Effetti delle variazioni climatiche e dell'impatto antropico su crescita e livelli di discriminazione isotopica del carbonio in una rete di faggete italiane
Author(s)
Date Issued
May 27, 2013
Type
Doctoral Thesis
Abstract
Tree-ring networks are effective tools to investigate how climate modulates tree growth variability along latitudinal/elevational transects. Multivariate classification/ordination approaches have already shown (Piovesan et al. 2005; Di Filippo et al. 2007) that in Italian beech populations are organized in distinct bioclimatic units. In the latitudinal sense we can distinguish 2 Bioclimatic Zones : the Temperate and the Mediterranean Zone, whose boundary falls on the Northern Apennines and coincides with the limit of Summer drought control on beech growth. In the Temperate Zone drought is absent and instead low temperatures become growth limiting. Each zone can be further divided into 3 Elevation Belts (low-elevation, mountain and high-mountain;), representing modulations with elevation of the dominant zone signal.
To investigate long-term growth trends and water use efficiency dynamics of mature beech forests in Italy according to their bioclimate position, naturalness degree and climate change,30 beech populations sampled (>20 trees cored/site) at different positions and elevations in Italy. Crossdated tree-ring series converted to BAI series using their current diameter at breast height (DBH).Raw site chronology building (representing long-term stand productivity). Bootstrapped correlation functions served to detect the main climatic factors limiting growth at the annual timescale (data not shown). Multidecadal stand productivity variations were obtained by interpolating 50-yr cubic smoothing splines to raw BAI chronologies. The main climate factors were equally filtered to check their long-term impact on BAI trends.Carbon isotope discrimination analysis was used to detect possible changes in water stress among beech sites with time.
The results showed deep differences in climatic constraints that are applied to the beech population along the distribution range of beech. In central Italy declining trends were observed at all elevations after BAI culmination between end of the 60s and mid-80s (Fig. 4). BAI culmination occurred in a cooler/wetter period (circa 1960-1970) and then followed the drying/warming trend began in the 70s. Productivity loss was the norm at low to mid elevations (up to 1400 m asl), ranging 20-50% in average BAI decrease (Fig. 6). In high-mountain stands the responses didn't converge, with unmanaged stands tendentially responding positively to warming.
The naturalness degree controlled how beech stands responded to climate changes. Old-growth stands followed more closely climate variations of the growing season. Managed stands peaked later (decreased water competition after logging), and at low-elevation suffered greater BAI losses in the recent dry period.
In the eastern Alps increasing trends were observed at all elevations (Fig. 5). BAI increase followed the January-September warming which started at end 70s. Old-growth stands followed more closely the climate variations. Growth trends maintained sustained increasing rhytms, with the exception of low-elevation sites, which are slowing down their productivity rates due to the recent emergence of July drought effects. We found no significant relationship between BAI and carbon discrimination in the Alpine beech populations (Fig. 7).
In Central Italy emerged a negative significant relationship between previous year BAI and carbon discrimination of the following year. We hypothesize a preconditioning effect of previous year BAI on current LAI (through a reserve effect on bud formation), which could favour tree predisposition to drought stress.
When comparing BAI and 13C in old-growth beech forests growing at different elevations in Central Italy (Fig. 8), we found that the high-mountain stand with stable BAI trends maintained a constant isotope discrimination; the lower elevation stand with decreasing BAI trend, instead experienced a parallel decrease in carbon isotope discrimination.
2
Diverging growth trends emerged between the Alps and Central Italy following recent climate changes. While in Central Italy the warming corresponded to increasing drought stress (reducing productivity), on the Alps it corresponded to more favourable growing season conditions and less cold Winters (increasing productivity). In Central Italy decreasing growth trends corresponded to decreases in carbon isotope discrimination. The naturalness degree controlled how beech stands responded to climate changes, with old-growth stands followed more closely climate variations.
Additional information
Dottorato di ricerca in Scienze e tecnologie per la gestione forestale e ambientale
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