Reconstruction of source water using the δ18O of tree ring phenylglucosazone: A potential tool in paleoclimate studies

The oxygen isotope ratios of tree ring cellulose have a great potential as proxy for the oxygen isotope ratios of source water, which is related to climate. However, source water isotopic signatures can be masked by plant physiological and biochemical processes during cellulose synthesis. To minimize biochemical effects in the recording of source water, we modified the cellulose molecule to phenylglucosazone, which only has oxygen attached to carbon 3–6 (O_C3–6) of the cellulose glucose moieties, thus eliminating the oxygen attached to carbon 2 of the cellulose glucose moieties (O_C-2). Here we developed a method to use small amounts of inter and intra-annual tree ring cellulose for phenylglucosazone synthesis. Using this new method we tested if the oxygen isotope ratios of source water reconstructed from tree ring phenylglucosazone (δ¹⁸O_sw^PG) and the observed source water (δ¹⁸O_sw^obs) would have a better agreement than those reconstructed from the tree ring cellulose molecule. Annual tree ring samples were obtained from Pinus sylvestris (1997–2003) (Finland) and Picea abies (1971–1992) (Switzerland) and intra-annual tree ring samples were obtained from Pinus radiata (October 2004–March 2006) (New Zealand), each near a meteorological station where precipitation and relative humidity (RH) were measured periodically. The δ¹⁸O of tree ring cellulose and tree ring phenylglucosazone for each of the three species were then used to back calculate the δ¹⁸O of source water according to a previous published empirical equation. As expected, the δ¹⁸O of tree ring phenylglucosazone was superior than cellulose in the reconstruction of source water available to the plant. Deviation between δ¹⁸O_sw^PG and δ¹⁸O_sw^obs was in part correlated with variation in atmospheric relative humidity (RH) which was not observed for the cellulose molecule. We conclude that this new method can be applicable to inter and intra-annual tree ring studies and that the use of the tree ring phenylglucosazone will significantly improve the quality of paleoclimate studies.     

Present and potential uses of the natural abundance of stable isotopes in plant science, with illustrations from the marine environment

The use of stable isotope natural abundance measurements in plant ecophysiological research is discussed in the context of studies of ¹³C/¹²C ratios in marine plants, with emphasis on the uniqueness of the information given by natural abundance measurements and of the importance of complementary data obtained by other techniques in making full use of the natural abundance data. (1) Inorganic C acquisition and assimilation in marine plants can involve diffusive entry of CO₂, or the occurrence of a CO₂-concentrating mechanism frequently involving active HCO₃^? influx. For diffusive CO₂ entry, the δ¹³C measurements can give unique information on the fractional limitation of photosynthesis by CO₂ transport which, with photosynthetic rate measurements, can be used to compute transport conductances. For active HCO₃^?, influx, the δ¹³C values uniquely permit computation of the ratio of the bidirection fluxes (influx/efflux) which, with photon yield data, can be used to given information on the mechanism of the efflux. The analyses are absolutely dependent on external (non-δ¹³C) data distinguishing between diffusive CO₂ entry and the occurrence of a CO₂ concentrating mechanism. (2) δ¹³C measurements on marine photolithotrophs and on members of other trophic levels collected from the sea can give unique data on food webs, with measurements of δ values for other isotopes and compositional data adding precision to the interpretations. (3) Measurements of in situδ¹³C values for extant marine photolithotrophs, compared with δ¹³C values for ancient atmospheric CO₂, can give unique information on the mechanism of atmospheric CO₂ draw-down at the start of glacials; other information permits more concrete conclusions to be drawn.     

Variations in bone collagen δC and δN values of fauna from Northwest Europe over the last 40?000 years

We report here a signal in the temporal variation of stable isotopes in protein from surviving animal bone in Northwest Europe over the past glacial cycle. There is a change in the average δ¹³C values of fauna in the Holocene, and there is also a significant reduction in δ¹⁵N values of herbivore bone collagen towards the end of the last glaciation, with a subsequent recovery soon after the start of the Holocene. This change is observed for several species and is restricted to those regions most affected by the glacial advance. Comparison with ice core data shows that there is a strong correlation between the average δ¹³C values of three herbivore species and ice core CO₂ concentration. The data presented here show how readily available faunal bone collagen δ¹³C and δ¹⁵N measurements provide a record of past climate and environmental change on a regional to continental scale.     

Disentangling the effects of acidic air pollution,atmospheric CO2, and climate change on recent growth of red spruce trees in the Central Appalachian Mountains

In the 45 years after legislation of the Clean Air Act, there has been tremendous progress in reducing acidic air pollutants in the eastern United States, yet limited evidence exists that cleaner air has improved forest health. Here, we investigate the influence of recent environmental changes on the growth and physiology of red spruce (Picea rubens Sarg.) trees, a key indicator species of forest health, spanning three locations along a 100 km transect in the Central Appalachian Mountains. We incorporated a multiproxy approach using 75‐year tree ring chronologies of basal tree growth, carbon isotope discrimination (?¹³C, a proxy for leaf gas exchange), and δ¹⁵N (a proxy for ecosystem N status) to examine tree and ecosystem level responses to environmental change. Results reveal the two most important factors driving increased tree growth since ca. 1989 are reductions in acidic sulfur pollution and increases in atmospheric CO₂, while reductions in pollutant emissions of NO_x and warmer springs played smaller, but significant roles. Tree ring ?¹³C signatures increased significantly since 1989, concurrently with significant declines in tree ring δ¹⁵N signatures. These isotope chronologies provide strong evidence that simultaneous changes in C and N cycling, including greater photosynthesis and stomatal conductance of trees and increases in ecosystem N retention, were related to recent increases in red spruce tree growth and are consequential to ecosystem recovery from acidic pollution. Intrinsic water use efficiency (iWUE) of the red spruce trees increased by ~51% across the 75‐year chronology, and was driven by changes in atmospheric CO₂ and acid pollution, but iWUE was not linked to recent increases in tree growth. This study documents the complex environmental interactions that have contributed to the recovery of red spruce forest ecosystems from pervasive acidic air pollution beginning in 1989, about 15 years after acidic pollutants started to decline in the United States.     

Temporal and spatial trends in marine carbon isotopes in the Arctic Ocean and implications for food web studies

The Arctic is undergoing unprecedented environmental change. Rapid warming, decline in sea ice extent, increase in riverine input, ocean acidification and changes in primary productivity are creating a crucible for multiple concurrent environmental stressors, with unknown consequences for the entire arctic ecosystem. Here, we synthesized 30 years of data on the stable carbon isotope (δ¹³C) signatures in dissolved inorganic carbon (δ¹³C‐DIC; 1977–2014), marine and riverine particulate organic carbon (δ¹³C‐POC; 1986–2013) and tissues of marine mammals in the Arctic. δ¹³C values in consumers can change as a result of environmentally driven variation in the δ¹³C values at the base of the food web or alteration in the trophic structure, thus providing a method to assess the sensitivity of food webs to environmental change. Our synthesis reveals a spatially heterogeneous and temporally evolving δ¹³C baseline, with spatial gradients in the δ¹³C‐POC values between arctic shelves and arctic basins likely driven by differences in productivity and riverine and coastal influence. We report a decline in δ¹³C‐DIC values (?0.011‰ per year) in the Arctic, reflecting increasing anthropogenic carbon dioxide (CO₂) in the Arctic Ocean (i.e. Suess effect), which is larger than predicted. The larger decline in δ¹³C‐POC values and δ¹³C in arctic marine mammals reflects the anthropogenic CO₂ signal as well as the influence of a changing arctic environment. Combining the influence of changing sea ice conditions and isotopic fractionation by phytoplankton, we explain the decadal decline in δ¹³C‐POC values in the Arctic Ocean and partially explain the δ¹³C values in marine mammals with consideration of time‐varying integration of δ¹³C values. The response of the arctic ecosystem to ongoing environmental change is stronger than we would predict theoretically, which has tremendous implications for the study of food webs in the rapidly changing Arctic Ocean.     

Role of land cover changes for atmospheric CO2 increase and climate change during the last 150 years

We assess the role of changing natural (volcanic, aerosol, insolation) and anthropogenic (CO₂ emissions, land cover) forcings on the global climate system over the last 150 years using an earth system model of intermediate complexity, CLIMBER‐2. We apply several datasets of historical land‐use reconstructions: the cropland dataset by Ramankutty & Foley (1999) (R&F), the HYDE land cover dataset of Klein Goldewijk (2001) , and the land‐use emissions data from Houghton & Hackler (2002) . Comparison between the simulated and observed temporal evolution of atmospheric CO₂ and δ¹³CO₂ are used to evaluate these datasets. To check model uncertainty, CLIMBER‐2 was coupled to the more complex Lund–Potsdam–Jena (LPJ) dynamic global vegetation model. In simulation with R&F dataset, biogeophysical mechanisms due to land cover changes tend to decrease global air temperature by 0.26°C, while biogeochemical mechanisms act to warm the climate by 0.18°C. The net effect on climate is negligible on a global scale, but pronounced over the land in the temperate and high northern latitudes where a cooling due to an increase in land surface albedo offsets the warming due to land‐use CO₂ emissions. Land cover changes led to estimated increases in atmospheric CO₂ of between 22 and 43 ppmv. Over the entire period 1800–2000, simulated δ¹³CO₂ with HYDE compares most favourably with ice core during 1850–1950 and Cape Grim data, indicating preference of earlier land clearance in HYDE over R&F. In relative terms, land cover forcing corresponds to 25–49% of the observed growth in atmospheric CO₂. This contribution declined from 36–60% during 1850–1960 to 4–35% during 1960–2000. CLIMBER‐2‐LPJ simulates the land cover contribution to atmospheric CO₂ growth to decrease from 68% during 1900–1960 to 12% in the 1980s. Overall, our simulations show a decline in the relative role of land cover changes for atmospheric CO₂ increase during the last 150 years.     

Leaf carbon isotope ratio and water use efficiency of urban roadside trees in summer in Kyoto city

The effect of summer climate on leaf carbon isotope composition (δ¹³C) of the major roadside tree species Prunus × yedoensis (P. yedoensis) was investigated in Kyoto city, Japan, to explore the implications for alterations in urban environments. Temperature and the vapor pressure deficit were higher at sites of higher traffic volumes, possibly affected by a heat island effect. The leaf δ¹³C of P. yedoensis trees was affected strongly by leaf carbon isotope discrimination (Δ), with much less effect of δ¹³C on atmospheric CO₂. Leaf Δ values in the summer were smaller at sites of higher traffic volumes with high atmospheric temperatures, suggesting a higher long-term water use efficiency (WUE) at these sites. Gas exchange measurements of P. yedoensis leaves indeed suggested a higher intrinsic WUE at sites of higher traffic volumes with high atmospheric temperatures. These results suggest that leaf Δ is related to the response of WUE to summer climates, and that leaf δ¹³C in urban areas is a useful tracer for understanding the influences of urban environments on plant photosynthetic processes.     

Regional differences in the carbon isotopic compositions of teak from two monsoonal regimes of India

Inter and intra-annual carbon isotope compositions (δ¹³C) of several annual growth rings of teak trees from two monsoonal regimes from India were studied and compared with the corresponding oxygen isotopic (δ¹⁸O) variations. In teak from both the regimes, amplitudes of intra-annual δ¹³C were ∼2-3 times lower than that observed in δ¹⁸O. Seasonal cycle with lower δ¹³C values at the middle and higher at ring boundaries was observed for teak from central India, dominated by the southwest monsoon. Positive correlations of intra-annual δ¹³C values with the corresponding δ¹⁸O values of the same rings and with relative humidity (RH) of the concurrent period suggest a dominant role of RH in controlling δ¹³C values of teak from central India. Intra-annual δ¹³C variations of teak from southern India, receiving both the southwest and northeast monsoons, revealed an initial decreasing trend followed by an increasing trend before culminating in depleted ¹³C values at the end of the growing season. No correlation was observed between intra-annual δ¹³C and δ¹⁸O variations of teak trees from southern India. Regional differences in the climatology of δ¹³C of atmospheric CO₂ or the lengths of growing season could be likely reasons for differing intra-annual δ¹³C variations of teak from the two climatic regimes.     

Interpreting species‐specific variation in tree‐ring oxygen isotope ratios among three temperate forest trees

Although considerable variation has been documented in tree‐ring cellulose oxygen isotope ratios (δ¹⁸O_cell) among co‐occurring species, the underlying causes are unknown. Here, we used a combination of field measurements and modelling to investigate the mechanisms behind variations in late‐wood δ¹⁸O_cell (δ¹⁸O_lc) among three co‐occurring species (chestnut oak, black oak and pitch pine) in a temperate forest. For two growing seasons, we quantified among‐species variation in δ¹⁸O_lc, as well as several variables that could potentially cause the δ¹⁸O_lc variation. Data analysis based on the δ¹⁸O_cell model rules out leaf water enrichment (Δ¹⁸O_lw) and tree‐ring formation period (Δt), but highlights source water δ¹⁸O (δ¹⁸O_sw) as an important driver for the measured difference in δ¹⁸O_lc between black and chestnut oak. However, the enriched δ¹⁸O_lc in pitch pine relative to the oaks could not be sufficiently explained by consideration of the above three variables only, but rather, we show that differences in the proportion of oxygen exchange during cellulose synthesis (p_ex) is most likely a key mechanism. Our demonstration of the relevance of some species‐specific features (or lack thereof) to δ¹⁸O_cell has important implications for isotope based ecophysiological/paleoclimate studies.     

Effects of elevated carbon dioxide on stomatal characteristics and carbon isotope ratio of Arabidopsis thaliana ecotypes originating from an altitudinal gradient

Stomatal functioning regulates the fluxes of CO₂ and water vapor between vegetation and atmosphere and thereby influences plant adaptation to their habitats. Stomatal traits are controlled by external environmental and internal cellular signaling. The objective of this study was to quantify the effects of CO₂ enrichment (CE) on stomatal density (SD)‐related properties, guard cell length (GCL) and carbon isotope ratio (δ¹³C) of a range of Arabidopsis thaliana ecotypes originating from a wide altitudinal range [50–1260 m above sea level (asl)], and grown at 400 and 800 ppm [CO₂], and thereby elucidate the possible adaptation and acclimation responses controlling stomatal traits and water use efficiency (WUE). There was a highly significant variation among ecotypes in the magnitude and direction of response of stomatal traits namely, SD and stomatal index (SI) and GCL, and δ¹³C to CE, which represented a short‐term acclimation response. A majority of ecotypes showed increased SD and SI with CE with the response not depending on the altitude of origin. Significant ecotypic variation was shown in all stomatal traits and δ¹³C at each [CO₂]. At 400 ppm, means of SD, SI and GCL for broad altitudinal ranges, i.e. low (<100 m), mid (100–400 m) and high (>400 m), increased with increasing altitude, which represented an adaptation response to decreased availability of CO₂ with altitude. δ¹³C was negatively correlated to SD and SI at 800 ppm but not at 400 ppm. Our results highlight the diversity in the response of key stomatal characters to CE and altitude within the germplasm of A. thaliana and the need to consider this diversity when using A. thaliana as a model plant.     

Assessing the effects of nitrogen deposition and climate on carbon isotope discrimination and intrinsic water‐use efficiency of angiosperm and conifer trees under rising CO2 conditions

The objective of this study is to globally assess the effects of atmospheric nitrogen deposition and climate, associated with rising levels of atmospheric CO₂, on the variability of carbon isotope discrimination (Δ¹³C), and intrinsic water‐use efficiency (iWUE) of angiosperm and conifer tree species. Eighty‐nine long‐term isotope tree‐ring chronologies, representing 23 conifer and 13 angiosperm species for 53 sites worldwide, were extracted from the literature, and used to obtain long‐term time series of Δ¹³C and iWUE. Δ¹³C and iWUE were related to the increasing concentration of atmospheric CO₂ over the industrial period (1850–2000) and to the variation of simulated atmospheric nitrogen deposition and climatic variables over the period 1950–2000. We applied generalized additive models and linear mixed‐effects models to predict the effects of climatic variables and nitrogen deposition on Δ¹³C and iWUE. Results showed a declining Δ¹³C trend in the angiosperm and conifer species over the industrial period and a 16.1% increase of iWUE between 1850 and 2000, with no evidence that the increased rate was reduced at higher ambient CO₂ values. The temporal variation in Δ¹³C supported the hypothesis of an active plant mechanism that maintains a constant ratio between intercellular and ambient CO₂ concentrations. We defined linear mixed‐effects models that were effective to describe the variation of Δ¹³C and iWUE as a function of a set of environmental predictors, alternatively including annual rate (N_rate) and long‐term cumulative (N_cum) nitrogen deposition. No single climatic or atmospheric variable had a clearly predominant effect, however, Δ¹³C and iWUE showed complex dependent interactions between different covariates. A significant association of N_rate with iWUE and Δ¹³C was observed in conifers and in the angiosperms, and N_cum was the only independent term with a significant positive association with iWUE, although a multi‐factorial control was evident in conifers.     

Increased water‐use efficiency during the 20th century did not translate into enhanced tree growth

Aim The goals of this study are: (1) to determine whether increasing atmospheric CO₂ concentrations and changing climate increased intrinsic water use efficiency (iWUE, as detected by changes in Δ¹³C) over the last four decades; and if it did increase iWUE, whether it led to increased tree growth (as measured by tree‐ring growth); (2) to assess whether CO₂ responses are biome dependent due to different environmental conditions, including availability of nutrients and water; and (3) to discuss how the findings of this study can better inform assumptions of CO₂ fertilization and climate change effects in biospheric and climate models. Location A global range of sites covering all major forest biome types. Methods The analysis encompassed 47 study sites including boreal, wet temperate, mediterranean, semi‐arid and tropical biomes for which measurements of tree ring Δ¹³C and growth are available over multiple decades. Results The iWUE inferred from the Δ¹³C analyses of comparable mature trees increased 20.5% over the last 40 years with no significant differences between biomes. This increase in iWUE did not translate into a significant overall increase in tree growth. Half of the sites showed a positive trend in growth while the other half had a negative or no trend. There were no significant trends within biomes or among biomes. Main conclusions These results show that despite an increase in atmospheric CO₂ concentrations of over 50 p.p.m. and a 20.5% increase in iWUE during the last 40 years, tree growth has not increased as expected, suggesting that other factors have overridden the potential growth benefits of a CO₂‐rich world in many sites. Such factors could include climate change (particularly drought), nutrient limitation and/or physiological long‐term acclimation to elevated CO₂. Hence, the rate of biomass carbon sequestration in tropical, arid, mediterranean, wet temperate and boreal ecosystems may not increase with increasing atmospheric CO₂ concentrations as is often implied by biospheric models and short‐term elevated CO₂ experiments.     

Inter-annual variation of tree-ring width, δ13C and δ18O in juvenile trees of five plantation poplar cultivars (Populus spp.)

The 10-year juvenile records of three hybrid poplar and two aspen cultivars (Populus spp.) from a short rotation coppice (SRC) were assessed by measuring tree-ring width (annual radial increment, i_r) and stable isotope ratios of carbon and oxygen (δ¹³C and δ¹⁸O) of α-cellulose. All cultivars showed common ‘juvenile trends’ that were modeled with nonlinear fit (NLF) functions. The i_r of all cultivars culminated in the middle of the juvenile phase. Within the first ten years, δ¹³C showed a gradual decrease of approximately 2.5 ‰ in all cultivars and δ¹⁸O showed an asymptotic increase which was variable among the poplar cultivars and which was more pronounced in two hybrid poplars. Potential causes of the juvenile inter-annual variability of δ¹³C and δ¹⁸O were discussed. Likely, the maturation related changes in hydraulic architecture, the canopy closure and the resulting increase of the proportion of shaded crown segments which have lower photosynthetic capacities had an effect on δ¹³C. An additional effect of changes in N nutrition on δ¹³C is assumed at the present SRC trial because the NO₃⁻ concentration had significantly decreased after ten years. Interpretation of δ¹⁸O data remained difficult given the lack of soil water δ¹⁸O records.The maturation effect and the respective interaction have resulted in large temporal variability in the present three investigated tree ring traits. However, the impact of two drought vegetation periods (2003 and 2006) was still reflected by the juvenile tree ring records of all traits. Different juvenile trends in the records of the stable isotope ratios δ¹³C and δ¹⁸O, and the trend slopes, which may vary between genetically different cultivars, must be considered in tree ring investigations of SRC poplars.     

Carbon isotope ratios of photolithotrophs from allt meall nan damh,a burn at ardeonaig,Perthshire, and their ecophysiological significance

Summary

δ¹³C measurements were made of dissolved inorganic C, and of submerged benthic cyanobacteria, algae and bryophytes, from Allt Meall nan Damh, a burn at Ardeonaig, Perthshire. The δ¹³C of the CO₂, HCO3/- and CO3/2- components of the inorganic C were computed, and the Δ values of the organic C in the photolithotrophs were then calculated relative to dissolved CO₂. The decreasing order of A values in the Ardeonaig Burn is Lemanea and bryophytes ≥ green macroalgae and Audouinella > diatom mats, which is the same as in the Dighty Burn. However, the Δ values of Lemanea and the bryophytes, which depend on diffusive CO₂ entry, are lower at Ardeonaig than in the Dighty Burn, suggesting greater diffusive limitation to photosynthesis in the Ardeonaig Burn. It is not easy to relate this difference in Δ values in Lemanea to the higher C:N atomic ratio in the Ardeonaig Burn (21.2 ± 0.64) than in the Dighty Burn (9.5–11.0). The Δ values relative to HCO3/- for the HCO3/--using diatom mat in the Ardeonaig Burn is also lower than that in the Dighty Burn; this is consistent with a greater diffusion limitation of photosynthesis in the thicker mats in the Ardeonaig Burn. The δ¹³C of a Lyngbya mat overlying a Lemanea population stranded by low summer water levels indicates that some of the C fixed by the HCO3/--using Lyngbya comes from respiration of low-δ¹³C inorganic C by the Lemanea which is shaded by the Lyngbya. The δ¹³C values of Mesotaenium in its mucilage sheath on a thinly vegetated bank is suggestive of predominant use of the higher CO₂ concentrations with lower δ¹³C from groundwater rather than of atmospheric CO₂ yielding lower dissolved CO₂ concentrations with a higher δ¹³C value.     

Foliar and litter needle carbon and oxygen isotope compositions relate to tree growth of an exotic pine plantation under different residue management regimes in subtropical Australia

Background and aim

Significant differences in tree growth were observed in an exotic pine plantation under different harvest residue management regimes at ages 2–10 years. However, the variations in tree growth between residue management treatments could not be explained by soil and foliar nutrient analyses, except by potassium (K) concentration. Therefore, this study determined the carbon isotope composition (δ¹³C) and oxygen isotope composition (δ¹⁸O) of current and archived foliar samples from the exotic pine plantation to establish relationships with foliar K concentration and tree growth indices as a means to determine changes in stomatal conductance (g_s) and photosynthetic rate (A_max) or water use efficiency (WUE), and therefore understand the variations in tree growth across treatments.

Methods

The harvest residue treatments were: (1) residue removal, RR₀; (2) single level residue retention, RR₁; and (3) double level residue retention, RR₂. Foliar δ¹³C and δ¹⁸O were determined for samples at ages 2, 4, 6 and 10 years, and the atmospheric ¹³C discrimination (Δ¹³C), intercellular CO₂ concentration (C_i) and WUE were determined from the δ¹³C data. Litter needle δ¹³C and δ¹⁸O were also determined over 15 months between ages 9 and 10 years. These parameters or variables where correlated to each other as well as to the periodic mean annual increment of basal area (PAIB) and the periodic mean annual increment of tree diameter at breast height (PAID) across the treatments and over time. Foliar δ¹³C and δ¹⁸O were also related to published data of foliar K concentrations of the same trees.

Results

Significant variations of foliar δ¹³C, and therefore WUE and Δ¹³C, across treatments were only observed at ages 4 and 10 years old, and foliar δ¹⁸O at age 4 years old only. The results showed increasing foliar δ¹³C, δ¹⁸O and WUE, and decreasing Δ¹³C and Ci, from RR₀ to RR₂ treatments. However, while the WUE was positively related to the PAID and PAIB at age 4 years, it was negatively related to PAID and PAIB at age 10 years old. Litter needle δ¹³C, indicative of WUE, was also negatively related to the PAID at age 10 years old. . At age 4 years, foliar δ¹³C and δ¹⁸O were positively related with a steep slope of 7.70 ‰ across treatments, and that both isotopes were positively related to foliar K concentrations. Similarly, δ¹⁸O was negatively related to the Δ¹³C. No significant relationship can be determined between foliar δ¹³C, or Δ¹³C, and δ¹⁸O at age 10 years old. In addition, WUE was increasing (p?<?0.001) and Δ¹³C and C_i decreasing (p?<?0.001) with decreasing PAID over time.

Conclusions

The variations at age 4 years in foliar δ¹³C or Δ¹³C and δ¹⁸O and increasing WUE with increasing growth rate suggest growth induced water-stress with increasing residue-loading rate as a result of the nutritional effect of the harvest residues on tree growth. At age 10 years, the negative relationships between WUE and PAID indicate nutrient limitation has an over-riding effect on δ¹³C variations rather than g_s. This was due to the lack of a significant relationship between foliar Δ¹³C and δ¹⁸O at this age, as well as over time.     

Changes in leaf photosynthetic characteristics and water use efficiency along with tree height of 4 tree species

As main photosynthetic organs, leaves are very sensitive to exterior environments. Water deficiency obviously affects the biological and physiological characteristics of leaves. Xylem pathways increase when trees grow tall, which results in the increase in water gravity as well as pathway resistance. Accordingly, the physiological characteristics of leaves change along with tree height. In this research, the photosynthetic characteristics and carbon isotope ratio (δ¹³C) in the leaves of 4 tree species, Platanus hispanica, Robinia pseudoacacia, Fraxinus chinensis and Ginkgo biloba, were measured. The results showed that the leaf photosynthetic rate (Pn), transpiration rate (Tr), stomatal conductance (Cond) and internal CO₂ concentration (Ci) reduced along with tree height, while the leaf δ¹³C increased along with tree height. The One Way ANOVA and LSD tests showed that the leaf photosynthetic characteristics and δ¹³C varied significantly at different tree heights (P < 0.05). The decrease in leaf photosynthetic capability and the increase in δ¹³C along with tree height indicate that the leaves at the tree tops suffer from water stress. These results support the hydraulic limitation hypothesis.     

Root exclusion through trenching does not affect the isotopic composition of soil CO2 efflux

Disentangling the autotrophic and heterotrophic components of soil CO₂ efflux is critical to understanding the role of soil system in terrestrial carbon (C) cycling. In this study, we combined a stable C-isotope natural abundance approach with the trenched plot method to determine if root exclusion significantly affected the isotopic composition (δ¹³C) of soil CO₂ efflux (R_S). This study was performed in different forest ecosystems: a tropical rainforest and two temperate broadleaved forests, where trenched plots had previously been installed. At each site, R_S and its δ¹³C (δ¹³C_Rs) tended to be lower in trenched plots than in control plots. Contrary to R_S, δ¹³C_Rs differences were not significant. This observation is consistent with the small differences in δ¹³C measured on organic matter from root, litter and soil. The lack of an effect on δ¹³C_Rs by root exclusion could be from the small difference in δ¹³C between autotrophic and heterotrophic soil respirations, but further investigations are needed because of potential artefacts associated with the root exclusion technique.     

The dendroclimatic value of oak stable isotopes

Tree-ring stable oxygen and carbon isotope ratios (δ¹⁸O and δ¹³C) are an important archive for climate reconstructions. However, it remains unclear whether the polyvinyl acetate emulsion, often used for the preservation and fixation of wood samples, influences δ¹⁸O and δ¹³C signals. Further uncertainties are associated with the possible effects of geographical origin and cambial age of historical samples. Here, we present annually-resolved and absolutely-dated δ¹⁸O and δ¹³C measurements of 21 living oaks (Quercus robur and Q. petraea) from the Czech Republic. We find that the δ¹⁸O and δ¹³C signals in the extracted alpha-cellulose are not affected by polyvinyl acetate treatment. Covering the entire 20^th century and reaching until 2018 CE, our dataset reveals spatial and temporal coherency within and between the individual δ¹⁸O and δ¹³C chronologies of different oak species, sample locations, and tree ages. Highly significant (p < 0.01) Pearson’s correlation coefficients of the site-specific δ¹³C and δ¹⁸O chronologies range from 0.48–0.77 and 0.36–0.56, respectively. The isotopic inter-series correlations of Q. robur and Q. petraea from the same site are 0.75 and 0.43 for the mean δ¹³C and δ¹⁸O values, respectively. Significant (p < 0.01) correlations of 0.49 and 0.84 are found for δ¹³C and δ¹⁸O, respectively, when all measurements from all sampling locations and tree ages are included. Our study shows that non-pooled oak δ¹⁸O and δ¹³C measurements from both species, different locations, and diverse tree ages can be combined into robust isotopic chronologies for climate reconstructions.     

Species-specific differences in temporal and spatial variation in δ13C of plant carbon pools and dark-respired CO2 under changing environmental conditions

Stable carbon isotope signatures are often used as tracers for environmentally driven changes in photosynthetic δ¹³C discrimination. However, carbon isotope signatures downstream from carboxylation by Rubisco are altered within metabolic pathways, transport and respiratory processes, leading to differences in δ¹³C between carbon pools along the plant axis and in respired CO₂. Little is known about the within-plant variation in δ¹³C under different environmental conditions or between species. We analyzed spatial, diurnal, and environmental variations in δ¹³C of water soluble organic matter (δ¹³C_WSOM) of leaves, phloem and roots, as well as dark-respired δ¹³CO₂ (δ¹³C_res) in leaves and roots. We selected distinct light environments (forest understory and an open area), seasons (Mediterranean spring and summer drought) and three functionally distinct understory species (two native shrubs—Halimium halimifolium and Rosmarinus officinalis—and a woody invader—Acacia longifolia). Spatial patterns in δ¹³C_WSOM along the plant vertical axis and between respired δ¹³CO₂ and its putative substrate were clearly species specific and the most δ¹³C-enriched and depleted values were found in δ¹³C of leaf dark-respired CO₂ and phloem sugars, ~?15 and ~?33 ‰, respectively. Comparisons between study sites and seasons revealed that spatial and diurnal patterns were influenced by environmental conditions. Within a species, phloem δ¹³C_WSOM and δ¹³C_res varied by up to 4 ‰ between seasons and sites. Thus, careful characterization of the magnitude and environmental dependence of apparent post-carboxylation fractionation is needed when using δ¹³C signatures to trace changes in photosynthetic discrimination.