Trends in thermal growing season length from years 1955-2020-A case study in hemiboreal forest in Estonia

We present the change in the thermal growing season based on a unique long-term climatological data set obtained at the Jarvselja Training and Experimental Forestry Centre in southeast Estonia. The data cover the years 1955-2020 and we employed the growing degree-days (GDD) algorithm on them. We defined the days when temperature is persistently above 5 degrees C as GDD5; when above 10 degrees C as GDD10. Our results showed that both GDD5 and GDD10 have increased by 14.8 (2.2 days/decade) and 18 (2.8 days/decade) days, respectively. The recent average growing season length is 204 days in case of GDD5 and 164 days in case of GDD10. Our results reveal that during the most recent decade, the length is stagnating and the onset in spring delays while the growing season's end extends towards winter. We find that the number of extreme cold (below -20 degrees C) dropped by a factor of 3.3 while extreme warm days (above +25 degrees C) rose by a factor of 2.6. Possible implications of these changes on the forest ecosystem are discussed.