The leaf size/number trade-off in herbaceous angiosperms

Aims In this study,we examined the extent to which between-species leaf size variation relates to variation in the intensity of leaf production in herbaceous angiosperms.Leaf size variation has been most commonly interpreted in terms of biomechanical constraints(e.g.affected by plant size limitations)or in terms of direct adaptation associated with leaf size effects in optimizing important physiological functions of individual leaves along environmental gradients(e.g.involving temperature and moisture).An additional interpretation is explored here,where adaptation may be more directly associated with the number of leaves produced and where relatively small leaf size then results as a trade-off of high‘leafing intensity’—i.e.number of leaves produced per unit plant body size.Methods The relationships between mean individual leaf mass,number of leaves and plant body size were examined for 127 species of herbaceous angiosperms collected from natural populations in southern Ontario,Canada.Important Findings In all,88%of the variation in mean individual leaf mass across species,spanning four orders of magnitude,is accounted for by a negative isometric(proportional)trade-off relationship with leafing intensity.These results parallel those reported in recent studies of woody species.Because each leaf is normally associated with an axillary bud or meristem,having a high leafing intensity is equivalent to having a greater number of meristems per unit body size—i.e.a larger‘bud bank’.According to the‘leafing intensity premium’hypothesis,because an axillary meristem represents the potential to produce either a new shoot or a reproductive structure,high leafing intensity should confer greater architectural and/or reproductive plasticity(with relatively small leaf size required as a trade-off).This greater plasticity,we suggest,should be especially important for smaller species since they are likely to suffer greater suppression of growth and reproduction from competition within multi-species vegetation.Accordingly,we tested and found support for the prediction that smaller species have not just smaller leaves generally but also higher leafing intensities,thus conferring larger bud banks,i.e.more meristems per unit plant body size.

Multi-scale phylogenetic structure in coastal dune plant communities across the globe

Aims Studies integrating phylogenetic history and large-scale community assembly are few,and many questions remain unanswered.Here,we use a global coastal dune plant data set to uncover the important factors in community assembly across scales from the local filtering processes to the global long-term diversification and dispersal dynamics.Coastal dune plant communities occur worldwide under a wide range of climatic and geologic conditions as well as in all biogeographic regions.However,global patterns in the phylogenetic composition of coastal dune plant communities have not previously been studied.Methods The data set comprised vegetation data from 18463 plots in New Zealand,South Africa,South America,North America and Europe.The phylogenetic tree comprised 2241 plant species from 149 families.We calculated phylogenetic clustering(Net Relatedness Index,NRI,and Nearest Taxon Index,NTI)of regional dune floras to estimate the amount of in situ diversification relative to the global dune species pool and evaluated the relative importance of land and climate barriers for these diversification patterns by geographic analyses of phylogenetic similarity.We then tested whether dune plant communities exhibit similar patterns of phylogenetic structure within regions.Finally,we calculated NRI for local communities relative to the regional species pool and tested for an association with functional traits(plant height and seed mass)thought to vary along sea–inland gradients.Important Findings Regional species pools were phylogenetically clustered relative to the global pool,indicating regional diversification.NTI showed stronger clustering than NRI pointing to the importance of especially recent diversifications within regions.The species pools grouped phylogenetically into two clusters on either side of the tropics suggesting greater dispersal rates within hemispheres than between hemispheres.Local NRI plot values confirmed that most communities were also phylogenetically clustered within regions.NRI values decreased with increasing plant height and seed mass,indicating greater phylogenetic clustering in communities with short maximum height and good dispersers prone to wind and tidal disturbance as well as salt spray,consistent with environmental filtering along sea–inland gradients.Height and seed mass both showed significant phylogenetic signal,and NRI tended to correlate negatively with both at the plot level.Low NRI plots tended to represent coastal scrub and forest,whereas high NRI plots tended to represent herb-dominated vegetation.We conclude that regional diversification processes play a role in dune plant community assembly,with convergence in local phylogenetic community structure and local variation in community structure probably reflecting consistent coastal-inland gradients.Our study contributes to a better understanding of the globally distributed dynamic coastal ecosystems and the structuring factors working on dune plant communities across spatial scales and regions.

Competition and body size in plants:the between-species tradeoff for maximum potential versus minimum reproductive threshold size

Aims According to traditional theory,superior competitive ability in plants generally requires relatively large plant body size.Yet even within the most crowded vegetation,most resident species are relatively small;species size distributions are right-skewed at virtually every scale.We examine a potential explanation for this paradox:small species coexist with and outnumber large species because they have greater‘reproductive economy’,i.e.they are better equipped—and hence more likely—to produce offspring despite severe size suppression from intense competition.Methods Randomly placed plots within old-field vegetation were surveyed across the growing season.Within each plot,the largest(MAX)and smallest(MIN)reproductive individuals of each resident species were collected for above-ground dry mass measurement.We tested three hypotheses:(i)smaller resident species(with smaller MAX size)have generally smaller reproductive threshold sizes;(ii)smaller resident species have greater‘reproductive economy’,i.e.a smaller MIN relative to MAX reproductive plant size;and(iii)MIN size predicts plot occupancy(species abundance within the community)better than MAX size.Important Findings The results supported the first and third,but not the second hypothesis.However,we could not reject the hypothesis that smaller species have greater reproductive economy,as it was not possible to record data for the largest potential plant size for each species—since even the largest(MAX)plants collected from our sampled plots were subjected to competition from neighbours under these natural field conditions.Importantly,contrary to conventional competition theory,more successful species(in terms of greater plot occupancy)had smaller minimum not larger(or smaller)maximum reproductive sizes.These results suggest that a small reproductive threshold size,commonly associated with relatively small potential body size,is generally more effective in transmitting genes into future generations when selection from neighbourhood crowding/competition is intense—at least within natural old-field vegetation.Accordingly,we propose a simple conceptual model that represents the basis for a fundamental paradigm shift in the predicted selection effects of crowding/competition on plant body size evolution.

Big plants--do they limit species coexistence?

Aims According to conventional theory,larger plant species are likely to inflict more intense competition on other(smaller)species.We tested a deducible prediction from this:that a larger species should generally be expected to impose greater limits on the number of species that can coexist with it.Methods Species richness was sampled under plant canopies for a selection of woody species(‘host’species)that display a wide range of adult sizes(from small shrubs to large trees),growing within natural vegetation of the Interior Douglas-fir zone of southern British Columbia,Canada.These data were compared with species richness levels sampled within randomly placed plots within the host species habitat.Important Findings A prominent host species size effect on species richness was detected but only narrowly at the small end of the species size range.Across most(90%)of the increasing size range of host species,the number of species residing under the host canopy showed no significant decrease relative to the number expected by random assembly,based on species richness within randomly defined equivalent areas within the habitat of the host species.This apparent‘null effect’,we suggest,is explained not because these larger species have no effect on community assembly.We postulate that larger species are indeed likely to be more effective in causing competitive exclusion of some smaller species(as expected from conventional theory),but that any potential limitation effect of this on resident species richness is offset for two reasons:(i)larger species also generate niche spaces that they cannot exploit under their own canopies and so have minimal impact(as competitors)on smaller species that can occupy these niches and(ii)certain other small species—despite small size—have effective competitive abilities under the severe competition that occurs within host neighbourhoods of larger species.These and other recent studies call for re-evaluation of traditional views on the role of plant size in affecting competitive ability and community assembly.

Leaf size versus leaf numbertradeoffs in dioecious angiosperms

Aims We explore the possible role of leaf size/number trade-offs for the interpretation of leaf size dimorphism in dioecious plant species.Methods Total above-ground biomass(both male and female)for three herbaceous dioecious species and individual shoots(from both male and female plants)for three woody dioecious species were sampled to record individual leaf dry mass,number of leaves,dry mass of residual above-ground tissue(all remaining non-leaf biomass),number of flowers/inflorescences(for herbaceous species)and number of branches.Important Findings For two out of three woody species and two out of three herbaceous species examined,male plants produced smaller leaves but with higher leafing intensity—i.e.more leaves per unit of supporting(residual)shoot tissue or plant body mass—compared with females.Male and female plants,however,did not differ in shoot or plant body mass or branching intensity.We interpret these results as possible evidence for a dimorphic leaf deployment strategy that promotes both male and female function,respectively.In male plants,capacity as a pollen donor may be favored by selection for a broadly spaced floral display,hence favoring relatively high leafing intensity because this provides more numerous axillary meristems that can be deployed for flowering,thus requiring a relatively small leaf as a trade-off.In one herbaceous species,higher leafing intensity in males was associated with greater flower production than in females.In contrast,in female plants,selection favors a relatively large leaf,we propose,because this promotes greater capacity for localized photosynthate production,thus supporting the locally high energetic cost of axillary fruit and seed development,which in turn requires a relatively low leafing intensity as a trade-off.

尚无证据表明顶端优势强的物种存在广义顶端优势潜在“成本”

破坏或去除植物顶端分生组织可能会弱化(欠补偿)、无影响(等补偿)或强化(超补偿)植物繁殖力和/或植物生长。后者为顶端优势潜在“成本”。本文选用了19种顶端生长占显著优势的垂直草本被子植物组成的自然种群为研究对象,去除(修剪)植物顶端分生组织来复制生长季节早期的植物,进而测试顶端优势潜在成本,又移除了修剪过顶端分生组织的植物与未修剪过顶端分生组织的植物(实验对照组)的邻近植物,并于开花结果后种子传播前收获所有植物,最后分别测定了地上植株(顶端)、叶片、种子及果实的干质量,并计算了每株植物的叶片、果实及种子的数量,从而得出以下预测:(i)去除顶端分生组织后,所研究物种(鉴于其生长表现为强顶端优势)的分枝强度增大,从而造成其繁殖力和/或生物量的超补偿;(ii)超补偿尤其使研究物种长出更多更小的叶片,腋生分生组织的芽库扩大,有利于分枝和/或果实生产。顶端优势的补偿能力差异很大,且其与叶片尺寸或出叶强度无显著种间关系,因此并无证据表明广义顶端优势潜在成本。总而言之,本文研究结果表明了物种特异性处理对分生组织分配模式的影响及其对局部资源可用性变化、种间物候变化、生活史特征与植食易感性的重大影响。

The relationship between individual seed quality and maternal plant body size in crowded herbaceous vegetation

Aims In most natural plant populations,there is a strong right-skewed dis-tribution of body sizes for reproductive plants-i.e.the vast majority are relatively small,suppressed weaklings that manage not just to survive effects of crowding/competition and other hazards but also to produce offspring.recent research has shown that because of their relatively large numbers,these relatively small resident plants collectively contribute most of the seed offspring production available for the population in the next generation.However,the success of these offspring will depend in part on their quality,e.g.reflected by seed size and resource content.accordingly,in the present study,we used material from natural populations of herbaceous species to test the null hypothesis that there is no significant relationship between body size variation in resident plants-resulting from between-site variation in the intensity of crowding/competition-and variation in the mass or N content of their individual seeds.Methods using populations of 56 herbaceous species common in eastern ontario,total above-ground dry plant mass,mean mass per seed and mean nitrogen(N)content per seed were recorded for a sample of the largest resident plants and also for the smallest reproduc-tive plants growing in local neighbourhoods with the most severe crowding/competition from near neighbours.Important Findingsmass per seed was numerically smaller from the smallest resident plants for most study species,but with few exceptions,this was not significantly different(P>0.05)from mass per seed from the larg-est resident plants.the results therefore showed no general effect of maternal plant body size on individual seed mass,or N content.this suggests that the reproductive output of the smaller half of the resident plant size distribution within these populations is likely to contribute not just most of the seed production available for the next generation but also seed offspring that are just as likely-on a per individual basis-to achieve seedling/juvenile recruitment success as the seed offspring produced by the largest resident plants.this conflicts with the traditional‘size-advantage’hypothesis for predicting plant fitness under severe competition,and instead supports the recent‘reproductive-economy-advantage’hypothesis,where competitive fitness is promoted by capacity to produce offspring that-despite severe body size suppression imposed by neighbour effects-in turn have capacity to produce grand-offspring.