Spatial Variation in Near-Ground Radiation and Low Temperature - Interactions with Forest Vegetation

Sammanfattning: Low temperature has a large impact on the survival and distribution of plants. Interactive effects with high irradiance lead to cold-induced photoinhibition, which may impact on the establishment and growth of tree seedlings. In this thesis, novel approaches are applied for relating the spatial variability in low temperature and irradiance to photosynthetic performance and growth of tree seedlings, and for modelling the micro- and local-scale spatial variations in low temperature for heterogeneous terrain. The methodologies include the development and use of a digital image analysis system for hemispherical photographs, the use of Geographic Information Systems (GIS) and statistical methods, field data acquisition of meteorological elements, plant structure, growth and photosynthetic performance. Temperature and amounts of intercepted direct radiant energy for seedlings on clear days (IDRE) were related to chlorophyll a fluorescence and the dry weight of seedlings. The combination of increased IDRE with reduced minimum temperatures resulted in persistent and strong photoinhibition as the season progressed, with likely implications for the establishment of tree seedlings at forest edges, and within shelterwood. For models of spatial distribution of low air temperature, the sky view factor was used to parameterise the radiative cooling, whilst drainage, ponding and stagnation of cold air, and thermal properties of the ground were all considered. The models hint at which scales and processes govern the development of spatial variations in low temperature for the construction of corresponding mechanistic models. The methodology is well suited for detecting areas that will be frost prone after clearing of forest and for comparing the magnitudes of impacts on low air temperature of forest management practices, such as shelterwood and soil preparation. The results can be used to formulate ground rules for use in practical forestry.