<p><strong>Table S1. </strong> The flowering onset data of <em>Kerria japonica</em> at Ibaraki, collected by Tsukuba Botanical Garden, and those at Fukushima, Toyama, and Yamagata, collected by the Japan Meteorology Agency, from 2003 to 2020. </p>
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<p><strong>Table S2. </strong> The results of Ljung-Box test, which were performed to test whether the flowering onset data for each study site, Fukushima, Ibaraki, Toyama, and Yamagata, exhibited an autocorrelation.</p>
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<p><strong>Table S3. </strong>The results of Pearson's correlation test, which were performed to test the correlation between flowering onset dates and daily mean temperature for each study site, Fukushima, Ibaraki, Toyama, and Yamagata. </p>
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<p><strong>Fig. S1. </strong>Gray curves indicate the daily temperature fluctuation from 2003 to 2020, dashed black lines indicate the average temperature through the 18 years, and solid black curves indicate the trends resulting from STL analysis in each study site.</p>