Forty years of growth-climate relationships in a progeny test of Pinus pinaster in Sardinia

The area of the Mediterranean basin is expected to be threatened by more severe and prolonged droughts and heat waves. Therefore, a more exhaustive knowledge about growth-climate responses in forest trees is necessary in order to adopt mitigation and adaptation strategies in forest management and planning. Climate change will cause shifts of the climate envelope for tree species, potentially leading to migration of species distribution. Under these circumstances, investigations on growth-climate relationships in trees of different provenances of the same species are important for the success of climate-smart forestry. Provenance trials are useful for understanding the response of this species to drought stress. We studied growth-climate relationships in 40-year-old trees of maritime pine (Pinus pinaster Ait.) from five provenances (Corsica, Portugal, Tuscany, and two native ones: Telti and Limbara) grown on four different sites in Sardinia island (Montes, Montarbu, Uatzo and Usinavà), Italy. In details, for all trees in each site, measurements of stem diameter at breast height (DBH) and plant height (H) were collected. For each site-provenance combination, two incremental cores were collected for each tree; successively, samples were cross-dated and standardized. Weather data (temperature and precipitation) were collected from CRU data online (http://www.cru.uea.ac.uk/). Differences in DBH and H were found among sites. In particular, the highest values for DBH and H were found in Montes and Uatzo, respectively. Instead, Montarbu showed the lowest mean values for both parameters. Differences among provenances were also observed. Specifically, in Montarbu, the greatest H were found for Tuscany and the lowest for Corsica (p<0.0001). The same pattern was also found for DBH, but without statistical significance (p>0.5). In Uatzo, Corsica showed the highest mean values for both H and DBH, while the lowest DBH was observed for Tuscany (p=0.0008), and the lowest H was found for Limbara (p<0.0001) provenance, respectively. No significant differences were found for both H and DBH in Montes. Finally, in Usinavà, Limbara showed significant higher values, for both H and DBH, compared to the other provenances (p<0.001). Temperature had a greater influence on growth traits in Montarbu, especially for spring-summer period, with Telti and Tuscany having the most significant correlation. Precipitation, instead, mostly affected Usinavà. On the other hand, in Montes and Uatzo, no significant correlations between climate and growth were observed. However, different climate-growth relationships were observed among provenances. In conclusion, our results suggest that, after 40 years of growth, greater H and DBH were found in the sites with lower temperature and higher precipitation (Uatzo and Montes). Interestingly, in Uatzo, Corsica showed the highest values of both DBH and H, while Limbara presented the lowest growth. Noteworthy, Limbara showed greater H in Usinavà (the warmest and driest site), whereas in the previous survey, Limbara had the lowest H. These results represent a further step in identifying potential genetic variation in tree growth and drought tolerance of maritime pine in Mediterranean conditions. Data collected in long-term experimental plots and repeated measurements are confirmed of fundamental importance to estimate the resilience of forest species to climatic changes.