Western Pomerania is located in the northwestern part of Poland. It is bordered by the Baltic Sea to the north, and by Germany to the west, across the Odra River. It is a young glacial area that was occupied by ice sheets several times during the last 100 thousand years. The last glaciation took place about 22 thousand years ago, and the ice sheet retreated about 14 thousand years ago. Terrain forms that resulted from ice sheet melting are well-preserved in the surface relief and form a typical post-glacial landscape with numerous lakes, frontal moraine belts, outwash plains, plateaux, kame terraces and eskers. The bedrock is dominated by tills, fluvioglacial sediments and ice-dammed lake clays. Some sediments (mostly in ice marginal valleys) have undergone aeolian transport and now form extensive dune fields The studied oak trees are growing in fresh mixed forests and fresh forests, on cambisols and chromisols, often in multi-species forests. The oldest trees (>400 years old) are growing on the Wolin Island (northern part of the study area), within the Wolin National Park. This area is a frontal moraine, characterized by considerable differences in elevation, and bordered by a high cliff on the seaward side. The oldest oak trees are few specimens growing as a remainder within a considerably younger stand. From the remainder of the study area, we selected old stands characterized by good health and growth dynamics. These oaks are growing in both continuous stands, and in isolated mid-field enclaves.
Samples were collected from individuals of native oak species (Quercus robur and Q. petraea) using Pressler borers, at 1.3 m above ground, at 24 study plots located in Western Pomerania (a total of 358 trees), between 1997 and 2021. In the laboratory, samples were mounted on boards, dried and sliced using a surgical blade in order to obtain a clear view of the tree-rings. Tree-ring widths (TRW) were measured with an accuracy of 0.01 mm under a stereomicroscope, using the TREE RINGS and LDB_Measure (ver. 1.0) programs. All samples were subsequently dated using standard cross-dating methods. Student’s t-test and Gl coherence coefficient (Gleichläufigkeitswert) were computed for the dendrochronological sequences, and visual similarity of the sequence variability was assessed. Based on this, we selected 72 sequences (from various sites in Western Pomerania), characterized by the highest statistical indices (t>4.5 and Gl>65%), and a high visual similarity. These were used to assemble a regional chronology (QUSP_WP, Quercus spp._Western Pomerania), whose quality was tested using Cofecha, part of the DPL software package.
The regional chronology QUSP_WP is based on 72 individual growth curves derived from oak trees growing at 24 sites in Western Pomerania. The assembled chronology spans 458 years, representing the period from 1564 to 2021. The mean tree-ring width equals 1.02 mm/year. The QUSP_WP chronology displays long-term fluctuations. Until about 1750, there is a clear decreasing trend, which is replaced by an increasing trend that lasts until about 1920. After that, another gradual, long-term decrease in tree-ring width is observed. Notably, a clear increase in tree-ring width is also observed in the last 20 years. The observed fluctuations are likely due to the diverse ages of the oak trees, whose tree-ring series were included in the assembled chronology, and from the impact of conditions within the habitats, in which the trees grow.
Tree-ring width of regional oak chronology (QUSP_WP, spans 458 years); 1st column - year, 2nd column - tree-ring width in mm.
(2024-08-20)
