Decoupled Litterfall and Canopy Signals Reveal Species‐Specific Leaf Phenology in a Wet Hawaiian Tropical Forest

Abstract

Leaf phenology in evergreen wet tropical forests reflects diverse life history strategies. Even as forest canopies maintain year‐round cover, individual species undergo distinct, asynchronous cycles of leaf turnover. Understanding the timing and drivers of leaf turnover is essential for predicting climate change impacts and informing conservation. In a wet montane forest in Hawai‘i dominated by Metrosideros polymorpha (‘ōhi‘a lehua), a resource‐conservative species, and Acacia koa (koa), a resource‐acquisitive species, we investigated species‐ and season‐specific leaf turnover. We aimed to (a) determine whether leaf litterfall timing corresponds to leaf replacement; (b) compare leaf litterfall and chemistry across species and seasons; and (c) identify species‐specific environmental drivers of litterfall. Using a 12‐year litterfall record, litter chemistry, machine‐learning classification of PhenoCam imagery, and local climate data, we show that M. polymorpha exhibits continuous leaf turnover and high green canopy cover during the wet season, and a discrete senescence‐abscission event during the dry season followed by leaf flush at the wet season onset. Canopy greenness did not track M. polymorpha litterfall, likely due to these varied seasonal dynamics. In contrast, A. koa leaf turnover occurred primarily during the dry season and canopy signals were tightly coupled with litterfall. A. koa invested more nutrients in individual leaves than M. polymorpha. M. polymorpha litterfall was best predicted by temperature while A. koa litterfall was best predicted by precipitation. These contrasting phenologies reflect divergent strategies and climate sensitivities, with implications for how co‐occurring tropical tree species mediate ecosystem function and respond to shifting temperature and precipitation regimes.

Citation

Bayliss, Shannon L. J.; Anderson, Craig; Dudley, Melanie Leilā; Litton, Creighton M.; Giardina, Christian P.; Pau, Stephanie. 2026. Decoupled Litterfall and Canopy Signals Reveal Species‐Specific Leaf Phenology in a Wet Hawaiian Tropical Forest. Biotropica. 58(4). https://doi.org/10.1111/btp.70237.