Late Spring Frost Trends and the Risk of Crop Losses: A Shift in Focus

For most stations, the analysis of trends in the time series of the last spring frost date at the height of 2 m showed a statistically significant trend indicating its earlier appearance. The trend calculated in this research study, ranging from 1.6 to 3.5 days per decade, and statistically significant changes confirm the previous results of research studies from the territory of Poland [7,23]. Any minor differences may result from the selection of the station.

Of course, Poland is not the only country in which such observations have been made. In the literature, one can find a number of studies, not only from Europe [14] but also from other continents, such as North America [24] or Asia [25], which show a statistically significant shift of the date of the last spring frost.

Despite this, in the last 10 years, in Europe, in at least the last 2 spring seasons (2011 and 2017), very large losses related to the occurrence of late and severe frosts were recorded. The acceleration of the last spring frost date calculated as the number of a consecutive day of the year does not seem to have a significant impact on reducing losses in agriculture and horticulture. In May 2011, significant losses in horticultural crops with a decrease in yields exceeding even 70% in many regions of Poland [26] were recorded. In 2017, the April and May strong frosts in Europe resulted in unprecedented losses of over euros 3.3 billion [27] in grapevines and fruit trees. One of the reasons for that may have been the change in the length of the growing season, including its earlier beginning.

Therefore, to correctly assess the changes in the risk associated with the occurrence of late frosts, they should be related to parameters that will allow to associate them with the plant’s development phase, e.g., growing degree days for different temperature levels or the day of frost occurrence calculated as a consecutive day of the growing season. In this study, referencing the last spring frost date to the beginning of the growing season resulted in considerable changes in the results of trend detection. While changes in the date of the last frosts, described as a day in a calendar year, undoubtedly take place, they practically do not change in relation to the growing season. Statistically significant changes during the growing season concerned only 1 station for measurements at the height of 2 m and 2 for measurements at the ground level.

According to [28], it is possible that the extension of the growing season duration, despite the decrease in the number of frost days per year, will increase the risk for plants. Moreover, the increase in the number of days with frost during the growing season is the most significant in regions where the growing season has lengthened the most, including Europe. In paper [29], the date of occurrence of heavy spring frosts (Tmin < -4°C) was compared to Growing Degree Days at 5°C. The obtained results show great spatial variability. In large areas of Europe and Asia, where the increase in the average annual air temperature is the highest, the risk of damage caused by late frosts increases.

No changes in the probability of crop losses or even an increase in this probability could be even more pronounced if they were related to the key stages of development of specific plant species. Such results were presented for 27 tree species in Europe [30].

It should also be remembered that the risk of crop losses caused by spring frosts may be different for different species and also for different populations within species depending on geographic location.