Climate changes have been observed in the Northern Hemisphere during the last century, causing a longer growing season and upslope expansion of forest, tree, and shrub limits. Here, a long-term historical perspective was used as a tool for investigating how climate change has impacted woody plants at the forest, tree, and shrub limits across the study region. For this, historical data about plant communities were used, including the position of the forest, tree, and shrub limits, gathered by the Reindeer Commission in Troms County (1914-1915). The historical data were compared to contemporary data from a re-visit study in 2022. In addition, variations in microclimatic factors were investigated by looking at the growing season length, distance to coast, slope, and aspect. Importantly, the growing season length was found to have increased across the study region with the largest increase along the coast. The increase in the growing season length led to an upslope shift in the vegetation limits, although not significantly so for the forest limit. The field layer had shifted from meadow to heath around the historical shrub limit, and non-significantly for the forest and tree limit zones, indicating that the alpine tundra is the most sensitive to the observed warming. Distance to coast and slope did not impact the shift in vegetation limits, while aspect impacted the forest limit shift. The results from this thesis indicate that future studies should expand their research beyond climate variables and explore factors such as herbivory and land use change

Predicting the net effect of climate change on communities requires understanding how increasing temperatures alter interactions between predators, herbivores, and plants. Over the last several decades, warming experiments have provided important information about how species and their interactions will respond to increasing temperatures. These studies typically examine climate warming by experimentally increasing temperature at a constant level (24 hours) or asynchronously during the daytime, relative to unwarmed control treatments. However, advances in climate models now project that increases in mean global temperatures have been disproportionately driven by increasing nighttime (minimum) temperatures rather than daytime (maximum) temperatures. The timing of warming could have important ecological implications. For example, while night warming could benefit an organism by increasing temperatures towards a more thermally-optimal environment, day warming could raise temperatures beyond a thermal optimum and induce heat-stress. Consequently, mismatching the timing of warming in experiments relative to actual temperature changes could generate misleading predictions about the effects of climate warming. My dissertation has evaluated climate-warming experiments by characterizing past methods, demonstrating present methods, and providing a foundation for future studies. I conducted a meta-analysis on past terrestrial predator-prey climate warming studies that revealed experimental temperatures rarely match model projections, and the magnitude of this mismatch correlated with increased changes in measured effects. Two experiments, one focused on predator functional traits and the other trophic cascades, showed that different types of warming treatments result in different effects of climate change. The context dependency of warming effects necessitates careful consideration of experimental treatments if studies are to accurately predict the effects of climate warming. Region specific climate data are now readily available. Moving forward, ecologists can use these models to inform their warming treatments and perform experiments with the highest level of realism.

No description available.

<p>Elevated temperatures accompanying climate warming are expected to have adverse effects on sensitive lichen species. This premise was examined by measuring the sensitivity of different lichen species to elevated temperatures in the laboratory and in the field. Laboratory studies involved the exposure of nine hydrated lichen species (Xanthoparmelia austro-africana, X. hyporhytida, Xanthoparmelia sp., Xanthomaculina hottentotta, Teloschistes capensis, Ramalina sp., Flavopuntelia caperata, Lasallia papulosa, Parmotrema austrosinensis) collected from sites of different aridity and mean annual temperature for 2 hourly intervals to temperatures ranging from 24&ordm / C to 48&ordm / C in a forced daft oven and measuring their respiration rates and maximum quantum yield of PSII. Field studies involved simultaneous hourly measurements of ground surface air temperatures and Lichen effective quantum yield of PSII of hydrated lichen species populations under ambient and artificially modified environmental conditions.</p>

The Mackenzie Delta region of the Northwest Territories, Canada, has experienced rapid climate warming in the past century resulting in rapidly thawing permafrost in this region. This thesis examines spatial and temporal changes to sediment organic carbon and mercury flux in lakes from thermokarst regions by comparing sediment cores from lakes with and without retrogressive thaw slumps on their shorelines. We show that sediments from lakes with permafrost thaw slump development on their shorelines (slump lakes) had higher sedimentation rates as well as lower total Hg, methyl mercury (MeHg), and labile OC fractions when compared to lakes where thaw slumps were absent. Total Hg and MeHg concentrations in sediments were correlated with total organic carbon (TOC), S2 (labile algal-derived OC), and inferred chlorophyll a content, indicating an association between autochthonous organic carbon and Hg in these sediments. Correlations between mercury and S2 in these study lakes generally support the hypothesis that algal-derived materials correlate with Hg concentration in sediments. We observed higher S2 concentrations in reference lakes than in slump lakes, likely due to uninterrupted algal production, lower dilution by flux of inorganic matter, and possibly better anoxic preservation in reference lakes compared to slump lakes. It is evident that thaw slump development in this thermokarst region increases inorganic sedimentation in lakes, while decreasing concentrations of organic carbon and associated Hg and MeHg in sediments.

Global climate models predict that the current trend of warming in the Arctic will continue over the next century. The productivity of arctic plants is often limited by short growing seasons with relatively low temperatures such that a warmer climate could have large impacts on plants and plant communities. This study characterised alpine plant responses to changes in temperature at an alpine tundra site near Whitehorse, Yukon, Canada. I examined relationships between plant productivity and natural temperature variations and assessed responses of plants exposed to an experimental warming treatment. Non-destructive measurements of reproductive and growth characteristics of four target species (Dryas octopetala, Lupinus arcticus, Polygonum viviparum, and Salix arctica) were taken annually from 1999 to 2008. There was no significant effect of the warming treatment (OTCs) on average daily mean temperatures as midday warming of up to 1.4 °C was largely offset by night time cooling in the OTCs. Vegetative measurements of target species showed no significant responses to OTC treatments. However, peduncles of D. octopetala and sections of P. viviparum inflorescences that produced bulbils were an average of 34.6 % and 64.7 % longer in OTCs than in controls, respectively. These treatment responses were likely due to plants responding to a factor other than temperature that was modified by the chamber. One vegetative and five reproductive characteristics were significantly related to annual variation in temperature. The summer of 2004 was exceptionally hot, and some species that did not respond to smaller fluctuations in temperature showed large changes in growth or reproduction in this year, perhaps indicating a non-linear response to temperature. Among the larger responses to the warm summer of 2004 was a shift in P. viviparum allocation from predominantly asexual to sexual means of reproduction. Measurements of plant community composition assessed at five-year intervals showed no differences in community composition between experimental plots and controls, and changes in composition over the study period were not uni-directional. In general, both individual plants and community composition were highly resilient to observed variation in summer temperatures. Other factors, such as nutrient availability, may be more important in determining plant responses to environmental change at this site than the direct effects of summer temperature variation.

The Mackenzie Delta region of the Northwest Territories, Canada, has experienced rapid climate warming in the past century resulting in rapidly thawing permafrost in this region. This thesis examines spatial and temporal changes to sediment organic carbon and mercury flux in lakes from thermokarst regions by comparing sediment cores from lakes with and without retrogressive thaw slumps on their shorelines. We show that sediments from lakes with permafrost thaw slump development on their shorelines (slump lakes) had higher sedimentation rates as well as lower total Hg, methyl mercury (MeHg), and labile OC fractions when compared to lakes where thaw slumps were absent. Total Hg and MeHg concentrations in sediments were correlated with total organic carbon (TOC), S2 (labile algal-derived OC), and inferred chlorophyll a content, indicating an association between autochthonous organic carbon and Hg in these sediments. Correlations between mercury and S2 in these study lakes generally support the hypothesis that algal-derived materials correlate with Hg concentration in sediments. We observed higher S2 concentrations in reference lakes than in slump lakes, likely due to uninterrupted algal production, lower dilution by flux of inorganic matter, and possibly better anoxic preservation in reference lakes compared to slump lakes. It is evident that thaw slump development in this thermokarst region increases inorganic sedimentation in lakes, while decreasing concentrations of organic carbon and associated Hg and MeHg in sediments.

Global climate models predict that the current trend of warming in the Arctic will continue over the next century. The productivity of arctic plants is often limited by short growing seasons with relatively low temperatures such that a warmer climate could have large impacts on plants and plant communities. This study characterised alpine plant responses to changes in temperature at an alpine tundra site near Whitehorse, Yukon, Canada. I examined relationships between plant productivity and natural temperature variations and assessed responses of plants exposed to an experimental warming treatment. Non-destructive measurements of reproductive and growth characteristics of four target species (Dryas octopetala, Lupinus arcticus, Polygonum viviparum, and Salix arctica) were taken annually from 1999 to 2008. There was no significant effect of the warming treatment (OTCs) on average daily mean temperatures as midday warming of up to 1.4 °C was largely offset by night time cooling in the OTCs. Vegetative measurements of target species showed no significant responses to OTC treatments. However, peduncles of D. octopetala and sections of P. viviparum inflorescences that produced bulbils were an average of 34.6 % and 64.7 % longer in OTCs than in controls, respectively. These treatment responses were likely due to plants responding to a factor other than temperature that was modified by the chamber. One vegetative and five reproductive characteristics were significantly related to annual variation in temperature. The summer of 2004 was exceptionally hot, and some species that did not respond to smaller fluctuations in temperature showed large changes in growth or reproduction in this year, perhaps indicating a non-linear response to temperature. Among the larger responses to the warm summer of 2004 was a shift in P. viviparum allocation from predominantly asexual to sexual means of reproduction. Measurements of plant community composition assessed at five-year intervals showed no differences in community composition between experimental plots and controls, and changes in composition over the study period were not uni-directional. In general, both individual plants and community composition were highly resilient to observed variation in summer temperatures. Other factors, such as nutrient availability, may be more important in determining plant responses to environmental change at this site than the direct effects of summer temperature variation.

<p>Elevated temperatures accompanying climate warming are expected to have adverse effects on sensitive lichen species. This premise was examined by measuring the sensitivity of different lichen species to elevated temperatures in the laboratory and in the field. Laboratory studies involved the exposure of nine hydrated lichen species (Xanthoparmelia austro-africana, X. hyporhytida, Xanthoparmelia sp., Xanthomaculina hottentotta, Teloschistes capensis, Ramalina sp., Flavopuntelia caperata, Lasallia papulosa, Parmotrema austrosinensis) collected from sites of different aridity and mean annual temperature for 2 hourly intervals to temperatures ranging from 24&ordm / C to 48&ordm / C in a forced daft oven and measuring their respiration rates and maximum quantum yield of PSII. Field studies involved simultaneous hourly measurements of ground surface air temperatures and Lichen effective quantum yield of PSII of hydrated lichen species populations under ambient and artificially modified environmental conditions.</p>

The Mackenzie Delta region of the Northwest Territories, Canada, has experienced rapid climate warming in the past century resulting in rapidly thawing permafrost in this region. This thesis examines spatial and temporal changes to sediment organic carbon and mercury flux in lakes from thermokarst regions by comparing sediment cores from lakes with and without retrogressive thaw slumps on their shorelines. We show that sediments from lakes with permafrost thaw slump development on their shorelines (slump lakes) had higher sedimentation rates as well as lower total Hg, methyl mercury (MeHg), and labile OC fractions when compared to lakes where thaw slumps were absent. Total Hg and MeHg concentrations in sediments were correlated with total organic carbon (TOC), S2 (labile algal-derived OC), and inferred chlorophyll a content, indicating an association between autochthonous organic carbon and Hg in these sediments. Correlations between mercury and S2 in these study lakes generally support the hypothesis that algal-derived materials correlate with Hg concentration in sediments. We observed higher S2 concentrations in reference lakes than in slump lakes, likely due to uninterrupted algal production, lower dilution by flux of inorganic matter, and possibly better anoxic preservation in reference lakes compared to slump lakes. It is evident that thaw slump development in this thermokarst region increases inorganic sedimentation in lakes, while decreasing concentrations of organic carbon and associated Hg and MeHg in sediments.