1,101 research outputs found
Linking fruit traits to variation in predispersal vertebrate seed predation, insect seed predation, and pathogen attack
The importance of vertebrates, invertebrates, and pathogens for plant communities has long been recognized, but their absolute and relative importance in early recruitment of multiple coexisting tropical plant species has not been quantified. Further, little is known about the relationship of fruit traits to seed mortality due to natural enemies in tropical plants. To investigate the influences of vertebrates, invertebrates, and pathogens on reproduction of seven canopy plant species varying in fruit traits, we quantified reductions in fruit development and seed germination due to vertebrates, invertebrates, and fungal pathogens through experimental removal of these enemies using canopy exclosures, insecticide, and fungicide, respectively. We also measured morphological fruit traits hypothesized to mediate interactions of plants with natural enemies of seeds. Vertebrates, invertebrates, and fungi differentially affected predispersal seed mortality depending on the plant species. Fruit morphology explained some variation among species; species with larger fruit and less physical protection surrounding seeds exhibited greater negative effects of fungi on fruit development and germination and experienced reduced seed survival integrated over fruit development and germination in response to vertebrates. Within species, variation in seed size also contributed to variation in natural enemy effects on seed viability. Further, seedling growth was higher for seeds that developed in vertebrate exclosures for Anacardium excelsum and under the fungicide treatment for Castilla elastica, suggesting that predispersal effects of natural enemies may carry through to the seedling stage. This is the first experimental test of the relative effects of vertebrates, invertebrates, and pathogens on seed survival in the canopy. This study motivates further investigation to determine the generality of our results for plant communities. If there is strong variation in natural enemy attack among species related to differences in fruit morphology, then quantification of fruit traits will aid in predicting the outcomes of interactions between plants and their natural enemies. This is particularly important in tropical forests, where high species diversity makes it logistically impossible to study every plant life history stage of every species
Theoretical Aspects of Microchannel Acoustofluidics:Thermoviscous Corrections to the Radiation Force and Streaming
AbstractWe study the effects of the temperature dependence of viscosity and density on the acoustic radiation force and the boundary-driven acoustic streaming in microchannel acoustofluidics. The acoustic streaming slip velocity for the bulk flow is calculated numerically taking these thermoviscous effects into account inside the micrometer-thin acoustic boundary layer and compare the results to recent analytical work in the literature. The acoustic radiation force is calculated for the case of an ultrasound wave scattering on a compressible, spherical particle suspended in a viscous, thermal conducting fluid. Using PrandtlâSchlichting boundary-layer theory, we include the viscosity and the volume thermal expansion coeffcient of the fluid and derive an analytical expression for the radiation force. The resulting force (valid for particle radius and boundary layers much smaller than the acoustic wavelength) is analyzed for microchannel acoustophoresis
Dead Wood Necromass in a Moist Tropical Forest : Stocks, Fluxes, and Spatiotemporal Variability
Woody debris (WD) stocks and fluxes are important components of forest carbon budgets and yet remain understudied, particularly in tropical forests. Here we present the most comprehensive assessment of WD stocks and fluxes yet conducted in a tropical forest, including one of the first tropical estimates of suspended WD. We rely on data collected over 8 years in an old-growth moist tropical forest in Panama to quantify spatiotemporal variability and estimate minimum sample sizes for different components. Downed WD constituted the majority of total WD mass (78%), standing WD contributed a substantial minority (21%), and suspended WD was the smallest component (1%). However, when considering sections of downed WD that are elevated above the soil, the majority of WD inputs and approximately 50% of WD stocks were disconnected from the forest floor. Branchfall and liana wood accounted for 17 and 2% of downed WD, respectively. Residence times averaged 1.9 years for standing coarse WD (CWD; > 20 cm diameter) and 3.6 years for downed CWD. WD stocks and inputs were highly spatially variable, such that the sampling efforts necessary to estimate true values within 10% with 95% confidence were > 130 km of transects for downed CWD and > 550 ha area for standing CWD. The vast majority of studies involve much lower sampling efforts, suggesting that considerably more data are required to precisely quantify tropical forest WD pools and fluxes. The demonstrated importance of elevated WD in our study indicates a need to understand how elevation above the ground alters decomposition rates and incorporate this understanding into models of forest carbon cycling.Peer reviewe
Nuclear spin conversion in formaldehyde
Theoretical model of the nuclear spin conversion in formaldehyde (H2CO) has
been developed. The conversion is governed by the intramolecular spin-rotation
mixing of molecular ortho and para states. The rate of conversion has been
found equal 1.4*10^{-4}~1/s*Torr. Temperature dependence of the spin conversion
has been predicted to be weak in the wide temperature range T=200-900 K.Comment: REVTEX, 16 pages + 5 eps figure
Testing Ecological Theory with Lianas
Lianas constitute a diverse polyphyletic plant group that is advancing our understanding of ecological theory. Specifically, lianas are providing new insights into the mechanisms that control plant distribution and diversity maintenance. For example, there is now evidence that a single, scalable mechanism may explain local, regional, and panâtropical distribution of lianas, as well as the maintenance of liana species diversity. The ability to outcompete trees under dry, stressful conditions in seasonal forests provides lianas a growth advantage that, over time, results in relatively high abundance in seasonal forests and low abundance in aseasonal forests. Lianas may also gain a similar growth advantage following disturbance, thus explaining why liana density and diversity peak following disturbance at the local, forest scale. The study of ecology, however, is more than the effect of the environment on organisms; it also includes the effects of organisms on the environment. Considerable empirical evidence now indicates that lianas substantially alter their environment by consuming resources, suppressing tree performance, and influencing emergent properties of forests, such as ecosystem functioning, plant and animal diversity, and community composition. These recent studies using lianas are transcending classical tropical ecology research and are now providing novel insights into fundamental ecological theory
Fixation of a Deleterious Allele under Mutation Pressure and Finite Selection Intensity
The mean fixation time of a deleterious mutant allele is studied beyond the
diffusion approximation. As in Kimura's classical work [M. Kimura, Proc. Natl.
Acad. Sci. U.S.A. Vol.77, 522 (1980)], that was motivated by the problem of
fixation in the presence of amorphic or hypermorphic mutations, we consider a
diallelic model at a single locus comprising a wild-type A and a mutant allele
A' produced irreversibly from A at small uniform rate v. The relative fitnesses
of the mutant homozygotes A'A', mutant heterozygotes A'A and wild-type
homozygotes AA are 1-s, 1-h and 1, respectively, where it is assumed that v<<
s. Here, we adopt an approach based on the direct treatment of the underlying
Markov chain (birth-death process) obeyed by the allele frequency (whose
dynamics is prescribed by the Moran model), which allows to accurately account
for the effects of large fluctuations. After a general description of the
theory, we focus on the case of a deleterious mutant allele (i.e. s>0) and
discuss three situations: when the mutant is (i) completely dominant (s=h);
(ii) completely recessive (h=0), and (iii) semi-dominant (h=s/2). Our
theoretical predictions for the mean fixation time and the quasi-stationary
distribution of the mutant population in the coexistence state, are shown to be
in excellent agreement with numerical simulations. Furthermore, when s is
finite, we demonstrate that our results are superior to those of the diffusion
theory that is shown to be an accurate approximation only when N_e s^2 << 1,
where N_e is the effective population size.Comment: 26 pages, 5 figures. Accepted by the Journal of Theoretical Biolog
Spike Penetration in Blast-wave-driven Instabilities
The problem of interest is the unstable growth of structure at density transitions affected by blast waves, which arise in natural environments such as core-collapse supernovae and in laboratory experiments. The resulting spikes of dense material, which penetrate the less dense material, develop broadened tips, but the degree of broadening varies substantially across both experiments and simulations. The variable broadening presumably produces variations in the drag experienced by the spike tips as they penetrate the less dense material. The present work has used semianalytic theory to address the question of how the variation in drag might affect the spike penetration, for cases in which the post-shock interface deceleration can be described by a power law in a normalized time variable. It did so by following the evolution of structure on the interface through the initial shock passage, the subsequent small-amplitude phase of Rayleigh-Taylor instability growth, and the later phase in which the spike growth involves the competition of buoyancy and drag. In all phases, the expansion of the system during its evolution was accounted for and was important. The calculated spike length is strongly affected by the drag attributed to spike tip broadening. One finds from such a calculation that it is not unreasonable for narrow spikes to keep up with the shock front of the blast wave. The implication is that the accuracy of prediction of spike penetration and consequent structure by simulations very likely depends on how accurately they treat the broadening of the spike tips and the associated drag. Experimental validation of spike morphology in simulations would be useful.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/98595/1/0004-637X_744_2_184.pd
Semiclassical transition in \phi^4 theory
We have shown an example of semiclassical transition in theory
with positive coupling constant. This process can be described by the classical
-invariant solution, considered on a contour in the complex time plane.
The transition is technically analogous to the one-instanton transition in the
electroweak model. It is suppressed by the factor , where
is Lipatov instanton action. This process describes a semiclassical
transition between two coherent states with much smaller number of particles in
the initial state than in the final state. Therefore, it could be relevant to
the problem of calculation of amplitudes for multiparticle production in
-type models.Comment: 26 pages, JHU-TIPAC-930013, (correct 2 typos, some notations
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