SAPFLUXNET: towards a global database of sap flow measurements.

Plant transpiration is the main evaporative flux from terrestrial ecosystems; it controls land surface energy balance, determines catchment hydrological responses and influences regional and global climate. Transpiration regulation by plants is a key (and still not completely understood) process that underlies vegetation drought responses and land evaporative fluxes under global change scenarios. Thermometric methods of sap flow measurement have now been widely used to quantify whole-plant and stand transpiration in forests, shrublands and orchards around the world. A large body of research has applied sap flow methods to analyse seasonal and diurnal patterns of transpiration and to quantify their responses to hydroclimatic variability, but syntheses of sap flow data at regional to global scales are extremely rare. Here we present the SAPFLUXNET initiative, aimed at building the first global database of plant-level sap flow measurements. A preliminary metadata survey launched in December 2015 showed an encouraging response by the sap flow community, with sap flow data sets from field studies representing >160 species and >120 globally distributed sites. The main goal of SAPFLUXNET is to analyse the ecological factors driving plant- and stand-level transpiration. SAPFLUXNET will open promising research avenues at an unprecedented global scope, namely: (i) exploring the spatio-temporal variability of plant transpiration and its relationship with plant and stand attributes, (ii) summarizing physiological regulation of transpiration by means of few water-use traits, usable for land surface models, (iii) improving our understanding of the coordination between gas exchange and plant-level traits (e.g., hydraulics) and (iv) analysing the ecological factors controlling stand transpiration and evapotranspiration partitioning. Finally, SAPFLUXNET can provide a benchmark to test models of physiological controls of transpiration, contributing to improve the accuracy of individual water stress responses, a key element to obtain robust predictions of vegetation responses to climate change.

[1]  F. Meinzer,et al.  A comparison of daily water use estimates derived from constant-heat sap-flow probe values and gravimetric measurements in pot-grown saplings. , 2007, Tree physiology.

[2]  C. Miniat,et al.  Predictive models for radial sap flux variation in coniferous, diffuse-porous and ring-porous temperate trees. , 2016, Tree physiology.

[3]  L. Sack,et al.  The determinants of leaf turgor loss point and prediction of drought tolerance of species and biomes: a global meta-analysis. , 2012, Ecology letters.

[4]  N. Breda,et al.  A generic model of forest canopy conductance dependent on climate, soil water availability and leaf area index , 2000 .

[5]  F. Valladares,et al.  Species-specific water use by forest tree species: from the tree to the stand , 2012 .

[6]  M. G. Ryan,et al.  Evaluating theories of drought-induced vegetation mortality using a multimodel-experiment framework. , 2013, The New phytologist.

[7]  Kathy Steppe,et al.  A comparison of sap flux density using thermal dissipation, heat pulse velocity and heat field deformation methods , 2010 .

[8]  Nathan Phillips,et al.  A comparison of daily representations of canopy conductance based on two conditional time- averaging methods and the dependence of daily conductance on environmental factors , 1998 .

[9]  Dennis D. Baldocchi,et al.  A comparison of methods for determining forest evapotranspiration and its components: sap-flow, soil water budget, eddy covariance and catchment water balance , 2001 .

[10]  D. Marshall.  Measurement of Sap Flow in Conifers by Heat Transport. , 1958, Plant physiology.

[11]  J. Sperry,et al.  What plant hydraulics can tell us about responses to climate-change droughts. , 2015, The New phytologist.

[12]  F. Do,et al.  Influence of natural temperature gradients on measurements of xylem sap flow with thermal dissipation probes. 1. Field observations and possible remedies. , 2002, Tree physiology.

[13]  H. Jones.  Irrigation scheduling: advantages and pitfalls of plant-based methods. , 2004, Journal of experimental botany.

[14]  L. Urban,et al.  Granier's Thermal Dissipation Probre (TDP) method for measuring sap flow in trees : theory and practice , 2004 .

[15]  Daniel M. Johnson,et al.  Xylem hydraulic safety margins in woody plants: coordination of stomatal control of xylem tension with hydraulic capacitance , 2009 .

[16]  Frederick C. Meinzer,et al.  Does water transport scale universally with tree size , 2005 .

[17]  James Regetz,et al.  Advances in global change research require open science by individual researchers , 2012 .

[18]  Ke Zhang,et al.  Ecosystem heterogeneity determines the ecological resilience of the Amazon to climate change , 2015, Proceedings of the National Academy of Sciences.

[19]  R. Ceulemans,et al.  Plasticity in hydraulic architecture of Scots pine across Eurasia , 2007, Oecologia.

[20]  R. B. Jackson,et al.  Hydraulic limits on maximum plant transpiration and the emergence of the safety-efficiency trade-off. , 2013, The New phytologist.

[21]  N. Nadezhdina,et al.  Sap flow measurements with some thermodynamic methods, flow integration within trees and scaling up from sample trees to entire forest stands , 2004, Trees.

[22]  R. Fernández,et al.  Stay wet or else: three ways in which plants can adjust hydraulically to their environment. , 2006, Journal of experimental botany.

[23]  Dario Papale,et al.  Database Maintenance, Data Sharing Policy, Collaboration , 2012 .

[24]  Hervé Cochard,et al.  Whole tree hydraulic conductance and water loss regulation in Quercus during drought: evidence for stomatal control of embolism? , 1996 .

[25]  Joshua M. Uebelherr,et al.  Ecophysiological variation of transpiration of pine forests: synthesis of new and published results. , 2017, Ecological applications : a publication of the Ecological Society of America.

[26]  D. Forrester,et al.  Enhanced water use efficiency in a mixed Eucalyptus globulus and Acacia mearnsii plantation. , 2010 .

[27]  F. Meinzer.  Co-ordination of vapour and liquid phase water transport properties in plants. , 2002, Plant, cell & environment.

[28]  A. Nardini,et al.  Global convergence in the vulnerability of forests to drought , 2012, Nature.

[29]  James S. Clark,et al.  The effects of elevated CO2 and nitrogen fertilization on stomatal conductance estimated from 11 years of scaled sap flux measurements at Duke FACE. , 2013, Tree physiology.

[30]  W. Schlesinger,et al.  Transpiration in the global water cycle , 2014 .

[31]  A. Granier.  Une nouvelle méthode pour la mesure du flux de sève brute dans le tronc des arbres , 1985 .

[32]  Daniel M. Johnson,et al.  Hydraulic safety margins and embolism reversal in stems and leaves: why are conifers and angiosperms so different? , 2012, Plant science : an international journal of experimental plant biology.

[33]  Nathan Phillips,et al.  Survey and synthesis of intra‐ and interspecific variation in stomatal sensitivity to vapour pressure deficit , 1999 .

[34]  M. G. Ryan,et al.  Evaluating different soil and plant hydraulic constraints on tree function using a model and sap flow data from ponderosa pine , 2001 .

[35]  G. Bonan.  Forests and Climate Change: Forcings, Feedbacks, and the Climate Benefits of Forests , 2008, Science.

[36]  S. Vicente‐Serrano,et al.  A Multiscalar Drought Index Sensitive to Global Warming: The Standardized Precipitation Evapotranspiration Index , 2009 .

[37]  Markus Reichstein,et al.  Climate and vegetation controls on the surface water balance: Synthesis of evapotranspiration measured across a global network of flux towers , 2012 .

[38]  G. Katul,et al.  Scaling xylem sap flux and soil water balance and calculating variance: a method for partitioning water flux in forests , 1998 .

[39]  Markus Reichstein,et al.  Linking plant and ecosystem functional biogeography , 2014, Proceedings of the National Academy of Sciences.

[40]  B. Köstner.  Evaporation and transpiration from forests in Central Europe – relevance of patch-level studies for spatial scaling , 2001 .

[41]  Stan D. Wullschleger,et al.  A review of whole-plant water use studies in tree. , 1998, Tree physiology.

[42]  Stephen S. O. Burgess,et al.  The contribution of fog to the water relations of Sequoia sempervirens (D. Don): foliar uptake and prevention of dehydration , 2004 .

[43]  C. Field,et al.  Three methods for monitoring the gas exchange of individual tree canopies: ventilated-chamber, sap-flow and Penman-Monteith measurements on evergreen oaks , 1994 .

[44]  N. McDowell,et al.  Mechanisms of plant survival and mortality during drought: why do some plants survive while others succumb to drought? , 2008, The New phytologist.

[45]  J. Pontailler,et al.  Transpiration of trees and forest stands: short and long‐term monitoring using sapflow methods , 1996 .

[46]  M. G. Ryan,et al.  Evidence that hydraulic conductance limits photosynthesis in old Pinus ponderosa trees. , 1999, Tree physiology.

[47]  J. Silvertown,et al.  Hydrological niches in terrestrial plant communities: a review , 2015 .

[48]  C. H. M. van Bavel,et al.  Measurement of mass flow of water in the stems of herbaceous plants , 1987 .

[49]  M. Adams,et al.  Measurement of sap flow in roots of woody plants: a commentary. , 2000, Tree physiology.

[50]  Bai Yang,et al.  The AmeriFlux Data Activity and Data System: An Evolving Collection of Data Management Techniques, Tools, Products and Services , 2013 .

[51]  T. S. Adams,et al.  Above- and belowground controls on water use by trees of different wood types in an eastern US deciduous forest. , 2013, Tree physiology.

[52]  Diego G. Miralles,et al.  Magnitude and variability of land evaporation and its components at the global scale , 2011 .

[53]  G. Katul,et al.  Interannual Invariability of Forest Evapotranspiration and Its Consequence to Water Flow Downstream , 2010, Ecosystems.

[54]  M. Zeppel.  Convergence of tree water use and hydraulic architecture in water‐limited regions: a review and synthesis , 2013 .

[55]  Functional convergence in water use of trees from different geographical regions: a meta-analysis , 2013, Trees.

[56]  P. Jarvis,et al.  Stomatal conductance, transpiration, and resistance to water uptake in a Pinussylvestris spacing experiment , 1984 .

[57]  K. Steppe,et al.  Sap-flux density measurement methods: working principles and applicability. , 2013, Functional plant biology : FPB.

[58]  Rafael Poyatos,et al.  A new look at water transport regulation in plants. , 2014, The New phytologist.

[59]  Frederick C. Meinzer,et al.  Stem water storage and diurnal patterns of water use in tropical forest canopy trees , 1998 .

[60]  Ram Oren,et al.  Baseliner: An open-source, interactive tool for processing sap flux data from thermal dissipation probes , 2016, SoftwareX.

[61]  A. Mäkelä,et al.  Predicting the decline in daily maximum transpiration rate of two pine stands during drought based on constant minimum leaf water potential and plant hydraulic conductance. , 2008, Tree physiology.