Final Report Summary - TRANSRESSOLID (Transcontinental research on a highly invasive plant species Solidago gigantea - Ecology and evolution in the native and introduced ranges)
The fundamental objective of TRANSRESSOLID project is to use a transcontinental approach to test the invasive success of Solidago gigantea, one of Europe’s most noxious plant invaders, that originates from North America. By placing a primary emphasis on this novel approach the project encompasses a wide range of cutting-edge research directions that exploit phenotypic plasticity, effects of soil biota and natural enemies, impact on native diversity, allelopathic effect, and genetic background in the native and in the invaded range. Different control methods against the species were also tested in Hungary.
Description of the work performed since the beginning of the project
In a transcontinental common garden experiment we tested the phenotypic plasticity of native and invasive giant goldenrod (Solidago gigantea) populations collected from both their native North American (32 populations) range and from their non-native European range (29 populations). We performed common garden experiments in both ranges that involved plant morphological, physiological investigations and the study of herbivore and pathogen resistance.
In a greenhouse experiment we tested, whether the competitive ability of 14 native and invasive populations differed on 11 common native North American competitors. The ecotypic variation of these same populations was also investigated.
We compared the magnitude of plant-soil feedbacks on native and exotic genotypes of Solidago gigantea. Feedbacks were assessed in soil collected across 14 sites sampled across the western part of Solidago’s native range in the United States.
We compared tolerance to the effects of simulated herbivory expressed by plants from seven Solidago gigantea populations from the native North American range to that expressed by plants from nine populations from the non-native European range. Populations were also collected along elevational gradients to compare ecotypic variation within and between ranges.
We conducted in total 604 vegetation plot surveys in different Solidago gigantea stands, and also worked on a closely related species S. canadensis to find out whether increasing stem density of Solidago corresponds with larger decreases in the abundance of other species in its non-native range than in its native range.
To test the allelopathic effect of Solidago we tested its root and shoot extracts on seven plant species from each range that occurred in our plots and that represented a range of relative abundances, from very common to occurring sporadically.
We tested genetic differences of native and non-native populations of Solidago gigantea. Ploidy and microsatellite analyses were carried out on the same individuals. The results from the microsatellite data were used to differentiate groups and to perform statistical analyses of the measured parameters.
We tested the short and long-term effects of different control methods on sites invaded by Solidago gigantea in Hungary. For this we set up a common garden to test the effect of mowing, herbicide treatment, and the addition of a native competitor (Tanacetum vulgare), and we surveyed the effectiveness of applied control options that have been used for several years in Hungarian National Parks (mowing, grazing by cattle and sheep, flooding).
Description of the main results achieved so far
In our transcontinental common garden experiment we found that plants from both ranges produced more biomass, grew taller and developed more rhizomes when grown in the non-native range. But in the native range the same populations were smaller and produced less rhizomes. The biomass production and the number of rhizomes was not effected by the seed origin, but the interaction between the seed origin and the destination continent was highly significant. Solidago gigantea populations from both ranges equally inhibited the growth of North American native species in a greenhouse experiment.
Solidago plants from the non-native range of Europe were more tolerant to herbivory than plants from the native range of North America. Furthermore, plants from European populations increased in total biomass and growth rate with elevation, but decreased in tolerance to herbivory. There were no relationships between elevation and growth or tolerance for North American populations.
Both native and exotic genotypes of Solidago suffered consistently negative and broadly similar plant-soil feedbacks when grown in North American soil. Although there was substantial variation among soils from different sites in the strength of feedbacks generated, the magnitude of feedbacks generated by North American genotypes of Solidago gigantea were strongly correlated with those generated in the same soil by European genotypes.
The number of species in plots in Europe, other than Solidago, declined sharply with increasing Solidago stem density, from roughly 31 species per 4 m2 plot when the invader was not present or rare to less than 10 species per plot when the invader reached 340-350 stems/4 m2 (R2=0.800; P<0.0001). In North America there was no relationship between the stem density of Solidago and the number of other species in plots (R2=0.020; P=0.276; Fig. 1).
Fig. 1 The relationship between Solidago stem density and total species richness, in North America and Europe.
In the case of Solidago canadensis we experienced a similar trend. In a countrywide survey we found that Solidago gigantea was one of the most important mid-successional dominant species with the strongest negative effect on diversity found of abandoned fields in Hungary.
Extracts made from Solidago roots suppressed the germination of some European species and the root and shoot growth of a suite of species, as a group, that co-occurred in Europe with Solidago, but did not have a significant effect on co-occurring North American species as a group. Extracts made from Solidago shoots had substantially weaker effects overall.
Native populations were hexaploids while the non-native populations were tetraploids. The two examined ploidy levels showed segregation in microsatellite composition too, and these genetic differences also manifested ecologically. Our results suggest that genetic changes strongly influence the ecological performance of Solidago and increasing its ability to invasion.
Short-term control options mainly affect the viability of the invader and less of the structure of the community, while long-term applications have stronger structural effects by increasing diversity. Moreover the combination of different methods can highly enhance the affectivity of its control.
Expected final results and their potential impact
Careful study of the biogeographic differences in the behavior of exotic species may challenge or broaden a number of conceptual paradigms. The results of this transcontinental research will help us to understand the invasive behavior of Solidago gigantea, which leads to the development of effective control methods against this species and may provide control solutions to other widespread invasive weed species such as Ambrosia artemisiifolia. We already started to study potential biocontrol agents in their natural environments. The results will provide critically valuable information for weed management to understand the intrinsic mechanisms which turn an introduced species into an invasive weed, giving clues on why and when to act, manage, contain and eventually eradicate invasions. The following fields are directly affected by the results of the proposed research: Plant ecology, Agriculture (weed control), Nature conservation (biodiversity)
Description of the work performed since the beginning of the project
In a transcontinental common garden experiment we tested the phenotypic plasticity of native and invasive giant goldenrod (Solidago gigantea) populations collected from both their native North American (32 populations) range and from their non-native European range (29 populations). We performed common garden experiments in both ranges that involved plant morphological, physiological investigations and the study of herbivore and pathogen resistance.
In a greenhouse experiment we tested, whether the competitive ability of 14 native and invasive populations differed on 11 common native North American competitors. The ecotypic variation of these same populations was also investigated.
We compared the magnitude of plant-soil feedbacks on native and exotic genotypes of Solidago gigantea. Feedbacks were assessed in soil collected across 14 sites sampled across the western part of Solidago’s native range in the United States.
We compared tolerance to the effects of simulated herbivory expressed by plants from seven Solidago gigantea populations from the native North American range to that expressed by plants from nine populations from the non-native European range. Populations were also collected along elevational gradients to compare ecotypic variation within and between ranges.
We conducted in total 604 vegetation plot surveys in different Solidago gigantea stands, and also worked on a closely related species S. canadensis to find out whether increasing stem density of Solidago corresponds with larger decreases in the abundance of other species in its non-native range than in its native range.
To test the allelopathic effect of Solidago we tested its root and shoot extracts on seven plant species from each range that occurred in our plots and that represented a range of relative abundances, from very common to occurring sporadically.
We tested genetic differences of native and non-native populations of Solidago gigantea. Ploidy and microsatellite analyses were carried out on the same individuals. The results from the microsatellite data were used to differentiate groups and to perform statistical analyses of the measured parameters.
We tested the short and long-term effects of different control methods on sites invaded by Solidago gigantea in Hungary. For this we set up a common garden to test the effect of mowing, herbicide treatment, and the addition of a native competitor (Tanacetum vulgare), and we surveyed the effectiveness of applied control options that have been used for several years in Hungarian National Parks (mowing, grazing by cattle and sheep, flooding).
Description of the main results achieved so far
In our transcontinental common garden experiment we found that plants from both ranges produced more biomass, grew taller and developed more rhizomes when grown in the non-native range. But in the native range the same populations were smaller and produced less rhizomes. The biomass production and the number of rhizomes was not effected by the seed origin, but the interaction between the seed origin and the destination continent was highly significant. Solidago gigantea populations from both ranges equally inhibited the growth of North American native species in a greenhouse experiment.
Solidago plants from the non-native range of Europe were more tolerant to herbivory than plants from the native range of North America. Furthermore, plants from European populations increased in total biomass and growth rate with elevation, but decreased in tolerance to herbivory. There were no relationships between elevation and growth or tolerance for North American populations.
Both native and exotic genotypes of Solidago suffered consistently negative and broadly similar plant-soil feedbacks when grown in North American soil. Although there was substantial variation among soils from different sites in the strength of feedbacks generated, the magnitude of feedbacks generated by North American genotypes of Solidago gigantea were strongly correlated with those generated in the same soil by European genotypes.
The number of species in plots in Europe, other than Solidago, declined sharply with increasing Solidago stem density, from roughly 31 species per 4 m2 plot when the invader was not present or rare to less than 10 species per plot when the invader reached 340-350 stems/4 m2 (R2=0.800; P<0.0001). In North America there was no relationship between the stem density of Solidago and the number of other species in plots (R2=0.020; P=0.276; Fig. 1).
Fig. 1 The relationship between Solidago stem density and total species richness, in North America and Europe.
In the case of Solidago canadensis we experienced a similar trend. In a countrywide survey we found that Solidago gigantea was one of the most important mid-successional dominant species with the strongest negative effect on diversity found of abandoned fields in Hungary.
Extracts made from Solidago roots suppressed the germination of some European species and the root and shoot growth of a suite of species, as a group, that co-occurred in Europe with Solidago, but did not have a significant effect on co-occurring North American species as a group. Extracts made from Solidago shoots had substantially weaker effects overall.
Native populations were hexaploids while the non-native populations were tetraploids. The two examined ploidy levels showed segregation in microsatellite composition too, and these genetic differences also manifested ecologically. Our results suggest that genetic changes strongly influence the ecological performance of Solidago and increasing its ability to invasion.
Short-term control options mainly affect the viability of the invader and less of the structure of the community, while long-term applications have stronger structural effects by increasing diversity. Moreover the combination of different methods can highly enhance the affectivity of its control.
Expected final results and their potential impact
Careful study of the biogeographic differences in the behavior of exotic species may challenge or broaden a number of conceptual paradigms. The results of this transcontinental research will help us to understand the invasive behavior of Solidago gigantea, which leads to the development of effective control methods against this species and may provide control solutions to other widespread invasive weed species such as Ambrosia artemisiifolia. We already started to study potential biocontrol agents in their natural environments. The results will provide critically valuable information for weed management to understand the intrinsic mechanisms which turn an introduced species into an invasive weed, giving clues on why and when to act, manage, contain and eventually eradicate invasions. The following fields are directly affected by the results of the proposed research: Plant ecology, Agriculture (weed control), Nature conservation (biodiversity)