Aseasonal rainforests in Southeast Asia are renowned for their community-wide synchrony of flower production, known as general flowering, followed by mast fruiting. The intermittent blooming and massive seed production involving hundreds of forest species in dozens of families provokes discussions on its impact on forest regeneration, as well as the evolution of this phenomenon.
To further our understanding of these forests, the Pasoh Seed Rain Project monitors the fluctuation of flower and seed rain at the Pasoh 50-ha Forest Dynamics Plot with passive litter traps. The first 247 traps were placed along forest trails in 2001, and another 89 traps were added in 2002 (Figure 1).
Assembly and installation of all traps follows the protocol of the BCI flower and seed rain program. All reproductive material is sorted into categories of flower, immature fruit, mature fruit (intact, fragments, capsules, and injured by animals) and seeds (intact and injured by animals). Numbers of fruits/seeds and the presence of flowers are recorded. Most reproductive material is identified to species or morphospecies, with a small portion (0.37%), consisting mostly of fragments, being unidentifiable. A reference collection including most categories of reproductive material for most species is stored on site with silica gel.
In addition to the 336 seed traps, three to four 1-m2 seedling plots were placed beside each seed trap. Between 2002 and 2016, all new seedling recruits were tagged, identified to species or morphospecies, and measured annually for survival, height, and, for seedlings less than 1 m tall, leaf number in these seedling plots. Seedlings left the census when they reached 2 m in height, or, for lianas, when tendrils appeared.
The project completes the life cycle for tree species censused in the 50-ha forest dynamics plot by documenting seed production, seedling recruitment, and seedling growth and survival. The project also provides the timing of flowering and seed fall with weekly temporal resolution for hundreds of general flowering species in an aseasonal Southeast Asian Dipterocarp forest.
In March 2015, we removed 16 traps that were located farther from existing trails. Weekly censuses began in August 2001, with interruptions for two weeks in December 2003, 37 weeks in February-October 2011, one week in October, 2014, eight weeks in January and February 2015, and 11 weeks in March-May 2020.
Phenological patterns in aseasonal rain forests of Southeast Asia
Aseasonal rainforests exhibit a renowned phenomenon of community-wide synchronized flowering referred to as general flowering events. Previous reports described the phenomenon with a focus on the dominant family of Dipterocarpaceae. The Pasoh seed rain project enables the study of phenologies for hundreds of non-Dipterocarp species during and outside general flowering events. This long-term, high resolution dataset permits exploration of spatial and temporal variation in flowering and fruiting phenologies. The data also allows analyses of the environmental triggers or approximate cues of general flowering events and tests for ultimate causes, such as phylogenetic constraints, resource limitation, pollination facilitation, predator satiation and promotion of seed dispersal.
Success of masting species
Seed dispersal and seedling establishment are density dependent. Species joining general flowering events may enjoy benefits including increases in seed survival through satiation of seed predators, increases in pollination success, and, possibly, seed dispersal through attraction of nomadic pollinators and seed dispersal agents. However, the high seedling densities that follow general flowering events may lead to reduced seedling survival. Seedling survival data provides an opportunity to compare seedling performance between masting and non-masting species.
Impact of climate change on seed-mediated forest regeneration
Climate change may affect flowering and fruiting in the tropics. The aseasonal ever-wet forests in SE Asia are projected to be more seasonal, which may influence timing and levels of synchrony in flowering and fruiting. Changes in fruiting frequency and the degree of synchrony will have consequences for the success of seeds and seedlings. Long-term monitoring of flower and seed production, as well as of seedling dynamics, permits investigations regarding the impact of climate change on forest regeneration and the consequences for forest species composition.
Pasoh Seed Rain Project PIs
- Yu-Yun Chen (National Dong Hwa University, Taiwan, R.O.C.)
- I-Fang Sun (National Dong Hwa University, Taiwan, R.O.C.)
- S. Joseph Wright (Smithsonian Tropical Research Institute, Panama)
- Stephen P. Hubbell (University of Georgia, U.S.A.)
- Tze Leong Yao (Forest Research Institute Malaysia, Malaysia)
- Musalmah bt. Nasardin
- Nur Supardi Md. Noor
- Christine Fletcher
- NSF (DEB-0108388, to Stephen P. Hubbell, S. Joseph Wright and I-Fang Sun, 2001–2007)
- Center for Tropical Forest Science and the Smithsonian Tropical Research Institute (2001, 2006–2015)
- ForestGEO, Smithsonian (2014, 2016-)
Logistical support was provided by the University of Georgia, U.S.A, Forest Research Institute Malaysia (FRIM), CTFS, and ForestGEO.
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