Premature leaf discoloration of European deciduous trees is caused by drought and heat in late spring and cold spells in early fall

While leaf discoloration of many temperate and boreal tree species has been delayed in response to global warming, climatic stress such as severe drought can cause premature discoloration. We investigated whether is synchronized within across under which conditions occurs (i.e. exceptionally early dates the lower 5 % percentile). analyzed 16’865 observations from nine deciduous (beech, silver birch, horse chestnut, larch, large-leaved linden, rowan, small-leaved lime, sweet sycamore) a phenological network Switzerland. considered 222 stations that were located between 200 1933 m covered period 1971–2018. Leaf most significantly by 1.5 3.3 days per decade 1996 2018. detected synchronous temporal pattern both species. For species, occurred years with very warm dry spring-summer or cool fall temperatures. Premature was observed 10-40 all during these years, whereas long-term mean 4-6 %. Growing degree May June higher water balance compared regular discoloration, especially for sensitive beech. Similarly, minimum temperatures September consistently Our study provides first systematic assessment occurs. expect future increase frequency severity droughts Europe will likely risk