Toward a general theory of plant carbon economics

Scaling of leaf area with biomass in trees reconsidered: constant metabolically active sapwood volume per unit leaf area with height growth

Hypoallometric (slope<1) scaling between metabolic rate and body mass is often regarded as near-universal across organisms. However, there are compelling reasons to question hypoallometric scaling in woody plants, where metabolic rate is directly proportional to leaf area. This leaf area must provide carbon to the volume of the metabolically active sapwood (VMASW). Within populations of a species, variants in which VMASW increases per unit leaf area with height growth (e.g. ⅔ or ¾ scaling) would have proportionally less carbon for growth and reproduction as they grow taller. Therefore, selection should favor individuals in which, as they grow taller, leaf area scales isometrically with shoot VMASW (slope=1). Using tetrazolium staining, we measured total VMASW and total leaf area (LAtot) across 22 individuals of Ricinus communis and confirmed that leaf area scales isometrically with VMASW, and that VMASW is much smaller than total sapwood volume. With the potential of the LAtot-VMASW relationship to shape factors as diverse as the crown area-stem diameter relationship, conduit diameter scaling, reproductive output, and drought-induced mortality, our work indicates that the notion that sapwood increases per unit leaf area with height growth requires revision.

Stretched sapwood, ultra-widening permeability and ditching da Vinci: revising models of plant form and function

BACKGROUND

The mechanisms leading to dieback and death of trees under drought remain unclear. To gain an understanding of these mechanisms, addressing major empirical gaps regarding tree structure-function relations remains essential.

SCOPE

We give reasons to think that a central factor shaping plant form and function is selection simultaneously favouring constant leaf-specific conductance with height growth and isometric (1:1) scaling between leaf area and the volume of metabolically active sink tissues ('sapwood'). Sapwood volume-leaf area isometry implies that per-leaf area sapwood volumes become transversely narrower with height growth; we call this 'stretching'. Stretching means that selection must favour increases in permeability above and beyond that afforded by tip-to-base conduit widening ("ultra-widening permeability"), via fewer and wider vessels or tracheids with larger pits or larger margo openings. Leaf area-metabolically active sink tissue isometry would mean that it is unlikely that larger trees die during drought because of carbon starvation due to greater sink-source relationships as compared to shorter plants. Instead, an increase in permeability is most plausibly associated with greater risk of embolism, and this seems a more probable explanation of the preferential vulnerability of larger trees to climate change-induced drought. Other implications of selection favouring constant per-leaf area sapwood construction and maintenance costs are departure from the da Vinci rule expectation of similar sapwood areas across branching orders, and that extensive conduit furcation in the stem seems unlikely.

CONCLUSIONS

Because all these considerations impact the likelihood of vulnerability to hydraulic failure versus carbon starvation, both implicated as key suspects in forest mortality, we suggest that these predictions represent essential priorities for empirical testing.

Earlier onset and slower heartwood investment in faster-growing trees of African tropical species

BACKGROUND AND AIMS

Heartwood plays an important role in maintaining the structural integrity of trees. Although its formation has long been thought to be driven solely by internal ageing processes, more recent hypotheses suggest that heartwood formation acts as a regulator of the tree water balance by modulating the quantity of sapwood. Testing both hypotheses would shed light on the potential ecophysiological nature of heartwood formation, a very common process in trees.

METHODS

We measured quantities of heartwood and sapwood, xylem conduits and the width and number of growth rings on 406 stems of Pericopsis elata with ages ranging from 2 to 237 years. A subset of 17 trees with similar ages but varying growth rate were sampled in a shaded (slower-growth) site and a sun-exposed (faster-growth) site. We used regression analysis and structural equation modelling to investigate the dynamics and drivers of heartwood formation.

KEY RESULTS

We found a positive effect of growth rate on the probability of heartwood occurrence, suggesting an earlier heartwood onset in faster-growing stems. After this onset age, heartwood area increased with stem diameter and age. Despite the similar heartwood production per unit stem diameter increment, shaded trees produced heartwood faster than sun-exposed trees. Tree age and hydraulics showed similar direct effects on heartwood and sapwood area of sun-exposed trees, suggesting their mutual role in driving the heartwood dynamics of sun-exposed trees. However, for shaded trees, only tree hydraulics showed a direct effect, suggesting its prominent role over age in driving the heartwood dynamics in limited growing conditions. The positive relationship between growth rate and maximum stomatal conductance supported this conclusion.

CONCLUSIONS

Heartwood area increases as the tree ages, but at a slower rate in trees where water demand is balanced by a sufficient water supply. Our findings suggest that heartwood formation is not only a structural process but also functional.

Temperature and Turgor "Limitation" and Environmental "Control" in Xylem Biology and Dendrochronology

Trees and other woody plants are immensely ecologically important, making it essential to understand the causes of relationships between tree structure and function. To help these efforts, we highlight persistent traditions in plant biology of appealing to environmental factors "limiting" or "controlling" woody plant features. Examples include the idea that inevitable drops in cell turgor with plant height limit cell expansion and thus leaf size and tree height; that low temperatures prohibit lignification of cells and thus the growth of woody plants at high elevation; and notions from dendrochronology and related fields that climate factors such as rainfall and temperature "control" growth ring features. We show that notions of "control," "limitation," and the like imply that selection would favor a given trait value, but that these would-be favored values are developmentally impossible to produce. Such "limitation" scenarios predict trait frequency distributions that are very narrow and are abruptly curtailed at the upper limit of developmental possibility (the right-hand side of the distribution). Such distributions have, to our knowledge, never been observed, so we see little empirical support for "limitation" hypotheses. We suggest that, as a more productive starting point, plant biologists should examine adaptation hypotheses, in which developmental possibility is wide (congruent with the wide ranges of trait variation that really are observed), but only some of the possible variants are favored. We suggest that (1) the traditional the proximate/ultimate causation distinction, (2) purging scenarios of teleology/anthropomorphism, and (3) stating hypotheses in terms of developmental potential and natural selection are three simple ways of making "limitation" hypotheses clearer with regard to biological process and thus empirically testable.

Divergence in functional traits in seven species of neotropical palms of different forest strata

Functional traits are morphological and physiological characteristics that determine growth, reproduction, and survival strategies. The leaf economics spectrum proposes two opposing life history strategies: species with an "acquisitive" strategy grow fast and exploit high-resource environments, while species with a "conservative" strategy emphasize survival and slow growth under low resource conditions. We analyzed intra and interspecific variation in nine functional traits related to biomass allocation and tissue quality in seven Neotropical palm species from understory and canopy strata. We expected that the level of resources of a stratum that a species typically exploits would determine the dominance of either the exploitative or conservative strategy, as well as degree of divergence in functional traits between species. If this is correct, then canopy species will show an acquisitive strategy emphasizing traits targeting a larger size, whereas understory species will show a conservative strategy with traits promoting efficient biomass allocation and survival in the shade. Two principal components (57.22% of the variation) separated palm species into: (a) canopy species whose traits were congruent with the acquisitive strategy and emphasized large size (i.e., diameter, height, carbon content, and leaf area), and (b) understory species whose traits were associated with efficient biomass allocation (i.e., dry mass fraction -DMF- and tissue density). As we unravel the variation in functional traits in palms, which make up a substantial proportion of the tropical flora, we gain a deeper understanding of how plants adapt to environmental gradients.

Plant subindividual functional diversity and carbon cycling

Plant subindividual trait variation is a neglected level of functional diversity that contributes to the variation of phenotypes and ecological communities. Disregarding the role of subindividual functional diversity (SFD) in nature may lead to incorrect understanding of spatial and temporal scales of relationships between trait diversity, ecosystem function, and carbon cycling.

Megaherbivores provide biotic resistance against alien plant dominance

While human-driven biological invasions are rapidly spreading, finding scalable and effective control methods poses an unresolved challenge. Here, we assess whether megaherbivores-herbivores reaching ≥1,000 kg of body mass-offer a nature-based solution to plant invasions. Invasive plants are generally adapted to maximize vegetative growth. Megaherbivores, with broad dietary tolerances, could remove large biomass of established plants, facilitating new plant growth. We used a massive dataset obtained from 26,838 camera stations and 158,979 vegetation plots to assess the relationships between megaherbivores, native plants and alien plants across India (~121,330 km2). We found a positive relationship between megaherbivore abundance and native plant richness and abundance, and a concomitant reduction in alien plant abundance. This relationship was strongest in protected areas with midproductive ecosystem and high megaherbivore density but it was lost in areas where thicket-forming alien plants predominated (>40% cover). By incorporating the role of ecosystem productivity, plants traits and densities of megaherbivores on megaherbivore-vegetation relationships, our study highlights a function of megaherbivores in controlling alien plant proliferation and facilitating diverse native plants in invaded ecosystems. The study shows great potential for megafauna-based trophic rewilding as a nature-based solution to counteract dominance of plant invasions.

Functional trait divergence associated with heteromorphic leaves in a climbing fig

Introduction

Plants that display heteroblasty possess conspicuous variations in leaf morphology between their juvenile and adult phases, with certain species retaining juvenile-like leaves even in adulthood. Nevertheless, the ecological advantages of maintaining two or more distinct leaf types in heteroblastic plants at the adult stage remain unclear.

Method

The aim of this study is to examine the adaptive significance of heteroblastic leaves sampled from branches with divergent functions (sterile and fertile branches) of mature Ficus pumila individuals by comparing their morphological, anatomical, and physiological characteristics.

Result

Leaves on sterile branches (LSs) exhibited a significantly larger specific leaf area, thinner palisade and spongy tissues, lower chlorophyll contents, and lower light saturation points than leaves on fertile branches (LFs). These results demonstrate that LSs are better adapted to low light environments, while LFs are well equipped to take advantages of high light conditions. However, both LFs and LSs have a low light compensation point with no significant difference between them, indicating that they start to accumulate photosynthetic products under similar light conditions. Interestingly, significant higher net photosynthetic rate was detected in LFs, showing they have higher photosynthetic capacity. Furthermore, LFs produced significant more nutrients compared to LSs, which may associate to their ability of accumulating more photosynthetic products under full light conditions and higher photosynthetic capacity.

Discussion

Overall, we observed a pattern of divergence in morphological features of leaves on two functional branches. Anatomical and physiological features indicate that LFs have an advantage in varied light conditions, providing amounts of photosynthetic products to support the sexual reproduction, while LSs adapt to low light environments. Our findings provide evidence that heteroblasty facilitates F. pumila to utilize varying light environments, likely associated with its growth form as a climbing plant. This strategy allows the plant to allocate resources more effectively and optimize its overall fitness.

Mammal traits and soil biogeochemistry: Functional diversity relates to composition of soil organic matter

Mammal diversity affects carbon concentration in Amazonian soils. It is known that some species traits determine carbon accumulation in organisms (e.g., size and longevity), and are also related to feeding strategies, thus linking species traits to the type of organic remains that are incorporated into the soil. Trait diversity in mammal assemblages - that is, its functional diversity - may therefore constitute another mechanism linking biodiversity to soil organic matter (SOM) accumulation. To address this hypothesis, we analyzed across 83 mammal assemblages in the Amazon biome (Guyana), the elemental (by ED-XRF and CNH analysis) and molecular (FTIR-ATR) composition of SOM of topsoils (401 samples) and trait diversity (functional richness, evenness, and divergence) for each mammal assemblage. Lower mammal functional richness but higher functional divergence were related to higher content of carbonyl and aliphatic SOM, potentially affecting SOM recalcitrance. Our results might allow the design of biodiversity management plans that consider the effect of mammal traits on carbon sequestration and accumulation in soils.

Plant water use theory should incorporate hypotheses about extreme environments, population ecology, and community ecology

Plant water use theory has largely been developed within a plant-performance paradigm that conceptualizes water use in terms of value for carbon gain and that sits within a neoclassical economic framework. This theory works very well in many contexts but does not consider other values of water to plants that could impact their fitness. Here, we survey a range of alternative hypotheses for drivers of water use and stomatal regulation. These hypotheses are organized around relevance to extreme environments, population ecology, and community ecology. Most of these hypotheses are not yet empirically tested and some are controversial (e.g. requiring more agency and behavior than is commonly believed possible for plants). Some hypotheses, especially those focused around using water to avoid thermal stress, using water to promote reproduction instead of growth, and using water to hoard it, may be useful to incorporate into theory or to implement in Earth System Models.

Trait diversity shapes the carbon cycle

Trait evolution is shaped by carbon economics at the organismal level. Here, we expand this idea to the ecosystem level and show how the trait diversity of ecological communities influences the carbon cycle. Systematic shifts in trait diversity will likely trigger changes in the carbon cycle.

Mammal and tree diversity accumulate different types of soil organic matter in the northern Amazon

Diversity of plants and animals influence soil carbon through their contributions to soil organic matter (SOM). However, we do not know whether mammal and tree communities affect SOM composition in the same manner. This question is relevant because not all forms of carbon are equally resistant to mineralization by microbes and thus, relevant to carbon storage. We analyzed the elemental and molecular composition of 401 soil samples, with relation to the species richness of 83 mammal and tree communities at a landscape scale across 4.8 million hectares in the northern Amazon. We found opposite effects of mammal and tree richness over SOM composition. Mammal diversity is related to SOM rich in nitrogen, sulfur and iron whereas tree diversity is related to SOM rich in aliphatic and carbonyl compounds. These results help us to better understand the role of biodiversity in the carbon cycle and its implications for climate change mitigation.

Carbon stress causes earlier budbreak in shade-tolerant species and delays it in shade-intolerant species

PREMISE

Climate change may lead to C stress (negative C balance) in trees. Because nonstructural carbohydrates (NSC) are required during metabolic reactivation in the spring, C stress might delay budbreak timing. This effect is expected to be greater in shade-intolerant than in shade-tolerant species, owing to the faster C economy in the shade-intolerant.

METHODS

We experimentally induced C stress in saplings of six temperate tree species that differed in their light requirements by exposing them to either full light or shade from summer to spring, then recorded the date of first budbreak for the individuals. Because the levels of C reserves that represent effective C stress may differ among species, we estimated the degree of C stress by recording survival during the experiment and measuring whole-sapling NSC concentrations after budbreak.

RESULTS

Shade reduced NSC concentrations and increased the sugar fraction in the NSC in all species. In the shade, shade-intolerant species had higher mortality and generally lower NSC concentrations than the shade-tolerant species, indicating a trend for more severe C stress in species with faster C economy. In shade-intolerant species, budbreak started earlier and proceeded faster in full light than in shade, but in shade-tolerant species budbreak was delayed in full light. The effects of the light environments on budbreak were not greater in shade-intolerant than in shade-tolerant species.

CONCLUSIONS

Our study reveals a correspondence between budbreak responses to light and the light requirements of the species. This finding confirms that C metabolism has a significant role in triggering budbreak and demonstrates that whether C stress accelerates or delays budbreak depends on the species' light requirements.

Above- and below-ground plant traits are not consistent in response to drought and competition treatments

BACKGROUND AND AIMS

Our understanding of plant responses to biotic and abiotic drivers is largely based on above-ground plant traits, with little focus on below-ground traits despite their key role in water and nutrient uptake. Here, we aimed to understand the extent to which above- and below-ground traits are co-ordinated, and how these traits respond to soil moisture gradients and plant intraspecific competition.

METHODS

We chose seedlings of five tropical tree species and grew them in a greenhouse for 16 weeks under a soil moisture gradient [low (drought), medium and high (well-watered) moisture levels] with and without intraspecific competition. At harvest, we measured nine above- and five below-ground traits of all seedlings based on standard protocols.

KEY RESULTS

In response to the soil moisture gradient, above-ground traits are found to be consistent with the leaf economics spectrum, whereas below-ground traits are inconsistent with the root economics spectrum. We found high specific leaf area and total leaf area in well-watered conditions, while high leaf dry matter content, leaf thickness and stem dry matter content were observed in drought conditions. However, below-ground traits showed contrasting patterns, with high specific root length but low root branching index in the low water treatment. The correlations between above- and below-ground traits across the soil moisture gradient were variable, i.e. specific leaf area was positively correlated with specific root length, while it was negatively correlated with root average diameter across moisture levels. However, leaf dry matter content was unexpectedly positively correlated with both specific root length and root branching index. Intraspecific competition has influenced both above- and below-ground traits, but interacted with soil moisture to affect only below-ground traits. Consistent with functional equilibrium theory, more biomass was allocated to roots under drought conditions, and to leaves under sufficient soil moisture conditions.

CONCLUSIONS

Our results indicate that the response of below-ground traits to plant intraspecific competition and soil moisture conditions may not be inferred using above-ground traits, suggesting that multiple resource use axes are needed to understand plant ecological strategies. Lack of consistent leaf-root trait correlations across the soil moisture gradient highlight the multidimensionality of plant trait relationships which needs more exploration.