Search results

In ecohydrology, it is generally assumed that xylem water reflects the water source used by plants. Several studies have reported isotopic enrichment within woody tissues, particularly during dormancy periods or after long periods of inactivity. However, little is known about the short-term dynamics of this process. Here we assessed the magnitude and dynamics of xylem isotopic enrichment in suberized twigs of pines and oaks. We performed a series of laboratory experiments, in which we monitored hourly changes in water content and isotopic composition under two contrasting scenarios of sap flow restriction. First, we simulated the effect of extreme hydraulic failure by excising twigs to restrict sap flow, while sealing the wounds to ensure that water loss took place only through the leaves or bark, as would be the case for evaporation in attached stems. Second, we studied the effect of reduced leaf transpiration by darkening with aluminium foil all the leaves of healthy, well-watered saplings growing in pot conditions. We found evidence of fast evaporative enrichment in metabolically active stems, as a consequence of a temporal decline in sap flow rates, and not necessarily linked to a traceable decline in stem water content. The excision experiments showed significant isotopic changes (~+1‰ in oxygen) appearing in <1 h. Similarly, the pot experiment showed a progressive increase in isotope composition (up to +8‰ in oxygen in a 3-day cycle) when the leaves were covered, and a rapid recovery to initial values when sap flow rates were re-established. We conclude that evaporative enrichment of xylem water in stems is a highly dynamic process that may have significant effects even during short periods of restricted water flow. This has important implications for the study of plant water uptake, as well as for ecosystem- and global-scale hydrological models. ; This research was supported by the Spanish Government through projects AGL 2012-40039-C02 and AGL 2012- 40151-C03, the PhD fellowship to P.M.-G. (FPU12/00648) and Ramón y Cajal contract to J.P.F. (RYC-2008-02050).

In the Mediterranean, mixed forests of Aleppo pine and holm oak are widespread. Generally considered a transition stage in the succession towards climax oak communities, niche segregation may also contribute to the prevalence of these communities. So far, there is increasing evidence of hydrological niche segregation, with the two species showing complementary water use and seasonal growth patterns. However, it remains unknown whether interspecific interactions affect the response to climate and the mid-term (decadal) growth and water-use efficiency of pines and oaks in mixed stands. Here, we combined tree-ring chronologies, built on different competition classes within a mixed stand, with a spatially explicit assessment of individual growth and wood carbon isotope discrimination (∆13C), as a proxy of intrinsic water-use efficiency, and compared these results with previously reported water uptake patterns. We found that competition with pines modulated the climate response of oaks, whereas pine climate response was insensitive to competition. On the other hand, pine density affected only pine growth, whereas oak competition affected both species. We conclude that the presence of pines had negligible or even positive effects on the oaks, but competition with neighbor oaks limited their ability to recover after drought. Conversely, pines experienced greater drought stress under competition, with both oaks and pines ; This research was funded by the Spanish Government through Projects CGL2011-26654 and RTI2018-094691-B-C31 (MCIU/AEI/FEDER, EU). J.P.F. was supported by Reference Group H09_20R (Gobierno de Aragón, Spain).

In the Mediterranean, mixed forests of Aleppo pine and holm oak are widespread. Generally considered a transition stage in the succession towards climax oak communities, niche segregation may also contribute to the prevalence of these communities. So far, there is increasing evidence of hydrological niche segregation, with the two species showing complementary water use and seasonal growth patterns. However, it remains unknown whether interspecific interactions affect the response to climate and the mid-term (decadal) growth and water-use efficiency of pines and oaks in mixed stands. Here, we combined tree-ring chronologies, built on different competition classes within a mixed stand, with a spatially explicit assessment of individual growth and wood carbon isotope discrimination (∆13C), as a proxy of intrinsic water-use efficiency, and compared these results with previously reported water uptake patterns. We found that competition with pines modulated the climate response of oaks, whereas pine climate response was insensitive to competition. On the other hand, pine density affected only pine growth, whereas oak competition affected both species. We conclude that the presence of pines had negligible or even positive effects on the oaks, but competition with neighbor oaks limited their ability to recover after drought. Conversely, pines experienced greater drought stress under competition, with both oaks and pines. ; This research was funded by the Spanish Government through Projects CGL2011-26654 and RTI2018-094691-B-C31 (MCIU/AEI/FEDER, EU). J.P.F. was supported by Reference Group H09_20R (Gobierno de Aragon, Spain).

Stable isotopes in tree rings provide climatic information with annual resolution dating back for centuries or even millennia. However, deriving spatially explicit climate models from isotope networks remains challenging. Here we propose a methodology to model regional precipitation from carbon isotope discrimination (Δ13C) in tree rings by (1) building regional spatial models of Δ13C (isoscapes) and (2) deriving precipitation maps from Δ13C-isoscapes, taking advantage of the response of Δ13C to precipitation in seasonally dry climates. As a case study, we modeled the spatial distribution of mean annual precipitation (MAP) in the northeastern Iberian Peninsula, a region with complex topography and climate (MAP = 303–1086 mm). We compiled wood Δ13C data for two Mediterranean species that exhibit complementary responses to seasonal precipitation (Pinus halepensis Mill., N = 38; Quercus ilex L.; N= 44; pooling period: 1975–2008). By combining multiple regression and geostatistical interpolation, we generated one Δ13 C-isoscape for each species. A spatial model of MAP was then built as the sum of two complementary maps of seasonal precipitation, each one derived from the corresponding Δ13C-isoscape (September–November from Q. ilex; December–August from P. halepensis). Our approach showed a predictive power for MAP (RMSE = 84 mm) nearly identical to that obtained by interpolating data directly from a similarly dense network of meteorological stations (RMSE = 80–83 mm, N= 65), being only outperformed when using a much denser meteorological network (RMSE=56–57 mm, N=340). This method offers new avenues for modeling spatial variability of past precipitation, exploiting the large amount of information currently available from tree-ring networks. ; This work was funded by the Marie Curie project SMARTREES (MC-ERG-246725, European Union, FP7), and Spanish project PALEOISOTREE (CGL2009-13079-C02-01). J.d.C. is supported by a FPI fellowship (MCINN, Spain). JPF is supported by the Ramón y Cajal programme (RYC-2008-02050, MCINN, Spain).

Submediterranean forests are considered an ecotone between Mediterranean and Eurosiberian ecosystems, and are very sensitive to global change. A decline of Scots pine (Pinus sylvestris L.) and a related expansion of oak species (Quercus spp.) have been reported in the Spanish Pre-Pyrenees. Although this has been associated with increasing drought stress, the underlying mechanisms are not fully understood, and suitable monitoring protocols are lacking. The aim of this study is to bring insight into the physiological mechanisms anticipating selective decline of the pines, with particular focus on carbon and water relations. For this purpose, we performed a sampling campaign covering two growing seasons in a mixed stand of P. sylvestris and Quercus subpyrenaica E.H del Villar. We sampled seasonally twig xylem and soil for water isotope composition (δ18O and δ2H), leaves for carbon isotope composition (δ13C) and stems to quantify non-structural carbohydrates (NSC) concentration, and measured water potential and leaf gas exchange. The first summer drought was severe for both species, reaching low predawn water potential (−2.2 MPa), very low stomatal conductance (12 ± 1.0 mmol m−2 s−1) and near-zero or even negative net photosynthesis, particularly in P. sylvestris (−0.6 ± 0.34 μmol m−2 s−1 in oaks, −1.3 ± 0.16 μmol m−2 s−1 in pines). Hence, the tighter stomatal control and more isohydric strategy of P. sylvestris resulted in larger limitations on carbon assimilation, and this was also reflected in carbon storage, showing twofold larger total NSC concentration in oaks than in pines (7.8 ± 2.4% and 4.0 ± 1.3%, respectively). We observed a faster recovery of predawn water potential after summer drought in Q. subpyrenaica than in P. sylvestris (−0.8 MPa and −1.1 MPa, respectively). As supported by the isotopic data, this was probably associated with a deeper and more reliable water supply in Q. subpyrenaica. In line with these short-term observations, we found a more pronounced negative effect of steadily increasing drought stress on long-term growth in pines compared with oaks. All these observations confer evidence of early warning of P. sylvestris decline and indicate the adaptive advantage of Q. subpyrenaica in the area. ; We thank Pilar Sopeña, Maria Josep Pau and Mireia Oromí for laboratory assistance on isotope and carbohydrate analyses; Instituto de Formación Agroambiental de Jaca, Unidad de Salud de los Bosques de Aragón, Miguel Ángel Lázaro and Jorge del Castillo for their support in field campaigns. The research was supported by the Spanish Government through projects AGL 2012-40039-C02 and AGL 2012-40151-C03, the PhD fellowship to P.M.G. (FPU12/00648) and Ramón y Cajal contract to J.P.F. (RYC-2008-02050).

1. Climate warming increases vulnerability to drought in Mediterranean water-limited forests. However, we still lack knowledge of the long-term physiological responses of coexisting pine species in these forests regarding their ability to cope with warming-induced drought stress. 2. We investigated spatiotemporal patterns of tree performance for five isohydric pines with partially overlapping ecological niches in the eastern Iberian Peninsula along an altitudinal gradient: Pinus halepensis = Pinus pinaster ≤ Pinus nigra ≤ Pinus sylvestris ≤ Pinus uncinata. 3. Using indexed tree-ring widths (TRWi), we assessed the changes in the temporal coherence of radial growth (growth synchrony, âC) over the period 1902-2011 across three elevation belts: low ≈1100 m; mid = 1615 m; high = 2020 m. We also examined by mixed modelling whether TRWi showed an increased coupling with leaf-level gas exchange (inferred from indexed carbon isotope discrimination, Δ13Ci) by enhanced stomatal regulation in response to an amplified regional drought stress. 4. Increasingly negative annual water balances (decrease in annual precipitation minus evapotranspiration = −4·8 mm year−1; 1970-2011) prompted more synchronous growth of coexisting pines between low- and mid-elevation belts, with âC rising from 0·25 ± 0·04 (1902-1951) to 0·62 ± 0·05 (1962-2011). This effect was coupled with tighter stomatal regulation at mid-elevation as indicated by high correlations between TRWi and Δ13Ci (>0·60 from the mid-1970s onwards) which resembled those found at low elevation. Simultaneously, TRWi vs. Δ13Ci uncoupling occurred at the high-elevation belt across species. 5.Weaker growth-climate relationships as elevation increased highlighted the major role of the altitude-dependent thermal gradient in growth responsiveness to drought; however, an intensified Δ13Ci response to spring water availability across elevation belts observed from mid-1970s onwards suggested regional shifts in tree physiological activity linked to earlier seasonal drought impacts. Warming-induced drought stress is spreading to higher altitudes in Iberian pinewoods as multispecies growth is linked to progressively tighter stomatal control of water losses reflected in wood Δ13C. ; T.A.S. and J.P.F acknowledge ERANET-Mundus (European Commission, Grant agreement 20112573) and the Ramón y Cajal program (RYC-2008-02050) respectively. A.A.K. benefitted from a visiting grant (University of Lleida). This study was supported by projects AGL2015-68274-C3-3-R (MINECO/FEDER) and 2014 SGR1141 (Catalan Government). We acknowledge P. Sopeña and M.J. Pau for technical assistance and G. Sangüesa-Barreda for field sampling.

Aims Harsh edaphic environments harbor species with different soil affinities. Plant's responses to specific edaphic constraints may be compromised against re- sponses to prevalent stresses shared with other semi- arid environments. We expect that species with high edaphic affinity may show traits to overcome harsh soil properties, while species with low affinity may respond to environmental constraints shared with arid environments. Methods We quantified the edaphic affinity of 12 plant species co-occurring in gypsum outcrops and measured traits related to plant responses to specific gypsum con- straints (rooting and water uptake depth, foliar accumu- lation of Ca, S and Mg), and traits related to common constraints of arid environments (water use efficiency, macronutrients foliar content). Results Plants in gypsum outcrops differed in their strategies to face edaphic limitations. A phylogenetic informed PCA segregated species based on their foliar Ca and S accumulation and greater water uptake depths, associated with plant responses to specific gypsum lim- itations. Species' gypsum affinity explained this segre- gation, but traits related to water or nutrient use efficien- cy did not contribute substantially to this axis. Conclusions Plant's specializations to respond to spe- cific edaphic constraints of gypsum soils do not limit their ability to deal with other non-specific environmen- tal constraints ; The author thanks the Yesaires team, es- pecially to Daniel A. Rodríguez Ginart, for making the fieldwork of quantification of species gypsum affinity possible. We thank Dr. Sara Palacio (IPE) and two anonymous reviewers for their helpful revisions and comments on the manuscript. RSM was supported by the Ministry of Science and Innovations (FPU grant FPU17/00629). JPF was supported by Grupo de Referencia H09_20R (Aragón regional government, Spain). Financial support was provided by the Valencian Regional Government (GV/2016/ 187) and the Spanish Ministry of Science, Innovation and Univer- sities ...

Stable isotopes are extensively used as tracers for the study of plant-water sources. Isotope-ratio infrared spectroscopy (IRIS) offers a cheaper alternative to isotope-ratio mass spectroscopy (IRMS), but its use in studying plant and soil water is limited by the spectral interference caused by organic contaminants. Here, we examine two approaches to cope with contaminated samples in IRIS: on-line oxidation of organic compounds (MCM) and post-processing correction. We assessed these methods compared to IRMS across 136 samples of xylem and soil water, and a set of ethanol– and methanol–water mixtures. A post-processing correction significantly improved IRIS accuracy in both natural samples and alcohol dilutions, being effective with concentrations up to 8% of ethanol and 0.4% of methanol. MCM outperformed the post-processing correction in removing methanol interference, but did not effectively remove interference for high concentrations of ethanol. By using both approaches, IRIS can overcome with reasonable accuracy the analytical uncertainties associated with most organic contaminants found in soil and xylem water. We recommend the post-processing correction as the first choice for analysis of samples of unknown contamination. Nevertheless, MCM can be more effective for evaluating samples containing contaminants responsible for strong spectral interferences at low concentrations, such as methanol. ; This research was supported by the Spanish Government projects CGL2013-48074-P, AGL 2012-40039-C02 and AGL 2012- 40151-C03, the Catalan Government project SGR 2014-274 and the European Research Council Synergy grant ERC-2013- SyG-610028 IMBALANCE-P. The Spanish Government funded the FPU predoctoral fellowship to P.M-G., the FPI predoctoral fellowship and travel grant to A.B., and the Ram on y Cajal Programme to J.P.F. (RYC-2008-02050) and S.P. (RYC- 2013-14164).

Water and carbon fluxes in forests are largely related to leaf gas exchange physiology varying across spatiotemporal scales and modulated by plant responses to environmental cues. We quantified the relevance of genetic and phenotypic variation of intrinsic water-use efficiency (WUEi, ratio of net photosynthesis to stomatal conductance of water) in Pinus sylvestris L. growing in the Iberian Peninsula as inferred from tree-ring carbon isotopes. Inter-population genetic variation, evaluated in a provenance trial comprising Spanish and German populations, was low and relevant only at continental scale. In contrast, phenotypic variation, evaluated in natural stands (at spatial level) and by tree-ring chronologies (at temporal inter-annual level), was important and ten- and threefold larger than the population genetic variance, respectively. These results points to preponderance of plastic responses dominating variability in WUEi for this species. Spatial phenotypic variation in WUEi correlated negatively with soil depth (r = − 0.66; p < 0.01), while temporal phenotypic variation was mainly driven by summer precipitation. At the spatial level, WUEi could be scaled-up to ecosystem-level WUE derived from remote sensing data by accounting for soil water-holding capacity (r = 0.63; p < 0.01). This outcome demonstrates a direct influence of the variation of leaf-level WUEi on ecosystem water and carbon balance differentiation. Our findings highlight the contrasting importance of genetic variation (negligible) and plastic responses in WUEi (large, with changes of up to 33% among sites) on determining carbon and water budgets at stand and ecosystem scales in a widespread conifer such as Pinus sylvestris. ; This work was supported by the Spanish Government [MINECO Grant Number AGL2015-68274-C3-3-R] and the Russian Science Foundation (Project Number 14-14-00219-P, mathematical approach). We acknowledge P. Sopeña and M.J. Pau for technical assistance and V. Muñoz, M. Sala and A. Teixidó for field sampling.

There is increasing evidence that the circadian clock is a signif- icant driver of photosynthesis that becomes apparent when environmental cues are experimentally held constant. We studied whether the composition of photosynthetic pigments is under circadian regulation, and whether pigment oscillations lead to rhythmic changes in photochemical efficiency. To address these questions, we maintained canopies of bean and cotton, after an entrainment phase, under constant (light or darkness) conditions for 30–48 h. Photosynthesis and quantum yield peaked at subjective noon, and non-photochemical quenching peaked at night. These oscillations were not associ- ated with parallel changes in carbohydrate content or xantho- phyll cycle activity. W e observed robust oscillations of Chl a/b during constant light in both species, and also under constant darkness in bean, peaking when it would have been night dur- ing the entrainment (subjective nights). These oscillations could be attributed to the synthesis and/or degradation of tri- meric light-harvesting complex II (reflected by the rhythmic changes in Chl a/b), with the antenna size minimal at night and maximal around subjective noon. Considering together the oscillations of pigments and photochemistry, the observed pattern of changes is counterintuitive if we assume that the plant strategy is to avoid photodamage, but consistent with a strategy where non-stressed plants maximize photosynthesis. ; The authors acknowledge the support of the following re- search grants: UPV/EHU-GV IT-624-13 and IT-1018-16 from the Basque Government and CTM2014-53902-C2-2-P from the Spanish Ministry of Economy and Competitiveness (MINECO) and the ERDF (FEDER). This study benefited from the CNRS human and technical resources allocated to the Ecotrons Research Infrastructures as well as from the state allocation 'Investissement d'Avenir' ANR-11-INBS- 0001, ExpeER FP7 Transnational Access programme, Ramón y Cajal fellowships (RYC-2012-10970 to V.R.D. and RYC-2008-02050 to J.P.F.), the Erasmus Mundus Master Course Mediterranean Forestry and Natural Resources Management (MEDfOR) and internal grants from UWS- HIE to V.R.D. and ZALF to A.G. EHU postdoctora ellowship and JdC-Incorporation fellowship (IJCI-2014- 22489) to B.F.M. are also acknowledged. We remain indebted to E. Gerardeau, D. Dessauw, J. Jean, P. Prudent (Aïda CIRAD), J.-J. Drevon, C. Pernot (Eco&Sol INRA), B. Buatois, A. Rocheteau (CEFE CNRS), S. Devidal, C. Piel, O. Ravel and the full Ecotron team, J. del Castillo, P. Martín, A. Mokhtar, A. Pra, S. Salekin (UdL), S. García-Muñoz (IMIDRA), Z. Kayler and K. Pirhofer-Walzl (ZALF) for out- standing technical assistance during experiment setup, plant cultivation or subsequent measurements.

Mesophyll diffusion conductance to CO 2 is a key photosynthetic trait that has been studied intensively in the past years. The intention of the present review is to update knowledge of g m, and highlight the important unknown and controversial aspects that require future work. The photosynthetic limitation imposed by mesophyll conductance is large, and under certain conditions can be the most significant photosynthetic limitation. New evidence shows that anatomical traits, such as cell wall thickness and chloroplast distribution are amongst the stronger determinants of mesophyll conductance, although rapid variations in response to environmental changes might be regulated by other factors such as aquaporin conductance.Gaps in knowledge that should be research priorities for the near future include: how different is mesophyll conductance among phylogenetically distant groups and how has it evolved? Can mesophyll conductance be uncoupled from regulation of the water path? What are the main drivers of mesophyll conductance? The need for mechanistic and phenomenological models of mesophyll conductance and its incorporation in process-based photosynthesis models is also highlighted. ; The study was financially supported by the Estonian Ministry of Science and Education (grant SF1090065s07), the Spanish Ministry of Science and Innovation through projects BFU2008-01072 (MEFORE), AGL2009-11310/AGR, BFU2011-23294 (MECOME) and CGL2009-13079-C02-01 (PALEOISOTREE), and the European Commission through European Regional Fund (the Estonian Center of Excellence in Environmental Adaptation), and the Marie Curie project MC-ERG-246725 (FP7). J.P.F. is supported by the Ramón y Cajal program (RYC-2008-02050). A.G. had a Swiss National Science Fellowship (PA00P3_126259). M.M.B. and C.R.W are supported by Future Fellowships from the Australian Research Council (FT0992063 and FT100100024). C.D. was supported by a grant from the French government and by the cooperation project Tranzfor (Transferring Research between EU and Australia–New Zealand on Forestry and Climate Change, PIRSES-GA-2008-230793) funded by the European Union.

Molecular clocks drive oscillations in leaf photosynthesis, stomatal conductance, and other cell and leaf-level processes over ~24 h under controlled laboratory conditions. The influence of such circadian regulation over whole-canopy fluxes remains uncertain; diurnal CO2 and H2O vapor flux dynamics in the field are currently interpreted as resulting almost exclusively from direct physiological responses to variations in light, temperature and other environmental factors. We tested whether circadian regulation would affect plant and canopy gas exchange at the Montpellier European Ecotron. Canopy and leaf-level fluxes were constantly monitored under field-like environmental conditions, and under constant environmental conditions (no variation in temperature, radiation, or other environmental cues). ; This study benefited from the CNRS human and technical resources allocated to the Research Infrastructure Ecotrons, as well as from the state allocation 'Investissement d'Avenir' ANR-11-INBS-0001; ExpeER Transnational Access program; Ramón y Cajal fellowships (RYC-2012-10970 to VRD and RYC-2008-02050 to JPF); the Erasmus Mundus Master Course Mediterranean Forestry and Natural Resources Management (MEDfOR); and internal grants from the Leibniz Centre for Agricultural Landscape Research to AG, and from the Western Sydney University's Hawkesbury Institute for the Environment and the Spanish Government (AGL2015-69151-R) to VRD.