Robin Lundell

Interactions between snow, canopy, and vegetation in a boreal coniferous forest

Background: Snow is known to have a major impact on the distribution of plants in arctic and alpine ecosystems; however, its impact on understorey vegetation in boreal forests is little reported. Aims: To study the effects of trees on the distribution of snow and examine the small-scale spatial relation between snow distribution and ground vegetation. Methods: Detailed spatial variation in snow depth and summer precipitation, canopy dimensions and locations of individual trees, and ground vegetation cover were observed in a coniferous forest, and the spatial correlations between these variables were examined. Results: Spatial patterns of snow remained unchanged throughout the winter and across two different winters. Snow depth showed significant correlations with different tree influence indices calculated based on the distance to the trunk, height, diameter or canopy extent. Dwarf shrub cover correlated positively with snow, and moss cover correlated negatively with the tree influence indices. The highest covers of Vaccinium myrtillus, V. vitis-idaea and Hylocomium splendens were observed on patches with thick snow cover. Linnaea borealis, in contrast, was absent from these patches. Pleurozium schreberi and Dicranum polysetum were most abundant on patches with moderate snow. Conclusions: Trees do not only affect ground vegetation through competition, but also have indirect effects associated with uneven snow cover. Our results suggest that, like arctic and alpine species, boreal forest understorey species show differences in their snow affinity.

Overwintering of Vaccinium vitis-idaea in two sub-Arctic microhabitats: a reciprocal transplantation experiment

Northern plants have to cope with a wide range of overwintering conditions, as the depth and physical properties of snow show high spatial variation in the Arctic. The overwintering of lingonberry (Vaccinium vitis-idaea) was studied in a reciprocal transplantation experiment between two sub-Arctic microhabitats in northern Finland. The experiment was set up in the autumn, and physiological traits related to overwintering were measured at the time of snowmelt in the following spring. The origin of the plants was not a significant source of variation for most of the traits measured, whereas major differences were observed between the two sites. Plants that overwintered at an exposed site above the treeline showed high relative winter damage, assessed by the electrolyte leakage of the leaves. No severe winter damage was observed in the plants that overwintered under a moderate snowpack at a sheltered birch forest site. These plants were able to maintain their photosynthetic capacity through the winter. A low capacity of photosystem II and a very low capacity of CO2 uptake were characteristic of the exposed site, where low temperatures and high irradiation predominate during late winter. However, photosynthetic capacity was recovered within a few days when the plants were kept under favourable conditions after the field experiment. The content of nonstructural carbohydrates was low, probably because of high respiratory losses under the snow. This short-term study suggests that lingonberry, which occupies a wide range of microhabitats in the present climate, may thrive even if the overwintering conditions change as a result of climatic warming.

Effects of snowmelt on the springtime photosynthesis of the evergreen dwarf shrub Vaccinium vitis-idaea

Background: The dwarf shrub Vaccinium vitis-idaea has been found to retain its photosynthetic capacity under snow in winter. At snowmelt, the plants are exposed to low temperatures and full light, which may lead to the inhibition of photosynthesis. Aims: To examine the changes in the photosynthesis of V. vitis-idaea at snowmelt and to determine their temporal extent. Methods: The photosynthetic capacity and the energy conversion efficiency of photosystem II (F v/F m) were determined in a natural spring snowmelt setting in field conditions and by creating an artificial snowmelt gradient. Results: F v/F m decreased rapidly after snowmelt and remained low for several weeks in the spring. The photosynthetic capacity remained at pre-snowmelt levels at first, but started decreasing about a week after snowmelt. The depression of photosynthesis lasted for several weeks. Conclusions: The inhibition or downregulation of photosynthesis in V. vitis-idaea takes place at snowmelt, not at the onset of winter as it does in many evergreen conifers. The snow protects the plants from the harshest winter conditions and permits metabolic activity. If snow cover becomes more intermittent in the future, there may be longer periods of photoinhibition, which will affect the carbon balance of plants.