1,721,058 research outputs found
Stocks and flows of coarse woody debris across a tropical rain forest nutrient and topography gradient
No full textLarge pieces of standing or fallen dead wood, known as coarse woody debris (CWD), play important roles in temperate forest carbon and nutrient cycles, and affect the abundance and distribution of many classes of organisms. CWD biomass and inputs are poorly documented in tropical rain forests (TRF), and the causes for their variation at landscape-scales in this biome have not been studied. We quantified standing and fallen CWD stocks and inputs in upland (non-swamp) old-growth TRF at the La Selva Biological Station, Costa Rica. We used a network of 18 0.5 ha plots sited in three edaphic conditions to analyze soil nutrient effects on CWD stocks and inputs controlling for topography, and to examine topographic effects controlling for soil nutrients. The edaphic conditions were flat inceptisols, flat ultisols, and steep ultisols. Chemical analyses confirmed the existence of an almost three-fold gradient in total P and K in the upper 1 m of soil. We also annually censused all live woody stems ≥10 cm diameter above buttresses in each plot in September/October from 1997 to 2000 to obtain data on stand structure and dynamics.Fallen CWD stocks averaged 46.3 Mg ha−1 (22.3 Mg C ha−1), while standing CWD averaged 6.5 Mg ha−1 (3.1 Mg C ha−1). There were no significant differences in volume or mass of standing or fallen CWD among edaphic conditions. Annual inputs of CWD averaged 4.9 Mg ha−1 (2.4 Mg C ha−1). Turnover time of fallen CWD was ca. 9 year. Neither stocks nor inputs were correlated with stand structure (number of trees per plot, plot basal area, or plot estimated above-ground biomass). Potential differences in CWD stocks and inputs among sites with different edaphic conditions may have been obscured by a 10-fold variation in tree mortality among plots and a two-fold variation in mean CWD input among years. Analysis of sample variance showed that stocks of CWD were adequately sampled with the 18 0.5 ha plot design, but that inputs were measured with low precision.At La Selva fallen and standing CWD stocks together equaled ca. 33% of estimated above-ground live woody biomass. Tropical rain forest CWD and its associated carbon are intermediate in pool size and turnover rate between fine litter and live trees. Our results show that scaling up TRF CWD estimates to larger spatial scales may be more constrained by the quality of data obtained over single landscapes than by variation due to zonal soil nutrient and topographic conditions. Both the magnitude and vagility of TRF CWD pools are likely to change with global climate change, but the overall direction of change is uncertain
Freshwater wetland plants respond nonlinearly to inundation over a sustained period
Full text linkPremise Wetland plants regularly experience physiological stresses resulting from inundation; however, plant responses to the interacting effects of water level and inundation duration are not fully understood. Methods We conducted a mesocosm experiment on two wetland species, sawgrass (Cladium jamaicense) and muhly grass (Muhlenbergia filipes), that co-dominate many freshwater wetlands in the Florida Everglades. We tracked photosynthesis, respiration, and growth at water levels of −10 (control), 10 (shallow), and 35 cm (deep) with reference to soil surface over 6 months. Results The response of photosynthesis to inundation was nonlinear. Specifically, photosynthetic capacity (Amax) declined by 25% in sawgrass and by 70% in muhly grass after 1–2 months of inundation. After 4 months, Amax of muhly grass in the deep-water treatment declined to near zero. Inundated sawgrass maintained similar leaf respiration and growth rates as the control, whereas inundated muhly grass suppressed both respiration and growth. At the end of the experiment, sawgrass had similar nonstructural carbohydrate pools in all treatments. By contrast, muhly grass in the deep-water treatment had largely depleted sugar reserves but maintained a similar starch pool as the control, which is critical for post-stress recovery. Conclusions Overall, the two species exhibited nonlinear and contrasting patterns of carbon uptake and use under inundation stress, which ultimately defines their strategies of surviving regularly flooded habitats. The results suggest that a future scenario with more intensive inundation, due to the water management and climate change, may weaken the dominance of muhly grass in many freshwater wetlands of the Everglades.publishedVersio
Biotic and abiotic controls on diurnal fluctuations in labile soil phosphorus of a wet tropical forest
No full textThe productivity of many tropical wet forests is generally limited by bioavailable phosphorus (P). Microbial activity is a key regulator of P availability in that it determines both the supply of P through organic matter decomposition and the depletion of bioavailable P through microbial uptake. Both microbial uptake and mineralization occur rapidly, and their net effect on P availability varies with soil moisture, temperature, and soil organic matter quantity and quality. Exploring the mechanisms driving P availability at. ne temporal scales can provide insight into the coupling of carbon, water, and nutrient cycles, and ultimately, the response of tropical forests to climate change. Despite the recognized importance of P cycling to the dynamics of wet tropical forests and their potential sensitivity to short-term fluctuations in bioavailable P, the diurnal pattern of P remains poorly understood. This study quantifies diurnal fluctuations in labile soil P and evaluates the importance of biotic and abiotic factors in driving these patterns. To this end, measurements of labile P were made every other hour in a Costa Rican wet tropical forest oxisol. Spatial and temporal variation in Bray-extractable P were investigated in relation to ecosystem carbon flux, soil CO(2) efflux, soil moisture, soil temperature, solar radiation, and sap-flow velocity. Spatially averaged bi-hourly (every two hours) labile P ranged from 0.88 to 2.48 mu g/g across days. The amplitude in labile P throughout the day was 0.61-0.82 mu g/g (41-54% of mean P concentrations) and was characterized by a bimodal pattern with a decrease at midday. Labile P increased with soil CO(2) efflux and soil temperature and declined with increasing sap flow and solar radiation. Together, soil CO2 efflux, soil temperature, and sap flow explained 86% of variation in labile P.National Science Foundation [BE/CBC ID 0421178
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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