Clarification of Factors Inhibiting Regeneration of Yezo Spruce (Picea jezoensis) at the Initial Stage of Natural Regeneration

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Investigations were conducted on factors that inhibit the natural regeneration of Yezo spruce (Picea jezoensis): fungi attacking seeds when covered with snow; changes in the water content of the soil layers which might damage seeds by dehydration during the period from germination to rooting; the relationship between the amount of incident light and growth rate during the growth period of young seedllings, etc. Racodium therryanum was indentified as a fungus which damages Yezo spruce seeds; this fungus was found mostly in the L, F and H layers (e.g., L layer infection rate - 70%); in the A layer, the quantity of the fungi decreased with soil depth. At a temperature of 0゜C, the percentage of healthy seeds became zero in 90 days. Measurements of the water content in the soil layers over two years showed that it fluctuated widely in the uppermost layer, L, ranging from 20 to 70%; especially, in June during germination, it was at 20%, extremely dry. Thus, it was concluded that an excessively dry L layer was an important factor, along with the fungi damage, that inhibited seed regeneration. The relatonship between the growth rate and the amount of light incident on young seedlings during their growth period was as follows: the growth was 2.6cm under an incident light level of 43 KWH/m^2 (17.5% of unobstructed sky) and 1.9cm at 21 KWH/m^2.