Charakterystyka okresu wegetacyjnego w Lasach Doświadczalnych SGGW w Rogowie w latach 1951–2020

The growing season length indices derived from air temperature are frequently used in climate monitoring applications as well as to predict the response of forest ecosystems to climate change. The indicator most widely used in the studies of forest ecosystems is the length of the thermal growing season (temp ≥5°C), less commonly the parameters of the forest growing season (temp ≥10°C). However, only a few studies used long-term series of temperature measurements in the forest. In this article, we determined the temporal changes in the parameters of the thermal (TGS) and forest (FGS) growing season in the Experimental Forests of the Warsaw University of Life Sciences in Rogów (51o40’N, 19o55’E, h = 194 m MSL) in the years 1951–2020. The analysis is based on the dataset (daily mean air temperature) obtained from a meteorological station located near the forest complex and from a forest under-canopy station located in a more than 120-year-old fresh mixed forest. The results show a significant extension of the growing season in 1951–2020, the TGS lasted on average 2.8 days/10 years, and the FGS 2.4 days/10 years. The extension of the TGS and FGS was a consequence of both its earlier start and later end. The start of the TGS was characterized by a statistically significant negative trend (1.3 days/10 years), and most changes were characteristic for the last three decades (4.4 days/10 years). The last 30 years were also characterized by a statistically significant trend towards the later end of the TGS. The TGS in 1991–2020 was longer than in 1951–1980 and 1971–2000 by 9 days, while in 1981–2010 by 5 days. Changes in the length of the TGS resulted primarily from its earlier beginning: in the multi-year period 1991–2020, the TGS started 7 days earlier than in 1951–1980. In the case of the FGS, these changes were weaker, although there was a statistically significant negative trend in the start dates and a positive trend in the FGS length. The FGS started almost a month later than the TGS (average on April 28) and ended 4 weeks earlier (average on October 5) and lasted 160 days. TGS in the forest was shorter than outside the forest by 3 days, and FGS by 1 day. The acceleration of the beginning of TGS during the last three decades was faster than the beginning of the frost-free period, indicating a possible increase in vegetation exposure to spring frost. This may pose a threat to the development of plants in the first phase of vegetation.