Identifying critical windows for reproduction: fine-scale temporal dynamics of effective pollination in a spring ephemeral

<b>Premise:</b> Flowering phenology is a key determinant of plant fitness, yet the fine-scale temporal dynamics of effective pollination within a flowering period remain poorly understood. Understanding the critical windows for seed production is essential for mechanistic understandings of flowering phenology and predicting how phenological shifts affect reproductive success.<b>Methods:</b> We investigated the spring ephemeral <i>Trillium camschatcense</i> at two sites with different warming rates (Obihiro and Hiroo) in northern Japan. The flowering period was divided into weekly intervals. We implemented timed bagging treatments: flowers were left unbagged for a single designated week to allow natural pollination by insects, and kept bagged during all other weeks to exclude pollinators. Bagging was conducted in combination with supplemental hand-pollination to quantify weekly pollination success and pollen limitation. Pollinator abundance and visitation frequency were also monitored.<b>Key r</b><b>esults:</b> Effective pollination was significantly skewed toward the early flowering period in Obihiro, where Diptera were the primary pollinators. Hiroo exhibited a more uniform distribution of effective pollination over time, relying mainly on Coleoptera. While significant pollen limitation occurred during individual weeks at both sites, pollination over the entire season cumulatively achieved high seed set and alleviated pollen limitation.<b>Conclusion</b><b>s</b><b>:</b> Our findings demonstrate that reproductive success relies on specific temporal windows that differ between populations. Highly concentrated effective pollination may increase sensitivity to rapid warming, as even slight phenological shifts can decouple these critical windows from the flowering period. Fine-scale evaluation provides mechanistic insights into the temporal dimension of pollination and climate change vulnerability that season-wide measurements overlook.