The 7-year study took place in a National Park in Chiang Mai, Thailand
During a 7-year study on seedling recruitment and mortality in a national park in Chiang Mai, Northern Thailand, an exceptionally strong El Niño event occurred, which led to a longer and stronger drought than expected. Seasonally dry tropical forests (SDTF’s), which are considered drought-tolerant, typically experience annual dry seasons, however, this study found that seedling mortality increased in these areas when severe and prolonged drought took place.
El Niño refers to a warming of the ocean surface, or above average sea surface temperatures, in the central and eastern equatorial Pacific Ocean and can disrupt normal global weather patterns. In the tropical regions of Southeast Asia, El Niño often reduces rainfall and causes drier forests. Since tropical rainforests in these areas usually experience rainfall all-year round without a dry season, drought induced by El Niño increases tree mortality. Seasonally dry tropical forests are considered to be better able to adapt to drought since they experience both wet and dry seasons. However, until now there had been very little understanding about the effects of El Niño on SDTF’s.
A team of researchers from Osaka City University, Osaka Metropolitan University and Kasetsart University have addressed this knowledge gap through monitoring seedling recruitment and mortality at an SDTF in Chiang Mai, Thailand, at monthly intervals over a 7-year period. During the study period, the El Niño event took place between 2014 and 2016, which resulted in reduced rainfall. The dry season in 2016 lasted approximately 3 months longer than normal. The research team subsequently found that the data collected showed a severe and prolonged drought which increased seedling mortality, even in the SDTF. Additionally, it was noted that mortality caused by drought was greater in evergreen forests at high elevations, whereas drought tends to be less severe, than deciduous forests at lower elevations, where severe drought is more frequently observed.
The researchers found that during the El Niño period, it was only in deciduous forests that the number of seedlings increase significantly. This was largely due to a large number of acorns produced by the deciduous Oak Quercus brandisiana (Fagaceae). El Niño has been known to stimulate mass flowering and fruiting, resulting in an increase in seedlings in tropical rainforests in Southeast Asia.
"This study takes us one step closer to understanding the impact of El Niño on seasonally dry tropical forests in Southeast Asia," said study lead author Prapawadee Nutiprapun, from the Graduate School of Science at Osaka City University.
"A decline in the number of seedlings that carry the next generation will affect the entire forest in the long run," concluded Professor Akira Itoh from Osaka Metropolitan University. "We believe that we have obtained useful basic knowledge to develop measures to conserve tropical forest ecosystems in consideration of future climate change."
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Journal Reference:
Prapawadee Nutiprapun, Sutheera Hermhuk, Satoshi Nanami, Akira Itoh, Mamoru Kanzaki, Dokrak Marod. Effects of El Niño drought on seedling dynamics in a seasonally dry tropical forest in Northern Thailand. Global Change Biology, 2022; DOI: 10.1111/gcb.16466