Invasive alien plants affect grassland ant communities, colony size and foraging behaviour

Ants are dominant members of many terrestrial ecosystems and are regarded as indicators of environmental changes. However, little is known about the effects of invasive alien plants on ant populations, particularly as regards the density, spatial distribution and size of ant colonies, as well as their foraging behaviour. We addressed these questions in a study of grassland ant communities on five grasslands invaded by alien goldenrods (Solidago sp.) and on five non-invaded grasslands without this plant. In each grassland, seven 100 m² plots were selected and the ant colonies counted. Ant species richness and colony density was lower in the plots on the invaded grasslands. Moreover, both of these traits were higher in the plots near the grassland edge and with a higher number of plant species in the grasslands invaded by goldenrods but not in the non-invaded ones. On average, ant colony size was lower on the invaded grasslands than the non-invaded ones. Also, ant workers travelled for longer distances to collect food items in the invaded areas than they did in the non-invaded ones, even after the experimental removal of some ant colonies in order to exclude the effect of higher colony density in the latter. Our results indicate that invasive alien goldenrods have a profound negative effect on grassland ant communities which may lead to a cascade effect on the whole grassland ecosystem through modification of the interactions among species. The invasion diminishes a major index of the fitness of ants, which is a colony’s size, and probably leads to increased foraging effort of workers. This, in turn, may have important consequences for the division of labour and reproductive strategies within ant colonies.     

Green Frogs Show Reduced Foraging Success in Habitats Invaded by Japanese knotweed

Habitat loss is causing amphibian population declines worldwide, so there is increased attention to forces that degrade remaining habitats. Terrestrial habitats surrounding wetlands are critical foraging areas for temperate anurans. We investigated plant community changes in two old fields invaded by Japanese knotweed (Fallopia japonica) and the foraging success of Green frogs (Rana clamitans) in invaded and non-invaded portions of those fields. Within each field, vegetation data were recorded in quadrats located along two transects bisecting the invasion fronts. We placed frogs in ‘foraging buckets’ along transects and measured their change in mass over a 38 h period. There were significant changes in vegetation structure and composition associated with Japanese knotweed invasion. Diverse assemblages of native plants that covered non-invaded plots were absent from areas invaded by Japanese knotweed. There was also a significant change in vegetation architecture between invaded and non-invaded habitats. Change in frog mass declined significantly along transects, with most frogs in non-invaded plots gaining mass and no frogs in invaded plots gaining mass. Most frogs from non-invaded plots but only two from invaded plots defecated shortly after removal from foraging buckets (verification of recent feeding). We hypothesize that Japanese knotweed invasions degrade terrestrial habitat quality for frogs by indirectly reducing arthropod abundance. Nonnative plant invasions may be another factor contributing to amphibian population declines.     

Size, activity and catabolic diversity of the soil microbial biomass in a wetland complex invaded by reed canary grass

Reed canary grass (Phalaris arundinacea, L.) invasion of wetlands is an ecological issue that has received attention, but its impact on soil microbial diversity is not well documented. The present study assessed the size (substrate-induced respiration), catabolic diversity (CLPP, community level physiological profiles) and composition (selective inhibition) of the soil microbial community in invaded (>95% P. arundinacea cover) and in non-invaded areas of a wetland occupied by native species grown either as a mixed assemblage (22 species) or as quasi-monotypic stands of Scirpus cyperinus (74% cover). The study also tested the hypothesis that decomposition of lignin- and phenolics-rich plant tissues would be fastest in soils exhibiting high catabolic diversity. Results showed that soil respiration, microbial biomass and diversity were significantly higher (P?<?0.03; 1.5 to 3 fold) in P. arundinacea-invaded soils than in soils supporting native plant species. Fungal to bacterial ratios were also higher in invaded (0.6) than in non-invaded (0.4) plots. Further, canonical discriminant analysis of CLPP data showed distinct communities of soil decomposers associated with each plant community. However, these differences in microbial attributes had no effect on decomposition of plant biomass which was primarily controlled by its chemical composition. While P. arundinacea invasion has substantially reduced plant diversity, this study found no parallel decline in the size and diversity of the soil microbial community in the invaded areas.     

Lespedeza cuneata invasion alters soils facilitating its own growth

Lespedeza cuneata (sericea) is an Asian legume introduced to the US in the 1930s for erosion control and forage, but it can strongly reduce the abundance of native grassland plants. One possible explanation for this high invasive capacity is that L. cuneata is able to alter soil conditions to either improve its own growth, or reduce growth of native plants. To test for soil alteration following invasion, we collected soil from a previous 3-year field experiment in which L. cuneata was established in or excluded from randomly selected plots in a restored grassland. These soil history treatments were crossed with soil autoclaving—to disrupt microbial communities potentially important to plant interactions. For each treatment combination, a native plant, Sorghastrum nutans, was grown with L. cuneata or a conspecific in a 12-week greenhouse experiment. Although we found no evidence for competitive effects on the native species, L. cuneata biomass increased when grown in soil with a L. cuneata history as opposed to non-invaded soil (F _{1, 16} = 4.79, P = 0.04). Additionally, nodulation of L. cuneata increased in invaded compared to non-invaded soil (F _{1, 16} = 6.01, P = 0.026). These results indicate that, within three years of invasion, L. cuneata is able to alter soils to facilitate its own growth and suggest that at least part of the invasive success of L. cuneata is linked to altered soil conditions.     

黄顶菊对入侵地群落动态及植物生长生理特征的影响

为明确黄顶菊对入侵地植物群落和土著植物生理生长的影响机制,采用同质园试验对入侵和非入侵土壤的植物群落开展了整个生育期动态监测,并分析了黄顶菊入侵对狗尾草、羽叶鬼针草、灰绿藜、地肤4种土著植物生长和生理特征的影响规律。结果表明:黄顶菊入侵土壤植物群落多样性指数低于非入侵地,且有季节性差异,随生育期的推进差异逐渐减小;黄顶菊对本地植物的生长指标有显著影响(P0.05),随时间变化显著,但存在物种差异;4种植物的净光合速率(Pn)、气孔导度(Cd)、蒸腾速率(Tr)在非入侵土壤生长显著高于入侵地土壤(P0.05);而4种植物在入侵土壤生长的比叶面积(SLA)、比根长(SRL)、比根面积(SRA)显著高于本地土壤(P0.05)。综上,黄顶菊入侵抑制了本地植物的光合效率,减少了生物量的积累,导致本地植物群落的生物多样性水平降低,但表现出季节差异;不同物种对黄顶菊入侵胁迫的响应表现种间特异性,为理解入侵种对群落结构影响和实现入侵生境恢复提供了理论依据。     

Allelopathic effects of three plant invaders on germination of native species: a field study

The ability of some invasive plant species to produce biochemical compounds toxic to native species, called allelopathy, is thought to be one of the reasons for their success when introduced to a novel range, an idea known as the Novel Weapons Hypothesis. However, support for this hypothesis mainly comes from bioassays and experiments conducted under controlled environments, whereas field evidence is rare. In a field experiment, we investigated whether three plant species invasive in Europe, Solidago gigantea, Impatiens glandulifera and Erigeron annuus, inhibit the germination of native species through allelopathy more than an adjacent native plant community. At three sites for each invasive species, we compared the germination of native species that were sown on invaded and non-invaded plots. Half of these plots were amended with activated carbon to reduce the influence of potential allelopathic compounds. The germination of sown seeds and of seeds from the seedbank was monitored over a period of 9 weeks. Activated carbon generally enhanced seed germination. This effect was equally pronounced in invaded and adjacent non-invaded plots, indicating that invasive species do not suppress germination more than a native plant community. In addition, more seeds germinated from the seedbank on invaded than on non-invaded soil, probably due to previous suppression of germination by the invasive species. Our field study does not provide evidence for the Novel Weapons Hypothesis with respect to the germination success of natives. Instead, our results suggest that if invasive species release allelopathic compounds that suppress germination, they do so to a similar degree as the native plant community.     

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

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

The impact of <Emphasis Type="Italic">Carpobrotus</Emphasis> cfr. <Emphasis Type="Italic">acinaciformis</Emphasis> (L.) L. Bolus on soil nutrients,microbial communities structure and native plant communities in Mediterranean ecosystems

Background and aims

Carpobrotus spp. are amongst the most impactful and widespread plant invaders of Mediterranean habitats. Despite the negative ecological impacts on soil and vegetation that have been documented, information is still limited about the effect by Carpobrotus on soil microbial communities. We aimed to assess the changes in the floristic, soil and microbial parameters following the invasion by Carpobrotus cfr. acinaciformis within an insular Mediterranean ecosystem.

Methods

Within three study areas a paired-site approach, comparing an invaded vs. a non-invaded plot, was established. Within each plot biodiversity indexes, C and N soil content, pH and microbial biomass and structure (bacterial and fungal) were assessed.

Results

Invaded plots showed a decrease of α-species richness and diversity. The least represented plant species in invaded plots were those related to grassland habitats. In all invaded soils, a significant increase of carbon and nitrogen content and a significant decrease of pH were registered. Carpobrotus significantly increased bacterial and fungal biomass and altered soil microbial structure, particularly favoring fungal growth.

Conclusions

Carpobrotus may deeply impact edaphic properties and microbial communities and, in turn, these strong modifications probably increase its invasive potential and its ability to overcome native species, by preventing their natural regeneration.

     

Neighbourhood association of Cortaderia selloana invasion,soil properties and plant community structure in Mediterranean coastal grasslands

Invasion by alien species is threatening the conservation of native plant communities and the integrity of ecosystems. To gain a better understanding of such impacts, many studies have examined the traits that make alien species successful invaders as well as the factors involved in community invasibility. However, it is necessary to link invader effects on community structure and on ecosystem processes in order to unravel the mechanisms of impact. Cortaderia selloana is a perennial grass native to South America that is invading abandoned agricultural lands close to coastal human settlements in Catalonia (NE Spain). In invaded pastures, we examined the association between C. selloana invasion, soil properties and vegetation structure changes in pastures, comparing the neighbourhood area of influence of C. selloana with areas far from C. selloana. Areas under the influence of C. selloana had lower total soil nitrogen values and higher C/N values than in areas far from C. selloana. Furthermore, the areas affected by C. selloana had lower species, family and life form richness and diversity, and less plant cover. In addition, C. selloana also increased the vertical vegetation structure and changed species composition (only 44% similarity between invaded and non-invaded areas). Our results point out that C. selloana has an effect on its neighbourhood leading to an increase in small-scale variability within invaded fields.     

Invasion by an exotic tree alters above and belowground ecosystem components

With the widespread introduction and invasion of exotic plants there is a need for studies that quantify alterations of basic ecosystem structure and function. Ecosystem invasion by Melaleuca quinquenervia significantly altered both above- and belowground ecosystem components in this study. We measured the quantity and nutrient concentration of the litterfall, litter layer, and soil; microbial biomass pools; and rates of potentially mineralizable nitrogen and soil oxygen demand. Annual litterfall was 4.9 times higher in the non-invaded sites and contained 1.9 times more phosphorus than invaded sites. Non-invaded plots contained a larger litter layer compared to invaded plots: 2.4 ± 1.2 kg m⁻² and 0.62 ± 0.3 kg m⁻² , respectively. Lower nutrient concentration and quantity of the litter layer in the invaded plots led to changes in the aboveground storage of nutrients. In the invaded plots there was four times less carbon, seven times less nitrogen, and ten times less phosphorus stored in the organic litter layer compared to the non-invaded plots. Microbial biomass nutrient pools were consistently lower at both the 0–5 cm and 5–15 cm depth in the invaded soils compared to non-invaded soils, indicating a plant mediated change. Although M. quinquenervia altered microbial community structure, microbial activities were not different between invaded and non-invaded plots at either depth as measured by rates of soil oxygen demand and potentially mineralizable nitrogen. These changes may affect both native plant growth and water quality, and may act to promote and maintain site dominance by M. quinquenervia.     

Plant Invasions Associated with Change in Root-Zone Microbial Community Structure and Diversity

The importance of plant-microbe associations for the invasion of plant species have not been often tested under field conditions. The research sought to determine patterns of change in microbial communities associated with the establishment of invasive plants with different taxonomic and phenetic traits. Three independent locations in Virginia, USA were selected. One site was invaded by a grass (Microstegium vimineum), another by a shrub (Rhamnus davurica), and the third by a tree (Ailanthus altissima). The native vegetation from these sites was used as reference. 16S rRNA and ITS regions were sequenced to study root-zone bacterial and fungal communities, respectively, in invaded and non-invaded samples and analyzed using Quantitative Insights Into Microbial Ecology (QIIME). Though root-zone microbial community structure initially differed across locations, plant invasion shifted communities in similar ways. Indicator species analysis revealed that Operational Taxonomic Units (OTUs) closely related to Proteobacteria, Acidobacteria, Actinobacteria, and Ascomycota increased in abundance due to plant invasions. The Hyphomonadaceae family in the Rhodobacterales order and ammonia-oxidizing Nitrospirae phylum showed greater relative abundance in the invaded root-zone soils. Hyphomicrobiaceae, another bacterial family within the phyla Proteobacteria increased as a result of plant invasion, but the effect associated most strongly with root-zones of M. vimineum and R. davurica. Functional analysis using Phylogenetic Investigation of Communities by Reconstruction of Unobserved States (PICRUSt) showed bacteria responsible for nitrogen cycling in soil increased in relative abundance in association with plant invasion. In agreement with phylogenetic and functional analyses, greater turnover of ammonium and nitrate was associated with plant invasion. Overall, bacterial and fungal communities changed congruently across plant invaders, and support the hypothesis that nitrogen cycling bacteria and functions are important factors in plant invasions. Whether the changes in microbial communities are driven by direct plant microbial interactions or a result of plant-driven changes in soil properties remains to be determined.     

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

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

Woody vegetation structure of Brazilian Cerrado invaded by Pteridium arachnoideum (Kaulf.) Maxon (Dennstaedtiaceae)

Biological invasion is one of the major components of global change. In the Brazilian Cerrado, the fern Pteridium arachnoideum (neotropical bracken) is a potentially invasive species which is expected to change the vegetation structure of savannas. We assessed effects of the dominance of P. arachnoideum on the diversity of woody Cerrado species. We compared three sites dominated by neotropical bracken with three sites without neotropical bracken. We used multivariate analyses of variance and rarefaction curves with confidence limits to test whether the groups of species found in invaded areas were different from those found in non-invaded ones. We also analyzed the density of individuals and species and the mean area at breast height per plot. We found significant floristic differences between the areas. The diversity and the densities in the invaded areas were lower than in non-invaded ones. However, we did not observe differences in the mean area at breast height of woody plants. Thus, the P. arachnoideum invasion is a potential threat to Cerrado biodiversity. Management efforts should be made to control the bracken, since its dominance in the plant community may lead to impoverishment and simplification of the savanna vegetation.     

Pine invasion decreases density and changes native tree communities in woodland Cerrado

ABSTRACT

Background: Invasive plants can negatively impact native communities, but the majority of the effects of these invasions have been demonstrated only for temperate ecosystems. Tropical ecosystems, including the Cerrado, a biodiversity hotspot, are known to be invaded by numerous non-native species, but studies of their impacts are largely lacking.

Aims: Our research aimed at quantifying how Pinus spp. presence and density affected Cerrado plant communities.

Methods: We sampled areas invaded and non-invaded by Pinus spp. to determine if pine invasion affected native tree richness, diversity, evenness, and density. We also evaluated if community composition differed between invaded and non-invaded sites.

Results: We found invaded plots had lower native tree densities than non-invaded plots and that Pinus spp. invasions changed native tree communities by reducing native species abundances.

Conclusion: Invasive pines had negative impacts on the native Cerrado tree community by reducing native plant density and changing species abundances. Reduced density and abundance at early invasion stages can result in reduction in biodiversity in the long term.     

Invasion of the alien shrub Prunus laurocerasus in suburban deciduous forests: Effects on native vegetation and soil properties

Most invading alien plants affect native biodiversity and ecosystem functioning. In a field survey, we assessed the impact of the invasive shrub Prunus laurocerasus on the native vegetation and soil properties in suburban deciduous forests in the region of Basel, Switzerland. We installed four pairs of plots in patches of P. laurocerasus and in adjacent not invaded areas in each of twelve forest areas. Native species richness, Shannon-diversity and species composition of the ground vegetation and shrub layer were assessed in each plot. Furthermore, in each plot we measured physical and soil chemical characteristics, enzyme activities and the carbon source utilization pattern of the soil microbial community using Ecoplates™. The maximum age of P. laurocerasus in each plot was determined using tree ring analysis, indicating the time elapsed since the invasive plant has established. A lower native plant species richness in both the ground vegetation and shrub layer was observed in plots with presence of P. laurocerasus. A different species composition of the ground vegetation was also found among plots with and without the invasive shrub. Plots invaded by P. laurocerasus had a lower soil moisture content than control plots. The intensity, diversity and substrate richness of the carbon sources were increased in soil from invaded plots compared to soil in control plots. However, the chemical soil characteristics examined and the activities of enzymes were not influenced by the invasive plant. The effects of P. laurocerasus became more pronounced with the time elapsed since the invasive plant has established. Thus, the removal of young P. laurocerasus individuals would be an appropriate management practice for this invasive shrub species.     

Changes in the plant community of a Brazilian grassland savannah after 22 years of invasion by Pinus elliottii Engelm

Abstract

Background: The invasion by Pinus elliottii is one of the most serious threats to the remaining native cerrado vegetation in São Paulo State, Brazil, causing biodiversity losses yet to be evaluated. We conducted a study in an area where P. elliottii began establishing in 1988.

Aims: To estimate diversity losses in the plant community and to understand the floristic and structural changes resulting from pine tree invasion of grassland savannah.

Methods: All plants taller than 50 cm were sampled in 35 plots (64 m² each) within an area densely invaded by P. elliottii and in 10 plots in non-invaded grassland savannah. Density, species richness, diversity, ground cover and spatial distribution were compared by Wilcoxon tests, non-metric multidimensional scaling and Payandeh indices.

Results: Twenty-two years after the arrival of the first invasive trees (founders), the grassland savannah has become a dense pine forest with 12,455 individuals ha^?1, a basal area of 26.44 m² ha^?1, a sparse native woody understory comprised of 16 species (H'?=?0.44), density of 1210 individuals ha^?1 and the herbaceous layer totally absent.

Conclusions: Invasion by Pinus elliottii has completely changed the structure of the grassland savannah and caused severe plant diversity losses. Native species surviving the invasion in the understory do not typically represent the previous composition and functional traits of the native vegetation.     

Patterns of woody plant invasion in an Argentinean coastal grassland

Coastal dune grasslands are fragile ecosystems that have historically been subjected to various types of uses and human activities. In Buenos Aires Province (Argentina), these areas are frequently afforested for urban and touristic development. The introduction and subsequent spread of exotic tree species is one of the main threats to conservation of natural grasslands as invasive trees strongly transform their structure and composition. The aim of this study was to identify patterns of woody plant invasion comparing plant communities and environmental variables between invaded and non-invaded areas surrounding the coastal village of Mar Azul, Argentina. Coastal grasslands in this area are being invaded by Populus alba (white poplar) and Acacia longifolia (coast wattle). The height of the saplings and the richness of the accompanying vegetation were evaluated in relation to the distance from the edge of the mature tree patches. Also, the cover, richness and diversity of all species in the invaded and non-invaded areas were measured, as well as soil pH, temperature and particle size. Negative correlations were found between the height of the saplings and distance to mature tree patches in all areas. The richness of the accompanying vegetation was negatively and positively correlated with the distance from the poplar and acacia area, respectively. The most abundant native species was Cortaderia selloana. Less cover, richness and diversity of native plant species and greater soil particle size were found in invaded areas, where the proportion of bare soil was higher. Also, a higher proportion of leaf litter in the invaded areas was registered. The results emphasize the invasive capacity of P. alba and A. longifolia advancing on the native communities and reducing their richness. Knowledge of the impact of invasive woody plants in coastal grasslands is important to design active management strategies for conservation purposes.     

The impact of Acacia saligna invasion on Italian coastal dune EC habitats

Alien species can represent a threat to several ecosystems because they can alter species relationships and ecosystem function. In Italy, Acacia saligna is a major invader and it forms dense stands in coastal environments. We analyze the impact of A. saligna in Italian Mediterranean dune systems. We randomly sampled coastal dune vegetation and investigated its floristic composition with ordination techniques. We compared species richness in invaded and non-invaded plots with rarefaction curves and analyzed the frequency of focal and ruderal species. A. saligna invaded Mediterranean scrub (habitats 2250* and 2260) and coastal Pinus dune wood (habitat 2270*) and it is particularly prevalent in sunny areas of habitat 2270*. We observed an increase in ruderal species and a decrease in focal species in the invaded plots of habitat 2270*. We suggest that more open and disturbed areas are more prone to A. saligna invasion.     

Relationships between Scotch broom (Cytisus scoparius), soil nutrients, and plant diversity in the Garry oak savannah ecosystem

Scotch broom (Cytisus scoparius), is a leguminous shrub, native to the Mediterranean, which has invaded most of the remaining Garry oak savannah ecosystems in Oregon, Washington, and British Columbia. Here, it is considered to be a threat to the native plant community. We tested the hypothesis that broom would increase available soil nitrogen by comparing soil nutrients in contiguous broom-invaded and non-invaded sites. We then looked for changes in patterns of diversity in the herbaceous community that might indicate a role of Scotch broom in changing conditions following its invasion. Finally we carried out greenhouse assays to test whether broom had a greater impact on the growth of a native and an introduced grass compared to that of a native shrub. Broom was associated with only a weak trend in increased soil nitrogen, but a significant decrease in soil phosphorus was observed. Patterns of plant diversity differed between two sites. At one site, 60% of the plants whose abundances increased in the broom-invaded plots were introduced species while native species abundances decreased in the broom-invaded plots compared to broom-free plots. At the other site, 60% of the plants that caused the differences between broom-invaded and un-invaded plots were native species that were less abundant in the broom-invaded plots. Finally, in greenhouse assays grass growth was not affected as a result of being grown with broom; however, grasses appeared to produce more flowers when grown with broom. We conclude that broom does not necessarily modify soil nitrogen availability but may deplete soil phosphorus availability and that broom invasion can be associated with increase of exotic species and/or the decline of native species.     

Understory Invasion by <Emphasis Type="Italic">Acacia longifolia</Emphasis> Alters the Water Balance and Carbon Gain of a Mediterranean Pine Forest

In water-limited ecosystems, where potential evapotranspiration exceeds precipitation, it is often assumed that plant invasions will not increase total ecosystem water use, because all available water is evaporated or transpired regardless of vegetation type. However, invasion by exotic species, with high water use rates, may potentially alter ecosystem water balance by reducing water available to native species, which may in turn impact carbon assimilation and productivity of co-occurring species. Here, we document the impact of invasion by an understory exotic woody species (Acacia longifolia) in a semi-arid Mediterranean dune pine forest. To quantify the effects of this understory leguminous tree on the water use and carbon fixation rates of Pinus pinaster we compare an invaded and a non-invaded stand. A. longifolia significantly altered forest structure by increasing plant density and leaf area index in the mid-stratum of the invaded forest. A. longifolia contributed significantly to transpiration in the invaded forest (up to 42%) resulting in a slight increase in stand transpiration in the invaded relative to non-invaded forest. More importantly, both water use and carbon assimilation rates of P. pinaster were significantly reduced in the invaded relative to non-invaded stand. Therefore, this study shows that exotic plant invasions can have significant impacts on hydrological and carbon cycling even in water-limited semi-arid ecosystems through a repartitioning of water resources between the native and the invasive species.