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Elevational change in woody tissue CO2 efflux in a tropical mountain rain forest in southern Ecuador
No full textMuch uncertainty exists about the magnitude of woody tissue respiration and its environmental control in highly diverse tropical moist forests. In a tropical mountain rain forest in southern Ecuador, we measured the apparent diurnal gas exchange of stems and coarse roots (diameter 1-4 cm) of trees from representative families along an elevational transect with plots at 1050, 1890 and 3050 m a.s.1. Mean air temperatures were 20.8, 17.2 and 10.6 degrees C, respectively. Stem and root CO2 efflux of 13 to 21 trees per stand from dominant families were investigated with an open gas exchange system while stand microclimate was continuously monitored. Substantial variation in respiratory activity among and within species was found at all sites. Mean daily CO2 release rates from stems declined 6.6-fold from 1.38 mu mol m(-2) s(-1) at 1050 m to 0.21 mu mol m(-2) s(-1) at 3050 m. Mean daily CO2 release from coarse roots decreased from 0.35 to 0.20 mu mol m(-2) s(-1) with attitude, but the differences were not significant. There was, thus, a remarkable shift from a high ratio of stem to coarse root respiration rates at the lowest elevation to an apparent equivalence of stem and coarse root CO2 efflux rates at the highest elevation. We conclude that stem respiration, but not root respiration, greatly decreases with elevation in this transect, coinciding with a substantial decrease in relative stem diameter increment and a large increase in fine and coarse root biomass production with elevation
Carbon Turnover and Carbon‐13 Natural Abundance under Land Use Change in Semiarid Savanna Soils of La Pampa, Argentina
No full textHalf-lives of 50 to 1.00 yr have been reported for native soil C in temperate Mollisols taken under cultivation, whereas some C fractions have shown mean residence times of hundreds of years. In the Tropics, C turnover is much more rapid. This lower C stability may be attributable to climate or to a mineral suite with low C stabilization potential typical of many tropical soils. We present data on the mineralization and accretion pattern of organic C under land use conversion in illitic loessial loams of the semiarid, warm temperate Province of La Pampa, Argentina. These soils should have a low C stabilization potential, and the semiarid temperate climate should provide for relatively slow turnover compared with tropical conditions. Using C-13-natural abundance and soil fractionation, we determined soil C derived from C3 and C4 plants under land use changes between native Calden savannah, pastures, and arable fields, specifically: (i) the conversion of C3 natural savannah to agriculture that includes (C4) maize in the rotation, (ii) the conversion of a 40-yr-old C4 pasture into arable land under C3 crops, and (iii) the reclamation of highly degraded C3 cultivated land with C4 pastures. Although none of the land cover changes represented 100% conversions between plants of C4 and C3 metabolism, C turnover could still be determined. All sites provided evidence for rapid C losses with C half-lives just above 10 yr and no evidence for long-term stabilized C in any soil fractions. Soil under long-term pasture or the natural vegetation of the region, with initial C contents between 24 and 33 mg C g(-1) bulk soil, lost 33 to 57% of this original bulk soil C within 12 to 18 yr of continuous cultivation. On degraded soils under restoration with C4 pasture, C accretion was also rapid but leveled off well below the original C levels. Theses results provide evidence for potentially irreversible soil degradation and corroborate local practical experience that these temperate, coarse-textured, illitic soils are highly susceptible to degradation and should be managed carefully
Diverging temperature response of tree stem CO2 release under dry and wet season conditions in a tropical montane moist forest
Get PDFIt is commonly presumed that plant respiratory CO2 release increases with increasing temperature. However, we report on very contrasting stem CO2 release (R (S))-temperature relationships of trees in a species-rich tropical montane forest of southern Ecuador under dry and wet season conditions. Rates of R (S) were low and completely uncoupled from the dial temperature regime during the humid season. In contrast, during the dry season, R (S) was generally higher and temperature sensitivity of R (S) differed greatly in degree and even in the direction of response, indicating that temperature might not be the only determinant of R (S). In order to explain the heterogeneity of R (S), we related R (S) to vapour pressure deficit, wind speed and solar radiation as important abiotic drivers influencing transpiration and photosynthesis. Stepwise multiple regression analyses with these meteorological predictors either were biased by high collinearity of the independent variables or could not enhance the ability to explain the variability of R (S). We assume maintenance respiration to dominate under humid conditions unfavourable for energy acquisition of the tree, thus explaining the pronounced uncoupling of R (S) from atmospheric parameters. In contrast, the drier and hotter climate of the dry season seems to favour R (S) via enhanced assimilatory substrate delivery and stem respiratory activity as well as elevated xylem sap CO2 imports with increased transpiration. In addition, tree individual differences in the temperature responses of R (S) may mirror diverging climatic adaptations of co-existing moist forest tree species which have their distribution centre either at higher or lower elevations
Going Beyond Counting First Authors in Author Co-citation Analysis
Get PDFThe 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
Get PDF“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
Get PDFWe 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
Patterns of wood carbon dioxide efflux across a 2,000-m elevation transect in an Andean moist forest
Get PDFDuring a 1-year measurement period, we recorded the CO(2) efflux from stems (R (S)) and coarse woody roots (R (R)) of 13-20 common tree species at three study sites at 1,050, 1,890 and 3,050 m a.s.l. in an Andean moist forest. The objective of this work was to study elevation changes of woody tissue CO(2) efflux and the relationship to climate variation, site characteristics and growth. Furthermore, we aim to provide insights into important respiration-productivity relationships of a little studied tropical vegetation type. We expected R (S) and R (R) to vary with dry and humid season conditions. We further expected R (S) to vary more than R (R) due to a more stable soil than air temperature regime. Seasonal variation in woody tissue CO(2) efflux was indeed mainly attributable to stems. At the same time, temperature played only a small role in triggering variations in R (S). At stand level, the ratio of C release (g C m(-2) ground area year(-1)) between stems and roots varied from 4:1 at 1,050 m to 1:1 at 3,050 m, indicating the increasing prevalence of root activity at high elevations. The fraction of growth respiration from total respiration varied between 10 (3,050 m) and 14% (1,050 m) for stems and between 5 (1,050 m) and 30% (3,050 m) for roots. Our results show that respiratory activity and hence productivity is not driven by low temperatures towards higher elevations in this tropical montane forest. We suggest that future studies should examine the limitation of carbohydrate supply from leaves as a driver for the changes in respiratory activity with elevation
Dispelling the Myths Behind First-author Citation Counts
Get PDFWe conducted a full-scale evaluative citation analysis study of scholars in the XML research field to explore just how different from each other author rankings resulting from different citation counting methods actually are, and to demonstrate the capability of emerging data and tools on the Web in supporting more realistic citation counting methods. Our results contest some common arguments for the continued
use of first-author citation counts in the evaluation of scholars, such as high correlations between author rankings by first-author citation counts and other citation
counting methods, and high costs of using more realistic citation counting methods that are not well-supported by the ISI databases. It is argued that increasingly available digital full text research papers make it possible for citation analysis studies to go beyond what the ISI databases have directly supported and to employ more
sophisticated methods
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