheric CO2 concentration ([CO2]) is a ubiquitous selective force that may strongly impact species distribution and vegetation functioning. Plant-plant interactions could mediate the trajectory of vegetation responses to elevated [CO2], because some plants may benefit more from [CO2] elevation than others. The relative contribution of plastic (within the plant's lifetime) and genotypic (over several generations) responses to elevated [CO2] on plant performance was investigated and how these patterns are modified by plant-plant interactions was analysed.

METHODS

Plantago asiatica seeds originating from natural CO2 springs and from ambient [CO2] sites were grown in mono stands of each one of the two origins as well as mixtures of both origins. In total, 1944 plants were grown in [CO2]-controlled walk-in climate rooms, under a [CO2] of 270, 450 and 750 ppm. A model was used for upscaling from leaf to whole-plant photosynthesis and for quantifying the influence of plastic and genotypic responses.

KEY RESULTS

It was shown that changes in canopy photosynthesis, specific leaf area (SLA) and stomatal conductance in response to changes in growth [CO2] were mainly determined by plastic and not by genotypic responses. We further found that plants originating from high [CO2] habitats performed better in terms of whole-plant photosynthesis, biomass and leaf area, than those from ambient [CO2] habitats at elevated [CO2] only when both genotypes competed. Similarly, plants from ambient [CO2] habitats performed better at low [CO2], also only when both genotypes competed. No difference in performance was found in mono stands.

CONCLUSION

The results indicate that natural selection under increasing [CO2] will be mainly driven by competitive interactions. This supports the notion that plant-plant interactions have an important influence on future vegetation functioning and species distribution. Furthermore, plant performance was mainly driven by plastic and not by genotypic responses to changes in atmospheric [CO2].

Effects of elevated CO2 on photosynthetic traits of native and invasive C3 and C4 grasses

BACKGROUND

Rising CO2 is expected to result in changes in plant traits that will increase plant productivity for some functional groups. Differential plant responses to elevated CO2 are likely to drive changes in competitive outcomes, with consequences for community structure and plant diversity. Many of the traits that are enhanced under elevated CO2 also confer competitive success to invasive species, and it is widely believed that invasive species will be more successful in high CO2. However, this is likely to depend on plant functional group, and evidence suggests that C3 plants tend to respond more strongly to CO2.

RESULTS

We tested the hypothesis that invasive species would be more productive than noninvasive species under elevated CO2 and that stronger responses would be seen in C3 than C4 plants. We examined responses of 15 grass species (eight C3, seven C4), classified as noninvasive or invasive, to three levels of CO2 (390, 700 and 1000 ppm) in a closed chamber experiment. Elevated CO2 decreased conductance and %N and increased shoot biomass and C/N ratio across all species. Differences between invasive and noninvasive species depended on photosynthetic mechanism, with more differences for traits of C3 than C4 plants. Differences in trait means between invasive and noninvasive species tended to be similar across CO2 levels for many of the measured responses. However, noninvasive C3 grasses were more responsive than invasive C3 grasses in increasing tiller number and root biomass with elevated CO2, whereas noninvasive C4 grasses were more responsive than invasive C4 grasses in increasing shoot and root biomass with elevated CO2. For C3 grasses, these differences could be disadvantageous for noninvasive species under light competition, whereas for C4 grasses, noninvasive species may become better competitors with invasive species under increasing CO2.

CONCLUSIONS

The ecophysiological mechanisms underlying invasion success of C3 and C4 grasses may differ. However, given that the direction of trait differences between invasive and noninvasive grasses remained consistent under ambient and elevated CO2, our results provide evidence that increases in CO2 are unlikely to change dramatically the competitive hierarchy of grasses in these functional groups.

Carbon dioxide fertilisation and supressed respiration induce enhanced spring biomass production in a mixed species temperate meadow exposed to moderate carbon dioxide enrichment

The rising concentration of carbon dioxide in the atmosphere ([CO2]) has a direct effect on terrestrial vegetation through shifts in the rates of photosynthetic carbon uptake and transpirational water-loss. Free Air CO2 Enrichment (FACE) experiments aim to predict the likely responses of plants to increased [CO2] under normal climatic conditions. The Giessen FACE system operates a lower [CO2] enrichment regime (480μmolmol-1) than standard FACE (550-600μmolmol-1), permitting the analysis of a mixed species temperate meadow under a [CO2] level equivalent to that predicted in 25-30 years. We analysed the physiological and morphological responses of six species to investigate the effect of moderate [CO2] on spring biomass production. Carbon dioxide enrichment stimulated leaf photosynthetic rates and supressed respiration, contributing to enhanced net assimilation and a 23% increase in biomass. The capacity for photosynthetic assimilation was unaffected by [CO2] enrichment, with no downregulation of rates of carboxylation of Rubisco or regeneration of ribulose-1,5-bisphosphate. Foliar N content was also not influenced by increased [CO2]. Enhanced [CO2] reduced stomatal size, but stomatal density and leaf area index remained constant, suggesting that the effect on gas exchange was minimal.

Competitive interactions between established grasses and woody plant seedlings under elevated CO₂ levels are mediated by soil water availability

The expansion of woody plants into grasslands has been observed worldwide and is likely to have widespread ecological consequences. One proposal is that woody plant expansion into grasslands is driven in part by the rise in atmospheric CO2 concentrations. We have examined the effect of CO2 concentration on the competitive interactions between established C4 grasses and woody plant seedlings in a model grassland system. Woody plant seedlings were grown in mesocosms together with established C4 grasses in three competition treatments (root competition, shoot competition and root + shoot competition) under ambient and elevated CO2 levels. We found that the growth of the woody plant seedlings was suppressed by competition from grasses, with root and shoot competition having similar competitive effects on growth. In contrast to expectations, woody plant seedling growth was reduced at elevated CO2 levels compared to that at the ambient CO2 level across all competition treatments, with the most plausible explanation being reduced light and soil water availability in the elevated CO2 mesocosms. Reduced light and soil water availability in the elevated CO2 mesocosms was associated with an increased leaf area index of the grasses which offset the reductions in stomatal conductance and increased rainfall interception. The woody plant seedlings also had reduced 'escapability' (stem biomass and stem height) under elevated compared to ambient CO2 levels. Our results suggest that the expansion of woody plants into grasslands in the future will likely be context-dependent, with the establishment success of woody plant seedlings being strongly coupled to the CO2 response of competing grasses and to soil water availability.

Sediment type affects competition between a native and an exotic species in coastal China

Different types of sediments in salt marsh have different physical and chemical characters. Thus sediment type plays a role in plant competition and growth in salt marsh ecosystems. Spartina anglica populations have been increasingly confined to upper elevation gradients of clay, and the niche sediment has changed. Because the niches of S. anglica and the native species Scirpus triqueter overlap, we conducted a greenhouse experiment to test the hypothesis that plant competition has changed under different types of sediments. Biomass and asexual reproduction were analyzed, and inter- and intraspecific competition was measured by log response ratio for the two species in both monoculture and combination under three sediment types (sand, clay and mixture of sand and clay). For S. anglica, biomass, ramet number and rhizome length in combination declined significantly compared with those in monoculture, and the intensity of interspecific competition was significantly higher than that of intraspecific competition under all sediments. For S. triqueter, the intensities of intra- and interspecific competition were not significantly different. This indicates that S. triqueter exerts an asymmetric competitive advantage over S. anglica across all sediments, but especially clay. Thus the sediment type changes competition between S. anglica and S. triqueter.

Invasive forb benefits from water savings by native plants and carbon fertilization under elevated CO2 and warming

As global changes reorganize plant communities, invasive plants may benefit. We hypothesized that elevated CO2 and warming would strongly influence invasive species success in a semi-arid grassland, as a result of both direct and water-mediated indirect effects. To test this hypothesis, we transplanted the invasive forb Linaria dalmatica into mixed-grass prairie treated with free-air CO2 enrichment and infrared warming, and followed survival, growth, and reproduction over 4 yr. We also measured leaf gas exchange and carbon isotopic composition in L. dalmatica and the dominant native C3 grass Pascopyrum smithii. CO2 enrichment increased L. dalmatica biomass 13-fold, seed production 32-fold, and clonal expansion seven-fold, while warming had little effect on L. dalmatica biomass or reproduction. Elevated CO2 decreased stomatal conductance in P. smithii, contributing to higher soil water, but not in L. dalmatica. Elevated CO2 also strongly increased L. dalmatica photosynthesis (87% versus 23% in P. smithii), as a result of both enhanced carbon supply and increased soil water. More broadly, rapid growth and less conservative water use may allow invasive species to take advantage of both carbon fertilization and water savings under elevated CO2 . Water-limited ecosystems may therefore be particularly vulnerable to invasion as CO2 increases.

Gas exchange, growth, and defense responses of invasive Alliaria petiolata (Brassicaceae) and native Geum vernum (Rosaceae) to elevated atmospheric CO2 and warm spring temperatures

PREMISE OF STUDY

Global increases in atmospheric CO2 and temperature may interact in complex ways to influence plant physiology and growth, particularly for species that grow in cool, early spring conditions in temperate forests. Plant species may also vary in their responses to environmental changes; fast-growing invasives may be more responsive to rising CO2 than natives and may increase production of allelopathic compounds under these conditions, altering species' competitive interactions.

METHODS

We examined growth and physiological responses of Alliaria petiolata, an allelopathic, invasive herb, and Geum vernum, a co-occurring native herb, to ambient and elevated spring temperatures and atmospheric CO2 conditions in a factorial growth chamber experiment.

KEY RESULTS

At 5 wk, leaves were larger at high temperature, and shoot biomass increased under elevated CO2 only at high temperature in both species. As temperatures gradually warmed to simulate seasonal progression, G. vernum became responsive to CO2 at both temperatures, whereas A. petiolata continued to respond to elevated CO2 only at high temperature. Elevated CO2 increased thickness and decreased nitrogen concentrations in leaves of both species. Alliaria petiolata showed photosynthetic downregulation at elevated CO2, whereas G. vernum photosynthesis increased at elevated temperature. Flavonoid and cyanide concentrations decreased significantly in A. petiolata leaves in the elevated CO2 and temperature treatment. Total glucosinolate concentrations and trypsin inhibitor activities did not vary among treatments.

CONCLUSIONS

Future elevated spring temperatures and CO2 will interact to stimulate growth for A. petiolata and G. vernum, but there may be reduced allelochemical effects in A. petiolata.

Industrial-age changes in atmospheric [CO2] and temperature differentially alter responses of faster- and slower-growing Eucalyptus seedlings to short-term drought

Climate change may alter forest composition by differentially affecting the responses of faster- and slower-growing tree species to drought. However, the combined effects of rising atmospheric CO2 concentration ([CO2]) and temperature on drought responses of trees are poorly understood. Here, we examined interactive effects of temperature (ambient, ambient + °C) and [CO2] (290, 400 and 650mu;l l(-1)) on drought responses of Eucalyptus saligna Sm. (faster-growing) and E. sideroxylon A. Cunn. ex Woolls (slower-growing) seedlings. Drought was imposed via a controlled reduction in soil water over 1-2 weeks, re-watering seedlings when leaves visibly wilted. In ambient temperature, the effect of drought on the light-saturated net photosynthetic rate (Asat) in E. saligna decreased as [CO2] increased from pre-industrial to future concentrations, but rising [CO2] did not affect the response in Eucalyptus sideroxylon. In contrast, elevated temperature exacerbated the effect of drought in reducing Asat in both species. The drought response of Asat reflected changes in stomatal conductance (gs) associated with species and treatment differences in (i) utilization of soil moisture and (ii) leaf area ratio (leaf area per unit plant dry mass). Across [CO2] and temperature treatments, E. saligna wilted at higher soil water potentials compared with E. sideroxylon. Photosynthetic recovery from drought was 90% complete 2 days following re-watering across all species and treatments. Our results suggest that E. saligna (faster-growing) seedlings are more susceptible to drought than E. sideroxylon (slower-growing) seedlings. The greater susceptibility to drought of E. saligna reflected faster drawdown of soil moisture, associated with more leaf area and leaf area ratio, and the ability of E. sideroxylon to maintain higher gs at a given soil moisture. Inclusion of a pre-industrial [CO2] treatment allowed us to conclude that susceptibility of these species to short-term drought under past and future climates may be regulated by the same mechanisms. Further, the beneficial effects of rising [CO2] and deleterious effects of elevated temperature on seedling response to drought were generally offsetting, suggesting susceptibility of seedlings of these species to short-term drought in future climates that is similar to pre-industrial and current climate conditions.

Nonindigenous Plant Advantage in Native and Exotic Australian Grasses under Experimental Drought, Warming, and Atmospheric CO2 Enrichment

A general prediction of ecological theory is that climate change will favor invasive nonindigenous plant species (NIPS) over native species. However, the relative fitness advantage enjoyed by NIPS is often affected by resource limitation and potentially by extreme climatic events such as drought. Genetic constraints may also limit the ability of NIPS to adapt to changing climatic conditions. In this study, we investigated evidence for potential NIPS advantage under climate change in two sympatric perennial stipoid grasses from southeast Australia, the NIPS Nassella neesiana and the native Austrostipa bigeniculata. We compared the growth and reproduction of both species under current and year 2050 drought, temperature and CO2 regimes in a multifactor outdoor climate simulation experiment, hypothesizing that NIPS advantage would be higher under more favorable growing conditions. We also compared the quantitative variation and heritability of growth traits in populations of both species collected along a 200 km climatic transect. In contrast to our hypothesis we found that the NIPS N. neesiana was less responsive than A. bigeniculata to winter warming but maintained higher reproductive output during spring drought. However, overall tussock expansion was far more rapid in N. neesiana, and so it maintained an overall fitness advantage over A. bigeniculata in all climate regimes. N. neesiana also exhibited similar or lower quantitative variation and growth trait heritability than A. bigeniculata within populations but greater variability among populations, probably reflecting a complex past introduction history. We found some evidence that additional spring warmth increases the impact of drought on reproduction but not that elevated atmospheric CO2 ameliorates drought severity. Overall, we conclude that NIPS advantage under climate change may be limited by a lack of responsiveness to key climatic drivers, reduced genetic variability in range-edge populations, and complex drought-CO2 interactions.

Linking trait differences to community dynamics: evidence from Eupatorium adenophorum and co-occurring native species during a three-year succession

Trait differences between invasive and native species are believed to be closely related to whether the former are successful. However, few studies have measured trait differences between invasive and native species directly under field conditions or during long term experiments. We examined the phenological pattern, plant height and biomass accumulation and allocation of Crofton weed (Eupatorium adenophorum Spreng.) and co-occurring native species in a community during a three-year succession. The phenological pattern of Crofton weed differed from that of co-occurring native species. Crofton weed had longer vegetative stage (when resources were more available), a higher biomass accumulation and a higher above/below-ground ratio compared to native species. Crofton weed was shorter than grasses and two forbs (Artemisia tangutica and Cynoglossum amabile) during its first year of growth, but was significantly taller than all other species during subsequent years. The dominance (calculated as the importance value) of Crofton weed was the highest among all other species and continually increased over time while the dominance of co-occurring native species decreased. This study provides direct field evidence that trait differences are important to plant invasion.

Jack-and-master trait responses to elevated CO2 and N: a comparison of native and introduced Phragmites australis

Global change is predicted to promote plant invasions world-wide, reducing biodiversity and ecosystem function. Phenotypic plasticity may influence the ability of introduced plant species to invade and dominate extant communities. However, interpreting differences in plasticity can be confounded by phylogenetic differences in morphology and physiology. Here we present a novel case investigating the role of fitness trait values and phenotypic plasticity to global change factors between conspecific lineages of Phragmites australis. We hypothesized that due to observed differences in the competitive success of North American-native and Eurasian-introduced P. australis genotypes, Eurasian-introduced P. australis would exhibit greater fitness in response to global change factors. Plasticity and plant performance to ambient and predicted levels of carbon dioxide and nitrogen pollution were investigated to understand how invasion pressure may change in North America under a realistic global change scenario. We found that the introduced Eurasian genotype expressed greater mean trait values in nearly every ecophysiological trait measured – aboveground and belowground – to elevated CO₂ and nitrogen, outperforming the native North American conspecific by a factor of two to three under every global change scenario. This response is consistent with “jack and master” phenotypic plasticity. We suggest that differences in plant nitrogen productivity, specific leaf area, belowground biomass allocation, and inherently higher relative growth rate are the plant traits that may enhance invasion of Eurasian Phragmites in North America. Given the high degree of genotypic variability within this species, and our limited number of genotypes, our results must be interpreted cautiously. Our study is the first to demonstrate the potential importance of jack-and-master phenotypic plasticity in plant invasions when facing imminent global change conditions. We suggest that jack-and-master invasive genotypes and/or species similar to introduced P. australis will have an increased ecological fitness, facilitating their invasion in both stressful and resource rich environments.

Synergistic interactions of CO2 enrichment and nitrogen deposition promote growth and ecophysiological advantages of invading Eupatorium adenophorum in Southwest China

Global environmental change and ongoing biological invasions are the two prominent ecological issues threatening biodiversity worldwide, and investigations of their interaction will aid to predict plant invasions and inform better management strategies in the future. In this study, invasive Eupatorium adenophorum and native congener E. stoechadosmum were compared at ambient and elevated atmospheric carbon dioxide (CO(2)) concentrations combined with three levels of nitrogen (N; reduced, control and increased) in terms of growth, energy gain, and cost. Compared with E. stoechadosmum, E. adenophorum adopted a quicker-return energy-use strategy, i.e. higher photosynthetic energy-use efficiency and shorter payback time. Lower leaf mass per area may be a pivotal trait for the invader, which contributed to an increased N allocation to Rubisco at the expense of cell walls and therefore to higher photosynthetic energy gain. CO(2) enrichment and N deposition synergistically promoted plant growth and influenced some related ecophysiological traits, and the synergistic effects were greater for the invader than for the native congener. Reducing N availability by applying sugar eliminated the advantages of the invader over its native congener at both CO(2) levels. Our results indicate that CO(2) enrichment and N deposition may exacerbate E. adenophorum's invasion in the future, and manipulating environmental resources such as N availability may be a feasible tool for managing invasion impacts of E. adenophorum.