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 9weeks. 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

Area modulates the effect of elevation but not of land use or canopy on tropical plant species richness

One of the few general patterns in ecology is the increase of species richness with area. However, factors driving species-area relationship (SAR) are under debate, and the role of human-induced changes has been overlooked so far. Furthermore, SAR studies in tropical regions, in particular in multilayered rain forests are scarce. On the other side, studies of global change-induced impacts on biodiversity have become increasingly important, particular in the tropics, where these impacts are especially pronounced. Here, we investigated if area modulates the effect of land use, elevation and canopy on plant species richness. For the first time we studied SAR in multilayered tropical forests considering all functional groups. We selected 13 natural and disturbed habitats on Kilimanjaro in Tanzania, distributed over an elevational range of 3700 m. In each habitat type, we set up three to six modified Whittaker plots. We recorded all plant species in 64 plots and 640 subplots and described SAR using the power function. Area consistently modulated effects of elevation on plant species richness, partly effects of land use but not effects of plant canopy. Thus, area needs to be taken into account when studying elevational plant species richness patterns. In contrast to temperate regions open and forest habitats did not differ in SAR, probably due to a distinct vertical vegetation zonation in tropical forests. Therefore, it is important to consider all vegetation layers including epiphytes when studying SAR in highly structured tropical regions

Early responses of wild plant seedlings to arbuscular mycorrhizal fungi and pathogens

Abstract Many plants form associations with arbuscular mycorrhizal fungi (AMF) because they profit from improved phosphorus nutrition and from protection against pathogens. Whereas mycorrhiza-induced pathogen protection is well understood in agricultural plant species, it is rarely studied in wild plants. As many pathogens infest plants in the first days after germination, mycorrhiza-induced pathogen protection may be especially important in the first few weeks of plant establishment. Here, we investigated interacting effects of {AMF} and the seedling pathogen Pythium ultimum on the performance of six- to seven-week-old seedlings of six wild plant species of the family Asteraceae in a full factorial experiment. Plant species differed in their response to AMF, the pathogen and their interactions. {AMF} increased and the pathogen decreased plant biomass in one and three species, respectively. Two plant species were negatively affected by {AMF} in the absence, but positively or not affected in the presence of the pathogen, indicating protection by AMF. This mycorrhiza-induced pathogen protection is especially surprising as we could not detect mycorrhizal structure in the roots of any of the plants. Our results show that even seedlings without established intraradical hyphal network can profit from AMF, both in terms of growth promotion in the absence of a pathogen and pathogen protection. The function of {AMF} is highly species-specific, but tends to be similar for more closely related plant species, suggesting a phylogenetic component of mycorrhizal function. Further studies should test a wider range of plant species, as our study was restricted to one plant family, and investigate whether plants profit from early mycorrhizal benefits in the long term.Die meisten Pflanzenarten bilden Symbiosen mit arbuskulärer Mykorrhiza, da sie so von einer verbesserten Phosphorverfügbarkeit und von einem Schutz vor Pathogenen profitieren. Zwar wurde Mykorrhiza-induzierter Schutz vor Pathogenen für landwirtschaftlich genutzte Arten gut untersucht, jedoch ist die Bedeutung dieses Nutzens für Wildpflanzen unklar. Viele Pathogene befallen Pflanzen in den ersten Tagen nach der Keimung. Daher könnte Mykorrhiza-induzierter Schutz insbesondere für die Etablierung von Pflanzen wichtig sein. In dieser Studie untersuchten wir in einem vollfaktoriellen Experiment interagierende Effekte von arbuskulärer Mykorrhiza und dem Pathogen Pythium ultimum auf das Pflanzenwachstum von Keimlingen von sechs Wildpflanzenarten, die der Familie der Asteraceaen angehören. Die sechs Pflanzenarten unterschieden sich in ihrer Reaktion auf Mykorrhiza, dem Pathogen und deren Interaktion. Mykorrhiza hatte auf eine Pflanzenart einen positiven und der Pathogen auf drei Arten einen negativen Effekt. Zwei Arten erfuhren einen negativen Einfluss von Mykorrhiza in Abwesenheit des Pathogen und einen positiven Einfluss in Anwesenheit des Pathogens, was auf Mykorrhiza-induzierten Schutz hindeutet. Diese Schutzfunktion ist insbesondere überraschend, da wir keine mykorrhizale Strukturen in den Pflanzenwurzeln finden konnten. Unsere Resultate zeigen, dass sogar Keimlinge ohne etabliertem intrazellulärem Hyphennetzwerk von arbuskulärer Mykorrhiza profitieren können, sowohl mittels direktem Biomassezuwachs als auch indirekt über Mykorrhiza-induzierten Pathogenschutz. Die Funktion von Mykorrhiza variiert zwischen Pflanzenarten, ist jedoch tendenziell ähnlich für nahe verwandte Arten, was auf eine phylogenetische Komponente hindeutet. Da unsere Untersuchung sich auf die Familie der Asteraceaen beschränkte, sollten weiterführende Experimente Pflanzenarten aus verschiedenen Familien miteinbeziehen und den Langzeiteffekt der mykorrhizalen Vorteile im Keimlingsstadium untersuchen

Invasive plant species do not create more negative soil conditions for other plants than natives

Abstract A major task in ecology is to establish the degree of generality of ecological mechanisms. Here we present results from a multi-species experiment that tested whether a set of invasive species altered the soil conditions to the detriment of other species by releasing allelopathic compounds or inducing shifts in soil biota composition, and whether this effect was more pronounced relative to a set of closely related native species. We pre-cultivated soil with 23 exotic invasive, 19 related native and 6 related exotic garden species and used plain soil as a control. To separate allelopathy from effects on the soil biota, we sterilized half of the soil. Then, we compared the effect of soil pre-cultivation and sterilization on germination and growth of four native test species in two experiments. The general effect of soil sterilization was positive. The effect of soil pre-cultivation on test species performance was neutral to positive, and sterilization reduced this positive effect. This indicates general absence of allelopathic compounds and a shift toward a less antagonistic soil biota by cultivation species. In both experiments, pre-cultivation effects did not differ systematically between exotic invasive, exotic garden or native species. Our results do not support the hypothesis that invasive plants generally inhibit the growth of others by releasing allelopathic compounds or accumulating a detrimental soil biota

The relative importance of immediate allelopathy and allelopathic legacy in invasive plant species

Abstract Some introduced invasive species may be competitively superior to natives because they release allelochemicals, which negatively affect native species. Allelochemicals can be immediately effective after being released but can also persist in soils, resulting in a legacy effect. However, to our knowledge there are no studies which distinguish between allelopathic legacy and immediate allelopathy of invasive species and also test for their relative importance and possible interdependence. We used eleven invasive species and tested whether they show immediate allelopathy and allelopathic legacy effects in a factorial pairwise competition experiment using field-collected soil (invaded/non-invaded) and activated carbon to neutralize allelochemicals. We grew two native and the invasive species in both monocultures and pairwise mixtures. In monocultures, the native species did not experience an allelopathic legacy effect of the invasives, suggesting that invaders generally lack persistent allelochemicals. However, the effects of invader allelochemicals were modulated by competitive interactions. In competition, immediate allelopathy decreased competitive ability of natives, while allelopathic legacy positively affected the natives. Moreover, immediate allelopathic and allelopathic legacy effects were strongly negatively correlated. Our results suggest that both immediately released allelochemicals and the allelochemical legacy of invasive species are important for plant performance under natural conditions, and that natives should be able to recover once the invaders are removed. To test whether immediate allelopathy is responsible for plant invasion success, further studies should compare allelopathic effects between invasive and closely related native species

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