Mortality and regeneration of elms in primeval stands of Bialowieza National Park, Poland

The outbreaks of Dutch elm disease (DED) in the 20th century have led to a drastic decrease in the population of elms (Ulmus spp.), across Europe and North America. This study aimed to assess the overall health status of elms in the Bialowieza National Park (BNP), Poland. Elms, predominantly wych elm Ulmus glabra, constitute only a small proportion of all trees present in the BNP. The diameter at breast height (DBH) of all living and standing dead elms within 22 sample plots in 1997, 2003, 2009 and 2015 was measured. A significant decrease in the population of living elms was observed between 1997 and 2009, but their number stabilized in 2015. The most significant differences were noticed for trees having DBH of 1-40 cm, while noticeable differences were not observed in the thicker trees between study periods. No differences were noted in the number of dead elms between the investigated periods. Moreover, no substantial differences in the basal area of live and dead trees were established. The tree-size distribution trajectories for live trees were unimodal and moderately positively skewed in all the study periods, with fewer juveniles compared to adults. Although these findings provide evidence for a decline in the population with the progress of years, our observations did not confirm this trend. A remarkable increase in the proportion of the youngest elms was observed in 2015, which suggests the regeneration of wych elms in BNP. Homogenization of tree-size distribution in different sample plots between 1997 and 2015 indicates that the mortality and growth rates of trees varied locally. Our results demonstrated that DED adversely affected the elm population in BNP at the turn of the 20th and 21st centuries. However, it is unlikely that this disease would lead to the extinction of elms in this area.