We investigated whether species richness, diversity and density of understory herbaceous plants differed along logging(gap) and grazing(primarily by cattle) disturbance gradients, and sought to identify drivers of richness, diversity and density of understory vegetation of logged sites. A factorial experiment was conducted in the mixed conifer forest of Gidakom in Western Bhutan. Levels of the logging treatment included small(0.15 – 0.24 ha), medium(0.25 – 0.35 ha) and large(0.36 – 1.31 ha) gaps. The grazing treatment included grazed(primarily by cattle) and ungrazed(where herbivores were excluded by a fence) plots nested within each gap. Data were collected from 12 gaps(4 replicates at each level of logging) using the point intercept method. Shannon Weaver Diversity and Margalef's indices were used to estimate species diversity and describe species richness, respectively. Soil samples were analyzed for pH and nutrients. The interaction effect of logging and grazing was significant(p≤0.001) only on species diversity. Relative to ungrazed areas, species diversity was significantly higher(0.01≤p≤0.05) in medium grazed gaps. Under grazed conditions, soil P was negatively correlated with gap size and species diversity. While species diversity was positivelycorrelated(0.01≤p≤0.05) with soil N in grazed plots species richness was positively correlated(0.001≤p≤0.01) with soil N in ungrazed plots. Relative density of Yushania microphylla and Carex nubigena were higher under ungrazed conditions. Our study suggests that the combined effect of cattle grazing and logging results in higher species diversity of understory vegetation in medium and grazed gaps in mixed conifer forests of Bhutan,whereas increase or decrease in relative density of major species is determined primarily by the independent effects of grazing and logging. From management perspective, forest managers must refrain from creating large gaps to avoid loss of nutrients(mainly P and N), which may eventually affect tree regeneration. Managers intending to maintain understory vegetation diversity must consider the combined effects of grazing and logging, ensuring low to moderate grazing pressure. 相似文献
The free-convection flow of an incompressible viscous fluid past an infinite, vertical plate for impulsive as well as uniformly accelerated motion of the plate is discussed when the plate temperatuve varies as the square root of time. The Laplace transform technique is used to obtain the expressions for velocity and skin-friction. The influence of the various parameters, entering into problem, on the velocity field and skin-friction is extensively discussed. 相似文献
Debris flow has caused severe human casualties and economic losses in landslide-prone areas around the globe. A comprehensive understanding of the morphology and deposition mechanisms of debris flows is crucial to delineate the extent of a debris flow hazard. However, due to inherent complex field topography and varying compositions of the flowing debris, coupled with a lack of fundamental understanding about the factors controlling the geomaterial flow, interparticle interactions and its final settlement resulted in a limited understanding of the flow behaviour of the landslide debris. In this study, a physical model was set up in the laboratory to simulate and calibrate the debris flow using PFC, a distinct element modelling-based software. After calibration, a case study of the Varunavat landslide was taken to validate the developed numerical model. Following validation with an acceptable level of confidence, several models were generated to evaluate the effect of slope height, slope angle, slope profile, and grain size distribution of the dislodged geomaterial in the rheological properties of debris flow. Both qualitative and quantitative analysis of the landslide debris flow was performed. Finally, the utility of retaining wall and their effect on debris flow is also studied with different retaining wall positions along the slope surface.
The eclipsing binary system UV Psc was observed in the near-infrared (JHK), and a wave-like distortions have been observed similar to those in the visible. The out-of-eclipse observations have shown real large amount of scatter. This cannot be attributed to the existence of spots on one of the components, which can explain the wave-like distorition of the light curves, but probably to the intrinsic variability of one of them. 相似文献
Summary. Electrical and magnetic fields due to arbitrarily oriented lightning over an imperfectly conducting ground have been obtained. The theory has been applied to K -changes, return strokes and horizontal lightning. The results are in good agreement with the experimental observations previously reported by many workers. 相似文献
Non-uniform propagation of weak discontinuities headed by wavefronts of arbitrary shape along bi-characteristics of a hyperbolic system through thermally-conducting and dissociating gases are studied. An explicit criterion for the growth and decay of weak waves along bi-characteristic curves in the characteristic manifold of the governing differential equations is given. The two fundamental theorems on the global behaviour of the time-dependent wave amplitude of a weak wave has been concluded. 相似文献
Mildly deformed granitoids exposed around Bilgi in the northernmost part of the eastern Dharwar craton are divided into two
groups viz. granodiorites and monzogranites. The granodiorites contain microgranular enclaves and amphibolite xenoliths, and
show low-Al TTG affinity with high SiO2 (71–74 %), Na2O, Y and Sr/Y, moderate to moderately high Mg#, Cr and Ni, low to moderate LILE, and low Nb and Ta. However, compared to similar
TTGs from different cratons the Bilgi granodiorites have distinctly higher K2O, K2O/Na2O, Rb and lower REE and Th. The amphibolite xenoliths are characterized by variable enrichment of K2O, Rb, Ba and Th and depletion of Ti, Zr and P compared to MORB. The microgranular enclaves are quartz diorite to granodiorite
in composition with high Mg, Ni and Cr, and compared to MORB, are enriched in LILE and depleted in Ti and Y. The monzogranites,
compared to the granodiorites, display higher SiO2, K2O and Rb with lower Mg#, although still maintaining the high Na2O, Ni and Cr and low REE character.
The Bilgi granodiorites are explained as transitional TTGs late synkinematic with respect to regional deformation. Geochemical
signatures and regional geological set up suggest that they are probably derived from partial melting of a highly depleted
slab material (metabasalt) followed by variable contamination or assimilation of intermediate crustal rocks in a subduction
zone set up. Late stage fluid activity on the granodioritic magma is probably responsible for the generation of monzogranites.
The amphibolite xenoliths predate the granodiorites and possibly represent fragments of a schist belt carried away by the
granitic magma. They are probably island arc basalt derived from mantle source that has been metasomatized by slab-derived
fluids. The microgranular enclaves are coeval with the Bilgi granodiorites and also likely to be island arc magmas derived
from mantle variably enriched in slab-derived and within-plate components. 相似文献