Geology topics

J. T. Fahnestock

Publications and source records attributed to J. T. Fahnestock.

3 recordsLinked to original sources

USGS2003

Effects of ungulates and prairie dogs on seed banks and vegetation in a North American mixed-grass prairie

The relationship between vegetation cover and soil seed banks was studied in five different ungulate herbivore-prairie dog treatment combinations at three northern mixed-grass prairie sites in Badlands National Park, South Dakota. There were distinct differences in both the seed bank composition and the aboveground vegetation between the off-prairie dog colony treatments and the on-colony treatments. The three on-colony treatments were similar to each other at all three sites with vegetation dominated by the forbs Dyssodia papposa, Hedeoma spp., Sphaeralcea coccinea, Conyza canadensis , and Plantago patagonica and seed banks dominated by the forbs Verbena bracteata and Dyssodia papposa . The two off-colony treatments were also similar to each other at all three sites. Vegetation at these sites was dominated by the grasses Pascopyrum smithii, Bromus tectorum and Bouteloua gracilis and the seed banks were dominated by several grasses including Bromus tectorum, Monroa squarrosa, Panicum capillare, Sporobolus cryptandra and Stipa viridula . A total of 146 seedlings representing 21 species germinated and emerged from off-colony treatments while 3069 seedlings comprising 33 species germinated from on-colony treatments. Fifteen of the forty species found in soil seed banks were not present in the vegetation, and 57 of the 82 species represented in the vegetation were not found in the seed banks. Few dominant species typical of mixed-grass prairie vegetation germinated and emerged from seed banks collected from prairie dog colony treatments suggesting that removal of prairie dogs will not result in the rapid reestablishment of representative mixed-grass prairie unless steps are taken to restore the soil seed bank.

South Dakota↗

USGS1999

Long-term experimental manipulation of winter snow regime and summer temperature in arctic and alpine tundra

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 CO 2 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.

Hydrological Processes↗

USGS1998

Winter and early spring CO2 efflux from tundra communities of northern Alaska

Carbon dioxide concentrations through snow were measured in different arctic tundra communities on the North Slope of Alaska during winter and early spring of 1996. Subnivean CO 2 concentrations were always higher than atmospheric CO 2 . A steady state diffusion model was used to generate conservative estimates of CO 2 flux to the atmosphere. The magnitude of CO 2 efflux differed with tundra community type, and rates of carbon release increased from March to May. Winter CO 2 efflux was highest in riparian and snow bed communities and lowest in dry heath, upland tussock, and wet sedge communities. Snow generally accrues earlier in winter and is deeper in riparian and snow bed communities compared with other tundra communities, which are typically windswept and do not accumulate much snow during the winter. These results support the hypothesis that early and deep snow accumulation may insulate microbial populations from very cold temperatures, allowing sites with earlier snow cover to sustain higher levels of activity throughout winter compared to communities that have later developing snow cover. Extrapolating our estimates of CO 2 efflux to the entire snow-covered season indicates that total carbon flux during winter in the Arctic is 13–109 kg CO 2 -C ha −1 , depending on the vegetation community type. Wintertime CO 2 flux is a potentially important, yet largely overlooked, part of the annual carbon cycle of tundra, and carbon release during winter should be accounted for in estimates of annual carbon balance in arctic ecosystems.

Journal of Geophysical Research D: Atmospheres↗