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This study focuses on how established shrub seedlings (Salix myrsinifolia x phycilifolia and Vaccinium vitis-idaea) develop in historical milking grounds. Historical milking grounds are cultural remains from the intensive reindeer herding era when the Sami migrated with closely controlled herds. Although the places were never fenced, the high concentration of reindeer close to the tenting grounds created patches of grass and forb dominated vegetation in areas outherwise dominated by deciduous or ericoid shrubs. Despite about 100 years of abandonment the shrubs have not come back and the milking grounds are still clearly visible in the landscape. One theory why the former milking grounds are so stable is that shrubs cannot establish from seedlings due to unfavorable abiotic conditions, or due to competitive dominance of already established forbs and grasses. I tested this hypothesis by planting shrub seedlings in the milking grounds and in reference areas, with and without neighboring vegetation and investigated seedling survival and growth. The results show that shrub seedlings are able to both survive and develop in milking grounds suggesting that the seedling state is not the limiting factor in shrub encroachment in the milking grounds.

The literature of disturbance ecology reveals that, under present climatic conditions, non-native plants have little or no role in high arctic tundra revegetation. Rather, it has been suggested that indigenous flora, especially long-lived perennial graminoids, are crucial to recovery. However, few long-term data are available on past impacts within productive sedge-meadows in the High Arctic, and none which consider the non-vascular flora. / This thesis combines biogeographical and patch dynamics perspectives to focus on $ geq$21 yr of natural and assisted recovery of vegetation and soils from a wide range of dated anthropogenic surface disturbances at three Canadian Arctic sites. Empirical, experimental and archival investigations were made among climatically similar, but widely disjunct, coastal lowlands of contrasting geologies on Baffin, Devon, and Cornwallis Islands. These data encompass minerotrophic and oligotrophic wetlands in which the vascular floras show minimal differentiation yet the sampled bryofloras share only 31.8% of their total taxa. The occurrences chosen for study are representative of the most widespread, small-scale human impacts in the North, including vehicular, pedestrian, construction, and pollution disturbances. / It was determined that rutting from even a single passage of a tracked vehicle in summer resulted in significant reductions in species richness and biomass. On slopes $ geq$2$ sp circ$, these same small ruts have drained large areas of peatlands, a serious cumulative impact. Long-term effects of drainage include the local extinction of populations of Sphagnum spp. and rhizomatous vascular aquatics, and changes in the chemistry and thermal regime of drained mineral soils. Other effects include significant changes in biomass and the concentrations of macronutrients in the leaves of dominant species. These effects were magnified in peatlands drained where multi-pass vehicle movements occurred. / Species richness displayed an inverse relationship with trampling intensity and the soils of heavily trampled ground remained severely compacted after 21 years. These patches were dominated by dense swards of ruderal grasses. Nutrient concentrations in the leaves of the latter and other colonizing and surviving species tended to increase with trampling intensity. Trampled patches and archaeological sites appeared selectively grazed by several herbivores. Although humans initiated the disturbances within these patches, it is the animals which are responsible for many of the dynamics of patch change over the long-term. / Classification and ordination procedures revealed linkages between the floristic associations of trampled meadows on Baffin Island and archaeological sites on Devon and Cornwallis Islands. One critical implication is that even low levels of human impact may give rise to ruderal plant communities which are extremely persistent. These patches are poor in terms of species richness, but contribute to habitat heterogeneity at the landscape level and comprise preferred forage for local vertebrate herbivores. / Archaeological excavation and restoration revealed that at least some stores of viable seed exist in both mesic and wet tundra soils and point to the importance of initial floristic composition (sensu Egler 1954). From a long-term perspective, the data establish that mesic tundra vegetation and soils are easily disturbed and recover much more slowly than their low arctic counterparts under similar disturbance regimes.

The consortia of microorganisms responsible for the hydrolysis of cellulose in situ are at present poorly characterized. Nonetheless, the importance of these communities is underscored by their capacity for converting biomass to greenhouse gases such as carbon dioxide and methane. The metabolic capacities of these organisms is particularly alarming considering the volume of biomass that is projected to re-enter the carbon cycle in Arctic tundra soil environments as a result of a warming climate. Novel cold-adapted cellulase enzymes also present enormous opportunities for a broad range of industries. DNA stable-isotope probing (DNA-SIP) is a powerful tool for linking the phylogenetic identity and function of cellulolytic microorganisms by the incorporation of isotopically labelled substrate into nucleic acids. By providing 13C-enriched glucose and cellulose to soil microcosms, it was possible to characterize the communities of microorganisms involved in the metabolism of these substrates in an Arctic tundra soil sample from Resolute Bay, Canada. A protocol for generating 13C-enriched cellulose was developed as part of this thesis, and a visual DNA-SIP protocol was generated to demonstrate the experimental outline. Denaturing gradient gel electrophoresis (DGGE) and 16S rRNA clone libraries were used to visualize changes in community structure and to identify prevalent, active phylotypes in the SIP incubations. Notably, predominant phylotypes changed over time and clustered based on substrate metabolism. Labelled nucleic acids identified by sequenced DGGE bands and 16S rRNA gene clone libraries provided converging evidence indicating the predominance of Clostridium and Sporolactobacillus in the 13C-glucose microcosms, and Betaproteobacteria, Bacteroidetes, and Gammaproteobacteria in the 13C-cellulose microcosms. Active populations consuming glucose and cellulose were distinct based on principle coordinate analysis of “light” and “heavy” DNA. A large portion of the recovered sequences possessed no close matches in the GenBank database, reflecting the paucity of data on these communities of microorganisms.

Diss. (sammanfattning) Uppsala : Univ., 2003. / Härtill 5 uppsatser.

Thesis (M.SC.) - Carleton University, 2007. / Includes bibliographical references (p. 154-161). Also available in electronic format on the Internet.

Cryoturbation is a fundamental soil forming process with large importance for ecosystem functioning in the Arctic. Recent investigations have emphasized the effect of climate change on cryoturbation, but contrast in their predictions on cryoturbation under future climate warming. This study analyzed to what extent conclusions on the response of cryoturbation to environmental conditions depend on: 1) the proxy of cryogenic activity used; 2) the spatial scale of environmental predictor variables; and 3) the mesoclimatic regime of the study site. As an example of cryoturbated soil, 48 non-sorted patterned-ground features were sampled at eight sites along an elevational and a precipitation gradient and vegetation gradients nested within each site in the Abisko area, northernmost Sweden. To quantify cryogenic activity, eight proxies of cryogenic activity were used. In addition, environmental data were obtained at two spatial scales from field surveys and existing geodata. The results suggest a significant correlation between most activity proxies. Cryogenic activity increased along the precipitation gradient and peaked at intermediate elevations, while within-site variation was similar to between-site variation. The response of cryogenic activity to environmental factors was largly independent of the proxy used but varied with the spatial scale of predictor variables and across mesoclimatic regimes, with precipitation and vegetation cover being the most important predictors. The study indicates that spatial scale and mesoclimate should be considered when assessing the sensitivity of cryoturbation to climate changes. The results therefore provide possible explanations for contrasting previous predictions on the fate of cryoturbated patterned-ground ecosystems under future climate warming.

Plants have an important role in the tundra carbon (C) cycle by storing C in primary production and thus potentially counteract the C released from thawing permafrost. Tundra vegetation is limited by nitrogen (N), which is predicted to increase with rising temperatures and increased snow depth. In permafrost systems, rooting depth will determine whether plants can access N in the deep soil which, with increasing snow depth, has the potential to turn into a significant N source. Increased plant-available N is thus expected to affect both plant productivity and vegetation composition. This study aims to investigate vegetation responses to increased temperature and snow depth in a permafrost system of moist tussock tundra by combining open-top chambers with a realistic snow manipulation (snowfences). The shallow-rooted shrubs, Betula nana and Rhododendron tomentosum, and the deep-rooted sedge Eriophorum vaginatum were analyzed for responses in growth and reproduction effort. Also, vegetation responses in terms of normalized difference vegetation index (NDVI) were investigated. Winter warming increased flower density of E. vaginatum while B. nana showed an increased shoot growth in response to winter warming, but only during mid-growing season. Although winter warming increased winter soil temperature and generated a trend of increased thaw depth, there were no responses in NDVI or further species-specific responses in reproduction effort, leaf and shoot growth, leaf production or leaf dry weight to warming treatments. These results indicate that E. vaginatum respond in reproduction effort while B. nana respond in (mid-season) growth to winter warming. In total, the warming treatments generated a weak response in tundra plants which indicate that tussock tundra might not be very responsive to short-term warming. These results suggest that tundra plants have a low ability to counteract increased releases of soil C in response to short-term warming.

In tundra, phosphorus (P) is an important macronutrient for plants and microorganisms. A major fraction of P exists as organic compounds in the topsoil which can be mineralized to bioavailable inorganic P. Since mineralization is positively related to temperature, climate warming is likely to increase P bioavailability but the extend of these changes may also depend on vegetation cover and soil properties. I assessed organic and inorganic P concentrations across an elevation and vegetation gradient in northern Sweden using one dimensional (1D) solution 31P nuclear magnetic resonance spectroscopy. I hypothesized that concentration of labile soil P will decrease with increasing elevation (decreasing temperature) and that soils with meadow vegetation will contain higher concentrations of labile P than heath soils. Concentration of labile P in the form of Resin-P and polyphosphates decreased with elevation whereas less labile orthophosphate monoesters increased. Across vegetation types, polyphosphates were more abundant in heath and meadow contained higher concentrations of monoesters. The inverse response of Resin-P and monoesters to elevation may be best explained by lowered organic P mineralization in colder climate. High concentrations of polyphosphates at the lowest elevation may indicate an increased presence of fungal communities associated with mountain birch forest. Heath seemed to be more dominated by fungal communities than meadow and higher concentration of monoesters in meadow indicated a higher soil sorption capacity. In a broader view, the results may suggest that a warmer climate increases mineralization of organic P in form of orthophosphate monoesters to more labile P forms. This effect might be enhanced by an upward movement of the tree line and might be more pronounced in heath than meadow soils due to a higher fungal activity.

Arctic soils store close to 14% of the global soil carbon. Most of arctic carbon is stored below ground in the permafrost. With climate warming the decomposition of the soil carbon could represent a significant positive feedback to global greenhouse warming. Recent evidence has shown that the temperature of the Arctic is already increasing, and this change is associated mostly with anthropogenic activities. Warmer soils will contribute to permafrost degradation and accelerate organic matter decay and thus increase the flux of carbon dioxide and methane into the atmosphere. Temperature and water availability are also important drivers of ecosystem performance, but effects can be complex and in opposition. Temperature and moisture changes can affect ecosystem respiration (ER) and gross primary productivity (GPP) independently; an increase in the net ecosystem exchange can be a result of either a decrease in ER or an increase in GPP. Therefore, understanding the effects of changes in ecosystem water and temperature on the carbon flux components becomes key to predicting the responses of the Arctic to climate change. The overall goal of this work was to determine the response of arctic systems to simulated climate change scenarios with simultaneous changes in temperature and moisture. A temperature and hydrological manipulation in a naturally-drained lakebed was used to assess the short-term effect of changes in water and temperature on the carbon cycle. Also, as part of International Tundra Experiment Network (ITEX), I determined the long-term effect of warming on the carbon cycle in a natural hydrological gradient established in the mid 90’s. I found that the carbon balance is highly sensitive to short-term changes in water table and warming. However, over longer time periods, hydrological and temperature changed soil biophysical properties, nutrient cycles, and other ecosystem structural and functional components that down regulated GPP and ER, especially in wet areas.