Effects of canopy conditions on the regeneration of major tree species in an old-growth Chamaecyparis obtusa forest in central Japan

Abstract

The regeneration mode of major tree species ( Chamaecyparis obtusa (Sieb. et Zucc.) Endl., Thujopsis dolabrata Sieb. et Zucc., Chamaecyparis pisifera (Sieb. et Zucc.) Endl., Quercus mongolica Fischer ex Turcz., Magnolia obovata Thunb. and Betula grossa Sieb. et Zucc.) in relation to four canopy conditions (closed evergreen conifer canopy, closed evergreen conifer canopy adjacent to canopy gap (gap-adjacent), canopy gap, and closed deciduous broad-leaved tree (DBL) species canopy) was studied from 1988 to 1998 in a 4-ha permanent plot in the Akasawa Forest Reserve, central Japan. The vertical distribution of stems and the major species composition in four height (canopy; ≥25 m tall overtopping neighboring trees, subcanopy; ≥25 m tall, middle; ≥10 m tall, and understory; <10 m tall) classes varied with canopy condition. The understory and middle layers were generally dominated by T. dolabrata, but this species in the subcanopy layer existed only under gap-adjacent. Canopy gaps were usually filled by DBL tree species, especially M. obovata and less often by T. dolabrata, while the probability that they were filled by Chamaecyparis sp. increased in larger gaps. C. obtusa and C. pisifera occurred more often under closed DBL canopy, T. dolabrata under gap-adjacent canopy, and Q. mongolica, M. obovata and B. grossa under canopy gaps. The mortality and recruitment rates of understory stems of major tree species also varied with canopy condition. Most mortality resulted from suppression with standing-dead and natural disturbances with broken trunks, although the relative importance varied with canopy condition and species. Generally, evergreen conifers died as a result of natural disturbances, especially in the 1998 frost disturbance under canopy conditions other than closed evergreen conifer canopy. In conclusion, canopy conditions affect the vertical population structure, demographic parameters, and causes of tree mortality of major tree species. Of the favored regeneration sites in this old-growth C. obtusa forest, C. obtusa and C. pisifera generally may regenerate under closed DBL canopy, T. dolabrata under gap-adjacent canopy, and Q. mongolica, M. obovata, and B. grossa under canopy gaps. Consequently, this semi-natural old-growth C. obtusa forest will gradually change into a more diverse forest.