Stand and landscape scale variability in the amount and diversity of coarse woody debris in primeval European boreal forests

Abstract

Factors influencing natural variability in coarse woody debris (CWD) as an indicator of ecosystem services in boreal forests need to be better defined. We analysed the CWD characteristics in 194 dynamic primeval boreal forest stands and 3 landscapes under different disturbance regimes. The volume of CWD in the spruce dominated forests linearly increased with increasing annual temperature. The CWD amount and species diversity were higher in spruce than in pine dominated stands. The CWD volume increased in relationship to the types of forest stand dynamics in the order: ‘even-aged’, ‘cohort’, ‘fine-scale gap’. The CWD volumes were 52.3; 207.0 and 146.8m3ha−1 and the dead:live wood volume ratios were 29%, 102% and 60% in the landscapes with disturbance regimes driven by periodic surface fires, cohort-replacing windthrow, and fine-scale gap dynamics, respectively. Mean dead:live wood volume ratios exceeded 100% in spruce stands with even-aged and cohort dynamics and in pine stands with even-aged and fine-scale gap dynamics. The CWD decay class diversity was higher in pine than in spruce dominated stands. The CWD volume distribution by decay class was the most even in forest stands driven by fine-scale gap dynamics in both spruce and pine dominated stands. The CWD position diversity was higher in spruce than in pine dominated forests. Fallen logs dominated among other CWD position types in the spruce dominated stands with cohort and fine-scale gap dynamics and in pine stands with cohort and even-aged dynamics. Pine forests with fine-scale gap dynamics stored the greatest volume of snags. In the spruce dominated stands, the proportion of leaning logs decreased and the proportion of snags and stumps increased among forest dynamic types in the following order: even-aged, cohort, fine-scale gap. The CWD diameter distribution had peaks in small-sized CWD in even-aged spruce stands and pine stands with fine-scale gap dynamics. Mid-sized CWD dominated in spruce stands driven by cohort and fine-scale gap dynamics and pine stands with even-aged and cohort dynamics. The large quantity of CWD that encompassed a wide range of variation in tree species, decay class, position type and size creates a diversity of CWD habitats for saproxylic organisms and ensures functional resilience in boreal forest ecosystems. Our results stress that mean annual temperatures and natural site-specific disturbance regimes should be taken into account when setting targets for CWD volumes and dead:live wood volume ratios for management and restoration of CWD in boreal forests.