Decomposition dynamics of coarse woody debris of three important central European tree species

Background： 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. Methods： Using a chronosequence approach, we assessed the decomposition rates of downed CWD of Fagus sylvatica, Picea obies and Pinus sylvestfis, 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. Results： Significantly higher decomposition constants （k） were found in logs of F. sylvotica （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. obies. 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. sylvatico, followed by P. sylvestris and P. obies, decreased with increasing decay stage to similar values for all species. Conclusions： 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.

Nutrient retention and release in coarse woody debris of three important central European tree species and the use of NIRS to determine deadwood chemical properties

Background: Coarse woody debris(CWD) is very important for forest ecosystems, particularly for biodiversity and carbon storage. Its relevance as a possible reservoir and source of nutrients is less clear, especially in central Europe.Methods: Based on a chronosequence of known ages of logs, we analyzed the nutrients stored in CWD of Fagus sylvatica, Picea abies, and Pinus sylvestris at different sites in Germany. To quantify nutrient concentrations, we assessed the use of Near Infrared Reflectance Spectroscopy(NIRS) to determine the chemical properties of CWD.Results: NIRS models were suitable to predict concentrations of C, N, P, lignin and extractives. Concentrations of most nutrients increased with mass loss, with the exception of potassium, which decreased for beech and pine and remained relatively constant for spruce. The highest nutrient concentrations(N, P, S, Ca and Mn, except Mg and K) were generally observed in highly decomposed spruce logs. The net effect of decreasing CWD mass and increasing nutrient concentrations was either a decreasing(N, P and K in beech; P, Mg, K and Mn in pine), constant(S, Ca and Mg in beech; N, S and Ca in pine) or increasing amount of nutrients(N, P, S and Ca in spruce; Mn in beech) in the logsover the course of decomposition. The C/N ratio decreased for all tree species, most markedly for spruce from ca. 1000 at the beginning of the decomposition process to 180 at 36 years. The N/P ratio converged to a value of about 30 forall three species. Lignin concentrations increased for spruce and beech and remained constant for pine.Conclusions: Our results indicate that most nutrients remain in CWD for long periods. Nutrients may be used and cycled by microorganisms within CWD, but with the exception of P(in beech), Mg(in pine) and K(in beech and pine), there appears to be little net nutrient export until two thirds of the mass is lost. Instead, N, P, S and Ca were accumulated in spruce logs, indicating that CWD became a net sink rather than a net source of some nutrients for several decades.