Warmer winters reduce the advance of tree spring phenology induced by warmer springs in the Alps

Mountain regions are particularly susceptible and influenced by the effects of climate change. In the Alps, temperature increased two times faster than in the Northern Hemisphere during the 20th century. As an immediate response in certain tree species, spring phenological phases, such as budburst and flowering, have tended to occur earlier. However, recent studies have shown a slowing down of phenological shifts during the last two decades compared to earlier periods, which might be caused by warmer winters. Indeed, cold temperatures are required to break bud dormancy that occurs in early fall; and dormancy break is a prerequisite for cell elongation to take place in spring when temperature conditions are warm enough. Here we aimed at evaluating the effects of winter warming vs. spring warming on the phonological shift along mountain elevation gradients. We tested the hypothesis that a lack of chilling temperature during winter delayed dormancy release and subsequently spring phonological phases. For this, we used eight years of temperature and phenological records for five tree species (Betula penctula, Fraxinus excelsior, Corylus avellana, Picea abies and Larix deridna) gathered with the citizen science program Phenoclim (www.phenoclim.org) deployed over the French Alps. Our results showed that for similar preseason (i.e. after dormancy break) temperatures, warmer winters significantly delayed budburst and flowering along the elevation gradient (+ 0.9 to + 5.6 days degrees C-1) except for flowering of Corylus and budburst of Picea. For similar cold winter temperatures, warmer preseasons significantly advanced budburst and flowering along the elevation gradient (- 5.3 to -8.4 days degrees C-1). On average, the effect of winter warming was 2.3 times lower than the effect of spring warming. We also showed that warmer winter temperature conditions have a significantly larger effect at lower elevations. As a consequence, the observed delaying effect of winter warming might be beneficial to trees by reducing the risk of exposure to late spring frost on a short term. This could further lead to partial dormancy break at lower elevations before the end of the 21st century, which, in turn, may alter bud development and flowering and so tree fitness.