Extreme climate conditions limit seed availability to successfully attain natural regeneration of Pinus pinaster in sandy areas of central Spain

Natural regeneration comprises different subprocesses, each of them driven by specific climatic and stand-related factors, which determine the success of natural regeneration. The objective of this study was to investigate the seed availability of maritime pine (Pinus pinaster Aiton). To meet this objective, seed rain was monitored for four different levels of stand density at the experimental site of Cuellar, Spain, during a 10-year period. A generalized linear mixed-effects model was fitted to test the effects of climatic variables and stand density on the annual seed production and seed rain. The climatic covariates were chosen among those that are thought to affect the key physiological phases governing these subprocesses: minimum temperature in October 2 years before dispersal (cone growing), April precipitation 1 year before dispersal (cone growing), and October-November precipitation 1 year before dispersal (cone maturation). No climate variable related to flowering or seed rain process was significant. Moreover, stand density was considered through a spatially explicit index called the seed-source index. Primary cone growth was limited by extreme cold events. Absence of precipitation limits secondary growth and hinders final cone ripening. It turns out that seed production and seed rain may be a bottleneck for natural regeneration of P. pinaster under low stand densities, especially under extreme climatic scenarios.