Anthropogenic transformations of habitat conditions in moderate climate peatlands frequently cause a decrease in the number of sites of occurrence and the size of the population of many valuable plant species, including the boreal relic Betula humilis. The objective of this paper was an attempt to relate the specifics of the occurrence of the glacial relic Betula humilis in the peatlands of Central-Eastern Poland, which developed under moderate climate conditions, to the conditions of the natural environment identified through research into the physical and chemical parameters of the groundwater, as well as botanical research into its habitat. The study results confirm that shrub birch has a broad range of ecological tolerance to the majority of the studied factors and can therefore be an indicator of habitat transformation. Important environmental factors affecting its abundance are water relations and the contribution of calcium hydroxide and phosphorus fractions. A condition favouring the proper functioning of individuals of the studied species is TP values lower than other obtained values, in the following range: 0.08–0.32; P-PO4: 0.1; TN: 2.2–21.2; N-NH4: 0.1–0.46; DOC: 24.6–55.9 (mg·dm−3), as well as higher than average pH values, in the following range: 5.34–5.95; Ca: 5.67–28.1; Mg: 0.56–2.41 (mg·dm−3) and EC: 72.1–142.3 (µS·cm−1).