Negative allometry of leaf xylem conduit diameter and double-wall thickness: implications for implosion safety.

Xylem conduits have lignified walls to resist crushing pressures. The thicker the double-wall (T) relative to its diameter (D), the greater the implosion safety. Having safer conduits may incur higher costs and reduced flow, while having less resistant xylem may lead to catastrophic collapse under drought. Although recent studies have shown that conduit implosion commonly occurs in leaves, little is known about how leaf xylem scales T vs D to trade off safety, flow efficiency, mechanical support, and cost. We measured T and D in > 7000 conduits of 122 species to investigate how T vs D scaling varies across clades, habitats, growth forms, leaf, and vein sizes. As conduits become wider, their double-cell walls become proportionally thinner, resulting in a negative allometry between T and D. That is, narrower conduits, which are usually subjected to more negative pressures, are proportionally safer than wider ones. Higher implosion safety (i.e. higher T/D ratios) was found in asterids, arid habitats, shrubs, small leaves, and minor veins. Despite the strong allometry, implosion safety does not clearly trade off with other measured leaf functions, suggesting that implosion safety at whole-leaf level cannot be easily predicted solely by individual conduits' anatomy.

Effects of atmospheric dust particles on common medicinal plants in an industrial area of West Bengal, India.

The exposure of atmospheric dust particles on four common medicinal plants (Ocimum sanctum, Andrographis paniculata, Catharanthus roseous, and Kaempferia galanga, which are available in the study area and cultivated by the local people for medicinal purposes) affects their growth, levels of essential biochemical constituents and heavy metal concentration. The plant species were grown by pot cultivation in an industrial area with high levels of coal dust to assess the capacity of heavy metals accumulation in their leaves and changes in allometry and biochemical parameters. The results showed that annual average SPM and dustfall varied between 195.88 to 645.97 µg/m3 and 17.55 to 41.16 g/m2/month, respectively. Dustfall at different polluted sites was 2.4, 2.1, 1.5, 1.4, and 2.3 times higher than at the control site. The most prevalent heavy metal in atmospheric particulate matter was Zn, followed by Pb, Ni, Cu, Co, and Cd. Plant allometry measurements such as height, stem width, root length, petiole length, and leaf area are shown to have a strong and significant (p<0.05) negative correlation with dustfall and SPM. Total chlorophyll and RWC were inversely proportional to the dust load present in all the species. Except for Andrographis paniculata, chlorophyll and leaf-extracted pH of plant species were moderately correlated with APTI, whereas no correlation was noticed for ascorbic acid. A positive correlation between SPM and heavy metals in leaves was observed. The results implied that the cultivation and collection of medicinal plants from the study area could be potentially toxic to human health.

Pith width, leaf size, and twig thickness.

PREMISE: To support large leaves, many woody plant species evolved a cost-effective way to thicken twigs. As an extension of E. J. H. Corner's rule that twig diameter increases with leaf size, we hypothesized that pith width also increases with leaf size. The benefit to the plant from the proposed relationship is that pith is a low-cost tissue that reduces the metabolic cost of large diameter twig production. METHODS: Leaf sizes and cross-sectional areas of bark, xylem, and pith of 81 species of trees and shrubs growing in Gainesville, Florida were measured and compared with standardized major axis regressions of pairwise species trait values and phylogenetically independent contrasts. RESULTS: Pith area increases with leaf size with or without accounting for phylogenetic relationships. In agreement with Corner's rule, overall twig diameter as well as bark and wood thickness also increase with leaf size. Thicker twigs showed more variation in relative pith, wood, and bark cross-sectional areas compared to thinner twigs. CONCLUSIONS: Investments in pith, a tissue of low density found in the centers of twigs, provides a low-cost way to increase twig circumference and thereby space for attachment of large leaves while increasing the overall second moment of area of twigs, which increases their ability to biomechanically support large leaves.

Prediction of photosynthetic light-response curves using traits of the leaf economics spectrum for 75 woody species: effects of leaf habit and sun-shade dichotomy.

PREMISE: Photosynthetic light-response (PLR) curves for leaves are important components of models related to carbon fixation in forest ecosystems, linking the Mitscherlich equation and Michaelis-Menten equation to traits of the leaf economics spectrum (LES). However, models do not consider changes in leaf habits (i.e., evergreen and deciduous) and within-canopy shading variation in these PLR curves. METHODS: Here, we measured the PLR curves in sun and shade leaves of 44 evergreen and 31 deciduous species to examine the relationships between variables of the Mitscherlich equation and Michaelis-Menten equation, leaf nitrogen (N) and phosphorus (P) content, and leaf mass per area (LMA). RESULTS: Small changes were caused by different leaf habits and shade variations in relationships linking variables of the two equations to leaf N and P content and LMA. Values of the scaling exponents for PLR curve parameters did not differ regardless of canopy position and leaf habit (P > 0.05). The PLR curves in species with different leaf habits (i.e., evergreen and deciduous) at different canopy positions could be predicted using the general allometric relations between leaf traits and PLR parameters in the two equations. For photosynthetic photon flux densities from 0 to 2000 µmol m-2 s-1 , approximately 71% (Mitscherlich equation) and 70% (Michaelis-Menten equation) of the net assimilation rates could be predicted. CONCLUSIONS: These findings indicate that leaf net assimilation rates can be predicted through the large available data for LES traits. Incorporation of values for these traits available in the LES databases into ecosystem models of forest productivity and carbon fixation warrants further investigation.

How plant allometry influences bud phenology and fruit yield in two Vaccinium species.

BACKGROUND AND AIMS: Understanding how plant allometry, plant architecture and phenology contribute to fruit production can identify those plant traits that maximize fruit yield. In this study, we compared these variables and fruit yield for two shrub species, Vaccinium angustifolium and Vaccinium myrtilloides, to test the hypothesis that phenology is linked to the plants' allometric traits, which are predictors of fruit production. METHODS: We measured leaf and flower phenology and the above-ground biomass of both Vaccinium species in a commercial wild lowbush blueberry field (Quebec, Canada) over a 2-year crop cycle; 1 year of pruning followed by 1 year of harvest. Leaf and flower phenology were measured, and the allometric traits of shoots and buds were monitored over the crop cycle. We hand-collected the fruits of each plant to determine fruit attributes and biomass. KEY RESULTS: During the harvesting year, the leafing and flowering of V. angustifolium occurred earlier than that of V. myrtilloides. This difference was related to the allometric characteristics of the buds due to differences in carbon partitioning by the plants during the pruning year. Through structural equation modelling, we identified that the earlier leafing in V. angustifolium was related to a lower leaf bud number, while earlier flowering was linked to a lower number of flowers per bud. Despite differences in reproductive allometric traits, vegetative biomass still determined reproductive biomass in a log-log scale model. CONCLUSIONS: Growing buds are competing sinks for non-structural carbohydrates. Their differences in both number and characteristics (e.g. number of flowers per bud) influence levels of fruit production and explain some of the phenological differences observed between the two Vaccinium species. For similar above-ground biomass, both Vaccinium species had similar reproductive outputs in terms of fruit biomass, despite differences in reproductive traits such as fruit size and number.

Variance in biomass-allocation fractions is explained by distribution in European trees.

Intraspecific variability in ecological traits confers the ability of a species to adapt to an ever-changing environment. Fractions of biomass allocation in plants (BAFs) represent both ecological traits and direct expressions of investment strategies and so have important implications on plant fitness, particularly under current global change. We combined data on BAFs of trees in > 10 000 forest plots with their distributions in Europe. We aimed to test whether plant species with wider distributions have more or less variable intraspecific variance of the BAFs foliage-woody biomass and shoot-root ratios than species with limited distribution. Irrespective of corrections for tree age and phylogenetic relatedness, the standard deviation in BAFs was up to three times higher in species with the most extensive distributions than in those with the least extensive distribution due to a higher genetic diversity. Variance in BAFs also increased with latitude. We show that a combination of 36% tree genetic diversity and 64% environmental variability explains variance in BAFs and implies that changes in genetic diversity occur quickly. Genetic diversity should thus play a key role in regulating species responses to future climate change. Loss of habitat, even if transient, could induce a loss of genetic diversity and hinder species survival.

Leaf mass per area, not total leaf area, drives differences in above-ground biomass distribution among woody plant functional types.

Here, we aim to understand differences in biomass distribution between major woody plant functional types (PFTs) (deciduous vs evergreen and gymnosperm vs angiosperm) in terms of underlying traits, in particular the leaf mass per area (LMA) and leaf area per unit stem basal area. We used a large compilation of plant biomass and size observations, including observations of 21 084 individuals on 656 species. We used a combination of semiparametric methods and variance partitioning to test the influence of PFT, plant height, LMA, total leaf area, stem basal area and climate on above-ground biomass distribution. The ratio of leaf mass to above-ground woody mass (MF /MS ) varied strongly among PFTs. We found that MF /MS at a given plant height was proportional to LMA across PFTs. As a result, the PFTs did not differ in the amount of leaf area supported per unit above-ground biomass or per unit stem basal area. Climate consistently explained very little additional variation in biomass distribution at a given plant size. Combined, these results demonstrate consistent patterns in above-ground biomass distribution and leaf area relationships among major woody PFTs, which can be used to further constrain global vegetation models.

Neighbourhood structure and light availability influence the variations in plant design of shrubs in two cloud forests of different successional status.

BACKGROUND AND AIMS: Plant design refers to the construction of the plant body or its constituent parts in terms of form and function. Although neighbourhood structure is recognized as a factor that limits plant survival and species coexistence, its relative importance in plant design is not well understood. We conducted field research to analyse how the surrounding environment of neighbourhood structure and related effects on light availability are associated with changes in plant design in two understorey plants (Palicourea padifolia and Psychotria elata) within two successional stages of a cloud forest in Costa Rica. METHODS: Features of plant neighbourhood physical structure and light availability, estimated using hemispherical photographs, were used as variables that reflect the surrounding environment. Measures of plant biomechanics, allometry, branching and plant slenderness were used as functional plant attributes that reflect plant design. We propose a framework using a partial least squares path model and used it to test this association. KEY RESULTS: The multidimensional response of plant design of these species suggests that decreases in the height-based factor of safety and increases in mechanical load and developmental stability are influenced by increases in maximum height of neighbours and a distance-dependence interference index more than neighbourhood plant density or neighbour aggregation. Changes in plant branching and slenderness are associated positively with light availability and negatively with canopy cover. CONCLUSIONS: Although it has been proposed that plant design varies according to plant density and light availability, we found that neighbour size and distance-dependence interference are associated with changes in biomechanics, allometry and branching, and they must be considered as key factors that contribute to the adaptation and coexistence of these plants in this highly diverse forest community.

Improved allometric models to estimate the aboveground biomass of tropical trees.

Terrestrial carbon stock mapping is important for the successful implementation of climate change mitigation policies. Its accuracy depends on the availability of reliable allometric models to infer oven-dry aboveground biomass of trees from census data. The degree of uncertainty associated with previously published pantropical aboveground biomass allometries is large. We analyzed a global database of directly harvested trees at 58 sites, spanning a wide range of climatic conditions and vegetation types (4004 trees ≥ 5 cm trunk diameter). When trunk diameter, total tree height, and wood specific gravity were included in the aboveground biomass model as covariates, a single model was found to hold across tropical vegetation types, with no detectable effect of region or environmental factors. The mean percent bias and variance of this model was only slightly higher than that of locally fitted models. Wood specific gravity was an important predictor of aboveground biomass, especially when including a much broader range of vegetation types than previous studies. The generic tree diameter-height relationship depended linearly on a bioclimatic stress variable E, which compounds indices of temperature variability, precipitation variability, and drought intensity. For cases in which total tree height is unavailable for aboveground biomass estimation, a pantropical model incorporating wood density, trunk diameter, and the variable E outperformed previously published models without height. However, to minimize bias, the development of locally derived diameter-height relationships is advised whenever possible. Both new allometric models should contribute to improve the accuracy of biomass assessment protocols in tropical vegetation types, and to advancing our understanding of architectural and evolutionary constraints on woody plant development.

Deviation from symmetrically self-similar branching in trees predicts altered hydraulics, mechanics, light interception and metabolic scaling.

The West, Brown, Enquist (WBE) model derives symmetrically self-similar branching to predict metabolic scaling from hydraulic conductance, K, (a metabolism proxy) and tree mass (or volume, V). The original prediction was Kα V(0.75). We ask whether trees differ from WBE symmetry and if it matters for plant function and scaling. We measure tree branching and model how architecture influences K, V, mechanical stability, light interception and metabolic scaling. We quantified branching architecture by measuring the path fraction, Pf : mean/maximum trunk-to-twig pathlength. WBE symmetry produces the maximum, Pf = 1.0. We explored tree morphospace using a probability-based numerical model constrained only by biomechanical principles. Real tree Pf ranged from 0.930 (nearly symmetric) to 0.357 (very asymmetric). At each modeled tree size, a reduction in Pf led to: increased K; decreased V; increased mechanical stability; and decreased light absorption. When Pf was ontogenetically constant, strong asymmetry only slightly steepened metabolic scaling. The Pf ontogeny of real trees, however, was 'U' shaped, resulting in size-dependent metabolic scaling that exceeded 0.75 in small trees before falling below 0.65. Architectural diversity appears to matter considerably for whole-tree hydraulics, mechanics, photosynthesis and potentially metabolic scaling. Optimal architectures likely exist that maximize carbon gain per structural investment.

Allocation of gross primary production in forest ecosystems: allometric constraints and environmental responses.

Understanding the allocation of gross primary production (GPP) and its response to climate is essential for improving terrestrial carbon (C) modelling. Here, we synthesize data on component GPP fluxes from a worldwide forest database to determine the allocation patterns of GPP across global gradients in climate and nitrogen deposition (Ndep ). Our results reveal that allocation of GPP is governed in an integrated way by allometric constraints and by three trade-offs among GPP components: wood production (NPPwood ) vs fine-root production (NPPfroot ), NPPwood vs foliage production (NPPfoliage ), and autotrophic respiration (Ra ) vs all biomass production components. Component fluxes were explained more by allometry, while partitioning to components was related more closely to the trade-offs. Elevated temperature and Ndep benefit long-term woody biomass C sequestration by stimulating allometric partitioning to wood. Ndep can also enhance forest C use efficiency by its effects on the Ra vs biomass production trade-off. Greater precipitation affects C allocation by driving the NPPwood vs NPPfoliage trade-off toward the latter component. These results advance our understanding about the global constraints on GPP allocation in forest ecosystems and its climatic responses, and are therefore valuable for simulations and projections of ecosystem C sequestration.

Broad-scale comparison of photosynthetic rates across phototrophic organisms.

We tested the existence of general patterns in the photosynthetic metabolism of oxygen-evolving organisms, based on a compilation of data for 315 species ranging from cyanobacteria to tree leaves. We used thickness and chlorophyll a concentration of the photosynthetic structure (cell, thallus, leaf) to scale differences in photosynthetic metabolism among plants, because of the demonstrated importance of these plant traits in regulating light absorption properties and photosynthetic rates of particular plant groups. We examined only the properties of the photosynthetic structure because this is the plant unit responsible for the photosynthetic process and thus is closely related to plant productivity, whereas there is a lack of general quantitative descriptors of the whole organism useful for such broad-scale comparisons, and few studies report net photosynthetic rates of whole organisms, including respiration rates of all non-photosynthetic structures. The results demonstrated that descriptors of plant metabolism such as maximum net photosynthesis, initial slope of the photosynthesis-irradiance (PI) curve and dark respiration display strong positive interrelationships. The metabolic rates declined with increasing thickness of the photosynthetic structures and more steeply for photosynthesis than respiration. Photosynthetic rates also changed with increment of volume of the photosynthetic structure resembling patterns that have been previously described for animal metabolism related to body weight. The strong relationship of metabolic rate and chlorophyll a concentration to the thickness of photosynthetic tissue reflects broad-scale patterns and not the adaptive response of individual or closely-related species of similar tissue thickness to varying environmental conditions. Thickness of the photosynthetic structures, therefore, plays an important role in the environmental control of plant performance and, consequently, it might have been an important driver of plant evolution, setting thresholds to the metabolism and productivity of phototrophic organisms.

The allometry of the weight of fruit on trees and shrubs in Barbados.

Branch sampling of branch diameter and fruit crop on 22 species of Barbadian trees and shrubs provided sufficient data to build regressions between plant size and fruit crop weight. Orchard plants bear much more fruit than wild, feral or garden plants of similar size, but this difference disappears in multiple regression of fruit crop weight (F in g, fresh mass) on branch or stem diameter (D in cm) and individual fruit weight (W in g): F=22D1.2 W0.57. This explains 89% of the variation in F and successfully predicts crop weight for wild tropical and temperate trees and shrubs, but underestimated the crops on commercial, temperate, fruit trees by an order of magnitude. Comparisons of crop weight for feral, wild, and garden plants (Ff) using a simple regression Ff=47D1.9 show that crop weight is a minor load relative to branch weight for larger branches. Although fruit crops represent a declining proportion of total plant weight as plants become larger, the crops become larger relative to leaf and twig weight and in this sense, reproductive investment increases in larger plants. Finally, our equations, combined with the self-thinning rule, suggest that stands of large species of fruit plants produce more fruit per unit of land area than stands of small ones.