Vegetation recovery on Mount Koma, Hokkaido has been slow after the catastrophic eruption in 1929. The seedling establishment is supported by Salix reinii, facilitates the plant colonization. Moreover, the size effects have not been examined well. To examine the size effects, field experiments on two pioneer plants, Miscanthus sinensis and Polygonum sachalinense grown in the disturbed area, were conducted. Lab-experiments were also performed to characterize the seed germination and seedling growth of these two herb species. The field experiments confirmed that large patches promoted the seedling germination of the two species. On the lab-experiments, Reducing temperature did not seem to affect germination and growth of both species while shading, optimum litter amount, and wet substrate were synchronized to the seed germination but affected differently to the seedling growth. In conclusion, these results indicated that the large patches facilitated more to the colonization of pioneer plants via seed germination and growth. Large patches acted as a nursery supporting the natural regeneration in the disturbed area by maintaining soil water, reducing strong light and/or protecting from overheating.