Convergence patterns and multiple species interactions in a designed plant mixture of five species

It is known that convergence and divergence can occur in complex plant communities, but the relative importance of biotic and abiotic factors driving these processes is less clear. We addressed this issue in an experiment using a range of mixed stands of five species that are common in Swiss fens (Carex elata, C. flava, Lycopus europaeus, Lysimachia vulgaris and Mentha aquatica) and two levels of water and nutrients. One hundred and seventy-six experimental mixtures were maintained in large pots (75 l) for two consecutive growing seasons in an experimental garden. The stands varied systematically in the initial relative abundance of each of the five species and in overall initial stand abundance. The changes in biomass over 2 years were modelled as linear functions of treatments and the initial biomass of each species. The dynamics of the system were mainly driven by differences in the identity of species and by a negative feedback mechanism but also by different abiotic conditions. In all mixtures, C. elata became more dominant over time, which caused an overall convergence of community composition. In addition, the rate of change of each species’ biomass was negatively related to its own initial abundance. Thus, a negative feedback further contributed to the convergence of communities. Species responded differently to water level and nutrient supply, causing community dynamics to differ among treatments. However, the different abiotic conditions only slightly modified the overall convergence pattern. Competitive interactions between more than two species were weaker than the negative feedback but still significantly influenced the species’ final relative abundance. The negative feedback suggests that there is niche partitioning between the species, which permits their coexistence. Electronic supplementary material The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

Within-species variation among populations of the Carex flava complex as a function of habitat conditions

Background: Variable habitat conditions contribute to morphological variability that plays a substantial part in evolution of plants. Understanding the extent of phenotypic plasticity has important implication for assigning individuals to taxa.

Aims: I analysed morphological variability among populations within species of the Carex flava group and tested to what extent morphological variability was associated with habitat conditions.

Methods: A total of 571 specimens from 20 populations of four species from the C. flava complex were sampled in Poland and tested by Discriminant Function Analysis (DFA). The relationship between morphological characters and habitat factors was explored by means of the redundancy analysis (RDA).

Results: Variability of the generative and vegetative traits was fairly similar in Carex lepidocarpa, C. flava and Carex demissa, while it was somewhat higher in Carex viridula. The morphological traits were mostly related to soil organic matter content, calcium and carbonate, as well as to habitat fertility (C:N ratio), elevation and habitat disturbance. The results obtained do not support the separation of C. viridula var. pulchella from C. viridula var. viridula.

Conclusions: Phenotypic variability in the species of the C. flava complex is related to habitat conditions and this can lead to the differentiation of morphotypes within species.     

Plasticity of seed output in response to soil nutrients and density in Abutilon theophrasti: implications for maintenance of genetic variation

 Seed output is determined by two processes: resource acquisition and the allocation of resources to seeds. In order to clarify how the reaction norm of seed output is controlled by the phenotypic expression of its two components, we examined the genetic components of plasticity of seed dry mass, plant size, and reproductive allocation under different conditions of soil nutrient availability and conspecific competition among eight families of Abutilon theophrasti. Without competition, the reaction norm of seed mass of the families crossed between the lowest and other nutrient levels, although neither of its components, plant size and reproductive allocation, showed such a response. The crossing reaction norm (i.e., reversal of relative fitnesses of different genotypes along the environmental gradient) of seed mass resulted from (1) a trade-off between plant size and reproductive allocation, and (2) changes in the relative magnitude of genetic variances in plant size and reproductive allocation with soil nutrient availability. While allocation was more important in determining seed mass under limiting nutrient conditions, plant size became more important under high-nutrient conditions. There were no significant genetic variances in seed mass, plant size, and reproductive allocation in the competition treatment, except at the highest nutrient level. The results show that plant competition mitigated the effects of genetic differences in plant performance among the families. We discuss the results in relation to maintenance of genetic variation within a population. Received: 16 August 1996 / Accepted 26 April 1997     

Responses of Fraxinus excelsior seedlings to grass-induced above- and below-ground competition

The competitive interactions between woody seedlings and herbaceous vegetation have received increasing interest in recent years. However, little is known about the relative contributions and underlying mechanisms of above- and below-ground competition between species. We used a novel experimental approach to assess the responses of Fraxinus excelsior seedlings to different combinations of root and shoot competition imposed by the grass Dactylis glomerata under greenhouse conditions. Seedling growth was significantly reduced by competition for soil resources, but neither biomass nor height were significantly affected by shoot competition for light. Competitive response indices based on biomass confirmed that below-ground competition was more important than above-ground competition, and indicated that root and shoot competition did not interact to influence plant growth. Fraxinus biomass allocation and seedling traits were almost all significantly affected by root competition; these responses varied depending on the trait examined. In contrast, morphological responses to shoot competition were limited. In the absence of root competition, seedlings showed a significant increase in the biomass allocated to leaves and a greater leaf area ratio in response to shoot competition. Our findings suggest that morphological modifications help to mitigate the negative effects of competition, but the expression of plasticity may be suboptimal due to resource constraints. The present study also highlights the importance of appropriate experimental controls and analysis to avoid confounding effects of experimental design and ontogeny on the interpretation of competitive responses.     

Effect of minor water depth treatments on competitive effect and response of eight wetland plants

Two facets of plant competition, competitive effect (CE) and competitive response (CR), can be used to explain plant community composition but our understanding of abiotic factors that may differentially affect species’ competitive ability is incomplete. We tested whether water-depth affected CE (ability to suppress neighbour) and CR (avoid suppression from neighbour), and if so whether there was consistence in the rank order of both measures of competition under different water depth treatments. CE and CR were measured and compared for eight wetland plant species (Carex lurida, Carex tribuloides, Elymus virginicus, Juncus tenuis, Lythrum salicaria, Phalaris arundinacea, Rumex orbiculatus and Verbesina alternifolia) at five different water-depth treatments (+2, 0, −2, −4 and −6 cm relative to the substrate). Overall, we found that mean CE was at its lowest value at +2 cm water depth, while mean CR was highest at +2 and −6 cm compared to the other water treatments. There was a significant variation of CE between species, with a defined hierarchical order. Pairwise CE rank order correlations between water depth treatments were significant but CR correlations were generally not. There was no significant correlation between CE and CR. CE was significantly correlated with biomass of species grown alone but CR was not. These findings indicate that CE may be used as a general measure to predict wetland species performance, and thus community assemblage, across a range of water depths. CR does not seem to demonstrate predicable patterns between species and water depth treatments. Our results suggest that competition intensity may be reduced in a non-resource-stressed flooded environment by a reduction in CE, but the corresponding increase in CR could dampen this effect on overall competitive ability.     

Biomass allocation in old-field annual species grown in elevated CO2 environments: no evidence for optimal partitioning

Increased atmospheric carbon dioxide supply is predicted to alter plant growth and biomass allocation patterns. It is not clear whether changes in biomass allocation reflect optimal partitioning or whether they are a direct effect of increased growth rates. Plasticity in growth and biomass allocation patterns was investigated at two concentrations of CO₂ ([CO₂]) and at limiting and nonlimiting nutrient levels for four fast‐ growing old‐field annual species. Abutilon theophrasti, Amaranthus retroflexus, Chenopodium album, and Polygonum pensylvanicum were grown from seed in controlled growth chamber conditions at current (350 μmol mol^?1, ambient) and future‐ predicted (700 μmol mol^?1, elevated) CO₂ levels. Frequent harvests were used to determine growth and biomass allocation responses of these plants throughout vegetative development. Under nonlimiting nutrient conditions, whole plant growth was increased greatly under elevated [CO₂] for three C3 species and moderately increased for a C4 species (Amaranthus). No significant increases in whole plant growth were observed under limiting nutrient conditions. Plants grown in elevated [CO₂] had lower or unchanged root:shoot ratios, contrary to what would be expected by optimal partitioning theory. These differences disappeared when allometric plots of the same data were analysed, indicating that CO₂‐induced differences in root:shoot allocation were a consequence of accelerated growth and development rates. Allocation to leaf area was unaffected by atmospheric [CO₂] for these species. The general lack of biomass allocation responses to [CO₂] availability is in stark contrast with known responses of these species to light and nutrient gradients. We conclude that biomass allocation responses to elevated atmospheric [CO₂] are not consistent with optimal partitioning predictions.     

Individual flowering phenology shapes plant–pollinator interactions across ecological scales affecting plant reproduction

The balance of pollination competition and facilitation among co-flowering plants and abiotic resource availability can modify plant species and individual reproduction. Floral resource succession and spatial heterogeneity modulate plant–pollinator interactions across ecological scales (individual plant, local assemblage, and interaction network of agroecological infrastructure across the farm). Intraspecific variation in flowering phenology can modulate the precise level of spatio-temporal heterogeneity in floral resources, pollen donor density, and pollinator interactions that a plant individual is exposed to, thereby affecting reproduction. We tested how abiotic resources and multi-scale plant–pollinator interactions affected individual plant seed set modulated by intraspecific variation in flowering phenology and spatio-temporal floral heterogeneity arising from agroecological infrastructure. We transplanted two focal insect-pollinated plant species (Cyanus segetum and Centaurea jacea, n = 288) into agroecological infrastructure (10 sown wildflower and six legume–grass strips) across a farm-scale experiment (125 ha). We applied an individual-based phenologically explicit approach to match precisely the flowering period of plant individuals to the concomitant level of spatio-temporal heterogeneity in plant–pollinator interactions, potential pollen donors, floral resources, and abiotic conditions (temperature, water, and nitrogen). Individual plant attractiveness, assemblage floral density, and conspecific pollen donor density (C. jacea) improved seed set. Network linkage density increased focal species seed set and modified the effect of local assemblage richness and abundance on C. segetum. Mutual dependence on pollinators in networks increased C. segetum seed set, while C. jacea seed set was greatest where both specialization on pollinators and mutual dependence was high. Abiotic conditions were of little or no importance to seed set. Intra- and interspecific plant–pollinator interactions respond to spatio-temporal heterogeneity arising from agroecological management affecting wild plant species reproduction. The interplay of pollinator interactions within and between ecological scales affecting seed set implies a co-occurrence of pollinator-mediated facilitative and competitive interactions among plant species and individuals.     

Physiological and growth responses of Centaurea maculosa (Asteraceae) to root herbivory under varying levels of interspecific plant competition and soil nitrogen availability

Summary Centaurea maculosa seedlings were grown in pots to study the effects of root herbivory by Agapeta zoegana L. (Lep.: Cochylidae) and Cyphocleonus achates Fahr. (Col.: Curculionidae), grass competition and nitrogen shortage (each present or absent), using a full factorial design. The aims of the study were to analyse the impact of root herbivory on plant growth, resource allocation and physiological processes, and to test if these plant responses to herbivory were influenced by plant competition and nitrogen availability. The two root herbivores differed markedly in their impact on plant growth. While feeding by the moth A. zoegana in the root cortex had no effect on shoot and root mass, feeding by the weevil C. achates in the central vascular tissue greatly reduced shoot mass, but not root mass, leading to a reduced shoot/root ratio. The absence of significant effects of the two herbivores on root biomass, despite considerable consumption, indicates that compensatory root growth occurred. Competition with grass affected plant growth more than herbivory and nutrient status, resulting in reduced shoot and root growth, and number of leaves. Nitrogen shortage did not affect plant growth directly but greatly influenced the compensatory capacity of Centaurea maculosa to root herbivory. Under high nitrogen conditions, shoot biomass of plants infested by the weevil was reduced by 30% compared with uninfested plants. However, under poor nitrogen conditions a 63% reduction was observed compared with corresponding controls. Root herbivory was the most important stress factor affecting plant physiology. Besides a relative increase in biomass allocation to the roots, infested plants also showed a significant increase in nitrogen concentration in the roots and a concomitant reduction in leaf nitrogen concentration, reflecting a redirection of the nitrogen to the stronger sink. The level of fructans was greatly reduced in the roots after herbivore feeding. This is thought to be a consequence of their mobilisation to support compensatory root growth. A preliminary model linking the effects of these root herbivores to the physiological processes of C. maculosa is presented.     

Size-dependent sex allocation in a monocarpic perennial herb, <Emphasis Type="Italic">Cardiocrinum cordatum</Emphasis> (Liliaceae)

The size-dependent sex allocation model predicts that the relative resource allocation to female function often increases with plant size in animal-pollinated plants. If size effects on reproductive success vary depending on the environmental conditions, however, the size dependency may differ among populations. We tried to detect site-specific variation in size-dependent sex allocation of a monocarpic hermaphrodite with reference to light availability. Multiple flowers and fruits were sampled from the individuals of Cardiocrinum cordatum, a monocarpic understory herb, and pollen, ovule and seed production were measured with reference to the plant size in two populations. Furthermore, frequency and foraging behavior of pollinator visitation was observed. Ovule production per flower increased with plant size in both populations, while pollen production per flower increased with size only in the population under sparse canopy. Therefore, proportional allocation to male function decreased with plant size in the population under closed canopy, but did not change in the population under sparse canopy. Pollinators usually visited only one flower per plant, indicating the negligible geitonogamous pollination in this species. Although seed production under closed canopy was lower than that under sparse canopy, seed-set rate per flower and seed mass per fruit were independent of plant size in either of the populations. Size-dependent sex allocation in this species was site-specific, suggesting that not only resource storage before reproduction (i.e., plant size) but also resource availability of environment throughout the reproductive process (i.e., light availability) affect reproductive performance in this species.     

<Emphasis Type="Italic">Acer rubrum</Emphasis> (red maple) growth is negatively affected by soil from forest stands dominated by its invasive congener (<Emphasis Type="Italic">Acer platanoides</Emphasis>, Norway maple)

Invasive species continue to alter the plant communities of the eastern United States. To better understand the mechanisms and characteristics associated with invasive success, we studied competition between two Acer species. In a greenhouse, we tested (1) the effect of forest soil type (beneath an invasive and native stand) on seedling growth of the invasive Acer platanoides (Norway maple) and native A. rubrum (red maple), and the (2) effects of full (above- and below-ground) and partial inter-specific competition on species growth. We found A. rubrum growth was negatively affected by soil from the invaded stand, as it had lower above-ground (32%) and below-ground (26%) biomass, and number of leaves (20%) than in the native soil. The root:shoot resource allocations of A. platanoides depended on soil type, as it had 14% greater root:shoot mass allocation in the native soil; this ability to change root:shoot allocation may be contributing to the ecological success of the species. Widely published as having a large ecological amplitude, A. rubrum may be a useful species for ecological restoration where A. platanoides has been present, but the impacts of A. platanoides on soil functioning and subsequent plant interactions must be addressed before protocols for native reintroductions are improved and implemented.     

Regulation of dauciform root formation and root phosphatase activities of sedges (<Emphasis Type="Italic">Carex</Emphasis>) by nitrogen and phosphorus

Aims

Dauciform roots (DR) are formed by some Cyperaceae under phosphorus (P) deficiency. To advance our understanding of their physiological function, I ask: Is DR formation regulated by shoot P status or external P supply? How does it respond to nitrogen (N)? Do DR enhance root monoesterase, diesterase or phytase activities and ability to utilize organic P?

Methods

Greenhouse experiments were carried out with two Carex species grown in sand with (1) different combinations of N and P supply, (2) local supply of N or P to root halves, and (3) different organic P forms.

Results

Carex flava produced DR in all treatments. The density of DR and phosphatase activities increased with N supply; they were regulated by shoot P status and external N (but not P) supply. All phosphatase activities increased with DR density. Carex muricata produced no DR and had lower diesterase activity than C. flava but both species grew equally well with diester-P.

Conclusions

DR and phosphatase activities are regulated by both N and P supply. Similar growth responses to nutrients in both species suggest small costs and benefits of DR under experimental conditions but confirmation is needed for plants grown on natural soils.

     

Costs of reproduction in life history of a perennial plant <Emphasis Type="Italic">Carex secalina</Emphasis>

We tested a hypothesis based on life history theory that examines reproductive costs incurred by individuals in consecutive years of their life. A multi-year dataset of resource allocation to vegetative and reproductive structures was analysed in Carex secalina — a perennial, monoecious sedge, reproducing only sexually. In a four-year garden experiment, we assessed above-ground biomass at the end of each season and reproductive allocation expressed as the total length of male and female spikes. The study was aimed at determining how size and age of a plant relates to its reproduction, and how the rate of reproduction affects the year-toyear biomass change in Carex secalina. We observed that after each reproductive episode, individuals had significantly smaller sizes and produced a lower number of generative tillers. The total production of reproductive structures decreased significantly with age in all populations. Moreover, the decrease in plant biomass was greater when more reproductive structures were produced in a previous year, which indicates that the plants incur costs of reproduction in terms of above-ground biomass production.     

Numerical analysis of morphology of natural hybrids between Carex hostiana and the members of Carex flava agg. (Cyperaceae)

Uni‐variate and multi‐variate statistical methods, based on data taken from dried specimens, were used to determine the morphological variance of Carex hostiana 3 Carex flava agg. hybrids and to establish their parents among members of the C. flava complex. The following hybrids were found: C. demissa 3 hostiana [C. 3 fulva], C. hostiana 3 lepidocarpa [C. ×leutzii] and C. flava 3 hostiana [C. 3 xanthocarpa]. The least variable traits, namely beak length, utricle length, ratio of beak length to the overall utricle length, female spike width, and width of the lowest bract, proved to be the most useful in delimiting the hybrids. Carex flava 3 hostiana specimens usually have long utricles and beaks, wide male and female spikes, as well as wide bracts and leaf blades. Carex hostiana 3 lepidocarpa specimens are characterized by relatively short beaks (with low ratio of beak length to the overall utricle length) and narrow bracts. Common features of C. demissa 3 hostiana specimens, on the other hand, are male spikes with long peduncles, usually longly parted female spikes and a long beak compared to the overall utricle length.     

Herbivory Tolerance and Compensatory Differences in Native and Invasive Ecotypes of Chinese Tallow Tree (<Emphasis Type="Italic">Sapium sebiferum</Emphasis>)

We studied competitive interactions among three species (Corynephorus canescens, Hieracium pilosella and Carex arenaria) of different early successional stages on sand dunes. Our study focused on the influence of competition and water availability on biomass allocation patterns and the plasticity of root responses. Plants were grown for one growing season in a simple additive (target–neighbour) design under low or ambient water supply. Overall competition intensity (e.g., above–and below–ground), as well as root competition alone, were compared using control plants grown without competitors. Our results show high competition intensity leading to an average target plant biomass reduction of 56 relative to controls. Competition was mostly below–ground. With increasing water availability, the competitive effect of H. pilosella on both of the other species decreased significantly. All other tested species combinations were not influenced by water availability. Soil moisture seemed to be a key factor determining the plasticity of root responses. Under limited water availability, strong competitors caused a significant decrease of response ratio (lnRR) based on root: shoot ratios for H. pilosella and C. arenaria and a decrease in lnRR based on specific root length (SRL) for C. arenaria. Under sufficient water supply, however, there was no significant effect of competition on root: shoot ratios for any of the species and only C. arenaria in competition with C. canescens showed a lower lnRR based on SRL. These water–related, species–specific changes of root morphology and allocation patterns may point to an adaptive response to competition.     

青藏高原不同生境下湿生扁蕾(Gentianopsis paludosa)个体大小依赖的繁殖分配

植物对资源的投资和分配是生态学中的重要问题,它反映了植物应对环境变化时的生活史策略。选择青藏高原东缘同一海拔下的嵩草草甸(Kobresia sp.meadow)、金露梅灌丛(Potentilla fruticosa shrub meadow)以及草甸-灌丛交错带3种生境类型,并以3种生境下的湿生扁蕾(Gentianopsis paludosa)为对象,研究了其繁殖分配特征。结果发现:(1)在种群水平上,在生境从草甸经交错带到灌丛的变化中,湿生扁蕾个体大小和繁殖分配比例逐渐增加;3个种群湿生扁蕾的总花数目没有显著差异,但草甸生境湿生扁蕾的蕾期花数目显著高于灌丛生境,而果期花数目则显著低于灌丛生境;(2)在个体水平上,湿生扁蕾的繁殖绝对投入与个体大小显著正相关,且各种群植株都存在繁殖所需的个体大小阈值,而繁殖阈值在生境从草甸经交错带到灌丛的过渡中逐渐减小;湿生扁蕾的繁殖相对投入与个体大小负相关,但相关系数随着生境从草甸经交错带到灌丛的过渡中逐渐减小;各种群花数目与湿生扁蕾植株个体大小显著正相关。研究表明,湿生扁蕾的繁殖投资存在大小依赖效应,但生境差异会对其繁殖投资和生活史策略造成显著影响,而这种影响主要是由不同生境下自然条件的不同造成的。同时,资源分配也与湿生扁蕾的遗传特性和延迟自交的繁育系统特征有关。湿生扁蕾这种不同生境下个体大小依赖的繁殖投资差异是湿生扁蕾与其生境长期适应和进化(生境选择)的结果。     

Individual resource allocation to vegetative growth and reproduction in subgenus <Emphasis Type="Italic">Cyclobalanopsis</Emphasis> (<Emphasis Type="Italic">Quercus</Emphasis>, Fagaceae) trees

The resource allocation for vegetative growth and female reproduction in three tree species of subgenus Cyclobalanopsis (Quercus, Fagaceae), i.e., Q. salicina, Q. sessilifolia, and Q. acuta, were examined on a per-individual basis in two consecutive reproductive seasons, in order to test whether these trees fit the predictions of the masting hypotheses about resource matching versus resource switching. Since the three Quercus species have a biennial fruiting habit, it takes 3 years for the observation of two reproductive events. Female flower and acorn production per tree were investigated by using a seed-trap method and a numerical analysis of seed dispersal. The net production of each individual was estimated as the sum of the annual increase in the dry mass of vegetative organs and reproductive investment per tree. In the data analyses, the three species were pooled, since all 12 sample trees of the subgenus apparently showed masting in the same year, with no exceptions. Female flower and acorn production per individual tree changed considerably between years. The net production per tree increased with tree size, but did not differ between years. Therefore, the reproductive allocation (proportion of a plant’s annual assimilated resources which are used for reproduction) differed dramatically between years. On the other hand, within a year, the reproductive allocation increased with increasing net production per tree. These results suggest that the switching of resource allocation between years within an individual are occurring in subgenus Cyclobalanopsis species, and the intensity of the switching increases with increasing tree size.     

Interactions between root and shoot competition among four plant species in an alpine meadow on the Tibetan Plateau

At low altitudes, the interactions between root and shoot competition on plant growth have been extensively studied, and such interaction effects were found to be positive, negative, or neutral. However, little is known about such effects at high altitudes where the environmental conditions are harsher than those at low altitudes. We carried out a field experiment in an alpine meadow in the northeast Tibetan Plateau to test the hypothesis that a negative interaction between root and shoot competition exists for alpine plant species. Root and shoot competition were experimentally manipulated in the four grass species (Kobresia humilis, Saussurea superba, Stipa aliena and Elymus nutans). We found that K. humilis and S. aliena grew better without competition, whereas S. superba grew better with shoot competition and E. nutans grew better with root competition. The interactions between root and shoot competition were negative in K. humilis, positive in S. superba, but neutral in S. aliena and E. nutans, suggesting that the interaction effects are species-specific. This study also suggested that alpine plants may trade off both plant–plant interactions and competition shift between root and shoot to adapt to stressful environments.     

Effects of pasture competition and specialist herbivory on the performance of Cirsium arvense

Combining specialist herbivory with interspecific plant competition can be an effective means of controlling pasture weeds. Cirsium arvense (Canada thistle, Californian thistle, creeping thistle) is one of the worst weeds of pastoral production systems in New Zealand (NZ). The oligophagous leaf-feeding beetle, Cassida rubiginosa, was recently released in NZ for control of C. arvense. To assess the impact of this biocontrol agent we conducted an outdoor potted-plant experiment with low and high densities of Cassida larvae combined with different levels of interspecific competition from typical NZ pasture species. Secondly, we carried out a field-release experiment to quantify the impact of high densities of Cassida under more natural conditions. Interspecific competition reduced all measured plant parameters of C. arvense except mean shoot height and base diameter. Herbivory by Cassida only reduced root biomass, and showed a weak additive response when combined with competition. All other measured parameters of C. arvense showed a substitutive response, with competition being the only factor having a significant impact on the weed. There were no significant synergistic interactions with competition and herbivory on C. arvense. Interestingly, the number of root buds per plant was significantly greater in the presence of herbivory by Cassida, suggesting that C. arvense may compensate for defoliation. Similar to the potted-plant experiment, Cassida had no significant effect on shoot growth and development in the field-release experiment. The results of this study indicate that competition from typical NZ pasture species is a more important factor than herbivory by Cassida, and unless Cassida reaches outbreak densities, it will likely have an insignificant impact on this weed.     

Effects of competition on root–shoot allocation in <Emphasis Type="Italic">Plantago lanceolata</Emphasis> L.: adaptive plasticity or ontogenetic drift?

We investigated how shoot and root allocation in plants responds to increasing levels of competitive stress at different levels of soil fertility. In addition, we analyzed whether different responses were due to adaptive plasticity or should be attributed to ontogenetic drift. Plantago lanceolata plants were grown during 18 weeks at five plant densities and four nutrient supply levels in pots in the greenhouse. Thereafter root and shoot biomass was measured. There were clear negative effects of increasing plant densities on plant weights revealing strong intraspecific competition. At the lower N-treatments, the proportional allocation to root mass increased with increasing competitive stress, indicating the important role of belowground competition. At the higher N-supply rate, the relationship between competitive stress and shoot to root ratio was neutral. These responses could not be attributed to ontogenetic drift, but could only be explained by assuming adaptive plasticity. It was concluded that at lower N-supplies belowground competition dominates and leads to increased allocation to roots, while at the higher N-supply competition for soil resources and light had balanced impacts on shoot and root allocation. An alternative hypothesis explaining the observed pattern is that light competition has far less pronounced impacts on root–shoot allocation than nutrient deprival.     

Above- and below-ground competition between the liana <Emphasis Type="Italic">Acacia kamerunensis</Emphasis> and tree seedlings in contrasting light environments

Proliferation of lianas in canopy gaps can restrict tree regeneration in tropical forests through competition. Liana effects may differ between tree species, depending on tree requirements for above- and below-ground resources. We conducted an experiment in a shade house over 12 months to test the effect of light (7 and 27% external irradiance) on the competitive interactions between seedlings of one liana species and three tree species and the contribution of both above- and below-ground competition. Seedlings of the liana Acacia kamerunensis were grown with tree seedlings differing in shade tolerance: Nauclea diderrichii (Pioneer), Khaya anthotheca (Non-Pioneer Light Demander) and Garcinia afzelii (Non-Pioneer Shade Bearer). Trees were grown in four competition treatments with the liana: no competition, root competition, shoot competition and root and shoot competition. Both root and root–shoot competition significantly reduced relative growth rates in all three tree species. After one year, root–shoot competition reduced growth in biomass to 58% of those (all species) grown in no competition. The root competition treatment had a more important contribution in the effect of the liana on tree growth. Tree seedlings did not respond to competition with the liana by altering their patterns of biomass allocation. Although irradiance had a great effect on tree growth and allocation of biomass, the interaction between competition treatments and irradiance was not significant. Nauclea diderrichii, the tree species which responded most to the effects of competition, showed signs of being pot-bound, the stress of which may have augmented the competition effects. The understanding of the interaction of above- and below-ground competition between lianas and trees and its moderation by the light environment is important for a proper appreciation of the influence of lianas on tropical forest regeneration.