Water use efficiency (WUE), oftentimes estimated as transpiration efficiency (TE): the amount of biomass produced with regard to the water used, has not yet been used as a breeding trait to select poplar genotypes with simultaneously high productivity and conservation of water. Before its application as a selection target, evidence must be presented showing that WUE or its estimators remain constant with age and across environmental conditions. We conducted a rainfall exclusion experiment in the field on two Populus euramericana (Moench.) and two Populus nigra (L.) genotypes, and assessed leaf-level (A/g(s)) and whole-plant WUE (DMT/WU as well as their components and related traits. Then, we aimed to compare these results with the same poplar genotypes grown in a glasshouse under contrasting water availability. Despite a reduction of soil water content and whole-plant transpiration, growth was stimulated in the rainfall exclusion plot, likely as a result of an increased nitrogen assimilation. However, TE values between the glasshouse and the field were similar, and genotype ranking remained fairly constant for transpiration, carbon isotopic discrimination (Delta, as a proxy for Wi) and TE. Moreover, even though the drivers of WUE in both experiments were different, increases of WUE measured as Delta or TE was not associated with lower biomass production. Relatively good agreement was found between Delta and TE in the field, absence of a similar correlation in the glasshouse is discussed. These results suggest that. may be a good proxy for TE, and could be used, both as a breeding target for genotype selection in glasshouses without impacting biomass production when planted in the field. However, reduced water availability modified the genotype ranking more significantly than between the field/glasshouse experiments, suggesting a diversity of poplar response to drought that should be considered in breeding strategies.

In order to record the seasonal changes in aboveground biomass production (trunk and branches) in a forest, changes in wood density must be taken into account. A 60-year-old beech forest displayed a large intra-annual variability in its aboveground woody biomass production efficiency. This variation followed a seasonal trend with a maximum during the summer while gross primary production was rather low.

It has been known for a long time that drought intensity is a critical variable in determining water stress of Mediterranean tree species. However, not as much attention has been paid to other drought characteristics, for example the timing of the dry periods. We investigated the impact of the timing and intensity of extreme droughts on growing season length, growth and water-use efficiency of three tree species, Pinus nigra ssp. Salzmannii J.F. Arnold, Quercus ilex ssp. ballota (Desf.) Samp. and Quercus faginea Lam. coexisting in a continental Mediterranean ecosystem. Over the study period (2009-13), intense droughts were observed at annual and seasonal scales, particularly during 2011 and 2012. In 2012, an atypically dry winter and spring was followed by an intense summer drought. Quercus faginea growth was affected more by drought timing than by drought intensity, probably because of its winter-deciduous leaf habit. Pinus nigra showed a lower decrease in secondary growth than observed in the two Quercus species in extremely dry years. Resilience to extreme droughts was different among species, with Q. faginea showing poorer recovery of growth after very dry years. The highest intra- and inter-annual plasticity in water-use efficiency was observed in P. nigra, which maintained a more water-saving strategy. Our results revealed that the timing of extreme drought events can affect tree function to a larger extent than drought intensity, especially in deciduous species. Legacy effects of drought over months and years significantly strengthened the impact of drought timing and intensity on tree function.

Climate change and particularly increasing frequency of drought events during the vegetation period may threaten tree vitality and forest biomass productivity in many temperate regions in the future. In that context, the identification of critical environmental factors and a better understanding of their impact on forests are decisive. The water balance is recognized as one of the most important soil factors for stand productivity in temperate forests. Hence, the consequences of short or long term climate change might vary considerably spatially in function of soil type within a given forest. Our study objective was to assess the impact of contrasting soil types on the water balance and stand growth of a beech (Fagus sylvatica) forest ecosystem of similar age and management during four climatically contrasting years. The experimental forest site of Montiers presents different soils with contrasting physicochemical properties (Dystric Cambisol, Eutric Cambisol and Rendzic Leptosol) monitored to quantify water fluxes and stand biomass increment. Using data collected over the period 2012-2015, including a particularly dry year (-24% precipitations in 2015), we also quantified the impact of water shortage on stand productivity at the annual scale as a function of soil type. We evidenced important differences in soil water holding capacities (SWHC) along the studied soil sequence, ranging between 57 mm for the Rendzic Leptosol downhill over limestone and 205 mm for the Dystric Cambisol uphill over detrital sediments. The results show that the canopy intercepted the same amount of incident rainfall in the three plots and that there were no significant differences in annual soil moisture dynamics among the studied soils. We evidenced different rooting patterns depending on soil type. Under a same climate and with stand, site exposition and solar radiation equivalency, trees transpiration was the evident primary driver of the stand potential to produce aboveground biomass. Soil water holding capacity, annual trees transpiration and aboveground biomass production increased in that order: Rendzic Leptosol < Eutric Cambisol < Dystric Cambisol. During the drier year 2015, the decrease in aboveground biomass productivity was of similar amplitude on the three soil types. (C) 2017 Elsevier B.V. All rights reserved.

Humans require multiple services from ecosystems, but it is largely unknown whether trade-offs between ecosystem functions prevent the realisation of high ecosystem multifunctionality across spatial scales. Here, we combined a comprehensive dataset (28 ecosystem functions measured on 209 forest plots) with a forest inventory dataset (105,316 plots) to extrapolate and map relationships between various ecosystem multifunctionality measures across Europe. These multifunctionality measures reflected different management objectives, related to timber production, climate regulation and biodiversity conservation/recreation. We found that trade-offs among them were rare across Europe, at both local and continental scales. This suggests a high potential for 'win-win' forest management strategies, where overall multifunctionality is maximised. However, across sites, multifunctionality was on average 45.8-49.8% below maximum levels and not necessarily highest in protected areas. Therefore, using one of the most comprehensive assessments so far, our study suggests a high but largely unrealised potential for management to promote multifunctional forests.

A daily 1-km Pan-European weather dataset can drive the BIOME-BGC model for the estimation of current and future beech gross primary production (GPP). Annual beech GPP is affected primarily by spring temperature and more irregularly by summer water stress. The spread of beech forests in Europe enhances the importance of modelling and monitoring their growth in view of ongoing climate changes. The current paper assesses the capability of a biogeochemical model to simulate beech gross primary production (GPP) using a Pan-European 1-km weather dataset. The model BIOME-BGC is applied in four European forest ecosystems having different climatic conditions where the eddy covariance technique is used to measure water and carbon fluxes. The experiment is in three main steps. First, the accuracy of BIOME-BGC GPP simulations is assessed through comparison with flux observations. Second, the influence of two major meteorological drivers (spring minimum temperature and growing season dryness) on observed and simulated inter-annual GPP variations is analysed. Lastly, the impacts of two climate change scenarios on beech GPP are evaluated through statistical analyses of the ground data and model simulations. The weather dataset can drive BIOME-BGC to simulate most of the beech GPP evolution in all four test areas. Both observed and simulated inter-annual GPP variations are mainly dependent on minimum temperature around the beginning of the growing season, while spring/summer dryness exerts a secondary role. BIOME-BGC can also reasonably predict the impacts of the examined climate change scenarios. The proposed modelling approach is capable of approximately reproducing spatial and temporal beech GPP variations and impacts of expected climate changes in the examined European sites.

Recent research has shown that interactions between species with different functional traits can promote forest ecosystem processes. In the context of climate change, understanding whether species interactions in mixed-species ecosystems can improve the adaptation of these ecosystems to extreme climatic events is crucial to developing new management strategies. In this study, we investigated the impact of species interactions on the sap flux density of three Mediterranean tree species ( Quercus faginea , Pinus nigra and Pinus sylvestris ) during a summer drought. Measurements of foliar carbon isotopic composition ( δ 13 C) were also conducted on the same trees. The decline in transpiration during drought was the greatest for P. sylvestris and the least pronounced for Q. faginea . For P. nigra and Q. faginea, the decrease in transpiration as the drought progressed was lower when these species where interacting with another tree species, particularly with P. sylvestris . In contrast, the decrease for P. sylvestris was higher when this species was interacting with another species. Differing drought effects were consistent with the δ 13 C values. We showed that the identity of the species present in the direct neighbourhood of a given tree can differentially influence water availability and water-use of these three co-existing Mediterranean tree species during a summer drought. Our findings suggest that species interactions play an important role in modulating the response of tree species to drought. Favouring tree species diversity in this region does not seem to be systematically beneficial in terms of soil water availability and water-use for all the interacting species.

Extreme climatic events such as intense droughts are becoming more frequent in Mediterranean regions, but our understanding of their impact on tree performance is still fragmentary. We analyzed growth and sap flow responses for a 3-year period including the most stressful drought over the last half century in the evergreen Pinus nigra and the deciduous Quercus faginea , two dominant tree species in the continental plateau of the Iberian Peninsula. Our aim was to quantify the differential impacts of this event on the performance of both species and their modulation by local microclimate. Growth was registered with digital dendrometers, and water use was assessed by continuously recording sap flow in 8–9 coexisting adult individuals of each species in two sites. Q. faginea spring growth rate decreased by 60 % during the dry year at the dry site, while the decrease in P. nigra was around 36 %. P. nigra exhibited larger sap flow reductions during the dry season and also larger decreases during the extreme year, but in contrast to Q. faginea , it was able to recover growth and sap flow values after the extreme drought. Minor microclimatic differences between sites had significant effects on growth and water use, with slightly more mesic conditions significantly attenuating the impact of drought on both species. Findings suggest that the study species were near to their tolerance thresholds, so that even moderate increases in the intensity and frequency of unusual droughts have important consequences for individual tree performance, and eventually species coexistence and ecosystem processes.

Functional convergence in water use of trees across species from diverse geographic locations was examined using data on tree water use parameters, with the intention of gaining an understanding on the capacity for water transport for trees with varying structural and functional traits. Wood density (ρw), which is reported to have a negative exponential relation with sap flow density (SFD), showed a bell-shaped curve when the daily SFD data from 101 tree species belonging to 35 angiosperm and gymnosperm families were plotted. The species came from 23 different geographical locations representing all continents. Trees were most efficient in water transport when the ρw was between 0.51 and 0.65 g cm−3. When the ρw increased or decreased from this range, there was a gradual fall in their water transport rate as indicated by lower daily SFD. The unexpected reduction in SFD with decreasing ρw is explained in terms of reduced conductance in the transport pathway, which is a precaution taken by the tree for avoiding cavitation or implosion in larger conducting tubes, which is characteristic of low density wood. The development of severe leaf water potential variations, which is frequently reported in such trees, supports this notion. The SFD versus ρw relation has a potentially wide applicability in predicting water use by forest stands with varying ρw. In addition, the occurrence of a high number of tree species with ρw values in the range of 0.51–0.65 g cm−3 across all continents examined points towards the importance of ρw in the evolutionary process as related to efficient functioning of the water transport mechanism.

Summary The oxygen stable isotope composition of plant organic matter (OM) (particularly of wood and cellulose in the tree ring archive) is valuable in studies of plant–climate interaction, but there is a lack of information on the transfer of the isotope signal from the leaf to heterotrophic tissues. We studied the oxygen isotopic composition and its enrichment above source water of leaf water over diel courses in five tree species covering a broad range of life forms. We tracked the transfer of the isotopic signal to leaf water‐soluble OM and further to phloem‐transported OM. Observed leaf water evaporative enrichment was consistent with values predicted from mechanistic models taking into account nonsteady‐state conditions. While leaf water‐soluble OM showed the expected 18O enrichment in all species, phloem sugars were less enriched than expected from leaf water enrichment in Scots pine (Pinus sylvestris), European larch (Larix decidua) and Alpine ash (Eucalyptus delegatensis). Oxygen atom exchange with nonenriched water during phloem loading and transport, as well as a significant contribution of assimilates from bark photosynthesis, can explain these phloem 18O enrichment patterns. Our results indicate species‐specific uncoupling between the leaf water and the OM oxygen isotope signal, which is important for the interpretation of tree ring data.

The interannual variability of CO 2 exchange by forest ecosystems in Europe was analyzed at site and regional scales by identifying critical periods that contributed to interannual flux anomalies. Critical periods were defined as periods in which monthly and annual flux anomalies were correlated. The analysis was first conducted at seven European forest flux tower sites with contrasting species and climatic conditions. Organizing Carbon and Hydrology in Dynamic Ecosystems (ORCHIDEE), a generic process‐based model, represented fairly well most features of the critical period patterns and their climate drivers at the site scale. Simulations at the scale of European forests were performed with ORCHIDEE integrated at a 0.25° spatial resolution. The spatial and temporal distributions of critical periods for canopy photosynthesis, ecosystem respiration, and net ecosystem exchange (NEE) as well as their underlying climate drivers were analyzed. The interannual variability in gross primary productivity (GPP) was explained by critical periods during spring and summer months. In contrast, the interannual variability in total ecosystem respiration (TER) was explained by critical periods occurring throughout the year. A latitudinal contrast between southern and northern Europe was observed in the distributions of critical periods for GPP and TER. The critical periods were positively controlled by temperature in northern Europe and by soil water availability in southern Europe. More importantly, the latitudinal transition between temperature‐driven and water‐driven critical periods for GPP varied from early spring to late summer. Such a distinct seasonal regime of critical periods was less clearly defined for TER and NEE. Overall, the critical periods associated with NEE variations and their meteorological drivers followed those associated with GPP.

Abstract. Winter CO2 fluxes represent an important component of the annual carbon budget in northern ecosystems. Understanding winter respiration processes and their responses to climate change is also central to our ability to assess terrestrial carbon cycle and climate feedbacks in the future. The factors influencing the spatial and temporal pattern of winter respiration (RECO) of northern ecosystems are poorly understood. For this reason, we analyzed eddy covariance flux data sets from 57 ecosystem sites ranging from ~35° N to ~70° N. Deciduous forests carry the highest winter RECO ratios (9.7–10.5 g C m−2 d−1), when winter is defined as the period during which air temperature remained below 0 °C. By contrast, wetland ecosystems had the lowest winter RECO (2.1–2.3 g C m−2 d−1). Evergreen needle-leaved forests, grasslands and croplands were characterized by intermediate winter RECO values of 7.4–7.9 g C m−2 d−1, 5.8–6.0 g C m−2 d−1, and 5.2–5.3 g C m−2 d−1, respectively. Cross site analysis showed that winter air or soil temperature, and the seasonal amplitude of the leaf area index inferred from satellite observation, which is a proxy for the amount of litter available for RECO in the subsequent winter, are the two main factors determining spatial pattern of daily mean winter RECO. Together, these two factors can explain 71% (Tair, ΔLAI) or 69% (Tsoil, ΔLAI) of the spatial variance of winter RECO across the 57 sites. The spatial temperature sensitivity of daily winter RECO was determined empirically by fitting an Arrhenius relationship to the data. The activation energy parameter of this relationship was found to decrease at increasing soil temperature at a rate of 83.1 KJ ° C-1 (r = −0.32, p < 0.05), which implies a possible dampening of the increase in winter RECO due to global warming. The interannual variability of winter RECO is better explained by soil temperature than by air temperature, likely due to the insulating effects of snow cover. The increase in winter RECO with a 1 °C warming based calculated from the spatial analysis was almost that double that calculated from the temporal analysis. Thus, models that calculate the effects of warming on RECO based only on spatial analyses could be over-estimating the impact.

Eddy covariance (EC) measurements are widely used to estimate the amount of carbon sequestrated by terrestrial biomes. The decision to exclude an EC flux from a database (bad quality records, turbulence regime not adequate, footprint problem,...) becomes an important step in the CO2 flux determination procedure. In this paper an innovative combination of existing assessment tests is used to give a relatively complete evaluation of the net ecosystem exchange measurements. For the 2005 full-leaf season at the Hesse site, the percentage of rejected half-hours is relatively high (59.7%) especially during night-time (68.9%). This result strengthens the importance of the data gap filling method. The data rejection does not lead to a real improvement of the accuracy of the relationship between the CO2 fluxes and the climatic factors especially during the nights. The spatial heterogeneity of the soil respiration (on a site with relatively homogenous vegetation pattern) seems large enough to mask an increase of the goodness of the fit of the ecosystem respiration measurements with a dependence on soil temperature and water content when the tests are used to reject EC data. However, the data rejected present some common characteristics. Their removal lead to an increase in the total amount of CO2 respired (24%) and photosynthesised (16%) during the 2005 full-leaf season. Consequently the application of our combination of multiple quality tests is able improve the inter-annual analysis. The systematic application on the large database like the CarboEurope and FLUXNET appears to be necessary.

Water use by mature trees of Acacia tortilis (Forsk.) Hayne ssp. raddiana (Savi) Brenan var. raddiana growing in the northern Sahel was continuously recorded over 4 years. Water use was estimated from xylem sap flow measured by transient heat dissipation. Concurrently, cambial growth, canopy phenology, leaf water potential, climatic conditions and soil water availability (SWA) were monitored. In addition to the variation attributable to interannual variation in rainfall, SWA was increased by irrigation during one wet season. The wet season lasted from July to September, and annual rainfall ranged between 146 and 367 mm. The annual amount and pattern of tree water use were stable from year-to-year despite interannual and seasonal variations in SWA in the upper soil layers. Acacia tortilis transpired readily throughout the year, except for one month during the dry season when defoliation was at a maximum. Maximum water use of about 23 l (dm sapwood area)(-2) day(-1) was recorded at the end of the wet season. While trees retained foliage in the dry season, the decline in water use was modest at around 30%. Variation in predawn leaf water potential indicated that the trees were subject to soil water constraint. The rapid depletion of water in the uppermost soil layers after the wet season implies that there was extensive use of water from deep soil layers. The deep soil profile revealed (1) the existence of living roots at 25 m and (2) that the availability of soil water was low (-1.6 MPa) down to the water table at a depth of 31 m. However, transpiration was recorded at a predawn leaf water potential of -2.0 MPa, indicating that the trees used water from both intermediary soil layers and the water table. During the full canopy stage, mean values of whole-tree hydraulic conductance were similar in the wet and dry seasons. We propose that the stability of water use at the seasonal and annual scales resulted from a combination of features, including an extensive rooting habit related to deep water availability and an effective regulation of canopy conductance. Despite a limited effect on tree water use, irrigation during the wet season sharply increased predawn leaf water potential and cambial growth of trunks and branches.

This study aims to assess the effects of corrections for disturbances such as an increased amount of dead roots and an increase in volumetric soil water content on the calculation of soil CO2 efflux partitioning. Soil CO2 efflux, soil temperature and superficial soil water content were monitored in two young beech sites (H1 and H2) during a trenching experiment. Trenching induced a significant input of dead root mass that participated in soil CO2 efflux and reduced the soil dissolved organic carbon content, while it increased superficial soil water content within the trenched plot. Annual soil CO2 efflux in control plots was 528 g C m−2 year−1 at H1 and 527 g C m−2 year−1 at H2. The annual soil CO2 efflux in trenched plots was 353 g C m−2 year−1 at H1 and 425 g C m−2 year−1 at H2. By taking into account annual CO2 efflux from decaying trenched roots, the autotrophic contribution to total soil CO2 efflux reached 69% at H1 and 54% at H2. The partitioning calculation was highly sensitive to the initial root mass estimated within the trenched plots. Uncertainties in the remaining root mass, the fraction of root C that is incorporated into soil organic matter during root decomposition, and the root decomposition rate constant had a limited impact on the partitioning calculation. Corrections for differences in superficial soil water content had a significant impact on annual respired CO2 despite a limited effect on partitioning.

A modelling study is performed in order to quantify the relative effect of allowing for the physiological properties of an undergrowth grass sward on total canopy water and carbon fluxes of the Le-Bray forest (Les-Landes, South-western France). The Le-Bray forest consists of maritime pine and an herbaceous undergrowth (purple moor-grass), which is characterised by a low stomatal control of transpiration, in contrast to maritime pine. A CO2-responsive land surface model is used that includes responses of woody and herbaceous species to water stress. An attempt is made to represent the properties of the undergrowth vegetation in the land surface model Interactions between Soil, Biosphere, and Atmosphere, CO2-responsive, ISBA-A-gs. The new adjustment allows for a fairly different environmental response between the forest canopy and the understory in a simple manner. The model's simulations are compared with long term (1997 and 1998) micro-meteorological measurements over the Le-Bray site. The fluxes of energy, water and CO2, are simulated with and without the improved representation of the undergrowth vegetation, and the two simulations are compared with the observations. Accounting for the undergrowth permits one to improve the model's scores. A simple sensitivity experiment shows the behaviour of the model in response to climate change conditions, and the understory effect on the water balance and carbon storage of the forest. Accounting for the distinct characteristics of the undergrowth has a substantial and positive effect on the model accuracy and leads to a different response to climate change scenarios.

Two main symptoms are typical of silver fir (Abies alba Mill., hereafter shortened to “fir”) and Norway spruce (Picea abies (L.) Karst., hereafter shortened to “spruce”) decline in Europe: (a) crown thinning; and (b) yellowing of old needles, the latter being mostly related to Mg deficiency on acid soils (Roberts et al. 1989; Bonneau 1989). Crown thinning, when not associated with strong mineral deficiencies, may be caused by various abiotic and biotic factors. In the Vosges, it has been mainly related to water stresses, often increased by high stand density (Lévy and Becker 1987). In fact, both needle loss (when not derived from acute deficiencies) and reduced growth appear to be long lasting effects of the sequence of dry years which occurred during the 1960s and 1970s.

The transpiration of a maritime pine canopy was determined from sap-flow measurements obtained from three experiments carried out at two sites in the Landes de Gascogne Forest, southwest France. From these data, the canopy stomatal conductance was estimated by inverting the Penman-Monteith equation, and a canopy conductance model which included the effects of air vapour pressure deficit, soil moisture deficit and irradiance was tested. Values of canopy transpiration simulated by the model for the three sites were compared with field data. The canopy stomatal conductance was decreasingly sensitive to air vapour pressure deficit, soil moisture deficit and global irradiance. There was better agreement between predicted and actual data for transpiration (R2 = 0.83–0.87) than for canopy stomatal conductance (R2 = 0.50–0.78). Values of daily transpiration simulated by the model were in good agreement with field data (R2 = 0.85–0.93).