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  • 1
    ISSN: 1432-1939
    Keywords: Key words Stable isotopes ; Plant-animal interactions ; Rocky Mountain National Park ; Riparian ecosystems ; Ungulates
    Source: Springer Online Journal Archives 1860-2000
    Topics: Biology
    Notes: Abstract To ascertain whether browsing or hydrologic conditions influence the physiological performance of Salix and whether Salix and graminoids (Carex) use and possibly compete for similar water resources, we quantified the in situ seasonal patterns of plant water and carbon relations over three growing seasons. Our studies were designed to address the physiological factors which may be responsible for poor woody plant regeneration in montane riparian habitats of Rocky Mountain National Park, Colo. As these systems act to insure the delivery of fresh water to downstream users, the maintenance of their integrity is critical. We quantified plant water potentials, instantaneous rates of carbon fixation, leaf carbon isotope discrimination (Δ), leaf nitrogen content and water sources using stable isotopes of water (δ18O). The carbon and water relations of Salix were significantly affected by winter browsing by elk and in some cases by landscape position with regard to proximity to active streams. Winter browsing of Salix by elk significantly increased summer plant water potentials and integrative measures of gas exchange (Δ), though browsing did not consistently affect instantaneous rates of photosynthesis, leaf nitrogen or the sources of water used by Salix. No effect of experimental manipulations of surface water conditions on Salix physiology was observed, likely due to the mesic nature of our study period. Using a two-member linear mixing model, from δ18O values we calculated that Salix appears to rely on streams for approximately 80% of its water. In contrast, the graminoid Carex derives almost 50% of its water from rainfall, indicating divergent water source use by these two life forms. Based on these findings, winter browsing by elk improved Salix water balance possibly by altering the shoot to root ratio which in turn leads to higher water potentials and higher degrees of season-long gas exchange, while experimental damming had in general no effect on the physiological performance of Salix plants. In addition, as the water sources of Salix and Carex were significantly different, competition for water may not influence the growth, development, and regeneration of Salix. Thus, under the conditions of our study, herbivory had a positive effect on the physiological performance of Salix, but it is still unclear whether these changes in physiology transcend into improved Salix regeneration and survivorship. However, under drier environmental conditions such as lower snowpacks and lower stream flows, the browsing resistance of Salix and ecosystem regeneration may be greatly hindered because the reliance of Salix on stream water makes it vulnerable to changes in surface water and hydrological conditions.
    Type of Medium: Electronic Resource
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  • 2
    ISSN: 1432-1939
    Keywords: Arctic ; Climate change ; Dryas octopetala ; L. ssp. octopetala ; Clonal growth ; Reproductive output
    Source: Springer Online Journal Archives 1860-2000
    Topics: Biology
    Notes: Abstract Opportunities exist in high Arctic polar semidesert communities for colonisation of unvegetated ground by long-lived clonal plants such as Dryas octopetala. This can be achieved by lateral spread of vegetative ramets, or by sexual reproduction and seedling recruitment. The objectives of this study were (1) to determine whether these two means of proliferation show differential sensitivity to contrasting components of the abiotic environment (temperature, soil nutrient and water availability) and (2) to evaluate the potential for D. octopetala to respond to climate change by an increase in cover and biomass in polar semi-desert communities. Factorial environmental manipulations of growing season temperature, soil nutrient and water status were conducted over 3 years at a polar semi-desert community in Svalbard, Norway (78°56.12′N, 11°50.4′E) and both clonal and sexual reproductive performance, together with instantaneous net photosynthesis (Pn), were recorded during the third season (1993). D. octopetala capitalised rapidly on an amelioration in the availability of inorganic nutrients (N, P and K) by an expansion in leaf area and biomass supported by increased Pn per unit leaf weight, and by apparent luxury uptake of nutrients (particularly P). Several facets of sexual reproductive development and seed viability were markedly improved by elevated temperatures or soil nutrient availability. Thus although D. octopetala is a long-lived clonal plant, with many traits characteristic of stress resistance syndrome, it showed considerable phenotypic plasticity in response to environmental manipulations. The results support the hypothesis that clonal growth confers survival potential during unfavourable years, together with the ability to capitalise on nutrient flushes and recycle nutrients internally. Continued investment in sexual reproduction ensures that seed setting is successful during favourable years, even if these occur infrequently. Cimate warming in the high Arctic could thus result in changes in the cover, biomass and the relative significance of clonal versus sexual proliferation of D. octopetala (and thus the genetic diversity of the population) but the long-term responses will probably be constrained by lack of available nutrients.
    Type of Medium: Electronic Resource
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  • 3
    ISSN: 1432-1939
    Keywords: Key words Plant functional types ; Selective removal ; Shortgrass steppe ; Soil water ; Stable isotopes
    Source: Springer Online Journal Archives 1860-2000
    Topics: Biology
    Notes: Abstract We conducted a study to test the predictions of Walter's two-layer model in the shortgrass steppe of northeastern Colorado. The model suggests that grasses and woody plants use water resources from different layers of the soil profile. Four plant removal treatments were applied in the spring of 1996 within a plant community codominated by Atriplex canescens (a C4 shrub) and Bouteloua gracilis (a C4 grass). During the subsequent growing season, soil water content was monitored to a depth of 180 cm. In addition, stem and leaf tissue of Atriplex, Bouteloua and the streamside tree Populus sargentii were collected monthly during the growing seasons of 1995 and 1996 for analysis of the δ18O value of plant stem water (for comparison with potential water sources) and the δ13C value of leaves (as an indicator of plant water status). Selective removal of shrubs did not significantly increase water storage at any depth in the measured soil profile. Selective removal of the herbaceous understory (mainly grasses) increased water storage in the top 60 cm of the soil. Some of this water gradually percolated to lower layers, where it was utilized by the shrubs. Based on stem water δ18O values, grasses were exclusively using spring and summer rain extracted from the uppermost soil layers. In contrast, trees were exclusively using groundwater, and the consistent δ13C values of tree leaves over the course of the summer indicated no seasonal changes in gas exchange and therefore minimal water stress in this life-form. Based on anecdotal rooting-depth information and initial measurements of stem water δ18O, shrubs may have also had access to groundwater. However, their overall δ18O values indicated that they mainly used water from spring and summer precipitation events, extracted from subsurface soil layers. These findings indicate that the diversity of life-forms found in this shortgrass steppe community may be a function of the spatial partitioning of soil water resources, and their differential use by grasses, shrubs, and trees. Consequently, our findings support the two-layer model in a broad sense, but indicate a relatively flexible strategy of water acquisition by shrubs.
    Type of Medium: Electronic Resource
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  • 4
    Electronic Resource
    Electronic Resource
    Oxford, UK : Blackwell Publishing Ltd
    Global change biology 2 (1996), S. 0 
    ISSN: 1365-2486
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Biology , Energy, Environment Protection, Nuclear Power Engineering , Geography
    Notes: In 1993 a project was set up at Upper Teesdale to investigate some of the effects of predicted increased temperatures and increased nutrient availability resulting from increased litter decomposition rates, and atmospheric deposition of nitrogen, on the interaction between bracken and heather, two species which cover large areas globally. 32 2 × 2 m plots were laid out on a hillside on, and just below, the interface between the two species. Half of the plots were placed on pure bracken, and the other half on the boundary. Within each vegetation type half of the plots were covered with open-topped polythene tents to simulate climate warming, while the other plots remained open. The second treatment consisted of additional fertilizer at 50 kg N ha−1 y−1 simulating increased decomposition rates at higher temperatures and increased atmospheric nitrogen deposition. The two treatments and their controls were combined in a 2 × 2 factorial design, with two replicates in each of two blocks.Results obtained in the first two summers indicated that fronds emerged earlier in spring and senesced later in autumn inside the tents, effectively lengthening the growing season. Fronds were taller when growing in warmer conditions, and carried more pinnae. In addition, the frond density was higher inside the tents, and together these effects contributed to greatly enhanced vigour of bracken in some of the conditions predicted by General Circulation Models. It seems likely that bracken will be more competitive, and have the potential to further encroach into heather dominated areas under these conditions.
    Type of Medium: Electronic Resource
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  • 5
    Publication Date: 1996-02-01
    Print ISSN: 1354-1013
    Electronic ISSN: 1365-2486
    Topics: Biology , Energy, Environment Protection, Nuclear Power Engineering , Geography
    Published by Wiley
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  • 6
    Publication Date: 1998-11-01
    Print ISSN: 0148-0227
    Electronic ISSN: 2156-2202
    Topics: Geosciences
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  • 7
    Publication Date: 1999-10-01
    Description: Three 60 m long, 2.8 m high snowfences have been erected to study long-term effects of changing winter snow conditions on arctic and alpine tundra. This paper describes the experimental design and short-term effects. Open-top fiberglass warming chambers are placed along the experimental snow gradients and in controls areas outside the fences; each warming plot is paired with an unwarmed plot. The purpose of the experiment is to examine short- and long-term changes to the integrated physical-biological systems under simultaneous changes of winter snow regime and summer temperature, as part of the Long-Term Ecological Research network and the International Tundra Experiment. The sites were at Niwot Ridge, Colorado, a temperate high altitude site in the Colorado Rockies, and Toolik Lake, Alaska, a high-latitude site. Initial results indicate that although experimental designs are essentially identical at the arctic and alpine sites, experimental effects are different. The drift at Niwot Ridge lasts much longer than do the Toolik Lake drifts, so that the Niwot Ridge fence affects both summer and winter conditions, whereas the Toolik Lake fence affects primarily winter conditions. The temperature experiment also differs in effect between the sites. Although the average temperature increase at the two sites is similar (daily increase 1.5°C at Toolik and 1.9°C at Niwot Ridge), at Toolik Lake there is only minor diurnal variation, whereas at Niwot Ridge the daytime increases are extreme on sunny days (as much as 7-10°C), and minimum nighttime temperatures in the chambers are often slightly cooler than ambient (by about 1°C). The experimental drifts resulted in wintertime increases in temperature and CO2 flux. Temperatures under the deep drifts were much more consistent and warmer than in control areas, and at Niwot Ridge remained very close to 0°C all winter. These increased temperatures were likely responsible for observed increases in system carbon loss. Initial changes to the aboveground biotic system included an increase in growth in response to both snow and warming, despite a reduced growing season. This is expected to be a transient response that will eventually be replaced by reduced growth. At least one species, Kobresia myosuroides, had almost completely died at Niwot Ridge three years after fence construction, whereas other species were increasing. We expect in both the short- and long-term to see the strongest effects of snow at the Niwot Ridge site, and stronger effects of temperature at Toolik Lake.Three 60 m long, 2.8 m high snowfences have been erected to study long-term effects of changing winter snow conditions on arctic and alpine tundra. This paper describes the experimental design and short-term effects. Open-top fiberglass warming chambers are placed along the experimental snow gradients and in controls areas outside the fences; each warming plot is paired with an unwarmed plot. The purpose of the experiment is to examine short- and long-term changes to the integrated physical-biological systems under simultaneous changes of winter snow regime and summer temperature, as part of the Long-Term Ecological Research network and the International Tundra Experiment. The sites were at Niwot Ridge, Colorado, a temperate high altitude site in the Colorado Rockies, and Toolik Lake, Alaska, a high-latitude site. Initial results indicate that although experimental designs are essentially identical at the arctic and alpine sites, experimental effects are different. The drift at Niwot Ridge lasts much longer than do the Toolik Lake drifts, so that the Niwot Ridge fence affects both summer and winter conditions, whereas the Toolik Lake fence affects primarily winter conditions. The temperature experiment also differs in effect between the the sites. Although the average temperature increase at the two sites is similar (daily increase 1.5°C at Toolik and 1.9°C at Niwot Ridge), at Toolik Lake there is only minor diurnal variation, whereas at Niwot Ridge the daytime increases are extreme on sunny days (as much as 7-10°C), and minimum nighttime temperatures in the chambers are often slightly cooler than ambient (by about 1°C). The experimental drifts resulted in wintertime increases in temperature and CO2 flux. Temperatures under the deep drifts were much more consistent and warmer than in control areas, and at Niwot Ridge remained very close to 0°C all winter. These increased temperatures were likely responsible for observed increases in system carbon-loss. Initial changes to the aboveground biotic system included an increase in growth in response to both snow and warming, despite a reduced growing season. This is expected to be a transient response that will eventually be replaced by reduced growth. At least one species, Kobresia myosuroides, had almost completely died at Niwot Ridge three years after fence construction, whereas other species were increasing. We expect in both the short- and long-term to see the strongest effects of snow at the Niwot Ridge site, and stronger effects of temperature at Toolik Lake.
    Print ISSN: 0885-6087
    Electronic ISSN: 1099-1085
    Topics: Architecture, Civil Engineering, Surveying , Geography
    Published by Wiley
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