Foliar δ13C Values Research Articles

Urban CO2 imprints on carbon isotope and growth of Chinese pine in the Beijing metropolitan region

Rapid urbanization has occurred globally and resulted in increasing CO2 emissions from urban areas. Compared to natural forests, urban forests are subject to higher atmospheric CO2 concentrations in view of strong urban-periurban-rural gradients of CO2 emissions. However, relevant insights in the CO2-associated urban imprints on the physiology and growth of regional forests remain lacking. By sampling foliage and tree rings of Chinese pine (Pinus tabuliformis) in the Beijing metropolitan region, China, we explored whether and how urban CO2 emissions affect stable carbon isotope ratios (δ13C) and tree growth spatially and/or temporally. The results indicate a significant decrease in foliar δ13C values towards the urban center and this pattern was mainly explained by the urban-periurban-rural gradients of CO2 emissions as surrogated by trunk road density. Tree-ring δ13C values showed a significant decrease over last four decades and this trend was mainly explained by rising levels of CO2 and secondarily mediated by the variations of aridity index during growing season. Moreover, annual basal area increment of Chinese pine was significantly accelerated during last two decades, being mainly driven by increasing CO2 emissions and secondarily mediated by climate variations. These findings reveal significant CO2-associated imprints of urbanization on plant growth and provide empirical evidences of significant CO2-induced alteration of carbon cycles in urban forests.

Carbon and nitrogen stable isotope variations in leaves of two grapevine cultivars (Chasselas and Pinot noir): Implications for ecophysiological studies

We investigated the within- and between-leaf variability in the carbon and nitrogen isotope composition (δ13C and δ15N) and total nitrogen (TN) content in two grapevine cultivars (Vitis vinifera cv. Chasselas and Pinot noir) field-grown under rain-fed conditions. The within-leaf variability was studied in discs sampled from base-to-tip and left and right regions from the margin to midrib. The intra- and interplant variability was studied by comparing leaves at different positions along the shoot (basal, median, apical). In leaves from both cultivars, a decrease in δ13C from base to tip was observed, which is in line with an upward gradient of stomatal density and chlorophyll concentration. Less important, but still significant differences were observed between the right and left discs. The leaf TN and δ15N values differed between cultivars, showed smaller variations than the δ13C values, and no systematic spatial trends. The intraleaf variations in δ13C, δ15N, and TN suggest that stomatal behavior, CO2 fixation, chlorophyll concentrations, and the chemical composition of leaf components were heterogeneous in the leaves. At the canopy scale, the apical leaves had less 13C and more 15N and TN than the basal leaves, indicating differences in their photosynthetic capacity and remobilizations from old, senescing leaves to younger leaves. Overall, this study demonstrates patchiness in the δ13C and δ15N values of grapevine leaves and species-specificity of the nitrogen assimilation and 15N fractionation. These findings suggest that care must be taken not to overinterpret foliar δ13C and δ15N values in studies based on fragmented material as markers of physiological and biochemical responses to environmental factors.

Relationship between climate and leaf carbon stable isotope of Hippophae.

We analyzed the relationship between carbon stable isotope characteristics of 131 Hippophae populations and environmental factors by measuring the foliar δ13C value in Hippophae. The results showed that the foliar δ13C values of Hippophae ranged from -24.65‰ to -29.11‰, with an average of -26.97‰. Hippophae species were C3 plants. For the foliar δ13C values, the coefficient variation at intraspecific level was higher than that at interspecific level, indicating that environmental factors should be main factors driving the variations of leaf δ13C. The δ13C values had no significant correlation with latitude and longitude, but were negatively correlated with altitude. The regression equation was δ13C(‰)=0.118VAP-0.007GST-0.000028RDA-20.721 (R2=0.212,P<0.0001). Water vapor pressure (VAP), growing season temperature (GST), and radiation (RDA) were the major factors affecting foliar δ13C values. Our results could provide a theoretical basis to understand the responses of Hippophae species to global climate change.

Spatial patterns of leaf δ13C and δ15N of aquatic macrophytes in the arid zone of northwestern China.

Analysis of stable isotope composition is an important tool in research on plant physiological ecology. However, large‐scale patterns of leaf‐stable isotopes for aquatic macrophytes have received considerably less attention. In this study, we examined the spatial pattern of stable isotopes of carbon (δ13C) and nitrogen (δ15N) of macrophytes leaves collected across the arid zone of northwestern China (approximately 2.4 × 106 km2) and attempted to illustrate its relationship with environmental factors (i.e., temperature, precipitation, potential evapotranspiration, sediment total carbon and nitrogen). Our results showed that the mean values of the leaf δ13C and δ15N in the macrophytes sampled from the arid zone were −24.49‰ and 6.82‰, respectively, which were far less depleted than those measured of terrestrial plants. The order of averaged leaf δ13C from different life forms was as follows: submerged > floating‐leaved > emergent. Additionally, our studies indicated that the values of foliar δ13C values of all the aquatic macrophytes were only negatively associated with precipitation, but the foliar δ15N values were mainly associated with temperature, precipitation, and potential evapotranspiration. Therefore, we speculated that water‐relation factors are the leaf δ13C determinant of macrophytes in the arid zone of northwestern China, and the main factors affecting leaf δ15N values are the complex combination of water and energy factors.

Root-zone CO2 enrichment increases biomass accumulation in lettuce and pepper grown hydroponically and aeroponically

Enhancing CO2 levels in commercial glasshouses is a widely used technique to increase productivity, but has high-energy costs and detrimental environmental impacts due to frequent ventilation of the glasshouse (to prevent plant diseases) releasing CO2 into the atmosphere. Previous studies suggest that root-zone (RZ) CO2 enrichment may be a more economic and sustainable alternative to aerial CO2 enrichment. These experiments aimed to compare the effects of RZ CO2 enrichment by adding either bicarbonate or gaseous CO2 into hydroponic and aeroponic systems respectively, and to determine the physiological mechanisms by which plants respond to RZ CO2. Root-zone CO2 enrichment (1500 ppm) of aeroponically-grown lettuce increased shoot dry weight by around 20% compared to those grown with 400 ppm RZ CO2. Supplying hydroponically grown plants with different HCO3- concentrations, that increased the levels of dissolved inorganic carbon (DIC), increased biomass accumulation of lettuce (10% increase at 1 and 5 mM HCO3-) and pepper (10% increase at 1 mM HCO3-). Plants exposed to 1 mM NaH13CO3 showed a significant increase of foliar δ13C values over time, therefore confirming the uptake of DIC by the roots. The d13C values of roots increased significantly over time, however, higher values at the beginning of H13CO3- exposure suggested root-to-shoot transport of DIC. Nutrient solution pH did not affect root carbon uptake, but shoot d13C values were lower in those plants exposed to lower pH levels (5.8) compared to those exposed to fluctuating pH (between 6.3 and 6.7), suggesting differences in root-to-shoot transport of DIC. Thus, root carbon uptake was independent of the form in which CO2 was provided (gaseous CO2 at pH 5.8; HCO3- at higher pHs). How this additional carbon promotes plant growth is still unclear. Potential mechanisms of action such as increased rates of photosynthesis, altered amino acid concentrations and changes in phytohormone concentrations will be investigated in future studies. © 2020 International Society for Horticultural Science. All rights reserved.

Housing Age and Affluence Influence Plant and Soil Nitrogen and Carbon Cycles in Two Semiarid Cities

AbstractWhile human activities have altered the urban nitrogen (N) and carbon (C) cycles, the relationships between social and biophysical processes in cities are not well understood. Here we evaluated relationships between sociodemographic variables (median household income and housing age) and N and C contents and stable isotope ratios of vegetation in the Los Angeles Metropolitan Area (LA), California, and the Salt Lake Valley, Utah. We hypothesized that (1) N content and stable isotope ratios would be negatively related to income via increased N deposition in lower‐income areas; (2) N and C content and N stable isotope ratios would have a positive relationship with age due to soil organic matter accumulation and increased exposure to N losses over time, respectively; and (3) δ13C would increase with income as a result of increased fossil fuel‐derived CO2 concentrations in lower‐income areas. We found that δ15N values decreased with median household income in both cities and N content decreased with income in LA. In addition, atmospheric NO2 concentrations decreased with income in LA. Soil N and C content increased with housing age. However, δ15N had opposing relationships with age in the two cities. Finally, foliar δ13C values were more isotopically depleted with both increasing household income and increasing NO2 concentrations in LA. These results show that urban foliar and soil isotopic composition are associated with sociodemographic variables and that affluence, as well as housing age, influences urban plant and soil function.

Effect of floods on the δ13C values in plant leaves: a study of willows in Northeastern Siberia.

Although stable carbon isotopic composition (δ13C) of plants has been widely used to indicate different water regimes in terrestrial ecosystems over the past four decades, the changes in the plant δ13C value under waterlogging have not been sufficiently clarified. With the enhanced global warming in recent years, the increasing frequency and severity of river floods in Arctic regions lead to more waterlogging on willows that are widely distributed in river lowland. To investigate the δ13C changes in plants under different water conditions (including waterlogging), we measured the δ13C values in the leaves of willows with three species, Salix boganidensis, S. glauca, and S. pulchra, and also monitored changes in plant physiology, under several major flooding conditions in Northeastern Siberia. The foliar δ13C values of willows varied, ranging from −31.6 to −25.7‰ under the different hydrological status, which can be explained by: (i) under normal conditions, the foliar δ13C values decrease from dry (far from a river) to wet (along a river bank) areas; (ii) the δ13C values increase in frequently waterlogged areas owing to stomatal closure; and (iii) after prolonged flooding periods, the δ13C values again decrease, probably owing to the effects of not only the closure of stomata but also the reduction of foliar photosynthetic ability under long period of waterlogging. Based on these results, we predict that plant δ13C values are strongly influenced by plant physiological responses to diverse hydrological conditions, particularly the long periods of flooding, as occurs in Arctic regions.

Relationship between foliar \u03b413C and sapwood area indicates different water use patterns across 236 Salix genotypes

Key messageThe relationship between sapwood area and foliar δ13C values varies among 236 Salix genotypes, indicating different water use patterns across these genotypes.The relationship between leaf δ13C and plant size (represented by e.g. total leaf area) has been used to analyze different water use patterns of plants. However, the total leaf area (TLA) is difficult to assess in trees. Our aims were to (i) identify a feasible predictor for TLA; (ii) estimate the effects of TLA on leaf-level δ13C and δ18O values; and (iii) evaluate whether the relationship between leaf-level δ13C and a TLA proxy can be used to discriminate between different water use patterns. Various leaf and shoot traits of up to 236 Salix genotypes field-grown in Sweden and Italy were assessed and analyzed. Accumulated shoot diameter and sapwood area (SA) calculated from it were the best predictors for TLA. The SA was significantly correlated with foliar δ13C, but not δ18O values in some genotypes. The effects of SA on foliar δ13C values varied significantly among genotypes, and the foliar δ13C–SA relationship could be used to discriminate between different water use patterns across 236 Salix genotypes. Our results demonstrate a great variability of water use patterns across taxonomically closely related plants, and may also have implications for Salix pre-breeding and selection for different drought conditions.

Growth stress response to sea level rise in species with contrasting functional traits: A case study in tidal freshwater forested wetlands

With rising sea levels, mortality of glycophytes can be caused by water and nutrient stress under increasing salinity. However, the relative effects of these two stressors may vary by species-specific functional traits. For example, deciduous species, with leaves typically emerging during low salinity periods of the year, may suffer less from water stress than evergreen species. We sampled two woody species with contrasting functional traits: the evergreen and N2-fixing waxmyrtle (Morella cerifera), and the deciduous and non-N2 fixing baldcypress (Taxodium distichum) along a coastal river (South Carolina, USA) showing an increasing pattern of plant mortality along a salinity gradient. We first analyzed oxygen and hydrogen isotope ratios of plant stem water and river water to determine changes in plant source water at different sites. Then we analyzed foliar carbon and nitrogen isotope ratios (δ13C and δ15N) along with nitrogen and phosphorus content (%N and %P) as proxies for the water and nutrient stress. Results showed that: (1) the two species had different water sources at the higher salinity sites; (2) foliar δ15N values of baldcypress decreased with higher salinity while retaining a constant δ13C value, and both of these isotope values were positively related with foliar %P, suggesting greater nutrient stress but minor water stress under high salinity; and (3) foliar δ13C values of waxmyrtle increased with higher salinity while retaining a constant foliar δ15N value, and neither of the values was significantly related to foliar nutrients, suggesting greater water stress but minor nutrient stress under high salinity. The different responses of the two species to high salinity may be related to their differences in leaf phenology and N2-fixation. Our results suggest that nutrient stress, particularly of P, can contribute to stress and eventual high mortality of baldcypress exposed to salt water intrusion.

Biotic and Abiotic Drivers of Sap Flux in Mature Green Ash Trees (Fraxinus pennsylvanica) Experiencing Varying Levels of Emerald Ash Borer (Agrilus planipennis) Infestation

While the relationship between abiotic drivers of sap flux are well established, the role of biotic disturbances on sap flux remain understudied. The invasion of the emerald ash borer (Agrilus planipennis Fairmaire, EAB) into North America in the 1990s represents a significant threat to ash trees (Fraxinus spp.), which are a substantial component of temperate forests. Serpentine feeding galleries excavated by EAB larvae in the cambial and phloem tissue are linked to rapid tree mortality. To assess how varying levels of EAB infestation impact the plant water status and stress levels of mature green ash (Fraxinus pennsylvanica Marshall) trees, we combined tree-level sap flux measurements with leaf-level gas exchange, isotopes, morphology and labile carbohydrate measurements. Results show sap flux and whole tree water use are reduced by as much as 80% as EAB damage increases. Heavily EAB impacted trees exhibited reduced leaf area and leaf mass, but maintained constant levels of specific leaf area relative to lightly EAB-impacted trees. Altered foliar gas exchange (reduced light saturated assimilation, internal CO2 concentrations) paired with depleted foliar δ13C values of heavily EAB impacted trees point to chronic water stress at the canopy level, indicative of xylem damage. Reduced photosynthetic rates in trees more impacted by EAB likely contributed to the lack of nonstructural carbohydrate (soluble sugars and starch) accumulation in leaf tissue, further supporting the notion that EAB damages not only phloem, but xylem tissue as well, resulting in reduced water availability. These findings can be incorporated into modeling efforts to untangle post disturbance shifts in ecosystem hydrology.

Latitudinal trends in foliar δ13C and δ15N of Quercus variabilis and their influencing factors.

We aimed to reveal the latitudinal trends in foliar δ13C and δ15N of Quercus variabilis, a widely distributed species in East Asia, associated with two ontogenetic stages (juvenile and mature trees) along a North-South transect (26°-40° N). The results showed that mature trees had higher foliar δ13C and δ15N values than juveniles. Foliar δ13C and δ15N values of trees with both ontogenetic stages were nonlinearly increased and decreased with latitude, respectively. No interaction between ontogenetic stage and latitude for the changes of foliar δ15N and δ13C indicated that both ontogenetic stages across the transect consistently responded to latitudinal environmental variations. Results from the random forest models indicated that foliar δ15N of Q. variabilis was mainly affected by soil nutrient contents, e.g., soil organic matter, phosphorus, nitrogen, whereas dominated factors for foliar δ13C were related to moisture, such as relative humidity, precipitation of growing season.

Seasonal dynamics of foliar δ13C and nutrient concentration of Chinese chastetree and spine jujube in foothill rock outcrop habitat of the southern Taihang Mountains, Central China

In this study, in situ investigation was conducted on Chinese chastetree (Vitex negundo var. heterophylla) and spine jujube (Ziziphus jujuba var. spinosa) during early, developed, and closing succession stages. Monthly data were recorded for foliar δ13C values and foliar N, P, and K concentrations from May to October. Foliar δ13C values declined with time and differences by succession stage varied by species. Older spine jujube displayed higher foliar N concentration while foliar P concentration was inversely related with N. Chinese chastetree had lower foliar K concentration than spine jujube. Foliar C/N ratio declined during the dry season and increased in the wet season. Foliar N/P ratio increased during the dry season and declined to its initial level until October, when it increased again. Foliar δ13C value was positively correlated with foliar N concentration. Foliar P and K concentrations were positively correlated to each other. In conclusion, the two shrubs had contrasting strategies of water use efficiency during their succession, but water use efficiency of both species was promoted by N availability and limited by P availability.

Testing the importance of a common ectomycorrhizal network for dipterocarp seedling growth and survival in tropical forests of Borneo

Background: Connections between mature trees and seedlings via ectomycorrhizal (EcM) hyphal networks existing in dipterocarp-dominated tropical rain forests of South-east Asia could have strong implications for seedling growth and survival and the maintenance of high diversity in such forests.Aim: To test whether EcM hyphal network connections are important for the growth and survival of dipterocarp seedlings.Methods: We conducted four independent experiments that prevented contact of experimental seedlings with an EcM network by using a series of fine meshes and/or plastic barriers. We measured the growth and survival (and foliar δ13C in one experiment) of seedlings of six dipterocarp species over intervals ranging from 11 to 29 months.Results: Seedling growth (diameter, height or leaf number) was unaffected by exclusion from the EcM network in three experiments and there were no differences in foliar δ13C values in the fourth. Seedling survival was reduced following exclusion from the EcM network in one experiment. Our results give little support to the hypothesis that dipterocarp seedlings growing in the shaded forest understorey benefit from being connected, through a common EcM network, to surrounding trees.Conclusions: We suggest that our negative results, in contrast to studies conducted in low diversity boreo-temperate or tropical forests, are due to these high diversity forests lacking host species-specific EcM fungi and therefore providing little opportunity for adaptive support of seedlings via hyphal networks.

On the influence of provenance to soil quality enhanced stress reaction of young beech trees to summer drought.

Climate projections propose that drought stress will become challenging for establishing trees. The magnitude of stress is dependent on tree species, provenance, and most likely also highly influenced by soil quality. European Beech (Fagus sylvatica) is of major ecological and economical importance in Central European forests. The species has an especially wide physiological and ecological amplitude enabling growth under various soil conditions within its distribution area in Central Europe. We studied the effects of extreme drought on beech saplings (second year) of four climatically distinct provenances growing on different soils (sandy loam and loamy sand) in a full factorial pot experiment. Foliar δ13C, δ15N, C, and N as well as above‐ and belowground growth parameters served as measures for stress level and plant growth. Low‐quality soil enhanced the effect of drought compared with qualitatively better soil for the above‐ and belowground growth parameters, but foliar δ13C values revealed that plant stress was still remarkable in loamy soil. For beeches of one provenance, negative sandy soil effects were clearly smaller than for the others, whereas for another provenance drought effects in sandy soil were sometimes fatal. Foliar δ15N was correlated with plant size during the experiment. Plasticity of beech provenances in their reaction to drought versus control conditions varied clearly. Although a general trend of declining growth under control or drought conditions in sandy soil was found compared to loamy soil, the magnitude of the effect of soil quality was highly provenance specific. Provenances seemed to show adaptations not only to drought but also to soil quality. Accordingly, scientists should integrate information about climatic pre‐adaptation and soil quality within the home range of populations for species distribution modeling and foresters should evaluate soil quality and climatic parameters when choosing donor populations for reforestation projects.

Nitrogen cycling in an extreme hyperarid environment inferred from δ(15)N analyses of plants, soils and herbivore diet.

Climate controls on the nitrogen cycle are suggested by the negative correlation between precipitation and δ15N values across different ecosystems. For arid ecosystems this is unclear, as water limitation among other factors can confound this relationship. We measured herbivore feces, foliar and soil δ15N and δ13C values and chemically characterized soils (pH and elemental composition) along an elevational/climatic gradient in the Atacama Desert, northern Chile. Although very positive δ15N values span the entire gradient, soil δ15N values show a positive correlation with aridity as expected. In contrast, foliar δ15N values and herbivore feces show a hump-shaped relationship with elevation, suggesting that plants are using a different N source, possibly of biotic origin. Thus at the extreme limits of plant life, biotic interactions may be just as important as abiotic processes, such as climate in explaining ecosystem δ15N values.

Variation in Populus euphratica foliar carbon isotope composition and osmotic solute for different groundwater depths in an arid region of China.

Water use efficiency (WUE) is an important trait associated with plant acclimation caused by water deficits, and δ13C is a good surrogate of WUE under conditions of water deficits. Water deficiency also enhances the accumulation of compatible solutes in the leaves. In this study, variations in foliar δ(13)C values and main osmotic solutes were investigated. Those included total soluble sugar (TSS), sucrose, free proline, glycine betaine (GB), and inorganic ionic (K+, Ca2+, and Cl-) content of Populus euphratica for different groundwater depths in a Ejina desert riparian forest, China. Results indicated that foliar δ13C values in the P. euphratica for different groundwater depths ranged from -29.14±0.06 to -25.84±0.04 ‰. Foliar δ13C signatures became richer as groundwater levels declined. TSS, sucrose, free proline, GB, and K+ were accumulated in P. euphratica foliage with developing plant growth and increasing groundwater depth. Ca2+ and Cl- content increased under stronger P. euphratica transpiration rates for shallower groundwater depths (1-2.5 m) and decreased for deeper groundwater depths (greater than 3.0 m). Moreover, correlations between δ13C, osmotic solutes, and groundwater depths showed that the primary osmotic solutes were TSS, sucrose, proline, GB, and K+. Correlations also showed that δ13C was not only a useful measure for P. euphratica-integrated WUE but also could be used as an indicator reflecting some physiological osmotic indexes.

Can tree-ring δ15N be used as a proxy for foliar δ15N in European beech and Norway spruce?

For long-term environmental investigations, tree-ring δ 15 N values are inappropriate proxies for foliar δ 15 N for both Fagus sylvatica and Picea abies under moderate N loads. Currently it is unclear whether stable nitrogen isotope signals of tree-rings are related to those in foliage, and whether they can be used to infer tree responses to environmental changes. We studied foliar and tree-ring nitrogen (δ15N) and carbon (δ13C) isotope ratios in European beech (Fagus sylvatica L.) and Norway spruce (Picea abies L.) from six long-term forest monitoring sites in Switzerland together with data on N deposition and soil N availability, as well as a drought response index over the last two decades. For both species, tree-ring δ15N and δ13C values were less negative compared to foliar δ15N and δ13C values, most likely due to recycling and reallocation of N within the tree and fractionation processes associated with the transport of sucrose and the formation of tree-rings, respectively. Temporal trends recorded in foliar δ15N were not reflected in tree-ring δ15N, with much higher variations in tree-rings compared to foliage. Soil N availability and N deposition were partially able to explain changes in foliar δ13C, while there were no significant correlations between environmental variables and either tree-ring or foliar δ15N. Our results suggest an uncoupling between the N isotopic composition of tree-rings and foliage. Consequently, tree-ring δ15N values are inappropriate proxies of foliar δ15N values under low-to-moderate N deposition loads. Furthermore, at such low levels of deposition, tree-ring δ15N values are not recommended as archives of tree responses to soil C/N or bulk N deposition.

Erratum to: Foliar δ13C values, photosynthetic pathway, and functional type for 238 common species on farm land and disturbed forest in Laos

Erratum to: Foliar δ13C values, photosynthetic pathway, and functional type for 238 common species on farm land and disturbed forest in Laos

N-Alkane biosynthetic hydrogen isotope fractionation is not constant throughout the growing season in the riparian tree Salix viminalis

Compound-specific δ2H values of leaf wax n-alkanes have emerged as a potentially powerful paleohydrological proxy. Research suggests terrestrial plant n-alkane δ2H values are strongly correlated with meteoric water δ2H values, and may provide information on temperature, relative humidity, evaporation, and precipitation. This is based upon several assumptions, including that biosynthetic fractionation of n-alkanes during synthesis is constant within a single species. Here we present a multi-isotope study of the n-alkanes of riparian Salix viminalis growing in Norwich, UK. We measured n-alkane δ2H, leaf water δ2H, xylem water δ2H, and bulk foliar δ13C and evaluated the variability of n-alkane δ2H values and net biosynthetic fractionation (εlw-wax) over a whole growing season. S. viminalis n-alkane δ2H values decreased by 40‰ between the start of the growing season in April and the time when they stabilized in July. Variation in leaf and xylem water δ2H did not explain this variability. εlw-wax varied from −116‰ during leaf expansion in April to −156‰ during the stable phase. This suggests that differential biosynthetic fractionation was responsible for the strong seasonal trends in S. viminalis n-alkane δ2H values. We suggest that variability in εlw-wax is driven by seasonal differences in the carbohydrate source and thus the NADPH used in n-alkane biosynthesis, with stored carbohydrates utilized during spring and recent occurring growing season assimilates used later in the season. This is further supported by bulk foliar δ13C values, which are 13C-enriched during the period of leaf flush, relative to the end of the growing season. Our results challenge the assumption that biosynthetic fractionation is constant for a given species, and suggest that 2H-enriched stored assimilates are an important source for n-alkane biosynthesis early in the growing season. These findings have implications for the interpretation of sedimentary n-alkanes and call for a careful design of calibration studies using contemporary samples.

Effects of sampling method on foliar δ (13)C of Leymus chinensis at different scales.

Stable carbon isotope composition (δ13C) usually shows a negative relationship with precipitation at a large scale. We hypothesized that sampling method affects foliar δ13C and its response pattern to precipitation. We selected 11 sites along a precipitation gradient in Inner Mongolia and collected leaves of Leymus chinensis with five or six replications repeatedly in each site from 2009 to 2011. Additionally, we collected leaves of L. chinensis separately from two types of grassland (grazed and fenced) in 2011. Foliar δ13C values of all samples were measured. We compared the patterns that foliar δ13C to precipitation among different years or different sample sizes, the differences of foliar δ13C between grazed and fenced grassland. Whether actual annual precipitation (AAP) or mean annual precipitation (MAP), it was strongly correlated with foliar δ13C every year. Significant difference was found between the slopes of foliar δ13C to AAP and MAP every year, among the slopes of foliar δ13C to AAP from 2009 to 2011. The more samples used at each site the lower and convergent P-values of the linear regression test between foliar δ13C and precipitation. Furthermore, there was significant lower foliar δ13C value in presence of grazed type than fenced type grassland. These findings provide evidence that there is significant effect of sampling method to foliar δ13C and its response pattern to precipitation of L. chinensis. Our results have valuable implications in methodology for future field sampling studies.