Start of the dry season as a main determinant of inter-annual mediterranean forest production variations

Recent investigations have highlighted the dependence of Mediterranean forest production on springrainfall. The current work introduces the concept of the start of the dry season (SDS) and performs athree-step analysis to determine the effect of SDS on Mediterranean forest production.Seven forest zones of Tuscany (Central Italy), which present differently pronouncedMediterranean features, are considered. First, a statistical analysis investigates theinfluence of spring water budget on forest Normalized Difference Vegetation Index(NDVI) inter-annual variations during July–August. The analysis is then extended toassess the impact of inter-annual SDS variability on forest gross primary production (GPP) simu-lated by a NDVI driven parametric model, modified C-Fix. These simulations lead to rank the consideredforest types according to the relevance of SDS in regulating inter-annual GPP variations. The applicationof similar statistical analyses to detrended tree ring-width time series of typical Tuscany forests confirmsthe existence of an eco-climatic gradient in the functional relevance of SDS. The influence of SDS ontree growth is attenuated moving from Mediterranean arid to temperate humid environments. Thesefindings are examined and interpreted from an eco-physiological viewpoint taking into considerationthe peculiarity of Mediterranean forest ecosystems. Next, relevant implications are discussed in view ofthe possible consequences of ongoing climate change.