The responses to shade of seedlings of very small-seeded tree and shrub species from tropical rain forest in Singapore

1. Newly germinated seedlings of six tree and shrub species with very small seeds (31–460 μg dry mass), one light-demanding and five shade-tolerant at the stage of establishment in the wild, were grown for 5 months in neutral shade houses with 0·5, 1, 3·5 and 7·5% daylight.
2. The ratio of yield in 7·5% to that in 1% was 8:1 for the light-demanding Melastoma malabathricum but only 2:1 for the confamilial shade-tolerator Pternandra echinata. The Pternandra, Urophyllum hirsutum, Ficus chartacea, Ficus grossularioides and Pellacalyx saccardianus showed a graded series of responses to irradiance, generally consistent with their apparent demands for light in the wild. In contrast, survival of very deep shade was not clearly related to light demand in nature.
3. The results support the conclusion drawn from observational studies that large seed size is not primarily adaptive in resisting shade but in resisting the associated risks of burial by litter, desiccation during dry spells, uprooting by birds and other kinds of damage by animals or falling debris.     

森林植物耐荫性及其形态和生理适应性研究进展

耐荫性是植物在低光环境下的生存和生长能力,对森林植物群落演替起重要作用,植物对遮荫的适应机制已成为生态学的研究热点.本文综述了森林植物的耐荫性及其在形态和生理方面的适应性,分析了森林植物在生长性状、生物量分配、树冠结构、叶片形态生理、叶片解剖结构、光合参数、碳水化合物分配、水分和养分的利用等方面对遮荫产生的可塑性响应,最后对目前研究存在的问题进行了分析,展望了未来的研究内容和方向.     

Growth responses of 15 rain-forest tree species to a light gradient: the relative importance of morphological and physiological traits

1. Growth of seedlings of 15 rain-forest tree species was compared under controlled conditions, at six different light levels (3, 6, 12, 25, 50 and 100% daylight).
2. Most plant variables showed strong ontogenetic changes; they were highly dependent on the biomass of the plant.
3. Growth rate was highest at intermediate light levels (25–50%) above which it declined. Most plant variables showed a curvilinear response to irradiance, with the largest changes at the lowest light levels.
4. There was a consistent ranking in growth between species; species that were fast growing in a low-light environment were also fast growing in a high-light environment.
5. At low light, interspecific variation in relative growth rate was determined mainly by differences in a morphological trait, the leaf area ratio (LAR), whereas at high light it was determined mainly by differences in a physiological trait, the net assimilation rate (NAR).
6. NAR became a stronger determinant of growth than LAR in more than 10–15% daylight. As light availability in the forest is generally much lower than this threshold level, it follows that interspecific variation in growth in a forest environment is mainly owing to variation in morphology.     

Differences in seed rain composition in small and large fragments in the northeast Brazilian Atlantic Forest

Tropical forests are seriously threatened by fragmentation and habitat loss. The impact of fragment size and forest configuration on the composition of seed rain is insufficiently studied. For the present study, seed rain composition of small and large forest fragments (8–388 ha) was assessed in order to identify variations in seed abundance, species richness, seed size and dispersal mode. Seed rain was documented during a 1‐year period in three large and four small Atlantic Forest fragments that are isolated by a sugarcane matrix. Total seed rain included 20,518 seeds of 149 species of trees, shrubs, palms, lianas and herbs. Most species and seeds were animal‐dispersed. A significant difference in the proportion of seeds and species within different categories of seed size was found between small and large fragments. Small fragments received significantly more very small‐sized seeds (<0.3 cm) and less large‐seeded species (>1.5 cm) that were generally very rare, with only one species in small and eight in large fragments. We found a negative correlation between the inflow of small‐sized seeds and the percentage of forest cover. Species richness was lower in small than in large fragments, but the difference was not very pronounced. Given our results, we propose changing plant species pools through logging, tree mortality and a high inflow of pioneer species and lianas, especially in small forest fragments and areas with low forest cover. Connecting forest fragments through corridors and reforestation with local large‐seeded tree species may facilitate the maintenance of species diversity.