This study examines the changing conditions at high elevation alpine treeline ecotones (ATEs) in the southern Rocky Mountains of Alberta, Canada. Using age reconstructions from core and disc samples removed from trees at ATEs throughout the Kananaskis Valley of Alberta, we were able to identify statistically significant correlations between warming winter temperatures and seedling establishment at elevations beyond the present treeline. Further, we were able to identify multi-phase patterns of upslope treeline encroachment, suggesting climate as a key driver of initial seedling establishment beyond modern treeline elevations succeeded by a secondary process of forest density infilling largely controlled by the availability of local viable seed sources and the alterations of local microclimates that promote heightened seedling establishment. The results suggest that ongoing anthropogenic climate warming will continue to drive the upslope encroachment of ATEs in Kananaskis Country, Alberta.

No description available.

Alpine and arctic treelines have been viewed as sensitive indicators of global climate change. While many treelines have advanced under warmer climate regimes in recent decades, the response has not been uniform. Some of this variability may be attributable to the impacts of herbivores. This study investigates the interacting effects of herbivory, climate, and understory vegetation on mountain birch establishment at treeline in the Scandes Mountains of northern Sweden. An extensive dendrochronological database was created to determine periods of establishment, which were then regressed against reindeer (Rangifer tarandus, L.) population data and historical climate data. Vegetation classifications were also created and analyzed to determine if establishment patterns vary by understory vegetation type. I have tested the hypothesis that tree establishment varies within the treeline ecotone and that high reindeer stocking levels negatively impact establishment. Weakly positive responses to herbivory were observed in patterns of tree establishment at treeline. This indicates that reindeer may modestly promote treeline advance at low densities, contradicting some previous research, but many of the results were not statistically significant. The climate variables found to have significant relationships with establishment were inconsistent across herding districts and aggregation levels. No connections between vegetation assemblages and establishment or between vegetation assemblages and reindeer use were observed.

Tree rings have long been used to make inferences about the environmental factors that influence tree growth. Great Basin bristlecone pine is a long-lived species and valuable dendroclimatic resource, but often with mixed growth signals; in many cases, not all trees at one location are limited by the same environmental variable. Past work has identified an elevational threshold below the upper treeline above which trees are limited by temperature, and below which trees tend to be moisture limited. This study identifies a similar threshold in terms of temperature instead of elevation through fine-scale topoclimatic modeling, which uses a suite of topographic and temperature-sensor data to predict temperatures across landscapes. We sampled trees near the upper limit of growth at four high-elevation locations in the Great Basin region, USA, and used cluster analysis to find dual-signal patterns in radial growth. We observed dual-signal patterns in ring widths at two of those sites, with the signals mimicking temperature and precipitation patterns. Trees in temperature-sensitive clusters grew in colder areas, while moisture-sensitive cluster trees grew in warmer areas. We found thresholds between temperature- and moisture-sensitivity ranging from 7.4 degrees C to 8 degrees C growing season mean temperature. Our findings allow for a better physiological understanding of bristlecone pine growth, and seek to improve the accuracy of climate reconstructions.

No description available.

This study examined if nutrient loading of phosphorus-rich pollen into small mountain lakes has a significant impact on lake productivity. Increased pollen input into lakes due to changes in vegetation (e.g., timberline advance) may increase lake production. Deteriorated pollen was recorded for frozen and freeze-dried sediment samples to determine if storage method effects pollen preservation. There were no strong relationships between pollen accumulation rates (PAR) and pigment concentrations for Sentinel Lake and Eiffel Lake. A lagged response of pigment concentrations to increased PAR was illustrated for Eiffel. Examination of pollen ratios and stomata suggests recent timberline advance for Eiffel, but pollen ratios were a poor indicator of timberline for Sentinel. Sediment storage methods did not play a significant role in differential preservation of pollen grains. Further investigation of the potential effect of PAR on lake productivity is required because timberline advance may alter lake productivity through increased pollen input. / Ecology

Temperature is commonly suggested to be the most important regulating factor for the position of the treeline. But also other abiotic and biotic processes may influence. To understand treeline shifts, it is necessary to improve the knowledge about the treeline forming species and their establishment, growth and survival. What are the drivers behind the shifts? The abundance of other vegetation is previously reported to facilitate growth of seedlings above treeline and also warmer temperatures are commonly observed to improve growth.   This study observed growth of mountain birch seedlings during one growing season. The relative importance of environmental factors in relation to the amount of growth and abundance was investigated. The study was conducted in the area of Abisko, Northern Sweden, using 4 sites, where transects were established just above the treeline. The sites differed in the amount of mean precipitation and aspect of the slopes. The vegetation composition around the seedlings and at the average treeline was observed, soil temperatures measured and the aspect of the slope estimated   Seedling growth was observed at all sites, with the highest amount in the sites with most precipitation, Pålnoviken and Katterjåkk. The observed soil mean and maximum temperatures were consistently highest in the southern facing slopes of Jiebrenjåkk and Pålnoviken. The vegetation was mostly dominated by dwarf shrubs, herbaceous plant cover, mosses and bare ground, and did not differ between the seedlings and the average treeline. The best model for growth was found to be the combination of the factors site, herbaceous plant cover, litter and soil mean temperature. Herbaceous plant cover was observed to improve the amount of growth in the drier sites of Pålnoviken and Jiebrenjåkk.   The finding of mountain birch seedlings growing in all vegetation types along the treeline, indicates that they have no preference of vegetation type for establishment. However, the negative influence of bare ground on growth supports the theory that abundance of vegetation facilitates growth of seedlings. The highest amount of growth was found in the sites with most precipitation, suggesting this to be an important factor for growth. In contrast to the expectations, warmer soil temperatures and the south facing slopes did not affect growth positively. This could be explained by the extremely high temperatures of the summer that may have induced drought. Finally, the improved growth by herbaceous plant cover in the drier sites may be because of their preference of moisture and nutrient rich soils, that could also support the growth of mountain birch seedlings.

Alpine treeline ecotones (ATE) are the transition zones between contiguous subalpine forest and open alpine tundra. Because of their transitional natures formed by different ecosystems in high mountain areas, there are a variety of acute interactions between different species, between vegetations and environmental factors, and between ecological pattern and process. These interactions, or feedbacks, are often nonlinear in nature and make alpine treeline ecotones sensitive to environmental change, especially climate change. Feedbacks or nonlinear interactions between pattern and process create a variety of distinctive yet sometime surprising alpine treeline patterns. These nonlinear interactions between pattern and process and their resultant various patterns are defined as spatial complexity. In this study, the research framework of complexity theory was adopted. Dynamical simulations of alpine treeline ecotone is used as basic research method, and local nonlinear interactions, or more specifically, positive feedbacks are considered the key mechanism driving alpine treeline dynamics. A cellular simulation was created with tree/no-tree states that change as a function of probabilities of tree establishment and mortality which are functions of the neighborhood and an underlying gradient; the former changes in space and time endogenously; the latter can change in space and time exogenously. Three research projects were conducted for this dissertation that explore the endogenous and exogenous aspects of alpine treeline dynamics. First, the endogenous dynamics of alpine treeline ecotones was examined, which indicates that local positive feedbacks originated from interactions between trees can create fractal spatial dynamics in space and time. Second, the impacts of geomorphologic factors that impose an exogenous spatial structure on alpine treeline dynamics, was examined, which shows that there is a geomorphic limit to the endogenous fractal alpine treeline dynamics. Third, the impacts of climate change that imposes an exogenous temporal structure on alpine treeline dynamics was examined, which suggests that the self-organization nature of alpine treeline dynamics will not be significantly affected by external climate change and the use of alpine treeline ecotones as potential indicator of climate change is called into question. Results of this study suggest further research using complexity theory is needed to improve our understanding of alpine treeline dynamics and their interactions with exogenous environmental factors.

<p> Statistical analyses between the relationship of tree-ring widths with variations in climate were used to examine the limiting climatic factors upon the growth rate of Picea mariana along the treeline and in hopes of reconstructing past climate. Tree-ring widths were used as a proxy measure of growth rates to establish the relationship of annual variances in temperature and precipitation on the growth rate of trees. Twelve tree cores were sampled from separate trees along the treeline northeast of Yellowknife, Northwest Territories. The trees, (with cores spanning 40 -160 years per tree), were compared to monthly and seasonal variations of both temperature and precipitations. A stepwise regression model was used to examine the climate-growth response. No climatic variables were found to illicit a general response pattern. The growth rate of trees along the treeline, northeast of Yellowknife do not appear to be dependent upon variations in the regional climate. Therefore, no climatic reconstructions were possible.</p> / Thesis / Bachelor of Arts (BA)

Abstract Air and soil temperatures, snow cover, serious snow load damage to coniferous trees, wind, topography and edaphic factors on the fells situated between 67°N and 68°N in Finnish Lapland are described and their influence on the location of the actual timberline and treeline is discussed. In addition the relation between annual climate conditions and pollen deposition in the timberline ecotone is analysed and the results of seedling density monitoring in the same environment are presented. The potential for the actual timberline and treeline to advance to a higher elevation is also discussed. The field studies were carried out on the fells of Aakenustunturi, Yllästunturi and Pyhätunturi. The average altitude of the actual timberline varies from 370 metres to 402 metres a.s.l. The actual timberline is hardly ever composed of a single tree species but featured alternating occurrences of Norway spruce (Picea abies), Scots pine (Pinus sylvestris) and mountain birch (Betula pubescens ssp. czerpanovii). The mean tetratherms on the southern and northern slopes (+10.3°C and +10.1°C, respectively), the mean maximum tetratherm on the southern slope (+15.1°C) and the corresponding measures for the treeline (460 m a.s.l), the minimum tetratherm (+6.3°C), mean July temperature (+12.6°C), biotemperature (+3.3°C) and minimum effective temperature sum (455 d.d.), coincide closest with the results of earlier studies. The maximum altitudes of the actual timberline are dictated by many climatic factors on southern and western slopes with a gentle inclination, and the forest cover gradually becomes thinner, in which case the actual timberline does not form any easily distinguishable line. The lowest altitudes of the actual timberline are the results of an extremely high proportion of block fields, slope steepness and snow patches on the northern and eastern slopes. On the precipitous and rocky slopes trees have difficulties in taking root and in obtaining nutrients and water, while as a consequence of snow patches the growing season may be too short for tree growth at all, and if trees exist there they are suffering from low soil temperature and parasitic snow fungi. Serious snow load damage to trees evidently hampers any advance in the actual timberline, as do avalanches and mires. The location of the treeline is the result of a combination of a great number of unfavourable conditions for tree regeneration, seedling establishment and tree growth, such as inadequate snow protection, extreme soil temperatures, almost total destruction of trees by the snow load, wind pressure, an often inadequate effective temperature sum and length of the growing season, night frost in early summer, and poor, dry soil suffering from excessive evaporation. Actual timberline responses to predicted climate warming will differ greatly from site to site in relation to the local topography, edaphic features and associated ecological limitations. Any advance in the treeline to a higher elevation is likely to be slower and at least less certain than that in the actual timberline. In addition, advances in the actual timberline and treeline may even be prevented by phenomena occurring along with climate change. A potential key factor in this is serious snow load damage to the trees.