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The ash species Fraxinus platypoda is the dominant canopy tree species at Ooyamazawa riparian forest. I investigated flowering, seed production, germination, seedling survival and growth, and structural measures in F. platypoda. Flowering and seed production demonstrated a clear masting pattern over a 28-year period. The seeds of F. platypoda germinated in most environments, but seedling survival was regulated by microtopographic factors. I suggest that gap formation processes may be vital to the establishment of canopy-height individuals. The overall forest structure and spatial distribution of F. platypoda suggested that all individuals of this species occurring at Ooyamazawa regenerated simultaneously following a large-scale disturbance event 200 years ago. Since this event occurred, saplings of F. platypoda have regenerated in canopy gaps. It is probable that F. platypoda succeeds as a dominant species in riparian forests by regenerating in response to disturbance at multiple scales.
KeywordsAdvanced sapling Dioecy Disturbance regime Flowering Germination Life history Microtopography Reproductive strategy Seed production Seedling
Native riparian forests are distributed along the Ooyamazawa stream in the Chichibu Mountains of the Kanto region, central Japan. In these forests, Fraxinus platypoda Oliv. (Oleaceae) is one of the dominant canopy tree species, coexisting with Pterocarya rhoifolia and Cercidiphyllum japonicum. The Chichibu Mountains harbor a very complex topography characterized by steep slopes, with tree diversity distributed among the resulting microhabitats. For example, Tsuga sieboldii and Chamaecyparis obtusa forests are distributed along ridges, Fagus crenata and Fagus japonica on mountain slopes, and F. platypoda, P. rhoifolia, and C. japonicum in valleys (Maeda and Yoshioka 1952; Tanaka 1985). The regeneration mechanisms of T. sieboldii, F. crenata, and F. japonica have been extensively studied by Suzuki (1979, 1980, 1981a, b), Nakashizuka and Numata (1982a, b) and Nakashizuka (1983, 1984a, b), and Ohkubo et al. (1988, 1996), respectively. These researchers examined forest regeneration within the context of gap dynamics theory. Gaps are open spaces within the forest canopy layer formed by die back, trunk breakage, and uprooted trees. In general, gaps occur on a small scale and are not often accompanied by soil disturbance, with the exception of uprooted trees.
On the other hand, disturbances within riparian zones vary in type, frequency, magnitude, and size compared to canopy gap formation on hillslopes. In steeper mountain regions, valley floor landforms are sculpted by fluvial processes and a variety of mass soil movement processes from tributaries and adjacent hillslopes (Gregory et al. 1991).
2.2 Study Species
2.3 Reproductive Traits
The timing of flowering in F. platypoda differs depending on altitude but, typically, occurs in mid-April (700 m a.s.l) to mid-May (1500 m a.s.l.). In the Ooyamazawa riparian forest (1500 m a.s.l.), flowering occurs from the beginning of May to mid-May. However, the exact timing fluctuates annually.
2.3.2 Seed Production
2.5 Seedling Survival
In other words, the distribution of F. platypoda seedlings is regulated by the microtopographic variation in forest floor vegetation. In riparian forests, the light environment depends not only on the presence of canopy gaps but also on gaps in the forest floor vegetation due to stream disturbance. The dynamics of F. platypoda seedlings are thought to be strongly influenced by the latter.
2.6 Seedling Growth
2.7 Forest Structure and Spatial Distribution
Fraxinus platypoda produces a large number of seeds once every few years, thus forming young advanced sapling communities within various microtopographic habitats. In particular, advanced sapling communities are formed in gravel deposits along mountain streams. These sapling communities continue to be regenerated after repeated destruction by mountain stream disturbances and subsequent regeneration on new gravel deposits. After large disturbances, F. platypoda regenerates within all river basins and also fills canopy gaps by advanced saplings when small gaps are formed. Thus, F. platypoda succeeds as a dominant species in riparian forests by regenerating in response to various scales of disturbances throughout its life history.
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