| Summary: | Climate change will be most pronounced at high latitudes where it will affect autumn, winter and spring disproportionally more than the growing season. Increasing or decreasing precipitation as snow, rising temperature and more frequent freeze-thaw cycles raise uncertainties about how the timing of snowmelt and the length of the growing season will change for northern plants. The timing of plant developmental stages (phenophases) in relation to snowmelt timing is important for avoiding harsh weather conditions in spring and ensuring a long growing season. In this doctoral dissertation, I investigate the effect of snowmelt timing and temperature conditions on plant phenology and plant stress. The first objective was to determine the natural variation in snowmelt timing on a small spatial scale in subarctic-alpine Finland. Such variation in the microclimate is created by the landscape topography and show the range of conditions plant populations are currently adapted to. Snowmelt timing varied by up to seven weeks within a few metres distance, so that early-melting plots were exposed to more frequent and more severe spring freezing events. This implies that plant populations are already exposed to the kind of climatic conditions which would be predicted from climate change. Secondly, I studied how the physiology of Vaccinium vitis-idaea and the phenology of seven subarctic-alpine plant species are affected by this natural variation in microclimate. The higher numbers of freezing events in early-melting plots were related to a higher reduction in the quantum yield of photosystem II (ΦPSII) in V. vitis-idaea, but not to increases in frost damage. This species therefore does not suffer substantial damage from the natural climatic variation in this habitat, implying that in this location it is likely highly resistant to the predicted changes in climate. Differences in snowmelt timing also led to three distinct patterns of phenological timing in subarctic-alpine plant species along the snowmelt gradient. These patterns ...
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