An intra-stand approach to identify intra-annual growth responses to climate in Pinus nigra subsp. laricio Poiret trees from southern Italy

The growth of Pinus nigra tree stands is known to be limited by spring-summer precipitation (P). We explored the intra-annual growth dynamics (early-wood EW and late-wood LW of tree-rings) and their responses to climate (in monthly, seasonal and annual scale) in Pinus nigra subsp. laricio at the intra-stand level in Calabria, at the pines' mesic to xeric distribution range. We used a variety of age detrending methods to assess how the adaptive potential to climate change of each tree varies within the even-aged forest stand. In years of wet climate, when precipitation (P) could infiltrate deeper below ground, higher growth rates occurred in 83% of trees, best explained by P accumulated over several previous years. The variability of EW increment was best explained by 3-5 previous year P (including the growth year) in 61% of trees, while LW increment was best explained by 1-3 year P in 78% of trees. This would suggest that in wet years most trees utilized not only surface but also deeper moisture pools using their taproot to produce both EW and LW. In contrast, during dry years, for 39% of trees the most significant predictor for EW was June rainfall. August P explained LW variability in 35% of the trees, while the influence of 1-3 year P on LW was reduced to 48%. Thus, under a drier climate ca. 1/3 of the trees within the stand significantly reduced their capacity to utilize deeper ground moisture, indicating higher vulnerability to drought stress. Multiple-year P appeared as the main climatic driver for growth in most trees, but only became evident through age detrending methods retaining low frequency growth variability. Our findings are the first to provide such insight into the wide spectrum of climatic factors that may drive P. laricio's inter-stand and inter-annual productivity. They also assist to identify the most vulnerable trees to drought stress within a forest stand. Such information could prove very useful in the application of silvicultural treatments (e.g., selective thinning) aiming to increase the resilience of tree stands to future drought intensification.