Estimation of Carbon Sequestration by Combining Remote Sensing and Net Ecosystem Exchange Data for Northern Mixed-Grass Prairie and Sagebrush–Steppe Ecosystems

Carbon sequestration was estimated a northern mixed-grass prairie site and a sagebrush–steppe site in southeastern Wyoming using an approach that integrates remote sensing, CO2 flux measurements, and meteorological data. Net ecosystem exchange (NEE) of CO2 was measured using aircraft and ground flux techniques and was linearly related to absorbed photosynthetically active radiation (APAR). The slope of this relationship is the radiation use efficiency (ε = 0.51 g C/MJ APAR); there were no significant differences in the regression coefficients between the two sites. Furthermore, ecosystem chamber measurements of total respiration in 1998 and 1999 were used to develop a functional relationship with daily average temperature; the Q10 of the relationship was 2.2. Using the Advanced Very High Resolution radiometer. Normalized Difference Vegetation Index and meteorological data, annual gross primary production and respiration were calculated from 1995 to 1999 for the two sites. Overall, the sagebrush– steppe site was a net carbon sink, whereas the northern mixed-grass prairie site was in carbon balance. There was no significant relationship between NEE and APAR for a coniferous forest site, indicating this method for scaling up CO2 flux data may be only applicable to rangeland ecosystems. The combination of remote sensing with data from CO2 flux networks can be used to estimate carbon sequestration regionally in rangeland ecosystems

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 herbivoreprairie 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

(Table 1) Climate characteristics of the four North American Tundra Experiment (ITEX) sites

Climate warming is expected to differentially affect CO2 exchange of the diverse ecosystems in the Arctic. Quantifying responses of CO2 exchange to warming in these ecosystems will require coordinated experimentation using standard temperature manipulations and measurements. Here, we used the International Tundra Experiment (ITEX) standard warming treatment to determine CO2 flux responses to growing-season warming for ecosystems spanning natural temperature and moisture ranges across the Arctic biome. We used the four North American Arctic ITEX sites (Toolik Lake, Atqasuk, and Barrow [USA] and Alexandra Fiord [Canada]) that span 10° of latitude. At each site, we investigated the CO2 responses to warming in both dry and wet or moist ecosystems. Net ecosystem CO2 exchange (NEE), ecosystem respiration (ER), and gross ecosystem photosynthesis (GEP) were assessed using chamber techniques conducted over 24-h periods sampled regularly throughout the summers of two years at all sites. At Toolik Lake, warming increased net CO2 losses in both moist and dry ecosystems. In contrast, at Atqasuk and Barrow, warming increased net CO2 uptake in wet ecosystems but increased losses from dry ecosystems. At Alexandra Fiord, warming improved net carbon uptake in the moist ecosystem in both years, but in the wet and dry ecosystems uptake increased in one year and decreased the other. Warming generally increased ER, with the largest increases in dry ecosystems. In wet ecosystems, high soil moisture limited increases in respiration relative to increases in photosynthesis. Warming generally increased GEP, with the notable exception of the Toolik Lake moist ecosystem, where warming unexpectedly decreased GEP >25%. Overall, the respiration response determined the effect of warming on ecosystem CO2 balance. Our results provide the first multiple-site comparison of arctic tundra CO2 flux responses to standard warming treatments across a large climate gradient. These results indicate that (1) dry tundra may be initially the most responsive ecosystems to climate warming by virtue of strong increases in ER, (2) moist and wet tundra responses are dampened by higher water tables and soil water contents, and (3) both GEP and ER are responsive to climate warming, but the magnitudes and directions are ecosystem-dependent