Geographic cline and climatic effects on cone characteristics of natural populations of Pinus densiflora throughout the Japanese archipelago

Major conifer species with a widespread geographic distribution are growing under diverse climatic conditions. For such species, populations in regions with low resource availability, where producing additional cones is costly, presumably have more individuals that produce larger, highly viable cones with a higher resource-use efficiency. Geographic variations in cone characteristics (cone and seed size, seed production and productivity per cone) were examined for 28 Pinus densiflora natural populations (628 trees) across the species’ distribution in Japan. The degree of phenotypic differentiation across populations (PST) varied among the ten characteristics investigated. The average PST value (0.139) was higher than the previously reported neutral genetic variation of the species (FST = 0.013). The more northeastern populations produced cones of larger size (in length, width and mass), had higher seed production (number of filled seeds, seed mass per cone) and had higher seed productivity (indicators for seed production per cone development cost). A generalized linear mixed model showed that geographic variation in cone size and seed productivity per cone of populations was correlated negatively with annual solar radiation and mean temperature. The larger cone size and higher seed productivity per cone observed in northeastern populations support the hypothesis of higher resource-use efficiency, and could result from a genetic or plastic response under lower temperature and less radiation.