Clonal integration and heavy-metal stress: responses of plants with contrasting evolutionary backgrounds

Physiological integration between ramets can ameliorate the growth and survival of clonal plants in spatially-heterogeneous environments, as ramets from favourable patches can provide support to those found in stressful patches. However, the advantage conferred by clonal integration might also depend on the evolutionary history of plants with regards to the presented stress. Here, we compared the benefit of clonal integration in response to the distribution of a heavy metal as a stress factor, and asked if this benefit would differ between ecotypes that have either undergone selection to tolerate heavy metals or not. In a greenhouse experiment, we grew pairs of connected and severed ramets of the metal hyperaccumulator Arabidopsis halleri, which originated from populations of either metalliferous or non-metalliferous soils. The ramets were grown in paired pots, which were contaminated with cadmium (Cd) either heterogeneously (100 or 0 ppm Cd per pot) or homogenously (50 ppm Cd per each pot). A. halleri ecotypes that originated from non-metalliferous soils performed better when ramets were connected and the distribution of Cd was heterogeneous. However, clonal integration had no effect on the performance of genotypes from metalliferous soils, regardless of the distribution of Cd. These results support the hypothesis that clonal integration is beneficial in stressful environments as long as the stress is patchily distributed, and particularly for plants that did not undergo selection to withstand it.     

Physiological integration modifies δ15N in the clonal plant Fragaria vesca,suggesting preferential transport of nitrogen to water-stressed offspring

Background and Aims

One of the most striking attributes of clonal plants is their capacity for physiological integration, which enables movement of essential resources between connected ramets. This study investigated the capacity of physiological integration to buffer differences in resource availability experienced by ramets of the clonal wild strawberry plant, Fragaria vesca. Specifically, a study was made of the responses of connected and severed offspring ramets growing in environments with different water availability conditions (well watered or water stressed) and nitrogen forms (nitrate or ammonium).

Methods

The experimental design consisted of three factors, ‘integration’ (connected, severed) ‘water status’ (well watered, water stressed) and ‘nitrogen form’ (nitrate, ammonium), applied in a pot experiment. The effects of physiological integration were studied by analysing photochemical efficiency, leaf spectral reflectance, photosynthesis and carbon and nitrogen isotope discrimination, the last of which has been neglected in previous studies.

Key Results

Physiological integration buffered the stress caused by water deprivation. As a consequence, survival was improved in water-stressed offspring ramets that remained connected to their parent plants. The nitrogen isotope composition (δ¹⁵N) values in the connected water-stressed ramets were similar to those in ramets in the ammonium treatment; however, δ¹⁵N values in connected well-watered ramets were similar to those in the nitrate treatment. The results also demonstrated the benefit of integration for offspring ramets in terms of photochemical activity and photosynthesis.

Conclusions

This is the first study in which carbon and nitrogen isotopic discrimination has been used to detect physiological integration in clonal plants. The results for nitrogen isotope composition represent the first evidence of preferential transport of a specific form of nitrogen to compensate for stressful conditions experienced by a member clone. Water consumption was lower in plants supplied with ammonium than in plants supplied with nitrate, and therefore preferential transport of ammonium from parents to water-stressed offspring could potentially optimize the water use of the whole clone.     

Shifting effects of physiological integration on performance of a clonal plant during submergence and de-submergence

Background and Aims

Submergence and de-submergence are common phenomena encountered by riparian plants due to water level fluctuations, but little is known about the role of physiological integration in clonal plants (resource sharing between interconnected ramets) in their adaptation to such events. Using Alternanthera philoxeroides (alligator weed) as an example, this study tested the hypotheses that physiological integration will improve growth and photosynthetic capacity of submerged ramets during submergence and will promote their recovery following de-submergence.

Methods

Connected clones of A. philoxeroides, each consisting of two ramet systems and a stolon internode connecting them, were grown under control (both ramet systems untreated), half-submerged (one ramet system submerged and the other not submerged), fully submerged (both ramet systems submerged), half-shaded (one ramet system shaded and the other not shaded) and full-shaded (both ramet systems shaded) conditions for 30 d and then de-submerged/de-shaded for 20 d. The submerged plants were also shaded to very low light intensities, mimicking typical conditions in turbid floodwater.

Key Results

After 30 d of submergence, connections between submerged and non-submerged ramets significantly increased growth and carbohydrate accumulation of the submerged ramets, but decreased the growth of the non-submerged ramets. After 20 d of de-submergence, connections did not significantly affect the growth of either de-submerged or non-submerged ramets, but de-submerged ramets had high soluble sugar concentrations, suggesting high metabolic activities. The shift from significant effects of integration on both submerged and non-submerged ramets during the submergence period to little effect during the de-submergence period was due to the quick recovery of growth and photosynthesis. The effects of physiological integration were not found to be any stronger under submergence/de-submergence than under shading/de-shading.

Conclusions

The results indicate that it is not just the beneficial effects of physiological integration that are crucial to the survival of riparian clonal plants during periods of submergence, but also the ability to recover growth and photosynthesis rapidly after de-submergence, which thus allows them to spread.     

松嫩平原两个生态型羊草实验种群对盐碱胁迫的生理响应

采用盆栽实验,对灰绿型和黄绿型两个生态型羊草(Leymus chinensis)在幼苗期分别用不同强度的NaCl、Na2CO3和混合盐碱胁迫,测定其叶片叶绿素含量、电解质外渗率和植株脯氨酸、Na／K等生理指标。结果表明,在3种盐碱胁迫条件下,灰绿型和黄绿型羊草的叶片叶绿素含量、电解质外渗率和植株脯氨酸、Na／K均与胁强之间呈直线y=a+bx变化,达到了显著或极显著水平,两个实验种群均具有耐盐碱能力,叶片叶绿素含量、电解质外渗率和植株脯氨酸、Na／K均有显著或极显著差异,它们在耐盐碱生理特性上已产生了分化,其中,灰绿型的耐盐碱能力强于黄绿型,本文为羊草生态型分化主导生态因子是土壤盐碱所致提供了的生理学证据。     

Physiological integration increases the survival and growth of the clonal invader <Emphasis Type="Italic">Carpobrotus edulis</Emphasis>

Clonal growth seems to be a common trait for many of the most aggressive invasive plant species. However, little research has been conducted to determine the role of clonality in the successful invasion of new areas by exotic species. Carpobrotus edulis (L.) N.E. Br. is a mat-forming succulent plant, native to South Africa that is invasive in coastal dunes of Australia, New Zealand, USA and Southern Europe. Although Carpobrotus edulis is a clonal plant, there is no information on the role of clonality for the invasion by this species, therefore the objective of this study was to test whether or not physiological integration improves the performance of C. edulis invading coastal sand dunes. To do that, a 6-month field experiment was designed in which the stolon connections between the apical ramets and the C. edulis mats were severed to prevent physiological integration. This treatment was applied to ramets growing under high and low competition with the native species. Apical ramets with intact stolon connections were used as control. Integration improved the survivorship and growth of apical ramets, both in high and low competition. Connected ramets showed a more pronounced increase of clonal growth (estimated as stolon length) during the experimental period and a higher total biomass and number of ramets at the completion of the experiment. In terms of survivorship, the benefit of integration was greater under high competition. Physiological integration can therefore be considered an important factor in the invasiveness of C. edulis, both in open space and in direct competition with the native plants.     

Arbuscular mycorrhizal fungi reduce effects of physiological integration in Trifolium repens

Background and Aims

One of the special properties of clonal plants is the capacity for physiological integration, which can increase plant performance through mechanisms such as resource sharing and co-ordinated phenotypic plasticity when plants grow in microsites with contrasting resource availabilities. However, many clonal plants are colonized by arbuscular mycorrhizal fungi (AMF). Since AMF are likely to reduce contrasts in effective resource levels, they could also reduce these effects of clonal integration on plasticity and performance in heterogeneous environments.

Methods

To test this hypothesis, pairs of connected and disconnected ramets of the stoloniferous herb Trifolium repens were grown. One ramet in a pair was given high light and low nutrients while the other ramet was given high nutrients and low light. The pairs were inoculated with zero, one or five species of AMF.

Key Results

Pairs of ramets grown without AMF developed division of labour and benefited from resource sharing, as indicated by effects of connection on allocation to roots, accumulation of mass, and ramet production. Inoculation with five species of AMF significantly reduced these effects of connection, both by inhibiting them in ramets given high nutrients and inducing them in ramets given high light. Inoculation with one species of AMF also reduced some effects of connection, but generally to a lesser degree.

Conclusions

The results show that AMF can significantly modify the effects of clonal integration on the plasticity and performance of clonal plants in heterogeneous environments. In particular, AMF may partly replace the effects and benefits of clonal integration in low-nutrient habitats, possibly more so where species richness of AMF is high. This provides the first test of interaction between colonization by AMF and physiological integration in a clonal plant, and a new example of how biotic and abiotic factors could interact to determine the ecological importance of clonal growth.Key words: Arbuscular mycorrhizal fungi, biomass allocation, clonal plant, division of labour, environmental heterogeneity, light availability, nutrients, white clover     

Trampling, defoliation and physiological integration affect growth, morphological and mechanical properties of a root-suckering clonal tree

Background and Aims

Grazing is a complex process involving the simultaneous occurrence of both trampling and defoliation. Clonal plants are a common feature of heavily grazed ecosystems where large herbivores inflict the simultaneous pressures of trampling and defoliation on the vegetation. We test the hypothesis that physiological integration (resource sharing between interconnected ramets) may help plants to deal with the interactive effects of trampling and defoliation.

Methods

In a field study, small and large ramets of the root-suckering clonal tree Populus simonii were subjected to two levels of trampling and defoliation, while connected or disconnected to other ramets. Plant responses were quantified via survival, growth, morphological and stem mechanical traits.

Key Results

Disconnection and trampling increased mortality, especially in small ramets. Trampling increased stem length, basal diameter, fibrous root mass, stem stiffness and resistance to deflection in connected ramets, but decreased them in disconnected ones. Trampling decreased vertical height more in disconnected than in connected ramets, and reduced stem mass in disconnected ramets but not in connected ramets. Defoliation reduced basal diameter, leaf mass, stem mass and leaf area ratio, but did not interact with trampling or disconnection.

Conclusions

Although clonal integration did not influence defoliation response, it did alleviate the effects of trampling. We suggest that by facilitating resource transport between ramets, clonal integration compensates for trampling-induced damage to fine roots.     

Partial mechanical stimulation facilitates the growth of the rhizomatous plant Leymus secalinus: modulation by clonal integration

Background and Aims

Mechanical stimulation (MS) often induces plants to undergo thigmomorphogenesis and to synthesize an array of signalling substances. In clonal plants, connected ramets often share resources and hormones. However, little is known about whether and how clonal integration influences the ability of clonal plants to withstand MS. We hypothesized that the effects of MS may be modulated by clonal integration.

Methods

We conducted an experiment in which ramet pairs of Leymus secalinus were subjected to three treatments: (1) connected ramet pairs under a homogeneous condition [i.e. the proximal (relatively old) and distal (relatively young) ramets were not mechanically stressed]; (2) connected ramet pairs under a heterogeneous condition (i.e. the proximal ramet was mechanically stressed but the distal ramet was not); and (3) disconnected ramet pairs under the same condition as in treatment 2. At the end of the experiment, we harvested all plants and determined their biomass and allocation.

Key Results

Clonal integration had no significant influence on measured traits of distal L. secalinus ramets without MS. However, under MS, plants with distal ramets that were connected to a mother ramet produced more total plant biomass, below-ground biomass, ramets and total rhizome length than those that were not connected. Partial MS exerted local effects on stimulated ramets and remote effects on connected unstimulated ramets. Partial MS increased total biomass, root/shoot ratio, number of ramets and total rhizome length of stimulated proximal ramets, and increased total biomass, root weight ratio, number of ramets and total rhizome length of connected unstimulated ramets due to clonal integration.

Conclusions

These findings suggest that thigmomorphogenesis may protect plants from the stresses caused by high winds or trampling and that thigmomorphogenesis can be strongly modulated by the degree of clonal integration.     

Differential influence of clonal integration on morphological and growth responses to light in two invasive herbs

Background and aims

In contrast to seeds, high sensitivity of vegetative fragments to unfavourable environments may limit the expansion of clonal invasive plants. However, clonal integration promotes the establishment of propagules in less suitable habitats and may facilitate the expansion of clonal invaders into intact native communities. Here, we examine the influence of clonal integration on the morphology and growth of ramets in two invasive plants, Alternanthera philoxeroides and Phyla canescens, under varying light conditions.

Methods

In a greenhouse experiment, branches, connected ramets and severed ramets of the same mother plant were exposed under full sun and 85% shade and their morphological and growth responses were assessed.

Key results

The influence of clonal integration on the light reaction norm (connection×light interaction) of daughter ramets was species-specific. For A. philoxeroides, clonal integration evened out the light response (total biomass, leaf mass per area, and stem number, diameter and length) displayed in severed ramets, but these connection×light interactions were largely absent for P. canescens. Nevertheless, for both species, clonal integration overwhelmed light effect in promoting the growth of juvenile ramets during early development. Also, vertical growth, as an apparent shade acclimation response, was more prevalent in severed ramets than in connected ramets. Finally, unrooted branches displayed smaller organ size and slower growth than connected ramets, but the pattern of light reaction was similar, suggesting mother plants invest in daughter ramets prior to their own branches.

Conclusions

Clonal integration modifies light reaction norms of morphological and growth traits in a species-specific manner for A. philoxeroides and P. canescens, but it improves the establishment of juvenile ramets of both species in light-limiting environments by promoting their growth during early development. This factor may be partially responsible for their ability to successfully colonize native plant communities.     

Coordination of leaf N,anatomy, photosynthetic capacity,and hydraulics enhances evergreen expansive potential

Key message

Reduced leaf longevity, N-fixation, and enhanced hydraulic capacity combined support greater shifts in seasonal photosynthetic capacity of an expansive understory evergreen woody species relative to co-occurring less expansive evergreen species.

Abstract

Physiological functioning typically declines with increased leaf life span. While an evergreen leaf habit is generally associated with reduced leaf N, physiological capacity, and slower growth, most expansive woody species are evergreens and/or N fixers. An evergreen leaf habit enables year-round activity and less investment in carbon and nutrients, while N-fixation enhances photosynthetic capacity. Our objective was to compare anatomy and physiology of three woody evergreens Ilex opaca Aiton (Aquifoliaceae), Kalmia latifolia L. (Ericaceae), and Myrica cerifera (Myricaceae) of varying leaf longevity, N-fixation capability, and known expansive potential in a deciduous forest understory to determine if seasonal physiological performance integrated these factors. We hypothesized that I. opaca (non-expansive) and K. latifolia (moderately expansive), which have longer leaf longevities, would have reduced physiological performance compared to M. cerifera (expansive), which has shorter leaf longevity, and symbiotically fixes atmospheric N. Stomatal conductance to water vapor, photosynthetic and hydraulic capacities, specific leaf area, and leaf %N decreased with increasing leaf life span; however, trends among species were not consistent seasonally. While hydraulic capacity remained constant throughout the year, photosynthetic capacity did not. During the growing season, M. cerifera displayed photosynthetic capacity similar to deciduous species, yet, during the winter, photosynthetic capacity was similar to the slower-growing evergreens. Reduced leaf life span, enhanced hydraulic capacity, and nitrogen fixation support the seasonal shift in photosynthetic capacity observed in M. cerifera. This “hybrid” strategy enables M. cerifera to maximize productivity during months of optimal conditions, thereby promoting rapid growth and expansion in the understory.     

Sexual differences in physiological integration in the dioecious shrub Lindera triloba: a field experiment using girdling manipulation

Background and Aims

It is important to consider the modular level when verifying sexual dimorphism in dioecious plants. Nevertheless, between-sex differences in resource translocation among modules (i.e. physiological integration) have not been tested at the whole-plant level. In this study, sexual differences in physiological integration were examined among ramets, within a genet in the dioecious sprouting shrub Lindera triloba, by a field experiment with girdling manipulation.

Methods

Female and male genets were randomly assigned to girdled or intact groups. Girdling of the main ramets was conducted in May 2009 by removing a ring of bark and cambium approx. 1 cm wide at a height of 80–100 cm. The effects of treatment and sex on ramet dynamics (mortality, recruitment and diameter growth) and inflorescence production during 1 year after girdling were examined.

Key Results

The diameter growth rate of main ramets of both sexes was lower at ground level (D₀) but higher at breast height (dbh) in girdled than in intact groups. In sprouted ramets with a dbh of 0–2 cm, males in girdled groups had lower growth rates at D₀ than those of intact groups, whereas no girdling effect was found for females. The main ramets in girdled groups produced more inflorescences than intact groups, irrespective of sex, but male ramets showed a greater response to the treatment than females.

Conclusions

In L. triloba, physiological integration exists at the whole-plant level, and sprouted ramets are dependent on assimilates translocated from main ramets, but this dependence weakens as sprouted ramets get larger. Female sprouted ramets can grow in a physiologically independent manner from the main ramet earlier than those of males. This study highlights the importance of considering modular structures and physiological integration when evaluating sexual differences in demographic patterns of clonal plants.     

Foraging responses of clonal plants to multi-patch environmental heterogeneity: spatial preference and temporal reversibility

Background and aims

Plant root placement is highly plastic in order to acquire patchily distributed nutrients and to ensure their survival, growth and reproduction. Considering the spatial extension of clonal organs, we selected two clonal plants (Leymus chinensis (Trin.) Tzvel. and Hierochloe glabra Trin.) to determine the spatio-temporal effects of environmental heterogeneity on belowground organs and newly-born ramets.

Methods

Small-scale and multi-patch heterogeneous environments were manipulated by creating four patches filled with different types of soil in a same pot. The four patches were composed of sandy soil, sandy loam, loam soil and humus soil, respectively. Ramet number, bud number, mean spacer length, rhizome length, and biomass allocation within each patch were measured to identify plant foraging responses.

Results

The preferential patch of L. chinensis was humus soil patch which was the highest in nutrient availability, whereas H. glabra preferred to place ramets in sandy loam and loam soil patches. When growing in homogeneous environments, both species randomly rooted their offspring ramets in the four compartments. In heterogeneous environments, foraging responses were detected in ramet placement, aboveground biomass and total rhizome length. However, there were no differences in bud number or belowground biomass among four types of patches in heterogeneous environments, which might suggest that there would be no inter-patch differences in seedling establishment in the next year.

Conclusions

Plants show selective allocation of offspring ramets to preferential patches in the presence of multi-patch environmental heterogeneity. Responses of H. glabra to multi-patch heterogeneity were faster than those of L. chinensis, demonstrating that the foraging patterns are species-specific. Clonal plants can rapidly respond to environmental heterogeneity, whereas foraging responses are potentially reversible over a longer temporal scale.     

Clonal integration facilitates spread of Paspalum paspaloides from terrestrial to cadmium‐contaminated aquatic habitats

• Cadmium (Cd) is a hazardous environmental pollutant with high toxicity to plants, which has been detected in many wetlands. Clonal integration (resource translocation) between connected ramets of clonal plants can increase their tolerance to stress. We hypothesised that clonal integration facilitates spread of amphibious clonal plants from terrestrial to Cd‐contaminated aquatic habitats.
• The spread of an amphibious grass Paspalum paspaloides was simulated by growing basal older ramets in uncontaminated soil connected (allowing integration) or not connected (preventing integration) to apical younger ramets of the same fragments in Cd‐contaminated water.
• Cd contamination of apical ramets of P. paspaloides markedly decreased growth and photosynthetic capacity of the apical ramets without connection to the basal ramets, but did not decrease these properties with connection. Cd contamination did not affect growth of the basal ramets without connection to the apical ramets, but Cd contamination of 4 and 12 mg·l^?1 significantly increased growth with connection. Consequently, clonal integration increased growth of the apical ramets, basal ramets and whole clones when the apical ramets were grown in Cd‐contaminated water of 4 and 12 mg·l^?1. Cd was detected in the basal ramets with connection to the apical ramets, suggesting Cd could be translocated due to clonal integration. Clonal integration, most likely through translocation of photosynthates, can support P. paspaloides to spread from terrestrial to Cd‐contaminated aquatic habitats.
• Amphibious clonal plants with a high ability for clonal integration are particularly useful for re‐vegetation of degraded aquatic habitats caused by Cd contamination.

     

Clonal integration affects allocation in the perennial herb <Emphasis Type="Italic">Alternanthera philoxeroides</Emphasis> in N-limited homogeneous environments

Most work on clonal growth in plants has focused on the advantages of clonality in heterogeneous habitats. We hypothesized (1) that physiological integration of connected ramets within a clone can also increase plant performance in homogeneous environments, (2) that this effect depends on whether ramets differ in ability to take up resources, and (3) that only ramets with relatively low uptake ability benefit. We tested these hypotheses using the perennial amphibious herb Alternanthera philoxeroides. We grew clonal fragments and varied numbers of rooted versus unrooted ramets, connection between the apical and basal parts of fragments, and availability of nitrogen. Patterns of final size and mass of fragments did not support these hypotheses. By some measures, severance did reduce the growth of more apical ramets and increase the growth of less apical ones, consistent with net apical transfer of resources. Rooting of individual ramets strongly influenced their growth: second and third most apical ramets each grew most when they were the most apical rooted ramet, and this pattern was more pronounced under higher nitrogen levels. This adds to the evidence that signalling between ramets is an important aspect of clonal integration. Overall, the results indicate that physiological integration between ramets within clones in homogeneous environments can alter the allocation of resources between connected ramets even when it does not affect the total growth of clonal fragments.     

Responses of CO2, CH4 and N2O fluxes to livestock exclosure in an alpine steppe on the Tibetan Plateau, China

Background and aims

The photochemical reflectance index (PRI) is correlated to photosynthetic efficiency and has been successfully applied at multiple scales for remote estimation of physiological functioning. However, interpretation of the PRI signal can be confounded by many different variables including declines in photochemical pigments. Our study was aimed at investigating PRI in response to salinity stress, and evaluating physiological and pigment responses of two co-occurring shrubs, Baccharis halimifolia and Myrica cerifera in laboratory studies.

Methods

Photosynthesis, water relations, chlorophyll fluorescence, hyperspectral reflectance and leaf pigment contents were measured following salinity treatment.

Results

Physiological measurements indicated that both species exhibit adaptations which protect PSII during periods of stress. Chlorophyll fluorescence parameters were affected in both species, but indicated that other photochemical reactions (e.g. photorespiration) were important for energy dissipation in absence of chlorophyll changes. After many days of reduced photosynthesis, photochemical changes were detectable using PRI indicating chronic stress.

Conclusions

Variations in PRI were not related to changes in pigments but strongly related to tissue chlorides indicating salinity effects on the PRI signal. Thus, PRI is an indicator of salinity stress in these coastal species and may be as an early signal for increasing salt exposure associated with rising sea-level and climate change.     

Clonal integration in Fragaria vesca growing in metal-polluted soils: parents face penalties for establishing their offspring in unsuitable environments

Clonal plants often establish descendent ramets in sites with contrasting presence of favourable and unfavourable factors. Connections between ramets allow translocation of essential resources from established ramets to developing ramets and, as consequence, integration confers net benefits to ramets growing under unfavourable conditions. Therefore, integrated ramets may survive in habitat patches that would be lethal to independent ramets or non-clonal plants. This experiment aimed to investigate the physiological and morphological responses of the clonal plant Fragaria vesca growing in heterogeneous substrate with patches of contrasting quality (i.e. uncontaminated or heavy-metal-contaminated). We observed that parents reduced their photosynthetic efficiencies and growth as consequence of maintaining their offspring. This cost did not affect survival of the parents. Physiological integration brings about benefits to offspring ramets growing both at uncontaminated and heavy-metal-contaminated soils. The benefits of integration were detected in both physiological and morphological traits, enhancing the survivorship of offspring ramets in the Cu-polluted soils. We conclude that integration improves the performance of developing ramets of F. vesca growing in heavy-metal-contaminated habitats, allowing clone systems to overcome the establishment risks and maintain their presence in these less favourable sites.     

Dynamics of distribution and performance of ramets constructing genets: a demographic�Cgenetic study in a clonal plant, Convallaria keiskei

Background and Aims

In clonal plants producing vegetative offspring, performance at the genet level as well as at the ramet level should be investigated in order to understand the entire picture of the population dynamics and the life history characteristics. In this study, demography, including reproduction and survival, the growth patterns and the spatial distributions of ramets within genets of the clonal herb Convallaria keiskei were explored.

Methods

Vegetative growth, flowering and survival of shoots whose genets were identified using microsatellite markers were monitored in four study plots for 3 years (2003–2005). The size structures of ramets in genets and their temporal shifts were then analysed. Their spatial distributions were also examined.

Key Results

During the census, 274 and 149 ramets were mapped in two 1 × 2 m plots, and 83 and 94 ramets in two 2 × 2 m quadrats. Thirty-eight genotypes were identified from 580 samples. Each plot included 5–18 genets, and most ramets belonged to the predominant genet(s) in each plot. Shoots foliated yearly for several years, but flowering ramets did not have an inflorescence the next year. A considerable number of new clonal offspring persistently appeared, forming a bell-shaped curve of the size structure of ramets in each genet. Comparing the structures modelled by the normal distributions suggested variation among ramets belonging to a single genet and variation among genets. Furthermore, spatial analyses revealed clumped and distant distributions of ramet pairs in a genet, in which the distant patterns corresponded to the linearly elongating clonal growth pattern of this species.

Conclusion

Characteristics of ramet performances such as flowering and recruitment of clonal offspring, in addition to growth, played a large part in the regulation of genet dynamics and distribution, which were different among the studied genets. These might be characteristics particularly relevant to clonal life histories.Key words: Clonal plant, Convallaria keiskei, demography, genet, genetic identification, growth pattern, life history, ramet, spatial distribution     

Comparison of phenotypic and genetic clone delineation in quaking aspen, <Emphasis Type="Italic">Populus tremuloides</Emphasis>

Key message

Clonal delineation at nuclear microsatellites and phenotypic traits showed high correspondence and revealed an important role of both sexual and clonal reproduction for stand genetic structure.

Abstract

Quaking aspen (Populus tremuloides Michx.) grows throughout the northern and central portions of North America. Reproduction occurs both sexually via seeds and clonally from root suckers. Clonal delineation using morphological/phenological traits, and more recently, highly variable nuclear microsatellites have shown considerable variation in the size of clonal assemblies, and the relative importance of sexual versus clonal reproduction across the species range. In order to provide reliable estimates of genet size (N/G; ramets per sampled genet) and genotypic diversity (G/N; genets/ramets), and to compare genetic and phenotypic clone delineation, we characterized 181 sampled stems (ramets) at seven nuclear microsatellites, and morphological and phenological traits from six clones (genet size ≥11). Genotypic diversity was moderate (G/N = 0.18) and within the range reported in other studies across North America. Multivariate statistics revealed a high correspondence between genetic and phenotypic clone delineation, both with and without predefined genetic groups (94.2 %, 81.7 %). Moderate average genet size (5.6 ramets per genet) and the occurrence of genetically distinct single-ramet genets surrounded by larger genets suggested intermediate levels of sexual reproduction contributing to the genetic structure of this stand. Significant differences among genets were found for phenological and morphological traits such as bark thickness and leaf shape. However, most clones showed no significant differences in diameter growth which was likely caused by poor drainage in this high clay soil that inhibited the expression of genetic differences in growth.

     

Conservation of functional traits leads to shrub expansion across a chronosequence of shrub thicket development

Key message

Robust physiology of Myrica cerifera across a chronosequence (i.e., space for time substitution) of shrub thicket age classes contributes to rapid cover expansion observed in the last 50 years.

Abstract

Many studies have documented the causes of woody expansion into grasslands, but few address unique morphological and physiological traits that facilitate expansion. Myrica cerifera, an evergreen N-fixer, is the dominant shrub on many barrier islands of the southeastern United States. Cover of Myrica cerifera has expanded by ~400 % on Hog Island, Virginia, in the past 50 years. Accretion of the northern end of the island has resulted in a chronosequence (i.e., space for time substitution) of both soil age and shrub thicket development. We investigated functional traits and physiological parameters related to light capture, processing and water balance of M. cerifera across shrub thickets of four age classes from ~10 to ~50 years. We hypothesized that light processing capabilities and hydraulic capacity would be reduced with thicket age. Spatial variation in morphology (i.e., leaf thickness, leaf area) and structure (i.e., leaf angle) related to light capture was observed. Yet, little or no differences were detected in stomatal density, photosynthetic pigments, electron transport rate (ETR) and hydraulic conductivity across sites. Previous research has shown declines in leaf N content, productivity and leaf litter production across the chronosequence. In contrast, we observed that physiology remains consistent despite considerable differences in thicket age and development. Myrica cerifera maintains high photosynthetic and hydraulic efficiency, factors which enable expansion and maintenance of shrub thickets in mesic coastal environments.