Variable seed behavior increases recruitment success of a hardwood tree, Zelkova serrata, in spatially heterogeneous forest environments

Variable seed behavior (e.g., seed heteromorphism) plays an important role in enhancing plant fitness by spreading risk in spatially and temporally changing environments. This adaptation is common in herbs but rare in trees. Zelkova serrata, one of the most important commercial tree species in Japan, has two types of seeds: shoot seeds that disperse via a seed-bearing shoot and single seeds that disseminate after separating from shoots. To evaluate whether the seeds of Z. serrata show heteromorphism, we compared several inherent ecological properties (e.g., seed mass, degree of seed dormancy, and dispersal ability) between the two seed types and assessed the effects of variable seed behavior on recruitment success in forest environments. The degree of seed dormancy did not differ between the two seed types. Seed mass increased with a delayed seed dispersal time but did not differ between the two types at any time. Seed dispersal ability was greater in shoot seeds than in single seeds. These inherent seed behaviors suggest that the two types of seeds do not reflect seed heteromorphism. The results, however, indicate that shoot seeds dispersed larger seeds over much greater distances than did single seeds, particularly at later seed dispersal times, whereas a greater number of single seeds were disseminated near adults. To evaluate the effects of variable seed dispersal ability on seedling establishment in forest environments, we measured seedling performance at different distances (≤27.5 m) from adults isolated from one another in the forest understory. Seedling density, survival, and growth did not vary with distance from adults (i.e., there was little negative distance-dependent seedling mortality), which suggests that single seeds can contribute to persistent seedling banks near conspecific adults. To evaluate the advantages of long-distance dispersal of shoot seeds for recruitment on disturbed sites (i.e., gaps), we compared the occurrence and relative abundance of Z. serrata among 1100 plots with different slopes; steep slopes are closely associated with gap creation (e.g., landslides). The occurrence and relative abundance of Z. serrata increased with slope inclination, which suggests that shoot seeds may enhance recruitment success in disturbed sites along steep slopes, thereby generating Z. serrata-dominated forests. This suggests that the variability in seed behavior of Z. serrata may enhance recruitment success in spatially heterogeneous forest environments. Knowledge of the unique reproductive behaviors and the wide range of habitats suitable for this species is important for forest managers when making sustainable timber production plans for Z. serrata.