Tree ring and water deficit indices as indicators of drought impact on black truffle production in Spain

Black truffle is a highly-prized fungus that constitutes an important economic resource in many Mediterranean forests of southern Europe. Black truffle production in these forests and in cultivated truffle orchards is subject to important interannual variations linked to climatic factors, among which summer water shortage is particularly relevant in dry regions such as Mediterranean Spain. Thus, in recent years irrigation has become widespread in Spanish black truffle orchards, although no long-term proxies of black truffle production exist and scientific guidelines for the irrigation of these orchards are lacking. In this study, we explore the possibilities and limitations of pine tree-ring width data (seasonal wood production) and agroclimatic indices to predict black truffle production in Spain, while gaining more in-depth knowledge on water requirements of truffle production. For this purpose, we used the national annual estimates of black truffle production and compared these values with several tree-ring width series, monthly climatic factors, and water balance-based indices. We found a strong linkage between summer water availability, black truffle production and Mediterranean pine latewood formation, with summer drought intensity reducing both black truffle and Pinus pinaster latewood production. The adjusted latewood index for P. pinaster and the accumulated water deficit from April to September were robust predictors of black truffle production. The predictive skills of these two variables were further confirmed with a calibration-verification technique. These variables can be used as climatic proxies of black truffle production in agronomic or silvicultural studies, in a sector where no large-scale network for monitoring soil water content exists, and many truffle orchards are small and have low possibilities for modernisation.