Oak Ridge National Laboratory

About: Oak Ridge National Laboratory is a facility organization based out in Oak Ridge, Tennessee, United States. It is known for research contribution in the topics: Neutron & Ion. The organization has 31868 authors who have published 73724 publications receiving 2633689 citations. The organization is also known as: ORNL.

Effects of fire size and pattern on early succession in yellowstone national park

Abstract: The Yellowstone fires of 1988 affected >250 000 ha, creating a mosaic of burn severities across the landscape and providing an ideal opportunity to study effects of fire size and pattern on postfire succession. We asked whether vegetation responses differed between small and large burned patches within the fire-created mosaic in Yellowstone National Park (YNP) and evaluated the influence of spatial patterning on the postfire veg- etation. Living vegetation in a small (1 ha), moderate (70-200 ha), and large (500-3600 ha) burned patch at each of three geographic locations was sampled annually from 1990 to 1993. Burn severity and patch size had significant effects on most biotic responses. Severely burned areas had higher cover and density of lodgepole pine seedlings, greater abundance of opportunistic species, and lower richness of vascular plant species than less severely burned areas. Larger burned patches had higher cover of tree seedlings and shrubs, greater densities of lodgepole pine seedlings and opportunistic species, and lower species richness than smaller patches. Herbaceous species present before the fires responded in- dividually to burn severity and patch size; some were more abundant in large patches or severely burned areas, while others were more abundant in small patches or lightly burned areas. To date, dispersal into the burned areas from the surrounding unburned forest has not been an important mechanism for reestablishment of forest species. Most plant cover in burned areas consisted of resprouting survivors during the first 3 yr after the fires. A pulse of seedling establishment in 1991 suggested that local dispersal from these survivors was a dominant mechanism for reestablishment of forest herbs. Succession across much of YNP appeared to be moving toward plant communities similar to those that burned in 1988, primarily because extensive biotic residuals persisted even within very large burned areas. However, forest reestablishment remained questionable in areas of old (>400 yr) forests with low prefire serotiny. Despite significant effects of burn severity and patch size, the most important explanatory variable for most biotic responses was geographic location, particularly as related to broad-scale patterns of serotiny in Pinus contorta. We conclude that the effects of fire size and pattern were important and some may be persistent, but that these landscape-scale effects occurred within an overriding context of broader scale gra- dients.

A theory of the spatial and temporal dynamics of plant communities

Abstract: An individual-based model of plant competition for light that uses a definition of plant functional types based on adaptations for the simultaneous use of water and light can reproduce the fundamental spatial and temporal patterns of plant communities. This model shows that succession and zonation result from the same basic processes. Succession is interpreted as a temporal shift in species dominance, primarily in response to autogenic changes in light availability. Zonation is interpreted as a spatial shift in species dominance, primarily in response to the effect of allogenic changes in water availability on the dynamics of competition for light. Patterns of succession at different points along a moisture gradient can be used to examine changes in the ecological roles of various functional types, as well as to address questions of shifts in patterns of resource use through time.

Onset of antiferromagnetism in heavy-fermion metals

Abstract: There are two main theoretical descriptions of antiferromagnets. The first arises from atomic physics, which predicts that atoms with unpaired electrons develop magnetic moments. In a solid, the coupling between moments on nearby ions then yields antiferromagnetic order at low temperatures1. The second description, based on the physics of electron fluids or ‘Fermi liquids’, states that Coulomb interactions can drive the fluid to adopt a more stable configuration by developing a spin density wave2,3. It is at present unknown which view is appropriate at a ‘quantum critical point’, where the antiferromagnetic transition temperature vanishes4,5,6,7. Here we report neutron scattering and bulk magnetometry measurements of the metal CeCu6-xAux, which allow us to discriminate between the two models. We find evidence for an atomically local contribution to the magnetic correlations which develops at the critical gold concentration (xc = 0.1 ), corresponding to a magnetic ordering temperature of zero. This contribution implies that a Fermi-liquid-destroying spin-localizing transition, unanticipated from the spin density wave description, coincides with the antiferromagnetic quantum critical point.