Do Clonal and Bud Bank Traits Vary in Correspondence with Soil Properties and Resource Acquisition Strategies? Patterns in Alpine Communities in the Scandian Mountains

Plant traits associated with resource acquisition strategies (specific leaf area (SLA), leaf dry matter content (LDMC), leaf size and plant height) change along gradients of soil properties, being the most conservative in a resource-poor environment and the most dynamic in a resource-rich environment. Clonal attributes also vary along soil and other environmental conditions. We hypothesized that in alpine communities in the Scandian Mts. (1) the average composition of traits in a plant assemblage in terms of i) the predominance of different clonal growth organ types, ii) the number of buds in the bud bank, iii) the distribution of the bud-bank (above- and below ground), iv) the distance of lateral spread and v) the longevity of plant – offspring connections would change along a gradient of soil properties and (2) that this variation would be in correspondence with that of traits associated with resource acquisition strategies (SLA, LDMC, leaf size and plant height). Analysis of clonal and bud bank traits for species of alpine communities supported our first hypothesis: with decreasing soil quality the most common clonal growth organs were rhizomes, and there was a predominance of perennial bud banks located at the soil surface or below-ground, low rates of lateral spread and long persistence of plant – offspring connections. Our second hypothesis was partly supported. As predicted, at the level of the plant assemblage, these clonal and bud bank traits were positively associated with LDMC, and negatively with leaf size and plant height. These observations reinforce the hypotheses about trade-offs between acquisition and retention strategies in plants. The only result that was in contradiction with our expectations was the lack of correspondence between clonal and bud bank traits and SLA that could be attributed to errors associated to the measurement of the area of narrow and small leaves or to the dependence of the SLA index on species-specific morphological attributes.     

Clonal vs leaf-height-seed (LHS) traits: which are filtered more strongly across habitats?

Plant functional traits are now frequently used instead of species identity to identify how plant species co-exist in assemblages. One notion is that species inhabiting the same environment have more characteristics in common than species from different habitats, leading to different prevailing dominant traits along environmental gradients, and also to a lesser diversity of traits in habitats that impose a stronger filter on these traits. Though such patterns have been demonstrated for different environmental drivers and different traits, studies using easily available traits connected to above ground processes (i.e. traits of the leaf-height-seed, or LHS, strategy scheme) are largely overrepresented in these analyses. Here we combined data on clonal and bud bank traits, representing the ability to reproduce and spread vegetatively, with LHS trait data and examined how these traits varied in relation to the vegetational composition of 29 Central-European habitat types. Our analysis focused on determining whether clonal/bud bank or LHS traits play an important role for environmental filtering along gradients approximated by Ellenberg indicator values (EIV) across these habitats. Our results show that clonal and bud bank traits are at least as – if not more – important for the differentiation of the 29 habitat types. Overall, diversity and dominance of clonal and bud bank traits was more strongly correlated with gradients of light availability, temperature, moisture, soil reaction, and nutrient availability across these habitats than it was the case for traits of the leaf-height-seed scheme. Our results call for a stronger integration of belowground traits into the functional traits approach in plant ecology and for an extension of efforts to collect such data.     

Intraspecific phenotypic variability of plant functional traits in contrasting mountain grasslands habitats

Empirical studies that link plants intraspecific variation to environmental conditions are almost lacking, despite their relevance in understanding mechanisms of plant adaptation, in predicting the outcome of environmental change and in conservation. Here, we investigate intraspecific trait variation of four grassland species along with abiotic environmental variation at high spatial resolution (n = 30 samples per species trait and environmental factor per site) in two contrasting grassland habitats in Central Apennines (Italy). We test for phenotypic adaptation between habitats, intraspecific trait-environment relationships within habitats, and the extent of trait and environmental variation. We considered whole plant, clonal, leaf, and seed traits. Differences between habitats were tested using ANOVA and ANCOVA. Trait-environment relationships were assessed using multiple regression models and hierarchical variance partitioning. The extent of variation was calculated using the coefficient of variation. Significant intraspecific differences in trait attributes between the contrasting habitats indicate phenotypic adaptation to in situ environmental conditions. Within habitats, light, soil temperature, and the availability of nitrate, ammonium, magnesium and potassium were the most important factors driving intraspecific trait-environment relationships. Leaf traits and height growth show lower variability than environment being probably more regulated by plants than clonal traits which show much higher variability. We show the adaptive significance of key plant traits leading to intraspecific adaptation of strategies providing insights for conservation of extant grassland communities. We argue that protecting habitats with considerable medium- and small-scale environmental heterogeneity is important to maintain large intraspecific variability within local populations that finally can buffer against uncertainty of future climate and land use scenarios.     

Multiple Regenerative Strategies of Short-Lived Species: An Effect on Geographical Distribution, Preference of Human-Made Habitats and Invasive Status

It is expected that plant species with multiple regenerative strategies have a wider ecological amplitude and geographical distribution. However there is still an anxious question why species with multiple regenerative strategies are not more frequent. In the presented work we asked whether short-lived plants with a potential bud bank i) have a wider geographical distribution, ii) occur more frequently in human-made habitats and iii) more frequently possess invasive status in comparison with the short-lived species without this additional regenerative strategy. Results show that short-lived plants with a potential bud bank are i) more widely geographically distributed, ii) under-represented in natural habitats and weakly over-represented in habitats disturbed by human activities and iii) contrary to expectation, less numerous among neophytes than species without a potential bud bank. This suggests that short-lived root-sprouters are more habitat-specialized when compared with such plants without adventitious bud-bank formation, but are more widely geographically distributed. A higher frequency of multiple regeneration strategies seems to be restricted by costs of having this ability in habitats or situations without a disturbance regime.     

Patterns of functional clonal traits and clonal growth modes in contrasting grasslands in the central Apennines,Italy

Abstract. Aim: Patterns of plant functional traits related to clonality (clonal growth modes; CGM) in plant communities were studied and hypotheses on the importance of the selected traits in plant communities supported by soils differing in moisture and nutrient status were tested. Material and Methods: Selected plant functional traits, such as the position of the mother‐daughter plants connections, length of spacers, frequency of multiplication, persistency of ramets connections, presence of storage organs and bud protection were studied in two contrasting plant communities (xeric and mesic abandoned pastures) typical of central Apennines, Italy. Results and Discussion: Clonality was shown to be of great importance in both mesic and xeric grasslands. The major differences between the two communities were due to the dominant CGMs: turf graminoids (having effective protection of growth meristems in dense tussocks) dominated xeric grasslands, while rhizomatous graminoids (typical of competitive resource‐rich environments) dominated mesic grasslands. Below‐ground CGOs (clonal growth organs), shorter spacers, higher multiplication potential, permanent ramet connection, large bud bank and increased importance of bud protection were found to be of importance in water stressed xeric grassland. Contrary to our expectations, the mesic (less stressed) grasslands have the higher number of clonal plants possessing storage organs.     

The Association of Dispersal and Persistence Traits of Plants with Different Stages of Succession in Central European Man-Made Habitats

Traits related to seed dispersal, clonality and bud bank affect the success or failure of plant species. Using data from 13 successional seres in various human-made habitats the spectra of traits associated with dispersal and persistence were compared to determine the traits that can be used to predict the occurrence of particular plant species at each stage in a succession and how the importance of these traits changes over time. Differences in the traits of species associated with primary and secondary successions were also studied. Species with seeds that are dispersed by water (hydrochory) decreased in abundance during the course of succession. Species with a splitting main root, monocyclic and dicyclic shoots also decreased in abundance. Species capable of forming a potential below-ground bud bank, hypogeogenous rhizome and retaining a long-term connection with clonal offspring increased in abundance. The results indicate that seed dispersal is more important in determining the species composition in the early stages of succession whereas bud banks and clonal traits are more important in the later stages and for colonizing a locality. Primary and secondary seres did not remarkably differ in the trait spectra of the species present indicating that these trends occur in both types of succession.     

Clonal Growth Forms in Eastern Ladakh, Western Himalayas: Classification and Habitat Preferences

Earlier observations that plant clonality, i.e., production of potentially independent offspring by vegetative growth, increase in importance in cold climates such as in arctic and alpine regions, have been recently questioned. However, lack of data obtained using a comparable methodology throughout different regions limit such comparisons. Here we present a classification of clonal growth forms for vascular plants from East Ladakh (an arid mountain range in NW Himalaya, India), and assess the relationship of these forms with multiple environmental gradients. Based on field assessment of clonality in 540 species we distinguished 20 growth forms, which were then grouped into four broader space occupancy strategies. Occurrence in communities and relationship with environmental characteristics and altitude were analyzed using multivariate methods. The most abundant growth form was represented by non-clonal perennial species with a pleiocorm having short branches, prevailing in steppes, Caragana shrubs and screes. The most abundant clonal species were those with very short epigeogenous rhizomes, such as turf graminoids prevailing in wet Kobresia grasslands. Two principal environmental gradients, together with several abiotic variables, affected space occupancy strategies: moisture and altitude. Non-spreading integrators prevailed on shaded rocky slopes, non-spreading splitters in wet grasslands and spreading splitters at the wettest sites. Spreading integrators were the least frequent strategy predominantly occurring at the most elevated sites. Because relevance of clonality decreased with altitude and different communities host different sets of clonal growth strategies, comparison with other cold climate regions should take multiple environmental gradients into account.     

Plant traits and regeneration of urban plant communities after disturbance: Does the bud bank play any role?

Questions: What is the relative role of the bud bank, seed and various species traits in the regeneration of urban plant communities after severe disturbances? Do invasive and exotic species, highly abundant in disturbed communities, regenerate better than native species after disturbance? Methods: Hand tilling was applied to three urban plant communities with and without additional herbicide treatment to exclude regeneration from the bud bank. Plant traits were determined from the literature and databases. Species responses to the treatments were evaluated with RDA analyses in CANOCO. Linear models were applied to identify traits that could predict the responses of species to disturbance. Results: The bud bank played a key role in regeneration in the plots without herbicide. In the plots with herbicide treatment, the seed bank was important in re‐establishing vegetation after disturbance. Exclusion of the bud bank by using herbicide allowed the establishment of small annuals, whereas biennials and perennials were successful in plots where the bud bank was not inhibited by herbicide. Exotic species with a long residence time in the local flora were successful in plots where regeneration from the bud bank was excluded, whereas species with short residence times or that were invasive were suppressed by both types of disturbance. Conclusion: In response to various types of disturbance, species with different regeneration strategies (either seeds or bud bank) were promoted. Exotic species were suppressed primarily by disturbance, which suggests that factors other than just regenerative capability contributed to the high abundance of exotics in urban communities.     

Underground organs of Brazilian Asteraceae: testing the CLO-PLA database traits

Not all plant traits from all regions have been standardized or databased. Some ecosystems, such as tropical grasslands, are under-represented in such databases owing to the difficulty in assessing bud banks and evaluating clonal growth. This study aimed to (i) determine whether Brazilian morphological traits of belowground organs can be translated into categories used in the CLO-PLA database and (ii) assess the applicability of clonal and bud bank traits standardized in the CLO-PLA database for Brazilian Aldama species, which have specialized belowground organs and are able to resprout. In all, 165 species, including herbs, subshrubs and shrubs, of 37 genera from different Brazilian ecosystems, were evaluated. Not all the traditional Brazilian morphological categories could be translated into CLO-PLA traits, resulting in a lower number of categories and loss of information regarding plant morphology. Furthermore, clonal and bud bank traits could be only partially evaluated for Aldama, since some traits showed seasonal variation. The CLO-PLA classification focused on the organs in relation to the soil surface, the connection between mother and daughter shoots, and the origin of buds from which daughter shoots sprout. In the Brazilian classification, by contrast, anatomical features or early ontogeny of the organ are very important. Nevertheless, our results might form the basis for future comparative studies across ecosystems and biomes, for which common trait standardization is necessary. However, further research is needed to assess the functional morphology of clonal and bud bank traits in tropical regions.     

Bud banks and their role in vegetative regeneration – A literature review and proposal for simple classification and assessment

While sexual regeneration of plants after disturbance is relatively well understood, vegetative regeneration has attracted some attention only recently. Its role along environmental gradients and across biomes is poorly known and standard methods for assessment are not yet established. We review current knowledge about the role of bud banks in vegetative regeneration and the diversity of their modes of functioning. The similarities and differences between bud banks and seed banks are illustrated, focusing on dormancy, dispersability, seasonal dynamics, longevity and storage of carbohydrates. We try to formulate some principles that unify bud bank functioning across habitats and growth forms: (1) the bud banks consist of all buds which may be used for vegetative regeneration, including those formed adventitiously only after injury; (2) vertical distribution of buds reflects avoidance of disturbance; (3) seasonal changes in the bud bank make vegetative regeneration sensitive to timing of disturbance; and (4) ability to form adventitious buds provides a potential for vegetative regeneration from roots, stumps and leaves. Based on these principles, a simple classification of bud banks is presented similar to the classification of seed banks. Bud bank traits are considered in relation to severity, timing and frequency of disturbance. These include vertical distribution and seasonal fluctuations in the number of buds. Methods for quantitative assessment of bud numbers and resprouting capacity are reviewed, and a new approach based on indirect bud counts is proposed. The suggested concept of bud banks may be widely used in studies focusing on plant functional traits in relation to disturbance regimes at the levels of plant individuals, populations and communities. Its further development may incorporate adjustments for areas with non-seasonal climate and refinements for some growth forms, such as epiphytes.     

Soil Moisture Rather than Soil Nutrient Regulates the Belowground Bud Bank of Rhizomatous Species <i>Psammochloa villosa</i> in Arid Sand Dunes

In arid and semi-arid sand dune ecosystems, belowground bud bank plays an important role in population regeneration and vegetation restoration. However, the responses of belowground bud bank size and composition to sand burial and its induced changes in soil environmental factors have been rarely studied. In arid sand dunes of Northwestern China, we investigated belowground bud bank size and composition of the typical rhizomatous psammophyte Psammochloa villosa as well as three key soil environmental factors (soil moisture, total carbon and total nitrogen) under different depths of sand burial. Total buds and rhizome buds increased significantly with increasing burial depth, whereas tiller buds first increased and then decreased, with a peak value at the depth of 20–30 cm. Soil moisture increased significantly with sand burial depth, and was positively correlated with the number of all buds and rhizome buds. Soil total carbon concentration first increased and then decreased with sand burial depth, and total nitrogen concentration was significantly lower under deep sand burial than those at shallow depths, and only the number of tiller buds was positively correlated with soil total nitrogen concentration. These results indicate that soil moisture rather than soil nutrient might regulate the belowground bud bank of P. villosa, and that clonal psammophytes could regulate their belowground bud bank in response to sand burial and the most important environmental stress (i.e., soil moisture). These responses, as the key adaptive strategy, may ensure clonal plant population regeneration and vegetation restoration in arid sand dunes.     

Quantifying the effects of ecological constraints on trait expression using novel trait‐gradient analysis parameters

Complex processes related to biotic and abiotic forces can impose limitations to assembly and composition of plant communities. Quantifying the effects of these constraints on plant functional traits across environmental gradients, and among communities, remains challenging. We define ecological constraint (C_i) as the combined, limiting effect of biotic interactions and environmental filtering on trait expression (i.e., the mean value and range of functional traits). Here, we propose a set of novel parameters to quantify this constraint by extending the trait‐gradient analysis (TGA) methodology. The key parameter is ecological constraint, which is dimensionless and can be measured at various scales, for example, on population and community levels. It facilitates comparing the effects of ecological constraints on trait expressions across environmental gradients, as well as within and among communities. We illustrate the implementation of the proposed parameters using the bark thickness of 14 woody species along an aridity gradient on granite outcrops in southwestern Australia. We found a positive correlation between increasing environmental stress and strength of ecological constraint on bark thickness expression. Also, plants from more stressful habitats (shrublands on shallow soils and in sun‐exposed locations) displayed higher ecological constraint for bark thickness than plants in more benign habitats (woodlands on deep soils and in sheltered locations). The relative ease of calculation and dimensionless nature of C_i allow it to be readily implemented at various scales and make it widely applicable. It therefore has the potential to advance the mechanistic understanding of the ecological processes shaping trait expression. Some future applications of the new parameters could be investigating the patterns of ecological constraints (1) among communities from different regions, (2) on different traits across similar environmental gradients, and (3) for the same trait across different gradient types.     

Environmental correlates of growth traits of the stoloniferous plant Potentilla palustris

Growth form is one of the important life history traits ultimately influencing plant fitness. Potentilla palustris is a stoloniferous plant growing in a range of habitats from densely vegetated wet meadows to acidic transitional fens, and its growth form varies according to habitat. In a four year multi-site comparative study, we investigated which biotic and abiotic characteristics influence most its growth traits. Vegetation composition and physiognomy, as well as numerous abiotic environmental variables, were recorded at 32 study sites located on an altitudinal gradient. Growth traits of P. palustris were best explained by the surrounding vegetation physiognomy and not by abiotic conditions, although the latter obviously represents the factors indirectly influencing its growth. Stolon length traits and branching were positively correlated with vegetation density and height, and negatively with altitude. Plants flowered more in taller vegetation, and leaf area was greater in wetter sites with lower vegetation cover. Potentilla palustris appeared to be well adapted to transitional fens, but its vegetative growth was fastest in wet meadows and alluvial habitats on highly organic humid soils. It produced more branches and larger leaves in alluvial habitats with open water, while it had enhanced generative reproduction in wet meadows. Species composition was less important than vegetation physiognomy. In less favorable habitat types, P. palustris prefers an escape strategy of linear growth. Internode length exhibited pronounced plasticity, increasing particularly in tall dense vegetation of lower altitude, whereas internode number remained fairly constant over various habitats. It is evident that both plastic low cost growth traits (internode elongation), and constant high cost traits (internode number) contribute to the P. palustris escape strategy under tall dense vegetation. Phenotypic plasticity enhances the potential of P. palustris to grow in a wide range of habitats and so increases plant fitness on regional scale.     

Plant traits: Their role in the regeneration of alpine plant communities in sub‐arctic Finland

Abstract. Question: How do the relative frequencies of plant traits (clonality, growth form, seed weight, diaspore morphology) vary during the life cycle and how does this affect regeneration? Location: Alpine meadow and heath communities at Kilpisjärvi, sub‐Arctic Finland. Methods: Control plots and three treatments were used to measure relative species abundances for five life cycle stages: standing vegetation, seed rain, seed bank and seedlings emerging in gaps and in closed vegetation. Results: The relative frequencies of plant traits varied between the life cycle stages. The meadows were dominated by weakly clonal herbs, small or intermediate seeds and unappendaged diaspores, while the heaths were dominated by clonal dwarf shrubs, small seeds and fleshy fruits. In the meadows, species with small seeds dominated during the seed rain and in the seedling stage in gaps, while species with intermediate seeds dominated the seed bank and the seedling stage in closed vegetation. Species with unappendaged diaspores dominated throughout the life cycle. In the heaths, seed bank and seedling stage were practically absent. Conclusions: The observed differences in plant trait spectra between life cycle stages indicate that important environmental factors differ among the stages. Small seeds are advantageous for dispersal, whereas intermediate seeds have a greater probability of germinating and establishing in closed vegetation. Appendages facilitate dispersal, whereas unappendaged diaspores favour seed burial. Although the plant growth form spectrum largely reflects environmental constraints during the regeneration cycle, information on seed weight and diaspore morphology improves our knowledge of the relative importance of morphological adaptations of sexual structures in different stages during the life cycle.     

Differential effects of clonal integration on performance in the stoloniferous herb Duchesnea indica, as growing at two sites with different altitude

Clonal integration may be adaptive and enhance the genet performance of clonal plants. Degree of clonal integration may differ between different environments . Here, a container experiment was used to determine how clonal integration affected the performance of the stoloniferous herb Duchesnea indica at two sites with different altitude along the transitional zone between the Qinghai-Tibet plateau and the Sichuan basin of Southwest China. In the experiment, the stolon between partially shaded two ramets experienced severing and intact treatments.We predicted that clonal integration would increase performance of whole clonal fragments and their shaded clonal parts at both sites. In both arctic and alpine environments, clonal plants may form highly integrated plant units (group of ramets).We predicted again that the reduction due to stolon severing in performance of whole clonal fragments and their shaded clonal parts would be greater at the site with high altitude than one with low altitude. The results indicated that the benefit for the shaded clonal parts and whole clonal fragments due to clonal integration was only observed at the site with high altitude. The results suggest that the performance of Duchesnea indica tends to be more responsive to the stolon severing at the site with high altitude than one with low altitude and support the second prediction. In addition, the effects of conditions of the sites and clonal integration on local morphological traits of ramets may be adaptive, five morphological traits of ramet-level (length of petiole, mean stolon internode length, specific petiole weight, specific stolon internode weight and specific leaf area) were investigated. Altogether, the results suggest that clonal integration might help D. indica plants to successfully inhabit the high-altitude habitat of the Qinghai-Tibet plateau of Southwest China, providing new evidences for the notion that clonal integration is an adaptive trait in stressful environments.     

Clonal splitters and integrators in harsh environments of the Trans-Himalaya

Individuals of clonal plants consist of physically and physiologically connected ramets. In splitters, they are integrated for a time shorter than ramet generation time (i.e. the time it takes to produce the first offspring ramet), whereas in integrators connections between ramets persist for a longer time. It has been predicted that integrators should prevail in stressful environments, such as habitats poor in nutrients, whereas splitters are expected to dominate in benign habitats, such as fertile areas with a moderate climate. I tested these predictions in four dry mountain areas of the Trans-Himalaya, in high altitudes subjected to multiple stresses. In accordance with the expectations I found that clonal plants with integrated ramets reach higher mean and maximum altitudes than splitters. Integrators were over-represented in nutrient-poor habitats, such as dry semi-deserts, sandy steppes and in subnival habitats, whereas splitters preferentially colonised mesic habitats, saline sites and wetlands. While there was no difference in the representation of splitters and integrators in habitats with an unstable surface, such as screes, dunes and water bodies, fully integrated clonal plants preferred very stable environments, such as banks of streams covered by closed-canopy vegetation. Most relationships between clonal integration and environmental factors were explainable by the phylogenetic relationship between the species, only the significant preference of splitters for shaded environments persisted in phylogenetically corrected analysis. The results indicate that clonal integration belongs to a set of evolutionarily conservative plant traits, usually shared by related species. Consequently, the adaptive value of clonal integration in individual habitats remains questionable.     

Direct and indirect effects of environmental factors,spatial constraints,and functional traits on shaping the plant diversity of montane forests

Understanding the relative importance of the factors driving the patterns of biodiversity is a key research topic in community ecology and biogeography. However, the main drivers of plant species diversity in montane forests are still not clear. In addition, most existing studies make no distinction between direct and indirect effects of environmental factors and spatial constraints on plant biodiversity. Using data from 107 montane forest plots in Sichuan Giant Panda habitat, China, we quantified the direct and indirect effects of abiotic environmental factors, spatial constraints, and plant functional traits on plant community diversity. Our results showed significant correlations between abiotic environmental factors and trees (r = .10, p value = .001), shrubs (r = .19, p value = .001), or overall plant diversity (r = .18, p value = .001) in montane forests. Spatial constraints also showed significant correlations with trees and shrubs. However, no significant correlations were found between functional traits and plant community diversity. Moreover, the diversity (richness and abundance) of shrubs, trees, and plant communities was directly affected by precipitation, latitude, and altitude. Mean annual temperature (MAT) had no direct effect on the richness of tree and plant communities. Further, MAT and precipitation indirectly affected plant communities via the tree canopy. The results revealed a stronger direct effect on montane plant diversity than indirect effect, suggesting that single‐species models may be adequate for forecasting the impacts of climate factors in these communities. The shifting of tree canopy coverage might be a potential indicator for trends of plant diversity under climate change.     

Disturbance is an important factor in the evolution and distribution of root-sprouting species

We address the neglected issue of ecological and evolutionary significance of root sprouting (RS) in plants. RS has been considered a sort of morphological curiosity. However, existing data of the Central European flora show that it occurs in about 10% of species. These species are therefore independent of a stem-derived bud bank in their resprouting. As sprouting from roots has been hypothesised to help plants survive disturbance, we used a large data set (2914 species with data on presence/absence of RS from Central Europe) to perform comparative analyses of its occurrence in disturbed habitats, evolution of RS in response to disturbance, and its distribution among individual plant lineages. To address these questions, we linked the data with species-level indicator values for disturbance, data on additional functional traits and phylogenetic data. We confirmed that RS ability is more frequent in plants growing in habitats subjected to disturbance, especially in annuals and clonal species. This contrasts with clonality via stem-based organs, which does not promote occurrence in disturbed habitats. Disturbance severity is the most important factor determining RS species distribution, whereas disturbance frequency plays a smaller role. RS is phylogenetically less conservative than sprouting from the stem-based belowground bud bank and thus can be easily acquired or lost in evolution, although these rates strongly differ between individual lineages. Evolution of RS seems to be driven largely by occurrence in disturbed habitats, and has appeared/disappeared independently of the presence of a stem-derived bud bank. Importantly, the data support the scenario in which colonisation of such habitats occurs prior to acquiring the RS ability, which develops only later. RS is hence a more important ecological trait than hitherto assumed. It constitutes an independent route of response to severe disturbance and its ecological effects and evolutionary patterns differ from stem-based clonality.     

植物种群更新的补充限制

补充限制基于生态位理论, 从种子萌发、幼苗存活和生长、繁殖体扩散等生活史阶段的种群统计特征及环境因素(土壤水分、养分、凋落物等)着手, 探讨种群的更新问题。种源限制和微生境限制是补充限制理论研究的核心内容, 但是哪个更为重要并没有统一的结论。种源限制与种子生产、土壤种子库和地下芽库中的繁殖体数量不足有关。其中, 气候的年际波动、土壤种子库寿命和动物的捕食都会影响种子生产在种群更新中的作用; 土壤种子库常被视为种群更新的保险库, 与地上种子雨共同促进种群更新, 但是, 如果土壤里种子具有较高的死亡率和休眠率, 将会降低种子库的作用; 地下芽库及其产生的无性分株对于种群更新的意义更多地体现在干扰后种群更强的恢复能力上。扩散限制是种群更新中的普遍现象, 与种子产量、散布能力、传播媒介、幼苗密度等因素有关。微生境限制主要表现为水分、养分、凋落物等非生物因素以及竞争、捕食等生物因素对种子的活力、萌发性、幼苗的存活力、物质分配等过程的影响, 其重要性随着植物生活史阶段而发生变化。未来需要进行综合的、长期的实验, 并应着重加强种源限制及相关生态过程的进化与生态相结合的机理性研究, 从而更深刻地认识和理解种群更新问题, 建立更为综合、系统的种群更新理论体系。     

Food choice behaviour may promote habitat specificity in mixed populations of clonal and sexual Potamopyrgus antipodarum

Genetic polymorphism along an environmental gradient may be maintained if disruptive selection on habitat-specific traits leads to a correlated response in traits that reduce gene flow between habitats. We studied a short-distance cline in a population of freshwater snails Potamopyrgus antipodarum in which sexual and clonal snails coexist. Sexuals and clones show a life history cline by depth: snails reproduce at a smaller size in shallower habitats. Clones are also structured genetically across habitats and seem not to mix, even though habitats are within the dispersal distance of the snails and the opportunity for gene flow via migration must be considerable. Because habitat preference may promote divergence in both clones and sexuals along the depth gradient, we investigated whether snails show habitat-specific food choice behaviour that could reduce migration. We tested the food choice behaviour of the snails by exposing them simultaneously to food from their home and adjacent habitats. Both juvenile and adult snails from the shallow shore bank and a mid-water macrophyte habitat preferentially grazed on the vegetation of their original habitats. We suggest that the observed genetic and life history cline may be maintained by food choice behaviour that may promote a partial barrier to gene flow between the habitats. Copyright 2000 The Association for the Study of Animal Behaviour.