The Impact of Climatic Warming on Earlier Wine-Grape Ripening in Northeastern Slovenia

Abstract

In this study, the development trends of bioclimatic parameters recorded at the Maribor and Murska Sobota climate stations from 1952 to 2022 and the dynamics of grape ripening in early-, medium-, and late-ripening grape varieties in the Podravje wine-growing region in Slovenia (north-eastern Slovenia) from 1980 to 2022 were investigated. Based on the data on soluble solids content, total acidity, and the recommended harvest date per year (until the technological ripeness of the grapes; 76°Oe), trends for shortening the growing period of the vines were calculated. Temperature changes have been more pronounced since 1980. The number of so-called hot days (with a maximum of T > 30 °C) has increased the most, which has the greatest impact on other bioclimatic parameters, e.g., the average temperature and growing degree days (GDDs) and the Huglin index (HI). For the period of 1980 to 2022, the trends were 0.44 °C (Murska Sobota) and 0.51 °C (Maribor) per decade, respectively. The trends were more pronounced for the average temperature in the period of May–June (TMJ). After 1980, the HI increased by about 10 units per year. As a result of the climate warming, grapes in north-eastern Slovenia ripened 26 (‘Sauvignon Blanc’) to 35 (‘Welschriesling’) days earlier. The trends showed a decrease in total acidity, which can be attributed to the higher temperatures during the growing season period, especially during the ripening period of the grapes (véraison). After 2010, the average temperatures during the growing season (1 April to 31 October) in Podravje were 1.6 °C higher than in the 1980s. In line with the earlier ripening of the grapes, the actual average temperature from 1 April to the harvest date was a further 1.0 °C higher. The higher temperatures in the late-ripening varieties ‘Riesling’ and ‘Furmint’ had a positive effect on the lower total acidity. Total annual precipitation and precipitation in the growing season for the period 1980 to 2022 in the Maribor area show decreasing trends of 6 mm/m² (p = 0.001) and 4 mm/m² (p = 0.012), respectively. In the eastern sub-wine-growing region of Podravje (Murska Sobota), the trends in precipitation were not significant.

1. Introduction

Many researchers have studied climate change in different regions [1,2,3,4,5] and its impact on viticulture [6,7,8], often with a particular focus on assessing and predicting the impact on grapevine development and the wine industry [9,10]. Warming has the potential to bring numerous risks and challenges that affect both the quality and quantity of grape production [11]. While changes in average temperatures are obvious and important, increasing attention is being paid to the analysis of extreme events due to their potential impact on viticulture [12,13,14]. The results show that the risk of unfavorable conditions during vine flowering decreases, while the risk of late frosts in spring increases due to the shift in the timing of bud break [15,16,17].

Throughout history, one of the main objectives of wine-growers in the different wine-growing regions has been to achieve maximum soluble solids contents without causing the berries to shrivel. In recent times, more consumers prefer lighter wines with moderate alcohol contents. In addition to the scenario described above, there is also global warming [18]. Worldwide, the phenology of grapevines has developed in step with the temperature trends caused by climate change in recent decades [19,20,21]. Analysis of the meteorological data shows a clear rise in temperatures. However, it is more likely that the most significant impacts will result from an increase in temperature, even if other climate variables, such as precipitation, are also considered [22]. A comparison of climatic and phenological data shows that the period between bud break and harvest has become earlier and shorter [23,24], and grapes ripen earlier under increasingly warmer conditions, which often has undesirable effects [25]. Post-flowering water requirements tend to increase due to climate, and as there is no clear evidence of a change in precipitation, the risks associated with dry summers are likely to increase in the future [26]. However, to fully understand how climate change contributes to changes in harvest dates, grapevine phenology and its relationship with climate must be analyzed over a longer time period, including data that predate anthropogenic interventions in the climate system [21,27,28]. During the growing season, grapevines require sustained daily average temperatures above 10 °C to initiate growth, followed by sufficient heat accumulation for fruit ripening [29]. However, temperature extremes during berry growth lead to stress, premature ripening, berry shedding, enzyme activation, and reduced flavor development [30]. Frost occurrence and timing are also important for grapevines, which benefit from a low risk of frost in spring and fall and a long frost-free season of 160 to 200 days or more. In terms of moisture requirements, grapevines should ideally have sufficient soil moisture for initial growth at the beginning of the growing season and then receive nominal amounts throughout the growing season (either naturally or through irrigation) [10].

The impact of climate change on the viticulture sector varies from region to region. The vines need both sufficient cold periods for hardening and fruit formation and sufficient warm periods to ripen quality fruit at an economically viable level without being overly stressed. The grapevine is therefore a model system for monitoring the effects of climate change because it has a long history, because it is grown in narrow climatic zones for which the individual varieties are best suited, and because the wines are tasted and rated for quality [27]. The predicted rise in global temperatures over the next half century could ultimately prove problematic for the wine industry. Minor changes in growing season temperatures could lead to shifts in varietal suitability in many regions [19] or require costly adaptation measures, particularly in soil management [31]. In addition, in regions such as Europe, where vines are not irrigated, either due to legal restrictions or for supply reasons, changes in the total amount of precipitation or its distribution over the year can have a significant impact on water availability for plants, especially in the warmer seasons [10].

Wine-growers traditionally select different wine-grape varieties according to the phenotypic characteristics that best suit their microclimates [32] and soils. They retain those varieties that produce consistent yields under local climatic conditions and have an appropriate balance of sugar, acidity, and other compounds [33]. Climate change, with its extreme weather conditions, will make it necessary to change varieties in many wine-growing regions, especially in regions with disease pressure and difficult growing conditions. New varieties (PIWI) are therefore better suited to climate-adapted and sustainable viticulture than traditional varieties are [34].

Climatic conditions during grape ripening have already changed, leading to a change in the composition of the grapes at harvest [17]. Grapes are being harvested with ever higher sugar levels, resulting in wines with higher alcohol levels [26,35]. In wine-growing regions around the world, the rise in temperature associated with climate change is responsible for earlier harvests. Determining the suitability of grape varieties in existing or new wine-growing regions is often based on temperature, without considering other factors. Sugar accumulation characteristics are also influenced by antecedent and concurrent climatic factors, such as the photosynthetically active radiation, temperature, and water status of the vine, regardless of whether this occurs before or after the mid-veraison [36]. Sugar is one of the most important metabolites in grape berries used for winemaking. Sugar concentration is a determining factor in the alcohol content of the finished wine, and its content during berry ripening is also involved in regulating the development of phenolic compounds that give the wine its color, flavor, and tannin structure [37,38].

Slovenia is a very small wine-growing region with different climatic zones (Mediterranean, continental, and Pannonian climates), where most of the world’s important grape varieties for quality wines are grown. Quantified data on climate development and bioclimatic indices during the growing season of grapevines and their possible effects on the earlier ripening of grapes are presented and discussed.

2. Data and Methods

2.1. Study Area

The long-term data available from the two meteorological stations (Maribor and Murska Sobota) in the Podravje wine-growing region in Slovenia were used for this study (Figure 1). The Podravje wine-growing region lies between the river Sava (SW) and the Hungarian border (NE). Geologically, the area is part of the former Pannonian Sea Basin and has a Pannonian continental transitional climate. The continental climate characteristics increase with increasing distance from the Alps. The vineyards (6000 ha) are predominantly planted with white grape varieties, on steep slopes with inclinations of 30–50%, and at altitudes of 250 to 350 m. The long-term average (1952–2022) for precipitation during the growing season (1 April to31 October) varies between 574 mm/m² (Murska Sobota) and 701 mm/m² (Maribor), and precipitation is very unevenly distributed throughout the year [39].

2.2. Climate Parameters and Grapevine Growing

Daily precipitation and temperature data (mean, maximum, and minimum) recorded at two meteorological stations (1952–2022) were used for the analysis. The data were taken from the archives of the Slovenian Environment Agency (SEA).

An analysis of the observed climate was carried out for the periods 1952–2022 and 1980–2022. The daily data from each station were divided into annual and vegetation periods and used to derive bioclimatic indices (

Table 1. Analyzed bioclimatic parameters.

Parameter	Parameter Description
Tavg	Average annual temperature, °C
Tmax	Average annual maximum temperature, °C
Tmin	Average annual minimum temperature, °C
GSTavg	Average growing season temperature (1 April to 31 October), °C
GSTmax	Average growing season maximum temperature (1 April to31 October), °C
GSTmin	Average growing season minimum temperature (1 April to 31 October), °C
TMJ	Average temperature May–October, °C
HI	Huglin index (1 April to 30 September) °C units
GDD	Growing degree days °C units
NDT30	Number of days with maximum temperature > 30 °C
NDF	Number of days with a minimum temperature <0 °C (frost occurrence)
NDFF	Number of days between the last frost and the first frost (length of frost-free period)
AP	Total annual precipitation, mm/m2
GSP	Total growing season precipitations (April to October), mm/m2

). For the growing season (1 April to 31 October), precipitation and temperature data (average, minimum, and maximum) of each station were summarized, as growing season averages trend to correlate significantly with wine production and quality [27]. To assess the signs of heat stress, the number of days with temperatures above 30 °C was determined. This temperature leads to the premature ripening of the grapes (a shorter vegetation period), lower total acidity, and fewer aroma compounds [24].

To simplify the global description of the weather during the growing season, there are several climatic indices. These indices enable qualified estimates of the effects of climate change on the development of the grapevine and the characteristics of the wine.

To obtain more information about the wine region and to determine general guidelines for the potential quality and style of the wine, the GDDs (growing degree days) [40] and the Huglin index (HI) [41] were calculated by summing the daily average temperatures above a base value of 10 °C (the sum of the effective temperatures), with values below 10 °C set to zero.

GDDs were calculated for the period of 1 April to 31 October (Winkler index—WI). Navratilova et al. (2021) [42] used the “shortened Winkler index”, which was recalculated for the period corresponding to the Huglin index. They believe that the original WI index is less indicative of the soluble solids content, according to their calculations. For the purposes of this article, GDDs were also calculated from 1 April to the harvest date (76°Oe; see Section 2.4), which better reflects the impact of warming on the earlier harvest date.

The Huglin index (HI) for the Northern Hemisphere is calculated using the following formula:

$$\mathrm{H}\mathrm{I}=\sum _{01.04}^{30.09}\mathrm{d}·\left[\frac{\left(\mathrm{T}\mathrm{a}\mathrm{v}\mathrm{g}-10\right)+\left(\mathrm{T}\mathrm{m}\mathrm{a}\mathrm{x}-10\right)}{2}\right],$$

where Tavg is the daily average air temperature (°C), Tmax is the daily maximum air temperature (°C), and d is the day length coefficient, which lies between 1.02 and 1.06 and between 40° and 50° north latitude. Base temperature = 10 °C. This index makes it possible to classify the wine-growing regions according to the sum of the temperatures required for the development of the vines and the ripening of the grapes.

Temperature extremes were calculated by the number of days with maximum temperatures >30 °C (NDT30) and average temperatures for the May–June period (TMJ). This parameter is important for predicting an increase in disease pressure (e.g., downy mildew), as more severe epidemics can be a direct result of more favorable temperature conditions in May and June [43].

2.3. Evaluated Varieties of Vitis vinifera L.

A long-term dataset (1980–2022) for several Vitis vinifera L. varieties has enabled a comprehensive assessment of varietal differences in terms of ripening time and relationships with climate and climate change in the Slovenian wine-growing region of Podravje. Data from the weekly monitoring of grape ripening in the period from 1980 to 2022 were collected and statistically analyzed for early-, medium-, and late-ripening grape varieties. Only in the Podravje wine-growing region were ripening data available for 15 varieties over such a long period. The data were collected at permanent locations in this region (Figure 1) and recorded by the Institute of Agriculture and Forestry in Maribor. Six varieties were selected as model grape varieties to assess the impact of climate change on vine ripening: ‘Bouvier’, the local early-ripening variety, ‘Chardonnay’ and ‘Sauvignon Blanc’, the two globally widespread varieties with medium–late ripening, ‘Blaufränkisch’, the local red variety, ‘Welschriesling’, the most widespread variety in Slovenia (especially in the Podravje wine-growing region), and ‘Furmint’, the late-ripening variety.

2.4. Monitoring of Grapevine Ripening

In the Podravje wine-growing region (Slovenia), the ripening of grapes has been monitored since 1980. For each variety, 100 berries were randomly sampled from about 25 plants (distance between plants was 1 m) from mid-véraison until technological maturity. The berries were always taken from the middle part of the bunch, with one from the inside and one from the outside of the bunch, so that there was a total of 50 berries on each side of the row. The degree of ripeness was determined according to the Slovenian wine law at the point when the total soluble solids had reached about 76°Oe (i.e., 76° on the Oechsle scale, a hydrometer scale that measures the density of grape must, which around 18°Brix; the limit for quality wine).

In poor vintages, the harvest date was determined according to the soluble solids content for quality wine or, in very poor vintages, according to the health of the grapes (vintages in the early 1980s). The focus was on the relationship between the bioclimatic and ripening parameters (total acidity) at the recommended harvest dates (76°Oe). Based on the data of soluble solids and total acidity in the grape juice and the recommended harvest date, the tendency towards an earlier ripening of the grapes and the correlations between the ripening parameters and the bioclimatic parameters were calculated for six varieties.

2.5. Data Evaluation and Statistical Analyses

The variables were evaluated using descriptive statistics and trend analysis. The average values of the parameters, the range of minimum and maximum values, and the processing of the linear trends of temperature, bioclimatic indices, and harvest date, as well as the linear correlations of temperatures, bioclimatic indices, and harvest date, were calculated. Since some of the parameters examined in the study were not normally distributed, a more stringent non-parametric Mann–Kendall trend test (MK test) with a significance level of 95% was applied to all series [44] implemented in R/Kendall [45]. The Mann–Kendall test, like other distribution-free or parametric tests, is very sensitive to the autocorrelation effect (persistence).

3. Result and Discussion

3.1. Climatic Structure and Temperatures Trends

The general climate for the period of 1952–2022 for the inland wine-growing region of Podravje in Slovenia was moderately continental, characterized by considerable seasonal temperature variations, cold winters, and moderately hot summers, with an average annual temperature of 10.3 °C (5.7 to 15.5 °C) for Maribor and 9.9 °C (4.8 to 15.3 °C) for Murska Sobota, with annual precipitation amounts of 998 mm/m² and 801 mm/m², respectively (

Table 2. Bioclimatic parameters for the two meteorological stations (Maribor and Murska Sobota) in the Slovenian wine-growing region of Podravje for the long-term period 1952–2022 and for the period 1980–2022, for which data on the ripening of grape are available. Figures in bold indicate significant trends (p ≤ 0.05).

Station/Period	Maribor 1952–2022					Maribor 1980–2022
	Variables					Variables
Parameters	Mean	SD	Trend	Tau	p	Mean	SD	Trend	Tau	p
			yr−1					yr−1
Tavg	10.3	0.99	0.037	0.628	0.001	10.8	0.82	0.048	0.524	0.001
Tmax	15.5	1.14	0.038	0.542	0.001	16.0	1.09	0.064	0.561	0.001
Tmin	5.7	1.06	0.042	0.66	0.001	6.4	0.71	0.034	0.420	0.001
GSTavg	15.8	0.99	0.037	0.589	0.001	16.4	0.80	0.044	0.482	0.001
GSTmax	21.5	1.15	0.038	0.491	0.001	22.1	1.09	0.063	0.526	0.001
GSTmin	10.6	1.02	0.041	0.615	0.001	11.3	0.66	0.024	0.300	0.005
TMJ	17.0	1.28	0.038	0.48	0.001	17.34	1.25	0.054	0.471	0.001
HI	1839	206	7.03	0.559	0.001	1947	187	10.08	0.491	0.001
GDD	1325	186	6.88	0.599	0.001	1432	154	8.10	0.480	0.001
NDT30	13.2	11.8	0.57	0.502	0.001	18	12.5	0.68	0.540	0.001
NDF	95	19	−0.56	−0.411	0.001	87	15.1	−0.39	−0.221	0.040
NDFF	206	22	0.53	0.340	0.310	214	20.8	0.30	0.118	0.272
AP	998	150	−2.88	−0.252	0.002	973	148	−5.8	−0.344	0.001
GSP	700	124	−1.68	−0.214	0.008	685	127	−3.9	−0.268	0.012
	Murska Sobota, 1952–2022					Murska Sobota, 1980–2022
Tavg	9.9	0.99	0.034	0.565	0.001	10.4	0.9	0.059	0.601	0.001
Tmax	15.3	1.16	0.036	0.520	0.001	15.8	1.1	0.066	0.528	0.001
Tmin	4.8	1.05	0.038	0.599	0.001	5.4	0.9	0.057	0.566	0.001
GSTavg	15.5	1.0	0.033	0.511	0.001	16.0	0.9	0.051	0.530	0.001
GSTmax	21.6	1.2	0.038	0.446	0.001	22.2	1.1	0.058	0.464	0.001
GSTmin	9.7	1.0	0.037	0.576	0.001	10.3	0.8	0.048	0.585	0.001
TMJ	16.8	1.25	0.037	0.475	0.001	17.48	1.28	0.058	0.375	0.001
HI	1831	213	6.43	0.478	0.001	1933	199.2	10.5	0.455	0.001
GDD	1278	186	6.21	0.532	0.001	1373	166.5	9.6	0.530	0.001
NDT30	14.1	12.1	0.37	0.447	0.001	19.1	12.5	0.55	0.428	0.001
NDF	110	18	−0.43	−0.374	0.050	105	17.4	−0.84	−0.431	0.001
NDFF	188	18	0.41	0.359	0.111	194	15.9	0.61	0.362	0.001
AP	801	112	−0.17	0.021	0.800	801	111.0	−0.07	0.013	0.908
GSP	574	94	0.28	0.037	0.655	576	92.5	0.52	0.069	0.523

). As far as the ripening potential of the grapes is concerned, the location was in the middle range (15.5 to 15.8 °C) (Table 2) [46]. The variability in temperature in the growing season (GSTavg, GSTmax, and GSTmin), the average temperature in the period of May to June (TMJ), the number of days with temperatures of <0 °C (NDF) and >30 °C (NDT30), and the frost-free period (NDFF) were similar at both stations (Table 2).

The values of growing degree days (GDDs) from 1 April to 31 October (Winkler index—WI) total 1278 to 1325 units, which places Podravje in Winkler region I (very cool—WI ≤ 1390), indicating a generally favorable climate for early-ripening grape varieties suitable for producing high-quality wines [40]. The average Huglin index (HI) values, which may be more appropriate than the WIs for European regions [47], date from 1831 to 1839 and place Podravje in Huglin’s at the beginning of the temperate climate type (HI-1; 1800 < HI ≤ 2100), which is suitable for ‘Pinot Noir’, ‘Traminer’, ‘Chardonnay’, ‘Riesling’, ‘Sauvignon Blanc’, and ‘Cabernet Franc’, for example [41].

The values for growing season precipitation (GSP) generally indicate that precipitation amounts decreased slightly for the entire period (1952–2022), but the trends for the Murska Sobota site were not significant. The variability in the GSP over this long-term period shows a variation of ~16% between years at both locations. The Murska Sobota site, with a total GSP amount of 574 mm, was drier (influenced by the Pannonian climate) than Maribor with 700 mm (

Table 3. Mean values and trends of total acidity in g/L and day in the year when the soluble solids content in grapes of 15 varieties was 76°Oe in the period 1980–2022 in the Podravje wine-growing region in Slovenia.

Parameters	Total Acidity g/L					Day in Year
Variety/Variable	Mean ± SD		Trend yr−1	Tau	p	Mean ± SD		Trend yr−1	Tau	p
‘Bouvier’	7.5	±1.10	−0.040	−0.273	0.010	247	±16.3	−0.99	−0.498	0.001
‘Müller Thurgau’	7.1	±1.16	−0.056	−0.421	0.001	258	±13.0	−0.65	−0.415	0.001
‘Muscat Ottonel’ *	6.0	±0.88	−0.035	−0.304	0.014	256	±13.5	−0.79	−0.364	0.003
‘Pinot Blanc’ **	8.9	±1.23	−0.036	−0.192	0.109	255	±14.5	−0.83	−0.414	0.001
‘Chardonnay’	10.2	±1.47	−0.072	−0.377	0.001	256	±15.5	−0.85	−0.482	0.001
‘Pinot Gris’	9.2	±1.57	−0.084	−0.458	0.001	253	±14.8	−0.76	−0.451	0.001
‘Sylvaner’ ***	8.6	±1.31	−0.029	−0.162	0.145	263	±16.7	−0.98	−0.490	0.001
‘Sauvignon Blanc’	10.3	±1.42	−0.053	−0.198	0.062	258	±16.2	−0.82	−0.455	0.001
‘Traminer’	8.2	±1.09	−0.035	−0.245	0.021	254	±15.7	−0.79	−0.469	0.001
‘Yellow Muscat’	8.3	±1.69	−0.095	−0.464	0.001	264	±17.5	−0.96	−0.502	0.001
‘Kerner’ ****	9.0	±1.32	−0.046	−0.180	0.138	255	±12.9	−0.76	−0.394	0.001
‘Blaufränkisch’	9.4	±1.08	−0.019	−0.057	0.601	266	±14.9	−0.83	−0.488	0.001
‘Welschriesling’	8.7	±1.39	−0.077	−0.508	0.001	271	±18.2	−1.05	−0.527	0.001
‘Riesling’	11.0	±1.76	−0.068	−0.270	0.011	271	±15.1	−0.85	−0.496	0.001
‘Furmint’	10.5	±1.88	−0.088	−0.412	0.001	279	±17.9	−1.06	−0.514	0.001

Data available after * 1990, ** 1988, *** 1982, and **** 1989; bold numbers indicate significant trends.

). However, drier conditions with more frequent and longer dry periods were more likely at both locations, as higher temperatures probably led to a higher evapotranspiration rates, as noted by Ramos et al. (2008) [10]. Precipitation becomes more intense with more intense dry spells. During the growing season, precipitation reaches ~70% of its annual amount.

The trends in the increase in the average temperature of the growing season (GSTavg) for the period 1952–2022 were 0.37 °C (Maribor) and 0.33 (Murska Sobota), and for the period 1980–2022, 0.44 to 0.51 °C per decade. The average temperature in the period of May to June (TMJ) shows the same significant trends for both locations (p = 0.001), namely, the TMJ increased more than 0.37 °C per decade in the whole study period and more than 0.54 °C per decade for the period of 1980–2022 (Table 2). These more favorable temperature conditions in May and June may lead to higher disease pressure (earlier powdery mildew infections). There were even more pronounced trends for NDT30, namely, 5.7 and 3.7 days and 6.8 and 5.5 days per decade, respectively (Table 2). An increased number of days with daily maximum temperatures of 30 °C is critical for optimal vine development and can lead to a reduction in photosynthesis, greater water deficiency, the premature ripening of grapes, and the drying of berries in early varieties, especially in the early ‘Bouvier’ variety in Slovenia [24]. Further, some days with temperatures above 30 °C during the ripening period could have been beneficial [9,11], especially for late-ripening varieties [24], such as ‘Riesling’ and ‘Furmint’, in this region.

After 2010, the average values of NDT30 increased three- to fourfold compared to the first decade after the 1980s (Figure 2). This was reflected in the trends of heat accumulation indices (HI and GDDs), which increased by about 6 (1952–2022) and even by about 10 units per year after 1980 (Table 2). Their values increased on average by around 100 units per decade (Figure 2).

The number of days with temperatures <0 °C (NDF) showed a decreasing trend, and the number of days between the last spring frost and the first fall frost (NDFF) increased (Table 2). In the period of 1980–2022, the trends in the average temperatures of the growing season (GSTavg) show a warming of 1.9 °C (Maribor) to 2.2 °C (Murska Sobota). Similar results were found in other European wine-growing regions, where growing seasons warmed by 1.7 °C and heat accumulation increased by 250–300 units in the 30–50 years. In Spain, heat accumulation (WI and HI) increased in the inland wine-growing regions, but not in the coastal regions [27].

Based on the categorization of wine-growing regions into climatic groups [48] and the increase in GSTavg in the last decade of the observation period, we can conclude that this wine-growing region is becoming suitable for the cultivation of some wine varieties from the warm-climate-ripening group, such as ‘Cabernet Franc’ and ‘Merlot’. If the warming trend continues in the next 30 years in a similar way to how it has since the 1990s (Figure 2), we can assume that the Podravje wine-growing region will completely transition to the warm-climate grape variety group [49]. Similar trends in bioclimatic parameters can also be observed for the period of 1980–2022 (Table 2), although some of them are more pronounced. In the Podravje wine-growing region, data on grape ripening are available for this period.

Precipitation trends were significant only for the Maribor site, i.e., for the period of 1952–2022, total annual precipitation (AP), and growing season precipitation (GSP) decreased by −2.9 and −1.7 mm/m² per year, respectively. After 1980, this downward trend was even more pronounced, with −5.8 L per year for AP and −3.9 L per year for GSP. After 2010, the total amount of GSP at the Maribor site decreased by 136 L and matched the amount at the Murska Sobota site (Figure 2), where the amount was stable throughout the study period. This indicates that the precipitation pattern of the Pannonian climate extends from east to west into the interior of the region. The seasonal precipitation amounts changed significantly. This could not be confirmed before 2010 [24], which was also pointed out by Tomasi et al. (2011) [49].

3.2. Grapevine Reactions to Climate Changes in the Podravje Wine-Growing Region

In the period of 1980–2022, all varieties show a trend towards earlier grape ripening (total soluble solids contents reached about 76°Oe) by slightly less than one day per year, with the exception of the varieties ‘Welschriesling’ and ‘Furmint’, where the trend is slightly more than one day per year (Table 3). The more pronounced trend for ‘Welschriesling’ in this region is not only due to climate change. In the last 15 to 20 years, this variety has been planted in sun-exposed sites, where it has mainly displaced the aromatic varieties. ‘Sauvignon Blanc’, for example, began to retreat from the sunniest locations to less sunny ones, mainly in order to preserve its primary aromas.

In the period of 1980–2022, trends towards an earlier harvest time of 43 days (‘Bouvier’), 37 days (‘Chardonnay’), 35 days (‘Sauvignon Blanc’), 36 days (‘Blaufränkisch’), 45 days (‘Welschriesling’), and 46 days (‘Furmint’) were observed, i.e., by 8–10 days per decade. Earlier grape ripening has been observed in many wine-growing regions [10,23,26]. In South Australia, ripening has advanced by 8 days per decade [25,26], while other values are estimated at 0.5 to 3 days per year [50], but most studies were conducted before 2010 and for a shorter period. After 2010, the harvest date for six varieties is on average one month earlier than in the period of 1980–1990. Taking into account the increase in GSTavg (Maribor station; 1.9 °C) in the studied period of 1980–2022, the combined trends of harvest date and climate result in an average shift of 18 (‘Sauvignon Blanc’) to 24 days (‘Furmint’) per 1.0 °C warming. Ramos et al. (2008) [10], found an earlier harvest for the ‘Chardonnay’ grape variety by about 5 days per 1 °C of warming in the growing season for the shorter period studied (1997–2006).

In the period of 1980–2022, the total acidity per decade decreased from 0.19 g/L for ‘Blaufränkisch’ to 0.95 g/L for ‘Gelber Muskateller’, while no trends were discernible for ‘Pinot Blanc’, ‘Sylvaner’, ‘Sauvignon Blanc’, ‘Kerner’, or ‘Blaufränkisch’ (Table 3). For the six varieties analyzed in more detail (‘Bouvier’, ‘Chardonnay’, ‘Sauvignon ‘Blanc’, ‘Blaufränkisch’, ‘Welschriesling’, and ‘Furmint’), the decreasing trends in total acidity were significant for ‘Chardonnay’ (R² = 0.374), ‘Welschriesling’ (R² = 0.477), and ‘Furmint’ (R² = 0.336), while the trends for earlier grape ripening were significant for all six varieties, from R² = 0.395 in ‘Sauvignon Blanc’ to R² = 0.566 for ‘Bouvier’ (Figure 3). The total acidity in grape juice decreased on average from 0.43 g/L (‘Blaufränkisch’) to 1.99 g/L (‘Furmint’) per 1 °C of warming. Figure 4 shows various bioclimatic parameters and the total acidity for six varieties (early-, medium-, and late-ripening varieties) for the first and last decades in the period of 1980–2022.

Even though the grapes are harvested at least one month earlier than in the period of 1980–1990, the total acidity content has fallen sharply as a result of the higher air temperatures. This has so far proved to be positive for late-ripening varieties (‘Furmint’) in particular, while it is negative for early varieties (‘Bouvier’). Early-ripening varieties are often subjected to greater dehydration and so-called forced ripening during the ripening period. As a result, undesirable astringency notes can occur later in the wine tasting. In cold climate regions such as Podravje, total acidity reduction can lead to a better balance between sugar and acidity, while in early-ripening varieties, acid reduction without acid correction in the cellar (as in warmer regions) can lead to less fruitiness in the wine. A warmer growing season usually results in an earlier harvest and a lower yield (possibly also due to spoilage, as was the case in 2022) as well as better wine quality if there was no excessive heat stress [5,10,25].

From the GSTavg values for each variety, it can be concluded that the grapes developed and ripened at a higher average temperature in the growing season after 2010, as shown by the GSTavg values for the meteorological growing season (1 April to 31 October) (

Table 4. Mean temperature (GSTavg) and total precipitation (GSP) in the growing season (1 April to 31 October) for the Maribor meteorological station and from 1 April to the day in the year with technological grape ripeness (76°Oe) and total acidity g/L content for six varieties in the Podravje wine-growing region in Slovenia in the period from 1980 to 2022, and by decades in this period (p ≤ 0.05).

Location- Variety	Variables/ Period	GSTavg ±SD		GSP ±SD		Total Acid. g/L ±SD		Day in yr ±SD
Maribor	1980–2022	16.4	±0.80	685	±127
	1980–1990	15.7	±0.56	727	±90
	1991–2000	16.2	±0.75	739	±142	1 April to 31 October
	2001–2010	16.7	±0.48	693	±97
	2011–2022	17.0	±0.57	591	±129
‘Bouvier’ 1 April to 76°Oe	1980–2022	17.4	±0.89	499	±150	7.5	±1.1	247	±16.3
	1980–1990	16.5	±0.73	577	±91	8.7	±1.1	262	±9.1
	1991–2000	17.3	±0.66	567	±170	7.0	±0.9	255	±11.2
	2001–2010	18.0	±0.65	495	±148	6.8	±0.3	243	±15.0
	2011–2022	18.0	±0.48	375	±83	7.3	±0.8	230	±8.0
‘Chardonnay’ 1 April to 76°Oe	1980–2022	17.5	±0.98	534	±163	10.2	±1.5	256	±15.5
	1980–1990	16.4	±0.60	613	±116	11.8	±1.1	270	±11.2
	1991–2000	17.3	±0.75	591	±180	10.6	±1.3	260	±12.4
	2001–2010	18.0	±0.78	529	±148	9.5	±0.6	251	±14.9
	2011–2022	18.1	±0.60	418	±133	9.2	±1.0	243	±9.3
‘Sauvignon Blanc’ 1 April to 76°Oe	1980–2022	17.4	±1.04	547	±164	10.3	±1.4	258	±16.2
	1980–1990	16.3	±0.83	640	±118	11.8	±1.3	272	±13.3
	1991–2000	17.3	±0.79	603	±175	10.1	±1.5	264	±8.4
	2001–2010	18.0	±0.76	520	±145	9.5	±0.6	251	±14.5
	2011–2022	18.1	±0.61	437	±132	9.8	±0.9	246	±11.4
‘Blaufrankisch’ 1 April to 76°Oe	1980–2022	17.4	±1.12	570	±171	9.4	±1.1	266	±14.9
	1980–1990	16.2	±0.79	663	±108	9.9	±1.4	279	±10.3
	1991–2000	17.2	±0.90	614	±193	9.5	±0.9	270	±11.7
	2001–2010	17.9	±0.92	580	±156	8.8	±0.7	262	±12.4
	2011–2022	18.2	±0.63	441	±138	9.3	±0.8	252	±9.8
‘Welschriesling’ 1 April to 76°Oe	1980–2022	17.3	±1.17	587	±179	8.7	±1.4	271	±18.2
	1980–1990	16.0	±0.84	683	±112	10.3	±1.1	289	±13.2
	1991–2000	17.1	±0.92	630	±206	9.0	±0.8	276	±14.0
	2001–2010	17.8	±0.96	610	±166	7.8	±0.6	269	±15.1
	2011–2022	18.2	±0.62	444	±134	7.7	±0.8	254	±10.2
‘Furmint’ 1 April to 76°Oe	1980–2022	17.2	±1.20	608	±169	10.5	±1.9	279	±17.9
	1980–1990	15.9	±0.80	689	±110	12.3	±1.7	296	±13.6
	1991–2000	16.9	±0.98	665	±200	10.3	±1.6	285	±13.6
	2001–2010	17.6	±0.98	623	±151	10.4	±1.3	275	±15.0
	2011–2022	18.1	±0.64	473	±119	9.0	±1.0	262	±7.8

). At the beginning of the observation period (1980–1990), the GSTavg was lower than in the last decade (2011–2022), as the grapes were ripe at the end of September or even in mid-October. After 2011, the average temperature in the vegetation period from 1 April to the ripening of the grapes is about 1 °C higher than the official GSTavg for the meteorological period (1 April to 31 October). For the period of 2011–2022, for example, the GSTavg is 17.0 °C (Table 4), but all varieties ripened at an average temperature of over 18 °C. This was mainly influenced by the so-called hot days. In the period of 1980–1990, there were 6 to 7 such days per year, and after 2010 there were 25 to 29 per year (Figure 4).

Precipitation during the growing season (1 April–31 October) at the Maribor site decreased from 727 mm/m² in the period of 1980–1990 to 591 mm/m² in the period of 2011–2022 (Figure 4), but the actual amount of precipitation during the growing season was lower for each variety (Table 4). From 1 April to the day of ripening of the individual varieties (76°Oe), the amount of precipitation was even lower, namely by an additional 202, 195, 203, 222, 239, and 216 mm/m² for the six varieties ‘Bouvier’, ‘Chardonnay’, ‘Sauvignon Blanc’, ‘Blaufränkisch’, ‘Welschriesling’, and ‘Furmint’, which corresponds to 135, 115, 113, 111, 109, and 98 mm/m² per 1 °C increase in temperature during the growing season, respectively (Table 4). For example, after 2010, the available precipitation in the growing season (1 April to harvest) was only 375 mm/m² for the early variety ‘Bouvier’ and 473 mm/m² for the late-ripening variety ‘Furmint’ (Table 4). Authors who studied climate change two decades ago found that precipitation would not decrease significantly in most cases [49]. Today, these estimates are too general, as they refer to the amount of precipitation in the growing season (1 April to 31 October) and do not reflect the actual amount of precipitation available to the individual varieties. In the Podravje wine-growing region, the amount of precipitation from 1 April to the technological ripeness of each variety was one-third less (Table 4) than in the period from 1 April to 31 October (which is mainly used in climate change research).

In the Podravje wine-growing region, the established method of soil care is the permanent maintenance of green cover on the soil in the vineyards, which has so far proven to be the most suitable method of soil care, both in terms of soil life [51] and erosion, as most of the vineyards are located on steep slopes [31]. Given climate change and trends in precipitation patterns, this method of soil management may be less suitable in the future.

When describing the positive effects of climate change on wine quality, particularly in the case of late-ripening grape varieties, it should not be overlooked that the development of viticulture and winemaking practices and the reduction in yields over the last 43 years have also had a significant influence on the improvement in wine quality. A one-sided assessment of the effects of climate change on wine quality is therefore inadmissible. However, should the trend of regional warming continue, as predicted by climate models [46], and should it continue in this region with the same dynamics as in the last 43 years, the Podravje wine-growing region will most likely experience poorer vintages, mainly due to lower total acidity, very high alcohol content, and other less desirable characteristics of the wine, possibly resulting in less balanced wines. A serious question arises as to whether it will be possible to maintain the same varieties in the future by adapting viticultural and oenological practices, as reported by Seguin and Garcia de Cortazar (2005) [22]. Due to the climatic changes observed in the wine-growing region that is the subject of this research, it will be necessary to exceed certain limits for the grape varieties that are currently defined for this region (‘terroir’).

For the available data (1980–2022), the relationships between the GDDs, the average temperature in the growing season (GSTavg), and the HI with the total acidity were calculated. The linear relationships between GDDs, GSTavg, and HI and total acidity are shown as individual variables in Figure 5. Total acidity showed high negative correlations with all climatic parameters for all six varieties except ‘Bouvier’. The most pronounced correlation was found between GDDs and total acidity (Figure 5).

The recalculated heat sum values of GDDs* (1 April to harvest date) and HI* (1 April to harvest date) for the individual varieties remain in the same range according to the Winkler (1974) [40] and Huglin (1978) [41] ripeness group classifications (

Table 5. Mean values of GDDs* and HI* from 1 April to the day in the year with technological grape maturity (76°Oe) for six varieties (‘Bouvier’, ‘Chardonnay’, ‘Sauvignon Blanc’, ‘Blaufränkisch’, ‘Welschriesling’, and ‘Furmint’) in the Podravje wine-growing region in Slovenia in the period from 1980 to 2022 (p ≤ 0.05).

Variety/Parameters	GDD*	±SD	HI*	±SD
‘Bouvier’	1191	±87.4	1683	±106.0
‘Chardonnay’	1259	±84.8	1782	±96.9
‘Sauvignon Blanc’	1275	±85.7	1804	±94.3
‘Blaufränkisch’	1320	±99.4	1873	±103.0
‘Welschriesling’	1341	±99.3	1902	±114.8
‘Furmint’	1371	±110.3	1954	±122.0

GDDs* and HI* recalculated for the period from 1 April to technological maturity (76°Oe).

), but the trends after 2010 show that these values are reached 26 to 34 days earlier than in the 1980s.

4. Conclusions

Global warming affects the growth of the vines and the composition of the berries, i.e., the onset of the phenological phase of ripening occurs earlier. When analyzing the trends of the time series of up to 43 years observed in the Podravje wine-growing region, we found that the average temperature in the growing season has increased by 1.6 °C since the 1980s. The increase in average temperature was most pronounced in the May–June period (TMJ), which can lead to an earlier and more intense occurrence of diseases, especially downy mildew. Climate warming has caused grapes to ripen about a month earlier than in the 1980s. The earlier ripening of the grapes is generally accelerated by the increased accumulation of sugar in the berries, which leads to a higher alcohol content in the wine. Due to the higher temperature during grape ripening, a downward trend in total acids was also observed. This was confirmed by correlations between the climate indices (GSTavg, GDDs, and HI) and total acidity. The current climate change in this wine-growing region has had a positive effect on the late-ripening variety ‘Furmint’, leading to a lower (more balanced) total acidity.