104 research outputs found

    The construction of Ï”\epsilon-splitting map

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    For a geodesic ball with non-negative Ricci curvature and almost maximal volume, without using compactness argument, we construct an Ï”\epsilon-splitting map on a concentric geodesic ball with uniformly small radius. There are two new technical points in our proof. The first one is the way of finding nn directional points by induction and stratified almost Gou-Gu Theorem. The other one is the error estimates of projections, which guarantee the nn directional points we find really determine nn different directions.Comment: to appear in Calculus of Variations and Partial Differential Equation

    Effects of experimental nitrogen additions on plant diversity in an old-growth tropical forest

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    Response of plant biodiversity to increased availability of nitrogen (N) has been investigated in temperate and boreal forests, which are typically N-limited, but little is known in tropical forests. We examined the effects of artificial N additions on plant diversity (species richness, density and cover) of the understory layer in an N saturated old-growth tropical forest in southern China to test the following hypothesis: N additions decrease plant diversity in N saturated tropical forests primarily from N-mediated changes in soil properties. Experimental additions of N were administered at the following levels from July 2003 to July 2008: no addition (Control); 50 kg N ha−1 yr−1 (Low-N); 100 kg N ha−1 yr−1 (Medium-N), and 150 kg N ha−1 yr−1 (High-N). Results showed that no understory species exhibited positive growth response to any level of N addition during the study period. Although low-to-medium levels of N addition (≀100 kg N ha−1 yr−1) generally did not alter plant diversity through time, high levels of N addition significantly reduced species diversity. This decrease was most closely related to declines within tree seedling and fern functional groups, as well as to significant increases in soil acidity and Al mobility, and decreases in Ca availability and fine-root biomass. This mechanism for loss of biodiversity provides sharp contrast to competition-based mechanisms suggested in studies of understory communities in other forests. Our results suggest that high-N additions can decrease plant diversity in tropical forests, but that this response may vary with rate of N addition

    Water balance of tropical eucalypt plantations in south-eastern

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    Abstract Monthly, seasonal and annual water balances of Eucalyptus urophylla plantations on the Leizhou Peninsula, southeastern China were estimated in 40 m × 40 m plots at two sites with contrasting soil types. The Jijia site is located on basalt-derived clay rich soils, while the Hetou site is characterised by coarse textured soils formed on Quaternary sediments. Observations of evaporative processes (overstorey canopy interception and transpiration, and soil evaporation), soil moisture dynamics, and climate variables were collected at both sites over 2 years. Canopy interception was measured by throughfall troughs and stemflow collectors, daily transpiration was measured by the heat pulse technique in year 1 and estimated from regressions with potential evapotranspiration and available soil water in year 2, soil evaporation was measured by periodic microlysimetry and used to derive a daily soil surface resistance-matric potential relationship for estimation of daily soil evaporation throughout the study period. Soil moisture storage was measured to 4 m depth and drainage estimated as the residual term in a water balance equation. Total annual evapotranspiration (E t ) was similar at 1118 and 1150 mm at Jijia and 969 and 1024 mm at Hetou for years 1 and 2, respectively, despite 20-30% higher rainfall in year 2. These values represent 71 and 66% of annual rainfall in year 1, and 54 and 50% in year 2. Transpiration did not exceed 600 mm in either year and annual soil evaporation was 15-26% of E t , with the higher values from Jijia. The higher rainfall in year 2 was predicted to produce an increase in drainage and runoff rather than tree water use. Dry season water balances showed E t exceeded or approached rainfall, indicating water use from deep soil or ground water storages following soil water depletion, particularly at Hetou. However, storages were replenished by high wet season recharge. The differences in soil properties between the sites resulted in a three-fold greater soil water store at Jijia that provided a supply for E s , and the sandier Hetou soils with poor water holding capacity had greater wet season drainage and higher dry season abstraction from deep storages. The water use of the eucalypts does not appear to be seriously deleterious for water supply in this area

    Underestimated ecosystem carbon turnover time and sequestration under the steady state assumption: a perspective from long‐term data assimilation

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    It is critical to accurately estimate carbon (C) turnover time as it dominates the uncertainty in ecosystem C sinks and their response to future climate change. In the absence of direct observations of ecosystem C losses, C turnover times are commonly estimated under the steady state assumption (SSA), which has been applied across a large range of temporal and spatial scales including many at which the validity of the assumption is likely to be violated. However, the errors associated with improperly applying SSA to estimate C turnover time and its covariance with climate as well as ecosystem C sequestrations have yet to be fully quantified. Here, we developed a novel model-data fusion framework and systematically analyzed the SSA-induced biases using time-series data collected from 10 permanent forest plots in the eastern China monsoon region. The results showed that (a) the SSA significantly underestimated mean turnover times (MTTs) by 29%, thereby leading to a 4.83-fold underestimation of the net ecosystem productivity (NEP) in these forest ecosystems, a major C sink globally; (b) the SSA-induced bias in MTT and NEP correlates negatively with forest age, which provides a significant caveat for applying the SSA to young-aged ecosystems; and (c) the sensitivity of MTT to temperature and precipitation was 22% and 42% lower, respectively, under the SSA. Thus, under the expected climate change, spatiotemporal changes in MTT are likely to be underestimated, thereby resulting in large errors in the variability of predicted global NEP. With the development of observation technology and the accumulation of spatiotemporal data, we suggest estimating MTTs at the disequilibrium state via long-term data assimilation, thereby effectively reducing the uncertainty in ecosystem C sequestration estimations and providing a better understanding of regional or global C cycle dynamics and C-climate feedback

    Transcriptome and Network Changes in Climbers at Extreme Altitudes

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    Extreme altitude can induce a range of cellular and systemic responses. Although it is known that hypoxia underlies the major changes and that the physiological responses include hemodynamic changes and erythropoiesis, the molecular mechanisms and signaling pathways mediating such changes are largely unknown. To obtain a more complete picture of the transcriptional regulatory landscape and networks involved in extreme altitude response, we followed four climbers on an expedition up Mount Xixiabangma (8,012 m), and collected blood samples at four stages during the climb for mRNA and miRNA expression assays. By analyzing dynamic changes of gene networks in response to extreme altitudes, we uncovered a highly modular network with 7 modules of various functions that changed in response to extreme altitudes. The erythrocyte differentiation module is the most prominently up-regulated, reflecting increased erythrocyte differentiation from hematopoietic stem cells, probably at the expense of differentiation into other cell lineages. These changes are accompanied by coordinated down-regulation of general translation. Network topology and flow analyses also uncovered regulators known to modulate hypoxia responses and erythrocyte development, as well as unknown regulators, such as the OCT4 gene, an important regulator in stem cells and assumed to only function in stem cells. We predicted computationally and validated experimentally that increased OCT4 expression at extreme altitude can directly elevate the expression of hemoglobin genes. Our approach established a new framework for analyzing the transcriptional regulatory network from a very limited number of samples

    The FLUXNET2015 dataset and the ONEFlux processing pipeline for eddy covariance data

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    The FLUXNET2015 dataset provides ecosystem-scale data on CO2, water, and energy exchange between the biosphere and the atmosphere, and other meteorological and biological measurements, from 212 sites around the globe (over 1500 site-years, up to and including year 2014). These sites, independently managed and operated, voluntarily contributed their data to create global datasets. Data were quality controlled and processed using uniform methods, to improve consistency and intercomparability across sites. The dataset is already being used in a number of applications, including ecophysiology studies, remote sensing studies, and development of ecosystem and Earth system models. FLUXNET2015 includes derived-data products, such as gap-filled time series, ecosystem respiration and photosynthetic uptake estimates, estimation of uncertainties, and metadata about the measurements, presented for the first time in this paper. In addition, 206 of these sites are for the first time distributed under a Creative Commons (CC-BY 4.0) license. This paper details this enhanced dataset and the processing methods, now made available as open-source codes, making the dataset more accessible, transparent, and reproducible.Peer reviewe