Effetti della gestione sul ciclo del carbonio nella cerreta della foresta demaniale di La Torre di Feudozzo (AQ)

Abstract

La gestione forestale sostenibile (GFS) è attualmente riconosciuta come lo strumento principale per il mantenimento del ruolo delle foreste quali regolatrici del sistema climatico, le quali contengono circa il 46% del carbonio organico terrestre sottoforma di biomassa arborea (359 Gt C) e nei suoli (787 Gt C) (WBGU, 1998). In Europa, il quadro di riferimento comune per verificare la sostenibilità della gestione delle foreste sono le Linee guida operative pan-europee, adottate durante la Seconda Conferenza Ministeriale sulla Protezione delle Foreste in Europa (MCPFE), tenutasi ad Helsinki nel 1993. Il presente lavoro si è posto l’obiettivo di valutare i cambiamenti che la gestione opera su un popolamento forestale e in particolare di stimare le dinamiche legate al ciclo del carbonio in una fustaia di cerro dell’Azienda Speciale del Demanio Forestale di Stato di La Torre di Feudozzo (AQ), in Abruzzo. Nel sito è presente un bosco di Quercus cerris L. che a partire dagli anni ’90 è stato sottoposto ai diradamenti e ai tagli di sementazione su più superfici. Questo ha portato alla formazione di quattro popolamenti diversificati, assimilabili ad una cronosequenza, di cui tre si trovano, rispettivamente, a 1 (08), 3 (06) e 12 (97) anni dal taglio di sementazione mentre uno è costituito da una fustaia transitoria a circa 40 anni dall’intervento di conversione, non ancoratrattata (70). Per ricostruire la storia della gestione del sito è stata svolta un’analisi su documenti cartacei e geografici in ambito GIS, che ha permesso di risalire agli eventi di gestione che hanno interessato la foresta negli ultimi 80 anni. Tali informazioni sono state derivate soprattutto dai due Piani di Assestamento redatti per conto dell’Azienda di Stato proprietaria della foresta fin dall’inizio del 1900. Le informazioni ottenute sono state successivamente riscontrate nei risultati dell’analisi dendrologica delle quattro particelle. Nel complesso, il taglio di sementazione ha portato, in media, ad una diminuzione del 50% dello stock di biomassa epigea con una semplificazione strutturale del popolamento. Lo stock totale, inteso come la somma dei cinque stock previsti dall’IPCC (1997), è risultato maggiore nella particella non tagliata con 370.8 tC ha-1 a differenza delle tagliate dove varia, invece, da 256.6 a 270.8 tC ha-1. In seguito al taglio è stato osservato, inoltre, una diminuzione dello stock di lettiera, 0.4 tC ha-1 nella 08 in confronto a 1.2 tC ha-1 della 70, ed un aumento della necromassa grossa e fine,rilasciata con le operazioni di esbosco (circa 10 tC ha-1). Il contenuto di azoto del suolo risulta inferiore nella particella a 12 anni dal taglio con 10.7 tN ha-1, rispetto alle 17.6 tN ha-1 della particella non tagliata. Questo è dovuto probabilmente alla diversa e più intensa gestione che è stata realizzata in questa parte della cerreta.

Sustainable forest management (SFM) has been recognized as the main instrument to preserve the role of forests as regulators of the global climate dynamics, since 46% of terrestrial organic carbon is stored in the aboveground biomass (359 Gt C) and forest soils (787 Gt C) (WBGU, 1998). In Europe, the common framework to control and promote the SFM are the Criteria and Indicators for Sustainable Forest Management as agreed by the Second Ministerial Conference on the Protection of Forests in Europe (MCPFE), held in Helsinki in 1993. In this study, the influence of forest management on the carbon cycle was investigated in a Turkey oak forest (Quercus cerris L.) in Abruzzo,. The study site is made up by a chronosequence consisting of four stands at different age after cut. Three stands are respectively 2 (08), 3 (06) and 12 (97) years after the shelterwood cut and one (70) has never been logged since the ’40s. To better understand the management history of the site, we traced the events that have affected the forest over the past 80 years from the previous forest management plans as well as from a GIS analysis in the last 13 years. The information obtained was subsequently confirmed by the results of the dendrological analysis performed for the four stands. The harvest removed about 50% of aboveground biomass which led to a simplification of the forest structure. Total carbon stock, defined as the sum of the five stocks defined by the IPCC (1997), was higher in the unmanaged stand with 370.8 tC ha-1, while the stock in the cut stands varied from 256.6 to 270.8 tC ha-1. After harvesting, we observed a decrease in litter stock (0.4 tC ha-1 of stand 08 compared to 1.2 tC ha-1 of stand 70) and an increase in coarse and fine deadwood derived from harvesting residues (about 10 tC ha-1). Soil nitrogen content was lower in the stand 12 years after harvesting with about 10.7 tN ha-1, respect to the untreated stand with 17.6 tN ha-1. This is probably due to different and more intensive management that was carried out in this part of the forest. After seed cutting, the reduced competition and the increased availability of resources for the remaining trees resulted in an expansion of crowns (LAI increase) and in an increasing of the Aboveground Net Primary Production (ANPP) of 3 tC ha-1 year-1 for stand 06 compared to 2.4 tC ha-1 year-1 for stand 70. The removal of canopy implied higher mean air and soil temperatures (Ts) during the growing season with greater temperatures fluctuations between day and night. Moreover gaps in the managed forest cover induced rapid increases of the soil water content (SWC) after precipitations but, at the same time, larger evaporations during the warm days. Effects of the microclimate and biomass variations were observed on soil respiration (RS) starting from three years after cutting. 11 Annual estimates of soil respiration and its temperature dependence in all stands were determined fitting data to Q10 and Gamma model (Khomik et al., 2009). Gamma model had the ability to take on logistical shapes and to fit the RS data better compared to the exponential Q10 at high Ts and low SWC values. In 2009, the total annual soil C emission estimated was higher in the 06 stand with 1937.1 gC m-2 year-1, followed by 97 (1645.7 gC m-2 year-1) and 08 (1488.1 gC m-2 year-1). The trend of the latter was similar to the 70 (1496 gC m-2 year-1). In addition, annual soil respiration was used to estimate the total belowground carbon allocation (TBCA) in the unmanaged stand where the assumption of a steady state condition with respect to total soil carbon storage was reasonable. Finally, we installed an eddy covariance (EC) mobile system to measure the net ecosystem exchange (NEE). Use of mobile EC system allowed a comparison of ecosystems with respect to the spatial patterns of management that generated a strong heterogeneity in surface. Furthermore, the chronosequence approach to studying vegetation dynamics with space-for-time substitution provided a significant insight into the patterns and mechanisms of stand carbon dynamics. During the first sampling period, from 30 April 2009 to August 14, the EC system was placed between the 97 and 06 stands, then, since August 14 we moved it at the boundary between the more recently cut stands (06 and 08) and the uncut one (70). Given the heterogeneity of the studied forest, we needed to precisely calculate the footprint contributions of specific surface areas around the mobile EC system. The analytical method proposed by Nefetl et al. (2008) provided a tool for calculating the source area after the Kormann and Meixner footprint equation (2001) combining the field measurements with a landuse information. It allowed us to discriminate all those cases in which the fluxes source area was spreaded over two different stands, especially during the second sampling period when the mobile EC System was placed in the middle of three different stands. Net ecosystem exchange (NEE) estimated over the sampling period ranged from -29 to +9 molCO2 m-2 s-1 in the stand 97 and from -20 to +15 molCO2 m-2 s-1 in the 06. Over the second period NEE ranged from -14 to +9 molCO2 m-2 s-1 in stand 08 and from -14 to +20 molCO2 m- 2 s-1 in stand 70. Respiration was slightly higher in the uncut stand, in relation to higher stand biomass and, at the same time, the approaching of the last part of the growing season. Cumulated net ecosystem exchange (NEE) for the period when single stands were measured, was -307 gC m-2 (GPP -461.6 gC m-2, Re 154.5 gC m-2) in stand 97 and -126.3 gC m-2 (GPP -502.3 gC m-2, Re 376 gC m-2) in stand 06, while for the second period cumulated NEE was -89.4 gC m- 2 (GPP -186.4 gC m-2, Re 97 gC m-2) in stand 08 and 20.2 gC m-2 (GPP -359 gC m-2, Re 379.8 gC m-2) for stand 70. 12 For the observed period, the site was estimated to be a carbon sink, with the exception of the unmanaged forest that, differently from the other stands, were measured until end of season (October). Anyhow, the annual carbon balance can be assessed after a full year of sampling including also the winter season. The multi-methodological approach to the study of carbon cycle has allowed to investigate the various factors that differently affect the dynamics of the ecosystem. In this study, the integration of multiple methods, such as structural analysis, forest inventories, fluxes and micrometeorological measurements allowed to relate the different aspects of the carbon cycle with the factors, mainly climatic and structural, that influenced the dynamics after management operations and along time. These tools, when individually applied, are not able to describe in the same way the complexity of the forest response to the modified ecological conditions after management. Understanding the effects of harvest on the carbon balance and its temporal dynamics is necessary for implementing sustainable forest management.