In the year 2016, passive biomonitoring studies were conducted in the forest areas of southern and north-eastern Poland: the Karkonosze Mountains (Kark), the Beskidy Mountains (Beskid), Borecka Forest (P. Bor), Knyszynska Forest (P. Kny), and Białowieza Forest (P. Bia). This study used bark from the tree, Betula pendula Roth. Samples were collected in spring (Sp), summer (Su), and autumn (Au). Concentrations of Mn, Fe, Ni, Cu, Zn, Cd, and Pb were determined for the samples using the atomic absorption spectrometry method with flame excitation (F-AAS). Based on the obtained results, the studied areas were ranked according to level of heavy-metal deposition: forests of southern Poland > forests of north-eastern Poland. Some seasonal changes in the concentrations of metals accumulated in bark were also indicated, which is directly related to their changing concentrations in the air during the calendar year, for instance, the winter heating season produces higher concentrations of heavy metals in the bark samples taken in spring. When deciding to do biomonitoring studies using bark, but also other biological materials, it is necessary to take into account the period in which the conducted research is done and the time when the samples are taken for analysis, because this will have a significant impact on the obtained results.