High frequency stable isotope signals as proxy for physiological responses to climate - Dual isotope approach at a European scale

Picea abies and Fagus sylvatica, are two of the most important tree species in Europe, and their responses to climate are being extensively investigated, especially at the limits of their distribution. However, their physiology at temperate sites is not yet fully understood. In a European tree-ring network, 10 sites along a climate gradient were sampled throughout Central Europe, and tree-ring width and stable isotope chronologies (C and O) were measured. The year-to-year variability of the isotopes time series for the last 100 years was analyzed in relation to tree-ring growth, spatial distribution, and seasonal climate.

Climate sensitivity of radial growth of both species was rather variable and site-dependent, and was strongest at the driest sites. On the contrary, variability in the isotopic ratios consistently responded to summer climate, particularly to vapor pressure deficit. The high δ¹⁸O coherence of the short-term variability between sites and species highlights the strength of the environmental signal in the O chronology also across long distances. On the contrary, δ¹³C shows lower correlations between sites and species, showing a stronger site-dependency, and a lower intra-annual variability. The generally positive correlation between the year-to-year differences in δ¹³C and δ¹⁸O across most sites demonstrates the strong role of stomatal conductance in controlling leaf gas exchange for these species. However, in the last decades, sites showed a dissimilar shift in the isotopes relationships, with the warmer sites showing an increase of either or both δ¹³C and δ¹⁸O and consequent decrease of photosynthetic rates and stomatal conductance, highlighting their dependency to atmospheric moisture demand and soil water availability.

Understanding the underlying physiological mechanisms controlling the short-term variation in tree-ring records will help with defining the performance of these ecologically and economically important tree species under future climate conditions.