Eloni Sonninen

Drought responses of Eucalyptus microtheca provenances depend on seasonality of rainfall in their place of origin

We exposed seedlings of 12 Eucalyptus microtheca F. Muell. provenances to well-watered and water-stressed growing conditions in a greenhouse experiment and investigated the effects of drought on various plant properties in the provenances. We found significant variation in total biomass, height, root mass/foliage area ratio,foliage area/stem cross sectional area ratio, specific leaf area (SLA), water-use efficiency (WUE) and carbon isotope composition (d 13 C) among the provenances. The observed inter-provenance variation was more pronounced in the water-stressed treatment than in the well-watered one. Drought increased root mass/foliage area ratio, foliage area/stem cross sectional area ratio, WUE, d 13 C and decreased total biomass, height, transpiration and SLA. We also analysed relationships between plant properties and climate of native habitats of the provenances and found that most properties were strongly correlated with mean driest quarter rainfall. The correlation was positive for total biomass, height, transpiration and SLA and negative for root mass/foliage area ratio, foliage area/stem cross sectional area ratio, WUE and d 13 C. Finally, we evaluated the intra-specific variation in foliage area/stem cross sectional area ratio in the context of tree hydraulic architecture: provenances from dry areas and trees grown under drought stress had more foliage per stem area ratio. However, their transpiration and the length of their hydraulic pathway were smaller and therefore the root to leaf water potential gradient might be smaller in these trees.

Spatial variability and temporal trends in water‐use efficiency of European forests

The increasing carbon dioxide (CO2 ) concentration in the atmosphere in combination with climatic changes throughout the last century are likely to have had a profound effect on the physiology of trees: altering the carbon and water fluxes passing through the stomatal pores. However, the magnitude and spatial patterns of such changes in natural forests remain highly uncertain. Here, stable carbon isotope ratios from a network of 35 tree-ring sites located across Europe are investigated to determine the intrinsic water-use efficiency (iWUE), the ratio of photosynthesis to stomatal conductance from 1901 to 2000. The results were compared with simulations of a dynamic vegetation model (LPX-Bern 1.0) that integrates numerous ecosystem and land-atmosphere exchange processes in a theoretical framework. The spatial pattern of tree-ring derived iWUE of the investigated coniferous and deciduous species and the model results agreed significantly with a clear south-to-north gradient, as well as a general increase in iWUE over the 20th century. The magnitude of the iWUE increase was not spatially uniform, with the strongest increase observed and modelled for temperate forests in Central Europe, a region where summer soil-water availability decreased over the last century. We were able to demonstrate that the combined effects of increasing CO2 and climate change leading to soil drying have resulted in an accelerated increase in iWUE. These findings will help to reduce uncertainties in the land surface schemes of global climate models, where vegetation-climate feedbacks are currently still poorly constrained by observational data.

Temperature sensitivity of soil carbon fractions in boreal forest soil.

Feedback to climate warming from the carbon balance of terrestrial ecosystems depends critically on the temperature sensitivity of soil organic carbon (SOC) decomposition. Still, the temperature sensitivity is not known for the majority of the SOC, which is tens or hundreds of years old. This old fraction is paradoxically concluded to be more, less, or equally sensitive compared to the younger fraction. Here, we present results that explain these inconsistencies. We show that the temperature sensitivity of decomposition increases remarkably from the youngest annually cycling fraction (Q10 < 2) to a decadally cycling one (Q10 = 4.2-6.9) but decreases again to a centennially cycling fraction (Q10 = 2.4-2.8) in boreal forest soil. Compared to the method used for current global estimates (temperature sensitivity of all SOC equal to that of the total heterotrophic soil respiration), the soils studied will lose 30-45% more carbon in response to climate warming during the next few decades, if there is no change in carbon input. Carbon input, derivative of plant productivity, would have to increase by 100-120%, as compared to the earlier estimated 70-80%, in order to compensate for the accelerated decomposition.

Modeling 13C discrimination in tree rings

Annual variations from 1877 to 1995 in tree-ring α-cellulose 13C/12C isotopic ratios for four subarctic Pinus sylvestris trees were determined, and, in conjunction with a recent record of atmospheric 13CO2/12CO2 ratios, the historical pattern of photosynthetic isotope discrimination, Δ13C, was evaluated. Year-to-year variability in Δ13C has been as much as 1.5‰ with the period 1900–1920 showing an extended period of unusually high photosynthetic discriminations. The summers during these years were, on average, unusually cold. Since 1920 a long term trend of increasing Δ13C of ∼0.016‰yr−1 is inferred. We compared measured Δ13C with those predicted on the basis of the theoretical relationship between Δ13C and the ratio of substomatal to ambient CO2 concentration, Ci/Ca using mechanistic equations for chloroplast biochemistry coupled with a stomatal conductance model. Two variations of a nonlinear optimal-regulation stomatal conductance model were compared. Although both models were based on the assumption that stomata serve to minimize the average transpiration rate for a given average rate of CO2 assimilation, one version of the model incorporated reductions in stomatal conductance in response to recent increases in atmospheric CO2 concentrations and the other did not. The CO2 sensitive stomatal model failed to describe the long-term increase in 13C discrimination, especially after 1950. The insensitive model gave good agreement, suggesting that an observed increase in subarctic Pinus sylvestris Δ13C since 1920 is attributable to recent increases in atmospheric CO2 concentrations with subsequent increases in the ratio of substomatal to ambient CO2 concentrations. The model was also capable of accounting for high frequency (year-to-year) variations in Δ13C, these differences being attributable to year-to-year fluctuations in the average leaf-to-air vapor pressure difference affecting stomatal conductance and hence Ci/Ca.

The influence of preservation method and time on the δ13C value of dissolved inorganic carbon in water samples

The precise delta(13)C value of dissolved inorganic carbon (DIC) is important for various types of ecological studies. Without a preservation agent, microbial degradation of organic compounds continues in water samples and the delta(13)C value of DIC will become more depleted with time. HgCl(2) or acidification is often used to prevent microbial activity in water samples collected for carbon isotope ratio analyses of DIC. Mercury compounds are toxic and result in waste disposal problems. Other inhibiting agents or preservation methods are therefore needed. Two possible solutions are to use copper sulphate (CuSO(4)) as a preservative agent or to acidify water samples with phosphoric acid (H(3)PO(4)) within 12 mL measurement Exetainers (septum-capped vials). We prepared a set of lake water samples in three types of vials: glass vials with silicone/PTFE septa, high-density polyethylene vials (HD-PE, scintillation vials) and Exetainers (12 mL) with butyl rubber septa. Samples in glass and PE vials were preserved with and without CuSO(4), whereas lake water was injected into the Exetainer and acidified with H(3)PO(4). Isotope ratios were measured in two laboratories over 6 months. The delta(13)C values of DIC systematically increased with storage time for samples preserved in glass and PE vials with and without CuSO(4). A strong correlation between a decrease of CO(2) concentration and an increase in DIC delta(13)C values was found. The delta(13)C values and DIC concentrations were stable for 6 months in acidified samples stored in Exetainers with butyl rubber septa. Therefore, we conclude that the best method for up to 6 months of storage is to inject samples in the field into butyl rubber septum capped Exetainers containing H(3)PO(4), thereby avoiding the use of preservatives.

Use of bomb-produced 14C to evaluate the amount of CO2 emanating from two peat bogs in Finland

We used moss increment counting to obtain well-defined samples of the topmost peat layers of two Sphagnum fuscum hummocks. The two ombrotrophic bogs, Lakkasuo in central Finland and Korvinsuo in eastern Finland, are of different ages, covering 3 and 9 ka, respectively. Using AMS dating, we traced bomb-produced 14C through the topmost parts of the two peat profiles. A well-defined 14C activity peak was found in both sequences dating the corresponding layer to AD 1965. A comparison between the maximum peat activities and the corresponding atmospheric values for the period of interest provides an opportunity to evaluate the amount of CO2 emanating from the decaying peat bog, and taken up by the living sphagnum plants. Considerable variations in 613C values were also observed. These variations indicate, at least partly, annual variations in the emission rate of C02 from decomposition of older peat in the bog, and are connected with climatic factors such as temperature and precipitation.

Contrasting effects of increased carbon input on boreal SOM decomposition with and without presence of living root system of Pinus sylvestris L.

AimsWe studied how the availability of carbon affects the decomposition of soil organic matter (SOM) in the presence of living root system compared to bare SOM treatments without roots.MethodsWe measured the effect of living Pinus sylvestris L. root systems on SOM C and N budgets, on the age of carbon in CO2 efflux, and on SOM protease enzyme activity. We examined the possible explanatory factors for increased SOM mineralization such as plant biomass, photosynthesis, microbial C, and protease enzyme activity in the soil.ResultsThe age of carbon respired during the decomposition of soil organic matter was significantly older in the presence of living root systems than in treatments lacking roots. If plants were present, glucose added directly in the SOM accelerated the C and N loss from the bulk material and decreased the rate of photosynthesis.ConclusionsWe conclude that the priming effect of SOM decomposition was affected by the presence of living root system. Our results stress the importance of including the plant–soil interactions in the SOM decomposition models used in climate change studies.

Adaptive Radiation along a Thermal Gradient: Preliminary Results of Habitat Use and Respiration Rate Divergence among Whitefish Morphs

Adaptive radiation is considered an important mechanism for the development of new species, but very little is known about the role of thermal adaptation during this process. Such adaptation should be especially important in poikilothermic animals that are often subjected to pronounced seasonal temperature variation that directly affects metabolic function. We conducted a preliminary study of individual lifetime thermal habitat use and respiration rates of four whitefish (Coregonus lavaretus (L.)) morphs (two pelagic, one littoral and one profundal) using stable carbon and oxygen isotope values of otolith carbonate. These morphs, two of which utilized pelagic habitats, one littoral and one profundal recently diverged via adaptive radiation to exploit different major niches in a deep and thermally stratified subarctic lake. We found evidence that the morphs used different thermal niches. The profundal morph had the most distinct thermal niche and consistently occupied the coldest thermal habitat of the lake, whereas differences were less pronounced among the shallow water pelagic and littoral morphs. Our results indicated ontogenetic shifts in thermal niches: juveniles of all whitefish morphs inhabited warmer ambient temperatures than adults. According to sampling of the otolith nucleus, hatching temperatures were higher for benthic compared to pelagic morphs. Estimated respiration rate was the lowest for benthivorous profundal morph, contrasting with the higher values estimated for the other morphs that inhabited shallower and warmer water. These preliminary results suggest that physiological adaptation to different thermal habitats shown by the sympatric morphs may play a significant role in maintaining or strengthening niche segregation and divergence in life-history traits, potentially contributing to reproductive isolation and incipient speciation.

Impacts of added dissolved organic carbon on boreal freshwater pelagic metabolism and food webs in mesocosm experiments.

We studied the response of pelagic community metabolism, bacterial community structure and the origin of crustacean zooplankton carbon following additions of two different sources of dissolved organic carbon (DOC), natural humic water and cane sugar, in two mesocosm experiments lasting 15-18 days. Experimental water with the natural plankton community originated from two boreal lakes in southern Finland having similar total organic carbon (ca. 11 mg L -1 ) and total phosphorus (ca. 13 μg L -1 ) content, but differing in total nitrogen content (ca. 1450 and 440 mg L -1 ). The experiments demonstrated limitation of bacterial activity mainly by biodegradable DOC in the high nitrogen lake water, and by nitrogen in the low nitrogen lake water, which was also reflected in total pelagic energy metabolism and zooplankton productivity. DOC additions caused changes in the bacterial community structure, shown by length heterogeneity analysis of the PCR amplified 16S rRNA gene (LH-PCR). Cane sugar, differing in stable carbon isotope value (δ 13 C ca.-11‰) from that of humic matter of boreal terrestrial vegetation (δ 13 C ca.-28‰), proved to be a good tracer of DOC via bacteria to metazooplankton, especially in the nitrogen-limited conditions. Utilization of detrital particulate terrestrial organic matter by zooplankton was also evident, but it appeared to be a poor quality food. Our results show that an increased loading of allochthonous DOC to boreal lakes as predicted under future climate scenarios will not necessarily stimulate bacterial production and hence carbon transfer up the food web. Whether this happens will be strongly dependent on the stoichiometry (C: N:P ratios) of the available resources.

A 520 year record of summer sunshine for the eastern European Alps based on stable carbon isotopes in larch tree rings

A 520-year stable carbon isotope chronology from tree ring cellulose in high altitude larch trees (Larix decidua Mill.), from the eastern European Alps, correlates more strongly with summer temperature than with summer sunshine hours. However, when instrumental records of temperature and sunshine diverge after AD1980, the tree ring time series does not follow warming summer temperatures but more closely tracks summer sunshine trends. When the tree ring stable carbon isotope record is used to reconstruct summer temperature the reconstruction is not robust. Reconstructed temperatures prior to the twentieth century are higher than regional instrumental records, and the evolution of temperature conflicts with other regional temperature reconstructions. It is concluded that sunshine is the dominant control on carbon isotope fractionation in these trees, via the influence of photosynthetic rate on the internal partial pressure of CO2, and that high summer (July–August) sunshine hours is a suitable target for climate reconstruction. We thus present the first reconstruction of summer sunshine for the eastern Alps and compare it with the regional temperature evolution.