Variability of thermal and precipitation conditions in the growing season in Poland in the years 1966–2015
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The aim of the study was to identify the thermal and precipitation conditions and their changes in the growing season in Poland in the years 1966–2015. Data on average daily air temperature and daily precipitation totals for 30 stations from the period of 1966–2015 were used. The data were obtained from the collections of the Institute of Meteorology and Water Management—National Research Institute. The growing season was defined as the period of average daily air temperature ≥ 5 °C. The mathematical formulas proposed by Gumiński (1948) were used to determine its start and end dates. In the growing season in Poland in the years 1966–2015, there were more significant changes in the thermal conditions than there were in the precipitation conditions. In terms of long-term trends over the study period, thermal conditions during the growing season are characterised by an increase in mean air temperature, an increase in the sum of air temperatures and an increasing occurrence of seasons classified as above-normal seasons. Precipitation conditions of the growing season show large temporal and spatial variations in precipitation and a predominance of normal conditions. The changes in precipitation were not statistically significant, except for Świnoujście.
In recent years, an increase in the average global air temperature has been observed. In the northern hemisphere, the 1983–2012 period was probably the warmest 30-year period in the last 1400 years (IPCC 2013). One consequence of the increase in air temperature is an extension of the growing season in Europe, which results in changes in the phenological stages of individual plants (Menzel and Fabian 1999; Chmielewski and Rötzer 2001; Chmielewski et al. 2004). In the coming years, the length of the growing season in Poland is forecast to increase. Nieróbca et al. (2013) indicate that, by 2030, the growing season in central Poland may have increased by 10–14 days over its length during the reference period 1971–2000 and, by 2050, by 18–27 days, while in south-western Poland, those increases are forecast to be 11–17 days and 22–30 days, respectively. A similar trend is predicted by Skaugen and Tveito (2004) for Norway by 2050.
Changes in thermal conditions may, however, also have adverse effects on agriculture (Nieróbca 2009). Reduced productivity of some crops may result from heat stress and deteriorating water balance. Increasing temperature may also promote the development of heat-loving weeds, pests or the emergence of new plant diseases. Changes in the start and end dates and length of the growing season, in addition to the consequences for plant ecosystems, may lead to a long-term increase in carbon storage, and changes in vegetation may affect the climate system (Linderholm 2006).
The aim of the study was to identify the thermal and precipitation conditions in the growing season in Poland in years 1966–2015 and to determine changes in the above conditions in the analysed multi-year period.
2 Data and methods
Based on the above data, the start and end dates of the growing season were determined. A period with an average daily air temperature ≥ 5 °C was considered as a growing season (Carter 1998; Skaugen and Tveito 2004; Linderholm et al. 2008; Szyga-Pluta 2011; Żmudzka 2012; Radzka 2013). The mathematical formulas proposed by R. Gumiński (1948) were used to determine the growing season’s start and end dates.
- t 1
Average temperature in the month preceding the threshold temperature
- t 2
Average temperature in the month following the threshold temperature
Number of days between the day with the threshold temperature and the 15th day of the preceding month
The number of days calculated on the basis of the above formula is added to the 15th day of the month preceding the threshold temperature. If the desired number is greater than 15, when adding, one should consider the actual number of days in a given month. The obtained date is the beginning or end of the separated period. The above method is commonly adopted in determining growing seasons and thermal seasons (Skowera and Kopeć 2008; Szyga-Pluta 2011; Kępińska-Kasprzak and Mager 2015; Czernecki and Miętus 2017). It was also used for the variability of seasons in individual years by Kossowska-Cezak (2005). Kępińska-Kasprzak and Mager (2015) compared the thermal vegetation period determined by the method of Gumiński with those determined by the method of Huculak and Makowiec (the latter consists in calculating the accumulated series of deviations of average daily air temperature values from appropriate threshold values) for the period 1966–2005 in Poland and they discussed the possibility of their use to determine the beginning and end of the growing season. They stated that in long-term datasets, both methods lead to similar results, and suggested their comparison with phenological observations and/or vegetation indicators obtained by satellite remote sensing. Bartoszek and Siłuch (2015) showed a significant accordance between average vegetation period onsets as determined by both the Gumiński and remote sensing methods in the decade 2001–2010.
In the next step, the average air temperature, the sum of the air temperatures and the sum of precipitation in the growing season in each year were calculated. The actual length of the growing season in particular years was taken into account to calculate these characteristics. Subsequently, the changes in the beginning and the end of the growing season and changes in the air temperature and precipitation totals in the analysed multi-year period were determined. The Mann–Kendall nonparametric test was used to determine the statistical significance (p < 0.05). In addition, for selected seasons, anomalies of monthly precipitation totals were calculated. Anomalies were calculated as the difference between the monthly precipitation total in a given year and the average monthly total from the multi-year period.
Scale range of the thermal classification according to Lorenc (1994)
3.1 Beginning and end of growing season
3.2 Thermal conditions of the growing season
Thermal classification of the growing season in Poland in the years 1966–2015
3.3 Precipitation conditions of the growing season
Precipitation classification of the growing season in the years 1966–2015 in Poland
4 Discussion and summary
In the years 1966–2015, the growing season in Poland on average started on 26 March and ended on 7 November—earliest in the south-west and the latest in the north and north-east. The end of the growing season took place earliest in the north-east, progressing to the south-west and along the north coast. Over the analysed period, the growing season started increasingly early; meanwhile, the end of the growing season was later and later. The recorded changes were statistically significant in the majority of the country except for the north-eastern regions, the Świętokrzyskie Mountains and the Lublin Upland. Results of Nieróbca et al. (2013) show a similar tendency. The increase in the length of the growing season in different regions of Poland was also confirmed by other authors, among others, Żmudzka and Dobrowolska (2001), Kożuchowski and Degirmendžić (2005), Olechnowicz-Bobrowska and Wojkowski (2006), Skowera and Kopeć (2008), Kijowska (2010), Krużel et al. (2015), Tylkowski (2015) and Graczyk and Kundzewicz (2016). A similar trend of changes has also been observed in other temperate regions, inter alia, in Scandinavia and Finland (Carter 1998; Irannezhad and Kløve 2015) and China (Song et al. 2009).
The thermal conditions of the growing season changed clearly in the analysed multi-year period. In the area of Poland, there was an increase in air temperature during the growing season. Apart from northern Poland and the vicinity of Płock and Białystok, the observed changes were statistically significant at a level of at least 0.05. The highest increase in air temperature was recorded in southern Poland, with the maximum in Rzeszów (0.26 °C/10 years) and Kraków and Wrocław (0.25 °C/10 years). A similar trend in most analysed stations in north-eastern Poland was noted by Radzka (2014), where in April, July and August significant changes in air temperature were recorded. The highest increase in this parameter (from 0.6 °C to 0.8 °C per 10 years) was observed in April. According to Radzka (2014), in stations in the east of the examined region, a significant temperature increase (from 0.3 °C to 0.5 °C per 10 years) also occurred in October. A significant temperature increase occurred, among others, in the warm part of the year and translated into a prolongation of the intensive growing season and a significant increase in thermal resources during the period of active growth of plants (Żmudzka 2012). The increase in temperature at the beginning of the growing season is the reason for the early phenological stages in Poland (Jabłońska et al. 2015). Skaugen and Tveito (2004) forecast a prolongation of the vegetation season due to the expected increase in air temperature in the warm mid-year by 2015. On the other hand, based on mesoscale data from the Beskids and Foothills, Bochenek et al. (2013) observed a slight downward trend in air temperature (− 0.05 °C/year) during the growing season.
At all stations, there was a marked increase in the frequency of above-normal seasons in the past 20 years, with a simultaneous decrease in seasons falling below normal. There was also a statistically significant increase in total air temperatures in Poland. The smallest changes took place in north-eastern Poland with a minimum in Białystok, 57.5 °C/10 years. Much more significant changes occurred in western and southern Poland.
Precipitation conditions in the years 1966–2015 did not change so much. Precipitation totals are characterised by high temporal variability. Żarski et al. (2014) find that the greatest variability occurs in August and the authors stress that it causes a climatic risk to growing plants.
In the years under study, the increase of precipitation in the growing season was observed at most of the stations. However, the changes were statistically significant only in Świnoujście (22 mm/10 years). The remaining coastal stations recorded slightly smaller increases, albeit much higher than in the rest of the country. According to Żarski et al. (2014), a significant trend in the increase in precipitation in Bydgoszcz in the period of 1981–2010 was found only in May. A reduction in total precipitation was found in the area extending from the Silesian Lowlands and Kraków-Częstochowa Upland through central Poland to the Masurian Lake District.
During the growing season in Poland, normal conditions prevailed in the period considered. Extremely dry seasons were observed at some stations. The seasons classified as extremely wet occurred much more often. There were no statistically significant changes found in the studied years.
In conclusion, it should be stated that in the growing season in Poland in the years1966–2015, there were more significant changes in thermal conditions than in precipitation conditions.
increase in the average air temperature in the growing season
increase in the sum of air temperatures in the growing season
increasing occurrence of seasons classified as above the norm
no significant changes (except for Świnoujście) in the sum of precipitation in the growing season
large temporal and spatial variability of precipitation in the growing season
prevalence of normal conditions
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