Sen Nishimura

Paternity analysis-based inference of pollen dispersal patterns, male fecundity variation, and influence of flowering tree density and general flowering magnitude in two dipterocarp species

BACKGROUND AND AIMS Knowledge of pollen dispersal patterns and variation of fecundity is essential to understanding plant evolutionary processes and to formulating strategies to conserve forest genetic resources. Nevertheless, the pollen dispersal pattern of dipterocarp, main canopy tree species in palaeo-tropical forest remains unclear, and flowering intensity variation in the field suggests heterogeneity of fecundity. METHODS Pollen dispersal patterns and male fecundity variation of Shorea leprosula and Shorea parvifolia ssp. parvifolia on Peninsular Malaysian were investigated during two general flowering seasons (2001 and 2002), using a neighbourhood model modified by including terms accounting for variation in male fecundity among individual trees to express heterogeneity in flowering. KEY RESULTS The pollen dispersal patterns of the two dipterocarp species were affected by differences in conspecific tree flowering density, and reductions in conspecific tree flowering density led to an increased selfing rate. Active pollen dispersal and a larger number of effective paternal parents were observed for both species in the season of greater magnitude of general flowering (2002). CONCLUSIONS The magnitude of general flowering, male fecundity variation, and distance between pollen donors and mother trees should be taken into account when attempting to predict the effects of management practices on the self-fertilization and genetic structure of key tree species in tropical forest, and also the sustainability of possible management strategies, especially selective logging regimes.

Impacts of dry and hazy weather in 1997 on a tropical rainforest ecosystem in West Sumatra, Indonesia

An equatorial rainforest in the coastal region of West Sumatra suffered a long period of dry and hazy weather in 1997. Canopy trees with higher growth rates before the event had reduced growth rates during the 8 months from June 1997 to an average of 23% of their 1996 growth rate. The size-dependency of growth depression in forest trees greater than 5 cm in stem diameter could be observed during this period. The weather induced defoliation, and high litterfall rates of leaves continued throughout the period. The impacts of the weather on growth rates were analyzed by using Saeki’s mathematical model for community production. This model suggested that the great reduction in growth-rates of active canopy trees would be caused mainly by the interacting effects of potential photosynthetic rate and leaf biomass because of their sensitive responses to drought. In the severe weather conditions, these variables were estimated to have decreased to more than 50% of their 1996 values.

Spatial patterns and habitat associations of Fagaceae in a hill dipterocarp forest in Ulu Gadut, West Sumatra

Spatial distribution patterns and habitat associations of Fagaceae species in a Fagaceae-codominated hill forest in Sumatra were investigated. Ten Fagaceae species believed to be zoochorous (animal-dispersed seed) and five codominant canopy and emergent anemochorous (wind-dispersed seed) species from Anacardiaceae and Dipterocarpaceae were studied. Five Fagaceae species and all codominant anemochorous species were significantly aggregated while the other five Fagaceae species showed a random distribution pattern. The median distance of small saplings from the nearest reproductively mature tree tended to be shorter for aggregated species than spatially random species. This implied that some Fagaceae species dispersed over longer distances than anemochorous species. Relationships between four habitat variables and distribution of the target species were examined with torus-translation tests. Three Quercus and one Lithocarpus species showed positive habitat associations. Two Quercus species aggregated at the preferred habitat, but the others were randomly distributed. Thus tree species with specific habitat preference do not only aggregate at the preferred habitat. The three ridge-specialist Quercus species showed gradual changes in habitat association, which could reflect avoidance of competition among the species. Most of the Lithocarpus species showed little correlation with habitat variables. Coexistence of the three Quercus species partly reflected subtle differences in topographical preferences. Distribution of five of the six Lithocarpus species was unrelated to topography, so other mechanisms must be sought to account for the maintenance of coexistence in this species-rich genus.

Can Tiny Thrips Provide Sufficient Pollination Service During a General Flowering Period in Tropical Rainforest

One of the most crucial and interesting topics in tropical rainforest research is the pollination systems adapted to mast species, which flower massively at multiyear intervals. Highly fecund flower thrips are the primary pollinators and can respond rapidly to such an abrupt increase of flowers. Because thrips are minute and rather weak flyers, however, it had remained unclear to what extent they could contribute to cross-pollination of mast species growing at low population densities. However, microsatellite DNA analyses of single pollen grains adhering to pollinators, combined with the direct observation of flower visitors with the aid of canopy observation systems, revealed the pollination efficiency of flower thrips during the masting period. In this study of Shorea acuminata, the principal flower visitors were flower thrips, accounting for more than 60% of the collected insects during the flowering time. The thrips visitation pattern was clearly synchronous with the flowering phenology of S. acuminata. However, the genetic analysis of pollen grains suggested geitonogamous self-pollination by flower thrips. Thus, flower thrips made a small contribution to the outcrossing of S. acuminata, although their high visitation frequency on S. acuminata synchronous with the flowering phenology of the trees brought about vast production of self-pollinated seeds. On the other hand, the greater genetic diversity of seeds than that of their adherent pollen load implied the existence of other pollinators that provide sufficient pollination service for S. acuminata. In this study, lygaeid and plant bugs (Hemiptera) were collected frequently (12.2%), placing them third after flower and stipule thrips. Lygaeid bugs typified by big-eyed bugs (Geocoris and Piocoris) are known to prey on thrips, and their visitations to S. acuminata trees increased with increasing numbers of flower thrips. Thus, it was considered that outbreaks of flower thrips may attract the visitation of lygaeid bugs to flowering S. acuminata trees as edible resources during the general flowering period, and such an ecological food chain would provide high levels of genetic diversity of seeds and promote sufficient pollination service for S. acuminata.

Fruiting behavior of dipterocarps in two consecutive episodes of general flowering in a Malaysian lowland rain forest

We examined fruiting behaviors of 24 dipterocarp species in a lowland rain forest of Peninsular Malaysia during two consecutive episodes of general flowering (GF). The first GF episode (GF2001) occurred from August 2001 to February 2002, and the second GF episode (GF2002) followed immediately, from March to September 2002. The magnitude of GF2002 was greater than that of GF2001 at the community level. Significant positive size dependence of fruiting behavior at the community level was found in both GF2001 and GF2002, but there was no significant association between the fruiting behaviors in GF2001 and GF2002 except for one species. These results imply that tree size was one of the explanatory factors for fruiting behavior of dipterocarp species, but there was no evidence that adjacent reproduction caused the absence of reproduction and decreased fecundity in the subsequent fruiting event. In contrast, strong spatial aggregation of fruiting trees was found in GF2001, suggesting that external factors may affect fruiting behavior of dipterocarps in a minor GF episode. Among the 12 study species, there were large variations in fruiting behavior, but growth type (e.g., fast-growing or slow-growing) did not simply explain the inter-specific pattern of fruiting behavior. Thus, tree size may account for fruiting behavior of dipterocarps during the consecutive GF episodes through species-specific differences in phonological responses to internal and external conditions.

Effects of root zone trenching on soil nitrogen dynamics in Japanese cedar and cypress plantations

The effects of root exclusion and planted tree species on soil nitrogen (N) dynamics were examined at two plantations, one planted with Japanese cedar and the other with Japanese cypress. We set up ten 1 × 1 × 0.2-m-deep trenched sites and ten untrenched control sites at each plantation. We measured the pool size and leaching of inorganic N at each site for 2 years and the net N mineralization 1 and 2 years after trenching. Despite similar soil conditions, the cedar plantation showed higher net N mineralization than the cypress plantation. Stopped tree uptake of N was expected to cause an increased pool size and leaching of inorganic N at the trenched sites. Nevertheless, we found no significant increase in those variables at both plantations. The trenched cypress sites showed no decrease in the net N mineralization during the 2 years after trenching. However, the net nitrification at the trenched cypress sites increased remarkably at the deeper horizons in comparison with that at the control sites. Enhanced nitrification might result from improved ammonium availability through root exclusion. Net N mineralization at the trenched cedar sites decreased more than 60% compared with that at the control sites 2 years after trenching. Higher nitrification potential at the cedar plantation and enhanced nitrification potential at the trenched cypress sites never resulted in increased leaching of N, due to added fine root litter which acted as an immobilization agent for excess N, thus preventing N loss.

Sprouting traits of Fagaceae species in a hill dipterocarp forest, Ulu Gadut, West Sumatra

The size distribution of a tree species gives important information about its regeneration strategy. For example, a tree species that regenerates primarily by sprouting will, in theory, have fewer seedlings than species that regenerate from seedlings, which generally form an L-shaped population size structure because of trade-offs in resource allocation between vegetative sprouting and sexual reproduction (Bellingham & Sparrow 2000, Loehle 2000). The results of some field studies suggest that the number of seedlings decreases with increasing dominance of multi-stemmed sprouters (Kruger et al . 1997, Zimmerman et al . 1994). In their study of four co-occurring species of Castanopsis (Fagaceae), Nanami et al . (2004) showed that species with a high frequency of sprouting adults had fewer juveniles and vice versa. This suggested that these species were able to co-exist as a result of trade-offs between investment in seedlings and sprouting. In this study, we investigated the sprouting trait of 17 co-occurring Fagaceae species in a Sumatran hill forest. The forest stand at this site shows a varied population structure across species, and hence, should have a varied life history. We tested the correlation between population skewness and proportion of sprouting trees, following the methods of Nanami et al . (2004). We hypothesized that the species that frequently form multi-stemmed sprouts will show a lower turnover of individuals because multi-stemmed architecture favours persistence (Bellingham & Sparrow 2009).