Age changes in the vegetative vs. reproductive allocation by module demographic strategies in a perennial plant

Retama sphaerocarpa (L.) Boiss. is aMediterranean shrub with a remarkably simplified metameric structure. Terminalyoungest shoots act as units of modular growth, being able to produce newshootsby basal axillary buds (at the base of the shoot) and inflorescencesby lateral axillary buds. In this study, we have analysed the structural andgrowth potential features of these modules, as well as theirdemographic proportions, regarding the allocation of newvegetative and reproductive growth in plants of different age. Reproductiveeffort is proportionally higher in older plants. This shift in the allocationstrategy with plant ontogeny is not attained with changes in the shoot modules(which maintain a constant size, nutrient composition and show a similarnew growth investment per module) but through a differentdemographic composition of the population ofmodulesaccording to their developmental fate (vegetative or reproductive).This indicates a high level of iterativity and a purely modular growth, sincethe attributes of the individual (age of the plants) do not seem toaffect those of the integrating modular units (growth performance of theshoots).     

Hierarchical pre-dispersal fitness assessment in a Mediterranean shrub plant

Under the concept of modularity, it is possible to recognise how seed production, as well as any other process affecting it, are hierarchically structured within fruits, within individual plants and within populations. In this work, we analysed the effects of pre-dispersal seed predation by insects upon a set of hierarchical levels in a population of the Mediterranean shrub plant Cistus ladanifer (“rock rose”) throughout a complete fruit-producing season (which takes place during the summer months). Almost all individual plants were predated, which implies that the effects of predation at the population level (regardless of the extent of predation within each individual) were virtually uniform. Within the individuals, however, the predation rate was close to a proportion of 0.5 (half of the fruits of each individual were predated), which indicates that this hierarchical level is likely to be subjected to a differential action of selection. Predation rates within the fruits showed an intermediate value (lower than that observed at the population level but higher than that at the individual level). According to these results, the pressure of phenotypic selection may therefore give rise to greater variation among fruits of the same individual than among seeds of the same fruit. In terms of the temporal patterns observed there was a large variation in the increments of predation along the fruiting season, which implies a high degree of heterogeneity in the temporal distribution of the effects of predation pressure on fitness. Besides its use in the specific example of the plant species studied in this work, the methodological procedure presented in this paper (integration of the temporal changes of different hierarchical levels) might be foreseen, in fact, as a useful tool for analysing the hierarchical structuring of fitness in modular organisms in general. This procedure allows to discriminate and integrate selection pressures and their effects across different phenotypic levels, from the infra-individual ones up to the population level.     

Temporal and spatial patterns of seed dispersal in two Cistus species (Cistaceae)

Cistus species are obligate seeding, early colonizers that follow disturbance, particularly fire, in Mediterranean ecosystems. We studied seed release, seed dispersal and soil seed populations in stands of Cistus ladanifer and C. libanotis. Seed release started in mid- to late summer (C. ladanifer) or in early autumn (C libanotis), and continued for a very extended period: 8-10 months in C. ladanifer, and for a mean of 16 months in C. libanotis. The xerochastic capsules of both species released seeds by successive dehiscence of the locules. All capsules begin to dehisce simultaneously at the start of the seed release period, but in C. libanotis capsule fragmentation replaced dehiscence early in the seed release period. In plants of both species, seed shadows were characterized by a peak of density beneath the plant canopy and a very short tail of much lower densities, indicating that seeds are concentrated beneath mother plants when dispersed. Nevertheless, in late May, at the onset of the fire season, soil seed densities beneath plant canopies were low compared with densities expected from seed shadows, but were apparently high enough to allow recovery of the stands if a disturbance, such as fire, had taken place. Seed-eating Bruchidae in summer and granivorous ants during the seed release period were apparently the main causes of seed losses. Results suggest that in both Cistus species, the staggered seed release could constitute an efficient risk-reducing trait. The plant pool of seeds existing throughout most of the year could be a relevant component of Cistus seed banks.     

Interpreting reproductive allometry: Individual strategies of allocation explain size-dependent reproduction in plant populations

Size-dependent or allometric relationships between reproductive and vegetative size are extremely common in plant populations. Reproductive allometry where plant size differences are due to environmental variability has been interpreted both as an adaptive strategy of plant growth and allocation, and as the product of fixed developmental constraints. Patterns of development are crucial in defining reproductive allometry but development is not fixed across individuals. For example, environmental adversity (e.g. resource impoverishment) tends to favor reproduction at relatively small sizes – an adaptive response to environmental adversity. While small individuals may have lower reproductive output than large individuals, all plants should maximize their reproductive output and relative allocation to reproduction may be constant across sizes. Thus, where individual plants within a population initiate reproduction at different sizes, no significant reproductive allometry is an appropriate null expectation. Reproductive allometry occurs in plant populations where initiating reproduction at small sizes yields relatively high or low reproductive size at final development. Both of these outcomes are common in plant populations. Our interpretation of reproductive allometry combines previous adaptive and developmental constraint interpretations, and is the first to successfully explain the range of relationships in plant populations where relative allocation has been observed to increase, decrease or remain constant will increasing plant size.     

Annual plant life histories and the paradigm of resource allocation

Summary Much of life history theory follows from the idea that natural selection acts on the allocation of resources to competing and independent demographic functions. This paradigm has stimulated much research on the life histories of annual plants. Models of whole-plant resource budgets that use optimal control theory predict periods of 100% vegetative and 100% reproductive growth, sometimes with periods of mixed growth. I show here that this prediction follows from the assumption of independence of the competing vegetative and reproductive compartments. The prediction is qualitatively unchanged even after relaxing important simplifying assumptions used in most models. Although it follows naturally from the assumptions of the models, this kind of allocation pattern is unlikely to occur in many plants, because it requires that (1) leaf and flower buds can never simultaneously be carbon sinks; and (2) organs that accompany flowers, such as internodes and bracts, can never be net sources of photosynthate. Thus while resources are doubtless important for annual plants, an exclusively resource-based perspective may be inadequate to understand the evolution of their life histories. Progress in research may require models that incorporate, or are at least phenomenologically consistent with, the basic developmental reles of angiosperms.     

Time of germination and establishment success after fire of three obligate seeders in a Mediterranean shrubland of central Spain

Aim Determining how differences in time of germination can affect plant establishment in plant communities that, after a disturbance, must reestablish from seeds under climatic conditions subject to extremes, such as the Mediterranean. Although early germination may be beneficial for survival in summer, when drought is severe, this may expose the seedlings to winter extremes, thus to higher mortality. Understanding how sensitive is the establishment of different species to temporal patterns of germination will help to understand the factors that control species distribution and community stability in disturbance‐prone environments, as well as its sensitivity to changes in weather patterns as climate changes. Methods An experimental fire was made in early fall in an old Cistus–Erica shrubland in Toledo (central Spain). After fire, germination, survival and growth of the three dominant seeder species (Cistus ladanifer, Erica umbellata and Rosmarinus officinalis) were monitored during the first 3 years after fire. Seedlings were tagged to identify their time of emergence, and divided into cohorts according to their month of germination. Differences in survival of the various cohorts were evaluated by means of a Wilcoxon (Gehan) statistic. Height of surviving, tagged plants was compared among cohorts by means of a Kolmogorov–Smirnov test. Results The year following fire was one of the driest on record, while the next one was one of the wettest. Germination was more abundant during the first than during the second year. Establishment was mainly from first‐year germination, as the majority of second‐year germinated seedlings died. Temporal patterns of germination within a year and between years varied between species. Seedling mortality was highest immediately following germination, not in summer. Mortality was related to time of germination: during a given period of time, the mortality of younger seedlings was higher than that of older ones. However, survival was not highest for the first cohorts. In general, the earlier the seedlings germinated the more vigourous they became, more clearly so for Cistus than for Rosmarinus, but differences tended to disappear with time. Overall, time of germination varied between species and affected differently seedling survival and vigour of the various species. Rosmarinus and Cistus had sufficient survivors to reestablish the initial population. Erica, despite abundant germination, suffered a strong population reduction. Main conclusions Mediterranean shrub species differ in their temporal patterns of germination and survival after fire. The effect of time of germination is complex: germinating early is advantageous since old seedlings fared better than younger ones when confronted with the same rigours. However, germinating early might expose the seedlings to greater hazards and the first cohort might not survive best. The temporal window for establishment is narrow and mainly restricted to the first year after fire. Second year seedlings, irrespective of most favourable conditions, survived very little. Missing the window of establishment might virtually lead to a population collapse, despite having very high germination, as found for Erica.     

Structural cost of shoot modules and its implications on plant potential fitness in a Mediterranean perennial shrub, Retama sphaerocarpa (L.) Boiss

This paper introduces a methodology to analyse the structural costs on plant potential fitness, empirically exemplified in the hierarchical shoot system of a Mediterranean perennial plant, Retama sphaerocarpa (L.) Boiss. During growing season every year (March-August), the terminal shoot (which is the basic unit of growth) develops inflorescences, flowers and fruits, as well as new shoots (first-, second- and third-order branching shoots) which have the potential to "behave" as terminal shoots in the following year. Different morphological and demographical aspects of the modules within the terminal shoot were measured in 100 terminal shoots selected from different plants of a natural population of R. sphaerocarpa. Complementary samples of 100 shoots of different branching orders were collected to obtain biomass estimations of the terminal shoots. We propose a simple procedure to estimate structural cost (biomass investment) on plant potential fitness (flowering buds) as a methodology for interpreting and comparing the consequences on fitness of different plant growth patterns. The results of this study exemplify how differential allocation patterns among plant structural modules, depending on their position within the shoot system, can be quantified to estimate their influence upon plant potential fitness.     

Cattle change plant reproductive phenology,promoting community changes in a post-fire Nothofagus forest in northern Patagonia,Argentina

Aims Variations in rates and length of flowering and fruiting not only affect the reproduction of a given plant species but also the behavior and reproduction of associated taxa. Flowering and fruiting variations may be influenced by herbivory, especially by large mammals. The aim of this study was to determine the effects of cattle browsing on the reproductive phenology of understory species in a subalpine post-fire Nothofagus forest in Patagonia.Methods The effects of herbivory on plant reproductive phenology were studied in a set of experimental exclosures (fenced plots) installed since 2001 in a post-fire N. pumilio forest, located in Nahuel Huapi National Park (NHNP), Argentina. We monitored the beginning and duration of each reproductive phenological stage: floral bud, open flower, immature fruit and mature fruit. We also counted the number of flowers, fruits, seeds and viable seeds of the dominant plants to assess whether browsing modifies temporal patterns of the flowering and fruiting periods.Important findings Cattle reduced the total number of species flowering and fruiting and changed the reproductive phenology of some species. We found that palatable species seem to be negatively affected by browsing in terms of reduced fitness due to changes in flowering and fruiting periods. In contrast, cattle benefitted the reproduction of non-palatable species and could promote the invasion of shade-intolerant exotic forbs such as Cirsium vulgare. The effects of livestock reported in this study are important to understanding how browsing could alter native species establishment and possibly alter successional trajectories during recolonization after fire.     

Inclusive fitness and reproductive strategies in dwarf mongooses

Dwarf mongooses (Helogale parvula) are small, communally breedingcarnivores found in woodland and tree-savanna throughout Africa.Within a pack, socially subordinate mongooses do not normallybreed, yet they invariably participate in all aspects of parentalcare. The primary alternative to tolerating reproductive suppressionis dispersal, which shortens the wait for dominance and breeding.Here, we calculate the annual inclusive fitness payoffs to thedispersing and nondispersing strategies for males and femalesof ages one to seven, using data from a 14-year study in SerengetiNational Park, Tanzania. Factors with effects on inclusive fitnessincluded relatedness to pack mates before and after dispersal,the effect of help on recipients' reproductive success, theprobability of dispersing successfully, the probability of attainingdominance, and reproductive success after attaining dominance.All of these factors differed between male and female dwarfmongooses. We compared the contributions of direct and indirectfitness to the total fitness of mongooses pursuing each of thestrategies, across a range of ages. In our population, dispersaland nondispersal both yielded direct and indirect payoffs atmost ages. For dispersers of both sexes, direct fitness wasthe primary component of total fitness but indirect fitnesswas substantial for young (< 2 years old) dispersers. Fornondispersers of both sexes, indirect fitness was the majorcomponent of total fitness among young mongooses (up to 2 or3 years), but direct fitness was the major component among oldermongooses. By comparing the inclusive fitness payoffs for thetwo strategies, we determined the range of ages at which dispersalshould be favored for each sex. These comparisons correctlypredicted that males should be more dispersive than femalesat all ages, and that males should disperse over a broader rangeof ages.     

Divergence in plant and microbial allocation strategies explains continental patterns in microbial allocation and biogeochemical fluxes

Allocation trade‐offs shape ecological and biogeochemical phenomena at local to global scale. Plant allocation strategies drive major changes in ecosystem carbon cycling. Microbial allocation to enzymes that decompose carbon vs. organic nutrients may similarly affect ecosystem carbon cycling. Current solutions to this allocation problem prioritise stoichiometric tradeoffs implemented in plant ecology. These solutions may not maximise microbial growth and fitness under all conditions, because organic nutrients are also a significant carbon resource for microbes. I created multiple allocation frameworks and simulated microbial growth using a microbial explicit biogeochemical model. I demonstrate that prioritising stoichiometric trade‐offs does not optimise microbial allocation, while exploiting organic nutrients as carbon resources does. Analysis of continental‐scale enzyme data supports the allocation patterns predicted by this framework, and modelling suggests large deviations in soil C loss based on which strategy is implemented. Therefore, understanding microbial allocation strategies will likely improve our understanding of carbon cycling and climate.     

Diurnal changes in allocation and partitioning of recently assimilated carbon in loblolly pine seedlings

We examined diurnal fluctuations in acquisition and partitioning of recently assimilated ¹⁴CO₂, and in subsequent allocation and partitioning to roots of loblolly pine (Pinus taeda L.) seedlings. Nonmycorrhizal seedlings were grown under optimal nutrient conditions in continuously flowin solution culture. Shoots of 15-week-old loblolly pine seedlings were labeled with ¹⁴CO₂ for 30 min at four separate labeling times: 1000, 1200, 1400 and 1600 h. Six whole plant harvests were conducted during a 48 h chase period, i.e. 0, 4, 8 12, 24 and 48 h after the end of the labeling and evacuation periods. Although assimilation of ¹⁴CO₂ was constant between 1000 and 1400 h, there were significant differences in partitioning of ¹⁴C-labeled assimilate in needles of all age classes. The highest percentage of recently assimilated ¹⁴CO₂ in the ethanol-soluble fraction of photosynthesizing tissue was observed near the beginning and end of the photoperiod. Partitioning of ¹⁴C in the ethanol-soluble fraction declined between the 1000 and 1400 h labeling eriods, and was accompanied by an increase in partitioning of recently assimilated ¹⁴CO₂ toward starch and a decrease in respiratory losses. These data suggest that most of the ¹⁴CO₂ assimilated at 1000 h was used to support shoot metabolic activities and possibly restore soluble sugar reserves. Peak starch accumulation in needles during the 1400 h labeling period, concomitant with minimal respiratory loss, indicated that photosynthate production exceeded demand and export out of source leaves. A possible feedback regulation of photosynthesis by starch and/or sugar accumulation may be responsible for the observed decline in assimilation of ¹⁴CO₂ during the 1600 h labeling period. Net accumulation of recently assimilated ¹⁴CO₂ in roots was correlated with assimilation rate of ¹⁴CO₂, but independent of partitioning of recently assimilated carbon in photosynthetic tissue. However, the percentage of total seedling ¹⁴C allocated to roots was essentially the same throughout the 48 h chase, regardless of time of labeling and assimilation rate. The data suggest a strong diurnal regulation of starch and soluble sugars synthesized from recently assimilated carbon in needles of loblolly pine seedlings that was independent of assimilation rate. Allocation and transport of recently assimilated carbon to roots of loblolly pine seedlings were not subject to short-term fluctuations in supply and demand.     

Temporally changing male reproductive success and resource allocation strategy in protandrousHeracleum lanatum (Apiaceae)

Few observations have been made on temporal changes in the siring success of flowers in the male stage. In this study, we estimated both male and female contributions to fitness for 21 plants of protandrous andromonoeciosHeracleum lanatum with differing dates of first flowering. The results of multiple regression analysis showed that total male fitness significantly increases with the advance of the first-flowering date but does not depend upon plant size, whereas female fitness increases with plant size but does not depend upon the first-flowering date. We also showed that the earlier-flowering plants have more late-blooming male flowers in their secondary umbels. Based on these results, we suggest that polymorphism of the early- and late-bloomers may be maintained by frequency-dependent selection through temporally changing male reproductive success.     

Testing for biases in selection on avian reproductive traits and partitioning direct and indirect selection using quantitative genetic models

Key life history traits such as breeding time and clutch size are frequently both heritable and under directional selection, yet many studies fail to document microevolutionary responses. One general explanation is that selection estimates are biased by the omission of correlated traits that have causal effects on fitness, but few valid tests of this exist. Here, we show, using a quantitative genetic framework and six decades of life‐history data on two free‐living populations of great tits Parus major, that selection estimates for egg‐laying date and clutch size are relatively unbiased. Predicted responses to selection based on the Robertson–Price Identity were similar to those based on the multivariate breeder's equation (MVBE), indicating that unmeasured covarying traits were not missing from the analysis. Changing patterns of phenotypic selection on these traits (for laying date, linked to climate change) therefore reflect changing selection on breeding values, and genetic constraints appear not to limit their independent evolution. Quantitative genetic analysis of correlational data from pedigreed populations can be a valuable complement to experimental approaches to help identify whether apparent associations between traits and fitness are biased by missing traits, and to parse the roles of direct versus indirect selection across a range of environments.     

The effects of inbreeding, genetic dissimilarity and phenotype on male reproductive success in a dioecious plant

Pollen fate can strongly affect the genetic structure of populations with restricted gene flow and significant inbreeding risk. We established an experimental population of inbred and outbred Silene latifolia plants to evaluate the effects of (i) inbreeding depression, (ii) phenotypic variation and (iii) relatedness between mates on male fitness under natural pollination. Paternity analysis revealed that outbred males sired significantly more offspring than inbred males. Independently of the effects of inbreeding, male fitness depended on several male traits, including a sexually dimorphic (flower number) and a gametophytic trait (in vitro pollen germination rate). In addition, full-sib matings were less frequent than randomly expected. Thus, inbreeding, phenotype and genetic dissimilarity simultaneously affect male fitness in this animal-pollinated plant. While inbreeding depression might threaten population persistence, the deficiency of effective matings between sibs and the higher fitness of outbred males will reduce its occurrence and counter genetic erosion.     

Effects of intraspecific competition on size variation and reproductive allocation in a clonal plant

Clonal plants grow in diameter rather than height, and therefore competition among genets is likely to be symmetric and to result in smaller variation in size of genets than in non-clonal plants. Moreover, clonal plants can reproduce both sexually and vegetatively. We studied the effects of density on the size of rosettes and of clones, variation in the size of rosettes and of clones, and allocation to sexual and vegetative reproduction in the clonal herb Ranunculus reptans . We grew plants from an artificial population of R. reptans in 32 trays at two densities. After four months, differences in density were still apparent, although clones in the low-density treatment had on average 155% more rosettes and 227% more rooted rosettes than clones in the high-density treatment. The coefficient of variation of these measures of clone size was 15% and 83% higher, respectively, in the low-density treatment. This indicates that intraspecific competition among clones of R. reptans is symmetric and increases the effective population size. Rooted rosettes were larger and varied more in size in the low-density treatment. The relative allocation of the populations to sexual and to vegetative reproduction was 19% and 13% higher, respectively, in the high-density treatment. Moreover, seeds produced in the high-density treatment had a 24% higher mass and a 7% higher germination percentage. This suggests that with increasing density, allocation to sexual reproduction increases more than allocation to vegetative reproduction in R. reptans , which corresponds to the response of some other species with a spreading growth form but not of species with a compact growth form. We conclude that intraspecific competition is an important factor in the life-history evolution of R. reptans because intraspecific competition affects its clonal life-history traits and may affect evolutionary processes such as genetic drift and selection through its effect on the effective population size.     

松嫩平原朝鲜碱茅无性系种群构件生物量结构及相关模型分析

从构件水平对松嫩平原碱化草甸朝鲜碱茅无性系种群各功能构件的生物量结构,各功能构件生物量与丛径之间的关系,以及各功能构件生物量之间的关系均建立相应的模型进行了定量分析。结果表明,在孕穗和果后营养两个生育期,朝鲜碱茅无性系种群各功能构件生物量及所占总生物量的比率具有相同的规律;各功能构件生物量与丛径之间的定量关系在孕穗期以直线函数模型相关性最大,在果后营养期以幂函数模型相关性最大;各功能构件生物量之间的定量关系除光合构件与支持构件在孕穗期以直线函数模型相关性最大外,其他各构件生物量之间在两个生长期均以幂函数模型相关性最大。