Topography affects the natural forest recovery on inland dunes in Central Europe

Topography, by spatially altering site conditions, affects ecological processes, e.g. natural forest recovery. Until now the early dynamics of naturally encroaching trees on inland dunes with regard to topography has not been sufficiently studied in detail, and became the aim of our study. To achieve this aim, in 2011 we established a 14.87 ha long-term research site located in the Torun artillery ground (N Poland), in one of the biggest inland dune fields of Central Europe. We surveyed twice (2011 and 2021) trees occurring in the site, and investigated their characteristics (GPS coordinates, height, diameter, age, annual stem increment, slenderness) regarding 5 topographical variants: north- (N), east(E), west- (W), and south-facing (S) slopes, as well as intra-dune depressions (D). We found that within the investigated 10 years timespan the number of trees increased almost fourfold (from 560 to 2016). The natural encroaching of trees was the most advanced in north-facing slopes (88 +/- 17 and 352 +/- 89 trees ha(-1 )in 2011 and 2021, respectively). In turn, in sunny exposures and in intra-dune depressions the process was the least advanced (in 2021: 64 +/- 21 and 25 +/- 15 trees ha(-1), respectively), which could be primarily linked to unfavourable moisture conditions occurring on south-facing slopes, and strong competitive effect of tall grasses on young trees regarding the depressions. By showing the rate and topographically controlled spatial pattern of trees encroachment, our study can be useful e.g. for practising managers when reforestation of inland dunes by natural means is planned.