青藏高原三种优势植物生物量分配的变化规律

该研究利用4个由高到低不同海拔的同质园实验,以青藏高原高寒草地优势植物垂穗披碱草(Elymus nutans)、矮嵩草(Kobresia humilis)和珠芽蓼(Polygonum viviparum)为对象,分析了植物个体根、茎、叶生物量分配及根冠比的变化规律及影响因素。结果表明:(1)植物个体根、茎、叶质量比和根冠比具有显著的种间差异;与垂穗披碱草和珠芽蓼相比,矮嵩草具有显著较高的根质量比而叶、茎质量比较低,所以其根冠比较高。(2)在向低海拔移栽的过程中,珠芽蓼叶质量比保持不变,茎质量比显著降低而根质量比显著升高,根冠比表现出显著上升的趋势;垂穗披碱草则相反,即叶、茎质量比显著升高而根质量比显著降低,根冠比表现出显著下降的趋势;矮嵩草根、茎、叶质量比和根冠比则无显著变化。(3)随着海拔降低,年均气温明显升高而年均降雨量明显降低,且在植物个体种源地和土壤基质保持一致的条件下,向低海拔移栽过程中温度是导致珠芽蓼根、茎、叶生物量分配及根冠比变化的重要因素,而水分是垂穗披碱草根、茎、叶生物量分配及根冠比变化的重要驱动因素;矮嵩草根、茎、叶生物量分配及根冠比受其遗传因素影响较大。因此,在将来暖干化的背景下,青藏高原高寒草地植物生物量的分配将会发生改变,导致它们对资源(光照、水分和土壤养分)获取和利用的变化而改变它们的种间关系,从而影响群落的物种多样性与组成,最终可能导致生态系统功能的变化。     

Effects of Elevated CO2 and Temperature on Biomass Growth and Allocation in a Boreal Bioenergy Crop (Phalaris arundinacea L.) from Young and Old Cultivations

An auto-controlled climate system was used to study how a boreal bioenergy crop (reed canary grass, Phalaris arundinacea L., hereafter RCG) responded to a warming climate and elevated CO₂. Over one growing season (April–September of 2009), RCG from young and old cultivations (3 years [3-year] and 10 years [10-year]) was grown in closed chambers under ambient conditions (CON), elevated CO₂ (EC, approximately 700 μmol?mol^?1), elevated temperature (ET, ambient + approximately 3 °C) and elevated temperature and CO₂ (ETC). The treatments were replicated four times. Throughout the growing season, the above-ground (leaf and stem biomass) and below-ground biomasses were measured six times, representing various developmental stages (early stages: the first three stages, and late stages: the last three stages). Compared to the growth observed under CON, EC enhanced RCG biomass growth over the whole growing season (p?<?0.05), whereas ET increased RCG biomass growth in early stages but decreased growth in late stages, regardless of the cultivation age. However, the negative effect of ET later in the growing season was partially mitigated by CO₂ enrichment. Compared to CON plants, the final total biomass was 18 % higher for 3-year plants and 8 % higher for 10-year plants grown under EC. In comparison, for 3-year and 10-year plants, the biomass was 5 and 3 % lower under ET and 7 and 4 % greater under ETC, respectively. Under EC, the below-ground growth contributed more to the total biomass growth compared to the above-ground portion. The opposite situation was observed under ET and ETC. The climate-related changes in biomass growth were smaller in the old cultivation than in the young cultivation due to the lower net assimilation rate and lower specific leaf area in the old cultivation plants.     

Seedling growth strategies in Bauhinia species: comparing lianas and trees

BACKGROUND AND AIMS: Lianas are expected to differ from trees in their growth strategies. As a result these two groups of woody species will have different spatial distributions: lianas are more common in high light environments. This study determines the differences in growth patterns, biomass allocation and leaf traits in five closely related liana and tree species of the genus Bauhinia. METHODS: Seedlings of two light-demanding lianas (Bauhinia tenuiflora and B. claviflora), one shade-tolerant liana (B. aurea), and two light-demanding trees (B. purpurea and B. monandra) were grown in a shadehouse at 25% of full sunlight. A range of physiological, morphological and biomass parameters at the leaf and whole plant level were compared among these five species. KEY RESULTS: The two light-demanding liana species had higher relative growth rate (RGR), allocated more biomass to leaf production [higher leaf mass fraction (LMF) and higher leaf area ratio (LAR)] and stem mass fraction (SMF), and less biomass to the roots [root mass fraction (RMF)] than the two tree species. The shade-tolerant liana had the lowest RGR of all five species, and had a higher RMF, lower SMF and similar LMF than the two light-demanding liana species. The two light-demanding lianas had lower photosynthetic rates per unit area (A(area)) and similar photosynthetic rates per unit mass (A(mass)) than the trees. Across species, RGR was positively related to SLA, but not to LAR and A(area). CONCLUSIONS: It is concluded that the faster growth of light-demanding lianas compared with light-demanding trees is based on morphological parameters (SLA, LMF and LAR), and cannot be attributed to higher photosynthetic rates at the leaf level. The shade-tolerant liana exhibited a slow-growth strategy, compared with the light-demanding species.     

Biomass allocation to leaves, stems and roots: meta-analyses of interspecific variation and environmental control

We quantified the biomass allocation patterns to leaves, stems and roots in vegetative plants, and how this is influenced by the growth environment, plant size, evolutionary history and competition. Dose-response curves of allocation were constructed by means of a meta-analysis from a wide array of experimental data. They show that the fraction of whole-plant mass represented by leaves (LMF) increases most strongly with nutrients and decreases most strongly with light. Correction for size-induced allocation patterns diminishes the LMF-response to light, but makes the effect of temperature on LMF more apparent. There is a clear phylogenetic effect on allocation, as eudicots invest relatively more than monocots in leaves, as do gymnosperms compared with woody angiosperms. Plants grown at high densities show a clear increase in the stem fraction. However, in most comparisons across species groups or environmental factors, the variation in LMF is smaller than the variation in one of the other components of the growth analysis equation: the leaf area : leaf mass ratio (SLA). In competitive situations, the stem mass fraction increases to a smaller extent than the specific stem length (stem length : stem mass). Thus, we conclude that plants generally are less able to adjust allocation than to alter organ morphology.     

Distinguishing the biomass allocation variance resulting from ontogenetic drift or acclimation to soil texture

In resource-poor environments, adjustment in plant biomass allocation implies a complex interplay between environmental signals and plant development rather than a delay in plant development alone. To understand how environmental factors influence biomass allocation or the developing phenotype, it is necessary to distinguish the biomass allocations resulting from environmental gradients or ontogenetic drift. Here, we compared the development trajectories of cotton plants (Gossypium herbaceum L.), which were grown in two contrasting soil textures during a 60-d period. Those results distinguished the biomass allocation pattern resulting from ontogenetic drift and the response to soil texture. The soil texture significantly changed the biomass allocation to leaves and roots, but not to stems. Soil texture also significantly changed the development trajectories of leaf and root traits, but did not change the scaling relationship between basal stem diameter and plant height. Results of nested ANOVAs of consecutive plant-size categories in both soil textures showed that soil gradients explained an average of 63.64-70.49% of the variation of biomass allocation to leaves and roots. Ontogenetic drift explained 77.47% of the variation in biomass allocation to stems. The results suggested that the environmental factors governed the biomass allocation to roots and leaves, and ontogenetic drift governed the biomass allocation to stems. The results demonstrated that biomass allocation to metabolically active organs (e.g., roots and leaves) was mainly governed by environmental factors, and that biomass allocation to metabolically non-active organs (e.g., stems) was mainly governed by ontogenetic drift. We concluded that differentiating the causes of development trajectories of plant traits was important to the understanding of plant response to environmental gradients.     

不同刈牧强度对冷蒿生长与资源分配的影响

利用野外实验与盆栽实验,对不同刈牧强度下冷蒿生长与资源分配影响的研究结果表明,按比例刈割冷篙的再生生长大于留茬高度刈割,在生长季前期,不刈割冷蒿净生长高于刈割处理,而进入生长季中后期(8月中旬以后),轻度刈割净生长高于不刈割处理,冷篙种群生物量分配的总体格局是根>叶>茎,刈牧明显影响冷蒿生物量分配格局,尤其是叶和花的分配,3／4刈割或留茬4cm刈割叶生物量分配显著高于其它各处理,而花的生物量及其分配显著低于其它处理,根、茎生物量分配各处理间差异不显著．冷蒿有性生殖分配随刈牧强度的增加而降低,繁殖方式发生了改变,优先将光合产物分配给再生茎以及繁殖方式转向营养繁殖,通过克隆生长维持和扩大种群是冷蒿对强度放牧的生态适应对策。     

Seasonal flooding regimes influence survival,nitrogen fixation,and the partitioning of nitrogen and biomass in Alnus incana ssp. rugosa

Alteration of natural flooding regimes can expose lowlands to waterlogged soil conditions during any month of the year. The seasonality of flooding may have profound effects on the carbon and nitrogen budgets of N-fixing alders (Alnus spp.), and in turn, may impact the C and N economy of extensive alder-dominated, wetland ecosystems, including those dominated by speckled alder (Alnus incana ssp. rugosa). To better understand this process, two-year-old, nodulated seedlings of speckled alder were subjected to late spring (May 10 – July 10), summer (July 10 – September 8), and fall (September 8 – November 8) flooding treatments. Alders were root-flooded outdoors in tanks containing an N-free nutrient solution and compared with unflooded alders at the experimental site. Flooding arrested N fixation, photosynthesis, and growth of alders without recovery in all flooding treatments for the remainder of the growing season. Late spring and summer flooding resulted in complete mortality of alders while all seedlings survived flooding in the fall. Fall flooding increased foliar N resorption by 140% over unflooded seedlings. Eighty-seven percent of the total N fixed and 89% of biomass accumulation for the entire growing season occurred in unflooded alders after July 10. In unflooded alders, nitrogen fixation rates per unit mass declined by 63% for nodules, 28% for leaves, and 48% for whole seedlings during the fall, while total N fixed per plant in the fall was similar to that fixed in the summer. The majority of newly fixed N in unflooded alders was allocated to leaves before September 8 and to roots/nodules combined after September 8. In unflooded plants, the greatest proportion of new biomass was partitioned to leaves before July 10, to stems between July 10 and September 8, and equally to stems and roots/nodules after September 8. Fall-flooded alders did not increase root or nodule biomass. Proportional allocation of plant resources were such that the ratio of N fixed to seedling growth of unflooded alders decreased by 19% during summer before rebounding by 6% in fall. Seasonality of flooding alters seedling survival, growth, and resource allocation, and may be a critical determinant of speckled alder recruitment and occurrence in wetlands.     

Biomass allocation is an important determinant of the tannin concentration in growing plants

BACKGROUND AND AIMS: Condensed tannins (CTs) in the diet affect consumers in a concentration-dependent manner. Because of their importance in plant defence against herbivores and pathogens as well as their potential application against gastrointestinal parasites of ruminants in agronomy, an understanding of the seasonal dynamics of CT concentrations during plant growth is essential. METHODS: Over a vegetation period, CT concentrations in leaves, stems and roots and the biomass proportions between these organs were investigated in Onobrychis viciifolia, Lotus corniculatus and Cichorium intybus. Based on the experimental data, a model has been suggested to predict CT concentrations in harvestable biomass of these species. KEY RESULTS: During the experiment, leaf mass fractions of plants decreased from 85, 64, 85 to 30, 18, 39 % d. wt in Onobrychis, Lotus and Cichorium, respectively, and proportions of stems and roots increased accordingly. While CT concentrations almost doubled in leaves in Onobrychis (from 52 to 86 mg g(-1) d. wt, P<0.001) and Lotus (from 25 to 54 mg g(-1) d. wt, P<0.001), they were stable at low levels in expanding leaves of Cichorium (5 mg g(-1) d. wt) and in stems and roots of all investigated species. Due to an inverse effect of the increasing CT concentrations in leaves and simultaneous dilution from increasing proportions of 'CT-poor' stems, CT concentrations in harvestable biomass were stable over time in all investigated species: 62, 26 and 5 mg g(-1) d. wt for Onobrychis, Lotus and Cichorium, respectively. CONCLUSIONS: As a consequence of the unequal distribution of tannins in different plant parts and due to the changing biomass proportions between them, various herbivores (e.g. a leaf-eating insect and a grazing ruminant) may find not only different concentrations of CT in their diets but also different CT dynamics during the season. For the prediction of seasonal variations of CT concentrations, biomass allocation and accumulation of none-CT plant material are likely to be as important predictors as the knowledge of CT synthesis and its regulation.     

Salt effects on functional traits in model and in economically important Lotus species

A common stress on plants is NaCl‐derived soil salinity. Genus Lotus comprises model and economically important species, which have been studied regarding physiological responses to salinity. Leaf area ratio (LAR), root length ratio (RLR) and their components, specific leaf area (SLA) and leaf mass fraction (LMF) and specific root length (SRL) and root mass fraction (RMF) might be affected by high soil salinity. We characterised L. tenuis, L. corniculatus, L. filicaulis, L. creticus, L. burtii and L. japonicus grown under different salt concentrations (0, 50, 100 and 150 mm NaCl) on the basis of SLA, LMF, SRL and RMF using PCA. We also assessed effects of different salt concentrations on LAR and RLR in each species, and explored whether changes in these traits provide fitness benefit. Salinity (150 mm NaCl) increased LAR in L. burtii and L. corniculatus, but not in the remaining species. The highest salt concentration caused a decrease of RLR in L. japonicus Gifu, but not in the remaining species. Changes in LAR and RLR would not be adaptive, according to adaptiveness analysis, with the exception of SLA changes in L. corniculatus. PCA revealed that under favourable conditions plants optimise surfaces for light and nutrient acquisition (SLA and SRL), whereas at higher salt concentrations they favour carbon allocation to leaves and roots (LMF and RMF) in detriment to their surfaces. PCA also showed that L. creticus subjected to saline treatment was distinguished from the remaining Lotus species. We suggest that augmented carbon partitioning to leaves and roots could constitute a salt‐alleviating mechanism through toxic ion dilution.     

生物量分配影响三种不同海拔起源的松树生长

松属的思茅松（Pinus kesiya var. langbianensis）、云南松（P. yunnanensis）和高山松（P. densata）是组成中国西南不同海拔针叶森林的主要树种,然而这三个树种在发育速度尤其是高生长方面表现出明显的差异。为了弄清引起这些变异的生理和形态学原因,本文将三种松树种植于同一环境下,对其光合作用、生物量分配、生长速率和叶片性状进行了研究。研究发现,与来源于高海拔的树种相比,低海拔的树种有更高的株高、以及更大的干物质重量、相对生长速率、叶质比、茎质比和比叶面积,但叶片氮含量、碳含量和根质比较低。高海拔树种的光合速率并不明显低于低海拔树种。相对生长速率和树高均与叶质比呈显著正相关,与根质比负相关,但与最大光合速率没有显著关系。这些结果表明,生物量的分配式样和长期的形态特性能够更好地预测不同海拔松树的生长表现。     

Allometric Effects of <Emphasis Type="Italic">Agriophyllum squarrosum</Emphasis> in Response to Soil Nutrients,Water, and Population Density in the Horqin Sandy Land of China

We monitored the allometric effects for greenhouse-grown Agriophyllum squarrosum plants in response to variations in population density and the availability of soil nutrients and water. Biomass allocations were size-dependent. The plasticity of roots, stems, leaves, and reproductive effort was “true” in response to changes in nutrient content. At a low level of soil minerals, plants allocated more resources to the development of roots and reproductive organs than to leaves, but data for stem allocations were consistent for tradeoffs between the effects of nutrients and plant size. The plasticities of leaf allocation and reproductive effort were “true” whereas those of root and stem allocations were “apparent” in response to fluctuations in soil water, being a function of plant size. Decreasing soil water content was associated with higher leaf allocation and lower reproductive effort. Except for this “apparent” plasticity of leaf allocation, none was detected with population density on biomass allocation. Architectural traits were determinants of the latter. For roots, the determining trait was the ratio of plant height to total biomass; for stems and reproduction, plant height; and for leaves, the ratio of branch numbers to plant height.     

百里香无性系的克隆生长特征

植物克隆生长及其与生态适应性的关系是当今植物种群生态学研究的热点和前沿课题,但目前小半灌木克隆生长的研究开展不多。百里香(Thymus serpyllum var. asiaticus)是一种具有地面匍匐茎的草本状小半灌木,可在土壤侵蚀剧烈、基岩大面积裸露的砒砂岩区形成百里香单优群落,在维持生态系统稳定方面具有重要的生态学作用。皇甫川流域是砒砂岩大面积分布的典型区域,在这一地区对百里香无性系的克隆生长进行研究,不仅具有重要的学术价值,而且在生态环境建设方面也具有一定的现实意义。在皇甫川流域选择含三级分株的百里香无性系,对其各级分株的总生物量、各构件生物量及数量、各构件生物量占总生物量的百分比及其月变化进行了研究。结果表明: 1)母株与子代相比,在总生物量、构件生物量及数量上占有绝对优势,而且具有体型大、结构复杂的特点; 2)对生物量分配格局的研究显示,母株根的生物量在总生物量中所占的比例最大,其叶所占的比例较低。子代叶的生物量在总生物量中所占的比例最大,其根所占的比例较低;3)不同级别分株在生物量分配上的差异,揭示了相互连接的分株在功能上的差别,母株可能更侧重于养分和水分的吸收,子株则更侧重于光合生产;4)构件枝、茎、花生物量分配比月变化显示,子1代各构件的生长规律与母株的基本一致,子2代与母株和子1代的相比差异较大,分析认为这可能是分株间不同程度的生理整合作用造成的结果。     

氮磷添加对巨桉幼苗生物量分配和C:N:P化学计量特征的影响

巨桉(Eucalyptus grandis)是一种优良的速生用材树种, 了解氮(N)和磷(P)对巨桉生长、养分限制、化学计量特征的影响对于科学合理施肥具有重要意义。该实验以巨桉无性系组培苗为研究对象, 通过在酸性紫色土中设置不同施N或施P梯度, 研究巨桉幼苗各器官(根、茎、叶)生物量及碳(C)、N、P的分配和化学计量特征以及巨桉生长的养分限制状况。结果表明: 施N处理对巨桉根茎叶及总生物量的影响极显著, 增加了地上部分的生物量比例而显著降低了根系的生物量比例; 施P对巨桉幼苗总生物量影响不显著, 但显著提高了根的生物量分配比例, 对茎和叶的生物量分配没有显著影响。施N或施P显著改变了巨桉幼苗的N、P含量和化学计量比, 同时也显著影响了土壤与植物N:P的关系。施N可以促使酸性紫色土条件下巨桉对N的吸收而抑制对P的吸收, 施P则促进巨桉幼苗对P的吸收。施N对巨桉幼苗根茎叶的C、N、P分配特征有极显著影响, 而施P对巨桉幼苗根茎叶的C、N、P分配没有显著影响。施N极显著降低了巨桉幼苗N的利用率, 显著提高了P的利用率, 而施P处理极显著降低了巨桉幼苗P的利用率。从巨桉生物量沿施肥梯度和N:P的变化规律可以判断, 当叶片N:P < 15时, 巨桉的生长主要受到N的限制作用。施N可以显著地提高根茎叶的N:P比值, 缓解巨桉缺N的现象, 施P则进一步加剧了N元素的缺乏。     

Biomechanical properties of the emergent aquatic macrophyte Sparganium erectum: Implications for fine sediment retention in low energy rivers

This paper is concerned with the biomechanical properties of the emergent aquatic macrophyte, Sparganium erectum. We present observations of adjustments in the physical characteristics and biomechanical properties of S. erectum during the growing season (April-November) from the River Blackwater, UK. When a pulling device is attached to plant stems to measure their resistance to uprooting, individual plants show remarkable strength in their above- and below-ground biomass (median stem strength when stems break away from the underground biomass, 78 N, median rhizome strength, 39 N) and high resistance to uprooting (median uprooting resistance when entire plants uproot, 114 N). This provides the potential for the species to protect and reinforce the generally soft, silty sediments that it often retains and within which its rhizomes and roots develop in lower energy river environments. There is a propensity for plant stems to break before the plant is uprooted at the beginning and end of the growth season, but for the stems to have sufficient strength in mid season for plant uprooting to dominate. This ensures that rhizome and root systems remain relatively undisturbed at times when the silty sediments in which they grow are poorly protected by above-ground biomass. In contrast, rhizome strength remains comparatively invariant through the growing season, supporting the plant's potential to have a protective/reinforcing effect on fine sediments through the winter when above ground biomass is absent.     

Impact of defoliation intensity and frequency on N uptake and mobilization in Lolium perenne

The aim of the study was to evaluate the impact of defoliation intensity, defoliation frequency, and interactions with N supply on N uptake, N mobilization from and N allocation to roots, adult leaves, and growing leaves. Plants of Lolium perenne were grown under two contrasted N regimes. Defoliation intensity treatments consisted of a range of percentage leaf area removal (0, 25, 50, 75, or 100%). These treatments were applied in parallel to a set of plants previously undefoliated, and to a second set of plants which had been defoliated several times at a constant height. A (15)N tracer technique was used to quantify N uptake, mobilization, and allocation over a 7 d period. A significant reduction in plant N uptake was observed with the removal of more than 75% of lamina area, but only with high N supply. As defoliation intensity increased, the amount of N taken up and subsequently allocated to growing leaves during the labelling period was maintained at the expense of N allocation to roots and adult leaves. Increasing defoliation intensity increased the relative contribution of roots supplying mobilized N to growing leaves and decreased the relative contribution of adult leaves. Defoliation frequency did not substantially alter N uptake, mobilization, and allocation between roots, adult and growing leaves on a plant basis. However, tiller number per plant was largely increased under repeated defoliation, hence indicating that allocation and mobilization of N to growing leaves, on the basis of individual tillers, was decreased by defoliation frequency.     

生物量分配影响三种不同海拔起源的松树生长

松属的思茅松（Pinus kesiya var．1angbianensis）、云南松（P．yunnanensis）和高山松（P．densata）是组成中国西南不同海拔针叶森林的主要树种,然而这三个树种在发育速度尤其是高生长方面表现出明显的差异。为了弄清引起这些变异的生理和形态学原因．本文将三种松树种植于同一环境下,对其光合作用、生物量分配、生长速率和叶片性状进行了研究。研究发现,与来源于高海拔的树种相比,低海拔的树种有更高的株高、以及更大的干物质重量、相对生长速率、叶质比、茎质比和比叶面积,但叶片氮含量、碳含量和根质比较低。高海拔树种的光合速率并不明显低于低海拔树种。相对生长速率和树高均与叶质比呈显著正相关,与根质比负相关,但与最大光合速率没有显著关系。这些结果表明,生物量的分配式样和长期的形态特性能够更好地预测不同海拔松树的生长表现。     

克里雅绿洲旱生芦苇根茎叶C、N、P化学计量特征的季节变化

旱生芦苇在水分限制、元素匮乏的环境条件下,经长期进化适应形成了自身独特的生理生态特征,研究其C、N、P化学计量特征随生长季节的变化规律有助于深入了解该植物生存和适应策略。系统分析了克里雅绿洲旱生芦苇根、茎、叶的C、N、P化学计量特征及其季节动态,深入探讨了不同生长季、不同器官以及两因素的交互作用对以上特征的影响。结果表明:旱生芦苇C、N、P含量均值分别为393.36、12.43、1.25 mg/g,C∶N、N∶P、C∶P均值分别为54.55、9.96、441.27。整个生长季内芦苇各器官间C、N、P平均含量的变化规律一致,为叶茎根,C、N、P化学计量比的变化规律不一致;芦苇C含量随生长季节的变化不断增加,N、P随季节的变化逐渐减少,C、N、P化学计量比随季节的变化规律也不尽相同。对芦苇C、N、P含量及其化学计量比整体变异来源分析显示,生长季节的变化对芦苇C、P、C∶N、C∶P变化的贡献大于器官间差异,器官间差异对芦苇N、N∶P变化的贡献大于生长季节的变化;说明芦苇生长发育过程中各生长季各器官对元素的吸收利用具有特异性。结合N、P元素含量及N∶P值的大小可知,研究区芦苇生长受到N、P共同限制,且更易受N元素的限制。     

BIOMASS ALLOCATION AND GEOMETRY OF THE CLONAL FOREST HERB LAPORTEA CANADENSIS: ADAPTIVE RESPONSES TO THE ENVIRONMENT OR ALLOMETRIC CONSTRAINTS?

Using a conceptual model, I predicted the direction of biomass allocation and geometric responses to several environmental variables for Laportea canadensis, a clonal forb dominating the herbaceous stratum of many North American floodplain and mesic forests. Laportea stems and plants, especially dominant ones, generally (60%) respond as predicted to canopy opening, conspecific leaf area and density, and poor drainage, but are merely reduced in growth when growing on sandier soils. However, allometric relationships explain most of the variation in geometry and allocation. Still, variation in geometry and allocation (as great as among 21 species of herbs studied by Givnish [1982]), helps explain the success of Laportea in a range of microenvironments. In upland forests, stems in canopy gaps are tallest but allocate relatively less biomass to leaves than shaded stems, suggesting that interherb competition is the major problem faced under canopy gaps. Leaf morphology also changes with increasing canopy opening—individual leaves are larger, heavier, and thicker and are displayed on more steeply ascending petioles. Floodplain plants respond to light gaps mainly with changes in leaf morphology and display. With increasing conspecific density and leaf production, Laportea stems in both uplands and floodplains grow taller, allocate relatively more biomass to stems, and display leaves higher on the stem. The allocation and geometry of taller stems are more independent of density, and more closely affected by tree-canopy opening, than are small stems. Intermediate soil textures in floodplains promote maximum Laportea production; variations in other factors are less important. Poorly drained soils in floodplains (heavy-textured soils at low elevations) cause decreased Laportea height and absolute leaf weight, but increase relative allocation to leaves and roots, as predicted. On the other hand, Laportea appears poorly adapted to sandier soils. Rather than responding to sandier soils as predicted, Laportea's overall growth is reduced. Geometric responses of Laportea to environment are mediated by allometric realities: an increase in height favored in productive environments produces a concomitant decrease in relative leaf allocation. Although predicted (presumably adaptive) shifts are significant when plant size is accounted for, most of the variation in allocation and geometry is due to allometry.     

The effect of nutrient availability on biomass allocation patterns in 27 species of herbaceous plants

We investigated allocation to roots, stems and leaves of 27 species of herbaceous clonal plants grown at two nutrient levels. Allocation was analyzed as biomass ratios and also allometrically. As in other studies, the fraction of biomass in stems and, to a lesser extent, in leaves, was usually higher in the high-nutrient treatment than in the low-nutrient treatment, and the fraction of biomass in roots was usually higher under low-nutrient conditions. The relationship between the biomass of plant structures fits the general allometric equation, with an exponent 1 in most of the species. The different biomass ratios under the two nutrient conditions represented points on simple allometric trajectories, indicating that natural selection has resulted in allometric strategies rather than plastic responses to nutrient level. In other words, in most of the species that changed allocation in response to the nutrient treatment, these changes were largely a consequence of plant size. Our data suggest that some allocation patterns that have been interpreted as plastic responses to different resource availabilities may be more parsimoniously explained as allometric strategies.     

放牧践踏对黄蒿功能性状的影响

植物功能性状是构成植物个体的基础,从性状角度揭示植物个体特征的变化机制尤为重要。该研究以半干旱沙质草地优势草本植物黄蒿为研究对象,探讨不同践踏强度在生长季早期对其功能性状的影响。结果表明：(1)在群落水平上,放牧践踏显著降低了生长季早期植物群落高度;而在个体水平上,黄蒿株高不是响应放牧践踏的敏感性状。(2)黄蒿的叶长、叶宽随践踏强度增加呈先增加后减少的趋势,在中度践踏强度下达到最高;茎直径随践踏强度的增加而增加;根系和全株性状随践踏强度增加无显著差异。(3)黄蒿的叶片长度、叶片宽度、单叶面积随叶片厚度的增加而减小,且叶片与一级根数目、根茎叶生物量之间均呈显著正相关关系;放牧践踏会影响黄蒿茎直径,但对其他表型性状没有显著影响;在生长发育过程中,黄蒿通过不同表型性状的非对称变化进行性状之间的权衡,践踏强度的增加对生长季早期黄蒿根茎叶生物量积累的影响很小。研究认为,黄蒿在生长季早期对放牧践踏具有较强的抵抗力,这对生长季早期半干旱沙地放牧压力的选择和物种保护具有重要的指导意义。