Jace T. Fahnestock

Winter Biological Processes Could Help Convert Arctic Tundra to Shrubland

Abstract In arctic Alaska, air temperatures have warmed 0.5 degrees Celsius (°C) per decade for the past 30 years, with most of the warming coming in winter. Over the same period, shrub abundance has increased, perhaps a harbinger of a conversion of tundra to shrubland. Evidence suggests that winter biological processes are contributing to this conversion through a positive feedback that involves the snow-holding capacity of shrubs, the insulating properties of snow, a soil layer that has a high water content because it overlies nearly impermeable permafrost, and hardy microbes that can maintain metabolic activity at temperatures of −6°C or lower. Increasing shrub abundance leads to deeper snow, which promotes higher winter soil temperatures, greater microbial activity, and more plant-available nitrogen. High levels of soil nitrogen favor shrub growth the following summer. With climate models predicting continued warming, large areas of tundra could become converted to shrubland, with winter processes like those described here possibly playing a critical role.

TUNDRA CO2 FLUXES IN RESPONSE TO EXPERIMENTAL WARMING ACROSS LATITUDINAL AND MOISTURE GRADIENTS

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 manipula- tions 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 108 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.

Wintertime CO2 efflux from Arctic soils: Implications for annual carbon budgets

Estimates of annual carbon loss from arctic tundra ecosystems are based nearly entirely on measurements taken during the growing season in part because of methodological limitations but also reflecting the assumption that respiration during winter is near zero. Measurements of CO2 flux during winter, however, indicate significant amounts of carbon loss from tundra ecosystems throughout the 240-day nongrowing season. In our study during the 1996 and 1997 nongrowing seasons, winter carbon losses ranged from 2.0 g CO2 m−2 season−1 in moist dwarf shrub communities to 97 g CO2 m−2 season−1 in natural snowdrift communities, with an average wintertime CO2 efflux of 45 g CO2 m−2 for all Low Arctic tundra communities (0.14 Pg CO2 yr−1 worldwide). These measurements indicate that current estimates of annual carbon loss from tundra ecosystems are low. Inclusion of wintertime losses of CO2 into annual carbon budgets increases the annual carbon efflux of arctic tundra ecosystems by 17% and changes some ecosystems from net annual sinks to net sources of CO2 to the atmosphere.

Cold-season Production of CO2 in Arctic Soils: Can Laboratory and Field Estimates Be Reconciled through a Simple Modeling Approach?

Abstract Microbial activity in arctic tundra soils has been evaluated through both lab incubations and field flux measurements. To determine whether these different measurement approaches can be directly linked to each other, we developed a simple model of soil microbial CO2 production during the cold season in tussock tundra, moss tundra, and wet meadow tundra in the Alaskan Arctic. The model incorporated laboratory-based estimates of microbial temperature responses at sub-zero temperatures with field measurements of C stocks through the soil profile and daily temperature measurements at the sites. Estimates of total CO2 production overestimated in situ cold season CO2 fluxes for the studied sites by as much as two- to threefold, suggesting that either CO2 produced in situ does not efflux during the cold season or that microbial respiration potentials are constrained by some other factor in situ. Average estimated winter CO2 production was near 120 g C m−2 in moist tundra and 60 g C m−2 in wet meadow tundra. Production was strongly seasonal, with most of the winter CO2 production happening early in the winter, before soils froze completely through. Roughly two-thirds of the total estimated CO2 production was from deep soils, largely mineral soils, in contrast to growing season CO2 dynamics.

Plant responses to defoliation and resource supplementation in the Pryor Mountains.

Field studies were conducted in 2 types of grasslands in the Pryor Mountain Wild Horse Range of northern Wyoming and southern Montana to examine plant biomass production and nitrogen responses to the separate and combined effects of graminoid defoliation and increased environmental resource (water or nutrients) supply. Short-term plant responses were monitored over 2 years which differed substantially in growing season precipitation. In the arid, low elevation grassland, total grass biomass was significantly lower in the dry year than the wet year in all treatments. Defoliation of the grasses did not reduce their aboveground biomass production in the wet year, but severely reduced it in the dry year, primarily because of a decrease in tiller density. Mass of remaining individual tillers increased with clipping in the dry year, and nitrogen concentrations of the grasses increased with clipping in both years. Irrigation alone increased total belowground biomass compared to the other treatments, but did not increase the aboveground biomass production of any plant functional group. Clipping plus irrigation resulted in greater total aboveground biomass production and higher nitrogen concentrations of the grasses than control or irrigated treatments. Clipping graminoids in the more mesic montane grassland did not decrease their biomass production in either year, but did increase their nitrogen concentrations and increase the collective aboveground biomass production of the other plant functional groups. Fertilization and fertilization plus clipping significantly increased total aboveground biomass production in both years, and total belowground biomass was greatest in fertilized plots.

Alpine Grassland CO2 Exchange and Nitrogen Cycling: Grazing History Effects, Medicine Bow Range, Wyoming, U.S.A

Abstract Our study examined carbon dioxide exchange and nitrogen cycling over two consecutive years (winter and summer) in a grazed alpine grassland and in an embedded long-term grazing exclosure to ascertain whether grazing history had resulted in divergent soil carbon attributes, CO2 exchange rates, and different vegetation C and N and soil N processes. Soil C and N concentrations and masses were significantly higher in the grazed than in the ungrazed area, though grass leaf N was higher in the ungrazed area, as was vegetation biomass. Detectable amounts of CO2 were lost from the grazed and ungrazed areas of this grassland during the winters of 1998, 1999, and 2000, and at 6 of 15 winter flux sample dates, CO2 efflux was greater in the grazed area than in the ungrazed area. The ungrazed area consistently gained more C during the summer months than the grazed area, with net CO2 exchange peaking in mid-July 1998 at nearly 5 μmol m−2 s−1 in the ungrazed area compared to <2 μmol m−2 s−1 in the grazed area. During the 2-yr study period, the grazed area was a carbon source of 170 g C m−2, while the ungrazed area was a carbon sink of 83 g C m−2. Lower N mineralization rates early and late in the summer (1999) in the grazed site at Libby Flats corresponded to reductions in net CO2 exchange and lower plant N content compared to the ungrazed exclosure. Based on these results, we suggest that: (1) long-term grazing in high-altitude rangelands can alter annual CO2 exchange and N dynamics; (2) temporal synchrony in C and N processes occur during the summer; that is, increased C exchange rates accompany increased N mineralization rates; and (3) integrative (total soil C and N) and instantaneous (CO2 exchange and vegetation N) measures of C and N dynamics may not necessarily lead to the same interpretation regarding C sequestration and N cycling in alpine grasslands.

Morphological and Physiological Responses of Perennial Grasses to Long-term Grazing in the Pryor Mountains, Montana

Abstract We evaluated in situ the effects of long-term grazing (>100 y) on the morphological (i.e., shoot height, leaf blade length, width and angle) and physiological (i.e., gas exchange and water relations) responses of the dominant perennial grass species from arid lowlands and more mesic uplands of the Pryor Mountain Wild Horse Range (PMWHR) in Montana. Pseudoroegneria spicata, the most abundant grass in the lowland communities, had shorter vegetative shoot heights and leaf blade lengths and narrower leaves in plants from grazed than long-term ungrazed sites. Similarly, vegetative and reproductive shoot heights of Festuca idahoensis and Elymus lanceolatus, common upland grass species, were shorter in plants from grazed than ungrazed sites. Leaf lengths of these upland grasses also were shorter and less erect in plants from grazed sites than ungrazed sites. The physiological responses of the dominant grasses to grazing were not consistent between species or sampling dates. Overall, photosynthetic rates, stomatal conductances and xylem pressure potentials were the same in over 65% of the comparisons between plants from grazed and ungrazed sites and were higher in grazed sites in only 11 to 22% of the grazed-ungrazed comparisons. Collectively, our results indicate that long-term grazing of grasses by wild horses and other herbivores in the PMWHR has resulted in morphological modification, but has not substantively altered physiological function.

Leaf isotopic (δ13C and δ15N) and nitrogen contents of Carex plants along the Eurasian Coastal Arctic: results from the Northeast Passage expedition

We conducted surrogate in-situ physiological performance measures (δ13C and δ15N) of Carex plants from 15 Eurasian Coastal Arctic sites. Leaf carbon isotope discrimination (LCID) of Carex plants exhibited significant differences between sites (populations). Additionally, LCID was inversely correlated with mean annual temperature and stomatal density, and to a lesser extent, with the depth of thaw. Leaf δ15N values of Carex plants exhibited significant differences between sites without differences among ramet age classes, and the leaf δ15N values were inversely correlated with mean annual precipitation. These ranges of Carex leaf gas exchange and mineral nutrition across the Eurasian Arctic may contribute to Carex’s dominance in coastal tundra systems. Also, the inverse correlation between LCID, precipitation, and temperature indicates that, as precipitation increases and temperatures continue to warm in Eurasia, leaf gas exchange may actually be lower in the future, leading to reductions in shoot growth and lower above-ground biomass production.

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.