The ability of plants to tolerate freezing limits their geographical distribution. Therefore, winter warming may shift a species' occurrence northwards and/or to higher altitudes. In Europe, the hemiparasitic vascular plant Viscum album (mistletoe) has two common and widespread subspecies: V. a. ssp. album and V. a. ssp. austriacum. The former has a more northern geographic distribution than the latter. Therefore we hypothesised that seeds of V. a. ssp. album are more tolerant to freezing than those of V. a. ssp. austriacum. From these two mistletoe subspecies V. a. ssp. austriacum is, in some managed forest areas, considered a novel threat to tree growth and forest health.
Berries of V. a. ssp. album were collected from Sweden, Poland and Switzerland and berries of V. a. ssp. austriacum were collected from Poland, Switzerland and Spain. After storage at -3 °C, seeds were extracted from the pulp of berries and exposed to eight different temperatures between -8 °C and -30 °C, with the storage temperature serving as the control. After freezing treatments, germination of seeds was monitored. In addition, differential thermal analysis was used to measure freeze tolerance of seeds.
The seeds of V. a. ssp. album tolerated lower temperatures than seeds of V. a. ssp. austriacum. The temperature at which 50% of seeds lost their ability to germinate (LT₅₀) was -15 °C in V. a. ssp. austriacum and between -15 °C and -19 °C in V. a. ssp. album. The results of differential thermal analysis to determine the freezing point of seeds supported these findings.
The freezing tolerance of mistletoe seeds was relatively well coupled with the winter climate at the edge of their current geographic distribution. Based on our results, the warming of winters may eliminate the abiotic barrier that has thus far limited mistletoes' expansion, opening a window of opportunity for these parasites to increase their abundance and shift their distribution range towards higher latitudes and altitudes.
Although mistletoes play important ecological roles in forest ecosystems, their recent increase has raised concern among forest managers, because they may cause a substantial reduction in tree growth in single species dominated stands. Increasing tree species diversity might be an effective method for limiting future mass infestations in homogeneous managed forests.