Jordi Voltas

Δ13C and tree-ring width reflect different drought responses in Quercus ilex and Pinus halepensis

Holm oak (Quercus ilex L.) and Aleppo pine (Pinus halepensis Mill) are representative of two different functional types of trees extensively found in the Mediterranean: evergreen sclerophyllous and drought-adapted conifers. The former is considered a partially drought-tolerant species, whereas the latter is a typically drought-avoiding, water-saving species. We postulated that contrasting strategies in response to water deficits in Q. ilex and P. halepensis would lead to a differential sensitivity to changes in water availability. To test this hypothesis, we compared the response of both species in growth rate (measured as radial increments) and intrinsic water use efficiency [WUEi, as inferred from carbon isotope discrimination (Δ13C) in wood samples] among sites from different provenance regions in NE Spain. We found significant differences in Δ13C and growth among provenance regions, partly explained by contrasting water availability. Wood Δ13C was positively related with precipitation and the ratio between precipitation and potential evapotranspiration (P / E). However, these relationships were stronger in P. halepensis (for P / E, r2=0.67, P <0.001) than in Q. ilex (r 2=0.42, P <0.01). In addition, radial growth was positively related with precipitation and Δ13C in P. halepensis (r 2=0.32 and r2=0.35, respectively, P <0.01), but not in Q. ilex. We concluded that P. halepensis was more sensitive than Q. ilex to water availability, showing faster increase in WUEi in response to water stress. We also found that the effect of north/south aspect on Δ13C and growth was site-specific, and unrelated to climatic variables.

Carbon and oxygen isotope ratios in wood constituents of Pinus halepensis as indicators of precipitation, temperature and vapour pressure deficit

Carbon and oxygen isotope compositions (δ13C, δ18O) in tree rings have been shown to bear relevant climatic signals. However, little is known about the interrelationship between both isotopes in wood constituents for species from other than relatively wet climates. We hypothesized that in a species adapted to temporary droughts (e.g. Pinus halepensis Mill.) the signal derived from δ18O in precipitation would be hidden by the strong variability in leaf transpirative enrichment. To test this assumption, we compared the effect of precipitation, temperature and vapour pressure deficit (VPD) on δ18O and δ13C along 23 sites covering the ecological range for this species. We extracted the cores from the south side of four to six adult dominant trees per aspect (north/south) within each site. For each aspect and site, fragments of the period 1975–1999 were pooled and milled to a fine powder. To further test the postulated need for cellulose purification in the assessment of climatic information, we studied these relationships in whole and extracted wood, holocellulose and lignin. In all wood fractions, δ13C was related to annual precipitation [r=−0.58 (P < 0.01) to −0.78 (P < 0.001)] and VPD [r= 0.53 (P < 0.01) to 0.57 (P < 0.01)]. In contrast, for δ18O only holocellulose showed consistent relationships with climatic data, being strongly significant for VPD [r= 0.66 (P < 0.001)]. However, it was unrelated to modelled δ18O in precipitation, confirming that transpirative enrichment (driven by VPD) dampened the source signal in P. halepensis. The relationships between δ13C and δ18O were generally poor, regardless of the wood constituent, suggesting that although both variables were somewhat related to transpirative demand, they were relatively independent. This was further confirmed by building stepwise models using both isotopes to predict annual and seasonal precipitation [r2= 0.34 (P < 0.01) to 0.68 (P < 0.001)], temperature [r2= 0.15 (P < 0.05) to 0.37 (P < 0.01)] and VPD [r2= 0.31 (P < 0.01) to 0.55 (P < 0.001)]. We concluded that, even when partially describing the same climate variables, the information underlying the two isotopes can be regarded as complementary.

Climate-related variability in carbon and oxygen stable isotopes among populations of Aleppo pine grown in common-garden tests

The Aleppo pine (Pinus halepensis Mill.) is found in the Mediterranean under a broad range of moisture and thermal conditions. Differences in severity and duration of water stress among native habitats may act as selective forces shaping the populations’ genetic make-up in terms of contrasting drought strategies. We hypothesised that these strategies should translate into intraspecific variation in carbon isotope composition (δ13C, surrogate of intrinsic water-use efficiency, WUEi) of wood holocellulose, and such variation might be linked to changes in oxygen isotope composition (δ18O, proxy of stomatal conductance) and to some climatic features at origin. Thus, we evaluated δ13C, δ18O, growth and survival for 25 Aleppo pine populations covering its geographic range and grown in two common-garden tests. We found intraspecific variability for δ13C and growth, with high-WUEi populations (which showed 18O-enriched holocellulose) having low growth. These results suggest stomatal regulation as common control for δ13C and productivity. We also detected sizeable relationships between δ13C and climate factors related to the magnitude and timing of drought such as the ratio of summer to annual rainfall. The main climate variable associated with δ18O was minimum temperature, but only in the coldest trial, suggesting differences in growth rhythms among sources. Overall, slow growing populations from highly-seasonal dry areas of the western Mediterranean exhibited a conservative water-use, as opposed to fast growing sources from the northernmost distribution range. The particular behaviour of the Mediterranean Aleppo pine as compared with other conifers demonstrates different selective roles of climate variables in determining intraspecific fitness.

Stable carbon and nitrogen isotopes and quality traits of fossil cereal grains provide clues on sustainability at the beginnings of Mediterranean agriculture

We present a novel approach to study the sustainability of ancient Mediterranean agriculture that combines the measurement of carbon isotope discrimination (Delta(13)C) and nitrogen isotope composition (delta(15)N) along with the assessment of quality traits in fossil cereal grains. Charred grains of naked wheat and barley were recovered in Los Castillejos, an archaeological site in SE Spain, with a continuous occupation of ca. 1500 years starting soon after the origin of agriculture (ca. 4000 BCE) in the region. Crop water status and yield were estimated from Delta(13)C and soil fertility and management practices were assessed from the delta(15)N and N content of grains. The original grain weight was inferred from grain dimensions and grain N content was assessed after correcting N concentration for the effect of carbonisation. Estimated water conditions (i.e. rainfall) during crop growth remained constant for the entire period. However, the grain size and grain yield decreased progressively during the first millennium after the onset of agriculture, regardless of the species, with only a slight recovery afterwards. Minimum delta(15)N values and grain N content were also recorded in the later periods of site occupation. Our results indicate a progressive loss of soil fertility, even when the amount of precipitation remained steady, thereby indicating the unsustainable nature of early agriculture at this site in the Western Mediterranean Basin. In addition, several findings suggest that barley and wheat were cultivated separately, the former being restricted to marginal areas, coinciding with an increased focus on wheat cultivation.

Combined use of δ13C, δ18O and δ15N tracks nitrogen metabolism and genotypic adaptation of durum wheat to salinity and water deficit

• Accurate phenotyping remains a bottleneck in breeding for salinity and drought resistance. Here the combined use of stable isotope compositions of carbon (δ¹³C), oxygen (δ¹⁸O) and nitrogen (δ¹⁵N) in dry matter is aimed at assessing genotypic responses of durum wheat under different combinations of these stresses. • Two tolerant and two susceptible genotypes to salinity were grown under five combinations of salinity and irrigation regimes. Plant biomass, δ¹³C, δ¹⁸O and δ¹⁵N, gas-exchange parameters, ion and N concentrations, and nitrate reductase (NR) and glutamine synthetase (GS) activities were measured. • Stresses significantly affected all traits studied. However, only δ¹³C, δ¹⁸O, δ¹⁵N, GS and NR activities, and N concentration allowed for clear differentiation between tolerant and susceptible genotypes. Further, a conceptual model explaining differences in biomass based on such traits was developed for each growing condition. • Differences in acclimation responses among durum wheat genotypes under different stress treatments were associated with δ¹³C. However, except for the most severe stress, δ¹³C did not have a direct (negative) relationship to biomass, being mediated through factors affecting δ¹⁸O or N metabolism. Based upon these results, the key role of N metabolism in durum wheat adaptation to salinity and water stress is highlighted.

Mineral accumulation, carbon isotope discrimination and indirect selection for grain yield in two-rowed barley grown under semiarid conditions

Mineral accumulation in vegetative plant parts and in mature kernels has been proposed as an indirect selection criterion, either in addition to, or in substitution of, carbon isotope discrimination (Δ), to assess grain yield of temperate cereals in Mediterranean areas. However, the association between mineral concentration, in different plant parts, and grain yield is not yet fully understood. In order to study these relationships, four rain-fed trials were established in northeastern Spain involving 10 two-rowed barley (Hordeum vulgare L.) cultivars. Carbon isotope discrimination and total ash concentration were measured at maturity in kernels and straw. As expected, the Δ values of kernels (Δ-K) and straw (Δ-S) were positively correlated within environments. By contrast, Δ-K and ash concentration in kernels were often negatively related within environments, which suggests that mineral accumulation in kernels is not associated with the transpiration efficiency of the plants during grain filling. The lack of a positive relationship between ash concentration in the straw and either Δ-K or Δ-S indicates that ash concentration in vegetative tissues sampled at maturity may be of limited value as a surrogate of Δ. Grain yield correlated positively with either Δ-K or Δ-S, and negatively with ash concentration in kernels, especially in the poorest rain-fed environments. However, the ash concentration in the straw was not consistently associated with grain yield. While our results confirm that Δ traits are valid indicators of grain yield under Mediterranean conditions, the use of ash concentration in kernels for screening purposes is not warranted at this time due to the lack of a more accurate understanding of the physiological mechanisms underlying mineral accumulation in kernels. Prediction of grain yield through multiple linear regression has shown, however, that ash concentration in kernels could be used as a complementary criterion to Δ in poor rain-fed environments.

Use of carbon isotope composition in monitoring environmental changes

In this work we will discuss the suitability of carbon isotope analysis in plants for the assessment of environmental changes and their effects on crops and natural systems. For C3 species, carbon isotope composition (δ13C) of plant tissues constitutes an integrated record of the miscellaneous climatic and physiological factors that affect carbon assimilation and/or stomatal conductance. Here we present a literature review on the relationship between different environmental parameters and δ13C in both herbaceous plants and trees, including some examples and case studies. We will also consider the applicability of some of these relationships in palaeoecological studies, as well as for the assessment of climate change dynamics and its implications. Major advantages and limitations of this technique are further discussed.

GRAIN SIZE AND NITROGEN ACCUMULATION IN SINK-REDUCED BARLEY UNDER MEDITERRANEAN CONDITIONS

Abstract In rainfed Mediterranean environments, grain growth of barley is often hindered by the occurrence of unpredictable terminal stresses such as drought or high temperature. To study the effect of an enhancement in the assimilate availability on final grain size, a 50% sink-reduction was done in the mainstems of three six-row barley cultivars (Hordeum vulgare L.). The study was conducted in 11 field trials during four years in northeast Spain. The average grain size of control spikes varied significantly among trials between 26.4 mg and 49.1 mg. The average increase of grain size in response to a 50% sink-reduction was 20%. These increments were progressively greater in trials with small grains of control spikes, indicating a major degree of source limitation in low-yielding rainfed environments. Total N content per grain showed a larger average increase (47.6%) in sink-reduced spikes. N accumulated uniformly across trials in response to sink manipulation, indicating that environmental conditions did not limit protein synthesis during grain filling. Dry matter increments in response to sink halving were mainly due to protein accumulation when environmental conditions determined large grains, suggesting that a sink limitation for carbohydrate accumulation may also occur under Mediterranean conditions.

Forests synchronize their growth in contrasting Eurasian regions in response to climate warming

Significance Forests dominate carbon fluxes in terrestrial ecosystems. We demonstrate how an intensified climatic influence on tree growth during the last 120 y has increased spatial synchrony in annual ring-width patterns within contrasting (boreal and Mediterranean) Eurasian biomes and on broad spatial scales. Current trends in tree growth synchrony are related to regional changes in climate factors controlling productivity, overriding local and taxonomic imprints on forest carbon dynamics. Enhanced synchrony is becoming a widespread, although regionally dependent, phenomenon related to warmer springs and increased temperature variability in high latitudes and to warmer winters and drier growing seasons in mid-latitudes. Forests play a key role in the carbon balance of terrestrial ecosystems. One of the main uncertainties in global change predictions lies in how the spatiotemporal dynamics of forest productivity will be affected by climate warming. Here we show an increasing influence of climate on the spatial variability of tree growth during the last 120 y, ultimately leading to unprecedented temporal coherence in ring-width records over wide geographical scales (spatial synchrony). Synchrony in growth patterns across cold-constrained (central Siberia) and drought-constrained (Spain) Eurasian conifer forests have peaked in the early 21st century at subcontinental scales (∼1,000 km). Such enhanced synchrony is similar to that observed in trees co-occurring within a stand. In boreal forests, the combined effects of recent warming and increasing intensity of climate extremes are enhancing synchrony through an earlier start of wood formation and a stronger impact of year-to-year fluctuations of growing-season temperatures on growth. In Mediterranean forests, the impact of warming on synchrony is related mainly to an advanced onset of growth and the strengthening of drought-induced growth limitations. Spatial patterns of enhanced synchrony represent early warning signals of climate change impacts on forest ecosystems at subcontinental scales.

Isotope‐ratio infrared spectroscopy: a reliable tool for the investigation of plant‐water sources?

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.