Interseasonal comparison of CO2 concentrations, isotopic composition, and carbon dynamics in an Amazonian rainforest (French Guiana)

Canopy CO₂ concentrations in a tropical rainforest in French Guiana were measured continuously for 5 days during the 1994 dry season and the 1995 wet season. Carbon dioxide concentrations ([CO₂]) throughout the canopy (0.02–38 m) showed a distinct daily pattern, were well-stratified and decreased with increasing height into the canopy. During both seasons, daytime [CO₂] in the upper and middle canopy decreased on average 7–10 μmol mol⁻¹ below tropospheric baseline values measured at Barbados. Within the main part of the canopy (≥ 0.7 m), [CO₂] did not differ between the wet and dry seasons. In contrast, [CO₂] below 0.7 m were generally higher during the dry season, resulting in larger [CO₂] gradients. Supporting this observation, soil CO₂ efflux was on average higher during the dry season than during the wet season, either due to diffusive limitations and/or to oxygen deficiency of root and microbial respiration. Soil respiration rates decreased by 40% after strong rain events, resulting in a rapid decrease in canopy [CO₂] immediately above the forest floor of about 50␣μmol mol⁻¹. Temporal and spatial variations in [CO₂]_canopy were reflected in changes of δ¹³C_canopy and δ¹⁸O_canopy values. Tight relationships were observed between δ¹³C and δ¹⁸O of canopy CO₂ during both seasons (r ² > 0.86). The most depleted δ¹³C_canopy and δ¹⁸O_canopy values were measured immediately above the forest floor (δ¹³C = −16.4‰; δ¹⁸O = 39.1‰ SMOW). Gradients in the isotope ratios of CO₂ between the top of the canopy and the forest floor ranged between 2.0‰ and 6.3‰ for δ¹³C, and between 1.0‰ and 3.5‰ for δ¹⁸O. The δ¹³C_leaf and calculated c _i/c _a of foliage at three different positions were similar for the dry and wet seasons indicating that the canopy maintained a constant ratio of photosynthesis to stomatal conductance. About 20% of the differences in δ¹³C_leaf within the canopy was accounted for by source air effects, the remaining 80% must be due to changes in c _i/c _a. Plotting 1/[CO₂] vs. the corresponding δ¹³C ratios resulted in very tight, linear relationships (r ² = 0.99), with no significant differences between the two seasons, suggesting negligible seasonal variability in turbulent mixing relative to ecosystem gas exchange. The intercepts of these relationships that should be indicative of the δ¹³C of respired sources were close to the measured δ¹³C of soil respired CO₂ and to the δ¹³C of litter and soil organic matter. Estimates of carbon isotope discrimination of the entire ecosystem, Δ_e, were calculated as 20.3‰ during the dry season and as 20.5‰ during the wet season. Received: 3 March 1996 / Accepted: 19 October 1996     

Leaf δ13C in Pinus resinosa trees and understory plants: variation associated with light and CO2 gradients

 Our objective was to evaluate the relative importance of gradients in light intensity and the isotopic composition of atmospheric CO₂ for variation in leaf carbon isotope ratios within a Pinus resinosa forest. In addition, we measured photosynthetic gas exchange and leaf carbon isotope ratios on four understory species (Dryopteris carthusiana, Epipactus helleborine, Hieracium floribundum, Rhamnus frangula), in order to estimate the consequence of the variation in the understory light microclimate for carbon gain in these plants. During midday, CO₂ concentration was relatively constant at vertical positions ranging from 15 m to 3 m above ground. Only at positions below 3 m was CO₂ concentration significantly elevated above that measured at 15 m. Based on the strong linear relationship between changes in CO₂ concentration and δ¹³C values for air samples collected during a diurnal cycle, we calculated the expected vertical profile for the carbon isotope ratio of atmospheric CO₂ within the forest. These calculations indicated that leaves at 3 m height and above were exposed to CO₂ of approximately the same isotopic composition during daylight periods. There was no significant difference between the daily mean δ¹³C values at 15 m (–7.77‰) and 3 m (–7.89‰), but atmospheric CO₂ was significantly depleted in ¹³C closer to the ground surface, with daily average δ¹³C values of –8.85‰ at 5 cm above ground. The light intensity gradient in the forest was substantial, with average photosynthetically active radiation (PAR) on the forest floor approximately 6% of that received at the top of the canopy. In contrast, there were only minor changes in air temperature, and so it is likely that the leaf-air vapour pressure difference was relatively constant from the top of the canopy to the forest floor. For red pine and elm tree samples, there was a significant correlation between leaf δ¹³C value and the height at which the leaf sample was collected. Leaf tissue sampled near the forest floor, on average, had lower δ¹³C values than samples collected near the top of the canopy. We suggest that the average light intensity gradient through the canopy was the major factor influencing vertical changes in tree leaf δ¹³C values. In addition, there was a wide range of variation (greater than 4‰) among the four understory plant species for average leaf δ¹³C values. Measurements of leaf gas exchange, under natural light conditions and with supplemental light, were used to estimate the influence of the light microclimate on the observed variation in leaf carbon isotope ratios in the understory plants. Our data suggest that one species, Epipactus helleborine, gained a substantial fraction of carbon during sunflecks. Received: 21 March 1996 / Accepted: 13 August 1996     

Influence of vegetation and soil CO2 exchange on the concentration and stable oxygen isotope ratio of atmospheric CO2 within a Pinus resinosa canopy

Measurements were made of the concentration and stable oxygen isotopic ratio of carbon dioxide in air samples collected on a diurnal basis at two heights within a Pinus resinosa canopy. Large changes in CO₂ concentration and isotopic composition were observed during diurnal time courses on all three symple dates. In addition, there was strong vertical stratification in the forest canopy, with higher CO₂ concentrations and more negative ¹⁸O values observed closer to the soil surface. The observed daily increases in ¹⁸O values of forest CO₂ were dependent on relative humidity consistent with the modelled predictions of isotopic fractionation during photosynthetic gas exchange. During photosynthetic gas exchange, a portion of the CO₂ that enters the leaf and equilibrates with leaf water is not fixed and diffuses back out of the leaf with an altered oxygen isotopic ratio. The oxygen isotope ratio of CO₂ diffusing out of a leaf depends primarily on the ¹⁸O content of leaf water which changes in response to relative humidity. In contrast, soil respiration caused a decline in the ¹⁸O values of forest CO₂ at night, because CO₂ released from the soil has equilibrated with soil water which has a lower ¹⁸O content than leaf water. The observed relationship between diurnal changes in CO₂ concentration and oxygen isotopic composition in the forest environment were consistent with a gas mixing model that considered the relative magnitudes of CO₂ fluxes associated with photosynthesis, respiration and turbulent exchange between the forest and the bulk atmosphere.     

A free-air system for long-term stable carbon isotope labeling of adult forest trees

Stable carbon (C) isotopes, in particular employed in labeling experiments, are an ideal tool to broaden our understanding of C dynamics in trees and forest ecosystems. Here, we present a free-air exposure system, named isoFACE, designed for long-term stable C isotope labeling in the canopy of 25 m tall forest trees. Labeling of canopy air was achieved by continuous release of CO₂ with a δ¹³C of −46.9‰. To this end, micro-porous tubes were suspended at c. 1 m distance vertically through the canopy, minimizing CO₂ gradients from the exterior to the interior and allowing for C labeling exposure during periods of low wind speed. Target for CO₂ concentration ([CO₂]) increase was ambient +100 μmol mol⁻¹. Canopy [CO₂] stayed within 10% of the target during more than 57% of the time and resulted in a drop of δ¹³C in canopy air by 7.8‰. After 19 labeling days about 50% of C in phloem sugars and stem CO₂ efflux were turned over and 20–30% in coarse root CO₂ efflux and soil CO₂. The isoFACE system successfully altered δ¹³C of canopy air for studying turn-over of C pools in forest trees and soils, highlighting their slow turn-over rates.     

Comparison of leaf water use efficiency of oak and sycamore in the canopy over two growing seasons

The seasonal trends in water use efficiency of sun and shade leaves of mature oak (Quercus robur) and sycamore (Acer pseudoplatanus) trees were assessed in the upper canopy of an English woodland. Intrinsic water use efficiency (net CO₂ assimilation rate/leaf conductance, A/g) was measured by gas exchange and inferred from C isotope discrimination (δ¹³C) methods. Shade leaves had consistently lower δ¹³C than sun leaves (by 1–2‰), the difference being larger in sycamore. Buds had distinct sun and shade isotopic signatures before bud break and received an influx of ¹³C-rich C before becoming net autotrophs. After leaf full expansion, δ¹³C declined by 1–2‰ gradually through the season, emphasising the importance of imported carbon in the interpretation of leaf δ¹³C values in perennial species. There was no significant difference between the two species in the value of intrinsic water use efficiency for either sun or shade leaves. For sun leaves, season-long A/g calculated from δ¹³C (72–78 μmol CO₂ [mol H₂O]⁻¹) was 10–16% higher than that obtained from gas exchange and in situ estimates of leaf boundary layer conductance. For shade leaves, the gas exchange–derived values were low, only 10–18% of the δ¹³C-derived values. This is ascribed to difficulties in obtaining a comprehensive sample of gas exchange measurements in the rapidly changing light environment.     

Biological and physical influences on the carbon isotope content of CO<Subscript>2</Subscript> in a subalpine forest snowpack,Niwot Ridge,Colorado

Considerable research has recently been devoted to understanding biogeochemical processes under winter snow cover, leading to enhanced appreciation of the importance of many winter ecological processes. In this study, a comprehensive investigation of the stable carbon isotope composition (δ¹³C) of CO₂ within a high-elevation subalpine forest snowpack was conducted. Our goals were to study the δ¹³C of biological soil respiration under snow in winter, and to assess the relative importance of diffusion and advection (ventilation by wind) for gas transport within snow. In agreement with other studies, we found evidence of an active microbial community under a roughly 1-m deep snowpack during winter and into spring as it melted. Under-snow CO₂ mole fractions were observed up to 3,500 μmol mol⁻¹, and δ¹³C of CO₂ varied from ~−22 to ~−8‰. The δ¹³C of soil respiration calculated from mixing relationships was −26 to −24‰, and although it varied in time, it was generally close to that of the bulk organic horizon (−26.0‰). Subnivean CO₂ and δ¹³C were quite dynamic in response to changes in soil temperature, liquid water availability, and wind events. No clear biologically-induced isotopic changes were observed during periods when microbial activity and root/rhizosphere activity were expected to vary, although such changes cannot be eliminated. There was clear evidence of isotopic enrichment associated with diffusive transport as predicted by theory, but simple diffusive enrichment (4.4‰) was not observed. Instead, ventilation of the snowpack by sustained wind events in the forest canopy led to changes in the diffusively-enriched gas profile. The isotopic influence of diffusion on gases in the snowpack and litter was greatest at greater depths, due to the decreased relative contribution of advection at depth. There were highly significant correlations between the apparent isotopic content of respiration from the soil with wind speed and pressure. In summary, physical factors influencing gas transport substantially modified and potentially obscured biological factors in their effects on δ¹³C of CO₂ within this subalpine forest snowpack.     

18O composition of CO2 and H2O ecosystem pools and fluxes in a tallgrass prairie: Simulations and comparisons to measurements

In this paper we describe measurements and modeling of ¹⁸O in CO₂ and H₂O pools and fluxes at a tallgrass prairie site in Oklahoma. We present measurements of the δ¹⁸O value of leaf water, depth‐resolved soil water, atmospheric water vapor, and Keeling plot δ¹⁸O intercepts for net soil‐surface CO₂ and ecosystem CO₂ and H₂O fluxes during three periods of the 2000 growing season. Daytime discrimination against C¹⁸OO, as calculated from measured above‐canopy CO₂ and δ¹⁸O gradients, is also presented. To interpret the isotope measurements, we applied an integrated land‐surface and isotope model (ISOLSM) that simulates ecosystem H₂¹⁸O and C¹⁸OO stocks and fluxes. ISOLSM accurately predicted the measured isotopic composition of ecosystem water pools and the δ¹⁸O value of net ecosystem CO₂ and H₂O fluxes. Simulations indicate that incomplete equilibration between CO₂ and H₂O within C₄ plant leaves can have a substantial impact on ecosystem discrimination. Diurnal variations in the δ¹⁸O value of above‐canopy vapor had a small impact on the predicted δ¹⁸O value of ecosystem water pools, although sustained differences had a large impact. Diurnal variations in the δ¹⁸O value of above‐canopy CO₂ substantially affected the predicted ecosystem discrimination. Leaves dominate the ecosystem ¹⁸O‐isoflux in CO₂ during the growing season, while the soil contribution is relatively small and less variable. However, interpreting daytime measurements of ecosystem C¹⁸OO fluxes requires accurate predictions of both soil and leaf ¹⁸O‐isofluxes.     

The carbon isotope ratio of plant organic material reflects temporal and spatial variations in CO2 within tropical forest formations in Trinidad

Summary A method of monitoring and collecting CO₂ samples in the field has been developed which has been used to study both temporal and spatial variations in canopy CO₂ isotopic signatures in two contrasting tropical forest formations in Trinidad. These have been related to vertical gradients in the carbon isotope ratio (¹³C) of organic material in conjunction with measurements of other environmental parameters. The ¹³C of leaf material from two canopies showed a gradient with respect to height, more negative values being found low in the understorey. The deciduous secondary forest, (Simla) showed a difference of 4.6 and the semi-evergreen seasonal canopy (Aripo), 2.8. The range of ¹³C values at Simla was 4 less negative than those at Aripo. In order to relate these measurements to the interaction between diffusion or carboxylation limitation, and source CO₂ effects, variations in environmental parameters through the canopy have been compared with changes in CO₂ partial pressure (P _a) and isotopic composition ¹³C throughout the day during the dry season. Values of P _a20 m above the ground at Aripo varied from 380 vpm at dawn to 340 vpm at midday, at which time the partial pressure 15 cm above the ground was 375 vpm. The CO₂ partial pressure did not stabilise during the course of the day, and there was good correlation (r ²=0.82) between _a and P _a, with more negative values of _a occuring in the understorey. Diuraal changes of 2 were evident at all canopy positions. In the more open canopy at Simla, these gradients were similar, but less marked. Leaf-air vapour pressure deficit (VPD) showed no relationship with height, possibly as a result of minimal water flux from both the soil and the canopy due to low soil water content; VPD was 1.5 kPa higher at midday than dawn. A 3° C temperature gradient between the understorey and upper canopy was observed at Aripo but not in the more open Simla canopy. CO₂ partial pressure stabilised for only 4 h in the middle of the day, while other parameters showed no stable period. The proportion of floor respired CO₂ reassimilated at Aripo has been calculated as 26%, 19%, and 8% for the periods 0600–1000, 1000–1400, and 1400–1800 hours. In order to quantify source CO₂ effects, measurements of the environmental parameters and assimilation rate must be made at all canopy positions and throughout the day.     

Biogeochemical properties of a tropical swamp forest ecosystem in southern Thailand

The distributions of organic matter in the tropical swamps in southern Thailand are reported. The concentrations of particulate and dissolved organic carbon (POC and DOC) in the Bang Nara River, which drains swamp forests and nearby paddy fields, were 2.9 ± 2.0 and 6.2 ± 1.3 mg C l⁻¹, respectively. Although the variation was large, DOC concentration in the Bang Nara River seemed to be higher than POC in November 1992 (DOC/POC ratio, 2.8 ± 2.2). River waters from the upland areas were characterized by low POC and DOC concentrations as compared with Bang Nara River water. The δ¹³C values of POC and river sediments were useful to distinguish between organic matter originating in upland and swamp areas. It is suggested that the distributions of organic matter and its isotopic composition reflect the difference in drainage characteristics between lowland swamp and upland areas. Isotopic analyses of plant leaves and soils revealed that the swamp forest ecosystems were characterized by low δ¹³C and low δ¹⁵N values, which suggested low efficiency of water use by plants and large contributions of atmospheric deposition of nitrogen, respectively. Although CO₂ recycling in the forest might be an important factor determining the δ¹³C values of understory plants, the main process in carbon metabolism of tropical swamp forests would be CO₂ exchange between the atmosphere and forest canopy. Received: May 1, 2001 / Accepted: September 28, 2001     

Influence of stand structure on carbon-13 of vegetation, soils, and canopy air within deciduous and evergreen forests in Utah, United States

Carbon isotope ratios (δ¹³C) were studied in evergreen and deciduous forest ecosystems in semi-arid Utah (Pinus contorta, Populus tremuloides, Acer negundo and Acer grandidentatum). Measurements were taken in four to five stands of each forest ecosystem differing in overstory leaf area index (LAI) during two consecutive growing seasons. The δ¹³C_leaf (and carbon isotope discrimination) of understory vegetation in the evergreen stands (LAI 1.5–2.2) did not differ among canopies with increasing LAI, whereas understory in the deciduous stands (LAI 1.5–4.5) exhibited strongly decreasing δ¹³C_leaf values (increasing carbon isotope discrimination) with increasing LAI. The δ¹³C values of needles and leaves at the top of the canopy were relatively constant over the entire LAI range, indicating no change in intrinsic water-use efficiency with overstory LAI. In all canopies, δ¹³C_leaf decreased with decreasing height above the forest floor, primarily due to physiological changes affecting c _i/c _a (> 60%) and to a minor extent due to δ¹³C of canopy air (< 40%). This intra-canopy depletion of δ¹³C_leaf was lowest in the open stand (1‰) and greatest in the denser stands (4.5‰). Although overstory δ¹³C_leaf did not change with canopy LAI, δ¹³C of soil organic carbon increased with increasing LAI in Pinus contorta and Populus tremuloides ecosystems. In addition, δ¹³C of decomposing organic carbon became increasingly enriched over time (by 1.7–2.9‰) for all deciduous and evergreen dry temperate forests. The δ¹³C_canopy of CO₂ in canopy air varied temporally and spatially in all forest stands. Vertical canopy gradients of δ¹³C_canopy, and [CO₂]_canopy were larger in the deciduous Populus tremuloides than in the evergreen Pinu contorta stands of similar LAI. In a very wet and cool year, ecosystem discrimination (Δ_e) was similar for both deciduous Populus tremulodies (18.0 ± 0.7‰) and evergreen Pinus contorta (18.3 ± 0.9‰) stands. Gradients of δ¹³C_canopy and [CO₂]_canopy were larger in denser Acer spp. stands than those in the open stand. However, ¹³C enrichment above and photosynthetic draw-down of [CO₂]_canopy below tropospheric baseline values were larger in the open than in the dense stands, due to the presence of a vigorous understory vegetation. Seasonal patterns of the relationship δ¹³C_canopy versus 1/[CO₂]_canopy were strongly influenced by precipitation and air temperature during the growing season. Estimates of Δ_e for Acer spp. did not show a significant effect of stand structure, and averaged 16.8 ± 0.5‰ in 1933 and 17.4 ± 0.7‰ in 1994. However, Δ_e varied seasonally with small fluctuations for the open stand (2‰), but more pronounced changes for the dense stand (5‰). Received: 15 April 1996 / Accepted: 19 October 1996     

Contributions of evaporation, isotopic non-steady state transpiration and atmospheric mixing on the delta18O of water vapour in Pacific Northwest coniferous forests

Changes in the ²H and ¹⁸O of atmospheric water vapour provide information for integrating aspects of gas exchange within forest canopies. In this study, we show that diurnal fluctuations in the oxygen isotope ratio (δ¹⁸O) as high as 4‰ were observed for water vapour (δ¹⁸O_vp) above and within an old‐growth coniferous forest in the Pacific Northwest region of the United States. Values of δ¹⁸O_vp decreased in the morning, reached a minimum at midday, and recovered to early‐morning values in the late afternoon, creating a nearly symmetrical diurnal pattern for two consecutive summer days. A mass balance budget was derived and assessed for the ¹⁸O of canopy water vapour over a 2‐d period by considering the ¹⁸O‐isoflux of canopy transpiration, soil evaporation and the air entering the canopy column. The budget was used to address two questions: (1) do δ¹⁸O values of canopy water vapour reflect the biospheric influence, or are such signals swamped by atmospheric mixing? and (2) what mechanisms drive temporal variations of δ¹⁸O_vp? Model calculations show that the entry of air into the canopy column resulted in an isotopically depleted ¹⁸O‐isoflux in the morning of day 1, causing values of δ¹⁸O_vp to decrease. An isotopically enriched ¹⁸O‐isoflux resulting from transpiration then offset this decreased δ¹⁸O_vp later during the day. Contributions of ¹⁸O‐isoflux from soil evaporation were relatively small on day 1 but were more significant on day 2, despite the small H₂¹⁶O fluxes. From measurements of leaf water volume and sapflux, we determined the turnover time of leaf water in the needles of Douglas‐fir trees as ≈ 11 h at midday. Such an extended turnover time suggests that transpiration may not have occurred at the commonly assumed isotopic steady state. We tested a non‐steady state model for predicting δ¹⁸O of leaf water. Our model calculations show that assuming isotopic steady state increased isoflux of transpiration. The impact of this increase on the modelled δ ¹⁸O_vp was clearly detectable, suggesting the importance of considering isotopic non‐steady state of transpiration in studies of forest ¹⁸O water balance.     

Functional diversity in an Amazonian rainforest of French Guyana: a dual isotope approach (δ15N and δ13C)

Functional aspects of biodiversity were investigated in a lowland tropical rainforest in French Guyana (5°2′N, annual precipitation 2200 mm). We assessed leaf δ¹⁵N as a presumptive indicator of symbiotic N₂ fixation, and leaf and wood cellulose δ¹³C as an indicator of leaf intrinsic water-use efficiency (CO₂ assimilation rate/leaf conductance for water vapour) in dominant trees of 21 species selected for their representativeness in the forest cover, their ecological strategy (pioneers or late successional stage species, shade tolerance) or their potential ability for N₂ fixation. Similar measurements were made in trees of native species growing in a nearby plantation after severe perturbation (clear cutting, mechanical soil disturbance). Bulk soil δ¹⁵N was spatially quite uniform in the forest (range 3–5‰), whereas average leaf δ¹⁵N ranged from −0.3‰ to 3.5‰ in the different species. Three species only, Diplotropis purpurea, Recordoxylon speciosum (Fabaceae), and Sclerolobium melinonii (Caesalpiniaceae), had root bacterial nodules, which was also associated with leaf N concentrations higher than 20 mg g⁻¹. Although nodulated trees displayed significantly lower leaf δ¹⁵N values than non-nodulated trees, leaf δ¹⁵N did not prove a straightforward indicator of symbiotic fixation, since there was a clear overlap of δ¹⁵N values for nodulated and non-nodulated species at the lower end of the δ¹⁵N range. Perturbation did not markedly affect the difference δ¹⁵N_soil−δ¹⁵N_leaf, and thus the isotopic data provide no evidence of an alteration in the different N acquisition patterns. Extremely large interspecific differences in sunlit leaf δ¹³C were observed in the forest (average values from −31.4 to −26.7‰), corresponding to intrinsic water-use efficiencies (ratio CO₂ assimilation rate/leaf conductance for water vapour) varying over a threefold range. Wood cellulose δ¹³C was positively related to total leaf δ¹³C, the former values being 2–3‰ higher than the latter ones. Leaf δ¹³C was not related to leaf δ¹⁵N at either intraspecific or interspecific levels. δ¹³C of sunlit leaves was highest in shade hemitolerant emergent species and was lower in heliophilic, but also in shade-tolerant species. For a given species, leaf δ¹³C did not differ between the pristine forest and the disturbed plantation conditions. Our results are not in accord with the concept of existence of functional types of species characterized by common suites of traits underlying niche differentiation; rather, they support the hypothesis that each trait leads to a separate grouping of species. Received: 18 August 1997 / Accepted: 14 April 1998     

Variations in leaf β13C along a vertical profile of irradiance in a temperate Japanese forest

The vertical profile of stable carbon isotope ratios (δ¹³C) of leaves was analyzed for 13 tree species in a cool-temperate deciduous forest in Japan. The vertical distribution of long-term averaged δ¹³C in atmospheric CO₂ (δ_a) was estimated from δ¹³C of dry matter from NADP-malic enzyme type C₄ plant (Zea mays L. var. saccharata Sturt.) grown at a tower in the forest for 32␣days, assuming constant Δ value (3.3‰) in Z. mays against height. The δ_a value obtained from δ¹³C in Z.␣mays was lowest at the forest floor (−9.30 ± 0.03‰), increased with height, and was almost constant above 10␣m (−7.14 ± 0.14‰). Then leaf Δ values for the tree species were calculated from tree leaf δ¹³ C andδ_a. Mean leaf Δ values for the three tall deciduous species (Fraxinus mandshurica, Ulmus davidiana, and Alnus hirsuta) were significantly different among three height levels in the forest: 23.1 ± 0.7‰ at the forest floor (understory), 21.4 ± 0.5‰ in lower canopy, and 20.5 ± 0.3‰ in upper canopy. The true difference in tree leaf Δ among the forest height levels might be even greater, because Δ in Z. mays probably increased with shading by up to ∼‰. The difference in tree leaf Δ among the forest height levels would be mainly due to decreasing intercellular CO₂ (C _i) with the increase in irradiance. Potential assimilation rate for the three tree species probably increased with height, since leaf nitrogen content on an area basis for these species also increased with height. However, the increase in stomatal conductance for these tree species would fail to meet the increase in potential assimilation rate, which might lead to increasing the degree of stomatal limitation in photosynthesis with height. Received: 30 September 1995 / Accepted: 25 October 1996     

Diurnal variation of Δ13CO2, ΔC18O16O and evaporative site enrichment of δH218O in Piper aduncum under field conditions in Trinidad

Concurrent measurements of gas exchange, instantaneous isotope discrimination (Δ) against ¹³CO₂ and C¹⁸O¹⁶O, and extent of ¹⁸O enrichment in H₂O at the evaporative sites, were followed in a tropical forest pioneer, Piper aduncum, on two different days in Trinidad during February 1995. Δ¹³CO₂ differed from that predicted from measurements of internal:external CO₂ concentration (C_i/C_a) and showed a wide range of values which decreased throughout the course of the day. Derivation of C_c (the CO₂ concentration at the carboxylation site) was not possible using carbon isotope discrimination under field conditions in situ and was derived assuming a constant value of internal transfer conductance (g_w). Under low rates of assimilation the derived C_c/C_a, like C_i/C_a, remained relatively stable over the course of both days and ΔC¹⁸O¹⁶O followed evaporative demand. Lower values of ΔC¹⁸O¹⁶O on day 2 occurred in response to the indirect effect of increased leaf-to-air vapour pressure deficits (VPD) and reduced stomatal conductance. For the first time, direct determination of the δH₂¹⁸O of transpired water vapour (δ_t) allowed derivation of evaporative site enrichment without the prerequisite of isotopic steady state (ISS) defined in the Craig and Gordon model. Generally, δ_t was less enriched than the source water (δ_s) in the morning and more enriched in the afternoon, which would be predicted from an increase and decrease in ambient VPD, respectively. On both days, leaves of P. aduncum approached ISS (indicated where δ_t≈δ_s) between 1300 and 1500 h. Evaporative site enrichment was maintained into the late afternoon, despite a decrease in ambient VPD. The data presented provide a greater insight into the natural variation in isotopic discrimination under field conditions, which may help to refine models of terrestrial biome discrimination.     

Isotope fractionation and 13C enrichment in soil profiles during the decomposition of soil organic matter

The mechanisms behind the ¹³C enrichment of organic matter with increasing soil depth in forests are unclear. To determine if ¹³C discrimination during respiration could contribute to this pattern, we compared δ¹³C signatures of respired CO₂ from sieved mineral soil, litter layer and litterfall with measurements of δ¹³C and δ¹⁵N of mineral soil, litter layer, litterfall, roots and fungal mycelia sampled from a 68-year-old Norway spruce forest stand planted on previously cultivated land. Because the land was subjected to ploughing before establishment of the forest stand, shifts in δ¹³C in the top 20 cm reflect processes that have been active since the beginning of the reforestation process. As ¹³C-depleted organic matter accumulated in the upper soil, a 1.0‰ δ¹³C gradient from −28.5‰ in the litter layer to −27.6‰ at a depth of 2–6 cm was formed. This can be explained by the 1‰ drop in δ¹³C of atmospheric CO₂ since the beginning of reforestation together with the mixing of new C (forest) and old C (farmland). However, the isotopic change of the atmospheric CO₂ explains only a portion of the additional 1.0‰ increase in δ¹³C below a depth of 20 cm. The δ¹³C of the respired CO₂ was similar to that of the organic matter in the upper soil layers but became increasingly ¹³C enriched with depth, up to 2.5‰ relative to the organic matter. We hypothesise that this ¹³C enrichment of the CO₂ as well as the residual increase in δ¹³C of the organic matter below a soil depth of 20 cm results from the increased contribution of ¹³C-enriched microbially derived C with depth. Our results suggest that ¹³C discrimination during microbial respiration does not contribute to the ¹³C enrichment of organic matter in soils. We therefore recommend that these results should be taken into consideration when natural variations in δ¹³C of respired CO₂ are used to separate different components of soil respiration or ecosystem respiration.     

Diurnal variations of needle water isotopic ratios in two pine species

Diurnal fluctuations of leaf water isotope ratios (δ¹⁸O and δD) were measured for Jeffrey (Pinus jeffreyi Balf.) and lodgepole (Pinus contorta Douglas ex Louden) pine. Two trees per species were sampled every few hours on 15–16 October 2005 and 19–20 June 2006. Diurnal gas exchange was measured during the summer sampling. In fall 2005, leaf water δ¹⁸O ranged from 0.7 to 9.0‰, and leaf water δD ranged from −70 to −50‰. In summer 2006, leaf water δ¹⁸O ranged from 7.7 to 20.7‰, and leaf water δD ranged from −61 to −24‰. Diurnal variation of leaf water isotope values typically reached a maximum in early afternoon, began decreasing around midnight, and reached a minimum in mid-morning. Both periods showed a high degree of enrichment relative to source water, with leaf water–source water enrichments ranging up to 37.8‰ for δ¹⁸O, and up to 95‰ for δD. Leaf water enrichment varied by season with summer enrichment being greater than fall enrichment. A steady-state model (i.e., modified Craig–Gordon modeling) for leaf water isotope compositions did not provide a good fit to measured values of leaf water. In summer, a non-steady state model provided a better fit to the measured data than the steady-state model. Our findings demonstrate substantial leaf water enrichment above source water and diurnal variations in the isotopic composition of leaf water, which has application to understanding short-term variability of atmospheric gases (water vapor, CO₂, O₂), climate studies based on the isotopic composition of tree rings, and ecosystem water fluxes.     

Controlling for anthropogenically induced atmospheric variation in stable carbon isotope studies

Increased use of stable isotope analysis to examine food-web dynamics, migration, transfer of nutrients, and behavior will likely result in expansion of stable isotope studies investigating human-induced global changes. Recent elevation of atmospheric CO₂ concentration, related primarily to fossil fuel combustion, has reduced atmospheric CO₂ δ¹³C (¹³C/¹²C), and this change in isotopic baseline has, in turn, reduced plant and animal tissue δ¹³C of terrestrial and aquatic organisms. Such depletion in CO₂ δ¹³C and its effects on tissue δ¹³C may introduce bias into δ¹³C investigations, and if this variation is not controlled, may confound interpretation of results obtained from tissue samples collected over a temporal span. To control for this source of variation, we used a high-precision record of atmospheric CO₂ δ¹³C from ice cores and direct atmospheric measurements to model modern change in CO₂ δ¹³C. From this model, we estimated a correction factor that controls for atmospheric change; this correction reduces bias associated with changes in atmospheric isotopic baseline and facilitates comparison of tissue δ¹³C collected over multiple years. To exemplify the importance of accounting for atmospheric CO₂ δ¹³C depletion, we applied the correction to a dataset of collagen δ¹³C obtained from mountain lion (Puma concolor) bone samples collected in California between 1893 and 1995. Before correction, in three of four ecoregions collagen δ¹³C decreased significantly concurrent with depletion of atmospheric CO₂ δ¹³C (n ≥ 32, P ≤ 0.01). Application of the correction to collagen δ¹³C data removed trends from regions demonstrating significant declines, and measurement error associated with the correction did not add substantial variation to adjusted estimates. Controlling for long-term atmospheric variation and correcting tissue samples for changes in isotopic baseline facilitate analysis of samples that span a large temporal range.     

Water status related photosynthesis and carbon isotope discrimination in species of the lichen genusPseudocyphellaria with green or blue-green photobionts and in photosymbiodemes

Summary Green lichens have been shown to attain positive net photosynthesis in the presence of water vapour while blue-green lichens require liquid water (Lange et al. 1986). This behaviour is confirmed not only for species with differing photobionts in the genusPseudocyphellaria but for green and blue-green photobionts in a single joined thallus (photosymbiodeme), with a single mycobiont, and also when adjacent as co-primary photobionts. The different response is therefore a property of the photobiont. The results are consistent with published photosynthesis/water content response curves. The minimum thallus water content for positive net photosynthesis appears to be much lower in green lichens (15% to 30%, related to dry weight) compared to blue-greens (85% to 100%). Since both types of lichen rehydrate to about 50% water content by water vapour uptake only green lichens will show positive net photosynthesis. It is proposed that the presence of sugar alcohols in green algae allow them to retain a liquid pool (concentrated solution) in their chloroplasts at low water potentials and even to reform it by water vapour uptake after being dried. The previously shown difference in δ¹³C values between blue-green and green lichens is also retained in a photosymbiodeme and must be photobiont determined. The wide range of δ¹³C values in lichens can be explained by a C₃ carboxylation system and the various effects of different limiting processes for photosynthetic CO₂ fixation. If carboxylation is rate limiting, there will be a strong discrimination of¹³CO₂, at high internal CO₂ partial pressure. The resulting very low δ¹³C values (-31 to-35‰) have been found only in green lichens which are able to photosynthesize at low thallus water content by equilibraiton with water vapour. When the liquid phase diffusion of CO₂ becomes more and more rate limiting and the internal CO₂ pressure decreases, the¹³C content of the photosynthates increases and less negative δ¹³C values results, as are found for blue-green lichens.     

Carbon isotopes and water use efficiency: sense and sensitivity

We revisit the relationship between plant water use efficiency and carbon isotope signatures (δ¹³C) of plant material. Based on the definitions of intrinsic, instantaneous and integrated water use efficiency, we discuss the implications for interpreting δ¹³C data from leaf to landscape levels, and across diurnal to decadal timescales. Previous studies have often applied a simplified, linear relationship between δ¹³C, ratios of intercellular to ambient CO₂ mole fraction (C _i/C _a), and water use efficiency. In contrast, photosynthetic ¹³C discrimination (Δ) is sensitive to the ratio of the chloroplast to ambient CO₂ mole fraction, C _c/C _a (rather than C _i/C _a) and, consequently, to mesophyll conductance. Because mesophyll conductance may differ between species and over time, it is not possible to determine C _c/C _a from the same gas exchange measurements as C _i/C _a. On the other hand, water use efficiency at the leaf level depends on evaporative demand, which does not directly affect Δ. Water use efficiency and Δ can thus vary independently, making it difficult to obtain trends in water use efficiency from δ¹³C data. As an alternative approach, we offer a model available at to explore how water use efficiency and ¹³C discrimination are related across leaf and canopy scales. The model provides a tool to investigate whether trends in Δ indicate changes in leaf functional traits and/or environmental conditions during leaf growth, and how they are associated with trends in plant water use efficiency. The model can be used, for example, to examine whether trends in δ¹³C signatures obtained from tree rings imply changes in tree water use efficiency in response to atmospheric CO₂ increase. This is crucial for predicting how plants may respond to future climate change. Electronic supplementary material The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

Effects of atmospheric CO2 enrichment on δ 13C, δ 15N values and turnover times of soil organic matter pools isolated by thermal techniques

CO₂ applied for Free-Air CO₂ Enrichment (FACE) experiments is strongly depleted in ¹³C and thus provides an opportunity to study C turnover in soil organic matter (SOM) based on its δ ¹³C value. Simultaneous use of ¹⁵N labeled fertilizers allows N turnover to be studied. Various SOM fractionation approaches (fractionation by density, particle size, chemical extractability etc.) have been applied to estimate C and N turnover rates in SOM pools. The thermal stability of SOM coupled with C and N isotopic analyses has never been studied in experiments with FACE. We tested the hypothesis that the mean residence time (MRT) of SOM pools is inversely proportional to its thermal stability. Soil samples from FACE plots under ambient (380 ppm) and elevated CO₂ (540 ppm; for 3 years) treatments were analyzed by thermogravimetry coupled with differential scanning calorimetry (TG-DSC). Based on differential weight losses (TG) and energy release or consumption (DSC), five SOM pools were distinguished. Soil samples were heated up to the respective temperature and the remaining soil was analyzed for δ ¹³C and δ ¹⁵N by IRMS. Energy consumption and mass losses in the temperature range 20–200°C were mainly connected with water volatilization. The maximum weight losses occurred from 200–310°C. This pool contained the largest amount of carbon: 61% of the total soil organic carbon in soil under ambient treatment and 63% in soil under elevated CO₂, respectively. δ ¹³C values of SOM pools under elevated CO₂ treatment showed an increase from −34.3‰ of the pool decomposed between 20–200°C to −18.1‰ above 480°C. The incorporation of new C and N into SOM pools was not inversely proportional to its thermal stability. SOM pools that decomposed between 20–200 and 200–310°C contained 2 and 3% of the new C, with a MRT of 149 and 92 years, respectively. The pool decomposed between 310–400°C contained the largest proportion of new C (22%), with a MRT of 12 years. The amount of fertilizer-derived N after 2 years of application in ambient and elevated CO₂ treatments was not significantly different in SOM pools decomposed up to 480°C having MRT of about 60 years. In contrast, the pool decomposed above 480°C contained only 0.5% of new N, with a MRT of more than 400 years in soils under both treatments. Thus, the separation of SOM based on its thermal stability was not sufficient to reveal pools with contrasting turnover rates of C and N. Responsible Editor: Bernard Nicolardot.