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