Alex T. K. Yee

Leaf litter depth as an important factor inhibiting seedling establishment of an exotic palm in tropical secondary forest patches

Intact tropical rainforests on continents and continental islands are considered to be relatively resistant to invasions by introduced plant species, but fragmentation and degradation may render them susceptible, especially to species from predominantly shade-tolerant families with centres of diversity in the tropics, such as palms. We investigated the seedling establishment patterns of the most common exotic palm species in Singapore’s secondary forest patches, the Macarthur palm (Ptychosperma macarthurii), in relation to landscape-level planting intensity, abiotic conditions, and recipient community composition. We first used conditional inference forests to narrow down the set of possible explanatory variables, followed by fitting generalised linear models with the forest patch and individual plots as random intercepts, to account for the nesting of plots within patches and overdispersion, respectively. The number of cultivated adults in the vicinity was not an important variable. Instead, leaf litter was the most important predictor of seedling density. Thick leaf litter in the disturbed and younger secondary forest matrix that surrounds old growth forest patches may therefore serve as a buffer against invasions, especially by small-seeded exotics. However, if adults of exotic species are allowed to establish unchecked, for example along forest trails that lead into the interior of the forested landscape, the seed rain may eventually reach old growth forest where leaf litter is typically thin. Further studies are required to determine if second-generation adults within invaded habitats contribute disproportionately more to propagule pressure than first-generation cultivated plants outside the invaded habitat.

Seed production and survival of four dipterocarp species in degraded forests in Singapore

Background: Seed production, germination, and seedling survival are crucial processes during community masting events in dipterocarp-dominated rain forest in South-east Asia. Aims: We examine how these processes function in fragmented remnants of dipterocarp forest in Singapore. Methods: Fruits were captured in seed traps and seed fates on the ground were tracked up to 5 months after fruiting for four dipterocarp species during a masting event in 2014; camera traps under each tree recorded seed predators. Results: Long-tailed macaques were the major seed predators, claiming 34% of pre-dispersal seeds. Insect seed predation, primarily by moth larvae, played a relatively minor role, claiming only 9% of seed pre-dispersal and 0.7% post-dispersal. The mean proportion of seeds that escaped post-dispersal predation but failed to germinate was between 5% and 37% for the four species. Conclusions: The non-viability of dipterocarp seeds can be at least as great a concern for forest regeneration as the rate of vertebrate pre-dispersal predation.

Pollination of Ficus elastica : India rubber re-establishes sexual reproduction in Singapore

Ficus elastica, otherwise known as India Rubber (although its geographical origins are unclear), was an important source of latex in the early 19th century and was widely cultivated in tropical Asia. Like all figs, F. elastica is dependent on tiny, highly specific wasps for pollination, and detailed studies based out of Singapore in the 1930s suggested that through the loss of its pollinator F. elastica was extinct in the wild. However, around 2005 wild seedlings of F. elastica began appearing in Singapore. We identified the pollinator as Platyscapa clavigera, which was originally described from F. elastica in Bogor in 1885. A visit to Bogor Botanical Gardens revealed that not only was F. elastica being pollinated by P. clavigera in the gardens, but there was clear evidence it had been reproducing naturally there over many decades. Although Singapore has a native fig flora of over 50 species, F. elastica went unpollinated for at least 70 years and probably from the time it was introduced during the 19th century. These observations illustrate the extraordinary specificity of this interaction and, through the fig’s ability to wait for its pollinators, demonstrates one way in which such highly specific interactions can be evolutionarily stable.

Vascular plant species richness and composition in two types of post-cultivation tropical secondary forest

Question How are plant communities in forests regenerating on post-cultivation land structured along environmental gradients, landscape context, and past land use? Location We investigated this for two types of post-agricultural fates: plantations abandoned with trees intact (abandoned-land forest) versus land that was cleared and left to regenerate into a forest (waste-woodland). The study sites were located in the tropical city-state of Singapore in Southeast Asia. Methods Five 20×20 m plots were surveyed for vascular plants in each of 11 patches of abandoned-land forest and nine patches of waste-woodland. For each plot, we estimated soil nutrient levels (nitrogen, potassium, phosphorous), canopy cover, leaf litter depth, distance to old-growth forest, and the size of the forest patch. Results For both forest types, increasing leaf litter and distance to old-growth forest is associated with lower species richness. Increasing soil nitrogen in abandoned-land forest and increasing soil potassium in waste-woodland is associated with lower total and native species richness, but not exotic species richness. Overall community composition is correlated with leaf litter, canopy cover, soil phosphorous and potassium, and distance to old-growth forest. Conclusions Different ways of land abandonment resulted in different successional trajectories that led to separate sets of environmental drivers of community patterns. Restoring such degraded forms of vegetation to native-rich communities may require management of soil nutrient levels and enrichment planting. This article is protected by copyright. All rights reserved.