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Global warming and drying trends, as well as the increase in frequency and intensity of droughts, may have unprecedented impacts on various forest ecosystems. We assessed the role of internal water storage (WS) in drought resistance of mature pine trees in the semi-arid Yatir Forest. Transpiration (T), soil moisture and sap flow (SF) were measured continuously, accompanied by periodical measurements of leaf and branch water potential (Ψleaf) and water content (WC). The data were used to parameterize a tree hydraulics model to examine the impact of WS capacitance on the tree water relations. The results of the continuous measurements showed a 5-h time lag between T and SF in the dry season, which peaked in the early morning and early afternoon, respectively. A good fit between model results and observations was only obtained when the empirically estimated WS capacitance was included in the model. Without WS during the dry season, Ψleaf would drop below a threshold known to cause hydraulic failure and cessation of gas exchange in the studied tree species. Our results indicate that tree WS capacitance is a key drought resistance trait that could enhance tree survival in a drying climate, contributing up to 45% of the total daily transpiration during the dry season.

The leaf economics spectrum1,2 and the global spectrum of plant forms and functions3 revealed fundamental axes of variation in plant traits, which represent different ecological strategies that are shaped by the evolutionary development of plant species2. Ecosystem functions depend on environmental conditions and the traits of species that comprise the ecological communities4. However, the axes of variation of ecosystem functions are largely unknown, which limits our understanding of how ecosystems respond as a whole to anthropogenic drivers, climate and environmental variability4,5. Here we derive a set of ecosystem functions6 from a dataset of surface gas exchange measurements across major terrestrial biomes. We find that most of the variability within ecosystem functions (71.8%) is captured by three key axes. The first axis reflects maximum ecosystem productivity and is mostly explained by vegetation structure. The second axis reflects ecosystem water-use strategies and is jointly explained by variation in vegetation height and climate. The third axis, which represents ecosystem carbon-use efficiency, features a gradient related to aridity, and is explained primarily by variation in vegetation structure. We show that two state-of-the-art land surface models reproduce the first and most important axis of ecosystem functions. However, the models tend to simulate more strongly correlated functions than those observed, which limits their ability to accurately predict the full range of responses to environmental changes in carbon, water and energy cycling in terrestrial ecosystems7,8., Three key axes of variation of ecosystem functional changes and their underlying causes are identified from a dataset of surface gas exchange measurements across major terrestrial biomes and climate zones.

The rate of change in atmospheric CO2 is significantly affected by the terrestrial carbon sink, but the size and spatial distribution of this sink, and the extent to which it can be enhanced to mitigate climate change are highly uncertain. We combined carbon stock (CS) and eddy covariance (EC) flux measurements that were collected over a period of 15 years (2001-2016) in a 55 year old 30 km2 pine forest growing at the semiarid timberline (with no irrigating or fertilization). The objective was to constrain estimates of the carbon (C) storage potential in forest plantations in such semiarid lands, which cover ~18% of the global land area. The forest accumulated 145-160 g C m-2 year-1 over the study period based on the EC and CS approaches, with a mean value of 152.5 ± 30.1 g C m-2 year-1 indicating 20% uncertainty in carbon uptake estimates. Current total stocks are estimated at 7,943 ± 323 g C/m2 and 372 g N/m2 . Carbon accumulated mostly in the soil (~71% and 29% for soil and standing biomass carbon, respectively) with long soil carbon turnover time (59 years). Regardless of unexpected disturbances beyond those already observed at the study site, the results support a considerable carbon sink potential in semiarid soils and forest plantations, and imply that afforestation of even 10% of semiarid land area under conditions similar to that of the study site, could sequester ~0.4 Pg C/year over several decades.

Climate change will have a considerable impact on forest productivity worldwide. Forecasting the magnitude of such impact, with multiple environmental stressors changing simultaneously, is only possible with the help of process-based models. In order to assess their performance, such models require careful evaluation against measurements. However, direct comparison of model outputs against observational data is often not reliable, as models may provide the right answers due to the wrong reasons. This would severely hinder forecasting abilities under unprecedented climate conditions. Here, we present a methodology for model assessment, which supplements the traditional output-to-observation model validation. It evaluates model performance through its ability to reproduce observed seasonal changes of the most limiting environmental driver (MLED) for a given process, here daily gross primary productivity (GPP). We analyzed seasonal changes of the MLED for GPP in two contrasting pine forests, the Mediterranean Pinus halepensis Mill. Yatir (Israel) and the boreal Pinus sylvestris L. Hyytiälä (Finland) from three years of eddy-covariance flux data. Then, we simulated the same period with a state-of-the-art process-based simulation model (LandscapeDNDC). Finally, we assessed if the model was able to reproduce both GPP observations and MLED seasonality. We found that the model reproduced the seasonality of GPP in both stands, but it was slightly overestimated without site-specific fine-tuning. Interestingly, although LandscapeDNDC properly captured the main MLED in Hyytiälä (temperature) and in Yatir (soil water availability), it failed to reproduce high-temperature and high-vapor pressure limitations of GPP in Yatir during spring and summer. We deduced that the most likely reason for this divergence is an incomplete description of stomatal behavior. In summary, this study validates the MLED approach as a model evaluation tool, and opens up new possibilities for model improvement.

Afforestation in semi-arid regions can potentially enhance the global carbon sink by increasing the terrestrial biomass. However, the survival of planted forests under such extreme environmental conditions is not guaranteed a priori, and critically depends on the surface–atmosphere exchange of energy. We investigate the pine forest Yatir in Israel, an example of a man-made semi-arid ecosystem, by means of large-eddy simulations. We focus on the interaction between surface–atmosphere exchange and secondary circulations that couple the isolated forest to the surrounding shrubland. The large-eddy simulations feature a grid resolution that resolves the forest canopy in several layers, and are initialized by satellite data and Doppler lidar, eddy-covariance and radiosonde measurements. We perform three large-eddy simulations with different geostrophic wind speeds to investigate the influence of those wind speeds on the surface–atmosphere exchange. We reproduce the measured mean updrafts above the forest and mean downdrafts above the shrubland, which increase in strength with decreasing geostrophic wind speed. The largest updrafts emerge above the older, denser part of the forest, triggering secondary circulations. The spatial extent of these circulations does not cover the entire forest area, although we observe a reduced aerodynamic resistance in the regions of updraft. Our simulations indicate that the enhanced surface–atmosphere exchange of the Yatir forest is not sufficient to compensate for the increased net radiation, due to the lower albedo of the forest with respect to the surroundings, resulting in higher air temperatures inside the forest. However, the difference between the forest and shrubland temperatures decreases with increasing geostrophic wind speed due to reduction in the aerodynamic resistance.

We investigate the effects of an isolated meso- $$\gamma $$ -scale surface heterogeneity for roughness and albedo on the atmospheric boundary-layer (ABL) height, with a case study at a semi-arid forest surrounded by sparse shrubland (forest area: $$28~\text{ km }^2$$ , forest length in the main wind direction: 7 km). Doppler lidar and ceilometer measurements at this semi-arid forest show an increase in the ABL height over the forest compared with the shrubland on four out of eight days. The differences in the ABL height between shrubland and forest are explained for all days with a model that assumes a linear growth of the internal boundary layer of the forest through the convective ABL upwind of the forest followed by a square-root growth into the stable free atmosphere. For the environmental conditions that existed during our measurements, the increase in ABL height due to large sensible heat fluxes from the forest ( $$600~\text {W~m}^{-2}$$ in summer) is subdued by stable stratification in the free atmosphere above the ABL, or reduced by high wind speeds in the mixed layer.

Estimations of ecosystem-level evapotranspiration (ET) and CO2 uptake in water-limited environments are scarce and scaling up ground-level measurements is not straightforward. A biophysical approach using remote sensing (RS) and meteorological data (RS–Met) is adjusted to extreme high-energy water-limited Mediterranean ecosystems that suffer from continuous stress conditions to provide daily estimations of ET and CO2 uptake (measured as gross primary production, GPP) at a spatial resolution of 250 m. The RS–Met was adjusted using a seasonal water deficit factor (fWD) based on daily rainfall, temperature and radiation data. We validated our adjusted RS–Met with eddy covariance flux measurements using a newly developed mobile lab system and the single active FLUXNET station operating in this region (Yatir pine forest station) at a total of seven forest and non-forest sites across a climatic transect in Israel (280–770 mm yr−1). RS–Met was also compared to the satellite-borne MODIS-based ET and GPP products (MOD16 and MOD17, respectively) at these sites.Results show that the inclusion of the fWD significantly improved the model, with R = 0.64–0.91 for the ET-adjusted model (compared to 0.05–0.80 for the unadjusted model) and R = 0.72–0.92 for the adjusted GPP model (compared to R = 0.56–0.90 of the non-adjusted model). The RS–Met (with the fWD) successfully tracked observed changes in ET and GPP between dry and wet seasons across the sites. ET and GPP estimates from the adjusted RS–Met also agreed well with eddy covariance estimates on an annual timescale at the FLUXNET station of Yatir (266 ± 61 vs. 257 ± 58 mm yr−1 and 765 ± 112 vs. 748 ± 124 gC m−2 yr−1 for ET and GPP, respectively). Comparison with MODIS products showed consistently lower estimates from the MODIS-based models, particularly at the forest sites. Using the adjusted RS–Met, we show that afforestation significantly increased the water use efficiency (the ratio of carbon uptake to ET) in this region, with the positive effect decreasing when moving from dry to more humid environments, strengthening the importance of drylands afforestation. This simple yet robust biophysical approach shows promise for reliable ecosystem-level estimations of ET and CO2 uptake in extreme high-energy water-limited environments.

Accurate determination of infrared (IR) emissivity is important for non-contact temperature measurement and for energy balance evaluation in systems that exchange radiation. A method for accurate measurement is proposed based on active modulation of the background radiation. The hemispherical directional reflectance is measured as a proxy for directional emissivity using an IR camera and an integrating sphere, while the background radiation is modulated using an IR emitter and a mechanical shutter. Measurement of the apparent temperature observed by the camera under two different illumination conditions allows the extraction of reflectance and emissivity. The accuracy of the measurement and its sensitivity to surface properties are analyzed, showing uncertainty values as low as 0.004 in some cases. Example measurements of natural and artificial surfaces are presented.

The temporal dynamics of water transport and storage in plants have major implications for plant functioning and survival. In trees, stress on the conductive tissue can be moderated by water storage. Yet, trees can survive high percent loss of conductivity (PLC, up to 80%), suggesting efficient recovery. We assess the role of tree water storage and PLC recovery based on simultaneous measurements of leaf transpiration, branch hydraulic conductivity, and stem sap-flow from different seasons in three study years in mature Pinus halepensis (Miller) trees in a semi-arid forest. During the wet season the rates of transpiration (T) and sap flow (SF) peaked at high morning and through the mid-day. During the dry season T peaked at ~9:00 and then decreased, whereas SF lagged T and fully compensated for it only in the evening, resulting in a mid-day water deficit of ~5 kg tree-1, and with up to 33% of daily T derived from storage. PLC of 30-40% developed during mid-day and subsequently recovered to near zero within 2-3 hr in the dry season (May, June, and September), but not in the wet season (January). The observed temporal decoupling between leaf water loss and soil water recharge is consistent with optimization of the trees’ water and gas exchange economy, while apparently facilitating their survival in the semi-arid conditions.

Drought‐related tree mortality had become a widespread phenomenon in forests around the globe. This process leading to these events and its complexity is not fully understood. Trees in the dry timberline are exposed to ongoing drought, and the available water for transpiration in the soil can determine their survival chances. Recent drought years led to 5%–10% mortality in the semi‐arid pine forest of Yatir (Israel). The distribution of dead trees was, however, highly heterogeneous with parts of the forest showing >80% dead trees (D plots) and others with mostly live trees (L plots). At the tree level, visible stress was associated with low pre‐dawn leaf water potential at the dry season (−2.8 MPa vs. −2.3 MPa in non‐stressed trees), shorter needles (5.5 vs. 7.7 mm) and lower chlorophyll content (0.6 vs. 1 mg/g dw). Trends in tree‐ring widths reflected differences in stress intensity (30% narrower rings in stressed compared with unstressed trees), which could be identified 15–20 years prior to mortality. At the plot scale, no differences in topography, soil type, tree age or stand density could explain the mortality difference between the D and L plots. It could only be explained by the higher surface rock cover and in stoniness across the soil profile in the L plots. Simple bucket model simulations using the site’s long‐term hydrological data supported the idea that these differences could result in higher soil water concentration (m³/m³) in the L plots and extend the time above wilting point by several months across the long dry season. Accounting for subsurface heterogeneity may therefore critical to assessing stand‐level response to drought and projecting tree survival, and can be used in management strategies in regions undergoing drying climate trends. A plain language summary is available for this article.

The future of forests and their productivity in dry environments will depend on both water availability through precipitation and ecosystem and plant water use characteristics. It is increasingly recognized that better understanding water use patterns and their response to change depends on our ability to partition evapotranspiration (ET). Here, we use chamber-based direct measurements of soil evaporation (Es) in a semi-arid Pinus halepensis forest to partition ET to Es and tree transpiration (Et), to assess the daily and seasonal changes and to compare annual-scale values with measurements carried out at the same site ten years earlier. The ecosystem is characterized by a high annual Es/ET ratio of 0.26, and an Et/ET of 0.63. Es diminished in the long dry season, but as much as 74 ± 5% of the residual flux was due to the re-evaporation of nighttime moisture adsorption, which may provide critical protection from soil drying. Over the 10 years observation period concurrent increase in the transpiration ratio (TR=Et/ET; +29%) and in leaf area index (LAI; +44%) were observed, with the ratio of TR/LAI remaining constant at ~0.31, and with persistently closed hydrological balance (ET/P of 0.94–1.07). The observed Et/ET values are similar to the estimated global mean values, but are attained at a much higher aridity index (5.5) than the mean one, demonstrating the potential for expanding forestation into dry regions.

The return-to-isotropy of turbulence across scales is studied for different thermal stratification and surface roughness regimes from measurements in the atmospheric boundary layer and it is compared with predictions of a homogenous model.

International audience; Short-term, intense heat waves (hamsins) are common in the eastern Mediterranean region and provide an opportunity to study the resilience of forests to such events that are predicted to increase in frequency and intensity. * The response of a 50-yr-old Aleppo pine (Pinus halepensis) forest to hamsin events lasting 1–7 d was studied using 10 yr of eddy covariance and sap flow measurements. * The highest frequency of heat waves was c. four per month, coinciding with the peak productivity period (March–April). During these events, net ecosystem carbon exchange (NEE) and canopy conductance (gc) decreased by c. 60%, but evapotranspiration (ET) showed little change. Fast recovery was also observed with fluxes reaching pre-stress values within a day following the event. NEE and gc showed a strong response to vapor pressure deficit that weakened as soil moisture decreased, while sap flow was primarily responding to changes in soil moisture. On an annual scale, heat waves reduced NEE and gross primary productivity by c. 15% and 4%, respectively. * Forest resilience to short-term extreme events such as heat waves is probably a key to its survival and must be accounted for to better predict the increasing impact on productivity and survival of such events in future climates.

The carbon sink intensity of the biosphere depends on the balance between gross primary productivity (GPP) of forest canopies and ecosystem respiration. GPP, however, cannot be directly measured and estimates are not well constrained. A new approach relying on canopy transpiration flux measured as sap flow, and water-use efficiency inferred from carbon isotope analysis (GPPSF ) has been proposed, but not tested against eddy covariance-based estimates (GPPEC ). Here we take advantage of parallel measurements using the two approaches at a semi-arid pine forest site to compare the GPPSF and GPPEC estimates on diurnal to annual timescales. GPPSF captured the seasonal dynamics of GPPEC (GPPSF = 0.99 × GPPEC , r(2) = 0.78, RMSE = 0.82, n = 457 d) with good agreement at the annual timescale (653 vs 670 g C m(-2) yr(-1) ). Both methods showed that GPP ranged between 1 and 8 g C m(-2) d(-1) , and the GPPSF /GPPEC ratio was between 0.5 and 2.0 during 82% of the days. Carbon uptake dynamics at the individual tree scale conformed with leaf scale rates of net assimilation. GPPSF can produce robust estimations of tree- and canopy-scale rates of CO2 uptake, providing constraints and greatly extending current GPPEC estimations.

Afforestation is an important approach to mitigate global warming. Its complex interactions with the climate system, however, makes it controversial. Afforestation is expected to be effective in the tropics where biogeochemical and biogeophysical effects act in concert; however, its potential in the large semi-arid regions remains insufficiently explored. Here, we use a Global Climate Model to provide a process-based demonstration that implementing measured characteristics of a successful semi-arid afforestation system (2000 ha, ~300 mm mean annual precipitation) over large areas (~200 million ha) of similar precipitation levels in the Sahel and North Australia leads to the weakening and shifting of regional low-level jets, enhancing moisture penetration and precipitation (+0.8 ± 0.1 mm d−1 over the Sahel and +0.4 ± 0.1 mm d−1 over North Australia), influencing areas larger than the original afforestation. These effects are associated with increasing root depth and surface roughness and with decreasing albedo. This results in enhanced evapotranspiration, surface cooling and the modification of the latitudinal temperature gradient. It is estimated that the carbon sequestration potential of such large-scale semi-arid afforestation can be on the order of ~10% of the global carbon sink of the land biosphere and would overwhelm any biogeophysical warming effects within ~6 years.

Carbonyl sulfide (COS) is a tracer of ecosystem photosynthesis that can advance carbon cycle research from leaf to global scales; however, a range of newly reported caveats related to sink/source strength of various ecosystem components hinder its application. Using comprehensive eddy-covariance and chamber measurements, we systematically measure ecosystem contributions from leaf, stem, soil, and litter and were able to close the ecosystem COS budget. The relative contributions of nonphotosynthetic components to the overall canopy-scale flux are relatively small (~4% during peak activity season) and can be independently estimated based on their responses to temperature and humidity. Converting COS to photosynthetic CO2 fluxes based on the leaf relative uptake of COS/CO2 , faces challenges due to observed daily and seasonal changes. Yet, this ratio converges around a constant value (~1.6), and the variations, dominated by light intensity, were found unimportant on a flux-weighted daily time-scale, indicating a mean ratio of daytime gross-to-net primary productivity of ~2 in our ecosystem. The seasonal changes in the leaf relative uptake ratio may indicate a reduction in mesophyll conductance in winter, and COS-derived canopy conductance permitted canopy temperature estimate consistent with radiative skin temperature. These results support the feasibility of using COS as a powerful and much-needed means of assessing ecosystem function and its response to change.

Estimations of ecosystem-level evapotranspiration (ET) and CO2 uptake in water-limited environments are scarce and scaling up ground-level measurements is not straightforward. A biophysical approach was previously proposed for ecosystem-level assessment relying on vegetation index and meteorological data (RS-Met) in temperate Mediterranean ecosystems. However, these RS-Met models have not been tested yet in extreme high-energy water-limited ecosystems that suffer from continuous stress conditions. Owing to the lack of ET and CO2 flux estimations in the Eastern Mediterranean, we examined the RS-Met approach using a newly developed mobile lab system and the single active Fluxnet station operating in this region, in seven forest and non-forest sites across a climatic transect in Israel (280–770 mm y−1). The RS-Met models were used with and without the addition of a seasonal drought stress factor (fDS), which was based on daily rainfall, temperature and radiation data. Results show that the RS-Met models with the inclusion of the fDS were significantly improved compared to the non-fDS models (r=0.64–0.91 compared to 0.05–0.80; P=0.06 and r=0.72–0.92 compared to r=0.56–0.90; P

Drought-induced productivity reductions and tree mortality have been increasing in recent decades in forests around the globe. Prescribed reduction in stand density, i.e. thinning, has been proposed as a management tool to improve forest sustainability in face of a warmer, drier future. Thinning should potentially reduce net stand water use and improve water-availability for remaining trees, thus reducing their subsequent drought vulnerability. However, few studies have directly measured these effects. In 2009 we established a large-scale thinning experiment in a semi-arid, 40-years-old pine afforestation. Study plots (70 × 70 m) were thinned to 100, 200, and 300 trees ha−1, and compared with unthinned control plots (210–400 trees ha−1), each at five replications. Stem and needle growth, and needle gas exchange were measured along 3–9 consecutive years at seasonal to annual temporal resolution. Measurements at the tree-scale were further up-scaled using both simple upscaling relationships and using an ecosystem model of coupled carbon, energy and hydrology (Regional Hydro Ecologic Simulation System, RHESSys). At the needle scale, photosynthesis was 70% higher at the 100 trees ha−1 than at 300 trees ha−1, whereas transpiration was merely 10% higher. Consequently, stem and needle growth increased by 100% and 20%, respectively. For most parameters, there was little change between 200 and 100 trees ha−1. Applying RHESSys at the stand-scale, these effects on tree physiology translated into 35% reduction in CO2 uptake and a 47% reduction in tree water-use, which was compensated for by increased evaporation from exposed soil. Our long-term measurements at the dry timberline highlight the role of thinning in enhancing the activity and growth of remaining trees, with increased water-use efficiency. Unexpectedly, this density reduction was associated with a relatively small decrease in forest carbon uptake. Light availability was a limiting factor in the higher density plots, even in our light-abundant forest.

© 2016 Elsevier Ltd Terrestrial ecosystem gross primary production (GPP) is the largest component in the global carbon cycle. The enhanced vegetation index (EVI) has been proven to be strongly correlated with annual GPP within several biomes. However, the annual GPP-EVI relationship and associated environmental regulations have not yet been comprehensively investigated across biomes at the global scale. Here we explored relationships between annual integrated EVI (iEVI) and annual GPP observed at 155 flux sites, where GPP was predicted with a log-log model: ln(GPP)=a×ln(iEVI)+b. iEVI was computed from MODIS monthly EVI products following removal of values affected by snow or cold temperature and without calculating growing season duration. Through categorisation of flux sites into 12 land cover types, the ability of iEVI to estimate GPP was considerably improved (R2 from 0.62 to 0.74, RMSE from 454.7 to 368.2 g C m−2 yr−1). The biome-specific GPP-iEVI formulae generally showed a consistent performance in comparison to a global benchmarking dataset (R2 = 0.79, RMSE = 387.8 g C m−2 yr−1). Specifically, iEVI performed better in cropland regions with high productivity but poorer in forests. The ability of iEVI in estimating GPP was better in deciduous biomes (except deciduous broadleaf forest) than in evergreen due to the large seasonal signal in iEVI in deciduous biomes. Likewise, GPP estimated from iEVI was in a closer agreement to global benchmarks at mid and high-latitudes, where deciduous biomes are more common and cloud cover has a smaller effect on remote sensing retrievals. Across biomes, a significant and negative correlation (R2 = 0.37, p

imbalance p paper contact with Roger Seco: email@rogerseco.cat We quantified springtime ecosystem-scale monoterpene fluxes from two similar Aleppo pine (Pinus halepensis Mill.) forests, located in Israel, that differed in the amount of received precipitation: Yatir in the arid south and Birya in the northern part of Israel (291 and 755 mm annual average rainfall, respectively). In addition to the lower water availability, during our measurement campaign the Yatir site suffered from a heat wave with temperatures up to 35 °C, which made the campaign-average net CO2 assimilation to occur in the morning (1 μmol m−2 s−1), with the rest of the daytime hours mainly dominated by net release of CO2. The milder conditions at Birya favored a higher net CO2 assimilation during all daytime hours (with average peaks higher than 10 μmol m−2 s−1). Despite these large differences in ambient conditions and CO2 net assimilation, daytime monoterpene emission capacities at both sites were comparable. While observed monoterpene fluxes were lower at Yatir than at Birya (hourly averages up to 0.4 and 1 mg m−2 h−1, respectively), the standardized hourly fluxes, after accounting for the differences in light, temperature and stand density between both sites, were comparable (0-1.3 mg m−2 h−1). The approach typically used by biogenic emission models overestimated monoterpene fluxes at Yatir when temperatures rose during the heat wave. This result, together with complementary leaf-level measurements showing that summertime monoterpene fluxes almost completely ceased at Yatir while being enhanced at Birya, highlight the interaction of water scarcity and high temperatures that drive monoterpene emissions from vegetation in such extreme climate zones and the need to further improve model performance.

This paper presents a multi-site (>20) analysis of the relative and absolute energy balance (EB) closure at European FLUXNET sites, as a function of the stability parameter xi, the friction velocity u., thermally-induced turbulence, and the time of the day. A focus of the analysis is the magnitude of EB deficits for very unstable conditions. A univariate analysis of the relative EB deficit as function of xi alone (both for individual sites and a synthesis for all sites), reveals that the relative EB deficit is larger for very unstable conditions (xi < -1.0) than for less unstable conditions (-0.02 > xi >= -1.0). A bivariate analysis of the relative EB deficit as function of both xi and u., however, indicates that for situations with comparable u. the closure is better for very unstable conditions than for less unstable conditions. Our results suggest that the poorer closure for very unstable conditions identified from the univariate analysis is due to reduced u. under these conditions. In addition, we identify that the conditions characterized by smallest relative EB deficits (elevated overall turbulence, mostly during day time) correspond to cases with the largest absolute EB deficits. Thus, the total EB deficit at the sites is induced mostly under these conditions, which is particularly relevant for evapotranspiration estimates. Further, situations with the largest relative EB deficits are generally characterized by small absolute EB deficits. We also find that the relative EB deficit does generally not correspond to the regression line of absolute EB deficit with the net radiation because there is a (positive or negative) offset. This can be understood from theoretical considerations. Finally, we find that storage effects explain a considerable fraction of the large relative (but small absolute) nocturnal EB deficits, and only a limited fraction of the overall relative and absolute EB deficits. (C) 2010 Elsevier B.V. All rights reserved.

Warmer summer conditions result in increased terpene emissions except under severe drought, in which case they strongly decrease. Water stress results in a reduction of the metabolism of plants and in a reorganization of their use of resources geared to survival. In the Mediterranean region, periods of drought accompanied by high temperatures and high irradiance occur in summer. Plants have developed various mechanisms to survive in these conditions by resisting, tolerating or preventing stress. We used three typical Mediterranean tree species in Israel, Pinus halepensis L., Quercus calliprinos and Quercus ithaburensis Webb, as models for studying some of these adaptive mechanisms. We measured their photosynthetic rates (A), stomatal conductance (g s), and terpene emission rates during spring and summer in a geophysical gradient from extremely dry to mesic from Yatir (south, arid) to Birya (north, moist) with intermediate conditions in Solelim. A and g s of P. halepensis were threefold higher in Birya than in Yatir where they remained very low both seasons. Quercus species presented 2–3-fold higher A and g s but with much more variability between seasons, especially for Q. ithaburensis with A and g s that decreased 10–30-fold from spring to summer. Terpene emission rates for pine were not different regionally in spring but they were 5–8-fold higher in Birya than in Yatir in summer (P

International audience; Measuring turbulent fluxes with the eddy covariance method has become a widely accepted and powerful tool for the determination of long term data sets for the exchange of momentum, sensible and latent heat, and trace gases such as CO2 between the atmosphere and the underlying surface. Several flux networks developed continuous measurements above complex terrain, e.g. AmeriFlux and EUROFLUX, with a strong focus on the net exchange of CO2 between the atmosphere and the underlying surface. Under many conditions basic assumptions for the eddy covariance method in its simplified form, such as stationarity of the flow, homogeneity of the surface and fully developed turbulence of the flow field, are not fulfilled. To deal with non-ideal conditions which are common at many FLUXNET sites, quality tests have been developed to check if these basic theoretical assumptions are valid. In the framework of the CARBOEUROFLUX project, we combined quality tests described by Foken and Wichura (1996) with the analytical footprint model of Schmid (1997). The aim was to identify suitable wind sectors and meteorological conditions for flux measurements. These tools were used on data of 18 participating sites. Quality tests were applied on the fluxes of momentum, sensible and latent heat, and on the CO2-flux, respectively. The influence of the topography on the vertical wind component was also checked. At many sites the land use around the flux towers is not homogeneous or the fetch may not be large enough. So the relative contribution of the land use type intended to be measured was also investigated. Thus the developed tool allows comparative investigations of the measured turbulent fluxes at different sites if using the same technique and algorithms for the determination of the fluxes as well as analyses of potential problems caused by influences of the surrounding land use patterns.

We applied a site evaluation approach combining Lagrangian Stochastic footprint modelling with a quality assessment approach for eddy-covariance data to 25 forested sites of the CarboEurope-IP network. The analysis addresses the spatial representativeness of the flux measurements, instrumental effects on data quality, spatial patterns in the data quality, and the performance of the coordinate rotation method. Our findings demonstrate that application of a footprint filter could strengthen the CarboEurope-IP flux database, since only one third of the sites is situated in truly homogeneous terrain. Almost half of the sites experience a significant reduction in eddy-covariance data quality under certain conditions, though these effects are mostly constricted to a small portion of the dataset. Reductions in data quality of the sensible heat flux are mostly induced by characteristics of the surrounding terrain, while the latent heat flux is subject to instrumentation-related problems. The Planar-Fit coordinate rotation proved to be a reliable tool for the majority of the sites using only a single set of rotation angles. Overall, we found a high average data quality for the CarboEurope-IP network, with good representativeness of the measurement data for the specified target land cover types.

International audience; This paper discusses the advantages and disadvantages of the different methods that separate net ecosystem exchange (NEE) into its major components, gross ecosystem carbon uptake (GEP) and ecosystem respiration (Reco). In particular, we analyse the effect of the extrapolation of night-time values of ecosystem respiration into the daytime; this is usually done with a temperature response function that is derived from long-term data sets. For this analysis, we used 16 one-year-long data sets of carbon dioxide exchange measurements from European and US-American eddy covariance networks. These sites span from the boreal to Mediterranean climates, and include deciduous and evergreen forest, scrubland and crop ecosystems. We show that the temperature sensitivity of Reco, derived from long-term (annual) data sets, does not reflect the short-term temperature sensitivity that is effective when extrapolating from night- to daytime. Specifically, in summer active ecosystems the long-term temperature sensitivity exceeds the short-term sensitivity. Thus, in those ecosystems, the application of a long-term temperature sensitivity to the extrapolation of respiration from night to day leads to a systematic overestimation of ecosystem respiration from halfhourly to annual time-scales, which can reach 425% for an annual budget and which consequently affects estimates of GEP. Conversely, in summer passive (Mediterranean) ecosystems, the long-term temperature sensitivity is lower than the short-term temperature sensitivity resulting in underestimation of annual sums of respiration. We introduce a new generic algorithm that derives a short-term temperature sensitivity of Reco from eddy covariance data that applies this to the extrapolation from night- to daytime, and that further performs a filling of data gaps that exploits both, the covariance between fluxes and meteorological drivers and the temporal structure of the fluxes. While this algorithm should give less biased estimates of GEP and Reco, we discuss the remaining biases and recommend that eddy covariance measurements are still backed by ancillary flux measurements that can reduce the uncertainties inherent in the eddy covariance data.

We applied a site evaluation approach combining Lagrangian Stochastic footprint modeling with a quality assessment approach for eddy-covariance data to 25 forested sites of the CarboEurope-IP network. The analysis addresses the spatial representativeness of the flux measurements, instrumental effects on data quality, spatial patterns in the data quality, and the performance of the coordinate rotation method. Our findings demonstrate that application of a footprint filter could strengthen the CarboEurope-IP flux database, since only one third of the sites is situated in truly homogeneous terrain. Almost half of the sites experience a significant reduction in eddy-covariance data quality under certain conditions, though these effects are mostly constricted to a small portion of the dataset. Reductions in data quality of the sensible heat flux are mostly induced by characteristics of the surrounding terrain, while the latent heat flux is subject to instrumentation-related problems. The Planar-Fit coordinate rotation proved to be a reliable tool for the majority of the sites using only a single set of rotation angles. Overall, we found a high average data quality for the CarboEurope-IP network, with good representativeness of the measurement data for the specified target land cover types.