Decay dynamics of Abies alba and Picea abies deadwood in relation to environmental conditions

In this study we analysed a dataset of 8661 logs of silver fir (Abies alba Mill.) and Norway spruce (Picea abies L., Karst) in mixed fir-spruce-beech stands in primeval and natural forests in four sites separated into the two macroclimatic categories according to mean annual temperature (“cold” and “warmer”) and according to mean annual precipitation (“mesic” and “humid”). We used “Bayesian Survival Trajectory Analysis” on a more than 40-year long time series (1972-2015), focusing on differences in the residence time of deadwood in different macroclimatic categories and two DBH classes. We also evaluated two qualitative characteristics of the downed logs: mortality mode and log position during decomposition. We calculated residence time and the time needed to reach the advanced decay stage. Our analyses confirmed the influence temperature and precipitation on modelled residence time. The residence time for silver fir logs in the DBH class 55+ cm in the “cold” site was 106 years, while in the “warmer” sites was 78 years. The residence time in the “mesic” site was 57 years, while in the “humid” sites was 90 years. It took 81 years for Norway spruce logs in the DBH class 55+ cm to completely decompose in the “cold” site. Suspended logs 11 took years longer to decay than those in contact with the ground. The modelled residence time of logs on wet sites was the same as that of logs at sites unaffected by water. These results can be utilised in biodiversity oriented forest management, as well as in modelling future amounts of deadwood. In order to maintain the continuous presence of silver fir and Norway spruce deadwood for those organisms that depend on it, it is necessary to supply deadwood at least once every 25-40 years (depending on climatic category and DBH class). During this time, approximately 50% of logs become completely decomposed and 50% remain in the last decay stage.