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Forests contribute to the sequestration of organic C and a key issue in C cycling in forest ecosystems can be linked to deadwood dynamics. Deadwood and litter act as important linkages between forest productivity and current community, and ecosystem processes. In forest ecosystems, coarse woody debris (CWD) influences the nutrient cycling, humus formation, carbon storage, fire frequency, water cycling and it represents also a habitat for many organisms. While a broad range of literature about CWD decay (above-ground) already exists, mechanisms describing the incorporation of the woody necromass into humus forms are rather poorly investigated. The objectives of this thesis are focused on providing a deeper understanding of deadwood decay processes in forest ecosystems located in the Mediterranean and Alpine montane areas. Moreover, this research project investigates the relationship between deadwood decay, altitude and exposure, exploring the decomposition timing in Apennine and Alpine forests, with the main aim to deeper understand the deadwood decay processes in these climatic contexts. In detail, the organic matter integration into the soil was investigated, focusing on the CWD decay and its incorporation in the soil organic matter (SOM) through the analysis of the wood biochemical compounds and soil chemical composition. A climosequence approach was used to investigate the decay processes, comparing along sites located on north- and south-facing slopes, at different elevations. An accurate sampling configuration and experimental procedure was set up: at each site of the climosequence, a field experiment using soil mesocosms (PVC tubes with deadwood inside) was tested. Data were collected in Apennine (Fagus sylvatica) and Alpine (Picea abies) forest types, in order to assess the variation of chemical and biochemical compounds in CWD during the decay progression. Lignin and cellulose amounts were quantified in 5 different decay stages of CWD in the Alpine sites (Picea abies and Larix decidua). Results showed that CWD decompose differently between Alps and Apennines, depending on the tree species, climate factors and soil composition. In detail, CWD of Fagus sylvatica decays very fast, while CWD decay progression of Picea abies is lower, as demonstrated by the analysis of lignin and cellulose in the different 5 decay stages. In conclusion, these results represent a contribution to the knowledge on CWD decay progression in Mediterranean and Subalpine forest ecosystems. However, further studies are needed to deeper explore the factors influencing the deadwood decay rates, in order to clarify the role of deadwood in contributing to the overall forest functioning at different scales. Le foreste contribuiscono al sequestro del C organico e le dinamiche di decomposizione della necromassa ricoprono un ruolo chiave nel ciclo del C negli ecosistemi forestali. Il legno morto e la lettiera agiscono come link importanti tra la produttività forestale, le comunità attuali ed i processi ecosistemici. Negli ecosistemi forestali, il coarse woody debris (CWD, necromassa grossolana) influenza il ciclo dei nutrienti, la formazione di humus, lo stoccaggio del carbonio, la frequenza degli incendi, il ciclo dell’acqua e rappresenta anche un habitat per molti organismi. Mentre esiste già un’ampia gamma di letteratura riguardante la decomposizione del CWD (sopra il suolo), i meccanismi che descrivono l’incorporazione della necromassa legnosa nelle forme di humus sono poco conosciuti. Gli obiettivi di questa tesi sono volti a fornire una conoscenza maggiore dei processi di decomposizione della necromassa negli ecosistemi forestali situati in aree montane Mediterranee ed Alpine. Inoltre, questo progetto di ricerca ha studiato le relazioni tra la decomposizione del legno morto, l’altitudine e l’esposizione, analizzando i tempi di decomposizione nelle foreste Appenniniche ed Alpine, con il principale scopo di capire maggiormente i processi di decomposizione della necromassa in questi contesti climatici. Nel dettaglio, è stata studiata l’integrazione della materia organica nel suolo, concentrando l’attenzione sulla decomposizione del CWD e sulla sua incorporazione nella materia organica del suolo (SOM), attraverso le analisi delle componenti biochimiche del legno e la composizione chimica del suolo. E’ stato utilizzato un approccio basato sulla climosequenza per studiare i processi di decomposizione, confrontando siti con diverse esposizioni, a diverse altitudini. E’ stato eseguito un campionamento accurato, basato su una procedura sperimentale: in ogni sito della climosequenza, sono stati usati i mesocosmi (tubi in PVC con/aventi all’interno legno morto) per realizzare un esperimento sul campo. I dati sono stati raccolti in diverse tipologie forestali Appenniniche (Fagus sylvatica) e Alpine (Picea abies), per valutare la variazione dei componenti chimici e biochimici del legno durante il progredire della decomposizione. Sono state inoltre misurate le quantità di lignina e cellulosa nei 5 diversi stadi di decomposizione del CWD nei siti Alpini (Picea abies e Larix decidua). I risultati mostrano che il CWD si decompone diversamente tra le Alpi e gli Appennini, a seconda della specie arborea, dei fattori climatici e della composizione del suolo. Nel dettaglio, il CWD del Fagus sylvatica si decompone molto velocemente, mentre la decomposizione del Picea abies è lenta, come dimostrato dalle analisi di lignina e cellulosa nei 5 stadi di decomposizione. In conclusione, questi risultati rappresentano un contributo alla conoscenza della progressione della decomposizione del CWD negli ecosistemi forestali Mediterranei e Subalpini. Comunque, necessitiamo di altri studi per conoscere più a fondo i fattori che influenzano i tassi di decomposizione della necromassa, e per spiegare il contributo che il legno morto fornisce alle funzioni delle foreste, a diverse scale. Dottorato di ricerca in Scienze, tecnologie e biotecnologie per la sostenibilità

Due to the large size (e.g. sections of tree trunks) and highly heterogeneous spatial distribution of deadwood, the timescales involved in the coarse woody debris (CWD) decay of Picea abies (L.) Karst. and Larix decidua Mill. in Alpine forests are largely unknown. We investigated the CWD decay dynamics in an Alpine valley in Italy using the chronosequence approach and the five-decay class system that is based on a macromorphological assessment. For the decay classes 1–3, most of the dendrochronological samples were cross-dated to assess the time that had elapsed since tree death, but for decay classes 4 and 5 (poorly preserved tree rings) radiocarbon dating was used. In addition, density, cellulose, and lignin data were measured for the dated CWD. The decay rate constants for spruce and larch were estimated on the basis of the density loss using a single negative exponential model, a regression approach, and the stage-based matrix model. In the decay classes 1–3, the ages of the CWD were similar and varied between 1 and 54 years for spruce and 3 and 40 years for larch, with no significant differences between the classes; classes 1–3 are therefore not indicative of deadwood age. This seems to be due to a time lag between the death of a standing tree and its contact with the soil. We found distinct tree-species-specific differences in decay classes 4 and 5, with larch CWD reaching an average age of 210 years in class 5 and spruce only 77 years. The mean CWD rate constants were estimated to be in the range 0.018 to 0.022y−1 for spruce and to about 0.012y−1 for larch. Snapshot sampling (chronosequences) may overestimate the age and mean residence time of CWD. No sampling bias was, however, detectable using the stage-based matrix model. Cellulose and lignin time trends could be derived on the basis of the ages of the CWD. The half-lives for cellulose were 21 years for spruce and 50 years for larch. The half-life of lignin is considerably higher and may be more than 100 years in larch CWD. Consequently, the decay of Picea abies and Larix decidua is very low. Several uncertainties, however, remain: 14C dating of CWD from decay classes 4 and 5 and having a pre-bomb age is often difficult (large age range due to methodological constraints) and fall rates of both European larch and Norway spruce are missing., Biogeosciences, 13 (5), ISSN:1726-4170

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 zone from deadwood to the 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 by focussing on semi-natural forests of Fagus sylvatica L. (beech). The decay process was studied in a field experiment (in the Majella mountains, Apennine Mountains, Italy) among an altitudinal transect at different climatic conditions. Beech wood blocks (mass, cellulose, lignin) having all an equal in size (5 cm × 5 cm × 2 cm) were placed in soil mesocosms to investigate over one year changes in the overall mass, cellulose and lignin content. The sites were along an altitudinal gradient, reflecting different climatic conditions. 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 climate data. Deadwood decayed very fast and followed an exponential trend. The decay rate constants of the deadwood mass significantly (positively) correlated with air temperature and soil moisture: the lower the temperature, the lower the evapotranspiration, 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