Enhanced effect of warming on the leaf-onset date of boreal deciduous broadleaf forest

The leaf-onset date is sensitive to climate warming. It is widely reported that the temperature sensitivity of the leaf-onset date (ST) of deciduous broadleaf forest (DBF) may decrease under dormancy-period warming. However, evidence of how boreal-DBF ST may generally change under dormancy-period warming is still lacking. Here, by analysing climate and satellite data, we find that, between 1982–1996 and 1998–2012, 74% of all 0.5° × 0.5° boreal-DBF-containing grid cells with a rise in boreal-DBF dormancy-period temperature exhibited an increase in boreal-DBF ST. We demonstrate that the observed general increase in boreal-DBF ST is largely attributable to a warming-related enhancement in dormancy-period chilling accumulation. Furthermore, we show that phenology models systematically underestimated the magnitude of the observed change in the mean boreal-DBF ST across all boreal-DBF-containing grid cells by a mean of 85%. This study has implications for improving phenology models and understanding the carbon cycle in boreal regions.