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Global climate change may lead to more extreme weather events and it is important to understand the effects of drought on tropical forests. Severe and prolonged drought increased seedling mortality even in seasonally dry tropical forests, showed a recent study in Thailand.
Scientists from Osaka Metropolitan University conducted a seven-year study on the growth and mortality of seedlings in a national park in Chiang Mai in northern Thailand.
The study was published in the journal Global Change Biology. The findings suggest future climate change may alter the species composition and spatial distribution of seedlings in Asian seasonally dry tropical forests.
Read more: Waiting for El Niño
The Southeast Asian tropical forests studied experience an annual dry season and are deemed more drought-tolerant than rainforests and the effects of El Niño-induced droughts may differ between the two.
El Niño is a local warming of surface waters, which takes place in the entire equatorial zone of the central and eastern Pacific Ocean. It affects the atmospheric circulation and direction of winds world-wide.
An extremely strong El Niño event occurred between 2014 and 2016, resulting in reduced rainfall. This took place during the study period and the researchers observed the dry season in 2016 was approximately three months longer than normal.
El Niño often reduces rainfall and causes drier forests in the tropical regions of Southeast Asia, according to the study. Since tropical rainforests there usually experience rainfall year-round with no dry season, El Niño-induced drought increases tree mortality.
The scientists looked at two types of forests in a seasonally dry tropical forest — a deciduous forest at lower elevations and evergreen forests at higher elevations.
El Niño has been known to stimulate mass flowering and fruiting, leading to an increase in seedlings in tropical rainforests in Southeast Asia.
However, seedling increased only in deciduous forests during the El Niño period. This was mainly due to a large number of acorns produced by the deciduous oak Quercus brandisiana (Fagaceae).
In addition, mortality due to the drought was greater in evergreen forests, where drought is usually less severe. Deciduous forests, in comparison, witness severe drought more frequently.
“A decline in the number of seedlings that carry the next generation will affect the entire forest in the long run,” said study author professor Akira Itoh, from the Graduate School of Science at Osaka City University.
“This study takes us one step closer to understanding the impact of El Niño on seasonally dry tropical forests in Southeast Asia,” said another author Prapawadee Nutiprapun, from the same institute.