We developed and tested a growth and yield model for simulating the temporal development of the main structural traits for chestnut coppices in the Mediterranean climate change hotspot. Although this study was focused on chestnut coppices at the southernmost distribution limit of the species, predictions revealed high yield potential of these forest stands, confirmed by the considerable wood volumes and growth trends (MAI and CAI). Aspects of coppice productivity have been often ignored in common growth modelling approaches (Vanclay, 1994; Pretzsch, 2009; Weiskittel et al., 2011). Therefore, this empirical growth and yield model might provide useful insights in production modelling and forest planning applied to chestnut coppices and aimed at balancing stand productivity and wood quality. Nevertheless, the specific geographical context and the complexity of environmental conditions and forest structures in which data were collected may limit comparisons with other modelling exercises (Bernetti, 1980; von Gadow and Hui, 1999).
Angelini et al. (2013) estimated, on average, MAI ranging from about 7.2 to 13.0 m3 ha− 1 year− 1 for 18–22 years old chestnut coppices in Central Italy. Ciancio et al. (2004), for 15–45 years old chestnut coppices in Calabria, found MAI ranging from 12 to 16 m3 ha− 1 year− 1. In Tuscany, Cutini (2001) recorded MAI of 17.6 and 12.8 m3 ha− 1 year− 1, for 15- and 38-years old chestnut coppices, respectively. The present study reports model results aligned with previous observations on chestnut coppices. With reference to the basal area, values of 25 m2 ha− 1 were observed for 11 years old chestnut coppices in Tuscany (Manetti et al., 2016), while values ranging from 18 to 42 m2 ha− 1 were recorded for 6–22 years old chestnut coppices in Lazio (Central Italy) (Mattioli et al., 2016). Again, results of the present study are consistent with previous research on chestnut coppices and, thus, the model exercise may help implementation processes and scaling procedures.
Although we are aware of the variability in local conditions across different geographical contexts, due to specific environmental factors and different management options, we believe that our model results clearly indicated high productivity of chestnut coppices in Mediterranean mountain systems. Indeed, shoot heights reached 22 m in 50 years old coppice stands, thus potentially ensuring wood-based products of high economic value. Particularly, the height-diameter curves revealed that, during the earlier stages after coppicing, shoots might show a considerable height increment. In fact, a relevant height-diameter curve differentiation was observed at these growth stages, probably due to the strong competition for light between shoots occurring on each single stump. When the coppice reached an age of 15–20 years and beyond, the height-diameter curve flattened, indicating a high level of spatial competition between shoots (Marziliano et al., 2013, 2019). At these ages, the competition is mainly affected by diametrical differentiation rather than by hypsometric variation, shoots growing more in diameter than in height.
Chestnut is considered the tree species with the highest capacity to provide multiple goods and services among Mediterranean forest species (Giannini et al., 2014), producing a variety of assortments other than firewood, even when it is managed as coppice stand. In this study, we highlighted the great potential of chestnut for producing different wood-based products when the rotation period of coppice stands is extended. In this context, although the organic layer and the mineral soil, as a large carbon stock in forest ecosystems, were not accounted for in this study, implications for carbon sinks and mitigation purposes are also important.
On the other hand, for the same assortments, wood quality traits might significantly vary, depending on the length of rotation periods. In fact, MOEd values revealed a negative trend as stand age increased (from to 25 to 50 years) while a positive trend at stand ages growing from 15 to 25 years. Moreover, a negative trend of MOEd values was observed as DBH increased, both in C15 and C50. According to Detter et al. (2008), chestnut wood-based products can be considered of good quality when the MOEd is not lower than 7200 MPa. However, in C15 and C50, we recorded MOEd values lower than this threshold value, for DBH higher than 18 cm (7000–7100 MPa) and 45 cm (5587–7100 MPa), respectively. Therefore, almost all large poles (assortments of the greatest size) attainable in C15 and most of beams and boards (assortments with considerable size) obtainable in C50 did not have the minimum quality requirements for being classified as adequate. Quality wood could, however, be produced at relatively high stand age and DBH when chestnut coppices grow in good site conditions and with appropriate silvicultural treatments (Manetti et al., 2020).
These results demonstrated that the advanced shoot ages, but also the high growth rates of chestnut coppices, negatively affect the wood quality. For this reason, the dynamics of stem radial increments might induce the production of a less-stiff mature wood, resulting in a significant loss of wood quality. Romagnoli et al. (2014) obtained similar results and observed, in coppice stands with age higher than 25–30 years, a decrease in the mechanical performance of chestnut wood near the threshold DBH of 35 cm. Therefore, to maximize and balance wood quality and stand productivity, coppicing in this Mediterranean context should be rethought in terms of strengths and weaknesses of the system, considering not only the shoot age, but also the shoot DBH at harvest (Genet et al., 2013), as well as shoot height and basal area (Marini et al., 2021).
We observed that, when the rotation cycle ranged between 25 and 30 years, wood-based products of high quality could be obtained, as well as a variety of assortments. Nevertheless, 20–30 years old chestnut stands, growing on favourable sites and properly managed, could be considered young and still have high growth rates (Cutini, 2001; Conedera et al., 2004; Manetti et al., 2009). Similarly, we observed MAI equal to 13.3 m3 ha− 1 yr− 1 at 26 years (year of culmination). Amorini et al. (1997) found that wood of good quality could be produced with rotation periods ranging between 30 and 50 years and with 2–4 medium-intensity thinning operations. It must be pointed out that, in many areas of Italy, common rotation periods range between 10 and 15 years (Ciancio et al., 2004). Such short rotation periods appear to be inadequate to ensure chestnut wood of good quality (Manetti et al., 2006). Indeed, only assortments of small dimensions and poor quality were obtained in the present study with short rotation periods. Nowadays, commercial operators and the timber market in general often require assortments of good wood quality, obtainable from the chestnut coppices by lengthening the rotation period currently adopted in most chestnut coppices to some extent (Angelini et al., 2013; Mattioli et al., 2016; Manetti et al., 2017). Extended rotation periods (in the range of 30–50 years) would also positively affect the provisioning of ecosystem services related to environmental issues (Gondard and Romane, 2005; Gondard et al., 2006).
Thinning would allow a greater average DBH of shoots to be obtained at earlier growth stages, with consequent differentiation of assortments (Mattioli et al., 2016). However, many studies have shown that marked increments in stem diameter after intensive thinning might induce less-stiff mature wood, resulting in a significant loss of wood quality at high DBH (Zhang, 1995; Ikeda et al., 2000; Wang et al., 2003; Štefancík et al., 2018). Marini et al. (2021) found a negative correlation between wood density and dominant height and diametric growth of chestnut coppice stands, and this was associated to tree ring width. Should stand density, i.e., the number of shoots per ha, be a factor affecting positively wood density and the related mechanical properties, the application of low to moderate thinning might favour the formation of wood-based products of good quality in chestnut coppices. Therefore, particular care should be considered when thinning is planned and executed, avoiding both strong intensity and late thinning. By modifying competition (the number of shoots per ha) and, thus, shoot diameter growth and stand basal area, thinning might increase the risk of ring shake (Fonti et al., 2002; Becagli et al., 2006; Romagnoli et al., 2014). All these negative effects would limit the use of chestnut wood for the most valuable assortments, i.e., those usable as structural elements (Fonti et al., 2002).