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Water Gradient (water + gradient)
Selected AbstractsCO2 exchange in three Canadian High Arctic ecosystems: response to long-term experimental warmingGLOBAL CHANGE BIOLOGY, Issue 12 2004Jeffrey M. Welker Abstract Carbon dioxide exchange, soil C and N, leaf mineral nutrition and leaf carbon isotope discrimination (LCID-,) were measured in three High Arctic tundra ecosystems over 2 years under ambient and long-term (9 years) warmed (,2°C) conditions. These ecosystems are located at Alexandra Fiord (79°N) on Ellesmere Island, Nunavut, and span a soil water gradient; dry, mesic, and wet tundra. Growing season CO2 fluxes (i.e., net ecosystem exchange (NEE), gross ecosystem photosynthesis (GEP), and ecosystem respiration (Re)) were measured using an infrared gas analyzer and winter C losses were estimated by chemical absorption. All three tundra ecosystems lost CO2 to the atmosphere during the winter, ranging from 7 to 12 g CO2 -C m,2 season,1 being highest in the wet tundra. The period during the growing season when mesic tundra switch from being a CO2 source to a CO2 sink was increased by 2 weeks because of warming and increases in GEP. Warming during the summer stimulated dry tundra GEP more than Re and thus, NEE was consistently greater under warmed as opposed to ambient temperatures. In mesic tundra, warming stimulated GEP with no effect on Re increasing NEE by ,10%, especially in the first half of the summer. During the ,70 days growing season (mid-June,mid-August), the dry and wet tundra ecosystems were net CO2 -C sinks (30 and 67 g C m,2 season,1, respectively) and the mesic ecosystem was a net C source (58 g C m,2 season,1) to the atmosphere under ambient temperature conditions, due in part to unusual glacier melt water flooding that occurred in the mesic tundra. Experimental warming during the growing season increased net C uptake by ,12% in dry tundra, but reduced net C uptake by ,20% in wet tundra primarily because of greater rates of Re as opposed to lower rates of GEP. Mesic tundra responded to long-term warming with ,30% increase in GEP with almost no change in Re reducing this tundra type to a slight C source (17 g C m,2 season,1). Warming caused LCID of Dryas integrafolia plants to be higher in dry tundra and lower in Salix arctic plants in mesic and wet tundra. Our findings indicate that: (1) High Arctic ecosystems, which occur in similar mesoclimates, have different net CO2 exchange rates with the atmosphere; (2) long-term warming can increase the net CO2 exchange of High Arctic tundra by stimulating GEP, but it can also reduce net CO2 exchange in some tundra types during the summer by stimulating Re to a greater degree than stimulating GEP; (3) after 9 years of experimental warming, increases in soil carbon and nitrogen are detectable, in part, because of increases in deciduous shrub cover, biomass, and leaf litter inputs; (4) dry tundra increases in GEP, in response to long-term warming, is reflected in D. integrifolia LCID; and (5) the differential carbon exchange responses of dry, mesic, and wet tundra to similar warming magnitudes appear to depend, in part, on the hydrologic (soil water) conditions. Annual net ecosystem CO2 -C exchange rates ranged from losses of 64 g C m,2 yr,1 to gains of 55 g C m,2 yr,1. These magnitudes of positive NEE are close to the estimates of NPP for these tundra types in Alexandra Fiord and in other High Arctic locations based on destructive harvests. [source] Liposomes in investigative dermatologyPHOTODERMATOLOGY, PHOTOIMMUNOLOGY & PHOTOMEDICINE, Issue 5 2001Daniel B. Yarosh Liposomes are microscopic spheres, usually composed of amphiphilic phospholipids. They may be useful without skin penetration if they simply protect or sequester compounds that would otherwise be unstable in the formulation. Liposomes that remain on the skin surface are useful as light-absorbers, agents to deliver color or sunscreens, or as depots for timed-release. Liposomes that penetrate the stratum corneum have the potential to interact with living tissue. Topically applied liposomes can either mix with the stratum corneum lipid matrix or penetrate the stratum corneum by exploiting the lipid-water interface of the intercellular matrix. There are at least four major routes of entry into the skin: pores, hair follicles, columnular spaces and the lipid:water matrix between squames. A major force driving liposome penetration is the water gradient, and flexible liposomes are best able to exploit these delivery opportunities. Some liposomes release their contents extracellularly. Topical application of photosensitizers may be enhanced by encapsulation in liposomes. Higher and longer-lasting drug concentrations may be produced in localized areas of skin, particularly at disease sites where the stratum corneum and the skin barrier function are disrupted. The liposome membrane should be designed to capture lipophilic drugs in the membrane or hydrophilic drugs in the interior. Other types of liposomes can be engineered to be taken up by cells. Once inside cells, the lysosomal sac and clatherin-coated pit are the dead-end destinations for liposomes unless an escape path has been engineered into the liposome. A novel method has been developed to allow delivery into cells of the skin, by escape from the lysosomal sac. These liposomes have been used to topical deliver active DNA repair enzymes from liposomes into epidermal cells and to enhance DNA repair of UV-irradiated skin. From these studies a tremendous amount has been learned about the relationship of DNA damage and skin cancer. Both mutations and immunosuppression appear to be essential to skin cancer and both are induced by DNA damage. DNA damage produces immediate effects by inducing the expression of cytokines, which means that DNA damage can induce signaling in neighboring, undamaged cells. The repair of only a fraction of the DNA damage has a disproportionate effect on the biological responses, clearly demonstrating that not all DNA damage is equivalent. This technology demonstrates that biologically active proteins can be delivered into the cells of skin, and opens up a new field of correcting or enhancing skin cell metabolism to improve human health. [source] A comparison of invasive and non-invasive dayflowers (Commelinaceae) across experimental nutrient and water gradientsDIVERSITY AND DISTRIBUTIONS, Issue 5-6 2004Jean H. Burns ABSTRACT Little is known about the traits and mechanisms that determine whether or not a species will be invasive. Invasive species are those that establish and spread after being introduced to a novel habitat. A number of previous studies have attempted to correlate specific plant traits with invasiveness. However, many such studies may be flawed because they fail to account for shared evolutionary history or fail to measure performance directly. It is also clear that performance is context dependent. Thus, an approach that corrects for relatedness and incorporates multiple experimental conditions will provide additional information on performance traits of invasive species. I use this approach with two or three pairs of invasive and closely related non-invasive species of Commelinaceae grown over experimental gradients of nutrient and water availability. Invasive species have been introduced, established, and spread outside their native range; non-invasive species have been introduced, possibly (but not necessarily) established, but are not known to have spread outside their native range. The invasive species had higher relative growth rates (RGR) than non-invasive congeners at high nutrient availabilities, but did not differ from non-invasive species at low nutrient availabilities. This is consistent with a strategy where these particular invasive species are able to rapidly use available resources. Relative growth rates were also higher for two out of three invasive species across a water availability gradient, but RGR did not differ in plasticity between the invasive and non-invasive species. This suggests that nutrient addition, but not changes in water availability, might favour invasion of dayflowers. This approach is novel in comparing multiple pairs of invasive and non-invasive congeners across multiple experimental conditions and allows evaluation of the robustness of performance differences. It also controls for some of the effects of relatedness that might confound multispecies comparisons. [source] Stress tolerance abilities and competitive responses in a watering and fertilization field experimentJOURNAL OF VEGETATION SCIENCE, Issue 6 2005P. Liancourt Abstract Question: Do water gradients produce patterns of responses to stress and competition similar to those induced by nutrient gradients? Location: French Alps. Methods: We established a split-plot design in a calcareous grassland, with watering and fertilization as main plot treatments and competition as subplot treatment. We followed individual and competitive responses of transplants of the three potential dominant grass species: Bromus erectus, Brachypodium rupestre and Arrhenatherum elatius, in all plots during two growing seasons. Changes in natural relative abundances of the three grass species were also monitored. Results: The growth and the relative abundance of A. elatius were primarily stimulated by nutrient addition and those of B. rupestre by water addition, whereas B. erectus decreased in abundance and had a very low flexibility with enhanced resource supply. Competition intensity increased for all species with both watering and fertilization and the ranking in competitive responses did not change with treatments: A. elatius > B. rupestre > B. erectus. Conclusions: Patterns of dominance were efficiently explained by stress tolerance abilities and competitive responses for dry and poor sites, and wet and rich sites for B. erectus and A. elatius respectively, whereas competitive responses were poor predictors of dominance for B. rupestre in wet and nutrient-poor sites. Further studies are needed to assess the potential role of other processes, such as increasing competitive effect on light with increasing age as well as interference, to explain the dominance of this conservative competitor type of species in wet and nutrient-poor sites. [source] | ||||||||||