Thesis (licentiate)--Swedish University of Agricultural Sciences, 2006. / Thesis documentation sheet inserted. Appendix reprints three journal articles co-authored with others. Includes bibliographical references. Also issued electronically via World Wide Web in PDF format; online version lacks appendix.

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.

The main objectives of this investigation were to determine the impact of climate on mountain birch (Betula pubecens ssp. czerepanovii (Orlova)) growth and to develop a regional chronology of autumnal moth outbreaks. To accomplish the objective, cores of mountain birch were taken from 21 sites in Norrbotten, Sweden. Tree-ring chronologies were developed for each site. Climatic influences were determined by correlating ring widths to climatic variables (average monthly temperature, average monthly precipitation and NAO). Outbreaks were recovered from the ring width indices using the non-host method with Scots pine (Pinus sylvestris (L.)) as the non-host. This method removes the climatic influence on growth to enhance other factors. Patterns of synchrony and regional outbreaks were detected using regression and cluster analysis techniques. The primary climatic influences on the tree ring growth of mountain birch are June and July temperatures; precipitation during October is of secondary importance. Climate explained 46% of yearly tree ring width variation. Outbreaks of the autumnal moth occur at varying time intervals depending on the scale of study. Intervals between outbreaks on the tree level are twice as long as at the plot level. On the regional scale plots within the same valley had more similar outbreak intervals and magnitudes of outbreaks. Elevation is a driver in determining the length of outbreaks and length between outbreaks. The percent monocormicity of a plot is also a determining factor of the length between outbreaks. This study is the first regional scale study on climate and outbreaks of the autumnal moth on mountain birch. The results complement research being conducted on autumnal moth larval densities and will help in modeling and assessing the effects of outbreaks with increasing climatic change.