Natural regeneration in Central-European subalpine spruce forests: Which logs are suitable for seedling recruitment?

Decaying logs form the major seedbed for trees in European subalpine Picea abies forests. However, many aspects related to seedling colonization pattern on logs remain unclear. The aim of this study was to analyze the relationships of P. abies (Norway spruce) seedling (height <15cm) and sapling (height ⩾15cm) densities on decaying logs in relation to stage of wood decay, log diameter, ground contact of decaying log, assumed cause of tree death, presence of species of wood-decaying fungi and coverage by surrounding plants in the subalpine old-growth forests of the Bohemian Forest and Ash Mountains, in the Czech Republic. We have focused on how logs with different origin differ in their properties and how these properties influence seedling abundance. Seedling densities peaked on the medium-decayed logs and decreased thereafter. Sapling densities continually increased as the decay progressed. Seedling and sapling densities followed negative binomial distributions, with many logs of all decay stages having low regeneration densities. The degree of ground contact, white-rot-causing Armillaria spp. presence, white-rot-causing Phellinus nigrolimitatus presence and log diameter were positively related to both seedling and sapling density. Also tree death as a result of wind uprooting was positively related to sapling density. Conversely, the presence of brown-rot-causing Fomitopsis pinicola and tree death as a result of bark beetle attack were negatively related to regeneration densities. The low cover of vegetation from sides positively affected seedling density; however, heavily covered logs were less occupied by seedlings. Our study provides evidence that large logs originating from wind uprooting or butt rot infection are most appropriate for retention to promote natural spruce regeneration in managed subalpine spruce forests.