Abstract
Coarse woody debris (CWD) is an important element of forest structure that needs to be considered when managing forests for biodiversity, carbon storage or bioenergy. To manage it effectively, dynamics of CWD decomposition should be known. Using a chronosequence approach, we assessed the decomposition rates of downed CWD of Fagus sylvatica, Picea abies and Pinus sylvestris, which was sampled from three different years of tree fall and three different initial diameter classes (>10 – ≤ 20 cm, >20 – ≤40 cm, >40 cm). Samples originating from wind throws in 1999 were collected along a temperature and precipitation gradient. Based on the decay class and associated wood densities, log volumes were converted into CWD mass and C content. Log fragmentation was assessed over one year for log segments of intermediate diameters (>20 – 40 cm) after 8 and 18 years of decomposition. Significantly higher decomposition constants (k) were found in logs of F. sylvatica (0.054 year−1) than in P. abies (0.033 year−1) and P. sylvestris (0.032 year−1). However, mass loss of P. sylvestris occurred mainly in sapwood and hence k for the whole wood may be overestimated. Decomposition rates generally decreased with increasing log diameter class except for smaller dimensions in P. abies. About 74 % of the variation in mass remaining could be explained by decomposition time (27 %), tree species (11 %), diameter (17 %), the interactive effects between tree species and diameter (4 %) as well as between decomposition time and tree species (3 %) and a random factor (site and tree; 9.5 %), whereas temperature explained only 2 %. Wood fragmentation may play a more important role than previously thought. Here, between 14 % and 30 % of the decomposition rates (for the first 18 years) were attributable to this process. Carbon (C) density (mgC · cm−3), which was initially highest for F. sylvatica, followed by P. sylvestris and P. abies, decreased with increasing decay stage to similar values for all species. The apparent lack of climate effects on decomposition of logs in the field indicates that regional decomposition models for CWD may be developed on the basis of information on decomposition time, tree species and dimension only. These can then be used to predict C dynamics in CWD as input for C accounting models and for habitat management.
Highlights
Coarse woody debris (CWD) is an important element of forest structure that needs to be considered when managing forests for biodiversity, carbon storage or bioenergy
The corresponding average diameters of logs were 34.8 (SD ± 14.8) cm for F. sylvatica, 33.9 (SD ± 13.9) cm for P. abies and 32.2 (SD ± 12.1) cm for P. sylvestris indicating that the effects of species were not biased through differences in log dimensions between species
Residence time per decay class The residence time per decay class increased with decay class. This increase was most pronounced in pine and least pronounced in Decomposition rates (k) and mass loss To our knowledge, this is the first study that systematically assessed CWD decomposition rates and dynamics of Fagus sylvatica, Picea abies and Pinus sylvestris across different sites and diameter classes in Central Europe
Summary
Coarse woody debris (CWD) is an important element of forest structure that needs to be considered when managing forests for biodiversity, carbon storage or bioenergy. Whereas the amount of CWD may comprise up to 30 % or even 40 % of the total timber volume in natural beech (Commarmot et al 2013) and spruce (Ranius et al 2003) forests, this share is typically less than 5 % in managed European forests (Bütler and Schlaepfer 2004; MCPFE 2007) This reduction in the amount and related quality of dead wood (Müller and Bütler 2010) has significant implications for its various functions. For the assessment of C stocks in dead wood as part of National Greenhouse Gas inventories, detailed information on C stored in dead wood of different species and their relationship with different decay stages (which are typically captured in inventories) is necessary. Information on residence times of CWD in different decay classes would be very helpful to forecast its dynamics and to calculate the input and output of different decay stages in order to conserve specific habitats of dead wood dependent species (Kruys et al 2002; Ranius et al 2003)
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