GAP CHARACTERISTICS AND GAP REGENERATION IN A SUB-ALPINE CONIFEROUS FOREST ON MT ONTAKE, CENTRAL HONSHU, JAPAN

Gap characteristics and gap regeneration were studied in three mature stands belonging to different community types in a subalpine coniferous forest on Mt Ontake, central Honshu, Japan. Gap disturbance regimes were remarkably similar among stands studied; percentage gap area to surveyed area, gap density and mean gap size were 7.3-8.5%, 17.8-20.0 ha(-1) and 40.8-42.5 m(2), respectively. The gap size class distributions were also similar and showed a strong positive skewness with a few large and many small gaps; gaps < 40 m(2) were most frequent and gaps > 200 m(2) were rare. Forty-five to 66% of gaps were due to the death of single canopy trees. Canopy trees more often died leaving standing dead wood (40-57%) or broken trunks (43-49%). Shade tolerant Abies mariesii and Abies veitchii, frequently, and Tsuga diversifolia, less frequently, regenerated in gaps, from advance regenerations recruited before gap formation. Picea jezoensis var, hondoensis may regenerate in gaps, from new individuals recruited after gap formation. The breakage of dense Sasa coverage and the mineral soils exposed by the uprooted plants that form gaps might provide regeneration opportunities for shade intolerant Betula. Of the important species limited to the ridge site, Chamaecyparis obtusa, frequently, and Thuja standishii, infrequently, regenerated from plants already in situ. Regeneration of Pinus parviflora was not seen. Based on the gap characteristics and gap regeneration behaviour of each species described, stand dynamics in each stand are discussed.