Assessment of various assisted migration approaches to determine the highest performance in Pinus halepensis seed sources

Assisted population migration in new forest plantations can enhance future success by using seed sources that are more adapted to future climate than the local seed source. It is particularly interesting in the Spanish Mediterranean, where the number of protective plantations with Pinus halepensis has increased. However, there is a lack of operational information on these practices. The aim of this work is to improve this gap and provide information to forest managers on how to accomplish this new practice. To this end, the response of six plantations in terms of growth and survival has been evaluated. These six sites are heterogeneous regarding: the seed source planted (between 7 and 11 different ones), the bioclimate (aridity gradient), and the period analysed (from one to 13 years). Twelve seed source selection approaches have been proposed and tested using these sites. The local-use approach performed well overall (54 % reliability) but showed 25 % less medium-term survival than the average. The genetic improvement approach achieved the highest growth (+13 %), although survival gains were limited (+2 %). The drought-duration approach showed the highest reliability (58 %) and favourable outcomes where differences appeared. In contrast, the use of population distribution models as an approach to select the suitable seed source, showed poor average reliability under climate change scenarios, but not for except the historical climate, and did not correlated to plantation performance. In conclusion, to improve the future performance of P. halepensis plantations, it is recommended to review traditional silvicultural practices and consider the use of seed sources from genetic improvement or from areas with longer drought periods than the target area.