Geographical Gradient of Fungal Decay Type in Norway Spruce Logs in Europe and Its Impact on Seedling Establishment

ABSTRACT Aim Norway spruce ( Picea abies ) is a dominant tree species across Eurasia, and is known to regenerate on decaying logs. Understanding its regeneration dynamics is crucial for predicting forest sustainability under climate change. The decay type of the logs, indicative of fungal decay capabilities of lignin and holocellulose and traditionally categorised into white rot and brown rot, markedly influences spruce seedling density on the logs, because brown rot reduces their establishment. As fungal distribution and wood decay types are affected by macroclimate, the success of spruce seedling establishment is expected to vary continentally and may be further influenced by climate change. The goal of the present study is to examine the relationship between decay type frequencies and spruce seedling densities on decaying logs, assessing effects on regeneration success along a climatic gradient. Location Fifteen sites in six European countries. Taxa Norway spruce ( Picea abies [L.] H. Karst.). Methods We collected wood samples from spruce logs naturally decaying in the forest to evaluate their decay types. Spruce seedling numbers were recorded on the logs. The occurrence of certain decay types, seedling density, and their relationships with climate, stand, and log variables were analysed using generalised linear mixed models (GLMM) and structural equation modelling (SEM). Results White rot type of decay was dominating in central Europe compared with southern and northern Europe, where brown rot was prevalent. Spruce seedling densities were positively correlated with white rot in heartwood, which was explained by high precipitation seasonality in central Europe. Main Conclusions Spruce seedling regeneration on decaying logs may be indirectly affected by climate through its influence on fungal‐mediated wood decomposition. Despite predictions of northward shifts in Norway spruce distribution due to climate change, slower regeneration rates on brown rot logs may limit northern expansion, potentially leading to a contraction of the species' range.