The continued increase in the atmospheric concentration of carbon dioxide due to anthropogenic emissions is predicted to lead to significant changes in climate. About half of the current emissions are being absorbed by the ocean and by land ecosystems, but this absorption is sensitive to climate as well as to atmospheric carbon dioxide concentrations, creating a feedback loop. General circulation models have generally excluded the feedback between climate and the biosphere, using static vegetation distributions and CO2 concentrations from simple carbon-cycle models that do not include climate change. Here we present results from a fully coupled, three-dimensional carbon–climate model, indicating that carbon-cycle feedbacks could significantly accelerate climate change over the twenty-first century. We find that under a 'business as usual' scenario, the terrestrial biosphere acts as an overall carbon sink until about 2050, but turns into a source thereafter. By 2100, the ocean uptake rate of 5 Gt C yr-1 is balanced by the terrestrial carbon source, and atmospheric CO2 concentrations are 250 p.p.m.v. higher in our fully coupled simulation than in uncoupled carbon models, resulting in a global-mean warming of 5.5 K, as compared to 4 K without the carbon-cycle feedback.

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Acceleration of global warming due to carbon-cycle feedbacks in a coupled climate model

There is growing recognition that classifying terrestrial plant species on the basis of their function (into 'functional types') rather than their higher taxonomic identity, is a promising way forward for tackling important ecological questions at the scale of ecosystems, landscapes or biomes. These questions include those on vegetation responses to and vegetation effects on, environmental changes (e.g. changes in climate, atmospheric chemistry, land use or other disturbances). There is also growing consensus about a shortlist of plant traits that should underlie such functional plant classifications, because they have strong predictive power of important ecosystem responses to environmental change and/or they themselves have strong impacts on ecosystem processes. The most favoured traits are those that are also relatively easy and inexpensive to measure for large numbers of plant species. Large international research efforts, promoted by the IGBP–GCTE Programme, are underway to screen predominant plant species in various ecosystems and biomes worldwide for such traits. This paper provides an international methodological protocol aimed at standardising this research effort, based on consensus among a broad group of scientists in this field. It features a practical handbook with step-by-step recipes, with relatively brief information about the ecological context, for 28 functional traits recognised as critical for tackling large-scale ecological questions.

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A handbook of protocols for standardised and easy measurement of plant functional traits worldwide

Summary 1. The leaf economics spectrum (LES) provides a useful framework for examining species strategies as shaped by their evolutionary history. However, that spectrum, as originally described, involved only two key resources (carbon and nutrients) and one of three economically important plant organs. Herein, I evaluate whether the economics spectrum idea can be broadly extended to water – the third key resource –stems, roots and entire plants and to individual, community and ecosystem scales. My overarching hypothesis is that strong selection along trait trade-off axes, in tandem with biophysical constraints, results in convergence for any taxon on a uniformly fast, medium or slow strategy (i.e. rates of resource acquisition and processing) for all organs and all resources. 2. Evidence for economic trait spectra exists for stems and roots as well as leaves, and for traits related to water as well as carbon and nutrients. These apply generally within and across scales (within and across communities, climate zones, biomes and lineages). 3. There are linkages across organs and coupling among resources, resulting in an integrated whole-plant economics spectrum. Species capable of moving water rapidly have low tissue density, short tissue life span and high rates of resource acquisition and flux at organ and individual scales. The reverse is true for species with the slow strategy. Different traits may be important in different conditions, but as being fast in one respect generally requires being fast in others, being fast or slow is a general feature of species. 4. Economic traits influence performance and fitness consistent with trait-based theory about underlying adaptive mechanisms. Traits help explain differences in growth and survival across resource gradients and thus help explain the distribution of species and the assembly of communities across light, water and nutrient gradients. Traits scale up – fast traits are associated with faster rates of ecosystem processes such as decomposition or primary productivity, and slow traits with slow process rates. 5. Synthesis. Traits matter. A single ‘fast–slow’ plant economics spectrum that integrates across leaves, stems and roots is a key feature of the plant universe and helps to explain individual ecological strategies, community assembly processes and the functioning of ecosystems.

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The world‐wide ‘fast–slow’ plant economics spectrum: a traits manifesto

Here, we analysed a wide range of literature data on the leaf dry mass per unit area (LMA). In nature, LMA varies more than 100-fold among species. Part of this variation (c. 35%) can be ascribed to differences between functional groups, with evergreen species having the highest LMA, but most of the variation is within groups or biomes. When grown in the same controlled environment, leaf succulents and woody evergreen, perennial or slow-growing species have inherently high LMA. Within most of the functional groups studied, high-LMA species show higher leaf tissue densities. However, differences between evergreen and deciduous species result from larger volumes per area (thickness). Response curves constructed from experiments under controlled conditions showed that LMA varied strongly with light, temperature and submergence, moderately with CO2 concentration and nutrient and water stress, and marginally under most other conditions. Functional groups differed in the plasticity of LMA to these gradients. The physiological regulation is still unclear, but the consequences of variation in LMA and the suite of traits interconnected with it are strong. This trait complex is an important factor determining the fitness of species in their environment and affects various ecosystem processes.

/pdf/causes-and-consequences-of-variation-in-leaf-mass-per-area-4q0s917bw1.pdf

Causes and consequences of variation in leaf mass per area (LMA): a meta-analysis.

Most tropical woody plants produce new leaves and flowers in bursts rather than continuously, and most tropical forest communities display seasonal variation in the presence of new leaves, flowers, and fruits. This patterning suggests that phenological changes represent adaptations to either biotic or abiotic factors. Biotic factors may select for either a staggering or a clustering of the phenological activity of individual plant species. We review the evidence for several hypotheses. The idea that plant species can reduce predation by synchronizing their phenological activity has the best support. However, because biotic factors are often arbitrary with respect to the timing of these peaks, it is essential also to consider abiotic influences. A review of published studies demonstrates a major role for climate. Peaks in irradiance are accompanied by peaks in flushing and flowering except where water stress makes this impossible. Thus, in seasonally dry forests, many plants concentrate leafing and flowering around the start of the rainy season; they also tend to fruit at the same time, probably to minimize seedling mortality during the subsequent dry season. Phenological variation at the level of the forest community affects primary consumers who respond by dietary switching, seasonal breeding, changes in range use, or migration. During periods of scarcity, certain plant products, keystone resources, act as mainstays of the primary consumer community.

https://www.jstor.org/stable/pdfplus/2097183.pdf

The Phenology of Tropical Forests: Adaptive Significance and Consequences for Primary Consumers*

This study compared the tissue water relations and seasonal changes in leaf water potential components of an evergreen tree,Morisonia americana, and two evergreen shrubs,Capparis verrucosa andC aristiquetae, with two deciduous trees,Humboltiella arborea andLonchocarpus dipteroneurus, and the deciduous vineMansoa verrucifera All these species coexist in a tropical dry forest in Venezuela Leaves of the evergreen species are sclerophyllous, while those of the deciduous species are mesophytic Leaf area to leaf weight ratios of fully mature leaves were about 75 and 170 cm2 g-1 in evergreen and deciduous species, respectively Seasonal fluctuations of leaf water content per unit of dry weight, water potential, and turgor pressure were smaller in evergreen than in deciduous species The analysis of tissue water relations using pressurevolume curves showed that evergreen species could develop a higher leaf turgor and lose turgor at lower leaf water potentials than deciduous species This was related to a lower osmotic potential at full turgor in evergreen (≃-30 MPa)_than in deciduous (≃-20 MPa) species, rather than to the elastic properties of leaf tissue The volumetric modulus of elasticity was 14 MPa in evergreen compared with 7-10 MPa in deciduous species Thus, leaf characteristics are important in determining the drought resistance of evergreen species of this tropical dry forest

Aspects of tissue water relations and seasonal changes of leaf water potential components of evergreen and deciduous species coexisting in tropical dry forests

Evidence from previous studies relates the significance of evergreeness and deciduousness to differences in leaf drought resistance and nutrient-use characteristics. Considering possible differences in leaf construction costs and lifespans in both plant types, I hypothesized that they should have contrasting strategies underlying their nitrogen-use efficiency (NUE) and water-use efficiency (WUE). Thus, leaves of six deciduous and four evergreen species were analysed to compare their construction and maintenance costs, maximum CO2 assimilation capacity (Amax) and potential (instantaneous) NUE and WUE. Leaf construction and maintenance costs were always higher in evergreen than in deciduous species. The largest Amax were recorded in some deciduous species. However, another deciduous species had Amax comparable to evergreen leaves. Therefore, a clear-cut difference between deciduous and evergreen leaves was not detected. However, NUE and WUE related to construction cost differences in the two plant groups. Indeed, NUE and WUE were largest in deciduous plants. Notwithstanding, overall carbon return per unit invested nitrogen may be increased in evergreens because leaves are photosynthetically active for longer periods. In fact, increases in the ratio between investment (construction cost) to potential payback (Amax) increase leaf life-spans (payback interval). Nevertheless, in tropical dry forests deciduous species are dominant and evergreen species are scarce. Carbon diversion to non-photosynthetic tissue (deep roots) and highest leaf cost may offset competitive capability of evergreen species. Key-words: Carbon assimilation, construction costs, deciduous, evergreen, maintenance cost, nitrogen-use efficiency, tropical dry forest, water-use efficiency

Cost-benefit relationships in deciduous and evergreen leaves of tropical dry forest species

Species of the ‘bana’ vegetation in the Amazonas region of equatorial South America have scleromorphic leaves. This leaf type, which characterizes the vegetation of Mediterranean climates, among others, has apparently evolved in this community in response to the oligotrophic soils and widely fluctuating water table. Anatomically, the leaves have several features commonly found in xeromorphic plants, including greater leaf and cuticle thickness, pubescent leaves and sunken stomata, and a high incidence of sclerenchyma. Concentrations of K and P decrease with leaf age, while N remains nearly constant and Ca increases. Concentrations of N and P are lower than in other sclerophyllous species, but the amount of these nutrients recovered before leaf shedding are similar. The correlation between P and N as expressed per unit dry weight is high (r=0.87; p 300 ppm) and Al (>1000 ppm).

General morphology, anatomical structure, and nutrient content of sclerophyllous leaves of the 'bana' vegetation of amazonas.

Drought-deciduous and evergreen species coexist in tropical dry forests. Drought-deciduous species must cope with greater seasonal leaf water-potential fluctuations than evergreen species and this may increase their susceptibility to drought-induced xylem embolism. The relationship between water transport efficiency and leaf life-span were determined for both groups. They differed in seasonal changes of both, wood water content (W c) and wood specific gravity (G). During the dry season, the W c in drought-deciduous species declined and the minimum value was recorded when leaf fall was complete. At this time, the volumetric fraction of gas (V g) increased indicating air entry into xylem vessels. In contrast, W c, G and V g changed only slightly throughout the year for evergreen species. Maximum hydraulic conductivity of drought-deciduous species was 2-6 times that of the evergreen species. but was severely reduced at leaf fall. In the evergreen species, similar water conductivities were measured during wet and dry seasons. The trade-off between xylem water transport capacity and leaf lifespan found in species coexisting in this forest reveals the existence of contrasting but successful adaptations to this environment. Drought-deciduous species maximize production in the short term with higher water transport efficiency which leads to the seasonal occurrence of embolisms. Conversely, the behaviour of evergreen species with reduced maximum efficiency is conservative but safe in relation to xylem embolism.

Trade-off between water transport efficiency and leaf life-span in a tropical dry forest.

The influence of leaf orientation on leaf temperature has been studied in an sclerophyll vegetation of the Amazon basin, which grows on white sandy soils of very low water retention capacity and variable depth of the water table. Leaf size of the species studied is mainly mesophyllous (sensu Raunkiaer). The high degree of leaf inclination in all species is very characteristic; 55% of the leaves present inclination angles (relative to the vertical) smaller than 45 degrees. Water potential is generally high, not being lower than -14 bars. Leaf resistance increases toward noon during the course of sunny days, indicating either water stress at leaf level or the influence of low relative humidity on stomata opening. Leaf temperature under sunny conditions reflects the influence of leaf orientation on the amount of radiation absorbed by the leaf. Temperature differences recorded range from 1.8--5.4 degrees C. The difference depends on leaf angle, leaf color and leaf diffusion resistance during the period of measurement. Analysis of the relationship between leaf angle and leaf temperature, using Gates leaf energy balance, shows that under the conditions prevailing at noon in sunny days, leaf angles smaller than 50 degrees are effective in reducing leaf temperature within a wide range of leaf resistances to water vapor transfer.

M. A. Sobrado

Papers

Aspects of tissue water relations and seasonal changes of leaf water potential components of evergreen and deciduous species coexisting in tropical dry forests

Cost-benefit relationships in deciduous and evergreen leaves of tropical dry forest species

General morphology, anatomical structure, and nutrient content of sclerophyllous leaves of the 'bana' vegetation of amazonas.

Trade-off between water transport efficiency and leaf life-span in a tropical dry forest.

Significance of leaf orientation for leaf temperature in an amazonian sclerophyll vegetation