Hitoshi Sakio

Molecular data and ecological niche modelling reveal a highly dynamic evolutionary history of the East Asian Tertiary relict Cercidiphyllum (Cercidiphyllaceae)

East Asias temperate deciduous forests served as sanctuary for Tertiary relict trees, but their ages and response to past climate change remain largely unknown. To address this issue, we elucidated the evolutionary and population demographic history of Cercdiphyllum, comprising species in China/Japan (Cercdiphyllum japonicum) and central Japan (Cercdiphyllum magnificum). Fifty-three populations were genotyped using chloroplast and ribosomal DNA sequences and microsatellite loci to assess molecular structure and diversity in relation to past (Last Glacial Maximum) and present distributions based on ecological niche modelling. Late Tertiary climate cooling was reflected in a relatively recent speciation event, dated at the Mio-/Pliocene boundary. During glacials, the warm-temperate C. japonicum experienced massive habitat losses in some areas (north-central China/north Japan) but increases in others (southwest/-east China, East China Sea landbridge, south Japan). In China, the Sichuan Basin and/or the middle-Yangtze were source areas of postglacial northward recolonization; in Japan, this may have been facilitated through introgressive hybridization with the cool-temperate C. magnificum. Our findings challenge the notion of relative evolutionary and demographic stability of Tertiary relict trees, and may serve as a guideline for assessing the impact of Neogene climate change on the evolution and distribution of East Asian temperate plants.

Effects of natural disturbance on the regeneration of riparian forests in a Chichibu Mountains, central Japan

Disturbances in the riparian area had a large effect on each stage of regeneration in riparian forests dominated by Fraxinus platypoda in the Chichibu Mountains, central Japan. F. platypoda adapted well to various disturbances in frequency and size such as landslides and canopy gap formation.The spatial distribution and age structure of F. platypoda were studied in relation to the disturbance regime of the riparian zone. The bell-shaped size and age distributions of F. platypoda suggest that F. platypoda trees were established synchronously in a large disturbance site caused by an earthquake landslide about 200 years ago. For the past 200 years, the topography has been stable and the canopy gap has been recovered by advance regenerated saplings.The distribution of F. platypoda saplings was restricted to an abandoned channel and part of a floodplain. These topographical sites formed by gravel provided safe sites for saplings because stream disturbances did not occur for a long time. Channel bars were under low shade stress because of the lack of herbs and a litter layer, which represents a safe site for seedling establishment. However, the seedling bank could have been destroyed by high frequent flooding caused by large typhoons and the establishment of seedlings might have been prevented for a long time.The regeneration process of F. platypoda was explicated based on the gap dynamics theory during the stable period of topography. On the other hand, an even-aged forest was established in a large scale disturbance site.Nomenclature: Ohwi & Kitagawa (1992)

Coexistence of three canopy tree species in a riparian forest in the Chichibu Mountains, central Japan

Three canopy tree species (Fraxinus platypoda, Pterocarya rhoifolia, andCercidiphyllum japonicum) coexist in riparian forests in the Chichibu Mountains of central Japan. We compared the forest structure and the reproductive characteristics of these species.F. platypoda was the dominant canopy species. It produced many saplings and grew in abandoned channels and floodplains, and was able to invade both large and small disturbance sites.P. rhoifolia was a subdominant species that occurred on the deposits of large-scale landslides and grew in patches containing even-aged trees.C. japonicum was the other subdominant species that produced few saplings and invaded large disturbance sites together withP. rhoifolia. Establishment sites ofC. japonicum were restricted to fine mineral soils and fallen logs. We found tradeoffs in reproductive characteristics (seed size, seed number, irregular seed production, and sprouting) among the three canopy species.F. platypoda andP. rhoifolia had large seeds and produced fruits irregularly.C. japonicum produced many small seeds every year and sprouted prolifically around the main stem. The causes of the coexistence mechanism of the three riparian canopy tree species may be both niche- and chance-determined to varying degrees. In riparian areas, the three canopy species were well-adapted to disturbances throughout their life-history.

Effects of flooding on growth of seedlings of woody riparian species

The effects of flooding on growth of seedlings were compared over a 7-month period (April–November) among six different woody species: Aesculus turbinata, Cercidiphyllum japonicum, Fraxinus platypoda, Pterocarya rhoifolia, Pterostyrax hispida, and Quercus mongolica var. grosseserrata. Flooding reduced the shoot length of F. platypoda, P. rhoifolia, C. japonicum, P. hispida, and Q. mongolica var. grosseserrata seedlings but did not affect that of A. turbinata seedlings. Among control seedlings, shoot elongation occurred once in A. turbinata and twice in F. platypoda and Q. mongolica var. grosseserrata; the other species continued to grow from April to August. Among the flooded plants of all species, shoot elongation occurred only once at the beginning of the growing season. On August 25, flooding significantly reduced the number of developed leaves as compared with control plants except for A. turbinata. In the flooded plants except for F. platypoda, leaf fall began on June 30; in controls, by contrast, the number of developed leaves increased until August 25. Flooding reduced the total dry weight increment in all species. The survival ratio of flooded plants after the experiment differed with species. All of the F. platypoda and A. turbinata seedlings survived the flooding treatment, while only 20% of P. hispida and 30% of Q. mongolica var. grosseserrata survived. Flooding seriously affected the growth of riparian pioneer species including P. rhoifolia, C. japonicum, P. hispida, and Q. mongolica var. grosseserrata. The effects of flooding on growth of the seedlings differed with the tree species because of differences in leaf-emergence pattern and physiological flood tolerance. The responses of tree seedlings to flooding reflected species habitats and growth patterns.

FACTORS INFLUENCING SEEDLING EMERGENCE AND SURVIVAL IN CERCIDIPHYLLUM JAPONICUM

Cercidiphyllum japonicumSieb. etZucc. is found in riparian forests in Japan, but the seedlings rarely regenerate more than coexisting tree species. We investigatedC. japonicum emergence and seedling survival in a nursery for 21 months. Bare soil, soil-with-litter, and gravel treatments and 3.0%, 10.9%, 22.7%, 60.1%, and 100% relative photosynthetic photon flux density (RPPFD) light conditions were tested. Seedling emergence depended on soil type and light conditions. Owing toC. japonicum’s small seed size, germinated seedlings could not penetrate the litter layer and became desiccated in gravel, but most seedlings emerged and survived in bare soil. These surviving seedlings needed quite bright light to germinate but not extreme light conditions. Initial mortality was high, but most of the seedlings that survived the first three months survived for the duration of the study, even under quite dark 10% RPPFD conditions. Current-year seedlings grew poorly under bright light conditions and rarely survived under the brightest light condition, when survival was probably negatively affected by desiccation. After one year, seedlings were able to use the higher light conditions more efficiently for growth. Such seedlings probably have a high chance of survival. Under low light conditions, both current- and second-year seedlings grew poorly. However, even small seedlings are likely to survive under low light conditions in a nursery, because the seedbed is level and nursery seedlings do not face all of the threats that are present in an actual forest.

Development of microsatellites in Machilus thunbergii (Lauraceae), a warm-temperate coastal tree species in Japan

PREMISE OF THE STUDY Microsatellite markers were developed and characterized in a typically coastal, widespread, and dominant tree species of the evergreen broadleaf forests, Machilus thunbergii, for comparison of the genetic diversity and structure of inland populations surrounding the ancient Lake Biwa and coastal populations in Japan. METHODS AND RESULTS Eighteen polymorphic microsatellites of this species were isolated using an improved technique for isolating codominant compound microsatellite markers. These isolated loci provided compound simple sequence repeat (SSR) markers with polymorphisms of three to 19 alleles per locus, with an average of 10.9. The expected and observed within-population heterozygosities ranged from 0.16 to 0.86 and from 0.13 to 0.72, respectively. CONCLUSIONS These markers may be useful tools for further investigation of the population genetic structure and biogeographic history of M. thunbergii in the warm-temperate zone of East Asia.

Difference between sprouting traits of Cercidiphyllum japonicum and C. magnificum

Cercidiphyllum japonicum and C. magnificum are deciduous tree species that produce large numbers of sprouts. They are found mainly in riparian and/or disturbed areas. C. japonicum is distributed in the montane zone in Japan, whereas C. magnificum is distributed mostly in the subalpine zone of central Japan. However, the two species sometimes coexist, e.g., on the talus slope at the valley head of the Chichibu Mountains. We investigated differences in sprouting traits in these two species by comparing class distributions of sprout diameter at breast height (DBH) and heights of individuals on the talus slope. Sprout DBH and individual height were smaller in C. magnificum as compared to C. japonicum. Moreover, the analysis of the DBH-class distribution of each species indicated that C. magnificum had numerous small sprouts and experienced high mortality, whereas C. japonicum had fewer small sprouts and low mortality. It is likely that the sprouting traits of C. magnificum make it more adapted to severe conditions in the subalpine zone than those of C. japonicum.

Nation-wide litter fall data from 21 forests of the Monitoring Sites 1000 Project in Japan

This data paper reports litter fall data collected in a network of 21 forest sites in Japan. This is the largest litter fall data set freely available in Japan to date. The network is a part of the Monitoring Sites 1000 Project launched by the Ministry of the Environment, Japan. It covers subarctic to subtropical climate zones and the four major forest types in Japan. Twenty-three permanent plots in which usually 25 litter traps were installed were established in old-growth or secondary natural forests. Litter falls were collected monthly from 2004, and sorted into leaves, branches, reproductive structures and miscellaneous. The data provide seasonal patterns and inter-annual dynamics of litter falls, and their geographical patterns, and offer good opportunities for meta-analyses and comparative studies among forests.

Adaptive plasticity in the life history strategy of a canopy tree species, Pterocarya rhoifolia , along a gradient of maximum snow depth

The Japanese wingnut Pterocarya rhoifolia is a riparian canopy tree species that grows under a variety of climate conditions, including heavy snowfall, despite the difficulties posed to canopy trees in such environments. This suggests that P. rhoifolia might adapt its life history strategy to different snowfall conditions. This study compared several life history traits of this tree species in a cool temperate mountainous area in central Japan along a gradient of maximum snow depth. The following trends were observed with increasing maximum snow depth: (1) diameter at breast height decreased, maximum stem length and tree height shortened, and trees tended toward a ‘dwarf shrub’ form; (2) the number of sprout stems increased significantly, and these sprouts contributed to maintaining the population; and (3) seed production decreased. Our results suggest trade-offs between clonal growth (sprouting) and sexual reproduction (seed production), and between sprouting and height growth. We concluded that the life history strategy P. rhoifolia demonstrated adaptive plasticity in response to a gradient of maximum snow depth.

General conclusions concerning riparian forest ecology and conservation

The aim of our research was to understand the dynamics and coexistence mechanisms of riparian forests with respect to natural disturbance and the life-history strategies of plant species and to contribute to the conservation and restoration of riparian ecosystems (see Chapter 1). There are many types of riparian vegetation in Japan (see Chapter 4). We have studied the vegetation of more than 20 riparian forests across Japan, from Hokkaido to Kyushu, and from headwater streams to lowland rivers. The studies were relatively broad, and included geography, vegetation science, population ecology, landscape ecology, and ecophysiology. In this chapter, I describe (1) the natural disturbance regime in riparian areas, (2) the life-history strategies of riparian trees, (3) the coexistence mechanisms of riparian trees, and (4) the conservation and management of riparian forests.