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Stable isotope mixing models are often used to quantify source contributions to a mixture. Examples include pollution source identification; trophic web studies; analysis of water sources for soils, plants; or water bodies, and many others. A common problem is having too many sources to allow a unique solution. We discuss two alternative procedures for addressing this problem. One option is a priori to combine sources with similar signatures so the number of sources is small enough to provide a unique solution. Aggregation should be considered only when isotopic signatures of clustered sources are not significantly different, and sources are related so the combined source group has some functional significance. For example, in a food web analysis, lumping several species within a trophic guild allows more interpretable results than lumping disparate food sources, even if they have similar isotopic signatures. One result of combining mixing model sources is increased uncertainty of the combined end-member isotopic signatures and consequently the source contribution estimates; this effect can be quantified using the IsoError model (http://www.epa.gov/wed/pages/models/isotopes/isoerror1_04.htm). As an alternative to lumping sources before a mixing analysis, the IsoSource mixing model (http://www.epa.gov/wed/pages/models/isosource/isosource.htm) can be used to find all feasible solutions of source contributions consistent with isotopic mass balance. While ranges of feasible contributions for each individual source can often be quite broad, contributions from functionally related groups of sources can be summed a posteriori, producing a range of solutions for the aggregate source that may be considerably narrower. A paleohuman dietary analysis example illustrates this method, which involves a terrestrial meat food source, a combination of three terrestrial plant foods, and a combination of three marine foods. In this case, a posteriori aggregation of sources allowed strong conclusions about temporal shifts in marine versus terrestrial diets that would not have otherwise been discerned.

Stable isotope analysis has become an important tool in studies of trophic food webs and animal feeding patterns. When animals undergo rapid dietary shifts due to migration, metamorphosis, or other reasons, the isotopic composition of their tissues begins changing to reflect that of their diet. This can occur both as a result of growth and metabolic turnover of existing tissue. Tissues vary in their rate of isotopic change, with high turnover tissues such as liver changing rapidly, while relatively low turnover tissues such as bone change more slowly. A model is outlined that uses the varying isotopic changes in multiple tissues as a chemical clock to estimate the time elapsed since a diet shift, and the magnitude of the isotopic shift in the tissues at the new equilibrium. This model was tested using published results from controlled feeding experiments on a bird and a mammal. For the model to be effective, the tissues utilized must be sufficiently different in their turnover rates. The model did a reasonable job of estimating elapsed time and equilibrial isotopic changes, except when the time since the diet shift was less than a small fraction of the half-life of the slowest turnover tissue or greater than 5-10 half-lives of the slowest turnover tissue. Sensitivity analyses independently corroborated that model estimates became unstable at extremely short and long sample times due to the effect of random measurement error. Subject to some limitations, the model may be useful for studying the movement and behavior of animals changing isotopic environments, such as anadromous fish, migratory birds, animals undergoing metamorphosis, or animals changing diets because of shifts in food abundance or competitive interactions.

Changes in climate associated with changes in atmospheric concentrations of CO 2 and other greenhouse gases might affect soil erosion by wind and water. Changes in erosion could in turn cause changes in productivity and sustainability of agricultural systems, and changes in air quality (PM 10 ) and water quality (sediment transport). Substantial effects on productivity may, however, only occur several decades after climate changes. This paper presents a procedure for assessing the potential effects of climate change on erosion and productivity. A preliminary screening process is used to identify and prioritize regions and management systems. Subsequent simulation of selected sites with the EPIC model is used to investigate potential practices to adapt agricultural systems to climate change. In some cases, proposed adaptation strategies might reduce sustainability if they are not matched to environmental conditions found at specific sites. As an example, the assessment procedure is applied to evaluate vulnerability and adaptation practices for a 20% increase in mean monthly wind speeds in the U.S. corn belt.

$\\bullet$ Here, we investigate fine-root production, mortality and standing crop of Douglas-fir (Pseudotsuga menziesii) seedlings exposed to elevated atmospheric CO2 and elevated air temperature. We hypothesized that these treatments would increase fine-root production, but that mortality would be greater under elevated temperature, leading to a smaller increase in standing crop. $\\bullet$ Seedlings were grown in outdoor, sun-lit controlled-environment chambers containing native soil. They were exposed in a factorial design to two levels of atmospheric CO2 and two levels of air temperature. Minirhizotron methods were used to measure fine-root length production, mortality and standing crop every 4 wk for 36 months. $\\bullet$ Neither elevated atmospheric CO2 nor elevated air temperature affected fine-root production, mortality, or standing crop. Fine roots appeared to root deeper in the soil profile under elevated CO2 and elevated temperature. $\\bullet$ Low soil nitrogen (N) levels apparently limited root responses to the treatments. This suggests that forests on nutrient-poor soils may exhibit limited fine-root responses to elevated atmospheric CO2 and elevated air temperature.

The effects of elevated CO^sub 2^ and N-fertilization on the architecture of Pinus ponderosa Dougl. ex P. Laws & C. Laws fine roots and their associated mycorrhizal symbionts were measured over a 4-year period using minirhizotron tubes. The study was conducted in open-top field-exposure chambers located near Placerville, Calif. A replicated (3 replicates), 3×3 factorial experimental design with three CO^sub 2^ concentrations [ambient air (354 μmol mol^sup -1^), 525 μmol mol^sup -1^, and 700 μmol mol^sup -1^] and three rates of N-fertilization (0, 100 and 200 kg ha^sup -1^ year^sup -1^) was used. Elevated CO^sub 2^ and N treatment had contrasting effects on the architecture of fine roots and their associated mycorrhizae. Elevated CO^sub 2^ increased both fine root extensity (degree of soil exploration) and intensity (extent that roots use explored areas) but had no effect on mycorrhizae. In contrast, N-fertilization had no effect on fine root extensity or intensity but increased mycorrhizal extensity and intensity. To better understand and model the responses of systems to increasing CO^sub 2^ concentrations and N deposition/fertilization it is necessary to consider these contrasting root architectural responses.[PUBLICATION ABSTRACT]

Four sizes of forest opening (0.016, 0.08, 0.4, 0.4, and 2.0 ha; two replicates each) were established in a Southern Appalachian forest to examine the effects of disturbance size on earl successional community structure and function. Solar radiation, soil temperature, and air temperature were all higher in large openings than small openings and increased from edge to center of disturbance patches. Aboveground net primary productivity (NPP) was 3-4 times as highe in larger (2.0 ha) as small (0.016 ha) openings, presumably in response to greater light availability in large patches. Stump and root sprouts of tree species accounted for the largest fraction of NPP in all patch sizes. Herbs, vines, shrubs, advance regeneration trees, and tree seedlings had progressively smaller NPP, respectively. Vegetation biomass reached 0.7-2.6% of undistributed forest levels and aboveground NPP reached 17-58% of forest levels by the 2nd yr after cutting. Plant species richness was generally higher in large than small patches. Tree species composition shifted considerably followed disturbance. Liriodendron tulipifera was important before and after logging. Large canopy dominants such as oaks and hickories were relatively unimportant sources of sprouts during early revegetation. Instead, minor canopy and understory species such as Robinia pseudoacacia, Halesia carolina, Acer rubrum, Cornus florida, and Magnolia fraseri were the major sprouters in all patch sizes. The N-fixing black locust (Robinia) was much more important in large than small openings. Disturbance size within the Southern Appalachians thus affects microenvironment, species composition, and NPP during early revegetation.

Factors controlling the structure of plant communities occupying soil-filled depressions on granite outcrops in southeastern United States were investigated by adding and/or removing species. The effects of the treatment on vascular plant density and cover, and on moss and lichen cover, bare ground, and some soil characteristics (depth, bulk density, pH, and organic matter content) were followed over 2 yr. Addition experiments performed in shallower soil island communities revealed that shallower soil species often significantly increased the establishment of deeper soil species. This positive effect might have resulted from indirect interactions mediated by mosses and lichens. Extreme abiotic factors (i.e., drought) seemed to have overridden this initial facilitative effect, however. Species removals performed in the tension zones of island communities indicated that, although physiological tolerances, and not biotic interactions, appeared to determine the lower limit of some species (annuals) on the depth gradient, for others (perennials), biotic interactions with shallower soil species seemed to be significant in restricting the populations to the deeper section of the gradient. Few soil characteristics responded to our experimental manipulations over the 2-yr study period. Succession occurs in the soil-filled depressions on granite outcrops, and because the spatial patterns studied here appear to be good analogs of the temporal patterns, our results suggest that mechanisms of facilitation followed by tolerance and inhibition are operative in the sere.

(1) Spacing patterns of shrubs were studied on a series of sites in the Mojave and Sonoran Deserts. Both aggregation and regularity in dispersion of individual shrubs were fairly common. Aggregation may result from vegetative reproduction or environmental heterogeneity, and regularity from competition among plants. (2) Small shrubs tend to be clumped, medium-sized ones tend to a random arrangement, and large shrubs tend to a regular pattern. This suggests the increasing importance of competition as the plants grow. (3) Further evidence of interference between plants was provided by the correlations of plant size with the distance to their neighbours. (4) Root systems were extensive enough to abut or overlap each other in the interplant spaces. (5) Most plants tended to have neighbours of the same species rather than other species. (6) None of these results depended on position along the considerable climatic gradients across the Mojave and Sonoran Deserts.