Division of Labor Brings Greater Benefits to Clones of Carpobrotus edulis in the Non-native Range: Evidence for Rapid Adaptive Evolution

Effects of kinship and integration between adjacent/non-adjacent ramets on the growth and feedback with soil biota in a clonal invader

Many invasive plants can reproduce through both seeds and clonal growth. In habitats, interacting seedlings may originate from the same mother, and interacting ramets originating from the same plant may not be adjacent to each other in the stolon, particularly for vines that can show curved growth. However, in a homogeneous environment, how kinship and integration between adjacent/non-adjacent ramets affect plant growth and feedback with soil biota has been less studied. We address these questions using an invasive stoloniferous vine Mikania micrantha. We found that sibling groups and stranger groups did not differ in biomass production, root allocation and feedback with soil biota, indicating that kin recognition is unlikely in M. micrantha. For two-ramet stolon fragments in which interacting ramets were adjacent to each other, older ramets allocated more biomass to roots than younger ramets when integrated, particularly in comparison with disconnected ramets from different genotypes, indicating that a division of labor was induced. For four-ramet stolon fragments in which there were two unrooted ramets between the two rooted, interacting ramets, integration increased biomass allocation to roots, possibly because only two of the four ramets could absorb belowground resources and a lower shoot allocation decreased aboveground light competition. When inoculated with soil biota conditioned by the four-ramet integrated fragments, plants of M. micrantha also increased biomass allocation to roots. These results indicate that the distance between interacting ramets in the stolon may affect the integration effect and feedback with soil biota in clonal plants.

Clonal integration benefits Calystegia soldanella in heterogeneous habitats

Land-use change and tourism development have seriously threatened the ecosystems of coastal protection forests and beaches. Light and nutrients are spatially heterogeneously distributed between the two ecosystems. Clonal plants, such as Calystegia soldanella, which play a crucial role in maintaining the ecological stability of coastal habitats, are likely to encounter diverse environments. In this study, we investigated clonal integration and the division of labour in C. soldanella under heterogeneous (high nutrient and low light [HNLL]; low nutrient and high light [LNHL]) and homogeneous habitats. We cultivated pairs of connected and severed ramets of C. soldanella in these environments. Our results showed the total biomass (TB) of connected ramets was higher than that of severed ramets in heterogeneous environments, suggesting clonal integration enhances growth in heterogeneous habitats. The root shoot ratio was significantly lower in HNLL than in LNHL conditions for connected ramets, demonstrating a division of labour in growth under heterogeneous conditions. However, parameters of clonal propagation of C. soldanella did not significantly differ between connected and severed ramets in heterogeneous environments, indicating no division of labour in clonal propagation. In homogeneous environments, the growth of C. soldanella did not benefit from clonal integration. Connected ramets in heterogeneous habitats exhibited higher TB than in homogeneous habitats. The TB of one ramet in HNLL was consistently higher than that in LNHL, irrespective of ramet's states, which suggests that high soil nutrients may enhance the growth. We conclude that C. soldanella has the capability of clonal integration to achieve high biomass in heterogeneous but not in homogeneous conditions, and the establishment of coastal protection forests (high nutrient and low light) may foster the growth of C. soldanella.

Invasive clonal plants possess greater capacity for division of labor than natives in high patch contrast environments

Invasion success of clonal plants is closely related to their unique clonal life history, and clonal division of labor is a crucial clonal trait. However, so far, it is unclear whether invasive alien clonal species generally possess a greater capacity for division of labor than native species and whether this pattern is affected by environmental conditions. To test whether patch contrast affects the differences in the capacity for division of labor between invasive alien and native clonal plants, we selected five pairs of exotic invasive and native clonal plant species that are congeneric and co-occurring in China as experimental materials. We grew the clonal fragment pairs of these invasive and native plants under high, low, or no contrast of reciprocal patchiness of light and nutrient, respectively, with ramet connections either severed (division of labor prevented) or kept intact (division of labor allowed). The results showed that connection significantly decreased the proportion of biomass allocated to roots in distal (younger) ramets, whereas it increased in proximal (older) ramets of all studied plants under high -contrast treatments. This clear pattern strongly indicated the occurrence of division of labor. Furthermore, the connection had a more pronounced effect on the pattern of biomass allocation of invasive alien plants, resulting in a greater increase in biomass for invasive alien plants compared to native plants. These findings suggest that the invasive alien plants possess a greater capacity for division of labor, which may confer a competitive advantage to them over natives, thus facilitating their invasion success in some heterogeneous habitats such as forest edges where light and soil nutrients show a high negative correlation.

Carbon and nitrogen stable isotope compositions provide new insights into the phenotypic plasticity of the invasive species Carpobrotus sp. pl. in different coastal habitats

The genus Carpobrotus N.E.Br. comprises several aggressive invasive species that threaten biodiversity in coastal areas worldwide. We studied the phenotypic plasticity of Carpobrotus sp. pl. invading four coastal habitats in the north-western Iberian Peninsula (coastal cliffs, disturbed areas, dunes and coastal forests). We measured morphological traits and carbon (δ13C) and nitrogen (δ15N) stable isotope compositions of Carpobrotus sp. pl. individuals collected in each habitat. Our results indicated that leaf carbon content (% C) and dry shoot weight were higher on cliffs and lower in mixed forests. In contrast, leaf hydration was higher in mixed forests and lower on cliffs. Leaf nitrogen content (% N) was higher in forests, which might be due to the presence of Acacia longifolia, an alien tree that accumulates N in the soil through symbiotic associations with N fixing bacteria. Differences in δ15N showed the use of different N sources in each habitat. Values were higher in disturbed areas with greater human activity and lower on cliffs and forests. δ13C was higher in cliffs and dunes, suggesting CAM activity where drought and salinity are more intense. Water use efficiency (iWUE) and δ13C were higher on cliffs and dunes, suggesting an adaptation and high tolerance of Carpobrotus sp. pl. to unfavourable conditions such as drought or salinity in the invaded areas.

Clonal integration benefits an invader in heterogeneous environments with reciprocal patchiness of resources, but not its native congener

Many of the world's most invasive plants are clonal, and clonal functional traits are suggested to contribute to their invasiveness. Clonal integration is one of the most important clonal functional traits, but it is still unclear whether clonal integration can benefit invasive alien clonal plants more than native ones in heterogeneous environments with reciprocal patchiness of resources and whether invasive plants show a higher capacity of division of labor than native ones in such environments. We grew connected (allowing clonal integration) and disconnected (preventing clonal integration) ramet pairs of an invasive plant Wedelia trilobata and its occurring native congener W. chinensis in the environment consisting of reciprocal patches of light and soil nutrients (i.e., a high-light but low-nutrient patch and a low-light but high-nutrient patch). Clonal integration greatly promoted the growth of the invasive species, but had no significant effect on the native one. Both invasive and native species showed division of labor in terms of morphology, biomass allocation, and/or photosynthetic physiology, but the capacity of labor division did not differ between the invasive and the native species. We conclude that in heterogeneous environments consisting of reciprocal patches of resources, which are common in nature, clonal integration can confer invasive plants a competitive advantage over natives, but this difference is not related to their capacity of labor division. This study highlights the importance of clonal integration for plants in heterogeneous environments and suggests that clonal integration can contribute to the invasion success of alien clonal plants.

Epigenetic and Phenotypic Responses to Experimental Climate Change of Native and Invasive Carpobrotus edulis

Despite the recent discoveries on how DNA methylation could help plants to adapt to changing environments, the relationship between epigenetics and climate change or invasion in new areas is still poorly known. Here, we investigated, through a field experiment, how the new expected climate scenarios for Southern Europe, i.e., increased temperature and decreased rainfall, might affect global DNA methylation in relation to phenotypic variation in individuals of clonal plant, Carpobrotus edulis, from its native (Southern African) and invaded (northwestern Iberian Peninsula) area. Our results showed that changes in temperature and rainfall induced phenotypic but not global DNA methylation differences among plants, and the climatic effects were similar for plants coming from the native or invaded areas. The individuals from the Iberian Peninsula showed higher levels of global methylation than their native counterparts from South Africa. We also observed differences between natives and invasive phenotypes in traits related to the pattern of biomass partitioning and to the strategies for water uptake and use and found an epigenetic contribution to phenotypic changes in some leaf traits, especially on the nitrogen isotopic composition. We conclude that the increased temperature and decreased rainfall projected for Southern Europe during the course of the twenty-first century may foster phenotypic changes in C. edulis, possibly endowing this species with a higher ability to successful cope the rapid environmental shifts. The epigenetic and phenotypic divergence that we observed between native and invasive plants suggests an intraspecific functional variation during the process of invasion. This result could indicate that phenotypic plasticity and global DNA methylation are related to the colonization of new habitats. Our findings reinforce the importance of epigenetic plasticity on rapid adaptation of invasive clonal plants.

Motherly care: How Leymus chinensis ramets support their offspring exposed to saline-alkali and clipping stresses

BACKGROUND

While clonal integration can improve the performance of rhizomatous plants, it remains unclear whether their clonal integration strategy changes under contrasting clipping and saline-alkali homogeneous and heterogeneous environments. Leymus chinensis is a clonal grass native to the Songnen grassland where heavy grazing and patchy saline-alkali stress are serious environmental and ecological problems. We hypothesized that L. chinensis overcomes these stresses through clonal integration, in particular the transfer of nitrogen and carbohydrates.

METHODS

A pot experiment was carried out with ¹⁵N isotope soil labeling method to study clonal integration strategy in the connected mother and daughter ramets of L. chinensis. The connected ramet pairs were grown in homogeneous (both connected ramets were treated) and heterogeneous (only daughter ramets were treated) environments with four treatments: control, clipping (60% aboveground biomass removal), saline-alkali (3.45 g of NaCl, NaHCO₃, and Na₂CO₃ per pot), and clipping × saline-alkali.

RESULTS

A significant amount (22.5%) of ¹⁵N was transferred from mother to daughter ramets under non-stressed conditions. When homogeneously stressing both mother and daughter ramets, N transfer was significantly reduced to 8.5--14.6%, independent of the nature of the stress. When only daughters were stressed (heterogeneous stress), saline-alkali stress led to a division of labor where daughters had enhanced photosynthesis, and mother ramets had increased ¹⁵N uptake and growth. Clipping only daughters reduced biomass and ¹⁵N uptake of both daughter and mother ramets.

CONCLUSIONS

Our results demonstrated that clonal integration also occurs in homogeneous favorable environments but is reduced under homogeneous stress. In heterogeneous environments, clonal integration is used to translocate resource after clipping and a division of labor is established to overcome saline-alkali stress. Clonal integration continued even when daughters were severely stressed by the combined treatments. Our findings suggest that these mechanisms are key to the success of L. chinensis in the Songnen grassland.

Interactions between soil properties and the rhizome-root distribution in a 12-year Moso bamboo reforested region: Combining ground-penetrating radar and soil coring in the field

Moso bamboo (Phyllostachys pubescens) plays an important role in mitigating climate change and ameliorating soil degradation because of its high carbon sequestration capacity and erosion resistance. Its strong underground rhizome-root systems form the basic framework of the aboveground system of Moso bamboo forest and define the basic ecological characteristics. However, studies on the relationship between the spatial distribution of roots and soil resources have often been neglected due to methodological limitations. The objective of this study was to test the detectability of rhizomes in the field by ground-penetrating radar (GPR) and to understand the interactions between rhizome-root systems and soil characteristics. The rhizome-root system distribution was investigated using GPR; and the soil texture, soil organic carbon and soil nutrients were investigated using a soil coring method to prepare 50-cm soil profiles. A few key findings were emphasized. First, the rhizome-root system was mainly distributed over a soil depth of 0-30 cm; and the rhizomes were larger in diameter (often greater than 1.0 cm). Therefore, GPR can accurately detect rhizomes in the field, making the non-invasive and long-term estimation of rhizome biomass and monitoring of changes in rhizome dynamics possible under field conditions. Second, the spatial heterogeneity of the soil moisture content, alkaline hydrolysed nitrogen and available phosphorus had a greater effect on the rhizomes spatial distribution than did the spatial heterogeneity of other soil characteristics. The rhizomes clonal growth led to increases in soil organic carbon, which promoted the amelioration of degraded soil. Third, the results provide insights for bamboo forest management, such as the application of GPR to prevent bamboo invasion and to determine the appropriate fertilizer level for a rhizome system. More field tests are needed to validate the application of GPR to rhizome systems and enhance the detection and quantification of rhizome systems in bamboo forest ecosystems.

Functional responses to climate change may increase invasive potential of Carpobrotus edulis

PREMISE

Biological invasions and climate change are major threats to biodiversity. It is therefore important to anticipate how the climate changes projected for Southern Europe would affect the ecophysiological performance of the invasive South African plant, Carpobrotus edulis (ice plant or sour fig), and its capacity to undergo rapid adaptive evolution.

METHODS

We manipulated the climate conditions in a field plot located on the island of Sálvora (northwest of the Iberian Peninsula) to establish a full factorial experiment with C. edulis plants transplanted from four native (southern African) and four invasive (northwestern Iberian Peninsula) populations. Throughout 14 months we measured growth and functional traits of this species under two temperatures (control vs. increased), and two rainfall levels (control vs. reduced).

RESULTS

Temperature increased photochemical efficiency and relative growth rate of C. edulis. Rainfall modulated some of the effects of temperature on C and N isotopic composition, and pigment contents. Invasive populations showed lower root mass allocation and higher survival rates, as well as increased water use efficiency, lipid peroxidation, chlorophyll, and xanthophyll cycle pigment contents than native populations.

CONCLUSIONS

The increased growth and physiological performances observed under our experimental conditions suggest that the expected climate changes would further promote the invasion of C. edulis. Differences between native and invasive genotypes in survival and functional traits revealed that populations have diverged during the process of invasion, what gives support to the invasiveness hypothesis. Our findings highlight the importance of analyzing intraspecific variability in functional responses to better predict how invasive species will respond to environmental changes.

Geographic patterns of seed trait variation in an invasive species: how much can close populations differ?

Seeds play a major role in plant species persistence and expansion, and therefore they are essential when modeling species dynamics. However, homogeneity in seed traits is generally assumed, underestimating intraspecific trait variability across the geographic space, which might bias species success models. The aim of this study was to evaluate the existence and consequences of interpopulation variability in seed traits of the invasive species Carpobrotus edulis at different geographical scales. We measured seed production, morphology, vigour and longevity of nine populations of C. edulis along the Catalan coast (NE Spain) from three differentiated zones with a human presence gradient. Geographic distances between populations were contrasted against individual and multivariate trait distances to explore trait variation along the territory, evaluating the role of bioclimatic variables and human density of the different zones. The analysis revealed high interpopulation variability that was not explained by geographic distance, as regardless of the little distance between some populations (< 0.5 km), significant differences were found in several seed traits. Seed production, germination, and persistence traits showed the strongest spatial variability up to 6000% of percent trait variability between populations, leading to differentiated C. edulis soil seed bank dynamics at small distances, which may demand differentiated strategies for a cost-effective species management. Seed trait variability was influenced by human density but also bioclimatic conditions, suggesting a potential impact of increased anthropogenic pressure and climate shifts. Geographic interpopulation trait variation should be included in ecological models and will be important for assessing species responses to environmental heterogeneity and change.

Demographic analysis of an Israeli Carpobrotus population

Carpobrotus species are harmful invaders to coastal areas throughout the world, particularly in Mediterranean habitats. Demographic models are ideally suited to identify and understand population processes and stages in the life cycle of the species that could be most effectively targeted with management. However, parameterizing these models has been limited by the difficulty in accessing the cliff-side locations where its populations are typically found, as well as accurately measuring the growth and spread of individuals, which form large, dense mats. This study uses small unmanned aerial vehicles (drones) to collect demographic data and parameterize an Integral Projection Model of an Israeli Carpobrotus population. We validated our data set with ground targets of known size. Through the analysis of asymptotic growth rates and population sensitivities and elasticities, we demonstrate that the population at the study site is demographically stable, and that reducing the survival and growth of the largest individuals would have the greatest effect on reducing overall population growth rate. Our results provide a first evaluation of the demography of Carpobrotus, a species of conservation and economic concern, and provide the first structured population model of a representative of the Aizoaceae family, thus contributing to our global knowledge on plant population dynamics. In addition, we demonstrate the advantages of using drones for collecting demographic data in understudied habitats such as coastal ecosystems.

A test of native plant adaptation more than one century after introduction of the invasive Carpobrotus edulis to the NW Iberian Peninsula

Background

Although the immediate consequences of biological invasions on ecosystems and conservation have been widely studied, the long-term effects remain unclear. Invaders can either cause the extinction of native species or become integrated in the new ecosystems, thus increasing the diversity of these ecosystems and the services that they provide. The final balance of invasions will depend on how the invaders and native plants co-evolve. For a better understanding of such co-evolution, case studies that consider the changes that occur in both invasive and native species long after the introduction of the invader are especially valuable. In this work, we studied the ecological consequences of the more than one century old invasion of NW Iberia by the African plant Carpobrotus edulis. We conducted a common garden experiment to compare the reciprocal effects of competition between Carpobrotus plants from the invaded area or from the native African range and two native Iberian plant species (Artemisia crithmifolia and Helichrysum picardii) from populations exposed or unexposed to the invader.

Results

Exposure of H. picardii populations to C. edulis increased their capacity to repress the growth of Carpobrotus. The repression specifically affected the Carpobrotus from the invader populations, not those from the African native area. No effects of exposition were detected in the case of A. crithmifolia. C. edulis plants from the invader populations had higher growth than plants from the species' African area of origin.

Conclusions

We found that adaptive responses of natives to invaders can occur in the long term, but we only found evidence for adaptive responses in one of the two species studied. This might be explained by known differences between the two species in the structure of genetic variance and gene flow between subpopulations. The overall changes observed in the invader Carpobrotus are consistent with adaptation after invasion.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12862-021-01785-x.

Capacity for clonal integration in introduced versus native clones of the invasive plant Hydrocotyle vulgaris

Clonal plants can make up a disproportionately high number of the introduced, invasive plant species in a region. Physiological integration of connected ramets within clones is a key ecological advantage of clonal growth. To ask whether clonal integration underlies the invasiveness of clonal plants, we tested the hypothesis that introduced clones of an invasive species will show higher capacity for integration than native clones of the same species. We conduct a greenhouse experiment on the widespread, perennial herb Hydrocotyle vulgaris. Clonal fragments consisting of pairs of connected ramets from seven sites in northwestern Spain where the species is native and seven sites in southeastern China where the species is introduced and invasive were grown for 79 days with the younger, apical ramet shaded to 30% of ambient light and the connection between ramets either severed or left intact. Severance decreased the final dry mass and ramet number of the apical ramet and its offspring in nearly all clones and increased the mass or ramet number of the basal portion of the fragment in about half of the clones, but these effects did not differ consistently between native and introduced clones. Severance did affect allocation more in introduced than in native clones, decreasing root/total mass more in apical portions and increasing it more in basal portions. Maintaining the connection between ramets caused introduced, but not native, clonal fragments to produce more leaf and less root mass and thus to lower allocation to roots. Regardless of severance, introduced clones accumulated about twice as much mass as native clones. Results suggest that introduced clones of a species can show greater effects of integration on allocation than native clones. In species such as H. vulgaris, this might increase competitiveness for light.

Drifting away. Seawater survival and stochastic transport of the invasive Carpobrotus edulis

Coastal areas are vulnerable and fluctuating habitats that include highly valuable spaces for habitat and species conservation and, at the same time, they are among the most invaded ecosystems worldwide. Occupying large areas within Mediterranean-climate coastlines, the "ecosystem engineer" Carpobrotus edulis appears as a menace for coastal biodiversity and ecosystem services. By combining the observation, current distribution, glasshouse experiment, and dispersion modeling, we aim to achieve a better understanding of the successful invasion process and potential dispersion patterns of C. edulis. We analyzed the response of plant propagules (seeds and plant fragments) to seawater immersion during increasing periods of time (up to 144 h). After 2 months of growth, plant fragments showed a total survival rate (100%) indicating high tolerance to salinity. During this time, fragment length was increased (up to 60%) and root length was higher than control in all cases. Also, immersed fragments consistently accumulated more biomass than control fragments. After two months of growth, photosynthetic parameters (F_v'/F_m', Φ_NO, and Φ_II) remained stable compared to control fragments. Physiologically, osmolyte and pigment content did not evidence significant changes regardless of immersion time. Based on the capacity of propagules to survive seawater immersion, we modeled the potential transport of C. edulis by combining an oceanic model (ROMS-AGRIF) with a particle-tracking model. Results indicated that propagules may travel variable distances maintaining physiological viability. Our model suggested that short-scale circulation would be the dominant process, however, long-scale circulation of propagules may be successfully accomplished in <6 days. Furthermore, under optimal conditions (southerly winds dominance), propagules may even travel large distances (250 km alongshore). Modeling transport processes, in combination with the dynamics of introduction and expansion, will contribute to a better understanding of the invasive mechanisms of C. edulis and, consequently, to design preventive strategies to reduce the impact of plant invasion.

Increased chilling tolerance of the invasive species Carpobrotus edulis may explain its expansion across new territories

Invasive plants are expanding their geographical distribution across new regions. Expansion modeling is crucial for geographic prioritization in management policies. However, the assumption of niche conservatism and the lack of information of the species physiological response to the environmental factors determining species presence may hinder predictions. In this study, we aimed to understand the expansion of the widely distributed plant Carpobrotus edulis in Europe. We contrasted introduced and native C. edulis ecological niches and explored the experimental response to temperature, a major determining factor for species distribution, of native and invasive individuals in terms of different biochemical markers. Niche analysis revealed an expansion of the introduced niche to occupy colder climates. Introduced and native individuals showed differential mechanisms facing low temperatures. Individuals from the native range showed an increased sensitivity to chilling, as reflected by photosynthetic pigment degradation, increased de-epoxidation of xanthophylls and the accumulation of the lipophilic antioxidant alpha-tocopherol. The found physiological differentiation towards an increased invasive chilling tolerance of invasive C. edulis individuals together with a high propagule pressure may explain the introduced climatic niche shift to colder climates observed, allowing the extensive expansion of this species in Europe.

Nutrient addition does not increase the benefits of clonal integration in an invasive plant spreading from open patches into plant communities

One benefit of clonal integration is that resource translocation between connected ramets enhances the growth of the ramets grown under stressful conditions, but whether such resource translocation reduces the performance of the ramets grown under favourable conditions has not produced consistent results. In this study, we tested the hypothesis that resource translocation to recipient ramets may reduce the performance of donor ramets when resources are limiting but not when resources are abundant. We grew Mikania micrantha stolon fragments (each consisting of two ramets, either connected or not connected) under spatially heterogeneous competition conditions such that the developmentally younger, distal ramets were grown in competition with a plant community and the developmentally older, proximal ramets were grown without competition. For half of the stolon fragments, slow-release fertiliser pellets were applied to both the distal and proximal ramets. Under both the low and increased soil nutrient conditions, the biomass, leaf number and stolon length of the distal ramets were higher, and those of the proximal ramets were lower when the stolon internode was intact than when it was severed. For the whole clone, the biomass, leaf number and stolon length did not differ between the two connection treatments. Connection did not change the biomass of the plant communities competing with distal ramets of M. micrantha. Although clonal integration may promote the invasion of M. micrantha into plant communities, resource translocation to recipient ramets of M. micrantha will induce a cost to the donor ramets, even when resources are relatively abundant.

Developmentally Programmed Division of Labor in the Aquatic Invader Alternanthera philoxeroides Under Homogeneous Soil Nutrients

Clonal traits can contribute to plant invasiveness, but little is known about the roles of division of labor (a key clonal trait) in homogeneous habitats. The hypothesis tested is that clonal integration allows division of labor and increases the overall performance of an invasive clonal plant, especially under higher soil nutrients. Clonal fragment pairs of aquatic invader Alternanthera philoxeroides (each with four ramets and a stolon apex) were grown in two homogenous habitats with high or low soil nutrient supply, and with stolon connections being either severed (clonal integration prevented) or kept intact (clonal integration allowed). Results showed that stolon connection allowed the division of labor within the clonal fragment, with basal ramets specializing in acquisition of belowground resources and apical ramets specializing in acquisition of aboveground expansion. Moreover, the capacity for division of labor was greater, which brought the clonal fragments of A. philoxeroides stronger clonal propagation and better performance in high nutrient habitats than in low nutrient habitats. The results supported our hypotheses that the developmentally programmed division of labor may facilitate the clonal expansion of this aggressive invader in some homogeneous habitats with high resource availability.

Nutrient addition increases the capacity for division of labor and the benefits of clonal integration in an invasive plant

Many of the most invasive plants are clonal, and clonal integration has been proposed as an important mechanism promoting invasiveness. When the availabilities of two essential resources are negatively correlated in space, clonal integration may benefit clonal plants through division of labor. We hypothesized that environments with reciprocal patchiness of light and soil water may induce division of labor, and nutrient addition may increase both the division of labor and the benefits of clonal integration. To test this, we grew pairs of connected and disconnected ramets of the clonal invader Mikania micrantha under negative spatial covariance of light and soil water such that the proximal ramets were grown under high light and low soil water conditions and the distal ramets were grown under low light and high soil water conditions. In half of the ramet pairs, both ramets of a pair received a nutrient addition treatment. The results showed that connection decreased the root to shoot ratio in proximal ramets and increased it in distal ramets, indicating that division of labor was induced. In addition, connection increased the root to shoot ratio of distal ramets more under high soil nutrient conditions than under low soil nutrient conditions, indicating that nutrient addition increased the division of labor. Connection increased plant biomass at the whole clonal fragment level, and this increase was larger under high soil nutrient conditions than under low soil nutrient conditions. This study showed, for the first time, that in environments with reciprocal patchiness of two essential resources, the capacity for division of labor and its influence on plant performance may depend on the availability of a third essential resource. Because invasive plants often can acquire a larger amount of soil resources than native plants, our study may also contribute to the understanding why clonality is related to invasiveness.

Herbivory induced non-local responses of the clonal invader Carpobrotus edulis are not mediated by clonal integration

The anthropogenic displacement of species around the world results in new environmental situations where native and exotic species coexist. Exotic plants have to face native herbivores, and interactions between introduced plants and native herbivores seem to play an important role in the invasiveness of some exotic plant species. We studied the role of clonal integration in induce morphological, physiological, and biochemical responses in the clonal invader Carpobrotus edulis against the attack of the native snail Theba pisana. Our results demonstrated the presence of labour division mediated by physiological integration, with a significant increase of photosynthesis potential (both at morphological and physiological) in un-attacked integrated ramets. This response could be especially important under herbivory, as the negative impact of T. pisana on the photosynthetic structures of attacked C. edulis ramets could be buffered by transferring the dependence of photosynthetic activity to the un-attacked ramets. Our results also showed a constitutive resistance in un-attacked apical ramets, showing a similar amount of defence compounds to those exhibited in the basal branches attacked by snails. Results reported a non-local compensatory response, which there was an increase of total biomass in apical ramets when their basal ramets were attacked by the herbivore. We interpret this result as a compensatory response, with these apical ramets increasing shoot biomass to compensate for the biomass loss due to a potential attack from herbivores. However, this non-local response was not mediated by physiological integration but probably due to belowground communication, with the presence of alarm signals released by root exudates. We conclude that the attack of this snail is not enough to be a possible biological control due to the compensatory response to this snail by C. edulis, favouring their expansion. Future studies should focus on unravelling the role of belowground communication in the defensive responses of C. edulis.

Contrasting phenotypic plasticity in the photoprotective strategies of the invasive species Carpobrotus edulis and the coexisting native species Crithmum maritimum

Photoprotective strategies vary greatly within the plant kingdom and reflect a plant's physiological status and capacity to cope with environment variations. The plasticity and intensity of these responses may determine plant success. Invasive species are reported to show increased vigor to displace native species. Describing the mechanisms that confer such vigor is essential to understanding the success of invasive species. We performed an experiment whereby two species were monitored: Carpobrotus edulis, an aggressive invasive species in the Mediterranean basin, and Crithmum maritimum, a coexisting native species in the Cap de Creus Natural Park (NE Spain). We analyzed their photoprotective responses to seasonal environmental dynamics by comparing the capacity of the invader to respond to the local environmental stresses throughout the year. Our study analyses ecophysiological markers and photoprotective strategies to gain an insight into the success of invaders. We found that both species showed completely different but effective photoprotective strategies: in summer, C. edulis took special advantage of the xanthophyll cycle, whereas the success of C. maritimum in summer stemmed from morphological changes and alterations on β-carotene content. Winter also presented differences between the species, as the native showed reduced F_v /F_m ratios. Our experimental design allowed us to introduce a new approach to compare phenotypic plasticity: the integrated phenotypic plasticity index (PP_int ), defined as the maximum Euclidian distance between phenotypes, using a combination of different variables to describe them. This index revealed significantly greater phenotypic plasticity in the invasive species compared to the native species.