Bringing together leaf trait data spanning 2,548 species and 175 sites we describe, for the first time at global scale, a universal spectrum of leaf economics consisting of key chemical, structural and physiological properties. The spectrum runs from quick to slow return on investments of nutrients and dry mass in leaves, and operates largely independently of growth form, plant functional type or biome. Categories along the spectrum would, in general, describe leaf economic variation at the global scale better than plant functional types, because functional types overlap substantially in their leaf traits. Overall, modulation of leaf traits and trait relationships by climate is surprisingly modest, although some striking and significant patterns can be seen. Reliable quantification of the leaf economics spectrum and its interaction with climate will prove valuable for modelling nutrient fluxes and vegetation boundaries under changing land-use and climate.

/pdf/the-worldwide-leaf-economics-spectrum-1uikdmil76.pdf

The worldwide leaf economics spectrum

The world's forests influence climate through physical, chemical, and biological processes that affect planetary energetics, the hydrologic cycle, and atmospheric composition. These complex and nonlinear forest-atmosphere interactions can dampen or amplify anthropogenic climate change. Tropical, temperate, and boreal reforestation and afforestation attenuate global warming through carbon sequestration. Biogeophysical feedbacks can enhance or diminish this negative climate forcing. Tropical forests mitigate warming through evaporative cooling, but the low albedo of boreal forests is a positive climate forcing. The evaporative effect of temperate forests is unclear. The net climate forcing from these and other processes is not known. Forests are under tremendous pressure from global change. Interdisciplinary science that integrates knowledge of the many interacting climate services of forests with the impacts of global change is necessary to identify and understand as yet unexplored feedbacks in the Earth system and the potential of forests to mitigate climate change.

/pdf/forests-and-climate-change-forcings-feedbacks-and-the-30ybh4qhzu.pdf

Forests and Climate Change: Forcings, Feedbacks, and the Climate Benefits of Forests

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.

/pdf/a-handbook-of-protocols-for-standardised-and-easy-2uo4112zhh.pdf

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

In the last decade, our understanding of the processes underlying plant response to drought, at the molecular and whole-plant levels, has rapidly progressed. Here, we review that progress. We draw attention to the perception and signalling processes (chemical and hydraulic) of water deficits. Knowledge of these processes is essential for a holistic understanding of plant resistance to stress, which is needed to improve crop management and breeding techniques. Hundreds of genes that are induced under drought have been identified. A range of tools, from gene expression patterns to the use of transgenic plants, is being used to study the specific function of these genes and their role in plant acclimation or adaptation to water deficit. However, because plant responses to stress are complex, the functions of many of the genes are still unknown. Many of the traits that explain plant adaptation to drought - such as phenology, root size and depth, hydraulic conductivity and the storage of reserves - are those associated with plant development and structure, and are constitutive rather than stress induced. But a large part of plant resistance to drought is the ability to get rid of excess radiation, a concomitant stress under natural conditions. The nature of the mechanisms responsible for leaf photoprotection, especially those related to thermal dissipation, and oxidative stress are being actively researched. The new tools that operate at molecular, plant and ecosystem levels are revolutionising our understanding of plant response to drought, and our ability to monitor it. Techniques such as genome-wide tools, proteomics, stable isotopes and thermal or fluorescence imaging may allow the genotype-phenotype gap to be bridged, which is essential for faster progress in stress biology research.

Understanding plant responses to drought — from genes to the whole plant

FLUXNET is a global network of micrometeorological flux measurement sites that measure the exchanges of carbon dioxide, water vapor, and energy between the biosphere and atmosphere. At present over 140 sites are operating on a long-term and continuous basis. Vegetation under study includes temperate conifer and broadleaved (deciduous and evergreen) forests, tropical and boreal forests, crops, grasslands, chaparral, wetlands, and tundra. Sites exist on five continents and their latitudinal distribution ranges from 70°N to 30°S. FLUXNET has several primary functions. First, it provides infrastructure for compiling, archiving, and distributing carbon, water, and energy flux measurement, and meteorological, plant, and soil data to the science community. (Data and site information are available online at the FLUXNET Web site, http://www-eosdis.ornl.gov/FLUXNET/.) Second, the project supports calibration and flux intercomparison activities. This activity ensures that data from the regional networks are intercomparable. And third, FLUXNET supports the synthesis, discussion, and communication of ideas and data by supporting project scientists, workshops, and visiting scientists. The overarching goal is to provide information for validating computations of net primary productivity, evaporation, and energy absorption that are being generated by sensors mounted on the NASA Terra satellite. Data being compiled by FLUXNET are being used to quantify and compare magnitudes and dynamics of annual ecosystem carbon and water balances, to quantify the response of stand-scale carbon dioxide and water vapor flux densities to controlling biotic and abiotic factors, and to validate a hierarchy of soil–plant–atmosphere trace gas exchange models. Findings so far include 1) net CO 2 exchange of temperate broadleaved forests increases by about 5.7 g C m −2 day −1 for each additional day that the growing season is extended; 2) the sensitivity of net ecosystem CO 2 exchange to sunlight doubles if the sky is cloudy rather than clear; 3) the spectrum of CO 2 flux density exhibits peaks at timescales of days, weeks, and years, and a spectral gap exists at the month timescale; 4) the optimal temperature of net CO 2 exchange varies with mean summer temperature; and 5) stand age affects carbon dioxide and water vapor flux densities.

/pdf/fluxnet-a-new-tool-to-study-the-temporal-and-spatial-2izxj9ikwz.pdf

FLUXNET: A New Tool to Study the Temporal and Spatial Variability of Ecosystem-Scale Carbon Dioxide, Water Vapor, and Energy Flux Densities

This review provides a theoretical framework and global maps for relations between nitrogen-(N)-nutrition and stomatal conductance, gs' at the leaf scale and flUXe!1 of water vapor and carbon dioxide at the canopy scale. This theory defines the boundaries for observed rates of maximum surface conductance, Gsmax, and its relation to leaf area index, A, within a range of observed max­ imum stomatal conductances. gsmax. Soil evaporation compensates for the reduced contribution of plants to total ecosystem water loss at A < 4. Thus, Gsmax is fairly independent of changes in A for a broad range of vegetation types. The variation of Gsmax within these boundaries can be explained by effects of plant nutrition on stomatal conductance via effects on assimilation. Relations are established for the main global vegetation types among (i) maximum stomatal conductance and leaf nitrogen concentrations with a slope of 0.3 mm s-I per mg N g-I, (ii) maximum surface conductance and stomatal conductance with a slope of 3 mm s-I in G per mm S-I in g, and (iii) maximum surface CO2 uptake and surface conductance with a slope of 1 /lmol m-2 s-1 in A per mm S-1 in G. Based on the distribution of leaf nitrogen in different vegetation types, predictions are made for maximum surface conductance and assimilation of carbon dioxide at a global scale. The review provides a basis for modeling and predicting feedforward and feedback effects between terres­ trial vegetation and global climate.

https://www.jstor.org/stable/pdfplus/2097327.pdf

Relationships among Maximum Stomatal Conductance, Ecosystem Surface Conductance, Carbon Assimilation Rate, and Plant Nitrogen Nutrition: A Global Ecology Scaling Exercise

A model is presented which solves simultaneously for leafâscale stomatal conductance, CO2 assimilation and the energy balance as a function of leaf position within canopies of wellâwatered vegetation. Fluxes and conductances were calculated separately for sunlit and shaded leaves. A linear dependence of photosynthetic capacity on leaf nitrogen content was assumed, while leaf nitrogen content and light intensity were assumed to decrease exponentially within canopies. Separate extinction coefficients were used for diffuse and direct beam radiation. An efficient Gaussian integration technique was used to compute fluxes and mean conductances for the canopy. The multilayer model synthesizes current knowledge of radiation penetration, leaf physiology and the physics of evaporation and provides insights into the response of whole canopies to multiple, interacting factors. The model was also used to explore sources of variation in the slopes of two simple parametric models (nitrogenâ and lightâuse efficiency), and to set bounds on the magnitudes of the parameters. For canopies low in total N, daily assimilation rates are â¼10% lower when leaf N is distributed uniformly than when the same total N is distributed according to the exponentially decreasing profile of absorbed radiation. However, gains are negligible for plants with high N concentrations. Canopy conductance, Gc should be calculated as Gc=AI(fslgsl+fshgsh), where I is leaf area index, fsi and fsh are the fractions of sunlit and shaded leaves at each level, and gsi and gsh are the corresponding stomatal conductances. Simple addition of conductances without this weighting causes errors in transpiration calculated using the âbigâleafâ version of the PenmanâMonteith equation. Partitioning of available energy between sensible and latent heat is very responsive to the parameter describing the sensitivity of stomata to the atmospheric humidity deficit. This parameter also affects canopy conductance, but has a relatively small impact on canopy assimilation. Simple parametric models are useful for extrapolating understanding from small to large scales, but the complexity of real ecosystems is thus subsumed in unexplained variations in parameter values. Simulations with the multilayer model show that both nitrogenâ and radiationâuse efficiencies depend on plant nutritional status and the diffuse component of incident radiation, causing a 2â to 3âfold variation in these efficiencies.

Leaf nitrogen, photosynthesis, conductance and transpiration : scaling from leaves to canopies

Maximum conductances for evaporation from global vegetation types

We used the eddy-correlation techn~que to investigate the exchange of COZ between an und~sturbed old-growth forest and the atmosphere at a remote Southern Hemi- sphere slte on 15 d between 1989 and 1990. Our goal was to determine how environmental factors regulate ecosystem CO, exchange. and to test whether present knowledge of leaf- level processes was sufficient to understand ecosystem-level exchange. On clear summer days the maxlmum rate of net ecosystem COY uptake exceeded 15 pmo1.m '.s I, about an order of magnitude greater than the maximum values observed on sunny days in the winter. Mean nighttime respiration rates varied between - -2 and -7 pmol.m-'.s-l. Nighttime CO, efflux rate roughly doubled with a 10°C increase in temperature. Daytime variation in net ecosystem C02 exchange rate was primarily associated with changes in total photosynthetically active photon flux density (PPFD). Air temperature. saturation deficit, and the diffuse PPFD were of lesser. but still significant, influence. These results are in broad agreement with expectations based on the biochemistry of leaf gas exchange and penetration of radiation through a canopy. However. at night. the short-term exchange of CO, between the forest and the atmosphere appeared to be regulated principally by atmospheric transport processes. There was a positive linear relationship between noc- turnal CO, exchange rate and downward sensible heat flux density. This new result has implications for the development of models for diurnal ecosystem CO, exchange. The daytime light-use efficiency of the ecosystem (CO, uptakehncident PPFD) was between 1.6 and 7.1 mmol/mol on clear days in the summer but decreased to 0.8 mmol/ mol after frosts on clear winter days. Ecosystem COZ uptake was enhanced by diffuse PPFD, a result of potentially global significance given recent increases in Northern Hemisphere haze.

https://www.nrs.fs.fed.us/pubs/jrnl/1994/ne_1994_hollinger_001.pdf

Carbon dioxide exchange between an undisturbed old-growth temperate forest and the atmosphere

Canopy-scale evaporation rate (E) and derived surface and aerodynamic conductances for the transfer of water vapour (gs and ga, respectively) are reviewed for coniferous forests and grasslands. Despite the extremes of canopy structure, the two vegetation types have similar maximum hourly evaporation rates (Emax) and maximum surface conductances (gsmax) (medians = 0.46 mm h-1 and 22 mm s-1). However, on a daily basis, median Emax of coniferous forest (4.0 mm d-1) is significantly lower than that of grassland (4.6 mm d-1). Additionally, a representative value of ga for coniferous forest (200 mm s-1) is an order of magnitude more than the corresponding value for grassland (25 mm s-1). The proportional sensitivity of E, calculated by the Penman-Monteith equation, to changes in gs is >0.7 for coniferous forest, but as low as 0.3 for grassland. The proportional sensitivity of E to changes in ga is generally ±0.15 or less.

Francis M. Kelliher

Papers

Relationships among Maximum Stomatal Conductance, Ecosystem Surface Conductance, Carbon Assimilation Rate, and Plant Nitrogen Nutrition: A Global Ecology Scaling Exercise

Leaf nitrogen, photosynthesis, conductance and transpiration : scaling from leaves to canopies

Maximum conductances for evaporation from global vegetation types

Carbon dioxide exchange between an undisturbed old-growth temperate forest and the atmosphere

Evaporation and canopy characteristics of coniferous forests and grasslands.