Crown allometries are less responsive than stem allometry to tree size and habitat variations in an Indian monsoon forest

While theoretical allometric models postulate universal scaling exponents, empirical relationships between tree dimensions show marked variability that reflects changes in the biomass allocation pattern. As growth of the various tree compartments may be controlled by different functions, it is hypothesized that they may respond differently to factors of variation, resulting in variable tree morphologies and potentially in trade-offs between allometric relationships. We explore the variability of tree stem and crown allometries using a dataset of 1,729 trees located in an undisturbed wet evergreen forest of the Western Ghats, India. We specifically test whether species adult stature, terrain slope, tree size and crown light exposure affect the relationships between stem diameter and stem height (stem allometry), and between stem diameter and crown width, crown area and crown volume (crown allometries). Results show that both stem and crown allometries are subject to variations in relation to both endogenous (tree size, species adult stature) and exogenous (terrain slope, crown light exposure) factors. Stem allometry appears to be more affected by these factors than are crown allometries, including the stem diameter–crown volume relationship, which proved to be particularly stable. Our results support the idea that height is a prevailing adjustment factor for a tree facing variable growth (notably light) conditions, while stem diameter–crown volume allometry responds more to internal metabolic constraints. We ultimately discuss the various sources of variability in the stem and crown allometries of tropical trees that likely play an important role in forest community dynamics.     

Tree shape plasticity in relation to crown exposure

Trees outside closed forest stands differ in the relation between stem diameter, height and crown volume from trees that grew with neighbours close by. Whether this plasticity in tree shape varies between species in relation to their light requirement is unknown. We purposefully sampled 528 trees ranging 5–100?cm diameter at breast height growing in a range of light conditions. Across ten broad-leaved species observed in Sumatra or Kalimantan, a generic relationship was found between light exposure of the crown and a light-dependent a _l parameter that modifies the height–diameter allometric equation (H?=?a _l D ^b ) from those for closed stands. In our results, vertical stretching is well predicted by light availability. In fully open conditions, trees are on average 31% shorter for the same diameter than under (partial) shade. Most of the stretching response occurs in all species as soon as some degree of lateral shading occurs. The response, however, varies by species (8–44% reduction) in a way apparently unrelated to species’ successional status. Crown volume varied less than stem height in its relationship with stem diameter across all light conditions tested. The scaling of crown volume with stem diameter, however, differed markedly between tree species.     

植物的表型可塑性、异速生长及其入侵能力

表型可塑性是指同一个基因型对不同环境响应产生不同表型的特性,特定性状的可塑性本身可以遗传,也可以接受选择而发生进化。植物个体的异速生长是指生物体某一特征的相对生长速率不等于第二种特征的相对生长速率的特性,该特性是由物种的遗传性决定的一种固定特征,植物往往朝着最佳的异速生长曲线进化。植物特定基因型在不同环境下,诸如生物量分配和种群几何学上的一些表型差异,既可由异速生长造成,也可由表型可塑性造成。植物本身的异速生长是一种"外观可塑性",而异速生长曲线的改变才是真正的可塑性。植物的表型可塑性、异速生长对于入侵植物的适应具有重要意义。干扰等异质性生境下表型可塑性成为物种生存扩散的有利性状,表型可塑性强的物种更有可能成为广布种。植物本身的异速生长特性或其异速生长曲线的改变都能影响其入侵能力。     

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.     

Stem and crown allometry in four congeneric species of dioecious tropical trees

Key message

Congeneric species showed similar stem and crown allometry, but differed in crown dimensions indicating that crown size is adaptive and variable despite mechanical restrictions.

Abstract

Morphological adaptations favor differential use of the space in tropical trees, but the variability in stem and crown allometry can be constrained by phylogenetic and mechanical factors. In addition, dioecious species show marked differences in their energy requirements related to reproduction, but little information is available about the role of shape and allometry on differential acquisition of energy between the sexes. We studied the stem and crown dimensions of congeneric dioecious trees to determine if there are: (i) differences in the allometry between the sexes, (ii) different average sizes among sympatric species, and (iii) differences in stem and crown allometry between sympatric and allopatric species. Two pairs of sympatric Virola (Myristicaceae) in Brazil and Costa Rica were studied. SMA regression models were used to investigate allometric relationships between diameter at breast height (DBH) and tree height, and between DBH and crown volume (CV). No sexual dimorphism in stem and crown morphology was observed in this study, indicating that differences in resource allocation for reproduction between the sexes do not impact the stem and crown structure in these species. Overall, low variability among the species was observed. Only one species differed in stem allometry and none differed in crown allometry. CV differed between sympatric species. Stem and crown allometry are related to structural stability and our results support similar mechanical restriction for these species. The ecological significance of differences in CV among canopy species remains to be explored.

     

Contrasted allometries between stem diameter,crown area,and tree height in five tropical biogeographic areas

Key message

Across five biogeographic areas, DBH-CA allometry was characterized by inter-site homogeneity and intra-site heterogeneity, whereas the reverse was observed for DBH-H allometry.

Abstract

Tree crowns play a central role in stand dynamics. Remotely sensed canopy images have been shown to allow inferring stand structure and biomass which suggests that allometric scaling between stems and crowns may be tight, although insufficiently investigated to date. Here, we report the first broad-scale assessment of stem vs. crown scaling exponents using measurements of bole diameter (DBH), total height (H), and crown area (CA) made on 4148 trees belonging to 538 species in five biogeographic areas across the wet tropics. Allometries were fitted with power functions using ordinary least-squares regressions on log-transformed data. The inter-site variability and intra-site (sub-canopy vs. canopy trees) variability of the allometries were evaluated by comparing the scaling exponents. Our results indicated that, in contrast to both DBH-H and H-CA allometries, DBH-CA allometry shows no significant inter-site variation. This fairly invariant scaling calls for increased effort in documenting crown sizes as part of tree morphology. Stability in DBH-CA allometry, indeed, suggests that some universal constraints are sufficiently pervasive to restrict the exponent variation to a narrow range. In addition, our results point to inverse changes in the scaling exponent of the DBH-CA vs. DBH-H allometries when shifting from sub-canopy to canopy trees, suggesting a change in carbon allocation when a tree reaches direct light. These results pave the way for further advances in our understanding of niche partitioning in tree species, tropical forest dynamics, and to estimate AGB in tropical forests from remotely sensed images.

     

Common allometric response of open-grown leader shoots to tree height in co-occurring deciduous broadleaved trees

Background and Aims

Morphology of crown shoots changes with tree height. The height of forest trees is usually correlated with the light environment and this makes it difficult to separate the effects of tree size and of light conditions on the morphological plasticity of crown shoots. This paper addresses the tree-height dependence of shoot traits under full-light conditions where a tree crown is not shaded by other crowns.

Methods

Focus is given to relationships between tree height and top-shoot traits, which include the shoot''s leaf-blades and non-leafy mass, its total leaf-blade area and the length and basal diameter of the shoot''s stem. We examine the allometric characteristics of open-grown current-year leader shoots at the tops of forest tree crowns up to 24 m high and quantify their responses to tree height in 13 co-occurring deciduous hardwood species in a cool-temperate forest in northern Japan.

Key Results

Dry mass allocated to leaf blades in a leader shoot increased with tree height in all 13 species. Specific leaf area decreased with tree height. Stem basal area was almost proportional to total leaf area in a leader shoot, where the proportionality constant did not depend on tree height, irrespective of species. Stem length for a given stem diameter decreased with tree height.

Conclusions

In the 13 species observed, height-dependent changes in allometry of leader shoots were convergent. This finding suggests that there is a common functional constraint in tree-height development. Under full-light conditions, leader shoots of tall trees naturally experience more severe water stress than those of short trees. We hypothesize that the height dependence of shoot allometry detected reflects an integrated response to height-associated water stress, which contributes to successful crown expansion and height gain.     

Geometric morphometrics and leaf phenotypic plasticity: assessing fluctuating asymmetry and allometry in European white oaks (Quercus)

Because plants are unable to move away from unfavourable habitats and environmental perturbations, leaf phenotypic plasticity facilitates light absorption and gas exchange. Oaks (Quercus spp.) are particularly known for their adaptability and plastic phenotypes, and leaf allometry and developmental instability may represent important mechanisms for their adaptation to environments and evolution. Because of its important role in the adaptation of plant populations to different environments, allometry can be involved in diversifying selection. Developmental instability is related to environmental perturbations and stresses by producing random deviations in structures characterized by bilateral symmetry, such as oak leaves. In addition, developmental instability can also arise from genetic bottlenecks or as a result of hybridization. The splitting of symmetric and asymmetric components of variation and their separate analysis allows the variability in leaf shape traits to be summarized, reducing the variation produced by developmental instability. The geometric morphometric approach is a useful method for the study of leaf asymmetry and allometric patterns. This method provides an important tool for the visualization of shape attributes that characterize species with highly variable leaf phenotypic patterns. In this study, leaf shape and size variability of three white oak species was investigated by means of a two‐dimensional landmark‐based method providing improved knowledge of variance partitioning, species discrimination, fluctuating asymmetry and allometric patterns of variation resulting from the different analyses. © 2015 The Linnean Society of London, Botanical Journal of the Linnean Society, 2015, 179 , 335–348.     

The expression of light-related leaf functional traits depends on the location of individual leaves within the crown of isolated Olea europaea trees

Background The spatial arrangement and expression of foliar syndromes within tree crowns can reflect the coupling between crown form and function in a given environment. Isolated trees subjected to high irradiance and concomitant stress may adjust leaf phenotypes to cope with environmental gradients that are heterogeneous in space and time within the tree crown. The distinct expression of leaf phenotypes among crown positions could lead to complementary patterns in light interception at the crown scale.Methods We quantified eight light-related leaf traits across 12 crown positions of ten isolated Olea europaea trees in the field. Specifically, we investigated whether the phenotypic expression of foliar traits differed among crown sectors and layers and five periods of the day from sunrise to sunset. We investigated the consequences in terms of the exposed area of the leaves at the tree scale during a single day.Key Results All traits differed among crown positions except the length-to-width ratio of the leaves. We found a strong complementarity in the patterns of the potential exposed area of the leaves among day periods as a result of a non-random distribution of leaf angles across the crown. Leaf exposure at the outer layer was below 60 % of the displayed surface, reaching maximum interception during morning periods. Daily interception increased towards the inner layer, achieving consecutive maximization from east to west positions within the crown, matching the sun’s trajectory.Conclusions The expression of leaf traits within isolated trees of O. europaea varies continuously through the crown in a gradient of leaf morphotypes and leaf angles depending on the exposure and location of individual leaves. The distribution of light-related traits within the crown and the complementarity in the potential exposure patterns of the leaves during the day challenges the assumption of low trait variability within individuals.     

Some autecological characteristics of early to late successional tree species in Venezuela

The breadth of the continuum concept of strategy with respect to succession was tested on 21 tree and shrub species common in either unlogged or logged stands, respectively, in the Forest Reserve of Caparo, Venezuela, by examining morphological, physiological and population characteristics. Based on a preliminary abundance analysis, `early', `mid' and `late' successional species as well as `generalists' were distinguished. Early successional species, i.e. Ochroma lagopus, Heliocarpus popayanensis and Cecropia peltata were similar in many autecological aspects, e.g. monolayered leaf arrangement, orthotropic architectural models, no adaptive reiteration, clumped distribution, but differed in gap association and distribution along a drainage gradient. Mid-successional species established themselves both in large and small gaps (> 300 m[sup2 ]; 80–300 m[sup2 ]) and showed a clumped to regular distribution pattern in logged areas; they exhibited more diverse crown and leaf characteristics than early successional species. Late successional species established themselves only in small gaps and understorey, and showed a regular spatial pattern in undisturbed areas. All late successional species displayed architectural models with plagiotropic lateral axes and showed a multilayered leaf arrangement. Adaptive reiteration was a common feature of late successional species which could be further subdivided into large, medium-sized and small trees, indicating different light requirements at maturity. Generalists were common treelet and shrub species in both disturbed and undisturbed sites where they are also capable of completing their life cycle. The light compensation point (LCP) of an individual plant was strongly influenced by its crown illuminance. Large late successional species showed the widest range of LCP values, reflecting the increasing light availability with increasing height in mature forest. On the basis of many autecological characteristics, it was found (i) that there is in fact a continuum of species strategies with respect to succession even among early and mid-successional species and (ii) that the latter group of species showed the widest breadth of autecological traits, reflecting the heterogeneous environment in which they establish and mature.     

Rejection of non-adaptive hypotheses for intraspecific variation in trophic morphology in gape-limited predators

Recent studies have interpreted intraspecific divergence in relative head sizes in snakes as evidence for adaptation of the trophic apparatus in gape-limited predators to local prey size. However, such variation might also arise from non-adaptive processes (such as allometry, correlated response, genetic drift, or non-adaptive phenotypic plasticity). We test predictions from these alternative hypotheses using data on the allometric relationship between head size and body size in two wide-ranging snake species: eight populations of adders ( Vipera berus ) and 30 populations of common gartersnakes ( Thamnophis sirtalis ). Our data enable strong rejection of the alternative (non-adaptive) hypotheses, because the relationship between head and body size differed significantly among populations, the geographic distance separating pairs of populations explained less than 1.5% of their divergence in allometric coefficients, and the within-population allometric coefficients were higher than the among-population coefficients in each species. In addition, the geographical variability of allometric coefficients in females did not parallel that in males, suggesting that allometric coefficients have evolved independently in the two sexes. Phenotypic plasticity also cannot explain the data, because laboratory studies show that the allometric relationship between head size and body size is relatively insensitive to differing growth rates. We conclude that the intraspecific head size divergence in these snakes is better explained by spatially heterogeneous selection to optimize prey handling ability, than by non-adaptive processes.     

Exploring ecological significance of tree crown plasticity through three-dimensional modelling

BACKGROUND AND AIMS: Morphogenetic plasticity may be as important as physiological plasticity in determining plant adaptability to changing environmental conditions. This study examines the importance of crown plasticity of trees in stands. METHODS: A three-dimensional forest simulator is used to explore the impact of crown shape plasticity on tree growth. Crown deformation is mediated through the local response to light and overall allometric constraints governing tree dimensions. By altering shape response parameters of Hevea brasiliensis the impact of increased or decreased plasticity is explored in a variety of competitive environments defined by various combinations of tree density and relative frequency of different strategies. The possible interactions between plasticity and growth rate and plasticity and below-ground competition are also explored. KEY RESULTS: Crown plasticity confers competitive superiority in all cases studied. Interactions with other processes may downplay or enhance this competitive advantage. CONCLUSIONS: Simulation results strongly suggest that crown plasticity does have a significant impact on tree performance in nature and that commonly observed crown shape deformation response of trees is of adaptive value.     

浙江天童常绿阔叶林演替阶段共有种的树木构型

树木构型是木本植物为响应光照变化在其空间建造结构上的配置模式和形态体现。研究演替不同阶段共有种构型的变化可以剔除植物谱系的影响, 反映植物构型特征与光资源供给性的关系。该研究在浙江宁波天童、南山和北仑3个次生演替序列上选择了5个演替共有种, 分4个群落高度层级, 对照分析了树高、冠幅深度和面积、枝条伸展方向、基径、叶片盖度和聚集度构型性状随演替的变化, 并分析了与冠幅曝光指数的线性关系。结果表明: 1)随着演替进行, 冠幅厚度和面积、叶片盖度、叶片聚集度和基径逐步增加, 但在个别相邻演替阶段增加不显著; 2)随着演替进行, 植物的垂直枝比例降低, 水平枝比例增加; 3)演替过程中植物冠幅曝光指数在各层级内都呈现出减小趋势; 4)构型性状和植物冠幅曝光指数间存在显著的线性回归关系(p < 0.001)。总之, 随着常绿阔叶林演替进行, 演替共有种构型的变化反映了物种功能类群由阳性先锋植物向耐阴植物的转化, 其中, 植物对光资源的适应是导致构型变化的主要原因。     

Variation in crown light utilization characteristics among tropical canopy trees

BACKGROUND AND AIMS: Light extinction through crowns of canopy trees determines light availability at lower levels within forests. The goal of this paper is the exploration of foliage distribution and light extinction in crowns of five canopy tree species in relation to their shoot architecture, leaf traits (mean leaf angle, life span, photosynthetic characteristics) and successional status (from pioneers to persistent). METHODS: Light extinction was examined at three hierarchical levels of foliage organization, the whole crown, the outermost canopy and the individual shoots, in a tropical moist forest with direct canopy access with a tower crane. Photon flux density and cumulative leaf area index (LAI) were measured at intervals of 0.25-1 m along multiple vertical transects through three to five mature tree crowns of each species to estimate light extinction coefficients (K). RESULTS: Cecropia longipes, a pioneer species with the shortest leaf life span, had crown LAI <0.5. Among the remaining four species, crown LAI ranged from 2 to 8, and species with orthotropic terminal shoots exhibited lower light extinction coefficients (0.35) than those with plagiotropic shoots (0.53-0.80). Within each type, later successional species exhibited greater maximum LAI and total light extinction. A dense layer of leaves at the outermost crown of a late successional species resulted in an average light extinction of 61% within 0.5 m from the surface. In late successional species, leaf position within individual shoots does not predict the light availability at the individual leaf surface, which may explain their slow decline of photosynthetic capacity with leaf age and weak differentiation of sun and shade leaves. CONCLUSION: Later-successional tree crowns, especially those with orthotropic branches, exhibit lower light extinction coefficients, but greater total LAI and total light extinction, which contribute to their efficient use of light and competitive dominance.     

Evidence of variant intra- and interspecific scaling of tree crown structure and relevance for allometric theory

General scaling rules or constants for metabolic and structural plant allometry as assumed by the theory of Euclidian geometric scaling (2/3-scaling) or metabolic scaling (3/4-scaling) may meet human's innate propensity for simplicity and generality of pattern and processes in nature. However, numerous empirical works show that variability of crown structure rather than constancy is essential for a tree's success in coping with crowding. In order to link theory and empiricism, we analyzed the intra- and inter-specific scaling of crown structure for 52 tree species. The basis is data from 84 long-term plots of temperate monospecific forests under survey since 1870 and a set of 126 yield tables of angiosperm and gymnosperm forest tree species across the world. The study draws attention to (1) the intra-specific variation and correlation of the three scaling relationships: tree height versus trunk diameter, crown cross-sectional area versus trunk diameter, and tree volume versus trunk diameter, and their dependence on competition, (2) the inter-specific variation and correlation of the same scaling exponents ([Formula: see text] and [Formula: see text]) across 52 tree species, and (3) the relevance of the revealed variable scaling of crown structure for leaf organs and metabolic scaling. Our results arrive at suggesting a more extended metabolic theory of ecology which includes variability and covariation between allometric relationships as prerequisite for the individual plant's competitiveness.     

Relationships among ontogenetic,static, and evolutionary allometry

The relationship between ontogenetic, static, and evolutionary levels of allometry is investigated. Extrapolation from relative size relationships in adults to relative growth in ontogeny depends on the variability of slopes and intercepts of ontogenetic vectors relative to variability in length of the vector. If variability in slopes and intercepts is low relative to variability in length, ontogenetic and static allometries will be similar. The similarity of ontogenetic and static allometries was tested by comparing the first principal component, or size vector, for correlations among 48 cranial traits in a cross-sectional ontogenetic sample of rhesus macaques from Cayo Santiago with a static sample from which all age- and sex-related variation had been removed. The vector correlation between the components is high but significantly less than one while two of three allometric patterns apparent in the ontogenetic component are not discernable in the static component. This indicates that there are important differences in size and shape relationships among adults and within ontogenies. Extrapolation from intra- or interspecific phenotypic allometry to evolutionary allometry is shown to depend on the similarity of genetic and phenotypic allometry patterns. Similarity of patterns was tested by comparing the first principal components of the phenotypic, genetic, and environmental correlation matrices calculated using standard quantitative genetic methods. The patterns of phenotypic, genetic, and environmental allometry are dissimilar; only the environmental allometries show ontogenetic allometric patterns. This indicates that phenotypic allometry may not be an accurate guide to patterns of evolutionary change in size and shape.     

Long‐term successional forest dynamics: species and community responses to climatic variability

Question: Are trees sensitive to climatic variability, and do tree species differ in their responses to climatic variability? Does sensitivity of forest communities to climatic variability depend on stand composition? Location: Mixed young forest at Walker Branch Watershed near Oak Ridge, East Tennessee, USA. Methods: Using a long‐term dataset (1967–2006), we analyzed temporal forest dynamics at the tree and species level, and community dynamics for forest stands that differed in initial species composition (i.e., chestnut oak, oak–hickory, pine, and yellow poplar stands). Using summer drought and growing season temperature as defined climate drivers, we evaluated relationships between forest dynamics and climate across levels of organization. Results: Over the four‐decade study period, forest communities underwent successional change and substantially increased in biomass. Variation in summer drought and growing season temperature contributed to temporal biomass dynamics for some tree species, but not for others. Stand‐level responses to climatic variability were related to the responses of component species, except in pine stands. Pinus echinata, the dominant species in pine stands, decreased over time due to periodic outbreaks of pine bark beetle (Dendroctonus frontalis). These outbreaks at Walker Branch could not be directly related to climatic conditions. Conclusions: The results indicate that sensitivity of developing forests to climatic variability is stand type‐dependent, and hence is a function of species composition. However, in the long term, direct effects of climatic variability on forest dynamics may be small relative to autogenic successional processes or climate‐related insect outbreaks. Empirical studies testing for interactions between forest succession and climatic variability are needed.     

Biomechanical constraints on tree architecture

Key message

Mechanical properties of wood constrain most conifers to an excurrent form and limit the width of tree crowns. Development of support tissue alters allometric relations during ontogeny.

Abstract

Biomechanical constraints on tree architecture are explored. Torque on a tree branch is a multiplicative function of mass and moment arm. As such, the need for support rises faster than branch length, which leads to increased taper as branch size increases. This violates assumptions of models, such as the pipe-model theory, for large trees and causes changing allometry with tree size or exposure. Thus, assumptions about optimal design for light capture, self-similarity, or optimal hydraulic architecture need to be modified to account for mechanical constraints and costs. In particular, it is argued that mechanical limitations of compression wood in conifers prevent members of this taxon from developing large branches. With decurrent form ruled out (for larger species), only a conical or excurrent form can develop. Wind is shown to be a major mortality risk for trees. Adaptations for wind include dynamic responses of wood properties and height. It is argued that an adaptation to wind could be the development of an open crown in larger trees to let the wind penetrate, thereby reducing wind-throw risk. It is thus argued that crown shape and branching may result not just from optimal light capture considerations but also from adaptation to and response to wind as well as from mechanical constraints. Results have implications for allometric theory, life history theory, and simulations of tree architecture.

     

An ontogenetic perspective on the study of sexual dimorphism, phylogenetic variability, and allometry of the skull of European ground squirrel, Spermophilus citellus (Linnaeus, 1766)

Most studies of morphological variability in or among species are performed on adult specimens. However, it has been proven that knowledge of the patterns of size and shape changes and their covariation during ontogeny is of great value for the understanding of the processes that produce morphological variation. In this study, we investigated the patterns of sexual dimorphism, phylogenetic variability, and ontogenetic allometry in the Spermophilus citellus with geometric morphometrics applied to cross-sectional ontogenetic data of 189 skulls from three populations (originating from Burgenland, Banat, and Dojran) belonging to two phylogenetic lineages (the Northern and Southern). Our results indicate that sexual dimorphism in the ventral cranium of S. citellus is expressed only in skull size and becomes apparent just before or after the first hibernation because of accelerated growth in juvenile males. Sexes had the same pattern of ontogenetic allometry. Populations from Banat and Dojran, belonging to different phylogroups, were the most different in size but had the most similar adult skull shape. Phylogenetic relations among populations, therefore, did not reflect skull morphology, which is probably under a significant influence of ecological factors. Populations had parallel allometric trajectories, indicating that alterations in development probably occur prenatally. The species’ allometric relations during cranial growth showed characteristic nonlinear trajectories in the two northern populations, with accelerated shape changes in juveniles and continued but almost isometric growth in adults. The adult cranial shape was reached before sexual maturity of both sexes and adult size after sexual maturity. The majority of shape changes during growth are probably correlated with the shift from a liquid to a solid diet and to a lesser degree due to allometric scaling, which explained only 20 % of total shape variation. As expected, viscerocranial components grew with positive and neurocranial with negative allometry.     

Factors controlling resource allocation in mountain birch

We present a comprehensive analysis of factors affecting resource allocation and crown formation in a subarctic birch tree, Betula pubescens ssp. czerepanovii (Orlova) Hämet-Ahti. Using biomass measurements and digitized data on tree architecture, we investigated several hypotheses on various factors that may modify plant growth. We also analyzed the extent to which different mechanisms operate at different scales, ranging from individual shoots to the whole branches or trees. Different factors affected allocation at different levels of organization. Stem age had a minor effect, suggesting that similar control mechanisms operate at all stages of development. Fates of individual shoots were affected by their local growing conditions as indicated, for example, by the dependence of long shoot production on light. Buds formed in the current long shoots were likely to become new long shoots. In the innermost crown parts, radial growth had priority compared to long shoot production. Elongation of individual long shoots was controlled by two conflicting factors. Long distance from the roots suppressed growth, probably indicating costs associated with resource transportation, whereas a high level of light augmented growth. In contrast, growth of entire branches was not so clearly related to the availability of resources, but showed limitation due to allometric scaling. This set a relationship between the maximum long shoot number and the overall branch size, and may indicate allometric constraints to the way a tree is constructed. Strict allometric relationships existed also between other structural traits of mountain birch, most of them similar at all levels of branching hierarchy. However, despite the upper level restrictions set by allometry, source-sink interactions and localized responses of individual shoots operated as local processes that directed allocation towards the most favourable positions. This may be a mechanism for achieving efficient tree architecture in terms of resource intake and costs of transportation.