Effect of Crown Asymmetry on Size-Structure Dynamics of Plant Populations

The effect of crown asymmetry on the size–structure dynamicsof populations was evaluated using a spatial competition modelincorporating crown asymmetry. Computer simulations were carriedout with various combinations of density levels, spatial patterns,and degrees of asymmetry in competition to assess how they modifythe effect of crown asymmetry on size–structure dynamics. In the model, crown asymmetry is expressed by the crown-vector,or the vector linking the stem base and the centre of the projectedarea of the crown on the horizontal plane. Crown-vectors areassumed to develop in the manner by which crowns repel eachother. As crown-vectors develop, the positions of the crown-centresmove. Competition between individuals is expressed by a neighbourhoodmodel, in which individual growth is determined by the distancefrom, and size of, the neighbours' crown-centres. Generally, populations of individuals which developed asymmetriccrowns had larger survivorship, larger mean size, smaller coefficientsof variation and skewness, and a more regular spatial patternthan populations of individuals which developed symmetric crowns.The effect of crown symmetry is generally stronger in populationswith high density and a clumped spatial pattern. The effectof mortality caused by one-sided competition on size-structuredynamics was similar to that of crown asymmetry; mortality increasedmean size, reduced size hierarchy, and made the spatial patternmore regular. Because mortality was heavier in populations withoutcrown asymmetry, its effect on size-structure dynamics cancelledout, or overwhelmed, the effect of crown asymmetry in latergrowth stages. If crown asymmetry is associated with a reductionin growth, the effect of crown asymmetry is reduced. Nevertheless,the resultant population structure is different from that ofpopulations without crown asymmetry. Competition; crown asymmetry; morphological plasticity; neighbourhood interference model; size-structure dynamics     

Plastic Response of Crown Architecture to Crowding in Understorey Trees of Two Co-dominating Conifers

Crown architecture and growth rate of trunk height, trunk diameterand lateral branches of understorey trees (5–10m tall)were compared between two co-dominating conifers,Abies sachalinensisandPiceaglehnii, in relation to the index of local crowding intensity,W,represented as a function of density, distance and basal areaof taller neighbours. For the two conifers, the growth of trunkheight and diameter was decreased and crowns became flat withincreasingW, keeping crown projection area. Self-pruning oflower branches was more intense inAbiesthan inPiceain crowdedconditions, while both conifers showed similar crown forms inless crowded conditions. These results suggest that the growthin lateral branches exceeded that in height in crowded conditions,especially inAbies. Tree age of both conifers increased withincreasingW, resulting from the low growth rate in crowded conditions.The age of the longest and lowest branch ofPicea, up to 150years, was positively correlated with the tree age ranging from70 to 250 years, whereas that ofAbieswas constant at around30 years irrespective of tree age varying from 40 to 140 years.This result agrees with the observation that agedAbieshad moreflat-shaped crowns than in agedPiceain crowded conditions. Theseresults suggest that each conifer adapted to crowding in differentways: high elongation of branches with high turnover rate forAbiesandviceversaforPicea. Abies; crown form; neighbourhood interference; Picea; plasticity     

Competitive Asymmetry Reduces Spatial Effects on Size-Structure Dynamics in Plant Populations

The growth of each individual in plant populations was simulatedby a spatial competition model for five density levels and fourdifferent spatial distribution patterns of individuals, varyingfrom highly clumped to regular. The simulation results wereanalysed using the diffusion model for evaluating the effectsof density and distribution pattern on the size-structure dynamicsin relation to the degree of competitive asymmetry. At low densities,changes in statistics of plant weight over time such as mean,coefficient of variation, skewness, and Box-Cox-transformedkurtosis differed greatly among spatial patterns, irrespectiveof the degree of competitive asymmetry. In completely symmetriccompetition, the spatial effect on size-structure dynamics remainedrelatively large irrespective of densities, although mean plantweight became similar among the spatial patterns with increasingdensity. However, the spatial effect diminished with increaseddensity in strongly asymmetric competition, when similar sizedistributions were realized irrespective of the spatial patterns.Therefore, it was concluded that: (1) irrespective of the degreeof competitive asymmetry, spatial pattern is important for size-structuredynamics at low densities; (2) spatial pattern is nearly immaterialunder strongly asymmetric competition at high densities; and(3) under crowded conditions, neighbourhood effects are muchmore apparent at the population level in less asymmetric competition.These processes and outcomes are linked to the forms of thefunctions of mean growth rate of individuals [G(t,x) function]and variance in growth rate [D(t,x) function]. These functionsare variable depending on the spatial pattern under symmetriccompetition, but are rather stable under strongly asymmetriccompetition at high densities irrespective of the spatial patterns.Therefore, size structure under strongly asymmetric competitioncan be regarded as a stable system, whereas that under symmetriccompetition is regarded as a variable system in relation tothe spatial pattern and process. From this, it was inferredthat: (1) the goodness-of-fit of spatial competition modelsfor crowded plant populations is higher in less asymmetric competition;and (2) higher species diversity in plant communities is associatedwith the lower degree of competitive asymmetry.Copyright 1994,1999 Academic Press Asymmetric competition, diffusion model, neighbourhood effect, size-structure stability, spatial competition model, spatial distribution pattern, species diversity, symmetric competition     

Nitrogen uptake and use by competing individuals in a Xanthium canadense stand

We studied differences in nitrogen uptake and use for plant growth among individuals competing in a natural dense stand of an annual herb, Xanthium canadense. Larger individuals took up more nitrogen than proportionately to their size, indicating that the competition for soil nitrogen was asymmetric among individuals, although it was more symmetric than the competition for light. The rate of nitrogen loss of individuals also increased with plant size. While smaller individuals shared smaller fractions of total plant nitrogen in the stand, they had higher nitrogen concentrations per unit mass. "Turnover" rates of nitrogen influx (r_in) and outflux (r_out) were defined as the rates of nitrogen uptake and loss per unit aboveground nitrogen, respectively. r_in was higher in larger individuals, whereas r_out was higher in smaller individuals. Consequently, the relative rate of nitrogen increment (r_in-r_out) was higher in larger individuals, whereas it was around zero in the smallest individuals. The mean residence time of nitrogen (MRT), defined as the inverse of r_out, was longer in larger individuals. Nitrogen productivity (NP), i.e. the growth rate per unit aboveground nitrogen, was higher in larger individuals. As the product of lifetime MRT and NP gives the nitrogen use efficiency (NUE), defined as biomass production per unit flux of nitrogen, higher MRT and NP observed in larger individuals would have contributed to their higher lifetime NUE. Shorter MRT in smaller individuals was caused by the abscission of leaves which contained relatively large fractions of total plant nitrogen. Xanthium canadense, as a competitive ruderal, tended to produce leaves at higher positions to acquire higher light levels at the expense of older leaves rather than to modify their productive structure to efficiently use low light levels as observed in shade-tolerant species.     

Stem extension and mechanical stability of Xanthium canadense grown in an open or in a dense stand

Background and Aims

Plants in open, uncrowded habitats typically have relatively short stems with many branches, whereas plants in crowded habitats grow taller and more slender at the expense of mechanical stability. There seems to be a trade-off between height growth and mechanical stability, and this study addresses how stand density influences stem extension and consequently plant safety margins against mechanical failure.

Methods

Xanthium canadense plants were grown either solitarily (S-plants) or in a dense stand (D-plants) until flowering. Internode dimensions and mechanical properties were measured at the metamer level, and the critical buckling height beyond which the plant elastically buckles under its own weight and the maximum lateral wind force the plant can withstand were calculated.

Key Results

Internodes were longer in D- than S-plants, but basal diameter did not differ significantly. Relative growth rates of internode length and diameter were negatively correlated to the volumetric solid fraction of the internode. Internode dry mass density was higher in S- than D-plants. Young''s modulus of elasticity and the breaking stress were higher in lower metamers, and in D- than in S-plants. Within a stand, however, both moduli were positively related to dry mass density. The buckling safety factor, a ratio of critical buckling height to actual height, was higher in S- than in D-plants. D-plants were found to be approaching the limiting value 1. Lateral wind force resistance was higher in S- than in D-plants, and increased with growth in S-plants.

Conclusions

Critical buckling height increased with height growth due mainly to an increase in stem stiffness and diameter and a reduction in crown/stem mass ratio. Lateral wind force resistance was enhanced due to increased tissue strength and diameter. The increase in tissue stiffness and strength with height growth plays a crucial role in maintaining a safety margin against mechanical failure in herbaceous species that lack the capacity for secondary growth.     

Stem Growth and Epicormic Branch Formation in Understorey Beech Trees (Fagus sylvatica L.)

A study of the formation of epicormic branches on suppressedbeech trees revealed developmental sequences that had not hithertobeen studied in depth. By combining architectural analysis andanatomy, it was possible to trace the history of the plant andthus fully understand the ontogenesis of the individuals studied.The main sequences observed were: (1) the formation of moribundtrees that were gradually covered in epicormic branches fromthe base of the stem to the crown following gradual forest closure;(2) the formation of this new crown of epicormic branches wascombined with a very sharp reduction in stem cambial activity(partial rings, if any) and modifications to the ligneous sections;and (3) resumed girth increment following the establishmentof epicormic branches. After describing in detail the methodsand individual variation, we highlight the spatial and temporalpatterns of the phenomena observed. Lastly, we put forward functionalhypotheses with a view to setting these results against a backgroundof forest tree ontogenesis in general. Copyright 2001 Annalsof Botany Company Fagus sylvatica L., beech, architecture, epicormic shoot, anatomy, growth, annual wood ring, missing ring     

Within-crown Foliar Plasticity of Western Hemlock, Tsuga heterophylla, in Relation to Stand Age

Anatomy and morphology of needles from six different positionswithin the crowns of western hemlock,Tsuga heterophylla (Raf.)Sarg., were studied to investigate the plastic response of leafstructure to crown position, in relation to stand age. Treeswere sampled from three stands, all of which originated followingcatastrophic fires, representing a chronosequence (15, 55 and145 years old) of stand development. Needles from the highestand outermost crown positions showed morphological and anatomicalresponses normally associated with ‘sun leaves’.As a measure of expressed plasticity, we calculated a relativetrait range (RTR) index to quantify differences in expressedtraits between needles from the upper outer crown (UO needles)and those from the lower inner crown (LI needles). RTR was positivefor most morphological and anatomical traits, indicating largertrait measurements on UO needles relative to LI needles (e.g.UO needles were thicker and had more vascular tissue than LIneedles). However, the degree to which sun and shade leaf traitswere expressed varied between stands. In the youngest stand,UO needles had higher maximal rates of photosynthesis than LIneedles (positive RTR_Ps), but this trend was reversed in theoldest stand (negative RTR_Ps). Mean RTR across all morphologicaland anatomical traits was higher in the 145-year-old stand thanin either the 55-year-old stand (P = 0·03) or the 15-year-oldstand (P < 0·01). While this probably relates mainlyto differences in light conditions between the three stands,it may also indicate a connection between ontogeny and phenotypicplasticity. Copyright 2001 Annals of Botany Company Foliar plasticity, needle anatomy and morphology, ontogeny, photosynthesis, stand development, sun–shade, Tsuga heterophylla(Raf.) Sarg., western hemlock     

Crown structure and vertical foliage distribution in 4-year-old plantation-grown Eucalyptus pilularis and Eucalyptus cloeziana

Tree growth and form are both influenced by crown architecture and how it effects leaf distribution and light interception. This study examined the vertical distribution of foliage in 4-year-old plantation-grown Eucalyptus pilularis Sm. and E. cloeziana F. Muell. trees. Leaf area (LA) distribution was determined at two different sites using allometric approaches to determine LA in crown sections and for whole trees. Leaf area was distributed more towards the upper crowns when canopies had been closed for longer. Leaf area was also skewed more towards the upper crowns for Eucalyptus pilularis than E. cloeziana. These species differences were consistent with differences in vertical light availability gradients as determined by point quantum sensors. Leaf area of individual branches was highly correlated with branch cross-sectional area (CSA) and whole-tree LA was closely related to stem CSA. Branch-level allometric relationships were influenced by site and crown position. However, the general allometric equations between stem size and whole-tree leaf area could be applied across sites. Results from this study suggest that pruning of live branches in these species should follow species-specific guides for the timing and height of pruning to optimise the effects on stem growth and form.     

Morphogenesis of Asymmetry and Symmetry in Lemna perpusilla Torr

The determination of which of the two reproductive pockets ofLemna perpusilla will produce the first daughter frond undernon-flowering conditions, or the flower under inductive conditions,is related to asymmetric gradients within the mother frond whichmay be the result of the asymmetric position of the axillaryfrond. Treatments that encourage vigorous growth of axillaryfronds may overcome the normal situation of asymmetry and causea morphological symmetry which is qualitatively different fromthe symmetry induced by treatments (TIBA) which prevent thedevelopment of axillary fronds. The origin and maintenance ofasymmetry in Lemna are discussed. Lemna perpusilla Torr., morphogenesis, asymmetry of fronds, tri-iodobenzoic acid     

Branching, nitrogen distribution, and carbon gain in solitary plants of an annual herb, Xanthium canadense

The theory of optimal nitrogen (N) distribution predicts that the carbon gain of plants will be maximised when leaves of higher irradiance have higher N content per area (N _area). Most previous studies have examined optimal N distribution without explicitly considering the branching status of plants. I investigated light environment, N distribution and photosynthetic traits of individual leaves of an herbaceous species, Xanthium canadense. X. canadense was grown solitary under high (HN) and low nutrients (LN). Light availability, leaf mass per unit area and N _area were measured for all leaves within plants. Daily photosynthesis of the plants of actual N distribution was compared with those of optimal and constant N distribution. Branch production was facilitated in HN but not in LN plants. N _area was correlated more with leaf order than with leaf light environment. Although N was more limited and the light environment was less heterogeneous within crowns in LN than in HN plants, leaf N distribution was closer to optimal in the latter. These results suggest that leaf N distribution was not optimised in solitary plants of X. canadense. Because this species often regenerates in a dense stand, leaf N distribution might be selected to maximise carbon gain only in such a stand. Leaf N distribution might thus be constrained by the regeneration strategy of the species.     

Crown Spread Patterns for Five Deciduous Broad-leaved Woody Species: Ecological Significance of the Retention Patterns of Larger Branches

Patterns of crown spread and branch retention of two shade-intolerantspecies (Betula platyphyllaandB. maximowicziana) were comparedwith three more tolerant species (Quercus mongolica,Acer sieboldianumandMagnoliaobovata). Branching height (height of the lowest living branch)rose more rapidly with age for the twoBetulaspecies than forthe shade tolerant species. Branching heights ofA. sieboldianumandM.obovatawere similar, irrespective of tree height and age, andlarger trees tended to produce wider crowns than theBetulaspecieswhen trees of similar height were compared. In all species,the branch basal area (cross-sectional area of a branch at itsbase) and the leaf area per branch generally increased as thebranch position on a stem became lower. Therefore, retaininglarger branches contributed significantly to the support ofa larger leaf area per tree. The number of larger branches (branchbasal area >80 cm²) for bothBetulaspecies was significantlysmaller than that of the shade tolerant species. The branchretention pattern ofBetulaspecies was probably a consequenceof intolerance of the leaves to shade. The decline ofBetulaspecieswith forest succession is likely to occur through their inabilityto retain branches with a large base area in closed forests.Copyright1997 Annals of Botany Company Shade tolerance; crown spread; branch retention; branch size; broad-leaved woody species; leaf area index per tree     

Patterns of branch permeability with crown depth among loblolly pine families differing in growth rate and crown size

Patterns in branch permeability with crown depth and permeability at the top of the main stem were analyzed for loblolly pine (Pinus taeda L.) trees from families selected on the basis of growth rate (fast, slow) and crown size (large, small). Analysis of variance with levels of crown size nested within levels of growth rate was used to test for differences in main stem permeability. Permeability at the top of the stem averaged 2.0×10^–12 m². There were no significant differences in permeability between families selected for fast and slow growth, but permeability was significantly lower for families selected for large crowns than for families selected for small crowns. Branch permeability averaged 0.74×10^–12 m² and decreased significantly with crown depth. Large-crown families had higher overall branch permeability than small-crown families. Average permeability in branches did not differ significantly between fast- and slow-growing families. Large crown-families had significantly larger current leaf area: total leaf area ratios in the lower two-thirds of the crown, and a weak but significant association was found between permeability and current leaf area: total leaf area ratios for a given relative crown depth. Our results suggest that ecotypic and adaptive processes simultaneously affect the overall patterns of stem and branch permeability in loblolly pine families.     

Correspondence between enamel hypoplasia and odontometric bilateral asymmetry in Australian twins

Four aspects of enamel hypoplasia of the maxillary central incisor and mandibular canine (hypoplasia presence, width, cumulative width, and crown position) were correlated with directional and fluctuating measures of bilateral odontometric asymmetry in a large panel (n = 950) of South Australian twins. Hypoplasia and asymmetry are thought to reflect general developmental disruption, but they show few correlations beyond the expected statistical type I error. This may relate to differences in their specific etiology, the composite nature of overall crown dimensions, a general lack of stress, and the extended period of formation of dental crowns. In contrast, asymmetry is marginally more detectable in a subsample separated according to hypoplastic teeth, suggesting that correspondence may be clearer in comparisons at the population rather than individual level. The most notable difference is the greater variability of asymmetry measures in hypoplastic individuals.     

A Canopy Photosynthesis Model for the Dynamics of Size Structure and Self-thinning in Plant Populations

A dynamic model for growth and mortality of individual plantsin a stand was developed, based on the process of canopy photosynthesis,and assuming an allometric relationship between plant heightand weight, i.e. allocation growth pattern of plant height andstem diameter. Functions G(t, x), for the mean growth rate ofindividuals of size x at time t, and M(t,x), for the mortalityrate of individuals of size x at time t, were developed fromthis model and used in simulations. The dynamics of size structurewere simulated, combining the continuity equation model, a simpleversion of the diffusion model, with these functions. Simulationsreproduced several well-documented phenomena: (1) size variabilityin terms of coefficient of variation and skewness of plant weightincreases at first with stand development and then stabilisesor decreases with an onset of intensive self-thinning; (2) duringthe course of self-thinning, there is a power relationship betweendensity and biomass per unit ground area, irrespective of theinitial density and of the allocation-growth pattern in termsof the allometric parameter relating plant height and weight.The following were further shown by simulation: (a) competitionbetween individuals in a crowded stand is never completely one-sidedbut always asymmetrically two-sided, even though competitionis only for light; (b) plants of ‘height-growth’type exhibit a greater asymmetry in competition than plantsof ‘diameter-growth’ type, (c) the effect of competitionon the growth of individuals in a crowded stand converges toa stationary state, even when the stand structure still changesgreatly. All of these theoretical results can explain recentempirical results obtained from several natural plant communities.Finally, a new, general functional form for G(t, x) in a crowdedstand is proposed based on these theoretical results, insteadof a priori or empirical growth and competition functions. Canopy photosynthesis, competition mode, continuity equation, self-thinning, simulation, size distribution     

The cause of the prevalent directions of the spiral grain patterns in conifers

 A new theory is presented on the cause of the prevalent directions of the spiral grain patterns found in conifers. The hypothesis is based upon the assumption that spiral grain has a function, i.e. that it represents a growth strategy to ensure survival of the trees. The mechanical function of the tree trunk is placed in focus, that is the ability of the trees to withstand external mechanical loads, mainly from wind. Spiral grain is an optimized growth feature when the trees are exposed to combined bending and torsion. Torsion occurs when the crown is asymmetric in the plane perpendicular to the wind direction. Systematic crown asymmetry, with heavier crowns on the south side, was confirmed by measuring the crown projections on 253 sparsely grown pines; 76.7% of the trees had longer branches on the south than on the north side, and the average length difference was 40.8 cm. By studying wind maps it was seen that most of the coniferous forests have prevailing westerly winds, which, when combined with the crown asymmetry, leads to a prevailing torque. Right-handed spiral grain in the outermost layers of mature trees is proposed to be a strategy to withstand this torque, i.e. to avoid stem breakage. Received: 30 June 1997 / Accepted: 17 November 1997     

Floral Induction of Xanthium strumarium in Long Days

An efficient method of inducing flowering in the normally strictshort-day plant, Xanthium strumarium L., in long days is described.Plants were grown in 16-h long days and subjected to two thermocycleswith a root application of gibberellic acid at the 8th h ofthe light period. A thermocycle was defined as follows. Forthe first 8 h of the 24-h cycle, the temperature was 4 °C;for the remaining 16 h, it was 23 °C. The light was on forthe first 16 h and off for the remaining 8 h. Xanthium strumarium L., floral induction, thermoperiod, gibberellic acid     

The Influence of the Arrangement of Neighbours on the Spatial Component of Resource Capture by an Individual Plant in Even-aged Monocultures

Data from experiments using extensive row-spacing treatments(Benjamin and Sutherland, 1992) were analysed to indicate thatmorphological responses to micro-spatial heterogeneity in resourcelevel are important in determining the growth response of field-growncarrot plants to the spatial distribution of neighbours. Thisinformation is important for the simulation of the growth ofindividual plants in response to the presence of neighbours.Copyright1993, 1999 Academic Press Crop yield, plant weight, plant density, plant spacing, plant arrangement, row crop, carrot, Daucus carota L., mathematical model, neighbourhood     

Description and Model of the Architecture of Four Genotypes of Determinate Autumn-sown White Lupin (Lupinus albus L.) as Influenced by Location, Sowing Date and Density

The structure of four new autumn-sown determine genotypes ofwhite lupin, grown with a combination of sowing dates and plantdensities at four locations in France was studied in 1990/91and 1991/92. Plant architecture was characterized in terms ofthe number of first-order branches produced and the distributionof leaves on these branches and how they varied with the numberof mainstem leaves and the position of the branch on the mainstem. The variation in the number of mainstem leaves on determinegenotypes was satisfactorily described by an earlier model developedfor indeterminate genotypes using the intensity of vernalizationrelative to accumulated temperature above 3°C as input.However the architecture of the first-order branches, definedby their number and the total number of leaves per branch ateach subtending position on the mainstem, differed and was influencedby genotype, location and sowing date. In four determinate genotypes,these two characters were positively correlated, and correlatedwith the number of leaves on the mainstem. Increasing densityfrom 10 to 40 plants m^-2 reduced the number of branches andtheir total number of leaves. Pruning 15 leaves from the mainstemat the beginning of stem elongation did not influence the developmentof the first-order branches. There appeared to be a functionalrelationship between the number of leaves on the mainstem andthe number of branches established very early in the growingseason. The profiles of number of leaves on the first-order brancheson the mainstem with branch position were modelled using twosub-models, one describing the mean number of leaves on thebranches actually present, and the second describing the frequentlyof occurrence of a branch at a particular position on the mainstem.Of the five model parameters, one, representing the minimalnumber of branches present on the plant, was correlated withthe number of first-order branches. Genotype, location, sowingdate and density affected the shape of the profiles.Copyright1993, 1999 Academic Press While lupin, Lupinus albus L., determine growth, architecture, model, branch     

Competition for Nitrogen and Potassium in Agropyron repens

Stands of Agropyron repens (couch grass) and single young sugar-beetplants were grown in pots, separately and in competition, withnitrogen and potassium each supplied at three rates in a factorialdesign. Both nitrogen and potassium increased relative growth-ratesof sugar-beet leaves and crowns and fresh-weight/dry-weightratios of all plant parts; the responses were usually curved,the high nutrient levels having little more effect than theintermediate levels. Competition decreased relative growth-ratesand freshweight/dry-weight ratios of all plant parts and alsoleaf-area ratio (F) and net assimilation rate (E). High ratesof nitrogen diminished the effects of competition on relativeleaf growth-rate, F, and on fresh-weight/dry-weight ratios,indicating competition for nitrogen, but high rates of potassiumdiminished only its effect on relative growth-rate of the crown,suggesting that competition for potassium had only a small effect. Competition depressed leaf nitrogen and potassium percentagesand total nitrogen and potassium uptake by the whole plant,but the depressions were small at high rates of nutrient supply,suggesting that the amounts of nitrogen and potassium availablewere nearly adequate for both sugar-beet and A. repens. When total dry weights and leaf areas of sugar-beet were comparedwith corresponding nutrient contents, variation in nitrogencontent alone could account for most of the effects of competitionon growth, but potassium depletion probably also contributeda little. E was correlated with potassium, but not nitrogen,per unit leaf area, but potassium differences could not adequatelyaccount for the effects of competition on E. Competition for potassium produced small effects compared withcompetition for nitrogen, in spite of large responses to addedpotassium, perhaps because potassium moves less readily in thesoil.