Uptake rates of thorium progeny in a semiarid environment.

The release rates and transformation processes that influence the mobility, biological uptake, and transfer of radionuclides are essential to the assessment of the health effects in the food chain and ecosystem. This study examined concentrations of 222Th in both soil and vegetation at a closed military training site, Kirtland Air Force Base (KAFB), New Mexico. Brazilian sludge was intentionally introduced into the topsoil in the early 1960s to simulate nuclear weapon accidents. Soil (60) and vegetation (120) samples were collected from 1996 to 2000 and analyzed for radionuclides and progeny. High-resolution gamma-ray spectroscopy was used to determine radionuclide activities. The results indicate that the thorium progeny were the predominant contaminant in soil and vegetation. Concentration ratios (CRs) were calculated based on actinium levels.

Ecotone Hierarchies.

Ecological phenomena are evident over a broad spectrum of space and time scales. Ecotones, being defined as zones of transition between adjacent ecological systems, also must occur over an equally broad spectrum of space and time scales. Scale-dependent constraints influence ecological phenomena and resulting zones of transition; however, in traditional ecotone studies little treatment has been given to these influences. This paper addresses aspects of the ecotone concept that relate to the strength of interactions between adjacent systems for a hierarchy of ecotones in a biome transition area in central New Mexico on the Sevilleta National Wildlife Refuge. Zones of transition occur at plant, population, patch, landscape, and biome levels in the hierarchy suggested here. Constraints differ across this hierarchy, primarily because of the different scales at which these constraints exert their influences. The basic strategy to understand these cross-scale influences must be to perform studies at different scales, and a hierarchical approach identifies those scales. This also is important for identifying the appropriate technologies that focus at the scales where transition zones between ecological systems/phenomena are expressed. A broad array of technologies are available for integrating the pattern-process relationships that occur across the many scales in ecological systems.

Gradient Analysis of Ecological Change in Time and Space: Implications for Forest Management.

Gradient analysis is a powerful technique to analyze for, and detect change in, the dynamics, structure, and function of ecosystems. Boundaries between zones or communities occur at distinctive locations along environmental gradients and are expected to be especially sensitive to environmental change. Gradient analysis can be performed at a range of scales, and allows integration and extrapolation of change across scales from those associated with communities to those of biomes. This review outlines the properties of gradients in space and time and uses an example of forests in the Rocky Mountain Physiographic Province to demonstrate constraints, the complex mosaics associated with distributional limits, transfers across boundaries, the role of disturbance, and threshold dynamics. A climate-change scenario is developed to hypothesize future changes in boundary movements, community mosaics, and ecosystem properties along elevational and latitudinal gradients in the Rocky Mountain Province. Mechanistic explanations of ecological phenomena that are necessary for management require information on: (1) the physical environmental constraints operating on the ecosystem; (2) the biota that operate within those constraints; and (3) the interactions among the biota and between the biota and environment. The relative importance of these three elements differs between environments and along environmental gradients. Biota and their interactions may account for much of the variance in system structure and function in mesic environments, while abiotic factors may limit biotic activity in less-favorable (arid) habitats. Plot studies that are analyzed as points along broader scale environmental gradients can provide quantitative information on the major driving variables, and broad-scale analyses of environmental factors along the gradient generate the information for extrapolating between sites and across scales. Modeling that includes such spatial gradients provides the foundation for local to regional management programs.

Organic matter and nutrient dynamics of the forest and forest floor in the Hubbard Brook forest.

The forest floor is a major reservoir of organic matter and nutrients for the ecosystem and as such it influences or regulates most of the functional processes occurring throughout the ecosystem. This study reports on the nutrient and organic matter content of the forest floor of the Hubbard Brook Experimental Forest during different seasons and attempts to correlate results from studies of vegetation, litter, decomposition, stemflow, throughfall, and soil. An organic matter budget is presented for an undisturbed watershed.Average weight of the forest floor on an undisturbed watershed ranged from 25,500 to 85,500 kg/ha. The weighted watershed average was 46,800 kg/ha. Although the F and H horizons did not vary significantly with time, the L horizon increased significantly during the period June to August largely as a result of a severe hail storm. The order of abundance of elements in the forest floor was Nτ;Ca≷Fe>S>P>Mn>K>Mg>Na>Zn>Cu. The concentrations of Ca, K, and Mn decreased with depth in the forest floor while N, P, S, Na, Fe, Zn, and Cu concentrations increased. N:P ratios were similar in decomposing leaf tissue, the forest floor, litterfall, and net stemflow plus throughfall suggesting a similar pattern of cycling. S was proportional to N and P in decomposing leaf tissue, the forest floor, and litterfall. Net stemflow and throughfall were affected by a relatively large input of SO4=-S from the atmosphere. Residence times for elements in the forest floor were affected by inputs other than litterfall (precipitation, stemflow, and throughfall). Calculation of residence times using all inputs caused smaller values than if litterfall alone was used. While all residence times were reduced, the major differences occurred for K, S, and Na. N and P showed relatively long residence times as a result of retranslocation and immobilization by decomposers. The slow turnover rate because of the strong demand and retention by all biota must account for the efficiency of the intrasystem cycling process for N and P. K showed the shortest residence time. A rapid and efficient uptake of K by vegetation seems to account for the efficient cycling of this element. The patterns of nutrient cycling are several depending on the chemical properties of the forest floor, and nutritional requirements of the biota.