Seasonal and annual changes in leaf δ13C in two co-occurring Mediterranean oaks: relations to leaf growth and drought progression

1. Changes of δ¹³C and its relation to leaf development, biochemical content and water stress were monitored over a 2 year period in two co-occurring Mediterranean oak species: the deciduous Quercus pubescens and the evergreen Quercus ilex .
2. The time course of leaf δ¹³C showed different patterns in the two species. Young Q. pubescens leaves had a high δ¹³C and a marked decrease occurred during leaf growth. In contrast, leaves at budburst and maturity did not differ significantly in the case of Q. ilex . We suggest that the difference between δ¹³C of young leaves was linked to differential use of reserves of carbon compounds in the two species.
3. δ¹³C values of mature leaves were negatively correlated with minimum seasonal values of predawn water potential, suggesting that a functional adjustment to water resources occurred.
4. There was a significant correlation between individual δ¹³C values for two successive years. This interannual dependence showed that δ¹³C rankings between trees were constant through time.     

Seasonal Variation of δ^13C of Four Tree Species： A Biological Integrator of Environmental Variables

Foliar δ^13C values, an indicator of long-term intercellular carbon dioxide concentration and, thus, of long-term water use efficiency （WUE） in plants, were measured for Pinus massoniana Lamb., P. elliottii Engelm., Cunninghamia laceolata （Lamb.） Hook., and Schima superba Gardn. et Champ. in a restored forest ecosystem in the Jiazhu River Basin. Seasonal variation and the relationship between the foliar δ^13C values of the four species and environmental factors （monthly total precipitation, monthly average air temperature, relative humidity, atmospheric pressure, and monthly total solar radiation and evaporation） were investigated. The monthly δ^13C values and WUE of the four species increased with increasing precipitation, air temperature, solar radiation, and evaporation, whereas δ^13C values of the four species decreased with increasing relative humidity and atmospheric pressure. Despite significant differences in δ^13C seasonal means for the four species, our results demonstrate a significant convergence in the responses of δ^13C values and WUE to seasonal variations in environmental factors among the species investigated and that the δ^13C signature for each species gives a strong indication of environmental variables.     

The influence of leaf thickness on the CO2 transfer conductance and leaf stable carbon isotope ratio for some evergreen tree species in Japanese warm-temperate forests

1. The influence of leaf thickness on internal conductance for CO₂ transfer from substomatal cavity to chloroplast stroma ( g _i) and carbon isotope ratio (δ¹³C) of leaf dry matter was investigated for some evergreen tree species from Japanese temperate forests. g _i was estimated based on the combined measurements of gas exchange and concurrent carbon isotope discrimination.
2. Leaves with thicker mesophyll tended to have larger leaf dry mass per area (LMA), larger surface area of mesophyll cells exposed to intercellular air spaces per unit leaf area ( S _mes) and smaller volume ratio of intercellular spaces to the whole mesophyll (mesophyll porosity).
3. g _i of these leaves was correlated positively to S _mes but negatively to mesophyll porosity. The variation in g _i among these species would be therefore primarily determined by variation of the conductance in liquid phase rather than that in gas phase.
4. δ¹³C was positively correlated to mesophyll thickness and leaf nitrogen content on an area basis. However, g _i values did not correlate to δ¹³C. These results suggest that difference in δ¹³C among the species was not caused by the variation in g _i, but mainly by the difference in long-term photosynthetic capacity.
5. Comparison of our results with those of previous studies showed that the correlation between leaf thickness and g _i differed depending on leaf functional types (evergreen, deciduous or annual). Differences in leaf properties among these functional types were discussed.     

Bulk leaf δ18O and δ13C reflect the intensity of intraspecific competition for water in a semi-arid tussock grassland

We investigated the extent to which plant water and nutrient status are affected by intraspecific competition intensity and microsite quality in a monodominant tussock grassland. Leaf gas exchange and stable isotope measurements were used to assess the water relations of Stipa tenacissima tussocks growing along a gradient of plant cover and soil depth in a semi-arid catchment of Southeast Spain. Stomatal conductance and photosynthetic rate decreased with increasing intensity of competition during the wet growing season, leading to foliar δ ¹⁸O and δ ¹³C enrichment. A high potential for runoff interception by upslope neighbours exerted strong detrimental effects on the water and phosphorus status of downslope S. tenacissima tussocks. Foliar δ ¹⁵N values became more enriched with increasing soil depth. Multiple stepwise regression showed that competition potential and/or rhizosphere soil depth accounted for large proportions of variance in foliar δ ¹³C, δ ¹⁸O and δ ¹⁵N among target tussocks (57, 37 and 64%, respectively). The results presented here highlight the key role that spatial redistribution of resources (water and nutrients) by runoff plays in semi-arid ecosystems. It is concluded that combined measurement of δ ¹³C, δ ¹⁸O and nutrient concentrations in bulk leaf tissue can provide insight into the intensity of competitive interactions occurring in natural plant communities.     

Correlations Between Foliar Stable Carbon Isotope Composition and Environmental Factors in Desert Plant Reaumuria soongorica （Pall.） Maxim.

Leaves of 407 individuals of Reaumuria soongorica （Pall.） Maxim. collected from the major distribution areas were measured to investigate the distribution characteristics of the stable carbon isotope in this desert plant, as well as correlations between δ^13C values and environmental factors. Results showed that δ^13C values in R. soongorica ranged from -22.77‰ to -29.85‰ and that the mean δ^13C value （-26.52‰） was higher than a previously reported δ^13C value for a different desert ecosystem. This indicates that R. soongorica belongs to the C3 photosynthetic pathway and has higher water use efficiency than other species. The correlations between δ^13C values and environmental factors demonstrated that the foliar δ^13C values in R. soongorica increased significantly with decreasing mean annual precipitation and mean relative humidity, and decreased with decreasing duration of sunshine and evaporation. The spatial distribution trend of δ^13C values in R. soongorica was not obvious and there was no significant correlation between the δ^13C values and mean annual temperature. We conclude that different distribution trends in δ^13C values for R. soongorica were likely caused by stomatal limitation rather than by nutrient-related changes in photosynthetic efficiency and that precipitation played an important role in the wide distribution range of R. soongorica. This pattern of δ^13C values for R. soongorica reinforced that it is a super-xerophil in terms of its adaptive strategies to a desert environment.     

Diurnal and seasonal variation in the carbon isotope composition of leaf dark-respired CO2 in velvet mesquite (Prosopis velutina)

We evaluated diurnal and seasonal patterns of carbon isotope composition of leaf dark-respired CO₂ ( δ ¹³C_l) in the C₃ perennial shrub velvet mesquite ( Prosopis velutina ) across flood plain and upland savanna ecosystems in the south-western USA. δ ¹³C_l of darkened leaves increased to maximum values late during daytime periods and declined gradually over night-time periods to minimum values at pre-dawn. The magnitude of the diurnal shift in δ ¹³C_l was strongly influenced by seasonal and habitat-related differences in soil water availability and leaf surface vapour pressure deficit. δ ¹³C_l and the cumulative flux-weighted δ ¹³C value of photosynthates were positively correlated, suggesting that progressive ¹³C enrichment of the CO₂ evolved by darkened leaves during the daytime mainly resulted from short-term changes in photosynthetic ¹³C discrimination and associated shifts in the δ ¹³C signature of primary respiratory substrates. The ¹³C enrichment of dark-respired CO₂ relative to photosynthates across habitats and seasons was 4 to 6‰ at the end of the daytime period (1800 h), but progressively declined to 0‰ by pre-dawn (0300 h). The origin of night-time and daytime variations in δ ¹³C_l is discussed in terms of the carbon source(s) feeding respiration and the drought-induced changes in carbon metabolism.     

Variation in leaf longevity of Pistacia lentiscus and its relationship to sex and drought stress inferred from leaf δ13C

1. Leaf formation, loss, retention, longevity and biomass on male branches of the evergreen mediterranean shrub Pistacia lentiscus , L. correlated strongly with water-use efficiency inferred from leaf δ¹³C across a gradient of precipitation on the island of Mallorca, Spain.
2. The correlations suggest that the leaf phenology is under control of drought-induced constraints on the carbon balance.
3. In fruiting female branches, the correlations between the inferred water-use efficiency and number of formed and retained leaves, leaf biomass and leaf longevity were non-significant. Leaf formation was strongly reduced by fruiting and the females compensated the reduced photosynthetic capacity by retaining older leaves for a longer time than male plants.
4. It is suggested that leaf longevity in females is under strong control of resource allocation to fruit formation which is 'overlaid' on the drought-induced carbon stress, which led to the observed longer leaf longevity in females than in males.     

Tracing carbon and oxygen isotope signals from newly assimilated sugars in the leaves to the tree-ring archive

The analysis of δ ¹³C and δ ¹⁸O in tree-ring archives offers retrospective insights into environmental conditions and ecophysiological processes. While photosynthetic carbon isotope discrimination and evaporative oxygen isotope enrichment are well understood, we lack information on how the isotope signal is altered by downstream metabolic processes.
In Pinus sylvestris , we traced the isotopic signals from their origin in the leaf water ( δ ¹⁸O) or the newly assimilated carbon ( δ ¹³C), via phloem sugars to the tree-ring, over a time-scale that ranges from hours to a growing season.
Seasonally, variable ¹³C enrichment of sugars related to phloem loading and transport did lead to uncoupling between δ ¹³C in the tree-ring, and the c _i/ c _a ratio at the leaf level. In contrast, the oxygen isotope signal was transferred from the leaf water to the tree-ring with an expected enrichment of 27‰, with time-lags of approximately 2 weeks and with a 40% exchange between organic oxygen and xylem water oxygen during cellulose synthesis.
This integrated overview of the fate of carbon and oxygen isotope signals within the model tree species P. sylvestris provides a novel physiological basis for the interpretation of δ ¹³C and δ ¹⁸O in tree-ring ecology.     

Correlations between net primary productivity and foliar carbon isotope ratio across a Tibetan ecosystem transect

Warming climate could affect leaf-level carbon isotope composition (δ¹³C) through variations in photosynthetic gas exchange. However, it is still unclear to what extent variations in foliar δ¹³C can be used to detect changes in net primary productivity (NPP) because leaf physiology is only one of many determinants of stand productivity. We aim to examine how well site-mean foliar δ¹³C and stand NPP co-vary across large resource gradients using data obtained from the Tibetan Alpine Vegetation Transects (1900–4900 m, TAVT). The TAVT data indicated a robust negative correlation between foliar δ¹³C and NPP across ecosystems (NPP=−2.7224δ¹³C-67.738, r²=0.60, p<0.001). The mean foliar δ¹³C decreased with increasing annual precipitation and its covariation with mean temperature and soil organic carbon and nitrogen contents. The results were further confirmed by global literature data. Pooled δ¹³C data from global literature and this study explained 60% of variations in annual NPP both from TAVT-measures and MODIS-estimates across 67 sites. Our results appear to support a conceptual model relating foliar δ¹³C and nitrogen concentration (N_mass) to NPP, suggesting that: 1) there is a general (negative) relationship between δ¹³C and NPP across different water availability conditions; 2) in water-limited conditions, water availability has greater effects on NPP than N_mass; 3) when water is not limiting, NPP increases with increasing N_mass.     

δ 13C of CO2 respired in the dark in relation to δ13C of leaf carbohydrates in Phaseolus vulgaris L. under progressive drought

The variations in δ ¹³C in both leaf carbohydrates (starch and sucrose) and CO₂ respired in the dark from the cotyledonary leaves of Phaseolus vulgaris L. were investigated during a progressive drought. As expected, sucrose and starch became heavier (enriched in ¹³C) with decreasing stomatal conductance and decreasing p _i/ p _a during the first half (15 d) of the dehydration cycle. Thereafter, when stomata remained closed and leaf net photosynthesis was near zero, the tendency was reversed: the carbohydrates became lighter (depleted in ¹³C). This may be explained by increased p _i/ p _a but other possible explanations are also discussed. Interestingly, the variations in δ ¹³C of CO₂ respired in the dark were correlated with those of sucrose for both well-watered and dehydrated plants. A linear relationship was obtained between δ ¹³C of CO₂ respired in the dark and sucrose, respired CO₂ always being enriched in ¹³C compared with sucrose by ≈ 6‰. The whole leaf organic matter was depleted in ¹³C compared with leaf carbohydrates by at least 1‰. These results suggest that: (i) a discrimination by ≈ 6‰ occurs during dark respiration processes releasing ¹³C-enriched CO₂; and that (ii) this leads to ¹³C depletion in the remaining leaf material.     

Carbon-Isotope Ratios and Photosynthetic Pathways in the Neotropical Family Rapateaceae

Abstract: The Rapateaceae is a small, mainly Neotropical family of terrestrial or occasionally epiphytic herbs that grow on mesic, nutrient-poor sites. Some recent studies suggest that the Rapateaceae may be closely related to the Bromeliaceae, one of the major families containing CAM plants. To investigate the photosynthetic pathway in Rapateaceae, the plant carbon-isotope ratio (δ¹³C) was determined for samples from dried herbarium specimens for 85 of the approximately 100 species in the family. The δ¹³C values ranged from - 37.7 to - 19.8 ‰. Most Rapateaceae showed δ¹³C values typical of C₃ plants. However, six species ( Kunhardtia rhodantha Maguire, Marahuacaea schomburgkii (Maguire) Maguire, Saxofridericia compressa Maguire, Stegolepis grandis Maguire, St. guianensis Klotzsch ex Körn. and St. squarrosa Maguire) showed δ¹³C values less negative than - 23 ‰, i.e., at the higher end of the range for C₃ plants and at the lower end of the distribution for plants exhibiting CAM. The δ¹³C values became significantly less negative with increasing altitude (regression analysis indicating a change from about - 30.7 ‰ at sea level to - 22.5 ‰ at 2500 m). Although other environmental factors and the type of tissue analysed may also influence δ¹³C values, these results suggest that some Rapateaceae may be capable of performing CAM. Further studies, including measurements of diel gas exchange patterns and leaf organic-acid fluctuations, would be needed to demonstrate CAM in Rapateaceae unequivocally, but living material of many of these enigmatic plants is difficult to obtain.     

How to account for the lipid effect on carbon stable-isotope ratio (δ13C): sample treatment effects and model bias

This study investigated the impact of lipid extraction, CaCO₃ removal and of both treatments combined on fish tissue δ¹³C, δ¹⁵N and C:N ratio. Furthermore, the suitability of empirical δ¹³C lipid normalization and correction models was examined. δ¹⁵N was affected by lipid extraction (increase of up to 1·65‰) and by the combination of both treatments, while acidification alone showed no effect. The observed shift in δ¹⁵N represents a significant bias in trophic level estimates, i.e. lipid-extracted samples are not suitable for δ¹⁵N analysis. C:N and δ¹³C were significantly affected by lipid extraction, proportional to initial tissue lipid content. For both variables, rates of change with lipid content (ΔC:N and Δδ¹³C) were species specific. All tested lipid normalization and correction models produced biased estimates of fish tissue δ¹³C, probably due to a non-representative database and incorrect assumptions and generalizations the models were based on. Improved models need a priori more extensive and detailed studies of the relationships between lipid content, C:N and δ¹³C, as well as of the underlying biochemical processes.     

Towards a predictive description of forest canopies from litter properties

1. Specific leaf area (SLA), carbon isotope ratio (δ¹³C) and leaf nitrogen content (N) were measured in litter in 1993 and in the litter layer and canopy in 1996 in a pedunculate Oak ( Quercus robur ) wood at Wytham, Oxford.
2. The frequency distribution of SLA of leaves falling through the late summer and autumn of 1993 changed with a greater contribution of leaves of high SLA earlier in the period. For the same SLA leaves falling earlier in the season had less negative δ¹³C and higher N content than leaves falling later.
3. The frequency distribution of SLA of litter leaves was similar in both years but litter leaves had lower N and less negative δ¹³C in 1996.
4. In the canopy, there was a highly significant negative relationship between height in the canopy and SLA in green and senescent leaves. The relationship between SLA and both δ¹³C ratio and leaf N was different between green, senescent and litter leaves.
5. The frequency distribution of foliage in the canopy estimated from the frequency of SLA in the litter and the vertical pattern of SLA in the canopy was similar to the distribution measured by the 'drop-line' method.     

Effects of water stress and nocturnal temperature on carbon allocation in the perennial grass, Leymus chinensis

Water deficit and high temperature often occur simultaneously, but their effects on plants are usually investigated separately. The aim of this study was to test how interactions between water stress and nocturnal warming affect carbon allocation in the perennial grass, Leymus chinensis . Plant biomass, dry mass allocation, ¹⁴C partitioning and carbon isotope composition (δ¹³C) were measured. Severe and extreme water stress during nocturnal warming decreased the allocation of dry mass and ¹⁴C partitioning below ground to the roots, but moderate water stress significantly increased the below-ground allocation of dry mass and ¹⁴C, especially at the lower night temperature. The δ¹³C values were more positive at day/night temperatures of 30/20°C than at 30/25°C, and greater in the roots than in the leaves. By plotting the δ¹³C values of the leaves against the δ¹³C values of the roots, the slopes of regressions were steeper at low than at high night temperature, also indicating that nocturnal warming reduces carbon allocation below ground to the roots. The results suggest that nocturnal warming may weaken acclimation during water stress in this species by regulating carbon allocation between source and sink organs.     

Natural 13C and 15N abundance of field-collected fungi and their ecological implications

The natural abundance of ¹³C and ¹⁵N was measured in basidiocarps of at least 115 species in 88 genera of ectomycorrhizal, wood-decomposing and litter-decomposing fungi from Japan and Malaysia. The natural abundance of ¹³C and ¹⁵N was also measured in leaves, litter, soil and wood from three different sites. ¹⁵N and ¹³C were enriched in ectomycorrhizal and wood-decomposing fungi, respectively, relative to their substrates. Ectomycorrhizal and wood-decomposing fungi could be distinguished on the basis of their δ¹³C and δ¹⁵N signatures. Although there was high variability in the isotopic composition of fungi, the following isotope- enrichment factors (ε, mean±SD) of the fungi relative to substrates were observed:
ε_{ectomycorrhizal fungi/litter} = 6.1±0.4‰¹⁵N
ε_{ectomycorrhizal fungi/wood} = 1.4±0.8‰¹³C
ε_{wood-decomposing fungi/wood} = −0.6±0.7‰¹⁵N
ε_{wood-decomposing fungi/wood} = 3.5±0.9‰¹³C
The basis of isotope fractionation in C metabolism from wood to wood-decomposing fungus is discussed.     

δ13C of wood in growth-rings indicates cambial activity of drought-stressed trees of Eucalyptus globulus

1. Growth, density and δ¹³C of wood and leaf area were measured in two adjacent stands of 6 year-old Eucalyptus globulus growing in the 600–700 mm year^–1 rainfall region of south-western Australia. Study sites were identical except for differences in the availability of water owing to physical properties of soil profiles and location of sites within the landscape.
2. Abundance of ¹³C (expressed as δ¹³C) in wood of trees growing on the drought-prone site (– 24·8‰±1·4) was greater than in other trees (– 25·8‰±1·2, P <0·001) throughout the 6 years and, with further development, the δ¹³C signatures of wood may become useful indices of drought-susceptibility in plantations within a few years of establishment. The seasonal pattern of δ¹³C of wood appeared to reflect seasonal variation in water availability and duration of cambial activity.
3. Basic density of wood of trees growing on the more drought-prone site (496±14·0 kg m^–3) was reduced compared to other trees (554±5·3 kg m^–3, P <0·001). δ¹³C of wood across boundaries of growth-rings suggested that drought stopped cambial activity resulting in less production of late wood and less dense wood.
4. The stand growing on the drought-prone site had reduced growth, wood yield and leaf area but identical specific leaf area. Annual growth was correlated with the previous season's rainfall. Together, these results suggested that within the same evaporative climate, drought reduces growth primarily by reducing leaf area and that there is a lag between onset of drought and reduced productivity.     

Relationships of grain δ13C and δ18O with wheat phenology and yield under water-limited conditions

Stable carbon isotope composition (δ¹³C) of dry matter has been widely investigated as a selection tool in cereal breeding programmes. However, reports on the possibilities of using stable oxygen isotope composition (δ¹⁸O) as a yield predictor are very scarce and only in the absence of water stress. Indeed, it remains to be tested whether changes in phenology and stomatal conductance in response to water stress overrule the use of either δ¹³C or δ¹⁸O when water is limited. To answer this question, a set of 24 genotypes of bread wheat ( Triticum aestivum ) were assayed in two trials with different levels of deficit irrigation and a third trial under rainfed conditions in a Mediterranean climate (northwest Syria). Grain yield (GY) and phenology (duration from planting to anthesis and from anthesis to maturity) were recorded, and the δ¹³C and δ¹⁸O of grains were analysed to assess their suitability as GY predictors. Both δ¹³C and δ¹⁸O showed higher broad-sense heritabilities ( H ²) than GY. Genotype means of GY across trials were negatively correlated with δ¹³C, as previously reported, but not with δ¹⁸O. Both isotopes were correlated with grain filling duration, whereas δ¹⁸O was also strongly affected by crop duration from planting to anthesis. We concluded that δ¹⁸O of grains is not a proper physiological trait to breed for suboptimal water conditions, as its variability is almost entirely determined by crop phenology. In contrast, δ¹³C of grains, despite being also affected by phenology, still provides complementary information associated with GY.     

Starvation and low feeding levels result in an enrichment of 13C in lipids and 15N in protein of Nile tilapia Oreochromis niloticus L.

The influence of different feeding levels below and slightly above maintenance on whole body δ¹³C and δ¹⁵N values of Nile tilapia Oreochromis niloticus was examined. The energy budget of each fish was determined by indirect calorimetry. The δ¹³C values of the lipid-free material of Nile tilapia fed below and slightly above maintenance level did not differ between the feeding groups, but the δ¹³C values in the lipids and the δ¹⁵N values of the lipid-free material showed small but significant differences. Those fish with a negative lipid retention had significantly higher δ¹³C values in the lipid fraction compared to fish that synthesized fatty acids. There was a significant negative correlation between the amount of energy metabolized by the fish and both the δ¹³C values in the lipids and the δ¹⁵N values of the lipid-free material. Fasting and feeding below the maintenance level may influence the isotopic composition of animals and should therefore be considered in ecological and nutritional studies.     

Stable carbon isotope fractionation by marine phytoplankton during photosynthesis

Abstract. In the marine environment, the range of values of carbon isotope fractionation between particulate tissue of phytoplankton and inorganic carbon can be more than 20‰ (− 35‰ < δ¹³C < − 14‰). This review considers the influence of seawater temperature, lipid content of phytoplanktonic cells, kinetic fractionation, and carbon pathway on δ¹³C values observed at sea.
In order to study the contribution of carboxylases (RUBISCO and the β-carboxylases phosphoenolpyruvate carboxylase, phosphoenoplpyruvate carboxykinase and pyruvate carboxylase) to variations of particulate δ¹³C values at sea, we present results obtained simultenously on carboxylase activities and δ¹³C in various environmental conditions. The lowest δ¹³C values are clearly associated with predominance of ribulose-1.5-bisphosphate carboxylase activity, but it was more difficult to explain the high δ¹³C values. Different hypotheses are discussed.     

Stable isotope analyses of feathers help identify autumn stopover sites of three long-distance migrants in northeastern Africa

The potential use of stable nitrogen (δ¹⁵N), carbon (δ¹³C) and hydrogen (δD) isotope ratios in feathers of marsh warblers Acrocephalus palustris , river warblers Locustella fluviatilis and whitethroats Sylvia communis was evaluated as a means to help identify the location and isotopic composition of autumn stopover sites in northeast Africa. Feather δD values were compared with regional precipitation δD maps averaged over autumn months. Compared with whitethroats, feather δ¹⁵N, δ¹³C, and δD values of marsh warblers and river warblers suggest the two warblers occupy and grow their feathers in geographic locations with relatively mesic environments, and with higher proportions of C3 (vs. C4) plants. However, δ¹³C values of marsh and river warblers were distinct enough to indicate use of different foodwebs. From previous studies, it is evident that during autumn stopover, river warblers moult their primaries in Ethiopia. It is likely that marsh warblers, like river warblers, stay in Ethiopia and/or in neighbouring regions. Based on feather δD values and regional δD precipitation maps, this region should lie between southeast Sudan and southwest Ethiopia. However, without additional regional isotopic maps in Africa, more precise locations of the stopover sites remain unclear. The relatively enriched δ¹⁵N and δ¹³C values of whitethroat feathers compared with the two other species, reflect the fact that whitethroats moult in relatively drier environments and/or with a lower proportion of C3 vs. C4 plants.