Timing of snow melt and phenology
Cold environments such as northern Finland are characterised by a long winter with permanent snow cover. The first snow already comes in October and the first patches of ground only become snow-free in May. This leaves only a very short growing season for plants which need warm temperatures for growth and development. As the snow melts in spring, plants can start growing but the timing of their development is important: If they start growing early, they have the advantage of a longer growing season. However, temperatures in the spring can still fall below zero which can damage newly growing plant shoots. If plants start growing late in the spring, they experience more stable and warmer temperatures, but they risk having too little time available to grow, reproduce and accumulate carbohydrates for the winter. So which is the best strategy?
Climate change is predicted to alter the amount and duration of snow cover in winter. For Finnish Lapland, the Finnish Meteorological Institute predicts that the duration of snow cover will be reduced by 20 % and the amount of snowfall by 60 % between the years 2071 to 2100. This will pose new challenges to the plants which have adapted to the specific snow regime in their habitats.
Researchers have used experiments in which they artificially changed the snow cover in a natural environment by adding or removing snow to study how plants respond to these simulated changes. However, variations in snow cover already exist in the natural environment. Such gradients ranging from early-melting areas where the snow melts in May, to late-melting areas where the snow only melts in July, can be found for example on mount Saana in Kilpisjärvi. The gradient is caused by the unevenness of the ground on the mountain slope, which leads to accumulation of snow cover in ditches and little snow cover on wind-swept ridges. As part of her doctoral studies, Friederike Gehrmann has used this natural gradient for assessing when developmental stages of plants, such as leaf unfolding and flower opening, occur in relation to early-, mid- or late-melting snow on mount Saana.
Her results show that the timing of snowmelt affects the timing of plant developmental stages and this effect varies depending on the plant species and the developmental stage. For example, lingonberry (Vaccinium vitis-idaea), usually opens its leaves and flowers faster in places where the snow melts late compared to early in the spring. Lingonberry therefore compensates for the late-melting snow by developing faster so that it doesn’t lose any time of the short summer. Dwarf birch (Betula nana) has a different strategy: the opening of the leaf buds and the unfolding of leaves always requires the same amount of days once the snow has melted, regardless of whether the snow melts early or late. This means that the development in dwarf birch is delayed and the growing season is shortened in places with late compared to early snowmelt. These different strategies for growth after snowmelt are due to the fact that plants have different requirements for growth. For example, some plants might be very sensitive to frost and cannot survive if they grow too early, whilst others are more frost resistant and need fewer days with high temperatures to grow.
Gehrmann F., Hänninen H., Liu C. & Saarinen, T. 2018. Phenological responses to small-scale spatial variation in snowmelt timing reveal compensatory and conservative strategies in subarctic-alpine plants. –Plant Ecology & Diversity, 10: 453–468. DOI: 10.1080/17550874.2018.1428693.