Trees need warm days, not just warm nights, to leaf out
Vitasse Y, Hoch G, Pepin S, Giardina F
Phenology
If you've noticed your beech or oak leafing out earlier some springs than others despite similar average temperatures, this explains why: it's the daytime warmth specifically, not the 24-hour average, that your trees are tracking.
Scientists tested four common European trees (beech, hornbeam, oak, and sycamore maple) by cooling their twigs through a fake winter, then warming them through a fake spring. They found that warmth during the day pushed buds to open much faster than the exact same warmth delivered at night; beech trees needed 51% more total warmth to bud if that warmth came at night instead of during the day. Each species responded differently, which means a warming climate won't just nudge all trees' spring schedules the same way.
Key Findings
Budburst occurred up to 12 days earlier under a 25°C day/10°C night regime than under 20°C/15°C with the same daily average, across all four species tested.
When night temperatures were warm instead of day temperatures (Experiment 2), species required 10% (Quercus robur/oak), 19% (Carpinus betulus/hornbeam), and 51% (Fagus sylvatica/beech) more cumulative warmth (degree days) to reach budburst; Acer pseudoplatanus (sycamore maple) showed no difference.
Shorter light-dark cycle frequency during chilling (4h/4h vs 12h/12h) delayed budburst in hornbeam after long chilling, with effects on budburst success depending on species and chilling duration.
chevron_right Technical Summary
Trees don't just count how much warmth they get in spring, they count when it arrives: daytime warmth triggers budburst far more effectively than the same warmth at night, and the pattern varies sharply by species.
Abstract Preview
Original paper
Species-specific budburst responses to diurnal temperature cycles during chilling and forcing in temperate deciduous trees.
Cold temperatures ('chilling') perceived by tree buds progressively release winter dormancy and initiate sensitivity to warmer temperatures ('forcing') required for budburst. Phenology of temperate...
open_in_new Read full abstractAbstract copyright held by the original publisher.
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Acer pseudoplatanus, known as the sycamore in the British Isles and as the sycamore maple in the United States, is a species of maple native to Central Europe and Western Asia. It is a large deciduous, broad-leaved tree, tolerant of wind and coastal exposure.