Sustained enhancement of photosynthesis in mature deciduous forest trees after 8years of free air CO2 enrichment

Carbon uptake by forests constitutes half of the planet's terrestrial net primary production; therefore, photosynthetic responses of trees to rising atmospheric CO2 are critical to understanding the future global carbon cycle. At the Swiss Canopy Crane, we investigated gas exchange characteristics and leaf traits in five deciduous tree species during their eighth growing season under free air carbon dioxide enrichment in a 35-m tall, ca. 100-year-old mixed forest. Net photosynthesis of upper-canopy foliage was 48% (July) and 42% (September) higher in CO2-enriched trees and showed no sign of down-regulation. Elevated CO2 had no effect on carboxylation efficiency (V cmax) or maximal electron transport (J max) driving ribulose-1,5-bisphosphate (RuBP) regeneration. CO2 enrichment improved nitrogen use efficiency, but did not affect leaf nitrogen (N) concentration, leaf thickness or specific leaf area except for one species. Non-structural carbohydrates accumulated more strongly in leaves grown under elevated CO2 (largely driven by Quercus). Because leaf area index did not change, the CO2-driven stimulation of photosynthesis in these trees may persist in the upper canopy under future atmospheric CO2 concentrations without reductions in photosynthetic capacity. However, given the lack of growth stimulation, the fate of the additionally assimilated carbon remains uncertai

Temperature versus species-specific influences on the stable oxygen isotope ratio of tree rings

Stable isotopic ratios integrate ecosystem variability while reflecting change in both environmental and biological processes. At sites, where climate does not strongly limit tree growth, co-occurring trees may display large discrepancies in stable oxygen isotopic ratios (δ18O) due to the interplay between biological processes (competition for light and nutrients, individual tree physiology, etc.) and climate. For a better quantification of the isotope variability within and among trees, the climatic and/or individual tree effects on seasonal δ18O variations in precipitation, soil water, leaf water and leaf organic material (whole leaf, cellulose and starch) and annual δ18O variations in tree-ring cellulose for Fagus sylvatica (Fs), Quercus robur (Qr), Carpinus betulus (Cb) and Pinus sylvestris (Ps) were studied in a mature temperate forest in Switzerland, using a mixed linear regression model technique. Furthermore, the influence of environmental factors on δ18O was assessed by means of three common isotope fractionation models. Our statistical analysis showed that except for Ps, a greater portion of δ18O variance in leaf compounds can be explained by individual tree effects, compared to temperature. Concerning tree-ring cellulose, only Fs and Ps show a significant temperature signal (maximum 12% of the variance explained), while the individual tree effect significantly explains δ18O for all species for a period of 38years. Large species differences resulted in a limited ability of the isotope fractionation models to predict measured values. Overall, we conclude that in a diverse mixed forest stand, individual tree responses reduce the potential extraction of a temperature signal from δ18

Flow of Deposited Inorganic N in Two Gleysol-dominated Mountain Catchments Traced with 15NO3− and 15NH4+

Abstract.: In two mountain ecosystems at the Alptal research site in central Switzerland, pulses of 15NO3 and 15NH4 were separately applied to trace deposited inorganic N. One forested and one litter meadow catchment, each approximately 1600m2, were delimited by trenches in the Gleysols. K15NO3 was applied weekly or fortnightly over one year with a backpack sprayer, thus labelling the atmospheric nitrate deposition. After the sampling and a one-year break, 15NH4Cl was applied as a second one-year pulse, followed by a second sampling campaign. Trees (needles, branches and bole wood), ground vegetation, litter layer and soil (LF, A and B horizon) were sampled at the end of each labelling period. Extractable inorganic N, microbial N, and immobilised soil N were analysed in the LF and A horizons. During the whole labelling period, the runoff water was sampled as well. Most of the added tracer remained in both ecosystems. More NO3− than NH4+ tracer was retained, especially in the forest. The highest recovery was in the soil, mainly in the organic horizon, and in the ground vegetation, especially in the mosses. Event-based runoff analyses showed an immediate response of 15NO3− in runoff, with sharp 15N peaks corresponding to discharge peaks. NO3− leaching showed a clear seasonal pattern, being highest in spring during snowmelt. The high capacity of N retention in these ecosystems leads to the assumption that deposited N accumulates in the soil organic matter, causing a progressive decline of its C:N rati

Carbon fluxes to the soil in a mature temperate forest assessed by 13C isotope tracing

Photosynthetic carbon uptake and respiratory C release from soil are major components of the global carbon balance. The use of 13C depleted CO2 (δ13C = −30‰) in a free air CO2 enrichment experiment in a mature deciduous forest permitted us to trace the carbon transfer from tree crowns to the rhizosphere of 100-120years old trees. During the first season of CO2 enrichment the CO2 released from soil originated substantially from concurrent assimilation. The small contribution of recent carbon in fine roots suggests a much slower fine root turnover than is often assumed.13C abundance in soil air correlated best with temperature data taken from 4 to 10days before air sampling time and is thus rapidly available for root and rhizosphere respiration. The spatial variability of δ13C in soil air showed relationships to above ground tree types such as conifers versus broad-leaved trees. Considering the complexity and strong overlap of roots from different individuals in a forest, this finding opens an exciting new possibility of associating respiration with different species. What might be seen as signal noise does in fact contain valuable information on the spatial heterogeneity of tree-soil interactio

δ 15N measurement of organic and inorganic substances by EA-IRMS: a speciation-dependent procedure

Little attention has been paid so far to the influence of the chemical nature of the substance when measuring δ 15N by elemental analysis (EA)-isotope ratio mass spectrometry (IRMS). Although the bulk nitrogen isotope analysis of organic material is not to be questioned, literature from different disciplines using IRMS provides hints that the quantitative conversion of nitrate into nitrogen presents difficulties. We observed abnormal series of δ 15N values of laboratory standards and nitrates. These unexpected results were shown to be related to the tailing of the nitrogen peak of nitrate-containing compounds. A series of experiments were set up to investigate the cause of this phenomenon, using ammonium nitrate (NH4NO3) and potassium nitrate (KNO3) samples, two organic laboratory standards as well as the international secondary reference materials IAEA-N1, IAEA-N2—two ammonium sulphates [(NH4)2SO4]—and IAEA-NO-3, a potassium nitrate. In experiment 1, we used graphite and vanadium pentoxide (V2O5) as additives to observe if they could enhance the decomposition (combustion) of nitrates. In experiment 2, we tested another elemental analyser configuration including an additional section of reduced copper in order to see whether or not the tailing could originate from an incomplete reduction process. Finally, we modified several parameters of the method and observed their influence on the peak shape, δ 15N value and nitrogen content in weight percent of nitrogen of the target substances. We found the best results using mere thermal decomposition in helium, under exclusion of any oxygen. We show that the analytical procedure used for organic samples should not be used for nitrates because of their different chemical nature. We present the best performance given one set of sample introduction parameters for the analysis of nitrates, as well as for the ammonium sulphate IAEA-N1 and IAEA-N2 reference materials. We discuss these results considering the thermochemistry of the substances and the analytical technique itself. The results emphasise the difference in chemical nature of inorganic and organic samples, which necessarily involves distinct thermochemistry when analysed by EA-IRMS. Therefore, they should not be processed using the same analytical procedure. This clearly impacts on the way international secondary reference materials should be used for the calibration of organic laboratory standards. Figure Control chart of the δ 15N value of IAEA-N1, IAEA-NO-3 and NH4NO3 analysed a) with oxygen injection (analytical cycle 70 s, oxygen for 60 s, sample start and stop at 18 s/20 s), b) with oxygen injection (analytical cycle 70 s, oxygen for 60 s, sample start and stop at 0 s/2 s and 5 s/7 s), c) without oxygen injection (analytical cycle 70 s, sample start and stop at 18 s/20 s

Carbon allocation in shoots of alpine treeline conifers in a CO2 enriched environment

With a new approach we assessed the relative contribution of stored and current carbon compounds to new shoot growth in alpine treeline conifers. Within a free air CO2 enrichment experiment at the alpine treeline in Switzerland, 13C-depleted fossil CO2 was used to trace new carbon in the two tree species Larix decidua L. and Pinus uncinata Ramond over two subsequent years. The deciduous L. decidua was found to supply new shoot growth (structural woody part) by 46% from storage. Surprisingly, the evergreen P. uncinata, assumed to use current-year photosynthates, also utilized a considerable fraction of storage (42%) for new wood growth. In contrast, the needles of P. uncinata were built up almost completely from current-year photosynthates. The isotopic composition of different wood carbon fractions revealed a similar relative allocation of current and stored assimilates to various carbon fractions. Elevated CO2 influenced the composition of woody tissue in a species-specific way, e.g. the water soluble fraction decreased in pine in 2001 but increased in larch in 2002 compared to ambient CO2. Heavy defoliation applied as an additional treatment factor in the second year of the experiment decreased the lipophilic fraction in current-year wood in both species compared to undefoliated trees. We conclude that storage may play an important role for new shoot growth in these treeline conifers and that altered carbon availability (elevated CO2, defoliation) results in significant changes in the relative amount of mobile carbon fractions in woody tissue. In particular, stored carbon seems to be of greater importance in the evergreen P. uncinata than has been previously though

Soil H218O labelling reveals the effect of drought on C18OO fluxes to the atmosphere

Concurrent and continuous measurements of the 18O/16O ratio in CO2 and H2Ov after a H2 18O labelling showed that drought reduces the 18O-equilibrium between CO2 and H2O at the shoot leve

Microbial assimilation of plant-derived carbon in soil traced by isotope analysis

The flow of new and native plant-derived C in the rhizosphere of an agricultural field during one growing season was tracked, the ratios in different soil C pools were quantified, and the residence times (τs) were estimated. For this the natural differences in 13C abundances of: (1) C4 soil (with a history of C4 plant, Miscanthus sinensis, cultivation), (2) C3 soil (history of C3 plant cultivation), and (3) C4/3 soil (C4 soil, planted with a C3 plant, Triticum aestivum) were used. Total amounts and δ13C values of total soil C, non-hydrolysable C, light fraction C, water-soluble C, microbial biomass C, and phospholipid fatty acids (PLFA) were determined. Using the δ13C values of soil C in a mixing and a 1-box model enabled the quantification of relative contributions of C3 plant and C4 plant C to the total amount of the respective C pools in the C4/3 soil and their τs. Compared to early spring (March), the percentage of C3 plant C increased in all pools in June and August, showing the addition of new C to the different soil C fractions. In August the contribution of new C to microbial biomass C and water-soluble C reached 64 and 89%, respectively. The τs of these pools were 115 and 147 days. The δ13C values of the dominant soil PLFA, 18:1ω7c, cy19:0, 18:1ω9c, 16:0, and 10Me16:0, showed wide ranges (−35.1 to −13.0‰) suggesting that the microbial community utilized different pools as C sources during the season. The δ13C values of PLFA, therefore, enabled the analysis of the metabolically active populations. The majority of δ13C values of PLFA from the C4/3 soil were closely related to those of PLFA from the C3 soil when T. aestivum biomass contributions to the soil were high in June and August. Specific populations reacted differently to changes in environmental conditions and supplies of C sources, which reflect the high functional diversity of soil microorganism

Tree-ring growth and stable isotopes (13C and 15N) detect effects of wildfires on tree physiological processes in Pinus sylvestris L

Forest fires may alter the physiological and growth processes of trees by causing stress in trees and modifying the availability of soil nutrient. We investigated if, after a high-severity fire, changes in tree-ring growth can be observed, as well as changes in the nitrogen and carbon isotope composition of tree rings of surviving trees. Two wildfires that occurred in Pinus sylvestris L. stands in Northern Italy, one at the beginning and one at the end of the vegetative season, were chosen as the focus of this study. After the fires, the surviving trees showed growth suppression with very narrow tree rings or locally absent rings. The carbon isotope ratio was more negative in tree rings formed in the 5years following fire, indicating better water supply in a situation of less competition. The nitrogen isotope ratio followed opposite trends in the two wildfire stands. In trees cored in the stand where the fire happened at the beginning of the vegetative season, there was no change in the nitrogen isotope ratio, whereas in samples collected in the other fire site, higher nitrogen isotope ratios were observed in the tree rings formed after the fire, reflecting changes in the soil nitrogen supply. Modifications in the growth and isotope composition of the fire-stressed trees disappeared from 6 to 10years after the fire. By studying trees before and after fire, we were able to show that fire affects not only the growth of surviving trees, but also their physiological processe

Summer temperature dependency of larch budmoth outbreaks revealed by Alpine tree-ring isotope chronologies

Larch budmoth (LBM, Zeiraphera diniana Gn.) outbreaks cause discernable physical alteration of cell growth in tree rings of host subalpine larch (Larix decidua Mill.) in the European Alps. However, it is not clear if these outbreaks also impact isotopic signatures in tree-ring cellulose, thereby masking climatic signals. We compared LBM outbreak events in stable carbon and oxygen isotope chronologies of larch and their corresponding tree-ring widths from two high-elevation sites (1800-2200m a.s.l.) in the Swiss Alps for the period AD 1900-2004 against isotope data obtained from non-host spruce (Picea abies). At each site, two age classes of tree individuals (150-250 and 450-550 years old) were sampled. Inclusion of the latter age class enabled one chronology to be extended back to AD 1650, and a comparison with long-term monthly resolved temperature data. Within the constraints of this local study, we found that: (1) isotopic ratios in tree rings of larch provide a strong and consistent climatic signal of temperature; (2) at all sites the isotope signatures were not disturbed by LBM outbreaks, as shown, for example, by exceptionally high significant correlations between non-host spruce and host larch chronologies; (3) below-average July to August temperatures and LBM defoliation events have been coupled for more than three centuries. Dampening of Alps-wide LBM cyclicity since the 1980s and the coincidence of recently absent cool summers in the European Alps reinforce the assumption of a strong coherence between summer temperatures and LBM defoliation events. Our results demonstrate that stable isotopes in tree-ring cellulose of larch are an excellent climate proxy enabling the analysis of climate-driven changes of LBM cycles in the long ter