Sapling growth and survivorship as a function of light in a mesic forest of southeast Texas, USA

For seven species in a mature mesic forest in southeast Texas, we estimated species-specific parameters representing radial growth in high light and low light for tree saplings. Shade-intolerant species had higher asymptotic growth rates and lower low-light growth than tolerant species. Inspection of species positions on graphs of low-light growth versus high-light growth suggested that there was a trade-off between these two processes across species. By linking functions of growth versus light and mortality versus growth, we also found that shade-intolerant species had higher mortality risk at low light and stronger sensitivity of mortality to light than shade-tolerant species. Moreover, we found that low-light survival and high-light growth were negatively correlated across species. In contrast to northern hardwood forests, where sapling survival in low light may be achieved at the expense of growth, our results suggested that shade-tolerant species in this southern mixed forest can grow faster as well as survive better than shade-intolerant species in low light. We conclude that both sapling growth and survival are important components of shade tolerance and their relationships may be system-specific.     

Does shade improve light interception efficiency? A comparison among seedlings from shade-tolerant and -intolerant temperate deciduous tree species

Here, we tested two hypotheses: shading increases light interception efficiency (LIE) of broadleaved tree seedlings, and shade-tolerant species exhibit larger LIEs than do shade-intolerant ones. The impact of seedling size was taken into account to detect potential size-independent effects on LIE. LIE was defined as the ratio of mean light intercepted by leaves to light intercepted by a horizontal surface of equal area. Seedlings from five species differing in shade tolerance (Acer saccharum, Betula alleghaniensis, A. pseudoplatanus, B. pendula, Fagus sylvatica) were grown under neutral shading nets providing 36, 16 and 4% of external irradiance. Seedlings (1- and 2-year-old) were three-dimensionally digitized, allowing calculation of LIE. Shading induced dramatic reduction in total leaf area, which was lowest in shade-tolerant species in all irradiance regimes. Irradiance reduced LIE through increasing leaf overlap with increasing leaf area. There was very little evidence of significant size-independent plasticity of LIE. No relationship was found between the known shade tolerance of species and LIE at equivalent size and irradiance.     

Carbon allocation to defense, storage, and growth in seedlings of two temperate broad-leaved tree species

Optimal carbon allocation to growth, defense, or storage is a critical trait in determining the shade tolerance of tree species. Thus, examining interspecific differences in carbon allocation patterns is useful when evaluating niche partitioning in forest communities. We hypothesized that shade-tolerant species allocate more carbon to defense and storage and less to growth compared to shade-intolerant species. In gaps and forest understory, we measured relative growth rates (RGR), carbon-based defensive compounds (condensed tannin, total phenolics), and storage compounds (total non-structural carbohydrate; TNC) in seedlings of two tree species differing in shade tolerance. RGR was greater in the shade-intolerant species, Castanea crenata, than in the shade-tolerant species, Quercus mongolica var. grosseserrata, in gaps, but did not differ between the species in the forest understory. In contrast, concentrations of condensed tannin and total phenolics were greater in Quercus than in Castanea at both sites. TNC pool sizes did not differ between the species. Condensed tannin concentrations increased with increasing growth rate of structural biomass (GRstr) in Quercus but not in Castanea. TNC pool sizes increased with increasing GRstr in both species, but the rate of increase did not differ between the species. Accordingly, the amount of condensed tannin against TNC pool sizes was usually higher in Quercus than in Castanea. Hence, Quercus preferentially invested more carbon in defense than in storage. Such a large allocation of carbon to defense would be advantageous for a shade-tolerant species, allowing Quercus to persist in the forest understory where damage from herbivores and pathogens is costly. In contrast, the shade-intolerant Castanea preferentially invested more carbon in growth rather than defense (and similar amounts in storage as Quercus), ensuring establishment success in gaps, where severe competition occurs for light among neighboring plants. These contrasting carbon allocation patterns are closely associated with strategies for persistence in these species’ respective habitats.     

Winning and Losing Tree Species of Reassembly in Minnesota’s Mixed and Broadleaf Forests

We examined reassembly of winning and losing tree species, species traits including shade and fire tolerance, and associated disturbance filters and forest ecosystem types due to rapid forest change in the Great Lakes region since 1850. We identified winning and losing species by changes in composition, distribution, and site factors between historical and current surveys in Minnesota’s mixed and broadleaf forests. In the Laurentian Mixed Forest, shade-intolerant aspen replaced shade-intolerant tamarack as the most dominant tree species. Fire-tolerant white pine and jack pine decreased, whereas shade-tolerant ashes, maples, and white cedar increased. In the Eastern Broadleaf Forest, fire-tolerant white oaks and red oaks decreased, while shade-tolerant ashes, American basswood, and maples increased. Tamarack, pines, and oaks have become restricted to sites with either wetter or sandier and drier soils due to increases in aspen and shade-tolerant, fire-sensitive species on mesic sites. The proportion of shade-tolerant species increased in both regions, but selective harvest reduced the applicability of functional groups alone to specify winners and losers. Harvest and existing forestry practices supported aspen dominance in mixed forests, although without aspen forestry and with fire suppression, mixed forests will transition to a greater composition of shade-tolerant species, converging to forests similar to broadleaf forests. A functional group framework provided a perspective of winning and losing species and traits, selective filters, and forest ecosystems that can be generalized to other regions, regardless of species identity.     

Carbohydrate storage,survival, and growth of two evergreen <Emphasis Type="Italic">Nothofagus</Emphasis> species in two contrasting light environments

A number of traits have been attributed important roles in tolerance of shade by plants. Some explanations emphasize traits enhancing net carbon gain; others emphasize energy conservation traits such as storage of non-structural carbohydrates (NSC). To date, cross-species studies have provided mixed support for the role of NSC storage in low-light survival. We examined NSC status, survival, biomass, and growth of large seedlings of two evergreen species of differing shade tolerance (Nothofagus nitida and N. dombeyi) grown in deep shade and 50% light for two growing seasons. We expected to find higher NSC concentration in the more shade-tolerant N. nitida and since allocation to storage involves sacrificing growth, higher growth rate in the shade-intolerant N. dombeyi. NSC concentration of both species was >twofold higher in 50% light than in deep shade, and in roots and stems did not differ significantly between species in either environment. NSC contents per plant were also similar between dead and living plants in deep shade. N. dombeyi outgrew N. nitida in 50% light, while this pattern was reversed in deep shade. Survival in deep shade was not correlated with NSC concentration. Leaf mass fraction was similar between species in 50% light, but lower in N. dombeyi in deep shade. Results provide little evidence of a link between carbohydrate storage and low-light survival in Nothofagus species, and support the view that understorey survival is primarily a function of net carbon gain. Patterns of variation in NSC concentration of the temperate species we studied are likely dominated by more important influences than adaptation to shade, such as limitation of growth or adaptation to cold stress.     

间伐对日本落叶松人工林林下更新木本植物组成和生长影响的时间效应

明确间伐对针叶人工林林下更新阔叶木本植物组成和生长影响的时间效应,可为促进人工纯林向针阔混交林转化、进而解决纯林生产与生态功能无法兼顾的问题提供参考。本研究以我国北方重要针叶人工林——日本落叶松人工林为例,比较间伐后短期(1～3年)、中期(4～9年)和长期(>9年)人工林林下更新木本植物组成的差异;并选择具有较高重要值且与落叶松互惠共生的3种更新树种蒙古栎、色木槭、糠椴,分析不同间伐时期人工林内光照(郁闭度)与更新木本植物生长的关系。结果表明: 在间伐样地内更新的阔叶木本植物达46种,不同间伐时期样地内共有且占据优势的乔木树种为色木槭,灌木树种为忍冬和卫矛。随间伐后时间的推移,更新物种数逐渐减少,但乔灌比呈增大的趋势;中性树种占据优势地位。间伐对更新树种生长影响的时间效应受树种耐荫性的影响,糠椴的基径和树高均高于蒙古栎和色木槭。随间伐后时间的延长,糠椴的基径、色木槭的树高对光照响应较敏感,分别倾向于采用“耐荫策略”和“避荫策略”以适应间伐后变化的环境。间伐对日本落叶松人工林林下更新阔叶木本植物组成和生长的影响具有明显的时间效应;在制定以促进人工林林下树种更新为目标的间伐措施时,应该考虑适当延长间伐间隔,从而保证糠椴和色木槭等阔叶树幼苗的生长,使其进入林冠层,促进形成针阔混交林,实现人工林的可持续发展。     

Photosynthetic sunfleck utilization potential of understory saplings growing under elevated CO<Subscript>2</Subscript> in FACE

Few studies have evaluated elevated CO₂ responses of trees in variable light despite its prevalence in forest understories and its potential importance for sapling survival. We studied two shade-tolerant species (Acer rubrum, Cornus florida) and two shade-intolerant species (Liquidambar styraciflua, Liriodendron tulipifera) growing in the understory of a Pinus taeda plantation under ambient and ambient+200 ppm CO₂ in a free air carbon enrichment (FACE) experiment. Photosynthetic and stomatal responses to artificial changes in light intensity were measured on saplings to determine rates of induction gain under saturating light and induction loss under shade. We expected that growth in elevated CO₂ would alter photosynthetic responses to variable light in these understory saplings. The results showed that elevated CO₂ caused the expected enhancement in steady-state photosynthesis in both high and low light, but did not affect overall stomatal conductance or rates of induction gain in the four species. Induction loss after relatively short shade periods (<6 min) was slower in trees grown in elevated CO₂ than in trees grown in ambient CO₂ despite similar decreases in stomatal conductance. As a result leaves grown in elevated CO₂ that maintained induction well in shade had higher carbon gain during subsequent light flecks than was expected from steady-state light response measurements. Thus, when frequent sunflecks maintain stomatal conductance and photosynthetic induction during the day, enhancements of long-term carbon gain by elevated CO₂ could be underestimated by steady-state photosynthetic measures. With respect to species differences, both a tolerant, A. rubrum, and an intolerant species, L. tulipifera, showed rapid induction gain, but A. rubrum also lost induction rapidly (c. 12 min) in shade. These results, as well as those from independent studies in the literature, show that induction dynamics are not closely related to species shade tolerance. Therefore, it cannot be concluded that shade-tolerant species necessarily induce faster in the variable light conditions common in understories. Although our study is the first to examine dynamic photosynthetic responses to variable light in contrasting species in elevated CO₂, studies on ecologically diverse species will be required to establish whether shade-tolerant and -intolerant species show different photosynthetic responses in elevated CO₂ during sunflecks. We conclude that elevated CO₂ affects dynamic gas exchange most strongly via photosynthetic enhancement during induction as well as in the steady state. Received: 1 April 1999 / Accepted: 16 August 1999     

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.     

Growth,biomass distribution and CO2 exchange of northern hardwood seedlings in high and low light: relationships with successional status and shade tolerance

The physiology, morphology and growth of first-year Betula papyrifera Marsh., Betula alleghaniensis Britton, Ostrya virginiana (Mill.) K. Koch, Acer saccharum Marsh., and Quercus rubra L. seedlings, which differ widely in reported successional affinity and shade tolerance, were compared in a controlled high-resource environment. Relative to late-successional, shade-tolerant Acer and Ostrya species, early-successional, shade-intolerant Betula species had high relative growth rates (RGR) and high rates of photosynthesis, nitrogen uptake and respiration when grown in high light. Fire-adapted Quercus rubra had intermediate photosynthetic rates, but had the lowest RGR and leaf area ratio and the highest root weight ratio of any species. Interspecific variation in RGR in high light was positively correlated with allocation to leaves and rates of photosynthesis and respiration, and negatively related to seed mass and leaf mass per unit area. Despite higher respiration rates, early-successional Betula papyrifera lost a lower percentage of daily photosynthetic CO₂ gain to respiration than other species in high light. A subset comprised of the three Betulaceae family members was also grown in low light. As in high light, low-light grown Betula species had higher growth rates than tolerant Ostrya virainiana. The rapid growth habit of sarly-successional species in low light was associated with a higher proportion of biomass distributed to leaves, lower leaf mass per unit area, a lower proportion of biomass in roots, and a greater height per unit stem mass. Variation in these traits is discussed in terms of reported species ecologies in a resource availability context.     

Mid-successional stand dynamics in a cool-temperate conifer-hardwood forest in northern Japan

Stand dynamics was studied over 13 years in a cool-temperate conifer-hardwood forest, northern Japan. A total 30 hardwood species and one conifer, Abies sachalinensis, larger than 1.5 cm DBH were recorded. The total stand density was 1677 trees ha⁻¹ at the beginning, decreasing to 1184 trees ha⁻¹ (30% reduction) over the study period, but the total stand basal area was almost unchanged (about 49 m² ha⁻¹). This large reduction in total density was mainly due to the death of saplings and infrequent recruitment. Number of recruits gradually decreased with time, while that of dead trees was constant. Cause of death of small trees was mainly due to suppression by tall trees. Skewness of the DBH frequency distribution varied among the species. A less skewed frequency distribution (i.e., few number of saplings) was shown by shade-intolerant species such as Populus maximowiczii and Betula maximowicziana, and a more skewed frequency distribution (i.e., large number of saplings) by shade-tolerant species such as Acer mono and Tilia japonica. DBH frequency distribution changed to less skewed patterns with reduction of density in most species during the census period. Rank of shade tolerance positively correlated with tree density and skewness, and negatively correlated with mean DBH. Skewness also positively correlated with recruitment rates. Furthermore, rank of shade tolerance positively correlated with seed size. These results suggest that shade-intolerant species regenerated immediately after disturbances by wide dispersal of small seeds, but their recruitment was interrupted after that. By contrast, shade-tolerant species were able to recruit even after the ceasation of recruitment of shade-intolerant species, but suffered severe mortality due to the increasing shading with the progress of stand development. This study suggests that the stand is still developing, with changes in species composition and size structure, and that species differences in shade tolerance and seed size are important for the stand structural changes.     

Contrasting patterns of diameter and biomass increment across tree functional groups in Amazonian forests

Species' functional traits may help determine rates of carbon gain, with physiological and morphological trade-offs relating to shade tolerance affecting photosynthetic capacity and carbon allocation strategies. However, few studies have examined these trade-offs from the perspective of whole-plant biomass gain of adult trees. We compared tree-level annual diameter increments and annual above-ground biomass (AGB) increments in eight long-term plots in hyper-diverse northwest Amazonia to wood density (rho; a proxy for shade tolerance), whilst also controlling for resource supply (light and soil fertility). rho and annual diameter increment were negatively related, confirming expected differences in allocation associated with shade tolerance, such that light-demanding species allocate a greater proportion of carbon to diameter gain at the expense of woody tissue density. However, contrary to expectations, we found a positive relationship between rho and annual AGB increment in more fertile sites, although AGB gain did not differ significantly with rho class on low-fertility sites. Whole-plant carbon gain may be greater in shade-tolerant species due to higher total leaf area, despite lower leaf-level carbon assimilation rates. Alternatively, rates of carbon loss may be higher in more light-demanding species: higher rates of litterfall, respiration or allocation to roots, are all plausible mechanisms. However, the relationships between rho and AGB and diameter increments were weak; resource availability always exerted a stronger influence on tree growth rates.     

Carbon Translocation as Affected by Shade in Saplings of Shade Tolerant and Intolerant Species

Carbon translocation was affected by shade in different tropical tree species differing in successional status and degree of shade tolerance. Plants of the early-successional shade-intolerant species Cecropia pachystachya and Schizolobium parahyba and of the late-successional shade-tolerant species Myroxylon peruiferum and Hymenaea courbaril were grown under full sun (FS) and natural shade treatments (NS) and assessed for [¹⁴C]-sucrose translocation. Most of the ¹⁴C was retained in the fed leaf after a 24 h translocation period. Under FS, the growing apical part of the plant was the most intense sink for most species. Shade affected growth and sink intensity differently in early and late successional species. Growth was more markedly affected in the early species. Whereas these continued to invest carbon into the growing apical part of the plant under shade conditions, the late successional species invested relatively more into other sinks. This revised version was published online in July 2006 with corrections to the Cover Date.     

Altering Light Availability to Restore Invaded Forest: The Predictive Role of Plant Traits

Many studies have demonstrated that reduced light availability, which can be manipulated at local scales by planting or seeding canopy species, can curtail the growth of invasive species and promote the growth of native species. Species differences in functional traits, such as light use and stress tolerance, may be used to determine how native and invasive species will respond to these resource manipulations. We altered light availability in a mesic Hawaiian forest understory and found that low light levels reduced the biomass and growth of two invasive grasses (Pennisetum clandestinum and Ehrharta stipoides) relative to two native shrubs (Pipturus albidus and Coprosma rhynchocarpa) and two native canopy species (Metrosideros polymorpha and Acacia koa). Native species generally displayed traits associated with shade tolerance (high quantum yield, chlorophyll content, and leaf mass per area), whereas the two invasive grasses displayed traits associated with shade intolerance (high photosynthetic rate and growth rate). Several key traits pertaining to light acquisition and shade tolerance (quantum yield, chlorophyll content, and leaf mass per area) predicted seedling survival in low‐light treatments. Our data suggest that differences in light use among native and invasive species can help to determine the utility of resource manipulation as a restoration tool and, more specifically, to predict which native species will be optimal for restoration efforts that manipulate light availability.     

Physiological responses to flooding and light in two tree species native to the Amazonian floodplains

In Amazonian floodplains, plant survival is determined by adaptations and growth strategies to effectively capture sunlight and endure extended periods of waterlogging. By measuring gas exchange, quantum efficiency of photosystem 2 (PSII), and growth parameters, we investigated the combined effects of flooding gradients and light on two common evergreen floodplain tree species, the light-tolerant Cecropia latiloba and the shade-tolerant Pouteria glomerata. Individual plants were subjected to different combinations of light and flooding intensity in short-term and long-term experiments. Plants of C. latiloba lost all their leaves under total submersion treatments (plants flooded to apex and with reduced irradiance) and showed highest maximum assimilation rates (A_max) in not flooded, high light treatments (6.1 μmol CO₂ m⁻² s⁻¹). Individuals of P. glomerata showed similar patterns, with A_max increasing from 1.9 μmol CO₂ m⁻² s⁻¹ under total flooding to 7.1 μmol CO₂ m⁻² s⁻¹ in not flooded, high light treatments. During the long-term flooding experiment, quantum efficiency of PSII (F_v/F_m) of C. latiloba was not affected by partial flooding. In contrast, in P. glomerata F_v/F_m decreased to values below 0.73 after 120 days of total flooding. Moreover, total submergence led P. glomerata to reduce significantly light saturation point (LSP), as compared to C. latiloba. For both species morphological adjustments to long-term flooding, such as the production of adventitious roots, resulted in reduced total biomass, relative growth rate (RGR) and leaf mass ratio (LMR). Growth increase in C. latiloba seemed to be more limited by low-light than by flooding. Therefore, the predominant occurrence of this species is in open areas with high light intensities and high levels of inundation. In P. glomerata flooding induced high reductions of growth and photosynthesis, whereas light was not limiting. This species is more abundant in positions where irradiance is reduced and periods of submergence are slightly modest. We could show that the physiological requirements are directly responsible for the flooding (C. latiloba) and shade (P. glomerata) tolerance of the two species, which explains their local distribution in Amazonian floodplain forests.     

Contrasting modes of light acclimation in two species of the rainforest understory

In tropical rainforests, the increased light associated with the formation of treefall gaps can have a critical impact on the growth and survivorship of understory plants. Here we examine both leaf-level and whole-plant responses to simulated light gap formation by two common shade-tolerant shrubs, Hybanthus prunifolius and Ouratea lucens. The species were chosen because they differed in leaf lifespans, a trait that has been correlated with a number of leaf- and plant-level processes. Ouratea leaves typically live about 5 years, while Hybanthus leaves live less than 1 year. Potted plants were placed in the understory shade for 2 years before transfer to a light gap. After 2 days in high light, leaves of both species showed substantial photoinhibition, including reduced CO₂ fixation, F _v/F _m and light use efficiency, although photoinhibition was most severe in Hybanthus. After 17 days in high light, leaves of both species were no longer photoinhibited. In response to increased light, Ouratea made very few new leaves, but retained most of its old leaves which increased photosynthetic capacity by 50%. Within a few weeks of transfer to high light, Hybanthus had dropped nearly all of its shade leaves and made new leaves that had a 2.5-fold greater light-saturated photosynthetic rate. At 80 days after transfer, the number of new leaves was 4.9-fold the initial leaf number. After 80 days in high light, Hybanthus had approximately tenfold greater productivity than Ouratea when leaf area, photosynthetic capacity, and leaf dark respiration rate were all taken into account. Although both species are considered shade tolerant, we found that their growth responses were quite different following transfer from low to high light. The short-lived Hybanthus leaves were quickly dropped, and a new canopy of sun leaves was produced. In contrast, Ouratea showed little growth response at the whole-plant level, but a greater ability to tolerate light stress and acclimate at the leaf level. These differences are consistent with predictions based on leaf lifespan and are discussed within the context of other traits associated with shade-tolerant syndromes. Received: 25 March 1999 / Accepted: 16 August 1999     

Influence of light level on the growth and morphology of saplings in a panamanian forest

Leaf spacing and aboveground growth were monitored in saplings of ten species in a range of light environments in a Panamanian lowland humid forest. One- to 2-m-tall individuals of the chosen species had intermediate to large leaves on stems with few or no branches. Saplings in high light environments grew faster in height and produced more biomass per unit leaf area than shaded saplings for all species. These growth responses involved morphological plasticity with greater extension per unit biomass increment increasing the height growth rate of gap-grown saplings and greater biomass allocation to leaves decreasing the whole plant light compensation point in shade. The relative performance of the species also varied across the light gradient and was related to differences in leaf lifespan and specific leaf mass. Light-demanding species grew as rapidly in shade as shade-tolerant species, but the shorter leaf lifespan of the former necessitates higher production rates to maintain a given leaf area, largely excluding light-demanders from shaded understory locations. Height growth rate was positively correlated with leaf spacing for each species, and differences between species in the height growth rate-internode length relationship were related to interspecific differences in specific leaf mass. Thus, sapling growth histories may be inferred from their morphologies.     

Interspecific variation in sapling mortality in relation to growth and soil moisture

To examine the causes of landscape variation in forest community composition, we have quantified sapling mortality as a function of growth and soil moisture for seven dominant species in transition oak-northern hardwood forests of the northeastern USA. We located saplings in sites that encompassed a wide range of variation in soil moisture and light availability. In mesic conditions, the probability of mortality decays rapidly with increasing growth among shade tolerant species and more gradually among shade intolerant species: the rank order of survivorship at low growth rates is Tsuga canadensis > Fagus grandifolia > Acer saccharum > Fraxinus americana > Acer rubrum > Quercus rubra > Pinus strobus . The relationship between probability of mortality and growth does not vary with soil moisture among species insensitive to drought: Tsuga canadensis , Quercus rubra, and Pinus strobus . However, probability of mortality increases substantially with decreasing soil water availability for the other four species. Acer saccharum and Fagus grandifolia have high mortality rates under xeric conditions even when their growth is not suppressed. Acer rubrum and Fraxinus americana exhibited a steady but more gradual increase in the probability of mortality with decreasing soil moisture. Among the five deciduous hardwood species we examined there is a weak inverse relationship between the ability to survive growth suppression, a measure of shade tolerance, and the ability to survive in xeric conditions, a measure of drought tolerance. Tsuga canadensis , however, is tolerant of growth suppression and exhibits high survivorship in xeric conditions, while Pinus strobus is intolerant of growth suppression but insensitive to soil moisture. Species differences in water-dependent mortality are consistent with the species distributions across landscape gradients of soil water availability.     

两种光强下木质藤本与树木幼苗的竞争关系

 为了探讨木质藤本和树木幼苗的相互作用关系,对两种光强(4%和35%的光强)、4种竞争处理下(全竞争、地上竞争、地下竞争和无竞争),一种 需光木质藤本(刺果藤(Byttneria grandifolia))和3个树种(耐荫种：五桠果木姜子(Litsea dilleniifolia)和绒毛番龙眼(Pometia tomentosa);需光种：羊蹄甲(Bauhinia variegata))幼苗的地上部分和地下部分的竞争关系进行了研究。结果表明：木质藤本的竞争显著影响 着3种树木幼苗的光合能力、形态特征和生长,但生长环境的不同光强影响地上部分竞争和地下部分竞争的相对强度。在低光下,地上部分竞争 比地下部分竞争对3种树木幼苗的相对生长速率(Relative growth rate,RGR)和光合能力造成更大的影响;而高光下,地下竞争对树木幼苗的 生长有更强的抑制作用。不同的竞争处理和光强对树木幼苗的生物量积累造成显著的影响。光强对3种树种的比叶面积(Specific leaf area, SLA)和叶面积比(Leaf area ratio,LAR)有显著的抑制作用,但竞争只对需光的羊蹄甲的SLA和LAR有显著影响。不同的光照和竞争处理之间, 同种植物表现出不同的表型特征。由于竞争的影响,苗木在形态上较为矮小、叶片数目较少、叶面积减小,但是长细比改变较少 。     

Phenotypic selection on leaf functional traits of two congeneric species in a temperate rainforest is consistent with their shade tolerance

Several studies across species have linked leaf functional traits with shade tolerance. Because evolution by natural selection occurs within populations, in order to explain those interspecific patterns it is crucial to examine variation of traits associated with shade tolerance and plant fitness at an intraspecific scale. In a southern temperate rainforest, two climbing plant species coexist but differ in shade tolerance. Whereas Luzuriaga radicans is most abundant in the shaded understory, L. polyphylla typically occurs in intermediate light environments. We carried out an intraspecific approach to test the hypothesis of differential selection patterns in relation to shade tolerance in these congeneric species. The probability of showing reproductive structures increased with specific leaf area (SLA) in L. polyphylla, and decreased with dark respiration in L. radicans. When reproductive output of fertile individuals was the fitness variable, we detected positive directional selection on SLA in L. polyphylla, and negative directional selection on dark respiration and positive directional selection on leaf size in L. radicans. Total light radiation differed between the microsites where the Luzuriaga species were sampled in the old-growth forest understory. Accordingly, L. radicans had a lower minimum light requirement and showed fertile individuals in darker microsites. L. radicans showed lower dark respiration, higher chlorophyll content, and greater leaf size and SLA than L. polyphylla. Results suggest that in more shade-tolerant species, established in the darker microsites, selection would favor functional traits minimizing carbon losses, while in less shade-tolerant species, plants displaying leaf traits enhancing light capture would be selected.     

Ecological filtering by a dominant herb selects for shade tolerance in the tree seedling community of coastal dune forest

The regeneration niche is commonly partitioned along a gradient from shade-tolerant to shade-intolerant species to explain plant community assembly in forests. We examined the shade tolerance of tree seedlings in a subtropical coastal forest to determine whether the ecological filtering effect of a dominant, synchronously monocarpic herb (Isoglossa woodii) selects for species at either end of the light response continuum during the herb’s vegetative and reproductive phases. Photosynthetic characteristics of seedlings of 20 common tree species and the herb were measured. Seedlings were grown in the greenhouse at 12–14% irradiance, and their light compensation points measured using an open-flow gas exchange system. The light compensation points for the tree species were low, falling within a narrow range from 2.1 ± 0.8 μmol m⁻² s⁻¹ in Celtis africana to 6.4 ± 0.7 μmol m⁻² s⁻¹ in Allophylus natalensis, indicating general shade tolerance, consistent with a high and narrow range of apparent quantum yield among species (0.078 ± 0.002 mol CO₂ mol⁻¹ photon). Rates of dark respiration were significantly lower in a generalist pioneer species (Acacia karroo) than in a forest pioneer (C. africana), or in late successional phase forest species. We argue that the general shade tolerance, and phenotypic clustering of shade tolerance, in many tree species from several families in this system, is a result of ecological filtering by the prevailing low light levels beneath the I. woodii understorey, which excludes most light-demanding species from the seedling community.