Safe for saplings not safe for seeds: Quercus robur recruitment in relation to coarse woody debris in Białowieża Primeval Forest, Poland

In forested ecosystems, oak saplings can be found in association with coarse woody debris (CWD) that offers protection against herbivore browsing. In this study we investigated whether CWD is already a safe site during the earlier stages of oak recruitment, i.e. at the seed and seedling phase, or whether these phases require different micro-environments reflecting spatial discordance. We tested our hypothesis in the Białowieża Primeval Forest in Poland, one of the last remaining examples of natural, lowland forest in Europe. We performed a seed removal experiment in two contrasting forest types (deciduous and coniferous), where we followed the fate of 576 acorns cached in plots with and without CWD. At all locations camera traps and rodent live traps were placed to assess species and abundance of responsible seed removers. To determine the spatial distribution of seedlings we surveyed transects (500 m × 2 m) in deciduous and coniferous forest (total 0.6 ha) and recorded for each sapling its association with CWD. Our experiment clearly showed that, despite a small sample size, acorn removal is higher when associated with CWD and higher in deciduous forest (100%) than in coniferous forest (58.2%). Both wild boar and rodents were responsible for the acorn removal, but wild boar had more impact in deciduous forest and rodents had more impact in coniferous forest. In line with our observed acorn removal, established seedlings were only found in coniferous forest, away from CWD. Our study suggests that spatial discordance occurs during the multiple stages of oak recruitment. The best place to survive for seed and seedling is in coniferous forest away from CWD, while the best place to survive for saplings is near CWD. Our findings may well explain the mechanisms behind recruitment limitation in our studied system, as well as in other forested ecosystems.