1. Introduction
Grapevine cultivation worldwide occurs at various latitudes; however, the majority of vineyards are established between the 30th and 50th parallels north and the 30th and 40th parallels south [
1], primarily due to favourable climatic conditions. In the Northern Hemisphere, the main concentration of viticulture is noted between the 30th and 40th parallels, such as in countries bordering the Mediterranean Sea or in California in the USA. Poland is a country situated on the northern edge of intensive grape cultivation (that is over the 50th parallel in the north) [
2]; however, this does not disqualify it from wine production development. Grape cultivation has been documented since medieval times, initially in the southern parts of the country [
3] and later in the west [
4]. Initially, grapes were grown solely for the needs of the clergy, near monasteries or churches, but over time, townspeople also began establishing their vineyards. In the 16th century, wine-producing regions in Poland were popular; however, in the 17th and 18th centuries, the wine industry experienced a crisis due to climatic and political conditions. Periods of increased or decreased grape cultivation were associated with thermal and precipitation conditions, as well as political situations and historical events. Based on vineyard records dating back to the medieval period, five wine-producing regions were identified: Podkarpackie, Małopolska, Małopolski Przełom Wisły, Sandomierz, and Zielona Góra [
5].
The Zielona Góra region proved to be particularly attractive, becoming the north-easternmost winegrowing area in Europe in which grape cultivation took place on an industrial scale [
4]. Winemaking traditions probably date back to AD 1150, when Flemish settlers brought the first grapevines. However, this date lacks confirmation in historical sources. The first documented mention of grape cultivation in the Zielona Góra region dates back to AD 1314 [
6]. Thus, it can be concluded that the winemaking tradition in this region has gone on intermittently since the Middle Ages [
7].
Currently, interest in wine and vineyard management is increasing in Poland. Each year sees a growing number of farms cultivating grapevines and an increase in cultivation area [
8,
9,
10] primarily due to climate warming. However, it is essential to remember that, alongside climatic conditions, habitat conditions play an incredibly significant role in vineyard management [
5], and the quality and quantity of wine produced are influenced by the “terroir” [
11]. Moreover, non-natural factors such as increased knowledge among growers, greater accessibility to vineyard management knowledge, the introduction of pest- and disease-resistant varieties, the growing trend in wine consumption, and relevant legal regulations significantly contribute to the noticeable upward trend in the number of vineyards [
12].
Nevertheless, it should be emphasised that natural conditions primarily influence the possibility of reviving winemaking traditions in Poland. The impact of climate change concerning grapevines (particularly its effect on wine quality) [
13,
14], the shifting boundaries of grape cultivation [
13], potential new cultivation areas [
15,
16], analyses of climate indicators [
17], and changes in phenology and frosts [
18] are frequently discussed in the literature. Few studies are focusing on Poland in this regard, addressing enotourism possibilities in wine-producing regions [
19,
20,
21,
22]; grape cultivation prospects in Poland [
23,
24], including the identification of existing winemaking problems [
25]; and the influence of weather conditions [
26,
27] climatic [
28,
29,
30,
31], and agroclimatic [
32] conditions on grape cultivation and their specific varieties and on wine quality [
33].
One of the aforementioned studies [
28] concentrates on analysing precipitation in the western part of Poland concerning grapevine requirements. However, there has not yet been a comprehensive assessment of the thermal conditions in the Zielona Góra winegrowing region. Such an analysis is crucial for proper cultivation, particularly considering the exceptional sensitivity of grapevines to climatic conditions [
34].
This research aimed to determine the agroclimatic conditions from 1951 to 2022 for grapevines in the Zielona Góra region and the directions of their changes. Such a study was justified by the lack of previous investigations of the agroclimate of the Zielona Góra winegrowing area, which is particularly crucial in the era of rapid climate warming. This research utilised selected agroclimatic indicators to assess conditions from the grapevine perspective.
3. Materials and Methods
This research was conducted based on data obtained from the resources of the Institute of Meteorology and Water Management—National Research Institute (IMGW-PIB) for the Zielona Góra station from the multi-year period 1951–2022. The station was chosen due to its location in the centre of the Zielona Góra winegrowing region and the length of the measurement series. The analysis covered over 70 years, so it was divided and analysed in seven decades. Daily average, maximum, and minimum air temperatures, as well as daily and monthly precipitation totals, were used to calculate agroclimatic indicators.
Agroclimatic indicators described in the Polish literature were included in the analysis.
Table 1 lists the applied agroclimatic indicators along with their characteristics and significance for grape cultivation, referencing the literature.
Based on the acquired data, the monthly and annual average air temperatures were calculated for each year, and then the average values for each decade and the entire multi-year period were determined. The warmest month was identified based on the calculations. Similar calculations were carried out for the minimum temperature in January. Absolute minimum air temperature values for the year and January were determined based on daily minimum air temperature values, and characteristic days with air temperatures ranging from −15 °C to −35 °C were identified. Additionally, days with air temperatures above 35 °C were determined based on maximum daily air temperatures.
Frost days were defined as days when the minimum temperature is below 0 °C and the maximum temperature is above 0 °C (t
min < 0 °C and t
max > 0 °C) [
42]. Based on the literature, a classification of frost intensity for grapevines was proposed:
Mild frost: from 0 to −2 °C.
Frost damaging to leaves: from −2 °C to −3 °C.
Frost damaging to buds: from −3 °C to −5 °C.
Frost damaging to the entire plant: below −5 °C.
SAT was calculated from the sum of average daily temperatures equal to or higher than 10 °C from the period 1 April to 31 October (Dawitaj’s method) [
43]. It was calculated using the following formula:
where
Td ≥ 10 °C
GDD (Winkler method) was calculated using the following formula:
where
Tbase is 10 °C.
To determine the beginning and end dates of the growing season, the mathematical method proposed by Gumiński [
44] was employed, with the following assumptions: the average monthly temperature occurs on the 15th day of the month, each month consists of 30 days, and temperature changes from month to month are uniform. Two formulas were utilised in the calculations:
For temperature increase:
For temperature decrease:
where the variables are as follows:
tp is the threshold temperature;
t1 is the average temperature in the month preceding the threshold temperature;
t2 is the average temperature in the month following the threshold temperature;
x is the number of days between the day with the threshold temperature and the 15th day of the preceding month.
The result obtained (the number of days) is added to the 15th day of the preceding month with respect to the threshold temperature. The determined date serves as the beginning or end of the growing season. In this article, the threshold temperature was considered as ≥8 °C.
Based on daily precipitation sums, the total precipitation for each month of the growing season and their cumulative values were calculated, divided into decades and for the entire multi-year period.
For the calculated indicators, average values for the entire multi-year period and decades were determined. Additionally, the rate and direction of changes in the aforementioned indicators over the considered multi-year period were determined using linear regression analysis, and statistical significance was assessed based on Student’s t-test at a significance level of p < 0.05.
Table 1.
Climatic and agroclimatic indicators for viticulture with their significance.
Table 1.
Climatic and agroclimatic indicators for viticulture with their significance.
No. | Climatic/Agroclimatic Indicator | Importance/Methods of Analysis | References |
---|
1. | average annual air temperature | Areas favourable for grape cultivation are those where the average annual air temperature is ≥8 °C. | [45] |
2. | average monthly air temperature of the warmest month of the year | Areas favourable for grape cultivation are those where the average monthly air temperature of the warmest month is ≥17 °C. | [45] |
3 | the dates of the first and last frosts and the length of the frost-free period in days | The occurrence of frost poses a threat to the profitability of grape cultivation. Frosts occurring in April and May are particularly dangerous. Temperature drops below −2 °C negatively affect fruiting, while frosts around −4 °C can lead to the loss of the entire yield. Frosts after October 15, around −3 °C, damage leaves and interrupt ripening, which is significant for varieties ripening in October. | [45,46] |
4. | absolute values of minimum air temperature | Different grape varieties have different frost tolerance. Depending on the variety, temperatures can range from −15 °C to −35 °C. Temperatures below −25 °C are a particular threat to the vines and break the winter dormancy phase. | [45,47] |
5. | sum of active temperatures (SAT) and Growing Degree Days (GDD) | The SAT, which stands for the sum of active temperatures, indicates the accumulated amount of warmth during the growing season. For very early varieties, it ranges from 2000 to 2200 °C, early varieties from 2200 to 2500 °C, early–medium varieties from 2500 to 2700 °C, medium–late varieties from 2700 to 2900 °C, and late varieties above 2900 °C. The value of the SAT may vary depending on the slope inclination, exposure, soil type, or wind protection. Ideally, this indicator should not fall below 2000 °C. The higher the SAT value, the later grapevine varieties can be cultivated in a given area. The GDD index is otherwise known as the Winkler index and describes the heat energy received by the crop maturity over a given time period to progress in development or growth processes. Winkler divided 5 viticulture zones according to the value of the GDD: I—values equal to or below 1371 °C, II—1372–1648 °C, III—1649–1927 °C, IV—1928–2204 °C, V—greater than or equal to 2204 °C. | [34,43,45,47] |
6. | the average monthly temperature in January, the minimum temperature in January, and the lowest values of minimum temperatures in January | The January temperature is representative for analysing the threat of severe frost. A monthly average January temperature below −1 °C is an indication for additional protective measures to prevent frost damage. Far better, exceeding the threshold of −1 °C indicates the monthly average minimum January temperature, and analysis of the lowest minimum temperature in January and the year allows one to assess the frost resistance of varieties and possible protective measures. | [48] |
7. | average number of hot days | Temperatures above 35 °C lead to the onset of heat acclimation, while temperatures above 40 °C negatively affect photosynthesis. They are particularly dangerous when they occur over periods of several days and are accompanied by a lack of water. | [49,50] |
8. | average length of the growing season | This is one of the most important climatic criteria used to assess the feasibility of growing grapevines. Vines begin and end the growing season at 8–10 °C and the season should not be shorter than 160 days. In this study, the growing season is defined as a period with an average daily air temperature of at least 8 °C. | [5,44,45] |
9. | precipitation during the growing season | Grapes require significant amounts of water, with its delivery during the growing season being particularly crucial. The greatest amounts of water are needed by the plant during intense growth and berry development, typically from mid-May to mid-August. Drought as well as excessive rainfall can contribute to poor flowering. Excessive rainfall during fruit ripening, in September for example, can cause fruit cracking and rotting. | [5,45] |
5. Discussion
The ongoing climate changes significantly influence the distribution of grapevines worldwide [
13]. The increasing average air temperature necessitates the adaptation of grapevines to climate change in current winegrowing regions or the relocation of cultivation zones to areas not traditionally associated with vineyards, such as further north [
51,
52]. This study focuses on the characterising climatic and agroclimatic conditions regarding grapevines in one of the revitalising wine regions in western Poland (the Zielona Góra region). The conducted research confirms that the Zielona Góra region is becoming increasingly suitable climatically for grapevine cultivation. The studies have shown a statistically significant increase in the average annual air temperature at a rate of 0.29 °C/10 years. The obtained results are consistent with earlier studies conducted in the country [
53,
54,
55,
56,
57]. Furthermore, the documented changes align with those occurring in other regions of Europe [
58] and worldwide [
59]. The prevailing thermal conditions are suitable for the cultivation of selected grapevine species, such as Riesling, Rondo, Regent, Aurora, Pinot Noir, and Solaris. Research by Koźmiński et al. [
30] indicates that 60% of the country’s area is conducive to intensive grapevine cultivation.
The average annual air temperature, as well as the average temperature of the warmest month, exceeds the threshold of 8 °C and 17 °C, respectively, and since the 1990s, sporadic years with lower values have occurred. Urban [
60] presented very similar results of a thermal conditions analysis in Zielona Góra at the turn of the 21st century in a climatological study covering the years 1961–2016. Similar results were obtained in the areas of the Wieliczka Foothills in the vineyards of Gaik-Brzezowa and Łazy for the years 1988–2007 [
32]. The main difference between vineyards in western and southern Poland lies in the air temperature values in January. The average monthly value for January (0.1 °C), the average minimum monthly temperature for January (−2.3 °C), and the average of the lowest minimum temperatures in January (−14.5 °C) during the period 1988–2007 were about 1–1.5 °C lower in southern Poland, particularly at the Gaik-Brzezowa station. Comparing the results with the Łazy station, the difference in these values is even greater [
32]).
The observed warming has also led to an earlier start and a later end of the vegetative period. Consequently, the period has extended, but the change in the start date had the greatest impact. Comparing the obtained results with those from other regions in Poland, it can be concluded that the Zielona Góra region experienced an earlier start and earlier end of the period. The difference between the analysed region and other regions in southern Poland [
32] was about (or over) two weeks for the start of the period and less than a week for the end. The prolongation of the vegetative period, its earlier start, and later end for Poland were confirmed by Tomczyk and Szyga-Pluta [
61] and Żmudzka [
62] in their research. Tomczyk and Szyga-Pluta [
61] note that the end of the vegetative period occurs earlier in the Wieliczka Foothills than in the Lubuskie region. Żmudzka [
62] indicates that the extension of the growing season and intensive vegetation create opportunities for introducing plants with higher thermal requirements, including grapevines, to Poland. It is forecasted that the vegetative season will significantly lengthen, with the longest vegetative period expected in southwestern Poland, reaching over 290 days under the RCP8.5 scenario (a scenario created for the business as usual formula that says that by the end of the century CO
2 concentrations are assumed to reach about 940 ppm and a radiative forcing of 8.5 [W/m
2] and the Earth’s average temperature will rise by 4.5 °C compared to the pre-industrial era) by the end of the 21st century [
63]. In this work, the growing season temperature threshold is 5 °C.
The observed warming has also influenced the length of the frost-free period in the analysed area. Similarly to the vegetative period, differences in the length of the frost-free period between the Zielona Góra region and regions in southern Poland were noted. This period was four days longer in western Poland than in Gaik-Brzezowa, while in Łazy it was as much as 22 days longer. This discrepancy in the duration of the frost-free period is likely due to the location of Gaik-Brzezowa above the Dobczyce Reservoir, which has a moderating effect on the climate and increases the minimum air temperature [
32].
The ongoing warming and the lengthening vegetative period do not eliminate the occurrence of spring and autumn frosts [
64]. Spring frosts are particularly perilous not only for grapevines cultivated in the west but also in other wine regions in Poland.
The year September 2008 was especially perilous for cultivation in central Poland, as on January 6, a minimum temperature of around −23 °C was recorded. Different varieties exhibited varying frost resistance, with ‘Riesling’ varieties coping best with low temperatures, while Polish varieties ‘Rondo’ and ‘Regent’ were highly susceptible to occasional freezing winters occurring every few years [
26]. It is worth noting that in that year, the air temperature in Zielona Góra was higher than in central Poland, which did not result in such numerous vineyard frosts. However, there are years when the air temperature drops below −20 °C, posing a risk of plant damage. As historical records indicate, frosts damaged vineyards in the Zielona Góra wine region in 1997, when temperatures dropped to −10 °C from April 11 to 13, accompanied by snowfall, and frosts with temperatures between −7 and −5 °C also occurred between May and June [
4]. In addition to frosts, extremely low winter air temperatures are also extremely dangerous. In 1956, due to February frosts, young seedlings were damaged, sub-surface roots froze, and even older bushes perished. The severe frosts that year resulted in a lack of a grape harvest. Moreover, frosts affected wine quality and yields in 1963 and 1971 [
4].
The increasing air temperature also contributes to raising the SAT values. Since the beginning of the 21st century, the sum of active temperatures has exceeded 3000 °C, allowing for the cultivation of even late grapevine varieties. Similar results were obtained by Kryza et al. [
31], who examined SAT values for the transboundary area of Germany, Poland, and the Czech Republic. In their studies, the Zielona Góra wine region performed very favourably. For the years 1970–2010, the SAT in the area examined by the authors ranged between 2500 and 3000 °C, depending on the location, with a rising and statistically significant trend of change. Additionally, Kryza et al. [
31] used the Growing Degree Days (GDD) indicator, which in the discussed area ranged from 1000 to 1200 °C, indicating suitable and good climatic suitability for late grapevine ripening. The GDD index is estimated as the sum of daily average temperature values from 01.04 to 31.10 (greater than or equal to 10 °C) minus a constant value of 10 °C. Favourable conditions for grapevine cultivation are present not only in the Zielona Góra wine region but also in eastern Germany [
31]. In southwestern Poland, in Wrocław, SAT values have consistently exceeded 3000 °C since 2013, confirming the suitability of southwestern Poland for the cultivation of late grapevine varieties [
30].
Although grapevines are considered to be drought-resistant plants [
65], they require monitoring in the face of decreasing precipitation during the growing season to achieve an adequate quantity and quality of yields. Precipitation conducive to grapevine cultivation should range from 500 to 800 mm, and in most places in the country, this sum is fulfilled [
5]. However, in addition to the annual precipitation sum, the appropriate distribution of precipitation during the growing season is crucial. Jagosz et al. [
28] analysed the water requirements for grapevines in the West Pomeranian, Lubusz, and Lower Silesian Voivodeships. The research found that Lubusz Voivodeship has the greatest variability in water needs for grapevines. In the era of climate change, the water demand for grapevines in the Lubusz Voivodeship is increasing by 9.1 mm/10 years. There is a need for additional irrigation of vineyards in very dry years during the June–August period [
28].
Research on the adaptation of grapevines to changing climates at similar and higher latitudes to Poland was conducted by Dunn et al. [
15] in Scotland. Interestingly, thermal analyses conducted for the whole year revealed numerous areas in Scotland suitable for grapevine cultivation. However, limiting the analysis to thermal conditions in summer alone revealed a small area suitable for growing this plant. Furthermore, adding lithological, topographic, or rainfall conditions to the analysis showed that despite favourable thermal conditions, Scotland is not be suitable for grapevine cultivation on an industrial scale. The results of the aforementioned studies are a basis for further analysis of the habitat conditions in the Zielona Góra wine region. It is important to consider the entirety of ‘terroir’ in the context of grapevine cultivation, not just selected parameters.
Future climate forecasts suggest significant changes that may profoundly impact grapevine cultivation and existing wine regions. The increasing average minimum and maximum air temperatures, the number of hot days, the decrease in the number of frost days in Poland [
56], and the lengthening of the vegetative period [
63] are revealing new areas suitable for grapevine cultivation, especially in southern and southwestern Poland, and are positively affecting yields and wine quality in current vineyards. In the future, wine regions in Southern Europe and the Mediterranean Basin will likely be vulnerable to reduced productivity, mainly due to droughts and water scarcity [
16,
66,
67]. Over the next 50 years, with the current increase in air temperature, the northern limit of grapevine cultivation in Europe may shift to around the 60th parallel of latitude [
68]. Therefore, it will be essential to relocate current grapevine cultivation from known wine regions to areas with cooler climates.