1,721,062 research outputs found
Forest floor fluxes drive differences in the carbon balance of contrasting boreal forest stands
Full text linkThe forest floor provides an important interface of soil-atmosphere CO2 exchanges but their controls and contributions to the ecosystem-scale carbon budget are uncertain due to measurement limitations. In this study, we deployed eddy covariance systems below- and above-canopy to measure the spatially integrated net forest floor CO2 exchange (NFFE) and the entire net ecosystem CO2 exchange (NEE) at two mature contrasting stands located in close vicinity in boreal Sweden. We first developed an improved cospectra model to correct below-canopy flux data. Our empirical below-canopy cospectra models revealed a greater contribution of large- and small-scale eddies in the trunk space compared to their distribution in the above-canopy turbulence cospectra. We found that applying the above-canopy cospectra model did not affect the below-canopy annual CO2 fluxes at the sparse pine forest but significantly underestimated fluxes at the dense mixed spruce-pine stand. At the mixed spruce-pine stand, forest floor respiration (R-ff) was higher and photosynthesis (GPP(ff)) was lower, leading to a 1.4 times stronger net CO2 source compared to the pine stand. We further found that drought enhanced Rff more than GPP(ff), leading to increased NFFE. Averaged across the six site-years, forest floor fluxes contributed 82% to ecosystem-scale respiration (R-eco) and 12% to gross primary production (GPP). Since the annual GPP was similar between both stands, the considerable difference in their annual NEE was due to contrasting R-eco, the latter being primarily driven by the variations in NFFE. This implies that NFFE acted as the driver for the differences in NEE between these two contrasting stands. This study therefore highlights the important role of forest floor CO2 fluxes in regulating the boreal forest carbon balance. It further calls for extended efforts in acquiring high spatio-temporal resolution data of forest floor fluxes to improve predictions of global change impacts on the forest carbon cycle
A managed boreal forest landscape in northern Sweden is a persistent net carbon sink despite large inter-annual weather anomalies
No full textThe future role of boreal forests in the global carbon cycle is uncertain given the rapid climate change in high latitudes. At the landscape scale, heterogeneity in stand age and land cover, contributions from terrestrial and aquatic fluxes, and harvest export may create complex carbon cycle-climate interactions. However, the integrated response of the net landscape carbon balance (NLCB) to inter-annual variations (IAVs) in environmental conditions is poorly understood. Here, we used tall-tower eddy covariance and stream monitoring to integrate terrestrial and aquatic carbon fluxes with harvest export for a 68 km2 boreal catchment in Sweden during 2016-2020. This actively managed forest landscape acted as a net carbon sink with a 5-year mean (+ standard deviation) NLCB of 128+55 g C m-2 yr-1. The NLCB IAV included a reduced sink (36 g C m-2 yr-1) during the cool/ cloudy year 2017. In the other four years, featuring a drought summer (2018) and an exceptionally warm/wet winter (2020), the landscape acted as a significant sink (127-180 g C m-2 yr-1). The NLCB IAV corresponded primarily to variations in landscape respiration, followed by GPP and harvest export, with negligible contributions from landscape CH4 and aquatic carbon fluxes. The NLCB IAV was not correlated to any single environmental factor. However, daily NLCB contrastingly responded to key environmental factors as a function of forest aboveground biomass and mire contributions. Overall, our study indicates that the annual carbon sink-strength of the managed boreal forest landscape may be resilient to a wide range of IAVs in environmental conditions
Two-level eddy covariance measurements reduce bias in land-atmosphere exchange estimates over a heterogeneous boreal forest landscape
No full textEstimates of land-atmosphere exchanges of carbon, energy, water vapor, and other greenhouse gases based on the eddy covariance (EC) technique rely on the fundamental assumption that the flux footprint area is homogeneous. We investigated the impact of source area heterogeneity on flux estimates in single-level EC measurements over a managed boreal forest landscape. For this purpose, we compared single-level measurements with those from a two-level approach consisting of concurrent EC measurements at 60 and 85 m above the ground. This two-level set-up provided a unique opportunity to obtain nearly congruent diel footprint areas by combining data from the higher and lower levels during day- and nighttime, respectively. We found that the variation in the averaged footprint area between day- and nighttime was reduced by up to 89% in the two-level approach compared to the single-level data at the higher level (85 m). Considering spring, summer, and fall months, the resulting relative potential bias in flux observations due to landscape heterogeneity was highest at short time steps (≤ daily) ranging between 35% and 325% for half-hourly data. During winter months, when stable atmospheric regimes prevailed during day and night, the footprints within the diel course nearly overlapped also at a given single level and hence no improvement of flux estimates was found. The absolute cumulated sums for the study period (excluding winter months) of gross primary production, ecosystem respiration, latent heat, and sensible heat flux were underestimated by about 28%, 52%, 5%, and 3%, respectively, whereas that of net ecosystem CO2 exchange was overestimated by about 109% in the single-level approach. Overall this study suggests that footprint heterogeneity may introduce considerable bias in single-level flux estimates — particularly at short time scales — with large implications for model-data fusion studies, site comparisons, and up- or downscaling of land-atmosphere exchange processes
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
Dispelling the Myths Behind First-author Citation Counts
Full text linkWe 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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