Annual and spatial variation in shoot demography associated with masting in Betula grossa: comparison between mature trees and saplings

Backgrounds and Aims

Shoot demography affects the growth of the tree crown and the number of leaves on a tree. Masting may cause inter-annual and spatial variation in shoot demography of mature trees, which may in turn affect the resource budget of the tree. The aim of this study was to evaluate the effect of masting on the temporal and spatial variations in shoot demography of mature Betula grossa.

Methods

The shoot demography was analysed in the upper and lower parts of the tree crown in mature trees and saplings over 7 years. Mature trees and saplings were compared to differentiate the effect of masting from the effect of exogenous environment on shoot demography. The fate of different shoot types (reproductive, vegetative, short, long), shoot length and leaf area were investigated by monitoring and by retrospective survey using morphological markers on branches. The effects of year and branch position on demographic parameters were evaluated.

Key Results

Shoot increase rate, production of long shoots, bud mortality, length of long shoots and leaf area of a branch fluctuated periodically from year to year in mature trees over 7 years, in which two masting events occurred. Branches within a crown showed synchronized annual variation, and the extent of fluctuation was larger in the upper branches than the lower branches. Vegetative shoots varied in their bud differentiation each year and contributed to the dynamic shoot demography as much as did reproductive shoots, suggesting physiological integration in shoot demography through hormonal regulation and resource allocation.

Conclusions

Masting caused periodic annual variation in shoot demography of the mature trees and the effect was spatially variable within a tree crown. Since masting is a common phenomenon among tree species, annual variation in shoot demography and leaf area should be incorporated into resource allocation models of mature masting trees.     

Size structure of current-year shoots in mature crowns

Characteristics of current-year shoot populations were examined for three mature trees of each of three deciduous broad-leaved species. For first-order branches (branches emerging from the vertical trunk) of the trees examined, lengths or diameters of all current-year shoots were measured. Total leaf mass and total current-year stem mass of first-order branches were estimated using an allometric relationship between leaf or stem mass and length or diameter of current-year stems. For each tree, the number of current-year shoots on a first-order branch was proportional to the basal stem cross-sectional area of the branch. On the other hand, first-order branches had shoot populations with size structures similar to each other. As a result, the leaf mass of a first-order branch was proportional to the basal stem cross-sectional area of the branch, being compatible with the pipe-model relationship. All current-year shoot populations had positively skewed size structures. Because small shoots have a larger ratio of leaf mass to stem mass than large shoots, first-order branches had an extremely large ratio of leaf mass to current-year stem mass. This biased mass allocation will reduce costs for current stem production, respiration and future radial growth, and is beneficial to mature trees with a huge accumulation of non- photosynthetic organs. The allometric relationships between leaf mass and basal stem diameter and that between leaf mass and current-year stem mass of first-order branches were each similar across the trees examined. Characteristics of shoot populations tended to offset inter-species diversity of shoot allometry so that branch allometry shows inter-species convergence.     

Responses of crown architecture in Betula pendula to competition are dependent on the species of neighbouring trees

We measured the growth responses of individual shoots and branches of Betula pendula when growing next to trees of the same species or Pinus sylvestris, Larix sibirica or Alnus glutinosa. We used the three-dimensionally digitized response variables and the size and distance of trees growing within a 5-m radius of the study trees to establish a relationship between tree performance and the effect of competing neighbouring tree species on crown architecture. B. pendula was able to modify its crown architecture and thus alter its strategy to compete with different neighbours. Trees of B. pendula growing beside species counterparts had the highest growth of new long shoots in relation to the already existing branch length [growth vigour (GV)], while GV was the lowest next to L. sibirica. With B. pendula or P. sylvestris as its main neighbour, B. pendula invested in short shoots by growing them rather densely in short branches with limited numbers, whereas with L. sibirica the number, length and angle of the branches were high. The competitive response was also strongly dependent on tree ontogeny and the shoot and branch characteristics were significantly affected by their location inside the crown. B. pendula was able to respond to the challenges posed by its neighbours, which was also reflected in the GV. The ability to maintain steady growth with alternative crown designs in different neighbourhoods reflects plasticity in the crown responses.     

Cost and probability of flowering at the shoot level in relation to variability in shoot size within the crown of Vaccinium hirtum (Ericaceae)

Size-related variation in the cost and probability of flowering among shoots within a crown of Vaccinium hirtum was investigated to clarify patterns and regulation of flowering at the shoot-module level, below the level of the individual. The apices of previous-year shoots differentiated into current-year shoots vegetatively (vegetative branches) or became reproductive by developing inflorescences (reproductive branches). Length growth and fate of current-year shoots were determined, and the future potential for reproduction was estimated using a matrix model of shoot dynamics. Reproductive branches had fewer current-year shoots and shorter total shoot lengths and thus had a reduced potential for reproduction compared with vegetative branches, indicating the cost of flowering at the shoot level. This cost of flowering was higher in longer shoots. The probability of the initiation of flowering in a shoot increased with increasing shoot length in shorter shoots, reached a maximum in medium-sized shoots, and decreased in longer shoots. The size-related changes in the probability of flowering at the shoot level can be largely explained by the size-dependent changes in shoot-level resource availability and cost of flowering.     

火灾后兴安落叶松长短枝变化及其对生存的影响

研究了兴安落叶松的长短枝习性及火灾对其影响.结果表明,短枝在春季发叶迅速,而长枝的生长对兴安落叶松的枝条及树冠结构的形成起决定性作用,且在1个长枝上能形成10.5个短枝.中度火烧使兴安落叶松的树冠及枝条受到很大伤害,火烧可刺激1级侧枝上37%的短枝芽变成长枝,加速了火烧后兴安落叶松树冠结构的恢复,火后第3年兴安落叶松的短枝总量达到火前水平的98.46%.     

Assessing Costs of Reproduction in Mountain Birch: The Importance of Considering the Modular Level

We studied the effects of production of male catkins on growthand the subsequent year's male catkin production in mountainbirch, Betula pubescens subsp. czerepanovii at both the shootand branch level. (Shoot is defined here as the product of asingle growing season while branch refers to a structure consistingof several shoots.) Like heterophyllous trees in general, mountainbirch canopy expansion takes place via the production of longshoots. We found phenotypic trade-offs between long shoot growthand male reproduction at the shoot level in the year of catkinproduction. Generative parental long shoots (long shoots withmale catkins) were significantly shorter than the vegetativeones (long shoots without a male catkin). In contrast, we foundno effects of male reproduction on the subsequent year's malecatkin production at the shoot level. Although the mean lengthof secondary long shoots (long shoots growing from the lateralbuds of parental long shoots) did not differ between vegetativeand generative parental long shoots, there was considerablebetween-individual variation in the response of individual trees.In addition, production of male catkins diminishes canopy expansionin mountain birch because the number of secondary long shootsproduced by generative parental long shoots was smaller thanthat of vegetative parental long shoots. At the branch level,the association between total long shoot growth and male catkinproduction was positive, i.e. no trade-off was found. This maybe because the strong sink strength (the ability of a branchto import assimilates from elsewhere in the tree) of branchesbearing reproductive long shoots masks possible trade-offs.We emphasize the importance of considering several levels ofthe modular hierarchy when analysing costs of reproduction inmodular organisms. Copyright 2000 Annals of Botany Company Hierarchical structure, modularity, mountain birch, short and long shoots, trade-off, Betula pubescens subsp.czerepanovii (Orlova) Hämet-Ahti     

The role of crown architecture, leaf phenology and photosynthetic activity in promoting complementary use of light among coexisting species in temperate forests

Mixed forests comprising multiple tree species with contrasting crown architectures, leaf phenologies, and photosynthetic activity, tend to have high ecosystem productivity. We propose that in such forests, differentiation among coexisting species in their spatial and temporal strategies for light interception, results in complementary use of light. Spatial differentiation among coexisting tree species occurs as a result of adaptation of crown architecture and shoot/leaf morphology to the spatially variable light conditions of the canopy, sub-canopy, and understory. Temporal differentiation occurs as a result of variation in leaf phenology and photosynthetic activity. The arrangement of leaves in both space and time is an important aspect of plant strategies for light interception and determines photosynthetic carbon gain of the plant canopy. For example, at the shoot level, morphological and phenological differentiation between long and short shoots reflects their respective shoot functions, indicating that spatial and temporal strategies for light interception are linked. Complementary use of light is a consequence of the spatiotemporal differentiation in light interception among coexisting species. Because coexisting species may show differentiation in strategies for resource acquisition (functional diversification) or convergence with respect to some limiting resource (functional convergence), the relative importance of various crown functions and their contribution to growth and survival of individuals need to be evaluated quantitatively and compared among coexisting species.     

Impact of leaf damage on growth of mountain birch shoots

Canopies of heterophyllous trees expand by production of long shoots. We have previously shown in mountain birch ( Betula pubescens ssp. czerepanovii ) that damage to internode leaves within long shoots does not impede shoot growth, indicating that long-shoot elongation occurs by means of external resources. To study to what extent leaves other than true long-shoot leaves are necessary for the normal growth of mountain birch long shoots, we simulated herbivore damage to the two basal leaves of shoots (which flush simultaneously with short-shoot leaves) and the short-shoot leaves nearest to the long shoot within the branch. Damage to the two basal long-shoot leaves significantly reduced long-shoot growth. Additional damage to short-shoot leaves, situated proximally to the long shoot, did not retard long-shoot growth any more than damage to basal leaves alone. To determine the extent to which short-shoot leaves within a large branch are responsible for the pooled long-shoot production of the branch, we clipped differing proportions of short-shoot leaves from such branches. We found small but significant reduction in the pooled length of the long shoots of the branch, presumably indicating a limited role in long-shoot elongation of current photosynthates within the branch. Our experiments indicate that long shoots are not independent modular units in their carbon economy.     

Uncaged larvae elicit a combination of local and integrated growth responses within mountain birch crown

The degree of autonomy shown by plant structures in response to natural herbivory remains uncertain. Previous studies have usually been based on simulated herbivory or the use of caged herbivores. In this study, experimental trees were subjected to long-lasting defoliation by uncaged insect larvae. The exact 3D architecture of the whole shoot system and the amount of local damage elicited by the larvae in individual tree shoots were measured to investigate herbivore-elicited changes in growth at all levels of branching hierarchy within the crowns of mountain birch ( Betula pubescens ssp. czerepanovii ). Multiple explanatory variables were derived from the data. Defoliation by uncaged larvae elicited a combination of local and integrated growth responses. Especially the integrated responses were clear and consistent during the two years of study and resulted in reduced elongation growth. The localised component most often included the effect of defoliation at several branching orders, including also changes in self-shading. This suggests the involvement of source–sink interactions or other changes in the control of resource allocation within a larger scale. Therefore, the assumption of autonomous modules may offer an overly limited view of plant responses to localised herbivory, which appear to involve both localised and integrated components.     

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.     

Individual variability in tree allometry determines light resource allocation in forest ecosystems: a hierarchical Bayesian approach

Tree species differences in crown size and shape are often highlighted as key characteristics determining light interception strategies and successional dynamics. The phenotypic plasticity of species in response to light and space availability suggests that intraspecific variability can have potential consequences on light interception and community dynamics. Species crown size varies depending on site characteristics and other factors at the individual level which differ from competition for light and space. These factors, such as individual genetic characteristics, past disturbances or environmental micro-site effects, combine with competition-related phenotypic plasticity to determine the individual variability in crown size. Site and individual variability are typically ignored when considering crown size and light interception by trees, and residual variability is relegated to a residual error term, which is then ignored when studying ecological processes. In the present study, we structured and quantified variability at the species, site, and individual levels for three frequently used tree allometric relations using fixed and random effects in a hierarchical Bayesian framework. We focused on two species: Abies alba (silver fir) and Picea abies (Norway spruce) in nine forest stands of the western Alps. We demonstrated that species had different allometric relations from site to site and that individual variability accounted for a large part of the variation in allometric relations. Using a spatially explicit radiation transmission model on real stands, we showed that individual variability in tree allometry had a substantial impact on light resource allocation in the forest. Individual variability in tree allometry modulates species’ light-intercepting ability. It generates heterogeneous light conditions under the canopy, with high light micro-habitats that may promote the regeneration of light-demanding species and slow down successional dynamics.     

Crown hollowing as a consequence of early shedding of leaves and shoots

Leafing pattern has long been considered as an important element characterizing the growth strategy of tree species; however, the consequences of leafing pattern for tree-crown formation have not been fully understood. To address this issue, the dynamic events (growth, birth, and death) of current-year shoots and leaves were investigated together with their location in saplings of a pioneer tree, Alnus sieboldiana. The leafing pattern was characterized by successive emergence and shedding of short-lived leaves. The combination of successive leafing and within-crown variation in leaf production brought about characteristic outcomes in crown morphology. In the outer crown, because of continuous leaf production, the shoots achieved great extension and enormous daughter shoot production, resulting in rapid expansion of the crown. In contrast, in the inner crown, due to early termination of leaf production, the shoots completely lost their leaves early in the growing season and consequently themselves died and were shed within the season. Such quick shedding of shoots caused “crown hollowing”, i.e., the interior crown consisted of primary branches with little secondary development or foliage. These dynamic features are an effective adaptive strategy in early succession but also may be a disadvantage to maintaining foliage for longer period. Crown maintenance associated with the longevity of structural components is thought to play an important role in survival strategy of tree species.     

Changes in Siberian Fir Crown Architecture under Impaired Homeostasis

The crown of the Siberian fir (Abies sibirica) is characterized by a specific organization, namely, a narrow-pyramidal form, apical dominance, tiering by types of sexualization, and whorled branching. The gravitational vector is clearly manifested at all levels of the organization of the crown, that is, in the architecture (diageotropism of branches), morphostructure (dorsoventrality of tissues and organs of shoots, as well as the location of reproductive organs in the tree crown and on the shoot), and physiological processes. It was shown that the internal state of the tree is a determining factor in graviperception. When Siberian fir trees weaken, the diageotropism of branches is lost, which can serve as a universal biomarker of impairment of homeostasis.

     

Growth and architecture of small honey mesquites under jackrabbit browsing: overcoming the disadvantage of being eaten

Browsing is an important mortality factor in seedlings and small plants. However, the induced changes in the architecture of plant survivors may influence subsequent browsing, opening the possibility of compensating for the damage done. How jackrabbit (Lepus californicus) browsing affects the growth and architecture of small individuals of honey mesquite, Prosopis glandulosa var. torreyana, a tree/shrub that produces spines at every node, was explored. Naturally established mesquites of unknown age were selected in one site, and 2-year-old mesquites were transplanted in another site. In both cases, half of them were exposed to jackrabbits and the other half were excluded as controls. After 4 years, shoot production (height, length and number of derived shoots) and plant growth (height and cover) increased 1.4-2.5-fold in naturally established controls relative to exposed plants, depending on the measured variable. In the transplant experiment, the increases were 2.8-7.1-fold in controls relative to exposed plants 2 years after initiation of the experiment. The net loss of biomass in treatment vs. control plants in these experiments suggests a negative response to browsing which has been defined as under-compensation. Alternative architectures in honey mesquites were evident at the end of the exclusion experiments: controls had long branches and an extended crown cover, while exposed plants had short branches and a compact crown cover. Results indicated that mesquites were able to grow under browser pressure by packing many stems in a compact matrix armed with spines and producing one or more shoots tall and wide enough to escape from jackrabbits.     

Sources of variability in plant resistance against insects: free caterpillars show strongest effects

Tests with herbivorous animals have shown great variability in foliage quality within individual plants. This may be a product of plant architecture and microclimate as well as a specific response to herbivory. We tested the level of constitutive and herbivore-induced resistance within the canopy of mountain birch ( Betula pubescens ssp. czerepanovii ) stems of different age, using shoots with different architectural position, function and age. We tested foliage both with uncaged larvae of Epirrita autumnata feeding freely on foliage and with growth trials in the laboratory. The only indication of systematic within-tree variability was low consumption by third instar larvae growing on long shoot leaves. This may protect long shoots that would suffer from reduced growth if their first leaves were consumed at an early stage of development. Stem age and architecture did not contribute to within-tree variation and the effects induced by herbivory were similar throughout the tree. A small number of caterpillars was sufficient to elicit responses in larval growth or consumption. This suggests that the presence of caterpillars may act as a cue of increased risk of future herbivory for mountain birch attacked by E. autumnata with poor mobility and regular outbreaks. Previous defoliation and young age of stems increased the disappearance rate of free larvae. The effect of defoliation suggests that induced resistance may act to prevent future buildup of E. autumnata populations. The small size and simple canopy architecture of young stems may in turn increase the disappearance rate by increasing the probability of predators finding prey. This may partly explain the low vulnerability of young mountain birch forests to E. autumnata outbreaks. Overall, changes in the disappearance rate were greater than changes in larval growth and consumption. This suggests that tests in the laboratory or inside cages may underestimate the efficacy of induced resistance against herbivores.     

Crown architecture of grafted Stone pine (Pinus pinea L.): shoot growth and bud differentiation

The singular umbrella-like crown shape of Stone pine can be interpreted as a consequence of primary shoot-growth patterns and posterior axis differentiation due to differential secondary growth and down-bending of branches. This paper centres on the first aspect, analysing the growth, branching and flowering behaviour of about 5,000 individual shoots on 27 grafted Stone pines. The data measurement on standing trees allowed to study correlations of topologic and geometric variables in the shoot and their ancestors. The only significant correlations were found with parameters of the mother shoot formed the previous year and with the number of cones born 3 years before by the respective ancestor. The fitted relationships between geometric and topologic shoot and branch variables are the first step of a structural model construction that can be completed with functional components like a radiation and a carbon allocation submodel, stressing the importance of the heavy Stone pine cones as carbon sinks, with a total annual allocation similar to stem wood. In conclusion, the Stone pine crown shape emerges as consequence of the lack of initial vigour differentiation between stem and main-branch apical meristems that favour the generalized sylleptic reiteration in the open-grown trees.     

Behavior of current-year shoots as a mechanism to determine the floral sex allocation at the level of individual tree and population in Siberian alder (Alnus hirsuta var. sibirica)

Floral sex allocation (weight of male flower buds over weight of female flower buds) was examined at the levels of current-year shoot, individual tree and population, and the tree individual level and population level floral sex ratio was explained as a consequence of the behavior of current-year shoots in the shoot-level monoecious (flowering current-year shoots have both male and female flowers) species, Siberian alder (Alnus hirsuta var. sibirica). The current-year shoot level floral sex allocation was not size-dependent and not different over years. However, in the year when the reproductive intensity was high, individual tree level floral sex allocation was size-dependent and the population level floral sex allocation was relatively female-biased. The female-biased floral sex allocation at the population level resulted from many gynoecious shoots (current-year shoots which have only female flowers). These results suggest that the floral sex allocation of Siberian alder was controlled not by changing the floral sex allocation of each current-year shoot, but by shifting the sex expression of current-year shoots from shoot-level monoecy to shoot-level gynomonoecy.     

Within-tree and among-tree variation in growth of Epirrita autumnata on mountain birch leaves

Abstract.

• 1 We studied among-tree and within-tree variation in the growth of larvae of a geometrid, Epirrita autumnata, on mountain birch (Betula pubescens ssp. tortuosa) leaves at different hierarchical levels: among trees (genets), among ramets within trees, among branches within ramets, among shoots within branches and among leaves within shoots. We used only short shoot leaves, which burst simultaneously in spring.
• 2 Trees accounted for most of the variation in larval growth rate, but there was substantial variation also among ramets within trees, among branches within ramets, and among short shoots within branches. Variation among leaves within short shoots was negligible. When the probabilities from different experiments were combined, the differences were statistically significant at the tree, ramet and branch levels, and approached significance at the shoot level.
• 3 In different experiments, larval growth was from 9% to 54% lower on the worst tree than on the best tree of the experiment. On average, larval growth rate was in different experiments from 11% to 32% lower on the worst ramet than on the best ramet within tree, from 8% to 18% lower on the worst than on the best branch within ramet, and from 12% to 30% lower on the worst than on the best shoot within branch.
• 4 The amount of among-and within-tree variation shown by our results may have ecological and evolutionary implications: among-tree variation should select for discrimination by ovipositing females and dispersing larvae, within-tree variation should select also for optimal foraging behaviour of larvae.

     

Crown development in a pioneer tree, Rhus trichocarpa, in relation to the structure and growth of individual branches

Based on an allometric reconstruction, the structure and biomass-allocation patterns of branches and current-year shoots were investigated in branches of various heights in the pioneer tree Rhus trichocarpa, to evaluate how crown development is achieved and limited in association with height. Path analysis was conducted to explore the effects of light availability, basal height and size of individual branches on branch structure and growth. Branch angle was affected by basal height, whereas branch mass was influenced primarily by light availability. This result suggests that branch structure is strongly constrained by basal height, and that trees mediate such constraints under different light environments. Previous-year leaf area and light availability showed positive effects on current-year stem mass. In contrast, branch basal height and mass negatively affected current-year stem mass. Moreover, the length of stems of a given diameter decreased with increasing branch height. Therefore the cost of biomass investment for a unit growth in length is greater for branches of larger size and at upper positions. Vertical growth rate in length decreased with increasing height. Height-dependent changes in stem allometry and angle influenced the reduction in vertical growth rate to a similar degree.     

Survival of three cool-temperate tree species in shade with lime and NPK applications

Knowledge of the site specificity of plastic reactions and shade tolerance of tree species within the complexity of natural forests is limited but critical to understanding forest regeneration. Paper birch (Betula papyrifera), yellow birch (B. alleghaniensis), and eastern hemlock (Tsuga canadensis) were planted in a randomized block design under natural deciduous canopy and within small canopy gaps with a single application of lime and NPK at time of planting in 2003. Saplings were excavated in 2004 and 2008 for analysis of biomass allocation and testing of foliar N. In 2008, mean survival rate differed significantly among species at 0.14 for paper birch, 0.57 for yellow birch, and 0.76 for eastern hemlock, respectively. Yellow birch emerged as a species of intermediate shade tolerance associated with a sharp decline in shoot development after two growing seasons, high leaf allocation of biomass, and foliar plasticity. Foliation and physiological adaptation of leaves appeared N-limited in yellow birch. Biomass allocation analysis revealed strong allometric growth in yellow birch and eastern hemlock across experimental treatments imposed. Increased soil reaction from pH 4.7 to 6.3 following lime applications had no discernable effect on the juvenile growth and survival of the three species during six growing seasons. The experiment underlined the need for experimental testing of species under natural forest canopy. Cyclical cutting of forests with moderate canopy opening is recommended for sustainable management of yellow birch and eastern hemlock.