Effect of microhabitat variability on restoration success of swamp willow Salix myrtilloides L. population

Maintaining local populations of rare boreal plant species living at the edge of their geographic ranges is particularly important as the climate continues to warm. In many European countries, their numbers have declined dramatically over the past few decades. In Poland, relict willow species are particularly at risk of extinction, including the swamp willow Salix myrtilloides. During a field experiment to reintroduce S. myrtilloides, 240 seedlings were planted within the quaking bog surrounding Lake Wiejki on the outskirts of the Knyszyńska Forest (northeastern Poland). Five years after the seedlings were introduced into the wild, the size of the population had increased to 272 individuals, probably due to increased vegetative proliferation caused by the browsing by moose. The flowering efficiency of individuals was initially relatively high (40.82%) but decreased almost twofold (24.27%) between 2019 and 2024. Monitoring over five years indicated that the height of the herb layer and the cover of competing species could affect the height of the aboveground shoots of S. myrtilloides. We found a significant statistical relationship between soil water pH and population restoration success, with lower pH promoted vegetative regrowth while limiting flowering.