Seedling Establishment of Subalpine Stone Pine (Pinus pumila) by Nutcracker (Nucifraga) Seed Dispersal on Mt. Yumori, Northern Japan

Abstract

Seedling establishment of Japanese stone pine, Pinus pumila, primarily depends on seed dispersal by Japanese nutcrackers (Nucifraga caryocatactes var. japonica). To assess how this mode of seed dispersal affects regeneration and habitat expansion of P. pumila, we made field observations concerning foraging behavior of nutcrackers, past annual production of pine cones, and age structure of established pine seedlings. Nutcrackers used various microsites for feeding and removing the pine seeds, especially on bare ground or around rocks. The pine seedlings occurred mainly in open patches sparsely covered with dwarf woody shrubs in wind-exposed sites, suggested that the nutcrackers preferred such open patches for caching. Age structure of the seedlings (<20 yr old) showed a clumped age distribution. Most of the younger pine seedlings (<10 yr old) grew as clusters, averaging four to five in number per cluster. Past cone production, estimated using a cone scar technique, varied annually, with some remarkable poor-crop years during the recent 12 yr. However, extensive recruitment of the pine seedlings did not necessarily follow a mast year. Fluctuation of the seedling recruitment was closely related to annual changes in early summer precipitation rather than air-temperature, indicating positive effect of water supply on seed germination. Our findings suggested that regeneration success of P. pumila was limited for two reasons: (1) spatially due to site preference of nutcrackers for seed caching, i.e., open patches at windy sites near the seed source of mature pine scrubs; and (2) temporally by annual fluctuation in climatic factors, especially soil moisture availability. Consequently, nutcrackers do not seem to help the pines expand more widely in the Japanese subalpine region.