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ABSTRACT: A current debate in ecology centers on the extent to which ecosystem function depends on biodiversity. Here, we provide evidence from a long-term field manipulation of plant diversity that soil microbial communities, and the key ecosystem processes that they mediate, are significantly altered by plant species richness. After seven years of plant growth, we determined the composition and function of soil microbial communities beneath experimental plant diversity treatments containing 1–16 species. Microbial community bio-mass, respiration, and fungal abundance significantly increased with greater plant diversity, as did N mineralization rates. However, changes in microbial community biomass, activity, and composition largely resulted from the higher levels of plant production associated with greater diversity, rather than from plant diversity per se. Nonetheless, greater plant pro-duction could not explain more rapid N mineralization, indicating that plant diversity affected this microbial process, which controls rates of ecosystem N cycling. Greater N availability probably contributed to the positive relationship between plant diversity and productivity in the N-limited soils of our experiment, suggesting that plant–microbe in-teractions in soil are an integral component of plant diversity's influence on ecosystem function.Ecology 01/2042; 84(8):2042-2050. DOI:10.1890/02-0433
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ABSTRACT: Understanding the structure of coronal mass ejections (CMEs) is one of the primary challenges in solar astrophysics. White-light coronagraphs make images of line-of-sight projections of the CME electron density (N e). The com-bination of the coronagraphs on the STEREO and SOHO spacecraft provides three simultaneous viewpoints that vary in angle with time, according to the spacecraft orbits. Three viewpoints are not enough to permit tomographic reconstruction via classical methods, but we argue here that recent advances in image processing methods that take into account prior information about the CME geometry may allow one to determine the CME density structure with only three viewpoints. The prior information considered here is that the CME is separated from a known (or simple) background by a closed surface, which may be described by a level set. We propose an alternating iterative procedure in which the surface is evolved via geometric partial differential equations in one step and the interior (and exterior) N e values are determined in the next step.The Astrophysical Journal 10/2015; 695(1):636-641. DOI:10.1088/0004-637X/695/1/636
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ABSTRACT: The H ii complex N 44 in the Large Magellanic Cloud (LMC) provides an excellent site to perform a detailed study of star formation in a mild starburst, as it hosts three regions of star formation at different evolutionary stages, and it is not as complicated and confusing as the 30 Doradus giant H ii region. We have obtained Spitzer Space Telescope observations and complementary ground-based 4 m uBVIJK observations of N 44 to identify candidate massive young stellar objects (YSOs). We further classify the YSOs into Types I, II, and III, according to their spectral energy distributions (SEDs). In our sample of 60 YSO candidates, ∼65% of them are resolved into multiple components or extended sources in high-resolution ground-based images. We have modeled the SEDs of 36 YSOs that appear single or dominant within a group. We find good fits for Types I and I/II YSOs, but Types II and II/III YSOs show deviations between their observed SEDs and models that do not include PAH emission. We have also found that some Type III YSOs have central holes in their disk components. YSO counterparts are found in four ultracompact H ii regions and their stellar masses determined from SED model fits agree well with those estimated from the ionization requirements of the H ii regions. The distribution of YSOs is compared with those of the underlying stellar population and interstellar gas conditions to illustrate a correlation between the current formation of O-type stars and previous formation of massive stars. Evidence of triggered star formation is also presented.The Astrophysical Journal 10/2015; 695(1):511-541. DOI:10.1088/0004-637X/695/1/511
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