Phylloclade development in the Asparagaceae: an example of homoeosis

Phylloclades are traditionally defined as flattened, determinate, leaf-like stems primarily on the basis of their axillary position. However, because the literature is replete with controversy over the morphological interpretation of these organs, a study of phylloclade development in comparison with leaf and stem development was undertaken in four closely related species of the Asparagaceae: Ruscus aculeatus, Danae racemosa, Semele androgyna and Asparagus densiflorus. Results reveal a continuum in phylloclade development from very leaf-like forms, such as those of Danae, via the more intermediate types of Ruscus, to the gradually more shootlike forms of Semele and Asparagus. This continuum results from a differential expression of stem (or shoot) and leaf characteristics in an axillary position. When stem (or shoot) and leaf features are combined, as in the fertile phylloclade of Ruscus, an intermediate organ is formed. Phylloclades are a form of evolutionary novelty that exemplifies the phenomenon of homoeosis, which is the transference of features from one organ to another. Developmentally, this means that leaf features are expressed by the axillary meristem.     

Stem turnover strategy of multiple-stemmed woody plants

We have investigated stem turnover strategy for Lindera umbellata, an understory shrub that sprouts from its rootstock under natural conditions to replace constituent stems, on the basis of the hypothesis that the multiple-stemmed form of woody species is an adaptation enabling efficient reproduction in high-stress environments. We tested the hypothesis that the timing of stem replacement maximizes sexual reproduction for the shrub. We developed a model for the time of optimum replacement of a stem by a daughter stem which maximizes the sexual reproduction of a shrub and tested the model using L. umbellata growing in the field. From the model, the optimum time of replacement of a stem with a daughter stem is when cumulative sexual reproduction per unit time for the stem is maximum. In practice, this will be the last age (t _opt) at which annual sexual reproduction in a stem can potentially exceed cumulative sexual reproduction per unit time for the stem. Half of the stems died at almost t _opt and had sexually mature daughter stems at that time. Other stems, however, died at times more remote from t _opt when daughter stems were sexually immature. It is thought that normal replacement of the latter stems was prevented by accidents such as breakage. We conclude that clumps of L. umbellata achieve efficient sexual reproduction by stem replacement at the optimum time, although accidents can, to some extent, determine when the stem actually dies.     

Diurnal and Seasonal Changes in Water Balance of Acer saccharum and Betula papyrifera

Diurnal and seasonal changes in plant water potential, leaf diffusion resistance, and stem radial changes of Acer saccharum and Betula papyrifera trees were studied in northern Wisconsin during the 1974 and 1975 growing seasons. Water potential decreased during the day, following relatively high values in the morning, and increased in the late afternoon and evening. Diurnal patterns and actual values of water potential varied with species, soil water availability, and factors influencing transpiration (e.g., solar radiation, vapor pressure deficit, and transpiration flux density). When plant water deficits were not severe, leaf resistance of both species was rather stable during the day. During severe droughts, however, leaf resistance increased (stomata closed) during the day when light intensity was high. Leaf resistance at high light intensity was higher in Acer than in Betula. Stomatal closure with decreasing light intensity varied between species and among Acer trees. Tree stems of both species shrank during the day, as internal water deficits developed, and they expanded as trees rehydrated during the night. Stems of Acer shrank more than those of Betula. The amount of daily stem shrinkage increased as the season progressed if the trees were not under severe water deficits. During severe droughts the amount of diurnal stem shrinkage decreased. Shrinkage of stems lagged behind water potential changes by 1 to 2 h in Acer and less than 1 h in Betula. The relationship between stem radius and leaf water potential was not constant throughout the growing season.     

Effects of Nitrogen Fertilizer on Growth and Yield of Spring Wheat

Nine amounts of nitrogen fertilizer, ranging from 0 to 200 kgN ha^–1, were applied to spring wheat cv. Kleiber in the3 years 1972-1974. In 1972 grain dry weight with 125 kg N ha^–1or more was 100 g m^–2 (23 per cent) greater than withoutnitrogen. Grain yield was unaffected by nitrogen in the otheryears. Leaf area at and after anthesis was increased throughoutthe range of nitrogen tested, most in 1972 and least in 1973.Consequently, the addition of 200 kg N ha^–1 decreasedthe amount of grain produced per unit of leaf area by approximately25 per cent in all years. The dry weight of leaves and stems at anthesis and maturitywas increased by nitrogen in all years, similarly to leaf area.However, the change in stem dry weight between anthesis andmaturity was not affected by nitrogen; stems increased in dryweight for about 20 days after anthesis and then decreased tovalues similar to those at anthesis. The uptake of CO₂ per unit area of flag leaf or second leaf(leaf below the flag leaf) was slightly decreased by nitrogenwhen the increase in leaf area caused by nitrogen appreciablydecreased the light intensity at the surface of these leaves.In spite of such decreases the CO₂ absorbed by flag and secondleaves per unit area of land was always increased by nitrogen,and relatively more than was grain yield. It is suggested that increases in respiratory loss of CO₂ withincreasing nitrogen fertilizer may explain why nitrogen increasedvegetative growth and leaf area relatively more than grain yield.     

The stem xylem of Patagonian shrubs operates far from the point of catastrophic dysfunction and is additionally protected from drought‐induced embolism by leaves and roots

Hydraulic architecture was studied in shrub species differing in rooting depth in a cold desert in Southern Argentina. All species exhibited strong hydraulic segmentation between leaves, stems and roots with leaves being the most vulnerable part of the hydraulic pathway. Two types of safety margins describing the degree of conservation of the hydraulic integrity were used: the difference between minimum stem or leaf water potential (Ψ) and the Ψ at which stem or leaf hydraulic function was reduced by 50% (Ψ – Ψ₅₀), and the difference between leaf and stem Ψ₅₀. Leaf Ψ₅₀ – stem Ψ₅₀ increased with decreasing rooting depth. Large diurnal decreases in root‐specific hydraulic conductivity suggested high root vulnerability to embolism across all species. Although stem Ψ₅₀ became more negative with decreasing species‐specific Ψ_soil and minimum stem Ψ, leaf Ψ₅₀ was independent of Ψ and minimum leaf Ψ. Species with embolism‐resistant stems also had higher maximum stem hydraulic conductivity. Safety margins for stems were >2.1 MPa, whereas those for leaves were negative or only slightly positive. Leaves acted as safety valves to protect the integrity of the upstream hydraulic pathway, whereas embolism in lateral roots may help to decouple portions of the plant from the impact of drier soil layers.     

Leaf growth of white mustard (Sinapis alba) in different environments

Summary The numbers of cells and area of fully expanded leaves were determined on successive leaves of Sinapis alba grown either in 8 hr. photoperiod (vegetative plants) or 16 hr. photoperiod (flowering plants) at a constant temperature of 20°C. In the 8 hr. photoperiod leaf 9 had the greatest area but leaf 12 had most cells. In 16 hr. photperiod leaf 5 had the greatest area but leaf 9 had most cells. The relationship between area and cell number of successive leaves on the main stem fell into 3 distinct phases: in phase (1), cell number increased at a greater rate than leaf area; in phase (2), leaf area decreased while cell number increased; in phase (3), cell number and leaf area decreased proportionally. For an increase in unit area, cell number increased more in 8 hr. than in 16 hr. photoperiod.Using final area and final cell number of successive leaves, by extrapolation the cell number of unit area of primordium has been deduced. Cell number per unit area increased in successive primordia up to a certain node after which it remained constant at succeeding nodes. It was found that in plants grown under different conditions the cell number per unit area in successive primordia increased at a constant logarithmic rate. That is, cells became progressively smaller. It is concluded that changes in cell size of successive primordia are not influenced by the environment but are under internal control.     

STEM AND LEAF GAS EXCHANGE OF TWO ARID LAND SHRUBS

Gas exchange studies were conducted on two shrub species found in cool shrub-steppe communities of the American West, big sagebrush (Artemisia tridentata subsp. tridentata Nutt.) and broom snakeweed (Gutierrezia sarothrae [Pursh] Britt. and Rusby), with a goal of evaluating characteristics and relative contributions of green stem and leaf material to total shoot CO₂ exchange at different temperatures. Variations in tissue temperature exerted a pronounced effect on CO₂ exchange—net photosynthesis and dark respiration—of green stems and leaves of both species. Definite temperature optima of net photosynthesis were noted, and dark respiration rates consistently increased with increases in temperature. Green stems of both species exhibited sizable dark respiration rates, although stem rates at all temperatures were lower than corresponding leaf rates. Artemisia tridentata did not exhibit sizeable green stem net photosynthesis even under conditions of optimal temperature and water availability, and leaf net photosynthesis rates were much lower than those of G. sarothrae. However, A. tridentata in general possessed a greater leaf biomass than G. sarothrae. Green stems of G. sarothrae exhibited considerable rates of net photosynthesis under both optimal and sub-optimal temperature and water availability conditions. A higher optimum temperature of net photosynthesis was noted for stems than for leaves of G. sarothrae. The adaptive significance of these interspecific differences in CO₂ exchange characteristics is discussed.     

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.     

Oligochaeta in Spartina stems: the microdistribution of Enchytraeidae and Tubificidae in a salt marsh,Sapelo Island,USA

The distribution and abundance of Enchytraeidae and Tubificidae in and around Spartina alterniflora plants in a tidal salt marsh on Sapelo Island, Georgia, USA were studied using two different sampling techniques: wet funnel extraction and stem dissection. At least 80% of all worms inhabited leaf sheaths at the bases of S. alterniflora plants, and densities were low in sediment, root and surface debris samples. Oligochaete densities were dependent on the position within the marsh, the height on stems and the stage of sheath decay. Six predominant species were identified and included Marionina appendiculata, Marionina spartinae, Marionina waltersi, Marionina paludis, and Monopylephorus parvus. Individual species were distributed differently on stems and enchytraeids were more common than tubificids on standing-dead and further up S. alterniflora stems. Estimates of oligochaete densities in salt marsh habitats are increased dramatically when the numbers of worms on stems are considered. Possible advantages of the stem microhabitat are discussed in relation to the biology and ecology of oligochaetes.     

Ants,Stem Borers,and Pubescence in Endospermum in Papua New Guinea1

The stems of the ant-plant, Endospermum labios Schodde, serve as colonization sites for the ant, Camponotus quadriceps F. Smith. They are also subject to damage by insect borers. We sampled young E. labios trees in distutbed forest to compare evidence of stem boring insect and stem miner damage in plants with and without colonies of C. quadriceps. Dissections of a subsample of plants showed that dipteran stem borers and stem miner damage were significantly more common in plants lacking C, quadriceps colonies than in plants with established colonies. Evidence from these dissections and from field counts of meristem damage caused by emerging borers suggested that coleopteran stem borers were also more abundant when ants were not present. In addition to the incidence of stem boring insects and ant colonies, we examined relative levels of leaf pubescence by measuring trichome density and leaf size for E. labios trees. We found that trichome density was significantly greater in trees with evidence of prior shoot damage (presumably from stem borer emergence at the meristem) but was not significantly related to the presence or absence of an ant colony. This prompted us to compare trichome density on leaves of nearby small trees and of different branches of the same tree, pairing a stem/branch that appeared damaged with one that appeared healthy. Trichome densities on leaves from damaged stems and branches were significantly greater than were trichome densities on healthy branches and stems. Based on these empirical data, we present several possible explanations for the patterns of association between ants, stem borers, and pubescence. Although feeding preference tests with a common folivore showed no effect of trichome densities on leaves, we suggest further study on how trichomes may affect ovipositing stem borers.     

Stem photosynthesis in a desert ephemeral,Eriogonum inflatum

Summary The gas exchange characteristics of photosynthetic tissues of leaves and stems of Eriogonum inflatum are described. Inflated stems were found to contain extraordinarily high internal CO₂ concentrations (to 14000 bar), but fixation of this internal CO₂ was 6–10 times slower than fixation of atmospheric CO₂ by these stems. Although the pool of CO₂ is a trivial source of CO₂ for stem photosynthesis, it may result in higher water-use efficiency of stem tissues. Leaf and stem photosynthetic activities were compared by means of CO₂ fixation in CO₂ response curves, light and temperature response curves in IRGA systems, and by means of O₂ exchange at CO₂ saturation in a leaf disc O₂ electrode system. On an area basis leaves contain about twice the chlorophyll and nitrogen as stems, and are capable of up to 4-times the absolute CO₂ and O₂ exchange rates. However, the stem shape is such that lighting of the shaded side leads to a substantial increase in overall stem photosynthesis on a projected area basis, to about half the leaf rate in air. Stem conductance is lower than leaf conductance under most conditions and is less sensitive to high temperature or high VPD. Under most conditions, the ratio C _i /C _a is lower in stems than in leaves and stems show greater water-use efficiency (higher ratio assimilation/transpiration) as a function of VPD. This potential advantage of stem photosynthesis in a water limited environment may be offset by the higher VPD conditions in the hotter, drier part of the year when stems are active after leaves have senesced. Stem and leaf photosynthesis were similarly affected by decreasing plant water potential.     

不同入侵程度紫茎泽兰碳氮磷生态化学计量特征研究

紫茎泽兰(Ageratina adenophora)是一种生态危害性较大的入侵植物。为探讨不同入侵程度下紫茎泽兰的化学计量特征,进一步揭示其营养策略与入侵机制,该研究以紫茎泽兰及其本土伴生种条叶猪屎豆(Crotalaria linifolia)为对象,测定研究了轻度入侵、中度入侵和重度入侵下紫茎泽兰根、茎、叶中碳(C)、氮(N)、磷(P)化学计量特征与其入侵地土壤养分状况,并进一步比较了紫茎泽兰和条叶猪屎豆的C、N、P化学计量特征。结果表明：(1)3种入侵程度下,紫茎泽兰叶N、P含量均显著大于根和茎N、P含量,将更多的N和P分配至叶,增加资源获取,以利快速生长。(2)3种入侵程度下,紫茎泽兰茎N∶P<根N∶P<叶N∶P,且茎N∶P在中度入侵下显著大于轻度入侵,在入侵过程中其茎呈现出较高的生长速率,可促进其获取更多环境资源,增强生长竞争优势。(3)与本土种条叶猪屎豆相比,紫茎泽兰具有更高的根P、茎P含量,根和茎C∶P、N∶P均显著小于条叶猪屎豆,各器官C∶N均显著大于条叶猪屎豆,显示出较高的养分利用效率及较低的资源需求量。(4)紫茎泽兰茎C和叶C、茎N和叶N均呈显著正相关,茎...     

Water loss from cut grass with special reference to hay-making

Water loss from cut grass was studied to determine factors limiting the drying process. An apparatus, used to measure water loss at 28±1°C from blotting paper and from leaves and stem internodes of cocksfoot, consisted of four channels in which air speed was controlled at 25–80 cm s^-1 and relative humidity at 7–68%. The maximum rate of water loss from wet blotting paper was 10500 mg water dm^-2 h^-1 but from leaves and stem internodes supplied with water it was less than 250 mg dm^-2 h^-1. The rate of loss from both plant specimens and blotting paper was linearly related to the vapour pressure differences between the specimen and the surrounding air but was not increased when air speed was changed from 40 to 80 cm s^-1. Grass specimens supplied with water had lower rates of water loss than wet blotting paper because of tissue resistances which were calculated for (a) untreated leaf and stem specimens, (b) rubbed leaves, (c) cut leaves, (d) leaves exposed to steam for 60 s. Treatments (b)-(d) greatly reduced tissue resistances. The rates of drying of leaves and stem internodes not supplied with water changed only slightly in response to faster air speeds but were significantly increased by treatments (b), (c), (c_s) (split stems), (d) and (e) (exposure to petroleum vapour for 60 s). The most effective treatments trebled the drying rates of leaves and increased the drying rates of stem internodes by 10 times. Reductions in relative humidity had little effect on drying rate following treatments (a), (b), (c) and (d), but when treatments (c_s) and (e) were given, additional significant increases in drying rates were obtained when the relative humidity was reduced. Grass specimens given the most effective treatments and dried under the most favourable conditions did not utilize the full drying capacity of the environment, for the rates of water loss from these specimens were at least three times lower than those from wet blotting paper. The results indicate that high rates of drying could be achieved at 28 ^oC or similar temperatures if practical treatments were developed to remove or greatly reduce the high resistance to water loss in cut grass.     

Utilization of exogenously supplied 14C-saccharose into primary metabolites and alkaloid production in Catharanthus roseus

Partitioning of exogenously supplied U-¹⁴C-saccharose into primary metabolic pool as sugars, amino acids, and organic acids was analyzed and simultaneous utilization for production of alkaloid by leaf, stem, and root in twigs and rooted plants of Catharanthus roseus grown in hydroponic culture medium was determined. Twigs revealed comparable distribution of total ¹⁴C label in leaf and stem. Stems contained significantly higher ¹⁴C label in sugar fraction and in alkaloids [47 kBq kg⁻¹(DM)] than leaf. In rooted plants, label in ¹⁴C in metabolic fractions in root such as ethanol-soluble, ethanol-insoluble, and chloroform-soluble fractions and in components such as sugars, amino acids, and organic acids were significantly higher than in stems and leaves. This was related with significantly higher content of ¹⁴C in alkaloids in stems and leaves. ¹⁴C contents in sugars, amino acids, and organic acids increased from leaf to stem and roots. Roots are the major accumulators of metabolites accompanied by higher biosynthetic utilization for alkaloid accumulation.     

Independence of stem and leaf hydraulic traits in six Euphorbiaceae tree species with contrasting leaf phenology

Hydraulic traits and hydraulic-related structural properties were examined in three deciduous (Hevea brasiliensis, Macaranga denticulate, and Bischofia javanica) and three evergreen (Drypetes indica, Aleurites moluccana, and Codiaeum variegatum) Euphorbiaceae tree species from a seasonally tropical forest in south-western China. Xylem water potential at 50% loss of stem hydraulic conductivity (P50_stem) was more negative in the evergreen tree, but leaf water potential at 50% loss of leaf hydraulic conductivity (P50_leaf) did not function as P50_stem did. Furthermore, P50_stem was more negative than P50_leaf in the evergreen tree; contrarily, this pattern was not observed in the deciduous tree. Leaf hydraulic conductivity overlapped considerably, but stem hydraulic conductivity diverged between the evergreen and deciduous tree. Correspondingly, structural properties of leaves overlapped substantially; however, structural properties of stem diverged markedly. Consequently, leaf and stem hydraulic traits were closely correlated with leaf and stem structural properties, respectively. Additionally, stem hydraulic efficiency was significantly correlated with stem hydraulic resistance to embolism; nevertheless, such a hydraulic pattern was not found in leaf hydraulics. Thus, these results suggest: (1) that the evergreen and deciduous tree mainly diverge in stem hydraulics, but not in leaf hydraulics, (2) that regardless of leaf or stem, their hydraulic traits result primarily from structural properties, and not from leaf phenology, (3) that leaves are more vulnerable to drought-induced embolism than stem in the evergreen tree, but not always in the deciduous tree and (4) that there exists a trade-off between hydraulic efficiency and safety for stem hydraulics, but not for leaf hydraulics.     

Vulnerability to cavitation of leaf minor veins: any impact on leaf gas exchange?

Vulnerability to cavitation of leaf minor veins and stems of Laurus nobilis L. was quantified together with that of leaflets, rachides and stems of Ceratonia siliqua L. during air‐dehydration of 3‐year‐old branches. Embolism was estimated by counting ultrasound acoustic emissions (UAE) and relating them to leaf water potential (Ψ_L). The threshold Ψ_L for cavitation was less negative in L. nobilis than in C. siliqua according to the known higher drought resistance of the latter species. Leaf minor vein cavitation was also quantified by infiltrating leaves with fluorescein at different dehydration levels and observing them under microscope. Distinct decreases in the functional integrity of minor veins were observed during leaf dehydration, with high correlation between the two variables. The relationship between leaf conductance to water vapour (g_L) and Ψ_L showed that stomata of L. nobilis closed in response to stem and not to leaf cavitation. However, in C. siliqua, g_L decreased in coincidence to the leaf cavitation threshold, which was, nevertheless, very close to that of the stem. The hypothesis that stem cavitation acts as a signal for stomatal closure was confirmed, while the same role for leaf cavitation remains an open problem.     

Wheat stem sawfly‐infested plants benefit from parasitism of the herbivorous larvae

1 Parasitoids Bracon cephi (Gahan) and Bracon lissogaster Muesebeck and their herbivorous host the wheat stem sawfly Cephus cinctus Norton, a pest of wheat Triticum aestivum, were investigated for yield in T. aestivum grown in the field. 2 Wheat stem sawfly‐infested stems had a higher yield potential than uninfested stems. However, final reproductive output was not significantly different between ears on infested stems that supported complete larval development compared with ears on uninfested stems. 3 Stems containing parasitized larvae and stems containing larvae that died before completing their development had a higher mean number of seeds and seed weight, when accounting for number of fertile spikelets of each ear, than either infested with live larvae and uninfested stems. 4 The results obtained suggest that larval feeding prevented infested stems from attaining their yield potential, and that the negative impact of the pest on wheat yield was reduced when late instar sawfly larvae were parasitized. Even though some feeding occurs before parasitism, this early damage has a comparatively low impact on yield. 5 This is the first study to show a yield benefit and enhanced plant fitness due to the wheat stem sawfly parasitoids B. cephi and B. lissogaster. This results from the maintenance of increased seed number and seed weight in the higher yielding stems that are preferentially infested by this pest.     

ROLE OF PARENCHYMA IN LINUM USITATISSIMUM LEAF TRACE PATTERNS

A new notation for leaf trace patterns was developed which is consistent with contemporary contact parastichy phyllotaxis notation. New computer-aided methods for generating accurate stem tissue maps were developed. Application of these methods resulted in clarification of the role that parenchyma differentiation plays in delimiting the procambial template for Linum usitatissimum L. stem vasculature through ontogeny. Study of the tissue maps for the various leaf trace patterns exhibited by Linum stems through ontogeny generated a set of observations which permits more rigorous definition of the developmental rules for vascular pattern formation. Long-known geometric principles of phyllotaxis were found applicable to leaf trace patterns.     

Shoot structure and dynamics of saplings and canopies of three deciduous broad-leaved trees of a coppice forest in central Japan

Three deciduous broad-leaved trees, Quercus serrata, Castanea crenata and Carpinus laxiflora, were the main constituents of a coppice forest in central Japan. The shoot elongation and leaf emergence modes of both saplings and the canopy of the three species were investigated. The shoot elongation modes of Q. serrata and C. crenata were the same in saplings and the upper layer of the canopy. The second shoots of these two species were formed after the first shoots were elongated. C. laxiflora was different between saplings and the upper layer of the canopy. In saplings, only the first shoots took a long time to elongate. In the upper canopy layer, higher order shoots were formed in the same way as in the other two species. In the lower layer of the canopy, all three species showed the same shoot elongation mode, in which only the first shoot and its duration of elongation was short. Leaf longevity, individual leaf area, leaf mass per unit leaf area and the stem mass per unit stem length of C. laxiflora were significantly shorter or significantly smaller than those of Q. serrata and C. crenata. The length of the stem per unit leaf area of C. laxiflora was three times that of Q. serrata and five times that of C. crenata. The elongation growth of C. laxiflora was highly efficient as it occurred with a small leaf area. The shoot dynamics and the shoot structure of C. laxiflora are more suitable for elongation growth than in Q. serrata and C. crenata. Furthermore, the shoot structures of the three species were compared and ecological characteristics of the three species are discussed. Received: 29 September 1998 / Accepted: 17 September 1999     

长期增温对树线交错带岷江冷杉幼苗异龄叶大小与出叶强度关系的影响

全球气候变暖强烈影响树线交错带植物的生活史策略,异龄叶大小-出叶强度权衡关系是常绿植物生活史策略的重要内容。以川西树线交错带的岷江冷杉（Abies faxoniana）幼苗为例,研究气候变暖对异龄叶大小与出叶强度关系的影响。通过开顶箱（Open-top chamber, OTC）对川西王朗自然保护区树线交错带的岷江冷杉进行模拟增温,采用标准化主轴估计（Standardized major axis estimation, SMA）方法研究了叶大小（单叶质量、单叶面积）与出叶强度（基于茎生物量、茎体积）间异速生长关系对长期增温的响应及其年际变化。结果表明：使用不同参数表征叶大小与出叶强度得到的结果存在差异;多年生小枝上存在单叶质量-出叶强度的负等速权衡关系,共同主轴随小枝年龄增加而向下漂移;长期增温并不影响单叶质量与出叶强度的异速生长关系,不同年龄小枝的异速生长常数对增温具有差异性响应。增温处理中当年生小枝在相同单叶质量下的出叶强度更低,以换取叶片总数的增加,使小枝具有更大的可塑性而适应增温。本研究提供了岷江冷杉幼苗协调异龄叶大小与出叶强度从而适应长期增温的证据,为评估树木生长随气候变化而加速提供了理论参考。