Climate-linked divergence in tree flowering and fruiting in an Eastern Himalayan tropical forest

Premise Long-term phenological patterns are increasingly being examined from the perspective of climate change and its potential effects. Climatic effects on plant phenology could involve the direct responses to changes in temperature, precipitation and solar irradiance, or could be mediated by these variables through exogenous teleconnections such as the El Niño Southern Oscillation (ENSO). The effects of climatic fluctuations on inter-annual variation in tropical phenology remain understudied.

We examined long-term patterns of tree flowering and fruiting intensity in a tropical forest site in the Eastern Himalayas between 2011 and 2024. Species-specific patterns were examined for 36 species. Long-term patterns were quantified using Generalized Additive Models, and splines were visualized to infer trends. Through Generalized Linear Mixed Models, we determined if there was a lagged phenological response to ENSO and temperature, precipitation and solar irradiance, and whether ENSO effects were being mediated through the latter group of variables or plant traits.

Between 2011 and 2019, trends in flowering and fruiting were significant for 17 and 23 species respectively. Flowering increased for 7 species, while fruiting declined for 8 species. Flowering peaked during El Niño, but this association did not appear to be mediated through climate variables, whereas fruiting showed a three-month positive lagged response to solar irradiance, independent of ENSO. The peak season of reproduction was the only trait determining species-specific responses to climate variables.

Our study highlights nonlinearity in long-term patterns of reproductive phenology, and the importance of solar irradiance in determining inter-annual fruit production. Competing Interest Statement The authors have declared no competing interest.