Seedling dynamics under the pressure of El Niño drought in a seasonally dry tropical forest in Northern Thailand

Abstract

El Niño’s impact on tropical forests is critical, as reduced rainfall and severe drought induced by this phenomenon affect species diversity and tree dynamics. As seedling is a crucial stage for forest regeneration to maintain population and species diversity in the forest, it is important to understand how El Niño-induced drought affects seedling dynamics. We monitored the seedling dynamics at monthly intervals for 7 years in a seasonally dry tropical forest (SDTF). We analyzed the differences in seedling recruitment and mortality during the El Niño and non-El Niño periods and compared two forest types in an SDTF: a deciduous dipterocarp forest (DDF) dominated by deciduous species, and an adjacent lower montane forest (LMF) with more evergreen species. The long-term data on seedling dynamics revealed that El Niño-induced drought triggered immense seedling mortality in both forest types. This effect was stronger in evergreen species, leading to higher mortality in the LMF during El Niño. However, El Niño increased seedling recruitment only in the DDF, mainly because of the massive recruitment of the deciduous oak, Quercus brandisiana (Fagaceae), which counterbalanced the seedling mortality in the DDF. Consequently, El Niño increased seedling density in the DDF while decreasing it in the LMF. Our findings showed that the El Niño-induced drought effects on seedling dynamics varied by forest type and leaf habit, suggesting that future changes in drought regimes may alter the species composition and spatial distribution of Asian seasonally dry tropical forests through differences in the response of seedlings to drought.