Plasticity in seedling morphology,biomass allocation and physiology among ten temperate tree species in response to shade is related to shade tolerance and not leaf habit

• Mechanisms of shade tolerance in tree seedlings, and thus growth in shade, may differ by leaf habit and vary with ontogeny following seed germination. To examine early responses of seedlings to shade in relation to morphological, physiological and biomass allocation traits, we compared seedlings of 10 temperate species, varying in their leaf habit (broadleaved versus needle‐leaved) and observed tolerance to shade, when growing in two contrasting light treatments – open (about 20% of full sunlight) and shade (about 5% of full sunlight).
• We analyzed biomass allocation and its response to shade using allometric relationships. We also measured leaf gas exchange rates and leaf N in the two light treatments.
• Compared to the open treatment, shading significantly increased traits typically associated with high relative growth rate (RGR) – leaf area ratio (LAR), specific leaf area (SLA), and allocation of biomass into leaves, and reduced seedling mass and allocation to roots, and net assimilation rate (NAR). Interestingly, RGR was not affected by light treatment, likely because of morphological and physiological adjustments in shaded plants that offset reductions of in situ net assimilation of carbon in shade. Leaf area‐based rates of light‐saturated leaf gas exchange differed among species groups, but not between light treatments, as leaf N concentration increased in concert with increased SLA in shade.
• We found little evidence to support the hypothesis of a increased plasticity of broadleaved species compared to needle‐leaved conifers in response to shade. However, an expectation of higher plasticity in shade‐intolerant species than in shade‐tolerant ones, and in leaf and plant morphology than in biomass allocation was supported across species of contrasting leaf habit.

     

Shade and salinity responses of two dominant coastal wetland grasses: implications for light competition at the transition zone

BackgroundCoastal wetlands are threatened by the increased salinity that may result from sea level rise. Salinity stress alters species zonation patterns through changes in competitive outcome between species differing in salinity tolerance. This study therefore aimed to understand how salinity and light affect two dominant and competing coastal wetland grasses that differ in salt tolerance, height and photosynthetic metabolism.MethodsThe C₄ species Spartina anglica and the C₃ species Phragmites australis were grown at five salinity levels (0, 7, 14, 21 and 28 ppt) and two light fluxes (100 % and 50 % of natural daylight) in an outdoor experimental setup for 102 d with full access to nutrients.Key ResultsSalinity reduced the biomass, height and shoot density of P. australis from 81.7 g dry weight (DW), 0.73 m and 37 shoots per pot at a salinity of 0 ppt to 16.8 gDW, 0.3 m and 14 shoots per pot at a salinity of 28 ppt. Biomass, height and shoot density of S. anglica did not respond or were only slightly reduced at the highest salinity of 28 ppt. High salinity also resulted in a higher tissue concentration of N and P in P. australis. Both species had low ability to acclimate to the lower light flux. Shade acclimation in S. anglica occurred via modest changes in specific leaf area, pigment content and biomass allocation.ConclusionsHigh salinity reduced traits important for light competition and increased the nutrient concentration in P. australis leaf and root biomass, while this was overall unaffected in S. anglica. This is likely to reduce the competitive ability of P. australis over S. anglica for light because at high salinities the former cannot effectively shade the lower-growing S. anglica. Neither species effectively acclimates to shade, which could explain why S. anglica does not occur in the understorey of P. australis at low salinities.     

Relative importance of photosynthetic traits and allocation patterns as correlates of seedling shade tolerance of 13 tropical trees

Among 13 tropical tree species on Barro Colorado Island, species with high seedling mortality rates during the first year in shade had higher reltive growth rates (RGR) from germination to 2 months in both sun (23% full sun) and shade [2%, with and without lowered red: far red (R:FR) ratio] than shade tolerant species. Species with higher RGR in sun also had higher RGR in shade. These interspecific trends could be explained by differences in morphological traits and allocation paterns among species. Within each light regime, seedlings of shade-intolerant species had lower root: shoot ratios, higher leaf mass per unit area, and higher leaf area ratios (LAR) than shade tolerant species. In contrast, leaf gas exchange characteristics, or acclimation potential in these traits, had no relationship with seedling mortality rates in shade. In both shade tolerant and intolerant species, light saturated photosynthesis rates, dark respiration, and light compensation points were higher for sungrown seedlings than for shade-grown seedlings. Differences in R:FR ratio in shade did not affect gas exchange, allocation patterns, or growth rates of any species. Survival of young tree seedlings in shade did not depend on higher net photosynthesis or biomass accumulation rates in shade. Rather, species with higher RGR died faster in shade than species with lower RGR. This trend could be explained if survival depends on morphological characteristics likely to enhance defense against herbivores and pathogens, such as dense and tough leaves, a well-established root system, and high wood density. High construction costs for these traits, and low LAR as a consequence of these traits, should result in lower rates of whole-plant carbon gain and RGR for shade tolerant species than shade-intolerant species in shade as well as in sun.     

Sapling biomass allocation and growth in the understory of a deciduous hardwood forest

Above- and belowground tissues of co-occurring saplings (0.1-1 m height) of Acer saccharum Marsh. (very shade tolerant), Acer rubrum L. (shade tolerant), Fraxinus americana L. (intermediate shade tolerant), and Prunus serotina Ehrh. (shade intolerant) were harvested from a forest understory to test the hypothesis that the pattern of biomass allocation varied predictably with shade-tolerance rank. The placement and length of branches along the main axis were consistent with the formation of a monolayer of foliage for the tolerant and intermediate species. Other morphological characteristics did not vary predictably with shade-tolerance rank. The maintenance of high specific leaf area (SLA; leaf area/leaf mass) and leaf area ratio (LAR; leaf area/sapling mass) is considered important for growth under extreme shade, yet these traits were not clearly related to the shade-tolerance rank of these species. Fraxinus americana, an intermediate species, had the highest LAR and growth rate in the understory, and with the exception of P. serotina, the very shade-tolerant A. saccharum had the lowest LAR. Prunus serotina maintained a large starch-rich tap root and shoot dieback was common, yielding the largest root/shoot ratio for these species. The observed allocation patterns were not similar to the long-standing expectation for the phenotypic response of juvenile trees to shade, but were consistent with three hypothetical "growth strategies" in the understory: (1) the low SLA and LAR of A. saccharum may provide a measure of defense against herbivores and pathogens and thus promote persistence in the understory, (2) the high SLA for F. americana and high LAR for F. americana and A. rubrum may enable these species to achieve high growth rates in shade, and (3) the large carbohydrate stores of P. serotina may poise this species for opportunistic growth following disturbance. The relative importance of resistance to herbivores and pathogens vs. the maintenance of high growth rates may be important in evaluating the patterns of biomass allocation in the understory.     

Relationship between morphological and physiological responses to waterlogging and salinity in Sporobolus virginicus (L.) Kunth

The effects of waterlogging and salinity on morphological and physiological responses in the marsh grass Sporobolus virginicus (L.) Kunth were investigated in a 4×2 factorial experiment. Plants were subjected to four salinity levels (0, 100, 200 and 400 mol m^–3 NaCl) and two soil inundation conditions (drained and flooded) for 42 days. Flooding at 0 mol m^–3 NaCl caused initiation of adventitious surface roots, increased internal acration and plant height, induced alcohol dehydrogenase activity (ADH), and decreased belowground biomass and the number of culms per plant. Salinity increase from 0 to 400 mol m^–3 NaCl under drained conditions increased leaf and root proline concentrations and decreased photosynthesis, aboveground biomass, number of culms per plant and number of internodes per culm. Concurrent waterlogging and salinity induced ADH activity and adventitious surface roots but decreased plant height and aboveground biomass. Internal air space increased with waterlogging from 0 to 100 mol m^–3 NaCl but further increases in salinity to 400 mol m^–3 reduced air space. Combined waterlogging and salinity stresses, however, had no effect on photosynthesis or on the concentrations of proline in leaves or roots. These results are discussed in relation to the widespread colonization by S. virginicus of a wide range of coastal environments varying in soil salinity and in the frequency and intensity of waterlogging.     

The influence of salinity on growth,biomass production and photosynthesis of Eucalyptus camaldulensis Dehnh. and Dalbergia sissoo Roxb. seedlings

The influence of NaCl salinity on growth, dry-matter production and leaf photosynthesis of seedlings of Eucalyptus camaldulensis Dehnh. and Dalbergia sissoo Roxb. was studied by imposing 4 levels (40, 80, 120 and 160 mM) of NaCl in pot culture. Salinity up to 160 mM did not affect plant survival, but did affect plant growth and dry-matter production depending upon the species and salt concentration. NaCl reduced leaf number and dry-weight of all the plant components, but increased stem dry-weight, especially in E. camaldulensis. Salinization also stimulated total dry-matter production at all the salinity levels in E. camaldulensis but only at 40 mM in D. sissoo. The two species varied in protein and chlorophyll concentration and in leaf photosynthetic rate. Protein and chlorophyll concentration of the plants fell at all the levels of NaCl, except at 40 mM, where stimulation in the photosynthetic carbon assimilation of the plants occurred. However, no distinct relationship between leaf photosynthetic rate and dry-matter production was found. The study indicated that low salt concentrations generally stimulated growth, biomass production and rate of photosynthesis in both the species, and E. camaldulensis appeared more NaCl salt-tolerant than D. sissoo.     

Responses to Light and Water Availability of Four Invasive Melastomataceae in the Hawaiian Islands

Plant invasion by Neotropical Melastomataceae is prominent in Hawaii. To understand life history traits of four successful invasive Melastomataceae, two shade-intolerant herbs (Arthrostema ciliatum and Tibouchina herbacea) and two shade-tolerant woody species (Clidemia hirta, a shrub, and Miconia calvescens, a tree) were subjected to three light levels and two watering regimes in a greenhouse. Plant height, leaf number and area, biomass allocation, relative growth rate (RGR), carbon assimilation (A), leaf nutrient content, leaf construction costs (CC), specific leaf mass (SLM), and leaf spectral properties were determined at the end of the experimental period. Plant size, total biomass, RGR, A, CC, and SLM decreased, whereas leaf light transmittance and leaf N increased under low light in all species. The effects of water stress were weaker than light-stress effects. Relative growth rate of herbs grown in sun and partial shade (0.046 and 0.033 g g-1 d-1, respectively) was higher than in the woody species (0.027 and 0.020 g g-1 d-1). Woody species allocated more biomass to leaf production than herbs, which allocated more biomass to stem production. Shade increased allocation of biomass to leaves, and water stress increased the root-shoot ratio in all species. Partial shade increased leaf area ratios more in the herbs (140%) than in woody species (68%). Miconia calvescens and C. hirta had higher leaf absorbance (92%) than both herbs (79%). Maximum A under all light treatments was similar in all species, and there was substantial acclimation to the different light levels. Leaf construction cost was higher in the apparently long-lived leaves of the woody species. Relative growth rate, carbon allocation, and SLM showed larger changes to light and water stress than A and related photosynthetic parameters. All species showed responses qualitatively similar to those of other tropical species including the high acclimation potential to light, but the herbs exhibited the largest quantitative responses. When compared with a large group of native species, the four melastomes appear to be better suited to capture and use light, which is consistent with their rapid spread in mesic and disturbed Hawaiian environments.     

Phenotypic plasticity and population differentiation in response to salinity in the invasive cordgrass <Emphasis Type="Italic">Spartina densiflora</Emphasis>

Salinity and tidal inundation induce physiological stress in vascular plant species and influence their distribution and productivity in estuarine wetlands. Climate change-induced sea level rise may magnify these abiotic stressors and the physiological stresses they can cause. Understanding the potential of invasive plants to respond to predicted salinity increases will elucidate their potential niche breadth. To examine potential phenotypic plasticity and functional trait responses to salinity stress in the invasive cordgrass Spartina densiflora, we collected rhizomes from four invasive populations occurring from California to Vancouver Island, British Columbia on the Pacific Coast of North America. In a glasshouse common garden experiment, we measured plant traits associated with growth and allocation, photosynthesis, leaf pigments, and leaf chemistry and calculated plasticity indices across imposed salinity treatments. Fifteen of 21 leaf chemistry, pigment, morphological and physiological traits expressed plastic responses to salinity. When averaged across all measured traits, degree of plasticity did not vary among sampled populations. However, differences in plasticity among populations in response to salinity were observed for 9 of 21 measured plant traits. Leaf chemistry and adaxial leaf rolling trait responses demonstrated the highest degree of plasticity, while growth and allocation measures were less plastic. Phenotypic plasticity of leaf functional traits to salinity indicates the potential of S. densiflora to maintain invasive growth in response to rising estuarine salinity with climate change.     

光强对两种入侵植物生物量分配、叶片形态和相对生长速率的影响

 比较研究了不同光强下生长的（透光率分别为12.5%、36%、50%、100%）两种入侵性不同的外来种——紫茎泽兰（Eupatorium adenophorum）和兰花菊三七（Gynura sp.）的生物量分配、叶片形态和生长特性。结果表明: 1）两种植物叶片形态对光环境的反应相似。弱光下比叶面积（SLA）、平均单叶面积（MLS）和叶面积比（LAR）较大,随着光强的升高,SLA、MLS、LAR和叶根比（LARMR）降低。2）100%光强下紫茎泽兰叶生物量比（LMR）、叶重分数（LMF）和叶面积指数高于低光强下的值,也高于兰花菊三七,支持结构生物量比（SBR）则相反。强光下紫茎泽兰叶片自遮荫严重,这可能是其表现入侵性的重要原因之一;兰花菊三七分枝较多,避免了叶片自遮荫,较多的分枝利于种子形成对其入侵有利。3）随生长环境光强的升高,两种植物的净同化速率（NAR）、相对生长速率（RGR）和生长对NAR的响应系数均升高（但100%光强下兰花菊三七RGR降低）,平均叶面积比（LARm）和生长对LARm的响应系数均降低,但不同光强下LARm对生长的影响始终大于NAR。4）随着光强的减弱,两种植物都增加高度以截获更多光能,但它们的生物量分配策略不同,紫茎泽兰根生物量比（RMR）降低,SBR增大,而兰花菊三七SBR降低,RMR增大。紫茎泽兰的生物量分配策略更好的反应了弱光环境中的资源变化情况。结论：紫茎泽兰对光环境的适应能力强于兰花菊三七。     

Salinity tolerance of 'Valencia' orange trees on rootstocks with contrasting salt tolerance is not improved by moderate shade

The effects of shading in combination with salinity treatments were studied in citrus trees on two rootstocks with contrasting salt tolerance to determine if shading could reduce the negative effects of salinity stress. Well-nourished 2-year-old 'Valencia' orange trees grafted on Cleopatra mandarin (Cleo, relatively salt tolerant) or Carrizo citrange (Carr, relatively salt sensitive), were grown either under a 50% shade cloth or left unshaded in full sunlight. Half the trees received no salinity treatment and half were salinized with 50 mM Cl- during two 9 week salinity periods in the spring and autumn interrupted by an 11 week rainy period. The shade treatment reduced midday leaf temperature and leaf-to-air vapour pressure deficit regardless of salinity treatments. In non-salinized trees, shade increased midday CO2 assimilation rate (A(CO2)) and stomatal conductance, but had no effect on leaf transpiration (E(lf)). Shade also increased leaf chlorophyll and photosynthetic water use efficiency (A(CO2)/E(lf)) in leaves on both rootstocks and increased total plant dry weight in Cleo. The salinity treatment reduced leaf growth and leaf gas exchange parameters. Shade decreased Cl- concentrations in leaves of salinized Carr trees, but had no effect on leaf or root Cl- of trees on Cleo. There were no significant differences in leaf gas exchange parameters of shaded and unshaded salinized plants but the growth reduction from salinity stress was actually greater for shaded than for unshaded trees. Shaded trees on both rootstocks had higher leaf Na+ than unshaded trees after the first salinity period, and this shade-induced elevated leaf Na+ persisted after the second salinity period in trees on Carr. Thus, shading did not alleviate the negative effects of salinity on growth and Na+ accumulation.     

Sand Accretion and Salinity as Constraints on the Establishment of Leymus arenarius for Land Reclamation in Iceland

Seed harvested from wild populations ofLeymus arenarius is sownextensively in Iceland to stabilize sandy barrens, on the coastand inland. Sand accretion can reach 50 cm over 3 months insummer near the outwash of glacial rivers on the south coastof Iceland and thus may be an important factor influencing survivaland growth ofL. arenarius . Newly germinated seedlings had great potential for elongationin darkness (etiolation). The length of the longest etiolatedleaf increased significantly with seed mass. The etiolationresponse proved to be a good predictor of their ability to emergefrom burial with sand. The mean length of etiolated shoots wasapprox. 16 cm and 40% of seedlings emerged when germinatingseeds were buried with 15 cm of sand, whereas none emerged fromburial under 20 cm of sand. A moderately high and sustainedrate of sand deposition (2–4 cm week^-1), applied to 10-weekold seedlings in a glasshouse experiment, significantly increasedleaf length and the allocation of biomass to shoots, such thatoverall biomass was slightly but not significantly increased. The growth responses of seedlings of one coastal populationand two inland populations ofLeymus arenarius were comparedwhen challenged with salinities ranging from 0 to 600 mM NaClin sand culture. The numbers of tillers produced by the coastalpopulation was stimulated by salinity in the range 200–400mM NaCl, unlike their inland counterparts. The total dry massof the coastal population was less adversely affected by highsalinity than that of the two inland populations, mainly becauseroot biomass was reduced less; total leaf area was also slightlyless reduced in the coastal population. The reclamation of sandbarrens in Iceland with high accretion rates would benefit fromsowing seeds from larger-seeded populations, at a depth of 5–10cm; at coastal reclamation sites, it would be preferable touse seed from the more salt-tolerant coastal populations. Leymus arenarius ; lyme grass; sand accretion; etiolation; seedling emergence; seed mass; salt tolerance; revegetation     

The response of an emergent sedge Bolboschoenus medianus to salinity and nutrients

The potential for nutrient load (30, 100 and 350 g N m⁻² per year) to alter plant performance under saline conditions (control, 4.5, 9 and 13 dS m⁻¹) was examined in the sedge Bolboschoenus medianus. Relative growth rates (RGR) across nutrient loadings ranged from 30.2 to 41.8 mg g⁻¹ per day in controls and were reduced to 20.9–28.5 mg g⁻¹ per day by salinities of 13 dS m⁻¹. Whilst higher nutrient loads generally increased RGR, the response was smaller at higher salinities. Responses to salinity and nutrient load were specific. Nutrient load increased the RGR via increases in the leaf area ratio (LAR). The LAR ranged from 1.9 to 2.1 m² kg⁻¹ across salinity treatments at 30 g N m⁻² per year, and increased to 2.5–2.8 m² kg⁻¹ at 350 g N m⁻² per year. Salinity reduced the RGR via a reduction in the net assimilation rate (NAR). The NAR in control plants ranged from 14.7 to 16 g m⁻² per day across nutrient loadings and decreased to 11–12 g m⁻² per day at 13 dS m⁻¹. Carbon isotope discrimination of leaves decreased by 2–3‰ in response to 13 dS m⁻¹ at the lower nutrient loadings. A prominent response of B. medianus to salinity was a change in biomass allocation from culms to tubers. In contrast, the response to nutrient load was characterised by a shift in biomass allocation from roots to leaves.     

Physiological behaviour of loquat and anger rootstocks in relation to salinity and calcium addition

The salinity tolerance of two commercial rootstocks used for loquat plants (Eribotrya japonica Lindl.), loquat and anger, was studied in a pot experiment. The plants were irrigated using solutions containing 5 and 50mM NaCl and 5 and 25mM calcium acetate for 4 months. The growth, tissue mineral content, water status, and leaf gas exchange responses to salt treatment with and without additional calcium were examined. Plant growth was not modified by salinity in anger (50mM), but was reduced in loquat; leaf biomass and stem diameter were particularly affected. However, Cl(-) levels leaf increased with salinity to a greater extent in anger, while the Na(+) content increased to the same extent in both species, indicating that ion transport from root to leaves was not inhibited in either species. Additional calcium (25mM) reduced Na(+) and Cl(-) concentrations in both species, but did not minimise the effects of salinity on the growth of salt-treated loquat plants. The decrease in K(+) concentrations had no effect on growth, as anger was the most tolerant rootstock and had lowest leaf K(+) content. Salinity reduced the Ca(2+) concentration in the roots of both species. However, when calcium was added, the concentration of Ca(2+) increased in the roots of salinised plants. Leaf water potential at pre-dawn decreased significantly in both species under saline conditions. Leaf gas exchange, stomatal conductance and, in particular, net CO(2) assimilation, decreased with salinity only in loquat, indicating that photosynthesis could be the growth-limiting factor in this species.     

Seed germination and seedling performance of two Mediterranean tree species,holm oak (shape Quercus ilex L.) and Aleppo pine (shape Pinus halepensis Mill.): a multifactor experimental approach

A two-level multifactor experimental approach was used to compare seed germination and seedling performance of two Mediterranean tree species: the early successional Aleppo pine (Pinus halepensis Mill.) and the late successional holm oak (Quercus ilex L.). In a first experiment germination rate was evaluated under the combined effects of shade, nitrogen availability, and pine or holm oak leaf litter. In a second experiment we tested for the effects of shade, nutrient availability, and litter type on seedling survival, growth and biomass allocation. Holm oak showed higher germination rates under shaded than under unshaded conditions, while Aleppo pine showed no differences between shaded and unshaded conditions. Nitrogen availability and litter type had no significant effect on germination of either species. Both species showed increased RGR, but also higher mortality rates, when grown in an enriched nutrient environment. While Aleppo pine showed no differences in RGR and mortality rate under different shading levels, RGR decreased and mortality increased for holm oak in full light. Increased radiation decreased LAR, SLA and height:diameter ratio, and increased RWR and R/S in both species, although Aleppo pine showed more pronounced changes. Unlike Aleppo pine, holm oak responded to increased nutrient availability by decreasing R/S and increasing LAR. From these results, no seed-seedling conflicts were found in either species, but a trade-off does seem to exist for holm oak between biomass allocation traits deployed in response to increased nutrient availability and radiation. Aleppo pine outperformed holm oak under most environmental conditions tested and showed a wider regeneration niche.     

滨海滩涂地带乌哺鸡竹和淡竹离子含量变化及其与生长及光合作用的关系

在沿海滩涂防护林带低盐区(0.1%)、中盐区(0.2%)和重盐区(0.4%) 3个盐分梯度下,研究了栽植10年的乌哺鸡竹和淡竹Na⁺、K⁺、Ca²⁺、Mg²⁺含量变化及其与生长和光合作用的相关关系.结果表明: 从低盐区到重盐区,乌哺鸡竹的立竹密度和地径分别下降30.4%和28.8%,降幅低于淡竹的44.1%和31.2%;两竹种单株生物量下降,地上器官生物量降幅均显著高于地下器官;乌哺鸡竹和淡竹净光合速率(P_n)和PSⅡ最大光化学效率(F_v/F_m)分别下降57.6%和67.7%、6.1%和7.4%,乌哺鸡竹耐盐能力比淡竹强.随着土壤含盐量的增大,乌哺鸡竹和淡竹各器官Na⁺含量逐渐增加,K⁺、Ca²⁺、Mg²⁺含量逐渐降低.两竹种根Na⁺积累较多,而地上部分K⁺含量较高.盐胁迫环境导致乌哺鸡竹根Ca²⁺含量与淡竹叶片Mg²⁺含量明显下降.两竹种的生物量、P_n、F_v/F_m与Na⁺含量呈显著负相关,与K⁺、Ca²⁺含量呈显著正相关.     

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.     

Tree seedling growth in natural deep shade: functional traits related to interspecific variation in response to elevated CO2

The mechanisms for species-specific growth responses to changes in atmospheric CO₂ concentration within narrow ecological groups of species, such as shade-tolerant, late-successional trees, have rarely been addressed and are not well understood. In this study the underlying functional traits for interspecific variation in the biomass response to elevated CO₂ were explored for seedlings of five late-successional temperate forest tree species (Fagus sylvatica, Acer pseudoplatanus, Quercus robur, Taxus baccata, Abies alba). The seedlings were grown in the natural forest understorey in very low and low light microsites (an average of 1.3% and 3.4% full sun in this experiment), and were exposed to either current ambient CO₂ concentrations, 500, or 660 µl CO₂ l^-1 in 36 open-top chambers (OTC) over two growing seasons. Even across the narrow range of successional status and shade tolerance, the study species varied greatly in photosynthesis, light compensation point, leaf dark respiration (R_d), leaf nitrogen concentration, specific leaf area (SLA), leaf area ratio (LAR), and biomass allocation among different plant parts, and showed distinct responses to CO₂ in these traits. No single species combined all characteristics traditionally considered as adaptive to low light conditions. At very low light, the CO₂ stimulation of seedling biomass was related to increased LAR and decreased R_d, responses that were observed only in Fagus and Taxus. At slightly higher light levels, interspecific differences in the biomass response to elevated CO₂ were reversed and correlated best with leaf photosynthesis. The data provided here contribute to a mechanistic process-based understanding of distinct response patterns in co-occurring tree species to elevated CO₂ in natural deep shade. I conclude that the high variation in physiological and morphological traits among late-successional species, and the consequences for their responses to slight changes in resource availability, have previously been underestimated. The commonly used broad definitions of functional groups of species may not be sufficient for the understanding of recruitment success and dynamic changes in species composition of old-growth forests in response to rising concentrations of atmospheric CO₂.     

Independent effects of drought and shade on growth,biomass allocation and leaf morphology of a flammable perennial grass Tetrarrhena juncea R.Br

Knowing the abundance of different plant species provides insights into the properties of vegetation communities, such as flammability. Therefore, a fundamental goal in ecology is identifying environmental conditions affecting the abundance of plant species across landscapes. Water and light are important environmental moderators of plant growth, and by extension, abundance. In the context of understanding forest flammability, the abundance of a flammable plant species in terms of its cover or biomass can shape the flammability of the whole vegetation community. We conducted a glasshouse experiment to determine the impact of drought and shade on growth, biomass allocation and leaf morphology of forest wiregrass Tetrarrhena juncea R.Br., a rhizomatous perennial grass. When it is abundant, this species is known to contribute substantially to the flammability of eucalypt forest understories (via both ignitability and combustibility). Contrasting hypotheses in the literature predict that drought can have a weaker, stronger, or independent (uncoupled) impact on plant growth when light is limiting. We used a randomized complete block design with ten treatments from the combination of two water levels (drought, well-watered) and five light levels (100%, 80%, 60%, 40%, 20%). Drought and shade were found to have independent effects on wiregrass growth, biomass allocation, and leaf morphology, supporting the uncoupled hypothesis. Growth showed greater plasticity in response to drought, while biomass allocation and leaf morphology showed greater plasticity in response to shade. Our results suggest that wiregrass is more likely to be abundant in terms of its cover and biomass when water is not limiting. Under these conditions, the increased wiregrass abundance could create a window of increased flammability for the forest ecosystem.

     

Grow slowly,persist, dominate—Explaining beech dominance in a primeval forest

Being able to persist in deep shade is an important characteristic of juvenile trees, often leading to a strong dominance of shade‐tolerant species in forests with low canopy turnover and a low disturbance rate. While leaf, growth, and storage traits are known to be key components of shade tolerance, their interplay during regeneration development and their influence on juveniles'' survival time remains unclear. We assessed the ontogenetic effects of these three traits on the survival time of beech (Fagus sylvatica), and Norway and sycamore maples (Acer pseudoplatanus, Acer platanoides) in a primeval beech forest. Biomass allocation, age, and content of nonstructural carbohydrates (NSC) were measured in the stems and roots of 289 seedlings and saplings in high‐ and low‐vitality classes. Saplings experienced a trade‐off between absolute growth rate (AGR) and storage (NSC) as the leaf area ratio (LAR) decreases with biomass development. High LAR but low AGR and low NSC corresponded to beech with a marked ability to persist in deep shade while awaiting canopy release. In turn, a comparably small LAR in combination with a high AGR and higher storage (NSC), as observed in Norway maple and sycamore maple, reduced sapling survival time, thus offering an explanation for beech dominance and maple disappearance in the undergrowth of old‐growth beech forests.     

LIGHT,TEMPERATURE AND SALINITY EFFECTS ON GROWTH,LEAF ANATOMY AND PHOTOSYNTHESIS OF DISTICHLIS SPICATA (L.) GREENE

The effects of light, temperature, and salinity on growth, net CO₂ exchange and leaf anatomy of Distichlis spicata were investigated in controlled environment chambers. When plants were grown at low light, growth rates were significantly reduced by high substrate salinity or low temperature. However, when plants were grown at high light, growth rates were not significantly affected by temperature or salinity. The capacity for high light to overcome depressed growth at high salinity cannot be explained completely by rates of net photosynthesis, since high salinity caused decreases in net photosynthesis at all environmental conditions. This salinity-induced decrease in net photosynthesis was caused largely by stomatal closure, although plants grown at low temperature and low light showed significant increases in internal leaf resistance to CO₂ exchange. Increased salinity resulted in generally thicker leaves with lower stomatal density but no significant differences in the ratio of mesophyll cell surface area to leaf area. Salinity and light during growth did not significantly affect rates of dark respiration. The mechanisms by which Distichlis spicata tolerates salt appear to be closely coulpled to the utilization of light energy. Salt-induced leaf succulence is of questionable importance to gas exchange at high salinity in this C₄ species.