In Vitro Digestibilities of Summer Forages Utilized by the Rivière George Caribou Herd

The Riviere George caribou herd (northern Quebec-Labrador, Canada) is thought to be regulated by forage limitations in its summer range. In such a situation, digestibilities of plants may strongly affect the diet choice and physical condition of animals. In vitro dry matter disappearance (IVDMD) of the most important summer forages of the Riviere George caribou herd was determined during fermentation periods of 12, 24, and 48 h using rumen fluid collected from a Holstein cow. IVDMD values for Cyperaceae (Eriophorum angustifolium and Carex rariflora), and shrubs (Betula glandulosa and Vaccinium uliginosum) collected in July and August were higher for long fermentation periods (48 h) than for shorter ones (12 and 24 h). Plants collected in early summer were also more digestible than those collected in late summer. Contrary to my prediction, both Cyperaceae were more digestible than the two shrubs in mid-July. However, no difference occurred in early August. The fermentation period did not affect the IVDMD of lichens; maximum digestibility was attained after 12 h. Alectoria ochroleuca and Cetraria spp. were more digestible than Cladina rangiferina, Cladina stellaris, and Stereocaulon paschale, likely because of their lower fibre content. These results suggest that the preference of caribou for Cladina spp. is not based on digestibility, but probably on the fact that these species are abundant in the Riviere George area. Variations in IVDMD seem to be explained by plant phenology, because longer fermentation periods were necessary to attain a high level of digestibility as summer progressed. Plant digestibility alone cannot explain caribou summer diet. Other variables such as plant constituents (e.g. protein) and relative abundance must also be considered.Il a été suggéré que l'abondance du troupeau de caribous de la rivière George (Québec nordique-Labrador, au Canada) était régulée par la disponibilité de nourriture dans les habitats d'été. Dans un tel système, la digestibilité des plantes peut affecter de façon importante le régime alimentaire et la condition physique des animaux. La disparition in vitro de la matière sèche (DIVMS) des principales espèces végétales composant la nourriture estivale des caribous de la rivière George a été déterminée durant des périodes de fermentation de 12, 24 et 48 heures, à l'aide de jus de rumen provenant d'une vache Holstein. Les valeurs de DIVMS des cypéracées (Eriophorum angustifolium et Carex rariflora) et des arbustes (Betula glandulosa et Vaccinium uliginosum) récoltés en juillet et août étaient plus élevées pour de longues périodes de fermentation (48 heures) que pour de courtes périodes (12 et 24 heures). Les plantes récoltées tôt durant l'été étaient aussi plus digestes que celles récoltées tard. Contrairement aux prédictions, les deux espèces de cypéracées étaient plus digestes que les deux espèces d'arbustes à la mi-juillet, mais pas au début août. La durée de fermentation n'a pas affecté la DIVMS des lichens, le maximum de digestibilité étant atteint après 12 heures. Alectoria ochroleuca et Cetraria spp. étaient plus digestes que Cladina rangiferina, Cladina stellaris et Stereocaulon paschale, probablement parce que leur contenu en fibres était plus faible. Ces résultats suggèrent que la préférence des caribous pour les espèces de Cladina n'est pas basée sur leur digestibilité mais probablement sur le fait que ces espèces sont abondantes dans la région de la rivière George. Les variations de DIVMS observées semblent s'expliquer par la phénologie de la végétation, des temps de fermentation plus longs étant nécessaires pour atteindre une digestibilité élevée vers la fin de l'été. La digestibilité des plantes seule ne peut donc expliquer le régime alimentaire du caribou, et d'autres variables comme la composition des plantes (e.g., le contenu en protéines) et leur disponibilité doivent aussi être considérées

Bromophenyl functionalization of carbon nanotubes : an ab initio study

We study the thermodynamics of bromophenyl functionalization of carbon nanotubes with respect to diameter and metallic/insulating character using density-functional theory (DFT). On one hand, we show that the activation energy for the grafting of a bromophenyl molecule onto a semiconducting zigzag nanotube ranges from 0.73 eV to 0.76 eV without any clear trend with respect to diameter within numerical accuracy. On the other hand, the binding energy of a single bromophenyl molecule shows a clear diameter dependence and ranges from 1.51 eV for a (8,0) zigzag nanotube to 0.83 eV for a (20,0) zigzag nanotube. This is in part explained by the transition from sp2 to sp3 bonding occurring to a carbon atom of a nanotube when a phenyl is grafted to it and the fact that smaller nanotubes are closer to a sp3 hybridization than larger ones due to increased curvature. Since a second bromophenyl unit can attach without energy barrier next to an isolated grafted unit, they are assumed to exist in pairs. The para configuration is found to be favored for the pairs and their binding energy decreases with increasing diameter, ranging from 4.34 eV for a (7,0) nanotube to 2.27 eV for a (29,0) nanotube. An analytic form for this radius dependence is derived using a tight binding hamiltonian and first order perturbation theory. The 1/R^2 dependance obtained (where R is the nanotube radius) is verified by our DFT results within numerical accuracy. Finally, metallic nanotubes are found to be more reactive than semiconducting nanotubes, a feature that can be explained by a non-zero density of states at the Fermi level for metallic nanotubes.Comment: 7 pages, 5 figures and 3 table

Skyrme Crystal In A Two-Dimensional Electron Gas

The ground state of a two-dimensional electron gas at Landau level filling factors near $\nu =1$ is a Skyrme crystal with long range order in the positions and orientations of the topologically and electrically charged elementary excitations of the $\nu=1$ ferromagnetic ground state. The lowest energy Skyrme crystal is a square lattice with opposing postures for topological excitations on opposite sublattices. The filling factor dependence of the electron spin-polarization, calculated for the square lattice Skyrme crystal, is in excellent agreement with recent experiments.Comment: 3 pages, latex, 3 figures available upon request from [email protected]

Linking alternative food sources to winter habitat selection of herbivores in overbrowsed landscapes

During winter, ungulates in boreal forests must cope with high energetic costs related to locomotion in deep snow and reduced forage abundance and quality. At high density, ungulates face additional constraints, because heavy browsing reduces availability of woody browse, the main source of forage during winter. Under these severe conditions, large herbivores might forage on alternative food sources likely independent of browsing pressure, such as litterfall or windblown trees. We investigated the influence of alternative food sources on winter habitat selection, by studying female white-tailed deer (Odocoileus virginianus) living in 2 landscapes with contrasted browse abundance, recently logged and regenerated landscapes, in a population at high density and on a large island free of predators. We fitted 21 female whitetailed deer with Global Positioning System (GPS) collars and delineated winter home ranges and core areas. We measured snow conditions in different habitat categories and sampled vegetation in the core areas and in the rest of the home ranges to determine how forage abundance, protective cover, and snow conditions influenced habitat selection within the home range. In both landscapes, deer were less likely to use open habitat categories as snow accumulated on the ground. At a finer scale, deer inhabiting the regenerated landscape intensively used areas where balsam fir cover was intermediate with greater balsam fir browse density than in the rest of the home range. In the recently logged landscape, deer were more likely to be found near edges between clear-cuts and balsam fir stands and in areas where windblown balsam fir trees were present; the latter being the most influential variable. Although balsam fir browse was sparse and mainly out of reach in this landscape, deer increased the use of areas where it was present. Our results offer novel insights into the resource selection processes of northern ungulates, as we showed that access to winter forage, such as woody browse and alternative food sources, depends on climatic conditions and stochastic events, such as abundant compacted snow or windthrows. To compensate for these scarce and unpredictable food supplies, deer selected habitat categories, but mostly areas within those habitat categories, where the likelihood of finding browse, litterfall, and windblown trees was greates

Commensurate-incommensurate transitions of quantum Hall stripe states in double-quantum-well systems

In higher Landau levels (N>0) and around filling factors nu =4N+1, a two-dimensional electron gas in a double-quantum-well system supports a stripe groundstate in which the electron density in each well is spatially modulated. When a parallel magnetic field is added in the plane of the wells, tunneling between the wells acts as a spatially rotating effective Zeeman field coupled to the ``pseudospins'' describing the well index of the electron states. For small parallel fields, these pseudospins follow this rotation, but at larger fields they do not, and a commensurate-incommensurate transition results. Working in the Hartree-Fock approximation, we show that the combination of stripes and commensuration in this system leads to a very rich phase diagram. The parallel magnetic field is responsible for oscillations in the tunneling matrix element that induce a complex sequence of transitions between commensurate and incommensurate liquid or stripe states. The homogeneous and stripe states we find can be distinguished by their collective excitations and tunneling I-V, which we compute within the time-dependent Hartree-Fock approximation.Comment: 23 pages including 8 eps figure