Increasing temperatures and atmospheric CO sub(2) concentrations will affect tree carbon fluxes, generating potential feedbacks between forests and the global climate system. We studied how elevated ...temperatures and CO sub(2) impacted leaf carbon dynamics in Norway spruce (Picea abies), a dominant northern forest species, to improve predictions of future photosynthetic and respiratory fluxes from high-latitude conifers. Seedlings were grown under ambient (AC, c. 435 mu mol mol super(-1)) or elevated (EC, 750 mu mol mol super(-1)) CO sub(2) concentrations at ambient, +4 degree C, or +8 degree C growing temperatures. Photosynthetic rates (A sub(sat)) were high in +4 degree C/EC seedlings and lowest in +8 degree C spruce, implying that moderate, but not extreme, climate change may stimulate carbon uptake. A sub(sat), dark respiration (R sub(dark)), and light respiration (R sub(light)) rates acclimated to temperature, but not CO sub(2): the thermal optimum of A sub(sat) increased, and R sub(dark) and R sub(light) were suppressed under warming. In all treatments, the Q sub(10) of R sub(light) (the relative increase in respiration for a 10 degree C increase in leaf temperature) was 35% higher than the Q sub(10) of R sub(dark), so the ratio of R sub(light) to R sub(dark) increased with rising leaf temperature. However, across all treatments and a range of 10-40 degree C leaf temperatures, a consistent relationship between R sub(light) and R sub(dark) was found, which could be used to model R sub(light) in future climates. Acclimation reduced daily modeled respiratory losses from warm-grown seedlings by 22-56%. When R sub(light) was modeled as a constant fraction of R sub(dark), modeled daily respiratory losses were 11-65% greater than when using measured values of R sub(light). Our findings highlight the impact of acclimation to future climates on predictions of carbon uptake and losses in northern trees, in particular the need to model daytime respiratory losses from direct measurements of R sub(light) or appropriate relationships with R sub(dark).
1. Are mature forests carbon limited? To explore this question, we exposed ca. 110-year-old, 40-m tall Picea abies trees to a 550-ppm CO sub(2) concentration in a mixed lowland forest in NW ...Switzerland. The site receives substantial soluble nitrogen (N) via atmospheric deposition, and thus, trees are unlikely N-limited. We used a construction crane to operate the free-air CO sub(2) release system and for canopy access. Here, we summarize the major results for growth and carbon (C) fluxes. 2. Tissue super(13)C signals confirmed the effectiveness of the CO sub(2) enrichment system and permitted tracing the continuous flow of new C in trees. Tree responses were individually standardized by pre-treatment signals. Over the five experimental years, needles retained their photosynthetic capacity and absorbed up to 37% more CO sub(2) under elevated (E) compared to ambient (A) conditions. However, we did not detect an effect on stem radial growth, branch apical growth and needle litter production. Neither stem nor soil CO sub(2) efflux was stimulated under elevated CO sub(2). The rate at which fine roots filled soil ingrowth cores did not significantly differ between A- and E-trees. 3. Since trees showed no stomatal responses to elevated CO sub(2), sap flow remained unresponsive, both in the long run as well as during short-term CO sub(2) on-off experiments. As a consequence, soil moisture remained unaffected. We trapped significantly more nitrate in the root sphere of E-trees suggesting a CO sub(2)-stimulated breakdown of soil organic matter, presumably induced by extra carbohydrate exudation ('priming'). 4. Synthesis. The lack of a single enhanced C sink to match the increased C uptake meant a missing C sink. Increased C transport to below-ground sinks was indicated by C transfer to ectomycorrhiza and on to neighbouring trees and by increased C export to soil. We conclude that these tall Picea abies trees are not C limited at current CO sub(2) concentrations and further atmospheric CO sub(2) enrichment will have at most subtle effects on growth, despite enhanced N availability. In a mature mixed forest, 40 m tall spruce trees exposed to 550 ppm CO sub(2) continuously over 5 years showed increased carbon uptake, but no significant increase in growth, nor in respiration. Nevertheless, an increase was observed in carbon transport to belowground sinks: carbon transfer to ectomycorrhiza and on to neighbouring trees, and carbon export to soil. These tall spruce trees are hence not carbon limited at current CO sub(2) concentrations.
Nitrite (NO sub(2) super(-)) as an important intermediate of the biological nitrogen cycle is particularly reactive in acidic soils and acts as a source of N sub(2)O and NO sub(x) (NO and NO sub(2)). ...However, abiotic and biotic pathways of NO sub(2) super(-)-driven N sub(2)O and NO sub(x) production in forest soil and the role of soil organic matter (SOM) in these processes are still unclear. In this study, NO sub(2) super(-) was applied to both unsterile and sterilized soil samples as well as to different SOM fractions from a Norway spruce forest. Biotic and abiotic N sub(2)O emission was measured with an infrared absorption analyzer and gas chromatography, while NO sub(x) emission was quantified with a chemiluminescence analyzer. Isotopic signatures of N sub(2)O ( delta super(15)N super(bulk), delta super(18)O, and super(15)N-N sub(2)O site preference) were analyzed with an isotope ratio mass spectrometer. After NO sub(2) super(-) addition, a large amount of NO sub(x) was emitted immediately, while N sub(2)O emission occurred 15-60 min later and was much lower compared to NO sub(x). Sterilization of soil decreased N sub(2)O emission significantly, but not NO sub(x) emission. The super(15)N site preference of N sub(2)O ranged from 7.98 to 11.58ppt for abiotic and 4.69-7.42ppt for biotic sources. The fulvic acid fraction contributed the most to abiotic N sub(2)O emission, while the fastest NO and N sub(2)O emission occurred after NO sub(2) super(-)application to the humin fraction, followed by the humic acid fraction. These results are important for the future understanding of NO sub(x) and N sub(2)O sources, as well as the use of isotopic signatures for source-partitioning N sub(2)O emission from soil. Figure not available: see fulltext.
Only scarce information is available on how organic C is incorporated into the soil during the decay and how (micro) climate influences this process. Therefore, we investigated the effect of exposure ...and elevation on the organic litter decomposition and C-stabilisation in acidic soils of an Alpine environment. An experiment with artificially super(13)C labelled Norway spruce needles was carried out at north- and south-exposed sites between 1200 and 2400 m a.s.l. in the Italian Alps using mesocosms. After 1 year, the super(13)C recoveries of the bulk soil were 18.6% at the north-facing slopes and 31.5% at the south-facing slopes. A density fractionation into a light (LF; less than or equal to 1.6 g cm super(-3)) and a heavy fraction (HF; >1.6 g cm super(-3)) of the soil helped to identify how the applied substrate was stabilised. At the northern slope, 10.5% of the substrate was recovered in the LF and 8.1% in the HF and at the south-facing slope 22.8% in the LF and 8.1% in the HF. The overall super(13)C recovery was higher at the south-facing sites due to restricted water availability. Although the climate is humid in the whole area, soil moisture availability becomes more important at south-facing sites due to higher evapotranspiration. However, at sites >1700 m a.s.l, the situation changed, as the northern slope had higher recovery rates. At such altitudes, temperature effects are more dominant. This highlights the importance of locally strongly varying edaphic factors when investigating the carbon cycle.
Relevant CO sub( 2 ) increase affects iWUE and growth potential of Alpine Norway spruce forests due to triggering of photosynthetic capacity. Minor effect on iWUE of tree size/age ontogenetic ...factors. An increase in European forest productivity has been widely reported, but evidences on its causal relationship with climate change are still scarce, though they are crucial to understand the mitigation potential of forests and their future dynamics. In the present study, we first assessed the changes in forest productivity of two even-aged Norway spruce forests. Consequently, we investigated the role of several environmental drivers, such as atmospheric CO sub(2) levels, temperature, and precipitation regimes on the intrinsic water-use efficiency (iWUE) temporal patterns of the above-mentioned forests. We applied a chronosequence approach, combining it with a multi-stable isotope analysis, including delta super(13)C and delta super(18)O, to infer tree responses to climate change over time in terms of iWUE changes. By this innovative methodology, we were able to separate environmental and age/size-related factors on iWUE changes. Results showed an increase in forest productivity in both sites, paralleled by a significant increase of iWUE, mainly triggered by a CO sub(2)-driven increase in photosynthetic capacity, rather than by a reduction of stomatal conductance. The paramount role of the increase in photosynthetic capacity was confirmed by a strong correlation between atmospheric CO sub(2) concentration and iWUE temporal patterns. The effect of size/age of trees on iWUE temporal changes resulted to be less defining than that of climate change.
Understanding the effects of elevated atmospheric CO sub(2) on carbon (C) relations of mature forest trees is central to understanding ecosystem C fluxes and pools in a future high-CO sub(2) world. ...Here, we investigated the CO sub(2)-induced photosynthetic enhancement and the diurnal variation in shoot carbon assimilation, stem CO sub(2) efflux and soil respiration associated with ca. 110-year-old and 37 m tall Norway spruce trees (Picea abies (L.) H. Karst.) growing under free air CO sub(2) enrichment (FACE) in a mixed, near-natural forest in Northern Switzerland. Diurnal measurements of these major C fluxes were conducted simultaneously on three occasions: one week before and after the start of CO sub(2) enrichment, and one year later. Under controlled leaf chamber conditions, an increase in the atmospheric CO sub(2) concentration of ca. 150 ppm above ambient stimulated light-saturated rates of photosynthesis in previous- and current-year upper-canopy shoots equally by 73 plus or minus 2%. In the course of the day such large differences in C assimilation between trees growing under elevated CO sub(2) (eCO sub(2)) and ambient conditions (aCO sub(2)) only occurred around midday under non-limiting light conditions. The CO sub(2) efflux rates from spruce stems (CE sub(stem)) and surrounding soil (R sub(soil)) shared a similar range during night- and daytime (3-5 mu mol m super(-2) s super(-1)) but were not stimulated by eCO sub(2). Both CE sub(stem) stem and R sub(soil) were still rising when photosynthesis approached evening light compensation potentially reflecting the time lag in assimilate allocation to stem tissue and fine roots. Our findings suggest a strong photosynthetic enhancement during the initial CO sub(2) enrichment phase but provide no evidence for an overall or daytime-dependent stimulation of respiratory CO sub(2) fluxes indicating that the extra C was not quickly returned to the atmosphere through respiratory processes in spruce stems or surrounding soil.
There is evidence of continued stimulation of foliage photosynthesis in trees exposed to elevated atmospheric CO sub(2) concentrations; however, this is mostly without a proportional growth response. ...Consequently, we lack information on the fate of this extra carbon (C) acquired. By a steady application of a super(13)CO sub(2) label in a free air CO sub(2) enrichment (FACE) experiment, we traced the fate of C in 37-m-tall, ca. 110-year-old Picea abies trees in a natural forest in Switzerland. Hence, we are not reporting tree responses to elevated CO sub(2) (which would require equally super(13)C labeled controls), but are providing insights into assimilate processing in such trees. Sunlit needles and branchlets grow almost exclusively from current assimilates, whereas shaded parts of the crowns also rely on stored C. Only 2.5 years after FACE initiation, tree rings contained 100 % new C. Stem-respiratory CO sub(2) averaged 50 % of new C over the entire FACE period. Fine roots and mycorrhizal fungi contained 49-56 and 26-43 % new C, respectively, after 2.5 years. The isotopic signals in soil CO sub(2) arrived 12 days after the onset of FACE, yet it contained only ca. 15 % new C thereafter. We conclude that new C first feeds into fast turnover C pools in the canopy and becomes increasingly mixed with older C sources as one moves away (downward) from the crown. We speculate that enhanced C turnover (its metabolic cost) along the phloem path, as evidenced by basipetal isotope signal depletion, explains part of the 'missing carbon' in trees that assimilated more C under elevated CO sub(2).
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