Kenji Seiwa

Importance of seed size for the establishment of seedlings of five deciduous broad-leaved tree species

Effects of seed size and phenology on the establishment of five deciduous broad-leaved tree species were examined in deciduous woodland. Treatments included absence and presence of litter in the forest understory, a small gap, and a large gap. Seedling emergence of large-seeded speciesQuercus mongolica var.grosseserrata andAcer mono was not reduced by accumulation of litter in the forest understory, but was promoted in the large gap where litter was less. Seedling emergence of small-seeded species,Alnus hirsuta, Cercidiphyllum japonicum andBetula platyphylla var.japonica, was reduced by the litter in almost all of the sites. Seedlings of large-seeded species avoid shade stress phenologically by unfolding all of their large leaves in a short period before canopy closure in the forest understory. These species had little mortality after seedling emergence. In contrast, small-seeded species have a longer duration of leaf emergence, shorter leaf longevity, and rapid leaf turnover in all the sites. These seedlings attained similar height to those of the large-seeded species at the end of the second year in the large gap, but survival and height growth rate decreased after canopy closure in the forest understory. We suggest that the importance of seed size in determining seedling establishment largely depends on the relationships between seasonal changes of environmental conditions and phenological traits of seedlings, which are related to seed size.

Effects of seed size and emergence time on tree seedling establishment: importance of developmental constraints

Abstract To evaluate the importance of developmental constraints in the determination of the relative importance of seed size and emergence time, early seedling performance of two woody species with contrasting growth phenology were observed under competitive conditions with tall herbs in an early successional habitat. The oak, Quercus mongolica var. grosseserrata, with determinate shoot development (a single leaf flush), showed a marked influence of seed size on subsequent seedling height growth and survival. Because of determinate growth, emergence time had negligible effects. The walnut, Juglans ailanthifolia, which continued to produce new leaves throughout the growing season (indeterminate development), showed a marked influence of time of emergence on seedling performance, because it affected the amount of growth that could be achieved; seed size, however, had negligible effects as the seedlings grew. These results suggest that relative importance of seed size and emergence time for early seedling performance is closely associated with developmental constraints (growth phenology).

Selfing and inbreeding depression in seeds and seedlings of Neobalanocarpus heimii (Dipterocarpaceae).

We evaluated the degree of selfing and inbreeding depression at the seed and seedling stages of a threatened tropical canopy tree, Neobalanocarpus heimii, using microsatellite markers. Selection resulted in an overall decrease in the level of surviving selfed progeny from seeds to established seedlings, indicating inbreeding depression during seedling establishment. Mean seed mass of selfed progeny was lower than that of outcrossed progeny. Since the smaller seeds suffered a fitness disadvantage at germination in N. heimii, the reduced seed mass of selfed progeny would be one of the determinants of the observed inbreeding depression during seedling establishment. High selfing rates in some mother trees could be attributed to low local densities of reproductive individuals, thus maintenance of a sufficiently high density of mature N. heimii should facilitate regeneration and conservation of the species.

POST‐DISPERSAL CHANGES IN THE SPATIAL DISTRIBUTION OF FAGUS CRENATA SEEDS

The spatial distribution of tree seeds is influenced by the behavior of seed dispersers and seed-killing agents. In this study we evaluated the relative importance of multiple seed-killing agents on changes in the spatial pattern of Fagus crenata seeds. The dispersal of seeds and causes of seed death following dispersal were investigated on a 0.6- ha study plot in northern Japan. Seeds were collected using seed traps (n = 110) in the autumn of 1995, as well as directly from the AO layer of the soil, including leaf litter, intermediate, and humus layers (n = 110), in the spring of 1996. A total of 44075 seeds were disseminated into the seed traps, including 17 964 mature seeds; 15% of these were viable the following spring, indicating a large decrease in the abundance of viable seeds during the post-dispersal seed stage. Spatial association of undamaged seeds changed from a positive to independent association between the different dispersal stages, indicating rearrangement of the spatial pattern of seeds through time. During this period, numbers of living seeds generally decreased beneath nonspecific adults compared to those located away from the adults. This change resulted from the spatially different behavior of several biotic and abiotic killing agents. Rodents and fungi attacked the seeds, depending on seed densities and presence of nonspecific adult crowns, respectively, but desiccation was independent of both seed density and nonspecific crowns. The multiple killing agents with different spatial behavior jointly influenced the distributions of recruits entering tree populations.

Seedling establishment of deciduous trees in various topographic positions

Abstract We investigated the effect of topography‐related environmental factors (i.e., ground‐surface stability and soil moisture) on seedling establishment of 8 deciduous tree species. A field experiment was carried out using canopy species, which were classified into 3 groups based on the spatial distribution of adult trees (ridge, slope and valley). Demographic parameters were compared among species during the early stage of seedling establishment among 3 topographic positions in combination with gap and canopy conditions. The percentage of emerging seedlings tended to be lower on the ridge irrespective of the adult topographical distribution patterns. There was no clear trend in seedling emergence among the species groups classified by their spatial distribution. Seedling survival during 2 growing seasons was significantly different among species, topographic positions and light conditions. On the ridge, seedlings of the species dominating ridge tops had greater survival than those of other species, probably due to differences in demand for soil moisture. On the slope, frequent physical damage caused by surface material movement was observed and some species showed greater adaptability to the disturbed slope habitat. Survival of all seedlings was highest in the valley plots. Light conditions were the critical factor for seedling survival in some species. The results of this study suggest that topography creates diverse habitats for the establishment of tree seedlings. In addition to soil moisture, surface material movement may be a significant factor affecting seedling establishment. Nomenclature:Satake et al. (1981‐1982,Satake 1989) Iwatsuki (1992).

Sexual vs. vegetative reproduction in relation to forest dynamics in the understorey shrub, Hydrangea paniculata (Saxifragaceae)

In the forest understorey, shrubs usually reproduce vegetatively rather than sexually, but the relative contribution of these two reproductive modes may vary with temporal changes in environmental conditions (e.g., light, substrates of forest floor) that are closely related to forest dynamics. To evaluate the occasional changes in reproductive modes of the clonal understorey shrub Hydrangea paniculata, the reproductive characteristics (i.e., flowering, seedling establishment, vegetative propagation, reproductive size and age) were investigated in four different developmental stages in a Japanese beech forest (early gap, EG; late gap, LG; building, BU; mature, MA). Flowering individuals occurred only in EG and LG, and a much greater number of seedlings was observed in both EG and LG than in BU and MA, mainly because canopy gaps provided sufficient light for flowering and suitable substrates (fallen trees and exposed mineral-soil) for seedling establishment. The flowering individuals originated from plants that had persisted in the shaded understorey until gap formation. In contrast to sexual reproduction, clonal fragmentation increased with forest development in the order LG, EG, BU, MA. This is because, in later stages of forest dynamics (BU and MA), large individuals, some of which had previously flowered in gap stages, were layered on the forest floor and subsequently produced a substantial number of clonal fragments by separation of the buried branches from the main stems which had decayed. This resulted in a large number of clonal fragments originating from a single mother plant. These results suggest that the reproductive modes of H. paniculata is strongly influenced by the changes in environmental conditions with respect to the dynamics of canopy trees. Reproduction from seed in gap stages, despite their short period, provide new genets in established populations, whereas vegetative propagation via fragmentation in closed-canopy stages enhance the stability of the population until the next disturbance.

Carbon allocation to defense, storage, and growth in seedlings of two temperate broad-leaved tree species

Optimal carbon allocation to growth, defense, or storage is a critical trait in determining the shade tolerance of tree species. Thus, examining interspecific differences in carbon allocation patterns is useful when evaluating niche partitioning in forest communities. We hypothesized that shade-tolerant species allocate more carbon to defense and storage and less to growth compared to shade-intolerant species. In gaps and forest understory, we measured relative growth rates (RGR), carbon-based defensive compounds (condensed tannin, total phenolics), and storage compounds (total non-structural carbohydrate; TNC) in seedlings of two tree species differing in shade tolerance. RGR was greater in the shade-intolerant species, Castanea crenata, than in the shade-tolerant species, Quercusmongolica var. grosseserrata, in gaps, but did not differ between the species in the forest understory. In contrast, concentrations of condensed tannin and total phenolics were greater in Quercus than in Castanea at both sites. TNC pool sizes did not differ between the species. Condensed tannin concentrations increased with increasing growth rate of structural biomass (GRstr) in Quercus but not in Castanea. TNC pool sizes increased with increasing GRstr in both species, but the rate of increase did not differ between the species. Accordingly, the amount of condensed tannin against TNC pool sizes was usually higher in Quercus than in Castanea. Hence, Quercus preferentially invested more carbon in defense than in storage. Such a large allocation of carbon to defense would be advantageous for a shade-tolerant species, allowing Quercus to persist in the forest understory where damage from herbivores and pathogens is costly. In contrast, the shade-intolerant Castanea preferentially invested more carbon in growth rather than defense (and similar amounts in storage as Quercus), ensuring establishment success in gaps, where severe competition occurs for light among neighboring plants. These contrasting carbon allocation patterns are closely associated with strategies for persistence in these species’ respective habitats.

Variable regeneration behaviour of Ulmus davidiana var. Japonica in response to disturbance regime for risk spreading

Recruitment-related traits (development and dispersal of seeds, emergence, leaf dynamics, growth and survival of seedlings) were studied in Japanese elm ( Ulmus davidiana var. japonica ) in relation to disturbance at the soil level (bare soil, litter and control) or the canopy level (forest edge (FE), small gaps (SG) and forest understorey (FU)), or both, in a riverside forest from 1990 to 1993. Seed germination was also tested in the laboratory. Litter accumulation severely inhibited seedling emergence. Seedling emergence from a single seed rain occurred in summer immediately after seed dissemination and in the following spring. In summer, seedling emergence was higher in FE than in FU. In the following spring more seedlings emerged in FU than in FE and SG, resulting in non-significant differences in the percentage emergence among the three sites in bare soil. Darkness or far-red light induced seed dormancy, but after chilling they started to germinate again under white or red light conditions. These results explained emergence phenology under changing light conditions in the field. In FU, the spring emerging cohort grew and survived better than the summer emerging one. In FE the reverse was true. Japanese elm avoids unfavourable periods by escaping either in time (dormancy) or in space (dispersal). Such traits compensate for late and compressed seed dissemination, and improve seedling growth and survival not only when disturbance occurs at both canopy and soil level (bare soil in FE) but also only at soil level (bare soil in FU), which may occur more frequently in riverside forests.

Gender-specific shoot structure and functions in relation to habitat conditions in a dioecious tree, Salix sachalinensis

Abstract In dioecious plants, females often incur greater reproductive costs than males due to seed production. This unequal reproductive cost can cause differences in the resource acquisition traits and the spatial segregation of the sexes. To evaluate how females compensate for the cost of reproduction, the differences in these traits between sexes were investigated in a dioecious tree, Salix sachalinensis, which is common in riverside forests. Females tend to inhabit the area near channel of rivers and streams and seem to be found at a lower ground level than males, indicating spatial segregation of the sexes. In the early growing season, females produced a greater number of vegetative shoots compared with reproductive shoots. In males the reverse was true, suggesting that females invest greater amounts of resources in photosynthetic organs than in reproductive organs at that time of the year. Females shed a greater number of vegetative shoots than males did during the growing season. In females, the shorter lifespan of vegetative shoots could enhance photosynthesis by placing new shoots in favorable light conditions. Both leaf weight ratio and leaf area ratio of vegetative shoots, and nitrogen content of the leaves, were found to be larger in females than males, but there was little difference in specific leaf area between the sexes. In females, shoot structure, phenology and physiology may compensate for their greater reproductive costs. Inhabiting wetter sites could result in a greater advantage to females than males, because such resources facilitate the photosynthetic capacity in females.

Pollen donor composition during the early phases of reproduction revealed by DNA genotyping of pollen grains and seeds of Castanea crenata

In plants, pollen donor composition can differ during the early phases of reproduction through various selection mechanisms favouring self, related or nonrelated pollen donors, but such differences have not been examined under natural conditions because paternity is difficult to analyse in a natural setting. Here, we performed paternity analyses based on microsatellite genotyping of individual pollen grains deposited on female flowers (n = 773) and seeds (n = 304) to evaluate pollen donor composition from three individuals of the insect-pollinated monoecious tree Castanea crenata in a natural forest. Spatial genetic structure was also investigated. A mean self-pollen rate of 90.2% was observed at the pollination stage, but a low selfing rate of 0.3% was observed at the seed stage. In outcross events, however, pairwise distance and relatedness between maternal and paternal parents were not different between pollination and seed stages. We also observed significant positive relatedness, based on clear fine-scale genetic structure of individual trees within 80 m of one another, and 71% of seeds were derived using pollen grains of related trees within 80 m. The results suggest that the mechanism of self-incompatibility strongly avoids self-pollen before seed production. However, the avoidance of biparental inbreeding was not obvious between pollination and seed stages.