Deadwood density, C stocks and their controlling factors in a beech-silver fir mixed virgin European forest

Abstract

Deadwood is a fundamental structural and functional component of forests, with a crucial role in supporting forest biodiversity, nutrient and carbon cycling. Precise deadwood density estimates and its relation to environmental factors are necessary to evaluate the biomass and carbon stocked in this component. In this study, we estimated dry deadwood density for two different tree species, silver fir (Abies alba) and European beech (Fagus sylvatica) and for three snags and five logs decomposition classes in a virgin mixed beech-fir forest in the Southern Carpathians. The goal was to assess how deadwood density is influenced by different abiotic (moisture, elevation, slope, aspect) and wood-related factors (rottenness, position along the piece, contact with the soil).

For snags, the mean dry density showed a reduced variability within decomposition classes (484–326 kg.m−3 for beech and 374–319 kg.m−3 for fir), compared to the logs (486–139 kg.m−3 for beech and 359–161 kg.m−3 for fir). While the mass moisture varied slowly in the first three decay classes (around 60–80 %), it increased sharply in the last two decay classes of logs (>140 % in the fourth class and > 350 % in the last one). The rottenness increased with the decay class in a similar way for both species. The contact of logs with the soil influenced positively the moisture, but the position of the sampling within the piece did not play any significant role in the variability of density. Based on density estimates per decay classes we estimated that the carbon (C) stored in deadwood varied greatly among the 21 plots from 0.36 to 41.16 MgC ha−1, with a mean value of 15.96 ± 2.36 (±SE) MgC ha−1.

Our study suggests that volume-based calculations might yield biased quantitative estimates of C stored in deadwood unless a local estimate of dead wood density corrected per species and decomposition class is applied. Moreover, the use of an averaged value of dry density instead of value for each decay class may result in an overestimation of 22% on the estimation of C stock. Thus, our study emphasises the importance of considering decay class-specific values in future inventories of C stocks in other forests and for other species. Furthermore, it could serve as a methodological basis for more specific research designed to uncover the potential influence of different forest management practices on dry deadwood density.