When a particularly pronounced period of drought in 2014 triggered massive flowering, or masting, a group of NUS researchers scrambled their resources to study the phenomenon’s links to forest regeneration in Singapore. Their findings were published online in the journal Plant Ecology & Diversity in December 2016.
Masting is thought to be explained by the predator satiation hypothesis, which theorises that trees withhold seed and fruit production for long periods to keep the predator population in check, and then release an oversupply of seeds during a masting event to escape predation and ensure the survival of more seedlings for forest regeneration.
However, with past logging and forest loss having taken its toll on the density of mature primary forest trees in Singapore, the team set out to find out whether the masting events are producing enough seeds for local forest regeneration.
Between June 2014 and February 2015, then Year 2 NUS Life Sciences students Rie Chong and Lorraine Tan studied two rainforest patches with the highest concentration of surviving mature dipterocarp trees in the Central Catchment Nature Reserve — along the MacRitchie Nature Trail and the Lornie Trail — under a team of supervisors from NUS Biological Sciences and the Lee Kong Chian Natural History Museum (LKCNHM). The dipterocarp family dominates the region’s forests and depend on masting for their regeneration. The team tracked both pre-dispersal and post-dispersal seeds for up to five months after fruiting, and camera traps under each tree recorded seed predators.
The results showed that long-tailed macaques were the major seed predators, claiming 34 per cent of pre-dispersal seeds.
It was also discovered that about one third of the fruits had fallen to the ground but neither germinated nor showed signs of predation or fungal attack.
“Studies have shown that in fragmented forests, dipterocarps are quite sensitive to reproductive isolation. Pollinators are unable to travel between trees if they are too far apart and as a result a lot of the seeds are produced by inbreeding. The resultant seeds tend to be smaller, have poor genetic make-up and are not able to survive even if they are not eaten,” explained Dr Chong Kwek Yan from NUS Biological Sciences.
This poses a concern for long-term forest regeneration. “The solution is to have more of these trees to ensure viability of their populations but it is not a solution that can happen overnight because it can take a hundred years for a seedling to become a mature tree and start participating in masting,” he added.
The team plans to conduct further studies into the phenomenon across different landscapes and tree species and over a longer period.