Seedling establishment of subalpine stone pine (Pinus pumila) by nutcracker (Nucifraga) seed dispersal on Mt. Yumori, northern Japan

Seedling establishment of Japanese stone pine, Pinus pumila, primarily depe nds 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 mode held observations concerning fo raging behavior of nutcrackers, past annual production of pine cones, and a ge structure of established pine seedlings. Nutcrackers used various micros ites for feeding and removing the pine seeds, especially on bare ground or around rocks. The pine seedlings occurred mainly in open patches sparsely c overed with dwarf woody shrubs in wind-exposed sites, suggested that the nu tcrackers preferred such open patches for caching. Age structure of the see dlings (<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 num ber per cluster. Past cone production, estimated using a cone scar techniqu e, varied annually, with some remarkable poor-crop years during the recent 12 yr. However, extensive recruitment of the pine seedlings did not necessa rily follow a mast year. Fluctuation of the seedling recruitment was closel y related to annual changes in early summer precipitation rather than air-t emperature, 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 se ed caching, i.e., open patches at windy sites near the seed source of matur e pine scrubs; and (2) temporally by annual fluctuation in climatic factors , especially soil moisture availability. Consequently, nutcrackers do not s eem to help the pines expand more widely in the Japanese subalpine region.