Quantifying decay progression of deadwood in Mediterranean mountain forests

Forests contribute to the sequestration of organic carbon (C). A key role in forest C cycling is played by deadwood. While a broad range of literature on deadwood decay (above-ground) exists, the mechanisms occurring in the transition phases from deadwood to humus are poorly understood. In particular, scarce information is available on the temporal patterns of wood compounds (such as lignin and cellulose) during decay processes.Our objective was to provide a deeper understanding on deadwood decay in a Mediterranean montane environment, with focus on semi-natural forests of Fagus sylvatica L. (beech). The decay progression of beech wood blocks (mass, cellulose, lignin) equal in size (5 cm x 5 cm x 2 cm) placed in soil mesocosms was studied during one year in the Apennines (Majella, Italy). The sites were located along an altitudinal gradient, reflecting different climatic zones. The effect of exposure (north- vs. south-facing slopes) was also considered. Deadwood, cellulose and lignin dynamics were related to soil parameters (pH, grain size, moisture, temperature) and climatic data. Deadwood decayed very fast. The decay rate constants of the deadwood mass significantly (positively) correlated with air temperature and soil moisture: the lower the temperature, the lower the evaporative demand, the higher the moisture availability, and the higher the decay rates. Lignin decayed more slowly than cellulose, resulting in average decay rate constants (k) between 0.368 and 0.382 y-1. Soil properties and topographic traits (slope and exposure) strongly influenced the decay processes. At south-facing sites (having an altitude < 1300 m a.s.l.), decay processes were lower owing, most likely, to drier conditions. A deeper understanding of deadwood decay progression on different temporal and spatial scales in Mediterranean mountain forests warrants further research, in order to define meaningful indicators of warming-induced changes on the linkages between C storage in deadwood and decomposition processes.