1,720,965 research outputs found
Tree rings indicate different drought resistance of a native (Abies alba Mill.) and a nonnative (Picea abies (L.) Karst.) species co-occurring at a dry site in Southern Italy
No full textClimate changes induced by the anthropogenic alteration of the atmospheric radiative balance are expected to change the productivity and composition of forest ecosystems. In Europe, the Mediterranean is considered one of the most vulnerable regions according to climatic forecasts and simulations. However, although modifications in the inter-specific competition are envisaged, we still lack a clear understanding of the ability of the Mediterranean vegetation to adapt to climate changes. We investigated how two co-occurring tree species commonly used in afforestation programmes, the native Abies alba Mill. and the nonnative Picea abies L. Karst., adapt to climate change by assessing their growth performance and physiological responses in relation to past climate variability. Growth was addressed by analysing tree-ring width and carbon and oxygen stable isotopes. Statistical relationships between isotopic value and monthly climate data suggest that the two species underwent ecophysiological adaptation to Mediterranean climatic constraints. These adaptations are also expressed in the ring-width data. Based on the carbon isotope ratio reflecting the stomatal response to drought, we found that the precipitation in the first period of the growing season, i.e. early spring, is a major factor influencing the annual growth of A. alba, which although native, proved to be sensitive to drought. P. abies, on the other hand, showed a higher tolerance to summer drought stress. These findings should help define criteria for sustainability and effective forest conservation in the Mediterranean region. © 2008 Elsevier B.V. All rights reserved
Climatic sensitivity of δ18O in the wood and cellulose of tree rings: Results from a mixed stand of Acer pseudoplatanus L. and Fagus sylvatica L.
No full textThe oxygen isotope composition in tree rings is commonly used to assess the impact of climatic factors on tree growth. However, the relationships between environmental variables and δ18O in whole wood and the wood components (e.g., cellulose, lignin) are still not completely clear - particularly for deciduous species. It is currently common procedure to consider only one wood constituent, a process that saves time. To test whether the δ18O of cellulose relates better to environmental variables than that of whole wood, we measured the oxygen isotopic composition of tree ring whole wood and cellulose, for the period 1916-1950, for two different broad-leaved species (Fagus sylvatica L. and Acer pseudoplatanus L.) on Monti Picentini (Southern Italy). Data show that δ18O in cellulose is enriched (4.7 ± 1.1‰) compared to the corresponding whole wood. The relationship between the δ18O of the cellulose and of whole wood is generally poor, especially for A. pseudoplatanus suggesting that the sources of variability for δ18O differ strongly between the whole wood and the cellulose constituents. Furthermore, we find that the cellulose fraction correlates strongly with climatic variables, while the whole wood fraction generally does not. Monthly temperature during the growing season is strongly correlated with δ18O of cellulose for F. sylvatica (r = 0.82) and A. pseudoplatanus (r = 0.63) but not with δ18O of whole wood. We found a relevant difference in the isotopic signals of the two species that could be related with difference in seasonal timing of growth. In contrast to F. sylvatica, early spring conditions have an influence on A. pseudoplatanus at our study site. This is evidenced by the high correlation between δ18O of cellulose and April and May precipitation, and the positive relationship between tree ring width and March and April mean annual temperature. The different climatic sensitivities of the two investigated species suggest that physiological properties, i.e. root system and stomatal conductance responses, or a variation in the proportions of wood constituents play an important role for the oxygen isotopic signal. We conclude that, even when partially obscured by site parameters (soil depth and structure, nutrient availability, etc.), δ18O in the cellulose compartment strongly reflects climate information. Our study suggests that cellulose extraction is a necessary step in studies seeking to investigate the climatic signatures in the δ18O of broad-leaved species. © 2008 Elsevier B.V. All rights reserved
Climatic isotope signals in tree rings masked by air pollution: A case study conducted along the Mont Blanc Tunnel access road (Western Alps, Italy)
No full textThree sites at about 1400 m a.s.l., were chosen for this study along the Mt. Blanc Motorway in Italy. Chronologies of stable isotope ratios (δ 13C, δ 18O, δ 15N), total N concentration and ring width of Larix decidua Mill. were analyzed to observe changes in growth and climatic signals in tree rings after significant changes in air pollution emissions occurred locally over time. The tunnel opened in 1968 and was closed for three years from March 1999 to March 2002. The obtained series from the three sites (Close = C, High = H and Far = F from the highway) for the analyzed periods, 1950-1970 (only sites H and F) and 1985-2008 (all sites), did not show any particular long-term change except site H that showed significant changes in δ 18O (enrichment), δ 15N (depletion) and total N (increase) δ 13C values at site C were enriched in the first year of the tunnel closure (1999), showing an opposite trend in δ 13C at site C, in comparison to the two control sites H and F, which cannot be explained by climatic factors. Since no great differences in δ 18O were recorded in 1999, this enrichment in δ 13C could be related to an enhancement in photosynthetic rate during periods of low air pollution loads. Opposite to δ 15N, total N concentration shows a generally good correlation between sites, and site F was measured as the most N enriched. In a correlation analysis performed on the two study periods between the climatic parameters and ring width, we did not find any clear relationships, whereas for the series of stable isotopes and total N, we found the strongest and most significant relationships only between δ 13C and summer (June to August) temperature (positive correlations) and precipitation (negative correlations) at sites H and F. These same relationships at site C were, instead, mostly insignificant, indicating an alteration of the climatic signal recorded in the δ 13C chronology, caused by direct exposure to the high level of air pollution at this site. Given that site C is more affected by pollution with respect to the other two sites (whereas the same climatic conditions influence tree growth at all sites), the lack of a climatic signal in the δ 13C chronology at this site can be ascribed to air pollution. Few other long-term changes were recorded by tree rings (e.g. at site H), indicating that trees probably record better the pollution events or the worsening of the environmental conditions rather than a lack of pollution for a relatively short time period in a polluted environment. © 2012 Elsevier Ltd
Volcanic explosive eruptions of the Vesuvio decrease tree-ring growth but not photosynthetic rates in the surrounding forests
No full textVolcanic eruptions impact the global and the hemispheric climate, but it is still unknown how and to what degree they force the climate system and in particular the global carbon cycle. In this paper, the relationships between individual eruptions (reconstructed for the past using written records), tree primary productivity (estimated using ring widths), photosynthetic rate and stomatal conductance (assessed by carbon and oxygen isotope data) are investigated, to understand the impact of volcanic eruptions on net primary production. Data from a mixed stand of Fagus sylvatica L. and Acer pseudoplatanus L. located in the area of the Vesuvio volcanic complex (Southern Italy) showed a significant decrease in ring width following each eruption. Isotope analyses indicate a change in climatic conditions after such events. Specifically, the lower oxygen isotope ratio in the tree-ring cellulose strongly suggests an increase in relative humidity and a decrease in temperature, with the latter resulting in a strong limitation to tree-ring growth. The carbon isotope ratio was only moderately but not significantly reduced in the years of volcanic eruption, suggesting no major changes in C fixation rates. This work is a case study on the effects of volcanic eruptions resulting in strong climatic changes on the local scale. This is an opportunity to explore the process and causal relationships between climatic changes and the response of the vegetation. Thus, we propose here a realistic model scenario, from which we can extrapolate to global scales and improve our interpretations of results of global studies. © 2007 Blackwell Publishing Ltd
Temporal dynamics in δ13C of ecosystem respiration in response to environmental changes
No full textWerner C, Unger S, Pereira JS, Ghashghaie J, Máguas C. Temporal dynamics in δ13C of ecosystem respiration in response to environmental changes. In: Dawson TE, Siegwolf RTW, eds. Stable isotopes as indicators of ecological change. Terrestrial ecology series. San Diego: Elsevier Academic Press; 2007: 193-210
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Variations on the Author
Full text link“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
Appropriate Similarity Measures for Author Cocitation Analysis
Full text linkWe provide a number of new insights into the methodological discussion about author cocitation analysis. We first argue that the use of the Pearson correlation for measuring the similarity between authors’ cocitation profiles is not very satisfactory. We then discuss what kind of similarity measures may be used as an alternative to the Pearson correlation. We consider three similarity measures in particular. One is the well-known cosine. The other two similarity measures have not been used before in the bibliometric literature. Finally, we show by means of an example that our findings have a high practical relevance.information science;Pearson correlation;cosine;similarity measure;author cocitation analysis
Response of mature Norway spruce (Picea abies) to elevated atmospheric CO²
Full text linkThe aim of this thesis was to identify pathways and rates of C allocation in tall forest trees, and to identify effects of elevated CO2 on respiratory processes and root growth. Correspondingly, this thesis is divided into three separate parts:
Chapter 2)
Long-term 13C labeling provides evidence for temporal and spatial carbon allocation patterns in mature Picea abies (published in Oecologia)
Chapter 3)
Respiratory fluxes and fine root responses in mature Picea abies trees exposed to elevated atmospheric CO2 concentrations (published in Biogeochemistry)
Chapter 4)
Photosynthetic enhancement and diurnal stem and soil carbon fluxes in a mature Norway spruce stand under elevated CO2 (published in Environmental and Experimental Botany)
The work was conducted at the Swiss canopy crane (SCC) research site in Hofstetten near Basel, Switzerland, and explored signals produced by free air CO2 enrichment (FACE) in 110-year-old, ca. 37m tall P. abies trees. Chapter 2 capitalizes on the isotopic signal carried by the CO2 gas used for CO2 enrichment, yet does not address effects of elevated CO2 as such, but rather deals with basic questions of C transfer in tall trees. Chapter 3 explores the longer-term CO2 effects on mature P. abies (i.e. 2.5 years), whereas chapter 4 reports short-term (diurnal) responses to elevated CO2. In the following, I will provide a summary of the results of the three chapters of my thesis, extended by a conclusion that links these chapters.
Chapter 2) Long-term 13C labeling of Picea abies
As a side effect, the FACE technique provided the unique opportunity to study C translocation within the tree body using the stable isotope 13C signal the FACE gas carries. Since control trees are not (can not) be similarly labeled with 13C the tree responses to elevated CO2 were not the subject of this chapter. Yet, FACE resembles a continuous 13C labeling of new assimilates. Tracking the fate of these assimilates over a period of 2.5 years in tall trees offers new insights in tree C relations under steady state conditions. We tracked 13C signals in mature P. abies trees at a high spatial and temporal resolution, i.e. from the canopy (needles and branchlets), down to the tree trunk (year rings and stem CO2 efflux), and into the soil compartment (fine roots, fungi, soil CO2 efflux). The following key questions were answered:
1. How long does it take for new C to arrive at a certain tissue type or respiratory flux?
2. What is the proportional contribution of newly assimilated C to concurrent tree tissue production and maintenance?
3. How long does it take until old C is replaced by new C in various tissues?
Generally, we observed a reduction of new assimilate investment with distance from the canopy, which can be explained by a progressive dilution of new C into the existing C storage pools in the tree. New sunlit needles (and adjacent branchlets) exhibited a nearly 100% share of new C, whereas shaded needles also used some older C. Stem wood isotope signals evidenced a complete exchange of old C by new C within 2 years. Fine roots contained only 49-56% new C, hence are using older C pools for a longer period of time. A surprisingly low fraction of novel C (26-43%) was recovered from fungal sporocarps, presumably related to the influence of neighboring trees that were not CO2 enriched. The first appearance of new C in soil and stem CO2 release occurred after 12 days, reflecting a lag due to the long transport distances in these 37m tall trees. The CO2 released by stems was composed of 50% new C already in the first year of FACE. In contrast, only ca. 15% new C contributed to soil CO2 efflux, reflecting the use of older substrates, and the influence of older roots and litter from neighboring trees blown in by wind.
These findings indicate a rapid contribution of new assimilates to tissue formation, and thus, a fast replacement of mobile C reserves with new C, and a progressive signal dilution from treetop to the bottom. The two-year replacement time in stem xylem shows that the storage pool is contributing substantially to tree ring formation. We speculate that the turnover of mobile C pools might be enhanced by elevated CO2, and the metabolic costs of this turnover might compensate for some of the extra C taken up at elevated CO2 concentrations, and thus, may explain the ‘missing C’ at the whole tree level. These metabolic costs are unlikely to produce measurable signals at tissue level, given the large heterotrophic volume of such trees.
Chapter 3) Responses of Picea abies to elevated CO2
Most FACE experiments revealed strong initial growth responses to elevated CO2 that diminished over the first 3 years (Körner 2006). Since growth in natural undisturbed systems is commonly not showing a continued stimulation under altered CO2 for reasons of nutrient supply, a step increase in CO2 concentration should induce overflow responses in terms of enhanced respiration and fine root expansion, the latter in order to forage for nutrients to balance the additional C input. In this web-FACE experiment, established in a natural Central European forest, we investigated mature ca. 110-year-old P. abies trees in their steady state of growth (C cycle coupled to the nutrient cycle; Körner 2006). In this publication we were particularly interested in:
1. Seasonal shifts in assimilate allocation;
2. Locations of C-investment;
3. Residence times (turnover) of mobile C pools.
We tracked the respiratory and fine root growth responses of these trees before and directly after the start of FACE, and for further 2.5 years. The CO2 concentration in the canopy (e.g. 540 ppm) was about twice the pre-industrial level. We anticipated a stimulation of CO2 release, and faster root expansion into root-free soil space (in-growth core method), but we also expected a weaker signal in these mature trees compared to young trees. Seasonal stem CO2 efflux did not show any sign of increase during the 2.5 years under elevated CO2. This result lines up with the lack of any stem radial growth response (ongoing work). Fine roots (<0.5-2 mm) did not accumulate more dry matter in the course of 2.5 years of CO2 fertilization. Interestingly, we observed a slight but significant reduction of CO2 release from the soil despite clear evidence by isotopic signals that novel assimilates arrived in the soil.
These data suggest that such mature trees do not even show a transient stimulation of respiration to a step increase of CO2, as was observed in other FACE experiments using much younger trees (Norby et al. 2010). Other growth-limiting factors appear to prevent more vigorous tree growth and thus, metabolism at high CO2 (Norby & Zak 2011). N limitation can be excluded at our site because of high N-deposition. A part of the extra C taken up by needles at elevated CO2 might have been allocated belowground, however, not to fine roots. Conversely, slightly reduced rather than increased rates of soil CO2 efflux implies that respiration of roots and/or soil organisms declined under elevated CO2, implying an overall reduced C allocation into the rhizosphere. We assume that extra C absorbed by foliage is either retained within the tree body (stored carbohydrates), recycled by respiration rates below detection limit across all heterotrophic plant tissues, or lost through enhanced leaching of dissolved organic or inorganic carbon (DIC/DOC).
In summary, we conclude that mature P. abies trees at our site are roughly C saturated at current CO2 concentrations. We find no indication of stimulated belowground metabolic activity (fine roots and soil CO2 efflux).
Chapter 5) Diurnal courses in P. abies under elevated CO2
Leaf-level photosynthetic stimulation in trees following a step increase of atmospheric CO2 was commonly observed in CO2 enrichment experiments, however, mostly without corresponding growth stimulation. Hence, the fate of this additional C input in tree still is not fully resolved, but C overflow mechanisms such as respiratory C losses might account for this C surplus. Since these potential variations in C fluxes might not be detectable on a daily basis, a response may emerge on shorter timescales (i.e. on a diurnal basis). This chapter (co-authorship) explored the diurnal variations in C fluxes (i.e. net photosynthesis, and CO2 efflux from the forest floor and the from stem) in mature P. abies trees exposed to elevated CO2 in the SCC web-FACE experiment. We tracked the diurnal variations of these fluxes on a summer day shortly before the onset of FACE, and twice during the FACE periods in summer 2009 and 2010.
Results from this study confirmed a CO2-induced photosynthetic stimulation shortly after the onset of FACE, and a change in magnitude throughout the day. Intriguingly, this stimulation of Anet diminished in the second year under FACE, indicating photosynthetic downregulation in these trees. The respiratory fluxes from P. abies stems, as observed on a seasonal basis (chapter 4), were not affected by high levels of CO2 whereas soil CO2 efflux decreased slightly with prolonged exposure to elevated CO2. Further, the diurnal patterns of CO2 release (stems and soil) were not altered by CO2 enrichment.
In conclusion, despite larger C input into the tree system in the first year of FACE, respiratory overflow mechanisms could not be observed even on a diurnal basis, corroborating our results obtained in chapter 4. Additionally, the photosynthetic downregulation observed at high CO2 confirms the assumption that these trees are C saturated.
Final conclusions
Stimulatory effects of elevated CO2 on tree growth are constrained by several growth-limiting factors, mainly availability of nutrients and other resources, and the developmental stage (age) of a tree. This thesis for the first time illuminates the current (chapter 2) and future (chapters 3 and 4) C balance of mature evergreen conifers subjected to prospective CO2 levels of 540 ppm in a near-natural forest in Switzerland. Isotopic labeling of fresh assimilates successfully depicted the pathways of C in these trees, thus provided basic insights into how P. abies trees handle the distribution of assimilates. We observed remarkable tree-specific variations in all pre-treatment measurements, emphasizing the importance of recording baseline conditions prior to any experiment. At current CO2 levels, all investigated tissues (except for needles in the sun), and respiratory fluxes depended only partly on new assimilates. The further away from the upper tree canopy, the greater the role of old C stores for new tissue formation and respiration. Since no aboveground growth stimulation was observed (ongoing works) despite higher but transient rates of photosynthesis, and since stem CO2 efflux remained unaffected by elevated CO2, we assume that the extra C assimilated in the first year is dissipated via respiration associated with C turnover (phloem) at rates below detection limit. These processes seem to be too small to be detectable but their accumulated rate along the entire phloem system might account for the unresolved ‘missing C’ at elevated CO2. We found no evidence for increased C investment belowground at elevated CO2 that might also account for some of the higher leaf-level C input at elevated CO2.
References
Körner C (2006) Plant CO2 responses: An issue of definition, time and resource supply. New Phytologist 172:393-411
Norby RJ, Warren JM, Iversen CM, Medlyn BE, McMurtie RE (2010) CO2 enhancement of forest productivity constrained by limited nitrogen availability. Proceedings of the National Academy of Sciences of the United States of America 107:19368-19373.
Norby RJ, Zak DR (2011) Ecological lessons from free-air CO2 enrichment (FACE) experiments. Annual Review of Ecology, Evolution, and Systematics 42:181-20
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