Leaf carbon isotope discrimination and nitrogen content for riparian trees along elevational transects

 Leaf carbon isotope discrimination (Δ), seasonal estimates of the leaf-to-air water vapor gradient on a molar basis (ω), and leaf nitrogen contents were examined in three riparian tree species (Populus fremontii, P. angustifolia, and Salix exigua) along elevational transects in northern and southern Utah USA (1500–2670 m and 600–1820 m elevational gradients, respectively). The ω values decreased with elevation for all species along transects. Plants growing at higher elevations exhibited lower Δ values than plants at lower elevations (P. fremontii, 22.9‰ and 19.5‰, respectively; P. angustifolia, 23.2‰ and 19.2‰, respectively; and S.␣exigua, 21.1‰ and 19.1‰, respectively). Leaf nitrogen content increased with elevation for all species, suggesting that photosynthetic capacity at a given intercellular carbon dioxide concentration was greater at higher elevations. Leaf Δ and nitrogen content values were highly correlated, implying that leaves with higher photosynthetic capacities also had lower intercellular carbon dioxide concentrations. No significant interannual differences were detected in carbon isotope discrimination. Received: 25 February 1996 / Accepted: 8 September 1996     

Seasonal, daily and diurnal variations in the stable carbon isotope composition of carbon dioxide respired by tree trunks in a deciduous oak forest

The stable C isotope composition (δ¹³C) of CO₂ respired by trunks was examined in a mature temperate deciduous oak forest (Quercus petraea). Month-to-month, day-to-day and diurnal, measurements were made to determine the range of variations at different temporal scales. Trunk growth and respiration rates were assessed. Phloem tissue was sampled and was analysed for total organic matter and soluble sugar ¹³C composition. The CO₂ respired by trunk was always enriched in ¹³C relative to the total organic matter, sometimes by as much as 5‰. The δ¹³C of respired CO₂ exhibited a large seasonal variation (3.3‰), with a relative maximum at the beginning of the growth period. The lowest values occurred in summer when the respiration rates were maximal. After the cessation of radial trunk growth, the respired CO₂ δ¹³C values showed a progressive increase, which was linked to a parallel increase in soluble sugar content in the phloem tissue (R = 0.95; P < 0.01). At the same time, the respiration rates declined. This limited use of the substrate pool might allow the discrimination during respiration to be more strongly expressed. The late-season increase in CO₂ δ¹³C might also be linked to a shift from recently assimilated C to reserves. At the seasonal scale, CO₂ δ¹³C was negatively correlated with air temperature (R = −0.80; P < 0.01). The diurnal variation sometimes reached 3‰, but the range and the pattern depended on the period within the growing season. Contrary to expectations, diurnal variations were maximal in winter and spring when the leaves were missing or not totally functional. By contrast to the seasonal scale, these diurnal variations were not related to air temperature or sugar content. Our study shows that seasonal and diurnal variations of respired ¹³C exhibited a similar large range but were probably explained by different mechanisms.     

特定化合物同位素分析技术在树木非结构性碳水化合物研究中的应用

特定化合物同位素分析(CSIA)可以实现对复杂基质中特定化合物稳定碳同位素组成(δ¹³C)的精确测定。应用此方法测定树木非结构性碳水化合物(NSC)中特定成分(如糖类、有机酸和糖醇)的δ¹³C,不仅能够追踪新同化的光合产物在树木中的运移及与外界的碳交换,还能够更敏感地指示树木生理状况对环境变化的响应。本文首先系统介绍了CSIA从样品采集、处理到δ¹³C测定的方法,然后综述了树木NSC中各成分之间及各成分在不同器官之间的δ¹³C差异,阐述了树木NSC的δ¹³C时间动态变化特征及内在机制,最后分析了NSC作为主要呼吸底物,其δ¹³C与树木呼吸释放CO₂的δ¹³C(δ¹³C_R)之间的联系,并针对CSIA分析技术在后光合分馏、树木逆境生理和年轮δ¹³C形成机制等研究的应用前景提出了展望。     

胡杨、灰叶胡杨种子营养化学成分的研究

对不同居群的胡杨、灰叶胡杨种子营养化学成分进行分析,结果表明,胡杨种子中可溶性糖、粗蛋白质、脂肪、淀粉的含量分别为9～12g·kg-1、38～44g·kg-1、21～62g·kg-1、1.9～2.3g·kg-1;灰叶胡杨种子中可溶性糖、粗蛋白质、脂肪、淀粉的含量分别为9～10g·kg-1、38～48g·kg-1、14～41g·kg-1、2.3～2.5g·kg-1.胡杨、灰叶胡杨不同居群种子中大量元素含量均以N最高,分别为6.0～6.9g·kg-1、6.3～7.6g·kg-1;微量元素含量排序一致,依次为Fe>Zn>Cu>Mn>Pb>Ni>Cd.统计分析表明,不同居群种子中可溶性糖、粗蛋白、粗脂肪、N、P、K、Ca、Mg的含量均呈现极显著差异;而淀粉与微量元素含量除一些差异不显著外,其余均达显著或极显著水平.     

Water use efficiency and carbon isotope composition of plants in a cold desert environment

Summary The effects of the availabilities of water and nitrogen on water use efficiency (WUE) of plants were investigated in a sagebrush steppe. The four species studied wereArtemisia tridentata (shrub),Ceratoides lanata (suffrutescent shrub),Elymus lanceolatus (rhizomatous grass), andElymus elymoides (tussock grass). Water and nitrogen levels were manipulated in a two-by-two factorial design resulting in four treatments: control (no additions), added water, added nitrogen, and added water and nitrogen. One instantaneous and two long-term indicators of WUE were used to testa priori predictions of the ranking of WUE among treatments. The short-term indicator was the instantaneous ratio of assimilation to transpiration (A/E). The long-term measures were 1) the slope of the relationship between conductance to water vapor and maximum assimilation and 2) the carbon isotope composition (¹³C) of plant material. Additional water decreased WUE, whereas additional nitrogen increased WUE. For both A/E and ¹³C, the mean for added nitrogen alone was significantly greater than the mean for added water alone, and means for the control and added water and nitrogen fell in between. This ranking of WUE supported the hypothesis that both water and nitrogen limit plant gas exchange in this semiarid environment. The short- and long-term indicators were in agreement, providing evidence in support of theoretical models concerning the water cost of carbon assimilation.     

Variation in carbon isotope discrimination within and among Sphagnum species in a temperate wetland

Field samples of bryophytes are highly variable in carbon isotope discrimination values (Δ, a measure of ¹³CO₂ uptake relative to ¹²CO₂), but it is unknown what affects Δ under field conditions, or how variation in Δ relates to bryophyte performance. This study employed field and greenhouse common garden studies to evaluate the influence of microsite, seasonal, and genetic variation on Δ in peatmosses. Three species of Sphagnum that occupy hollow (S. recurvum), carpet (S. palustre), and hummock (S. tenerum) habitats were sampled for relative growth rates (RGR), C:N ratio, and Δ throughout a growing season. Values of Δ ranged from 19.0 to 27.1‰. This variation was unrelated to species (P=0.61). However, Δ varied seasonally (P<0.001), with lower discrimination in the spring (mean 22.5‰), followed by summer (23.8‰) and winter (24.7‰). There was also significant microsite variation (P=0.015) which disappeared when plants were grown in a common garden. In both spring and summer, microsite variation in Δ was inversely related to RGR (P<0.001), but unrelated to C:N ratios (P>0.08). These results suggest that environmental, not genetic, variation at microsites affects Δ in non-vascular plants. However, environmental control of Δ is unlike that in vascular plants where water limitation lowers chloroplastic demand and increases resistance to carbon uptake. In non-vascular plants, water limitation lowers chloroplastic demand and decreases resistance to carbon uptake. These processes have additive effects and generate high spatial and seasonal variability in Δ. Received: 29 April 1999 / Accepted: 8 November 1999     

Aerial decay influence on the stable oxygen and carbon isotope ratios in tree ring cellulose

Sub-fossil wood is often affected by the decaying process that introduces uncertainties in the measurement of oxygen and carbon stable isotope composition in cellulose. Although the cellulose stable isotopes are widely used as climatic proxies, our understanding of processes controlling their behavior is very limited. We present here a comparative study of stable oxygen and carbon isotope ratios in tree ring cellulose in decayed and non-decayed wood samples of Swiss stone pine (Pinus cembra) trees. The intra-ring stable isotope variability (around the circumference of a single ring) was between 0.1 and 0.5‰ for δ¹⁸O values and between 0.5 and 1.6‰ for δ¹³C values for both decayed and non-decayed wood. Observed intra-tree δ¹⁸O variability is less than that reported in the literature (0.5–1.5‰), however, for δ¹³C it is larger than the reported values (0.7–1.2‰). The inter-tree variability for non-decayed wood ranges between 1.1 and 2.3‰ for δ¹⁸O values, and between 2 and 4.7‰ for δ¹³C values. The inter-tree differences for δ¹⁸O values are similar to those reported in the literature (1–2‰ for oxygen and 1–3‰ for carbon) but are larger for δ¹³C values. We have found that the differences for δ¹⁸O and δ¹³C values between decayed and non-decayed wood are smaller than the variation among different trees from the same site, suggesting that the decayed wood can be used for isotopic paleoclimate research.     

基于树轮稳定碳同位素的张北杨树防护林退化原因解析

三北防护林是京津冀地区重要的生态屏障,但近十年来张北县4/5的杨树防护林出现退化现象,接近1/3的杨树濒临死亡或枯死,导致杨树防护林生态功能下降.本研究用稳定碳同位素方法追溯研究了退化与未退化杨树年轮中δ¹³C值和内在水分利用效率(WUE_i)的差异,分析导致杨树退化和死亡的原因及其来源.结果表明:相同年龄杨树的直径随着退化程度增加而下降,退化杨树树轮的δ¹³C值变化范围为-25.26‰～-22.97‰,未退化杨树为-26.15‰～-23.50‰,从1997年开始,退化杨树δ¹³C值高于未退化杨树.退化与未退化杨树WUE_i值从1997年开始出现差异但不显著,2002年后其差异达到显著水平,退化与未退化WUE_i的差值ΔWUE_i连续正值可能是退化与未退化杨树产生分化的重要原因之一.退化和未退化杨树WUE_i与降雨量、相对湿度和潜在蒸散量(ET₀)的相关关系均不显著,但与气温和地下水埋深呈极显著的线性关系.1997年极端干旱事件是杨树林退化的起点,随后土地利用方式的改变导致地下水的过度使用,加剧了干旱持续时间和强度,进而加速杨树防护林的退化和死亡.     

Comparison of seasonal variations in water-use efficiency calculated from the carbon isotope composition of tree rings and flux data in a temperate forest

Tree-ring δ(13) C is often interpreted in terms of intrinsic water-use efficiency (WUE) using a carbon isotope discrimination model established at the leaf level. We examined whether intra-ring δ(13) C could be used to assess variations in intrinsic WUE (W(g), the ratio of carbon assimilation and stomatal conductance to water) and variations in ecosystem WUE (W(t) , the ratio of C assimilation and transpiration) at a seasonal scale. Intra-ring δ(13) C was measured in 30- to 60-μm-thick slices in eight oak trees (Quercus petraea). Canopy W(g) was simulated using a physiologically process-based model. High between-tree variability was observed in the seasonal variations of intra-ring δ(13) C. Six trees showed significant positive correlations between W(g) calculated from intra-ring δ(13) C and canopy W(g) averaged over several days during latewood formation. These results suggest that latewood is a seasonal recorder of W(g) trends, with a temporal lag corresponding to the mixing time of sugars in the phloem. These six trees also showed significant negative correlations between photosynthetic discrimination Δ calculated from intra-ring δ(13) C, and ecosystem W(t), during latewood formation. Despite the observed between-tree variability, these results indicate that intra-ring δ(13) C can be used to access seasonal variations in past W(t).     

Variation in leaf carbon isotope discrimination in Encelia farinosa: implications for growth,competition, and drought survival

Population-level variation in the leaf carbon isotope discrimination () values was examined in Encelia farinosa, a common Sonoran Desert shrub. There was approximately a 2 range in values among different plants. These differences in values among neighboring plants were maintained through time, both under conditions when neighbors were present and after neighbors had been removed. Individuals with high values were found to have an accelerated growth rate when these plants were released from competition for water. Individuals with low values were better able to persist through long-term drought. These data suggest possible tradeoffs between conditions favoring high- and low--value plants within a natural population. Given the temporal variability in precipitation between years and spatial variability in microhabitat quality in the Sonoran Desert, variation in values among E. farinosa plants will be maintained within a population.     

植物呼吸释放CO2碳同位素变化研究进展

稳定性碳同位素是研究碳循环的有效手段。植物呼吸释放CO_2的碳同位素(δ~(13)C_R)变化是研究植物或生态系统与大气碳交换的重要方法,并可以揭示植物的生理过程、碳分配方式及其对环境变化的响应。介绍了目前国内外关于植物δ~(13)C_R变化的研究概况,植物不同器官δ~(13)C_R值及其日变化幅度趋势一致:叶片根系树干/茎,不同功能群植物其呼吸释放CO_2碳同位素组成存在差异。但植物δ~(13)C_R值日变化与呼吸底物的相关性在不同的研究中结果并不一致。导致植物呼吸δ~(13)C_R发生变化的主要原因为光合同位素效应、呼吸底物的供给及呼吸代谢中间产物利用、碳代谢相关酶的活性、LEDR(light enhanced dark respiration)、植物的遗传特性及外部环境改变。目前国际上已有较多关于导致植物呼吸δ~(13)C_R发生变化原因的研究,但内在机制的研究尚未完善。该领域研究在国内鲜有报道,因此,亟需加强我国关于植物δ~(13)C_R短期变化及其潜在呼吸代谢机制的研究。     

不同生活型绿化植物叶片碳同位素组成的季节特征

通过测定北京地区不同生活型绿化植物叶片的碳同位素组成(δ13C值),从植物种和生活型两个方面研究植物水分利用效率的自然可变性。结果表明:所测定的75种植物(隶属于35科65属)的叶片的δ13C值变幅,春季为-30.7‰--23.4‰,夏季为-31.5‰--25.1‰,秋季为-31.4‰--23.9‰;落叶灌木种间差异不显著(p=0.114),而常绿乔木(p=0.005)、落叶乔木(p0.001)、常绿灌木(p=0.022)、草本植物(p0.001)和藤本植物(p=0.001)的种间差异显著或极显著;同一生活型植物叶片的δ13C季节差异显著,春季叶片的δ13C值显著大于夏秋两季(常绿乔木除外),不同生活型植物叶片的δ13C值在春、夏、秋3个季节差异都达到了极显著水平(春季p=0.001、夏季p0.001、秋季p0.001),且叶片的δ13C值表现出乔木树种灌木树种藤本植物草本植物、常绿植物落叶植物的规律。因此,植物种和生活型均会引起植物叶片δ13C值的变化,但δ13C受生活型变化的影响较大,表明不同生活型植物的水分利用效率具有明显差异。     

Distribution of carbon isotope composition of modern soils on the Qinghai-Tibetan Plateau

This paper presents a large data set on carbon isotope composition (¹³C) of modern soils which were collected under the main vegetation communities along an altitude of 1250–5500m above sea level in the Qinghai-Tibetan Plateau. The ¹³C values of 198 samples range from –28.6 to –15.1 versus PDB and exhibit a clean relation to different vegetation communities from forest (–25.9±1.2) to shrub (–24.7±1.4), steppe (–23.1±1.3), alpine meadow (–23.6±0.7), alpine desert steppe (–21.3±1.6), and alpine desert (–18.9±2.5). We attributed the observed variability in ¹³C values to that the mean annual precipitation (MAP) and the mean annual temperature (MAT) are the main factors controlling the distribution of vegetation types in the Tibetan Plateau, which causes the change in carbon isotope composition of modern soils at any given altitude. The result of both linear and nonlinear regression analyses also confirms that MAP and MAT are the major factors affecting the ¹³C values of surface soils. In the absence of favorable moisture and temperature conditions, low pCO₂ alone is not sufficient to cause the distinct changes in carbon isotope composition of modern soils in the Tibetan Plateau. This study provides some fundamental information on the carbon isotope composition of terrestrial carbon pools and bears some practical significance for the use of carbon isotope data to document vegetation changes and environmental conditions of the high plateau in the past.     

Stable carbon isotope composition of bone hydroxylapatite: significance in paleodietary analysis

The stable carbon isotope composition of the structural carbonate derived from animal bone hydroxylapatite (δ¹³C_B-HA) could record an animal’s diet. These records provide critical evidence for different paleontological disciplines, e.g., paleodiet analyses, and paleoclimate reconstructions. Compared to those of other body tissues, such as bone collagen or teeth enamel hydroxylapatite, δ¹³C_B-HA values record information on the whole diet of an animal in its last years. δ¹³C_B-HA can be applied to fossil animals of various body sizes. The δ¹³C analytical instruments available only require that prepared bone samples be approximately 2–5 mg for precise measurement, allowing δ¹³C_B-HA analysis to be feasible on most vertebrate fossils without destructive sampling, especially on small mammals or birds whose teeth are not large enough for sampling or are lost. Moreover, δ¹³C_B-HA can be used from different times or under less than ideal burial environments. For fossils dating back to Devonian or buried in hot and humid regions, dietary information has been completely lost in bone collagen during post-depositional processes but still remained in the δ¹³C_B-HA values because hydroxylapatite is less influenced by diagenetic effects after deposition. In addition, systematic methods such as X-ray diffraction and Fourier transform infrared spectroscopy have been developed to qualitatively or semiquantitatively assess the influence of diagenesis on bone hydroxylapatite to ensure the credibility of the δ¹³C_B-HA values. With the above merits, δ¹³C_B-HA analysis is therefore becoming an increasingly important method in paleodiet-related research. Currently, applications of the δ¹³C_B-HA method on fossil animals are primarily focused on two aspects, namely, paleodietary reconstruction of fossil animals with uncertain diets and paleoenvironmental reconstruction based on the δ¹³C_B-HA values of fossil herbivores. The published researches, combined with our new results from early birds, demonstrate the considerable significance of the δ¹³C_B-HA method in paleontological and paleoenvironmental research. Notably, the δ¹³C_B-HA-based paleodietary analysis of early vertebrates, especially the large number of small birds or mammals discovered in the past decades would be an important work in the near future.     

陆生植物稳定碳同位素组成与全球变化

分析了大气CO₂浓度、温度、降水和海拔高度等环境因素对陆生植物稳定性碳同位素组分的影响及其作用机理,综述了国内外碳稳定同位素技术在全球变化研究中的进展和应用,如重建大气CO₂浓度变化,揭示温度、降水对树木生长的“滞后效应”和“幼龄效应”,确定不同光合型植物随海拔高度的分布变化,以及通过碳稳定同位素技术揭示不同时间尺度上和不同气候条件下的植物水分利用效率变化及不同生活型植物的水分利用效率差异,并探讨研究中存在的问题及其研究前景.     

树木年轮年内高分辨率稳定同位素记录:方法、进展和展望

树轮稳定同位素比率能有效地记录树木生长过程中气候环境变化信息及树木的生理响应机制。年内高分辨率树轮稳定同位素比率则能够提供更为详细的气候环境信息,揭示树木对季节尺度气候环境变化的生理生态响应机制,在古气候和全球变化生态学研究方面显示出巨大的潜力。本文收集了1990年以来发表的树轮年内高分辨率稳定同位素比率研究论文,从样品剥离方法、α-纤维素化学提取方法以及应用等方面综述了其研究进展,展望了年内高分辨率树轮稳定同位素记录研究的潜力和未来发展方向。     

双向标记培养植物测定大气二氧化碳稳定碳同位素组成

基于植物能够利用体内的碳酸酐酶来催化碳酸氢根离子生成二氧化碳和水作为底物进行光合作用的特性,采用两种δ13CPDB值差值大于10‰的碳酸氢钠分别作为外源碳酸氢根离子的碳同位素标记物,通过室内双向水培诸葛菜和芥菜型油菜实验,分别向水培处理液里添加已知δ13CPDB值的碳酸氢钠并培养24 h,利用同位素比值质谱(IRMS)技术,测定并计算了两个时间、两种环境下的大气二氧化碳稳定碳同位素日平均组成。结果表明:在环境1(不同浓度的Na HCO3处理液)下所得到的δCa值与添加到处理液中的碳酸氢根离子的浓度有关;在环境2(不同浓度的PEG处理液)下所得到的δCa值与添加到处理液中的PEG的浓度无关;两种环境下所测得的大气二氧化碳稳定碳同位素日平均组成δCa值与实验中培养的植物种类无关,而与添加到培养液中碳酸氢根离子的浓度及植物的生长速率有关。数据重现性好,结果准确可靠,可以高精度的测定不同待测环境下大气二氧化碳稳定碳同位素比值,其可为以后监测不同时间、不同地点的大气二氧化碳碳同位素组成及来源提供非常有效的方法和信息。     

Carbon isotope composition of canopy leaves in a tropical forest in Panama throughout a seasonal cycle

The seasonal variation in ¹³C values was measured in leaves from 17 upper canopy, five mid- canopy and in four gap tree species, as well as in five epiphyte and five vine species, in a seasonally dry lowland tropical forest at Parque Natural Metropolitano near Panama City, Republic of Panama. No seasonal variation was detected in the ¹³C values of mature exposed leaves from either the upper or mid- canopy. However, canopy position did influence the ¹³C value. The mean isotopic composition of leaves from the mid- canopy was more negative than that of the upper canopy throughout the year. The ¹³C value was also influenced by leaf development, with juvenile leaves on average 1.5 less negative than mature leaves. The five epiphyte species exhibited ¹³C values that were typical of crassulacean acid metabolism (CAM). Codonanthe uleana, with isotopic values of –19.9 to –22.1, is only the second species in the Gesneriaceae reported to express CAM, whereas values between –14.6 and –22.0 indicate that Peperomia macrostachya can exhibit different degrees of CAM. The isotopic composition of exposed mature leaves from the vines showed little interspecific variation and was similar to the upper-canopy leaves of the trees.     

Altitude trends in conifer leaf morphology and stable carbon isotope composition

The natural ratio of stable carbon isotopes (δ¹³C) was compared to leaf structural and chemical characteristics in evergreen conifers in the north-central Rockies, United States. We sought a general model that would explain variation in δ¹³C across altitudinal gradients. Because variation in δ¹³C is attributed to the shifts between supply and demand for carbon dioxide within the leaf, we measured structural and chemical variables related to supply and demand. We measured stomatal density, which is related to CO₂ supply to the chloroplasts, and leaf nitrogen content, which is related to CO₂ demand. Leaf mass per area was measured as an intermediate between supply and demand. Models were tested on four evergreen conifers: Pseudotsuga menziesii, Abies lasiocarpa, Picea engelmannii, and Pinus contorta, which were sampled across 1800 m of altitude. We found significant variation among species in the rate of δ¹³C increase with altitude, ranging from 0.91‰ km^–1 for A. lasiocarpa to 2.68‰ km^–1 for Pinus contorta. Leaf structure and chemistry also varied with altitude: stomatal density decreased, leaf mass per area increased, but leaf nitrogen content (per unit area) was constant. The regressions on altitude were particularly robust in Pinus contorta. Variables were derived to describe the balance between supply and demand; these variables were stomata per gram of nitrogen and stomata per gram of leaf mass. Both derived variables should be positively related to internal CO₂ supply and thus negatively related to δ¹³C. As expected, both derived variables were negatively correlated with δ¹³C. In fact, the regression on stomatal density per gram was the best fit in the study (r ²=0.72, P<0.0001); however, the relationships were species specific. The only general relationship observed was between δ¹³C and LMA: δ¹³C (‰)=–32.972+ 0.0173×LMA (r ²=0.45, P<0.0001). We conclude that species specificity of the isotopic shift indicates that evergreen conifers demonstrate varying degrees of functional plasticity across environmental gradients, while the observed convergence of δ¹³C with LMA suggests that internal resistance may be the key to understanding inter-specific isotopic variation across altitude. Received: 1 June 1999 / Accepted: 2 November 1999     

Temporal dynamics of the carbon isotope composition in a Pinus sylvestris stand: from newly assimilated organic carbon to respired carbon dioxide

The (13)C isotopic signature (C stable isotope ratio; delta(13)C) of CO(2) respired from forest ecosystems and their particular compartments are known to be influenced by temporal changes in environmental conditions affecting C isotope fractionation during photosynthesis. Whereas most studies have assessed temporal variation in delta(13)C of ecosystem-respired CO(2) on a day-to-day scale, not much information is available on its diel dynamics. We investigated environmental and physiological controls over potential temporal changes in delta(13)C of respired CO(2) by following the short-term dynamics of the (13)C signature from newly assimilated organic matter pools in the needles, via phloem-transported organic matter in twigs and trunks, to trunk-, soil- and ecosystem-respired CO(2). We found a strong 24-h periodicity in delta(13)C of organic matter in leaf and twig phloem sap, which was strongly dampened as carbohydrates were transported down the trunk. Periodicity reappeared in the delta(13)C of trunk-respired CO(2), which seemed to originate from apparent respiratory fractionation rather than from changes in delta(13)C of the organic substrate. The diel patterns of delta(13)C in soil-respired CO(2) are partly explained by soil temperature and moisture and are probably due to changes in the relative contribution of heterotrophic and autotrophic CO(2) fluxes to total soil efflux in response to environmental conditions. Our study shows that direct relations between delta(13)C of recent assimilates and respired CO(2) may not be present on a diel time scale, and other factors lead to short-term variations in delta(13)C of ecosystem-emitted CO(2). On the one hand, these variations complicate ecosystem CO(2) flux partitioning, but on the other hand they provide new insights into metabolic processes underlying respiratory CO(2) emission.