Defense mechanisms against grazing: a study of trypsin inhibitor responses to simulated grazing in the sedge Carex bigelowii

Trypsin inhibitors have been suggested to constitute an inducible defense in the sedge Carex bigelowii , and some former studies suggest that this might be a cause for the cyclic population dynamics in many alpine and arctic small mammals, for example lemmings ( Lemmus lemmus ). We investigated this further by using a method of simulated grazing (clipping) at different intensities, in three different habitats with varying resource availability, with different harvest times (hours after clipping), and two different stages of ramets (reproductive/vegetative) in a study from the Swedish mountain range. Our results do not indicate that C. bigelowii has an inducible defense constituted by an increase in trypsin inhibitor activity (TIA), but rather that the amount of soluble plant proteins (SPP) is lowered in wounded plants. The responses were somewhat different in the three habitats, with ramets growing in the marsh showing the highest ratio of TIA to SPP, due to low amounts of SPP. We did not find any significant effects of harvest time, or of the stage of the ramet that could support the hypothesis of an inducible defense. To conclude, we could not find any evidence for an inducible defense consisting of trypsin inhibitors in Carex bigelowii ramets, but we did find variations in the amount of SPP that may have nutritional consequences for herbivores.     

Clonal plant allocation to daughter ramets is a simple function of parent size across species and nutrient levels

The evolution of clonal growth is a widespread phenomenon among plant species, characterized by the production of genetically identical clonal fragments (ramets) via rhizomes or stolons that form an interconnected clonal organism (genet). Clonal plant species are known to differ in their investment into ramet production, and exhibit considerable variation in ramet morphology both within and among species. While patterns of resource allocation are thought to be linked to a number of plant characteristics, many analyses are limited by uncertainty in how clonal plants determine the morphology and resources allocated to new ramets. In this study, we attempted to discern what aspects of parent ramets best predicted resource allocation to new daughter ramets, and the relationship between resource allocation and daughter ramet rhizome morphology. We grew two sedge species, Schoenoplectus tabernaemontani and Eleocharis elliptica, in a greenhouse under two levels of fertilizer addition. By harvesting daughter ramets that had initiated stem production, yet remained aphotosynthetic, we were able to isolate parental investment into non-independent daughter ramets at a point where daughter ramet spacer length became fixed. Our results indicate that parent ramets allocated a non-linear proportion of parent rhizome biomass to the production of daughter ramets. Moreover, this relationship was unaffected by environmental nutrient availability. Daughter ramet biomass, in turn, was strongly correlated with daughter ramet spacer length. These observations shed light on key processes governing clonal growth in plants, and their potential application in unifying allocational and morphological perspectives to explore the fitness implications of variability in clonal growth.     

Effects of ramet clipping and nutrient availability on growth and biomass allocation of yellow nutsedge

A glasshouse experiment was conducted to examine how the interactions of nutrient availability and partial ramet clipping affect growth, reproduction and biomass allocation of Cyperus esculentus, an invasive sedge. The plants sprouting from tubers were grown at low and high nutrient levels, and were subject either to no clipping, one, two or three clippings, with each clipping cutting half of the existing ramets at soil level. Our results show that nutrient availability and clipping frequency tended to independently affect most of growth, reproduction and biomass allocation parameters of Cyperus esculentus examined in the present study. Increased supply of nutrients led to an increase in plant productivity and its associated traits. All of the traits, except for the number of ramets, displayed a decreasing pattern with increasing clipping frequency, indicating that Cyperus esculentus had undercompensatory responses to ramet clipping. It is likely that the patterns of plants response to clipping are species specific, and depend on morphological characters of species. Its susceptibility to ramet clipping can offer opportunities for controlling this invasive species through mechanical methods such as mowing. Clipping had little effects on biomass allocation; however, root weight fraction increased with increasing clipping frequency. While nutrient availability and clipping frequency had no influence on leaf carbon concentration at harvest, both of them increased leaf nitrogen concentration, and hence reduced leaf C/N ratio.     

Long-term population dynamics of two Carex curvula species in the Central Alps on native and alien soils

The demography of two closely related alpine sedges, Carex curvula subsp. curvula and Carex curvula subsp. rosae (=C. curvula and C. rosae) has been investigated on their typical sites in the Central Alps. Both species proliferate vegetatively and develop dense tussocks but they show different dominance behaviours in their respective grasslands. It was hypothesized that this may be caused by different growth abilities. The main aim of the study was to compare the vegetative growth of the species under field conditions, under competition-free conditions and under changed soil conditions. An attempt was also made to clarify whether vegetative growth is density dependent. Permanent plots were established in the respective grasslands of the two species and the ramet density was counted over 3 years. Groups of 10 and of 30 ramets of each species were grown in pots with typical and with alien substrate and their growth was observed for 5 years at the field site. The grassland populations of both species were very stable and the overall ramet growth rate (λ) was close to 1.0. Within the pots, both species reached a high ramet number. Only the group of 30 ramets of C. curvula on alien soil could not recover from the transplantation shock. Within the pots, C. rosae showed a greater ramet turnover and a higher increase in ramets than C. curvula. On their native substrate, both species had a significantly higher ramet increase than on the alien substrate. Ramet growth was found to be density dependent for both species, the increases recorded for the groups of 10 being significantly greater than for the groups of 30. Although C. curvula produced fewer ramets than C. rosae, the aboveground dry weight of the former was significantly higher. This may be decisive for its greater competitive success in closed grasslands. Received: 12 April 1997 / Accepted: 12 December 1997     

Effects of simulated climate change on phenology and life history traits in Carex bigelowii

Climate change may have large effects on plants, especially in the Arctic. At two different sites, we studied the effects of enhanced temperature by using open-top chambers (OTCs) on the clonal sedge Carex bigelowii , a common plant in arctic and alpine tundra. At the subarctic-alpine site Latnjajaure, northern Sweden, overall flowering phenology was accelerated by open-top chambers (OTCs) during the five years of treatment. For this protogynous population, male flower phenology accelerated more than female flower phenology, which resulted in an increased gender phase overlap. Sexual reproductive effort at the ramet-level increased, both in male and female functions. Smut fungus incidence was not different among treatments, but a Dipteran seed predator attacked more ramets in the OTCs in one of five years. However, neither the fungus nor the seed predators affected plant growth or seed set measurably. Ramet-level growth increased in the OTCs at Latnjajaure, but decreased in the OTCs at the maritime subarctic site Thingvellir, Iceland after three years of treatment. At Latnjajaure, the initial ramet- level responses were still evident after five years, while responses at the ramet population level disappeared: there was no difference in flowering frequency of ramets in the fifth year of the warming treatment. This is interpreted as either meristem limitation or internal resource depletion. At Thingvellir flowering frequency was unaffected by warming treatment, while ramet production decreased. Ramet production was unaffected by the treatment at Latnjajaure. These site differences in responses could be the consequences of differences in climate or site specific conditions. The trade-off (negative correlation) between the number and size of vegetative offspring at Latnjajaure disappeared under enhanced temperatures (within the OTCs). This trade-off was not apparent under the warmer conditions at Thingvellir.     

Ramet dynamics in a centrifugally expanding clonal sedge: a matrix analysis

Carex humilis is a clonal sedge that can form distinct rings of densely aggregated ramets. We hypothesize that rings form because both production of new ramets and ramet dispersal are positively correlated to ramet size. This would lead to an overrepresentation of fast-moving and large ramets with high ramet production at the periphery, whereas slow-moving and small ramets with low ramet production would mainly be found in the interior of rings. We use matrix models to analyse how ramet populations both at the periphery and in the interior develop in the absence of ramet dispersal. We found that the stable size class distributions of ramets predicted by the models were not different from the distributions found in the field. Also, the asymptotic ramet population growth rates (λ₁) were the same. Hence, we conclude that rings would form even in the absence of a link between ramet dispersal and ramet production. Further analysis of the matrix models showed that the ramet population increases at the periphery but decreases in the interior of rings because medium and large ramets produce fewer large ramets in the interior than at the periphery. We also found that the temporal variance in λ₁ and transitions rates during the four study years was much higher at the periphery than in the interior. Our results suggest that rings may form because C. humilis ramets use below-ground resources from a much larger area than the one covered by the shoots. As the clone grows larger, the soil volume available to the ramets in the interior decreases because their access to soil outside the ring is cut-off by the ramets at the periphery. Ramet density in the interior is therefore decreasing.     

Strawberry plant relationship through the stolon

Stolon is an elongated, two-node, vegetative, axillary shoot, which supports the ramet (rooted rosette) until it is completely independent on its own roots. The reciprocal capacity of the ramets, in a single runner chain, to sustain the growth and share locally abundant resources or to tolerate a local stress, is still in debate. This capacity may play an important role for improving nursery plant production and for better understanding the natural clonal multiplication. To describe strawberry stolon action, in plant-to-plant relationship, bare-rooted Camarosa ramets, joint in couples by their own stolons (generally, second and third ramet in a runner chain) were transplanted in two pots. The couples of ramets were treated in a factorial experiment with decortication (peeling a 2-mm ring of bark from the stolon), removal of root system or glyphosate application to one of the two ramets. In the studied system, the older ramet was referred as mother and the other as daughter. The two ramets were very similar in age and seem to act with a very limited hierarchic prevalence of the mother. When the root system of one ramet was eliminated, leaf number and chlorophyll content had a very slight decrease, independently in the mother ramet or in the daughter. The decortication did not reduce water integration, in any group of plants, but limited assimilate allocation towards the daughter ramet when the mother ramet had a severe root cut (not vice versa). The glyphosate action resulted localized in the sprayed ramet, which reduced chlorophyll content within 2 days and expired after 4 days.     

扎龙湿地不同生境芦苇分株的形态特征及生物量分配的异速关系

不同环境条件下的植物个体可以表现出形态特征的分异和物质分配的权衡与调整。采用大样本抽样调查与统计分析方法,比较研究扎龙湿地不同生境芦苇(Phragmites Australis)生殖株和营养株的形态特征以及生物量分配的异速关系。结果表明：在9月末,盐碱生境、旱生生境、湿生生境和水生生境芦苇分株的生长表现出较大的生态可塑性,株高和株重均以盐碱生境最小,水生生境最大,最大值与最小值的比值分别为1.3—3.3和1.8—5.1,分株生长在种群间的变异度高于种群内,与营养株相比,生殖株的变异度较低;分株的支持分配与生产分配的比值为1.8—4.2,生产分配以盐碱生境最高,以水生生境最低,而支持分配和生殖分配表现与生产分配相反的序位;生殖株的花序长和花序重与株高间呈直线函数形式增长,株高和株重低于种群平均值的20%和35%的分株不进行有性生殖;叶重、叶鞘和茎重以及分株重与株高间呈幂函数形式的异速生长关系。植物通过改变个体的形态特征以及调整构件间生物量分配适应不同环境,而受遗传因素控制的构件间生长关系却相对稳定。     

Effects of inoculation with native arbuscular mycorrhizal fungi on clonal growth of Potentilla reptans and Fragaria moschata (Rosaceae)

Many clonal plants live in symbiosis with ubiquitous arbuscular mycorrhizal (AM) fungi, however, little is known about their interaction with respect to clonal reproduction and resource acquisition. The effects of arbuscular mycorrhiza on the growth and intraclonal integration between ramets of two stoloniferous species were studied experimentally in a nutritionally homogenous soil environment. Two species coexisting at the same field site, Potentilla reptans and Fragaria moschata, were selected as model plants for the study. Pairs of their ramets were grown in neighbouring pots with each ramet rooted separately. Four inoculation treatments were established: (1) both mother and daughter ramets remained non-inoculated, (2) both ramets were inoculated with a mixture of three native AM fungi from the site of plant origin, (3) only mother or (4) daughter ramet was inoculated. The stolons connecting the ramets were either left intact or were disrupted. Despite the consistent increase in phosphorus concentrations in inoculated plants, a negative growth response of both plant species to inoculation with AM fungi was observed and inoculated ramets produced fewer stolons and fewer offspring ramets and had lower total shoot dry weights as compared to non-inoculated ones. A difference in the extent of the negative mycorrhizal growth response was recorded between mother and daughter ramets of P. reptans, with daughter ramets being more susceptible. Due to AM effect on ramet performance, and thereby on the source-sink relationship, inoculation also significantly influenced biomass allocation within clonal fragments. Physiological integration between mother and daughter ramets was observed when their root systems were heterogeneous in terms of AM colonization. These results hence indicate the potential of mycorrhizal fungi to impact clonal growth traits of stoloniferous plant species, with possible consequences for their population dynamics.     

Effects of the loss of clonal integration on four sedges that differ in ramet aggregation

Although clonal growth is a dominant mode of plant growth in wetlands, the importance of clonal integration, resource sharing among ramets, to individual ramet generations (mother and daughter) and entire clones of coexisting species has not been well investigated. This study evaluated the significance of clonal integration in four sedge species of varying ramet aggregations, from clump-forming species (Clumpers –Carex sterilis, Eleocharis rostellata), with tightly aggregated ramets (rhizomes<1cm), to runner species (Runners –Schoenoplectus acutus, Cladium mariscoides), with loosely aggregated ramets. We manipulated clonal integration by either severing connections between target mother and daughter ramets or leaving connections intact, and then planted them in an intact neighborhood of a fen in Michigan, USA. We measured growth parameters of original and newly produced ramets over two growing seasons and conducted a final biomass harvest, to address four hypotheses. First, we expected integrated clones to accumulate more biomass than severed clones. However, final clone-level biomass and ramet production were the same for both treatments in all species although severing initially stimulated ramet production by Schoenoplectus and produced a more compact ramet aggregation in Cladium. Second, we hypothesized that mother ramets would experience a cost of integration, through reduced ramet or biomass production, while daughters would experience a benefit, through increased resource availability from mothers. Mother ramets of Cladium suffered a cost from integration, while Schoenoplectus mothers suffered a slight cost and Carex daughters saw a slight benefit. Finally, we hypothesized that integration would be more active in runner species than in clumper species. Indeed, we documented more active integration in runners than clumpers, but none of the study species were dependent upon integration for growth or survival once daughter ramets were established with their own roots and shoots. This study demonstrates that integration between established ramets may not be the most important advantage to clonal growth in this wetland field site. The loss of integration elicited varied responses among coexisting species in their natural habitat, somewhat but not completely related to their growth form, suggesting that a combination of plant life history traits contributes to the dependence upon clonal integration among established ramets of clonal species.     

青藏高原东缘过路黄在资源交互斑块性生境中的克隆内资源共享

在青藏高原和四川盆地过渡带,分别于618m和1800m两个海拔高度上研究匍匐茎克隆植物过路黄(Lysimachia christinae)在资源交互斑块性生境中的克隆内资源共享及其对生长的影响.结果显示, 在海拔1800m处,与资源的空间同质性处理(Ⅰ) 和(Ⅱ)相比, 资源的空间异质性处理(Ⅲ)和(Ⅳ)下过路黄整个克隆片段的生物量和分株数均获得显著增加;在海拔618m处,与资源的空间同质性处理(Ⅰ) 和(Ⅱ)相比,资源的空间异质性处理(Ⅲ)和(Ⅳ)下过路黄整个克隆片段生物量显著增加.在海拔618m和1800m处,生长在低光高养条件下的远端分株, 若与高光低养的近端分株相连, 相比连接到低光高养的近端分株, 它们分配更多的生物量到地下部分;在海拔1800m处,生长在高光低养条件下的远端分株, 若与低光高养的近端分株相连, 相比连接到高光低养的近端分株, 它们分配更多的生物量到地上部分.在海拔618m和1800m处,生长在高光低养条件下的近端分株, 若与低光高养的远端分株相连, 相比连接到高光低养的远端分株, 它们分配更多的生物量到地上部分.处于资源交互斑块性生境中的过路黄发生了克隆内分工,依靠相连分株间的功能分化, 克隆植物能有效的利用异质性分布的资源, 缓解资源交互斑块性分布对克隆植物生长的不利影响.通过间隔子(匍匐茎或根状茎),相连分株间能够相互传递和共享由不同分株获得的资源,这种资源共享能够提高克隆植物在异质性生境中的存活与生长.同时,方差分析显示环境异质性和海拔的交互作用显著影响克隆片段的生物量和分株数.相比于海拔618m,在海拔1800m处克隆内资源共享对克隆植物生长表现的影响更大.     

Optimal allocation to vegetative and sexual reproduction in plants: the effect of ramet density

Vegetative reproduction is a very common alternative by which plants can contribute to the next generations. There are many considerations predicting which mode of reproduction, vegetative or sexual, should be favored and numerous experimental studies to verify them. However, the results are inconsistent especially when the effect of plant density is considered. I apply here a dynamic optimization model to predict the rate of vegetative and sexual reproduction in plants as a response to changes in the local plant density. The population is assumed to occupy a heterogeneous environment consisting of patches in which growth and reproduction of plants are possible and unfavorable space between them. As the environment is globally stable, the seeds, which can disperse without restriction, exhibit a constant recruitment rate. The ramets are assumed to settle only within the patch of the mother plant. The rate of ramet production effects local density, which in turn determines ramet recruitment. The optimal strategy maximizes the expected lifetime genetic contribution, realized via both vegetative and sexual reproduction. The solutions obtained under these assumptions are dualistic. The model predicts that different approaches applied in studying the effect of ramet density should give opposite outcomes. When the comparison is between patches in natural populations, a positive relationship between relative ramet allocation and density is expected. When the density is experimentally manipulated or its effect is analyzed across different successional stages, a negative relationship should be found. The results seem to be confirmed by empirical studies.     

Effects of clonal integration on plant plasticity in Fragaria chiloensis

The ability of clonal plants to transport substances between ramets located in different microsites also allows them to modify the plastic responses of individual ramets to local environmental conditions. By equalising concentrations of substances between ramets, physiological integration might decrease responses to local conditions. However, integration has also been observed to increase plasticity and induce novel plastic responses in ramets. To ask how integration modifies plant plasticity in the clonal herb, Fragaria chiloensis, ramets were given either low light and high nitrogen or high light and low nitrogen, simulating a pattern of resource patchiness in their native habitat. Ramets in contrasting light/nitrogen treatments were either connected or single. Effects of light/nitrogen and connection were measured at three levels of morphological organisation, the organ, the ramet, and the clonal fragment. Connection between ramets reduced or had no effect on plastic responses in leaf size at the level of the plant organ. This suggested that integration dampened certain plastic responses. Connection induced a new plastic response at the level of the clonal fragment, an increase in allocation to vegetative reproduction in patches of low light and high nitrogen. It is concluded that clonal integration can have different effects on plant plasticity at different levels of plant organisation. It appears that, at least in this species, integration can increase plasticity at the level of the clonal fragment and concentrate vegetative reproduction in particular microsite types.     

松嫩平原全叶马兰种群不同龄级分株的生长策略

全叶马兰为根蘖性多年生草本菊科植物,种群由3个龄级无性系分株组成,在松嫩平原6月份,是全叶马兰营养生长的旺盛期,研究结果表明,分株的生长与生产力以1龄级最低,2、3龄级依次增高,3个龄级分株的生物量分配均为茎>叶>主根,叶的生物量分配为1龄级>2龄级>3龄级,茎和主根的均为1龄级<2龄级<3龄级,3个龄级分株总重与分株高度、分株叶重与茎重、分株主根重与枝条重之间均为幂函数异速生长规律,主根的生物量分配则随着分株高度的增加呈线性减少,全叶马兰无性系分株的生长与分配策略是在分株幼小时将较多的物质分配给叶,以保证物质生产器官叶的建造上,随着分株生长逐渐将生产的物质分配到茎和主根的生长与贮藏;分株不向主根投入较多的能量,但在分株幼小时向主根中分配的物质相对较多,随着分株的生长则按比例较小,并保持相对稳定的分配比率     

Clonal Transgenerational Effects Transmit for Multiple Generations in a Floating Plant

Environmental conditions of a parent plant can influence the performance of their clonal offspring, and such clonal transgenerational effects may help offspring adapt to different environments. However, it is still unclear how many vegetative generations clonal transgenerational effects can transmit for and whether it depends on the environmental conditions of the offspring. We grew the ancestor ramets of the floating clonal plant Spirodela polyrhiza under a high and a low nutrient level and obtained the so-called 1^st-generation offspring ramets of two types (from these two environments). Then we grew the 1^st-generation offspring ramets of each type under the high and the low nutrient level and obtained the so-called 2^nd-generation offspring ramets of four types. We repeated this procedure for another five times and analyzed clonal transgenerational effects on growth, morphology and biomass allocation of the 1^st- to the 6^th-generation offspring ramets. We found positive, negative or neutral (no) transgenerational effects of the ancestor nutrient condition on the offspring of S. polyrhiza, depending on the number of vegetative generations, the nutrient condition of the offspring environment and the traits considered. We observed significant clonal transgenerational effects on the 6^th-generation offspring; such effects occurred for all three types of traits (growth, morphology and allocation), but varied depending on the nutrient condition of the offspring environment and the traits considered. Our results suggest that clonal transgenerational effects can transmit for multiple vegetative generations and such impacts can vary depending on the environmental conditions of offspring.     

Hydraulic conductivity and PAT determine hierarchical resource partitioning and ramet development along Fragaria stolons

Co-ordination of metabolic and physiological activity between plant parts is key to the control of growth and development. Here the movement of resources and their allocation between mother plants and daughter ramets along Fragaria stolons was quantified with respect to hierarchy. Gradients of internodal ramet leaf water potential (ψ) and stolon and ramet hydraulic conductivities (L) were measured together with apparent stolon IAA movement via the polar auxin transport pathway (PAT). These processes are linked with measurements of stolon vascular development. The pattern of tissue differentiation and lignification in sequential stele sections of stolons demonstrated the rapid acquisition of the capacity for water transport, with transpiration potentially varying systematically with stolon lignification and the acropetal decline in stolon xylem ψ. Stolon and ramet L declined acropetally, with L across older ramets being significantly lower than that of the connecting stolons. The capacity for polar IAA transport increased with stolon age; this was due to increased transport intensity in older tissues. The partitioning of dry matter was strongly hierarchical with younger ramets smaller than older ramets, while foliar concentrations of N, P, and K were greater for the younger ramets. The results show that stolon anatomy develops rapidly at the apical end, facilitating hierarchical ramet development, which is evident as a basipetal increase in L. The rapid development of transport tissue functionality enables young unrooted ramets to acquire water, in order to supply an expanding leaf area, as well as mineral ions disproportionally with respect to older ramets. This facilitates colonization and self-rooting of apical ramets. The unidirectional increase in basipetal PAT along stolons facilitates hierarchical ramet development.     

竹类植物对异质生境的适应——表型可塑性

竹类植物是一类以木本为主的克隆植物,凭借表型可塑性的优势,对异质生境具有很强适应能力。然而,目前对竹类植物表型可塑性的实现方式及其异质生境适应对策未见系统总结,从而在一定程度上限制了竹类生态学的发展。从形态可塑性、选择性放置、克隆整合和克隆分工等4个方面对竹类植物的表型可塑性研究进行分析和梳理,结果表明:竹类植物在异质生境中具有明显的表型可塑反应,主要采用形态可塑性、选择性放置和克隆整合来适应异质生境,而克隆分工的普遍性仍有待验证;目前侧重于研究构件形态和生物量分配格局,而很少深入探讨形态、生理和行为等可塑性机理。今后竹类植物表型可塑性研究重点在于:1)克隆整合的格局与机理;2)克隆整合对生态系统的影响;3)克隆分工的形成及其与环境关系;4)表型可塑性的等级性及环境影响;5)不同克隆构型的表型可塑性特征及其内在机制。     

Clonal plasticity in response to rhizome severing and heterogeneous resource supply in the rhizomatous grass Psammochloa villosa in an Inner Mongolian dune, China

In a field experiment, Psammochloa villosa plants were subjected to rhizome severing. Severing rhizomes reduced growth in the young, detached rhizome segments compared to the controls in terms of all measured clonal growth-related characters, i.e. number of rhizomes and shoots, total rhizome length and total number of rhizome nodes. In a container experiment, the control ramets received uniform water and nutrient supply but in heterogeneous treatments high and low levels of water and nutrient supply, respectively were established. The number of ramets, total rhizome length, dry weight per ramet and biomass allocation to the rhizome had higher values at high water and nutrient supply, while spacer length (length of rhizome between shoots) and rhizome internode length were not affected. The local response of ramets given low water supply was enhanced due to connection to a well watered parent ramet in terms of number of ramets, total rhizome length and dry weight per ramet. A remote effect was not observed in the other treatments or in the other measured characters.     

Evolutionarily stable allocation to vegetative and sexual reproduction in plants

I apply the resource allocation approach to construct an ESS model aimed at predicting the rate of vegetative and sexual reproduction in plants. The model provides an integrated explanation of why the mode of reproduction should vary between populations and species, and what factors should affect the trade-off between ramet and seed production. It follows that seed production, usually considered to be less effective and more costly than ramet production, may be maintained in the population. The production of ramets should be favored when their recruitment rate is high or the costs are low. The intensity of vegetative reproduction is also related to conditions which affect seed production directly. The rate of ramet production should be inversely correlated with factors favoring sexual reproduction, such as lower cost of seeds or high opportunity for colonization of new patches, etc. Under such conditions it is beneficial to allocate more resources available for reproduction into seeds, at the expense of ramet production. The model emphasizes the need to consider both modes of plant reproduction whenever one of them is theoretically or empirically studied.     

太白山独叶草无性系分株营养元素含量对异质环境的响应

以分布在太白山国家森林公园3个海拔高度的游击型克隆植物独叶草为研究对象,分析测定了不同分株营养器官中C、N、P、K和Mg含量.结果表明：分株年龄、海拔梯度以及分株年龄与海拔梯度的交互作用对独叶草分株各营养器官全碳含量的影响不显著(P>0.05),对全氮、全磷、K和Mg含量的影响程度各不相同,说明独叶草无性系分株通过自身的生理代谢调整了各器官中营养元素的分配.在高海拔生境中,分株年龄显著影响独叶草无性系分株各器官的K含量、根状茎和根中的全氮含量(P<0.05).除了中海拔(2 800～2 900 m) 独叶草无性系分株的K与全氮、全磷和Mg含量相关性不明显外,其他海拔各元素两两间均呈显著相关(P<0.05).这表明高海拔生境中生理可塑性对克隆植物生长的贡献可能比低海拔更大,强的生理可塑性有助于克隆植物在高山环境中有效地利用资源,实现种群扩展、开拓新的生境.这也可能是独叶草在长期演化过程中对异质生境的一种生理响应.