Seasonal Precipitation and Continentality Drive Bimodal Growth of Gymnosperm Trees and Shrubs in the Mediterranean
Tree phenology is sensitive to climate warming and to changes in seasonal precipitation. Long xylogenesis records are scarce, thus limiting our ability to analyse how radial growth responds to climate variability. Alternatively, process-based growth models can be used to simulate intra-annual growth dynamics and to better understand why growth bimodality varies along temperature and precipitation gradients. We used the Vaganov-Shashkin (VS) growth model to analyse the main climatic drivers of bimodality of eight conifers (four pines and four junipers) inhabiting 11 sites across Spain. We selected sites with different continentality degree and spring/autumn precipitation ratios since we expected to find pronounced bimodal growth in less continental sites with spring and autumn precipitation peaks. The VS model predicted annual growth rates (ring-width indices) at all sites as a function of daily temperature and soil moisture data. Bimodal growth patterns clustered into less continental sites showing low spring/autumn precipitation ratios. This finding agrees with observed climate-growth associations showing that growth was enhanced by wet-cool prior winter to current spring conditions, but also by wet autumn conditions in the most bimodal sites. We observed stronger tendency of pine species for growth bimodality compared to junipers. We discuss the spatial variability of climate drivers in bimodality growth pattern and how increasing continentality and shifts in seasonal precipitation could affect growth. Bimodality could be an advantageous response to overcome summer drought in Mediterranean forests, and the ability of individual species to reactivate growth during autumn after summer quiescence might determine their adaptability to increasing summer drying of the Mediterranean region.