Phenological fluctuations induce seasonal asymmetric warming in boreal forests.

Phenological shifts in boreal forests alter land-atmosphere energy exchange, but annual averages mask pronounced seasonal contrasts in the magnitude and pathways of this feedback. Here we show that climate-driven phenological instability generates a systematic warming asymmetry: surface warming from leaf area loss exceeds the cooling from an equivalent gain. During the growing season, additional foliage yields diminishing cooling returns as transpiration approaches its physiological limits, constraining latent heat flux. During the dormant season, the canopy-snow albedo contrast makes shortwave radiation dominant; because this radiative perturbation is converted into surface temperature highly non-linearly, foliage loss still produces a net warming exceeding the cooling from an equivalent gain. Per unit leaf area change, the median asymmetry index indicates warming roughly 2.0-2.3 times stronger than cooling in spring and autumn, and about an order of magnitude stronger in winter, where a very small greening-side sensitivity inflates the ratio; in summer both responses approach the detection limit and the asymmetry is not directionally robust. Phenological instability therefore acts as a seasonally structured positive biophysical feedback in northern ecosystems.