The key importance of coarse woody debris (CWD) for biodiversity is well acknowledged. However, its role in terrestrial nutrient and carbon cycles is less studied, in particular in central Europe. We analysed the decay process of spruce Picea abies (L.) Karst., the most common tree species in Switzerland. The aims were: (i) to examine the usefulness of ultrasonic wave measurements for characterising of decay processes; (ii) to assess changes in physical and chemical variables of CWD during the decay process in relation to site-specific humus forms. We analysed 25 logs, five per decomposition class within a five-class system, for their density, moisture, C, N and P contents, lignin and cellulose. We also applied ultrasonic measurements to the radial axis of decaying logs using the Sylvatest-Duo^® tool. In addition, we described eight soil profiles below the sampled logs and analysed the soil samples for total C, N and P and water pH. All the soils sampled were classified as humiferous Brunisol (eutric Cambisol) with various humus types.
The propagation speed of ultrasonic waves was found to be directly proportional to the average tree density and inversely proportional to C content. These preliminary results point out the potential usefulness of this technique for further studies of wood decay. Wood density was found to decrease during wood decay (434-308 mg g⁻¹), whereas moisture increased (94-258%). Carbon and lignin concentrations remained stable, while N and P contents both increased between classes 3 and 5 (N: 0.41-1.26 mg g⁻¹ and P: 0.01-0.06 mg g⁻¹).
These general decay patterns are in accordance with previous studies of other tree species and of P. abies in other geographic regions. However, we did find some site-specific patterns, e.g. N and P were lower and wood density declined less than in other studies. Climatic factors at the study site slow down biological activity and they also seem to explain the morphology of the humus forms and their variations. We found no concordance between the humus morphology and the wood-decay state. We recommend performing long-term experiments in Central European forests to investigate the different factors that may influence CWD decomposition, such as edaphic and climatic conditions, in a controlled way.