Restrictions on natural regeneration of storm-felled spruce sites by silver birch (Betula pendula Roth) through limitations in fructification and seed dispersal

Particularly after disturbance events, the early successional colonist Betula pendula Roth is experiencing renewed silvicultural interest with respect to the natural regeneration of large disturbed forest areas. In a case study, we therefore studied the seed dispersal of B. pendula from two adjacent spruce stands to large storm-felled sites at high altitudes in the Thuringian Forest (Germany) over a 2-year period. We applied inverse modelling to describe the distance-dependent seed distribution using a negative exponential kernel and seed production function of birch seed trees. Maximum seed numbers of 2015 n m−2 (non-mast year) and 9557 n m−2 (medium year) occurred within 40‒50 m distance to a seed tree. The predicted seed production rate of a birch seed tree with a reference dbh of 20 cm was approximately 350,000 n tree−1 (non-mast year) and 1,500,000 n tree−1 (medium year). Regardless of the seed crop, the dispersal distances were similar in both years. The isotropic model showed mean dispersal distances of 86 and 97 m (uphill) and 367 and 380 m (downhill) for the 2 years of seed sampling. No directionality in seed dispersal was found. The findings showed birch seed dispersal to be strongly influenced by site inclination, seed tree position (valley, slope or plateau) and distance to the storm-felled site. Furthermore, the seed shadow is influenced by the number of seed sources. Therefore, risk-adapted forest management should include the ‘spatial optimization’ of birch seed trees, ideally creating a network of small seed tree groups scattered more or less regularly within pure conifer forests.