Effects of El Niño drought on seedling dynamics in a seasonally dry tropical forest in northern Thailand

As El Niño is predicted to become stronger and more frequent in the
future, it is crucial to understand how El Niño-induced droughts will
affect tropical forests. Although many studies have focused on tropical
rainforests, there is a paucity of studies on them, particularly in Asia,
and few studies have focused on seedling dynamics, which are expected to
be strongly affected by drought. Seedlings in seasonally dry tropical
forests (SDTFs) are generally more drought-tolerant than those in the
rainforests, and the effects of El Niño-induced droughts may differ
between SDTF and tropical rainforests. In this study, we explored the
impact of El Niño-induced drought at an SDTF in northern Thailand by
monitoring the seedling dynamics at monthly intervals for seven years,
including a period of strong El Niño. The effects were compared between
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. El Niño-induced drought
increased seedling mortality in both forest types. The effect of drought
was stronger in evergreen than in the deciduous species, resulting in
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
compensated for the mortality of seedlings in the DDF. As a result, El
Niño increased seedling density in the DDF and decreased it in the LMF.
This is the first long-term study to identify the differences in the
impacts of El Niño on seedlings between the two forest types, DDF and LMF,
and two leaf habits, evergreen and deciduous, in Southeast Asia. Our
findings suggest that future climate change may alter the species
composition and spatial distribution of seedlings in Asian SDTFs.