18 research outputs found
Dynamics of vortex tangle without mutual friction in superfluid He
A recent experiment has shown that a tangle of quantized vortices in
superfluid He decayed even at mK temperatures where the normal fluid was
negligible and no mutual friction worked. Motivated by this experiment, this
work studies numerically the dynamics of the vortex tangle without the mutual
friction, thus showing that a self-similar cascade process, whereby large
vortex loops break up to smaller ones, proceeds in the vortex tangle and is
closely related with its free decay. This cascade process which may be covered
with the mutual friction at higher temperatures is just the one at zero
temperature Feynman proposed long ago. The full Biot-Savart calculation is made
for dilute vortices, while the localized induction approximation is used for a
dense tangle. The former finds the elementary scenario: the reconnection of the
vortices excites vortex waves along them and makes them kinked, which could be
suppressed if the mutual friction worked. The kinked parts reconnect with the
vortex they belong to, dividing into small loops. The latter simulation under
the localized induction approximation shows that such cascade process actually
proceeds self-similarly in a dense tangle and continues to make small vortices.
Considering that the vortices of the interatomic size no longer keep the
picture of vortex, the cascade process leads to the decay of the vortex line
density. The presence of the cascade process is supported also by investigating
the classification of the reconnection type and the size distribution of
vortices. The decay of the vortex line density is consistent with the solution
of the Vinen's equation which was originally derived on the basis of the idea
of homogeneous turbulence with the cascade process. The obtained result is
compared with the recent Vinen's theory.Comment: 16 pages, 16 figures, submitted to PR
Mapping density, diversity and species-richness of the Amazon tree flora
Using 2.046 botanically-inventoried tree plots across the largest tropical forest on Earth, we mapped tree species-diversity and tree species-richness at 0.1-degree resolution, and investigated drivers for diversity and richness. Using only location, stratified by forest type, as predictor, our spatial model, to the best of our knowledge, provides the most accurate map of tree diversity in Amazonia to date, explaining approximately 70% of the tree diversity and species-richness. Large soil-forest combinations determine a significant percentage of the variation in tree species-richness and tree alpha-diversity in Amazonian forest-plots. We suggest that the size and fragmentation of these systems drive their large-scale diversity patterns and hence local diversity. A model not using location but cumulative water deficit, tree density, and temperature seasonality explains 47% of the tree species-richness in the terra-firme forest in Amazonia. Over large areas across Amazonia, residuals of this relationship are small and poorly spatially structured, suggesting that much of the residual variation may be local. The Guyana Shield area has consistently negative residuals, showing that this area has lower tree species-richness than expected by our models. We provide extensive plot meta-data, including tree density, tree alpha-diversity and tree species-richness results and gridded maps at 0.1-degree resolution
Consistent patterns of common species across tropical tree communities
Trees structure the Earth’s most biodiverse ecosystem, tropical forests. The vast number of tree species presents a formidable challenge to understanding these forests, including their response to environmental change, as very little is known about most tropical tree species. A focus on the common species may circumvent this challenge. Here we investigate abundance patterns of common tree species using inventory data on 1,003,805 trees with trunk diameters of at least 10 cm across 1,568 locations1,2,3,4,5,6 in closed-canopy, structurally intact old-growth tropical forests in Africa, Amazonia and Southeast Asia. We estimate that 2.2%, 2.2% and 2.3% of species comprise 50% of the tropical trees in these regions, respectively. Extrapolating across all closed-canopy tropical forests, we estimate that just 1,053 species comprise half of Earth’s 800 billion tropical trees with trunk diameters of at least 10 cm. Despite differing biogeographic, climatic and anthropogenic histories7, we find notably consistent patterns of common species and species abundance distributions across the continents. This suggests that fundamental mechanisms of tree community assembly may apply to all tropical forests. Resampling analyses show that the most common species are likely to belong to a manageable list of known species, enabling targeted efforts to understand their ecology. Although they do not detract from the importance of rare species, our results open new opportunities to understand the world’s most diverse forests, including modelling their response to environmental change, by focusing on the common species that constitute the majority of their trees