Arctic tundra vegetation communities are rapidly responding to climate warming with increases in aboveground biomass, particularly in deciduous shrubs. This increased shrub density has the potential to dramatically alter the functioning of tundra ecosystems through its effects on permafrost degradation and nutrient cycling, and to cause positive feedbacks to global climate change through its impacts on carbon balance and albedo. Experimental evidence indicates that tundra plant growth is most strongly limited by soil nutrient availability, which is projected to increase with warming. Therefore research to date into the mechanisms driving tundra 'shrub expansion' has taken a 'bottom-up' perspective, overlooking the potential role of herbivory in mediating plant-soil interactions. In this thesis, I integrate the impacts of climate warming and caribou browsing on tundra vegetation community structure, and specifically investigate if increases in soil fertility with warming might lead to changes in vegetation biomass and chemistry that could fundamentally alter herbivore-nutrient cycling feedbacks, shifting the role of caribou browsing from restricting shrub growth to facilitating it. Using experimental greenhouses, nutrient addition plots, and caribou exclosures at Daring Lake Research Station in the central Canadian low Arctic, I showed that warming increased soil nutrient availability and plant biomass, and that caribou browsing restricted tundra shrub growth under present conditions. Plant and soil nutrient pool responses to warming demonstrated that increased growing season temperatures enhanced tundra plant growth both by increasing soil nutrient availability and by inferred increases in the rate of photosynthesis, however that the former process was comparatively more limiting. Species- and plant part-specific changes in biomass and chemistry with warming and fertilization clearly indicated the rate and magnitude of change in soil fertility substantially alters plant community structure. Nonetheless, since plant nutrient concentrations decreased with warming and plant responses to browsing were independent of soil fertility, I did not find evidence for a shift from caribou decelerating to accelerating nutrient cycling with warming. Altogether this research indicates effective conservation and management of Rangifer populations is critical to understanding how climate change will affect tundra vegetation trajectories and ultimately tundra ecosystem carbon balances. / Thesis (Ph.D, Biology) -- Queen's University, 2013-06-07 15:13:21.698

Long-term warming experiments in arctic tundra have resulted in reduced moss cover and increased vascular plant cover. As mosses have a major impact on microclimatic conditions, changes in community composition can potentially alter direct and indirect drivers of productivity and decomposition, which are low in arctic ecosystems. This can potentially change the carbon balance. Therefore, it is of importance to investigate how the effects of warming on ecosystem processes depend on the identity of the moss species dominating the vegetation. Furthermore, because subarctic climate is expected to get warmer and wetter, it is essential to examine how warming effects depend on variation in precipitation as well. Thus, Gross Primary Production (GPP) and Ecosystem Respiration (Reco) fluxes, moss growth, and decomposition rate, were measured in plot pairs along a natural precipitation gradient in subarctic tundra Sweden. Each pair was dominated by one of three common moss species (Hylocomium splendens, Pleurozium schreberi and Sphagnum spp.), and were subjected to moss removal and Open Top Chamber (OTC) warming treatment combinations. Moss growth measurements were taken as a measure of productivity and Tea Bag Index was used as a measure of decomposition rate. Warming effects on GPP depended on both species and precipitation; Hylocomium and Sphagnum had highest productivity in warmed high-precipitation plots, whereas the effect was negative in all cases for Pleurozium. No significant interactions were found for the other response variables, but there was a positive effect of warming on Reco across all treatments, as well as a negative effect on decomposition. Warming induced increases in GPP and respiration, and decrease in decomposition rate, could imply that the increase in CO2 emissions from arctic ecosystems will be smaller than predicted, as the sequestration of CO2 in that case would exceed what is being emitted. Still, it is a complex matter and therefore crucial to further investigate the role of moss species in relation to warming effects on ecosystem processes.

L'augmentation de la décomposition de la matière organique des cryosols arctiques sous l'effet du réchauffement et de la dégradation du pergélisol contribuerait à une rétroaction positive sur les changements climatiques. Nous étudions le fonctionnement biogéochimique de deux Cryosols: un cryosol histique (H) et un cryosol turbique (T), en conditions naturelles et réchauffés. Les profils ont été instrumentés à Salluit (Nunavik, Canada) et les mesures ont été faites pendant les étés 2010 et 2011. Le réchauffement augmente la respiration de l'écosystème (ER) de manière plus intense pour H que pour T, bien que ER pour H soit plus faible. La sensibilité thermique de ER (Q10) est supérieure pour T que pour H et diminue avec le réchauffement. L'étude montre que les cycles journaliers de ER en fonction de la temperature forment des hystérésis. La variance de ER est mieux expliquée en utilisant la température minimale de la journée et la profondeur du front de dégel pour H. Pour T, l'ajout de la vitesse du vent et la radiation solaire améliore l'explication de la variance de ER. Nous montrons trois dynamiques spécifiques aux écosystèmes nordiques: 1) ER dépendant des propriétés du sol et de la solution du sol ; 2) rôle de variables thermo-indépendantes sur ER et 3) variations journalières du Q10 et interannuelles de la respiration basale. La décomposition de la matière organique est la principale source de CO2 pour H alors que les processus végétaux contrôlent ER pour T. Nos résultats contribuent à la compréhension et à l'extrapolation des mesures ponctuelles dans les écosystèmes de toundra, améliorant ainsi la modélisation du cycle du carbone dans les cryosols. / Increased organic mater decomposition rate in Arctic Cryosols due to warming and to permafrost thawing can lead to the release of greenhouse gases, thus potentially creating a positive feedback on climate change. We studied the biogeochemical functioning of two different permafrost-affected soils (i.e. Cryosols): a Histic Cryosol (H) and a Turbic Cryosol (T), both in natural conditions and under an experimental warming. Profiles were instrumented in Salluit (Nunavik, Canada) and monitored during summers 2010 and 2011. The induced warming increased CO2 fluxes in both soils; this impact was however more striking at H even if ER was lower than at T. Temperature sensitivity of ER (Q10) was higher at T than at H and decreased both with warming. We highlighted that diurnal ER cycles as a function of temperature showed hysteretic loops. Linear models performed to explain ER variance were improved adding daily minimum temperature and thaw front depth at H. In contrast at T, adding wind speed and solar radiation in models improved the ER variance explanation. We showed three specific CO2 flux dynamics related to northern ecosystems: 1) the large difference of ER depending on soil properties and soil solution composition; 2) environmental variables strongly alter CO2 fluxes and 3) the diurnal Q10 variations and the inter annual variability of basal respiration. Our results support the assumption that organic matter decomposition might be the major source of CO2 at H while plant-derived processes dominated ER at T. Our results contribute to understand and extrapolate the numerous punctual measurements of CO2 fluxes from tundra ecosystems improving carbon cycle modeling in Cryosols.