A Study of Observed Climate Change Effects on Grapevine Suitability in Oltenia (Romania)

Abstract

Viticulture represents an important agricultural sector in Oltenia, which is one of the Romanian regions most affected by temperature increases. The main purpose of the present study was to analyze the changes in climate suitability for grapevine and wine production against this climate context in the region. Two specific bioclimatic indices were applied, namely the bioclimatic index and the oenoclimate aptitude index, both reflecting the cumulated influence of temperature, actual sunshine duration, and precipitation amounts on the grapevine during the growing season (1 April–30 September). The indices were calculated as average values for the period 1961–2020. In order to emphasize potential shifts in suitability, the mean, maximum, and minimum values were calculated for two distinct periods, 1961–1990 and 1991–2020. The results of the analysis underlined three distinct suitability changes: the area suitable for quality red wines shifting northwards (on average, about 30′ of latitude or 55.5 km), including the eastern part of the Getic Subcarpathians, which is not currently part of any winegrowing region; the emerging new areas suitable for quality white wine (the western part of the Subcarpathians); and a potentially overly hot climate developing in Southern Oltenia where grapevine varieties are currently grown. Thus, the development of adequate adaptation strategies for viticulture to climate change in the region should be considered in the near future.

1. Introduction

Climate conditions have a critical influence on plant (natural and cultivated) phenology. In the case of grapevine, climate (mainly temperature and actual sunshine duration and, to a lesser extent, precipitation amounts) has a greater influence compared to other factors, such as soil type and topography [1]. It affects not only the yield but also the quality of grapes and, implicitly, the specific character or flavor of wine, as climate is one of the key factors of terroir [2]. Wine grape varieties and the types of wine production are mainly determined by regional climate suitability, which represents a stable characteristic of a wine area over time [3,4]. Grapevine (Vitis spp.) is generally resilient to fluctuations in the main meteorological parameters. However, extremely high or low values of these parameters diminish the quality of grapes, even if the plant is not irrecoverably damaged. In fact, premium wine grapes can be obtained within narrow climate ranges [5].

The documented changes currently taking place or those projected based on simulations [6] pose challenges for viticulture and, more broadly, for agriculture. In Europe, the highest climatic strain is mainly felt in traditional wine-growing zones from Spain, Portugal, Italy, and even France as a consequence of the increase in thermal stress and dryness during the growing season [7]. In France, changes in the phenology of grape varieties have been registered in the last decades, mainly triggered by temperature increases [8]. The same situation is documented in Italy [9,10] and Portugal [11,12]. In Spain, higher temperatures compared to the historical averages lead to negative effects on wine-growing practices [13]. Similarly, an almost 4 °C increase in the average annual temperature and prolonged drought periods in the last 45 years in Santorini (Greece) is documented to have a serious impact on yield [14]. This climate context (increasing temperatures, particularly during summer) expands areas suitable for grape cultivation northwards, mainly in Central and Northern Europe. This increase in suitability was identified in the Poland–Germany–Czech Republic transboundary area [15], Germany [16], Romania [17,18,19], and Slovenia [20].

Shifts in viticultural distribution are expected to occur on an even greater scale in the near future compared to the current situation because of climate change—mainly temperature increase and reduction in precipitation amounts. Under the A1B SRES emission scenario, Malheiro et al. [21] indicate detrimental impacts in southern Europe and new potential areas suitable for viticulture in Western and Central Europe, especially for the future periods of 2041–2070 and 2071–2100. Progressive shifts to the north–northwest of the currently cultivated area in France, together with the contraction of other wine regions, were identified by Moriondo et al. [22] (under the A2 and B2 SRES scenarios). The displacement of climate suitability for grapevines up to 55°N was highlighted by Fraga et al. [23] for both the RCP4.5 and RCP8.5 climate change scenarios. Cardell et al. [24] indicate the potential loss of traditional grape-growing regions in the Mediterranean area, together with the emergence of new suitable regions northwards in Western and Central Europe under the same climate change scenarios. Morales-Castilla et al. [25] point out the loss of 65–68% of the current suitable surface in Italy and Spain under a 2 °C warming scenario and up to 90% under a 4 °C warming scenario, as well as considerable northward shifts for different grapevine varieties. Sgubin et al. [26] also emphasize a northward shift of grapevine suitability of up to 3° of latitude for the RCP4.5 scenario and 8° of latitude for the RCP4.5 scenario by the end of the century. The study also emphasizes that a 2 °C global temperature increase compared to the preindustrial level would reduce grapevine-suitable areas within the traditional regions by 4%/°C rise.

In Romania, the modification of climate conditions has been widely analyzed in the past two decades, both at the national and regional levels. Thus, the main findings underscore the trends identified at the European level, i.e., significant increases in temperature and sunshine duration throughout all seasons except for autumn [27,28], including positive temperature extremes (maximum temperature above ≥35 °C) [29,30,31] and more frequent and intense heat waves (maximum temperatures ≥35 °C during the day and ≥20 °C during the night for summer heatwaves) [32,33,34]; a certain stability of the precipitation amounts, with no significant positive or negative trends [27] but with a significant increase in the frequency of rain showers [35]; and an inhomogeneous pattern in drought trends [36,37]. Oltenia, particularly the southern low area, faces a significant temperature and potential evapotranspiration increase [38,39,40,41], as well as an augmentation in drought [37,42], thermal continentality [43], and aridity [44,45,46,47,48]. Considering these aspects, the grapevine might be affected by both heat and hydric stress in the near future.

The influence of climate conditions on the grapevine has been examined in several studies at the national level [4,18,49,50,51], although the main focus has remained at the regional level. Costea et al. [52], Iliescu et al. [53], Baciu (Ropan) et al. [54], and Chedea et al. [55] presented different aspects related to the suitability of climate conditions for grapevines in Transylvania (central Romania). Nistor et al. [56] focused on the impact of climate variability on berry and wine composition in western Romania, highlighting that hot weather favored sugar accumulation in berries and wine and high temperatures combined with water stress resulted in low-acidity/high-pH grapes. In eastern Romania—Moldavia—the impact of climate change on viticultural potential was assessed in different vineyards by Irimia et al. [17,57], Pușcalău et al. [58], and Zaldea et al. [59]. The studies emphasized an increase in all temperature-related indices and sunshine duration, the region displaying high suitability for quality white wine varieties, and an increasing suitability for red wine varieties on the basis of climate change.

In southern Romania, Bucur and Dejeu [60] and Bucur et al. [61] studied the effects of climate change on grape yield and quality based on climate data from a maximum of three stations, indicating that a temperature increase triggered higher sugar content in grapes and reduced titratable acidity. Onache et al. [62] analyzed the influence of the climate conditions of 2019 on different oenological parameters, underlining that alcohol concentration and antioxidant activity depend not only on climate conditions but also on the type of soil. None of the aforementioned studies approached climate suitability or the impact of climate change on grapevine suitability across the entire Oltenia region.

Viticulture represents an important economic segment in Europe. For example, in 2023, in the E.U. states alone, vineyards covered a surface of 3.3 mil. ha (45%) out of 7.2 mil. ha of vineyard area at the global level, according to the International Organisation of Vine and Wine [63]. As stated by the European Commission (Directorate-General for Agriculture and Rural Development) [64], the wine industry represents the largest EU agri-food sector in terms of exports (7.6% of agri-food value exported in 2020). The same report [63] points to Romania as a major winegrowing country, ranking the 8th in the world in terms of vineyard surface (187,000 ha representing 2.6% of the global area under vine in 2023) and the 11th in terms of wine production (4.6 million hl in 2023). However, it is not among the main wine exporters, which underscores a need for different marketing strategies to take up commercial space on the European market and beyond. In order to increase visibility and sales, wine producers should increase participation in international events (e.g., influential competitions and festivals), build a better social media presence, access digital marketing tools, invest in wine tourism, etc.

Based on climate change trends, viticulture in Romania is also highly vulnerable, particularly in the southern part of the country. Any changes in the climate suitability of the main European wine regions would also have important economic and social consequences in Romania. The goals of this research are (1) to analyze the observed differences in active temperatures, growing season temperature, precipitation amounts, and actual sunshine duration between two periods: 1961–1990 and 1991–2020; (2) to determine potential changes (shifts, possible extension, shrinkage, etc.) in the wine-growing regions compared to the current territorial limits, as well as observed changes in their climate suitability for grapevine cultivation, based on the bioclimatic index (Ibcv) and the oenoclimate aptitude index (IAOe); and (3) to determine the trends of the selected bioclimatic indices based on observational series over the 1961–2020 interval.

2. Materials and Methods

2.1. Study Area

Located in the southwestern part of Romania, the Oltenia region includes all landforms (plain, hills, and mountains), starting from the Danube Floodplain in the south to the Carpathian Mountains—Parâng and Retezat Godeanu—in the north (Figure 1). Consequently, there is a great altitudinal range, from about 30 m to 2519 m, the highest altitude corresponding to the Parângul Mare Peak [65]. The predominance of altitudes below 500 m, the southern exposure of the landforms, as well as the generally adequate climate and soil conditions determine the suitability of Oltenia for grapevine cultivation.

The cultivated surface has gradually decreased over the past two decades because of various causes: changes in ownership structure, extreme weather events, increased incidence of different pests and diseases, etc. Based on data supplied by the National Institute of Statistics [66], the cultivated surface reached its peak in 2002 (44,092 ha), while the least cultivated surface was documented in 2023 (28,229 ha, of which 19,129 ha were hybrid vines). Such figures rank Oltenia the third in Romania (17.10%), after the Southeast Region, which holds 41.35% of the total surface, and the Northeast Region, with 17.29%.

Figure 1. Location of the study area, meteorological stations, and present viticultural zoning. (a) Romania location within Europe; (b) Oltenia location within Romania; (c) Oltenia region: meteorological stations and wine-growing centers within the two studied regions: Muntenia and Oltenia hills wine region (MOHwr) and Sands and other favorable areas in southern Romania (Swr). GIS processing relying on data from the public domain: Geo-spatial [67,68], Copernicus Land Monitoring Service [69], and Natural Earth [70]. The wine-growing centers are located according to the Romanian legislation [71].

Figure 1. Location of the study area, meteorological stations, and present viticultural zoning. (a) Romania location within Europe; (b) Oltenia location within Romania; (c) Oltenia region: meteorological stations and wine-growing centers within the two studied regions: Muntenia and Oltenia hills wine region (MOHwr) and Sands and other favorable areas in southern Romania (Swr). GIS processing relying on data from the public domain: Geo-spatial [67,68], Copernicus Land Monitoring Service [69], and Natural Earth [70]. The wine-growing centers are located according to the Romanian legislation [71].

According to the Romanian legislation in force [72], grapevine plantations are territorially grouped in wine-growing zones, wine-growing regions, vineyards, wine-growing centers, and wine-growing fields. The Ministerial Order 1508/17.12.2018 [73] indicates two wine-growing regions in Oltenia, i.e., Muntenia and Oltenia Hills (MOHwr) and Sands and other favorable areas in southern Romania (Swr), respectively (Figure 1). These two regions are also indicated in the Council Regulation (EC) 491/2009 [74] as belonging to the European wine-growing zone CI and CII. If Swr belongs only to CII, MOHwr has centers located within both wine-growing zones, but predominantly within CI. The same Order stipulates that, within MOHwr, there can be found one geographical indication (GI), namely Oltenia Hills, and five Protected Designations of Origin (POD)—Drăgășani POD, Sâmburești POD, Mehedinți POD, Segarcea POD, and Banu Mărăcine POD. The current legislation regarding Romanian viticultural zoning [71] recognizes twenty-eight wine-growing centers in Oltenia, of which four are independent, while the rest are grouped into eight more homogenous vineyards, being predominantly located within MOHwr (Figure 1). For the present study, the grapevine area is illustrated according to Corine Land Cover 2012 and 2018 [69].

Both red (Burgund Mare, Cabernet Franc, Cabernet Sauvignon, Dornfelder, Marselan, Merlot, Pinot noir, Sangiovese, Syrah, Touriga Nacional, Touriga Franca, and Zinfandel) and white (Chardonnay, Italian Riesling, Muscat Ottonel, Pinot gris, Rhine Riesling, Sauvignon, Ugni blanc, and Viognier) international cultivars are grown in Oltenia. Besides those, Romanian red varieties, such as Alutus, Băbească neagră, Fetească neagră, Negru de Drăgășani, and Novac, as well as white varieties, such as Crâmpoşie Selecţionată, Fetească albă, Fetească Regală, and Tămâioasă Românească, are also grown [73]. Four of these Romanian varieties, namely Alutus, Crâmpoşie Selecţionată, Negru de Drăgășani, and Novac, are specific to the analyzed region, as they were created at the Drăgășani Research Station for Viticulture and Vinification. Alutus is the most recently cultivated variety [75].

2.2. Climate Variables and Data Sources

The climate data (monthly values of mean temperatures, precipitation amounts, and actual sunshine duration) for the 14 meteorological stations located within the analyzed region were provided by the National Meteorological Administration (Figure 1 and

Table 1. Geographical co-ordinates of the considered meteorological stations.

No.	Meteorological Station	Altitude (m)	Latitude	Longitude
1.	Dr.-T. Severin **	77	44°38′	22°38′
2.	Calafat *	61	43°59′	22°57′
3.	Bechet *	36	43°47′	23°57′
4.	Băilești *	57	44°01′	23°20′
5.	Craiova **	192	44°19′	23°52′
6.	Caracal *	106	44°06′	24°22′
7.	Slatina **	172	44°26′	24°21′
8.	Bâcleș **	313	44°29′	23°07′
9.	Târgu Logreşti **	265	44°53′	23°42′
10.	Drăgășani **	280	44°40′	24°17′
11.	Apa Neagră (Padeș)	258	45°00′	22°52′
12.	Târgu Jiu **	203	45°02′	23°16′
13.	Polovragi	531	45°11′	23°49′
14.	Râmnicu Vâlcea	237	45°06′	24°22′

Note: the meteorological stations are displayed from west to east on landforms (the Danube Floodplain, Oltenia Plain, the Getic Piedmont, and the Getic Subcarpathian). * Swr wine region; ** MOHwr wine region; unmarked stations are not presently included in a wine-growing zone.

). Four meteorological stations are located within Swr and seven stations within MOHwr. As one of the objectives of this study is to assess the potential extension of the climate suitability northwards of the present limits of MOHwr, three meteorological stations located in the Subcarpathian (i.e., Apa Neagră, Polovragi, and Râmnicu Vâlcea) were also taken into consideration as part of MOHwr. The data cover a period of 60 years (1961–2020), except for Slatina, which started its activity in 1977. The evolution of climate data—particularly temperature variables that displayed significantly increasing trends beginning with the 1990s—underscores the consideration of two distinct intervals (1961–1990 and 1991–2020) within the larger period of 1961–2020.

2.3. Methods

2.3.1. Bioclimatic Indices

Climate suitability for grapevine growing and wine production is expressed by means of bioclimatic indices specific to viticulture, which incorporate two or more climate parameters. These indices enable the identification of different changes in the current climate conditions in relation to grapevine and indicate the suitable grapevine varieties. The most widely used indices are the Winkler index (WI) and the Huglin heliothermal index (HI), both illustrating the importance of temperature (heat accumulation) for sugar content and acidity [76,77].

In the present study, the assessment of climate suitability for grapevine is based on the analysis of two bioclimatic indices (

Table 2. Bioclimatic indices, equations, suitability for different wine types, and sources.

Bioclimatic Index and Abbreviation	Period	Equation, Suitability	Sources
Bioclimatic index (Ibcv, units)	1 April to 30 September	$\mathrm{I}\mathrm{b}\mathrm{c}\mathrm{v}={\displaystyle \frac{\mathrm{A}\mathrm{S}\mathrm{D}*\mathsf{\Sigma }\mathrm{T}\mathrm{a}}{\mathrm{P}\mathrm{P}*\mathrm{N}\mathrm{d}}}/10$ (1) 4.0–8.0 QWW; 8.0–15.0 QRW; <4.0 and >15.0 unsuitable for grapevine growing	[57,78,82]
Oenoclimate aptitude index (IAOe, units)	1 April to 30 September	$\mathrm{I}\mathrm{A}\mathrm{O}\mathrm{e}=\mathrm{A}\mathrm{S}\mathrm{D}+\sum \mathrm{T}\mathrm{a}-\left(\mathrm{P}\mathrm{P}-250\right)$ (2) <3793 = class IV (unsuitable for grapevine growing); 3793–4300 = class III (suitable for WTW + SW + WD); 4301–4600 = class II (suitable for QWW + RTW); >4600 = class I (suitable for QRW)	[57,79]

Note: Nd—number of days in the growing season (1 April–30 September); PP—precipitation amount (mm, 1 April–30 September); ASD = actual sunshine duration (hours, 1 April–30 September; ΣTa = sum of daily average temperatures >10 °C (1 April–30 September); 10—temperature base; 250 = mm, minimum precipitation amount needed for unirrigated grapevines.

), which are considered representative for viticultural zoning in Romania: the bioclimatic index (Ibcv) [78] and the oenoclimate aptitude index (IAOe) [79]. The main advantage of these indices is that they incorporate the combined influence of the most important climate parameters for grapevine, namely temperature, insolation, and precipitation, providing a much-detailed perspective with regard to climate suitability [4]. Thus, starting from the values registered by these parameters, we calculated the sum of active temperatures (∑Ta, °C), namely temperatures above 10 °C, which is considered the threshold for grapevine to begin the growth cycle [80,81], the precipitation amount (PP, mm), and the actual sunshine duration (ASD, hours) in the course of the growing season, which influence the adequate development of vine [57]. In Romania, the growing season is defined as the period between 1 April and 30 September; in April, temperatures are above the 10 °C threshold and grapevines start their growing cycle; in September, they reach maturity and are harvested even in the northern parts of the country. These indices can illustrate local variations in the temperate continental climate areas [57], and they are mainly used in Romania to assess the suitability of a region to obtain different types of wines: white table wines (WTW), sparkling wines (SW), wines for distillates (WD), quality white wines (QWW), red table wines (RTW), and quality red wines (QRW).

In order to capture the importance of each meteorological parameter that is part of the formula, they were also analyzed separately. Temperature represents the most significant factor for obtaining quality grapes and wine, as it conditions sugar accumulation, anthocyanin content, volatile compounds, etc. [83,84]. With regard to ∑Ta, Oșlobeanu et al. [85] quoted by Irimia et al. [19] indicate that ∑Ta < 2600 °C is restrictive for grapevine, values between 2600 and 2850 °C are suitable for WTW, WS, and WD, values between 2850 and 3180 °C for QWW and RTW, and values >3180 °C for QRW. Besides ∑Ta, we also analyzed the average temperature of the growing season (GST), as it is considered a good indicator for defining the suitability of various cultivars. GST values ranging between 13 and 21 °C indicate the potential of a region to produce quality wines. The thresholds are 15.1 and 17 °C for QWW, respectively, and 17.1 and 21 °C for QRW [86,87].

In terms of precipitation, it is considered that the amount registered during the growing season (PP) should range between 250 and 390 mm in order to obtain QRW [57]. Amounts below the 250 mm threshold are not restrictive but require irrigation and are suitable for QWW + RTW, while amounts above 390 mm indicate suitability for WTW, SW, and WD. However, higher PP values are often associated with a higher probability of pests and diseases [88]. ASD is also important, as it influences the accumulation of anthocyanins [89] and the phenolics composition [90]. In order to assess the suitability of the region from this viewpoint, the following thresholds [57] were used: <1280 h (improper for grapevine), 1280–1450 h (suitable for WTW, SW, and WD), 1451–1550 h (suitable for QWW and RTW), and 1551–1610 (suitable for QRW).

2.3.2. Trend Analysis

The presence of increasing or decreasing trends was assessed based on the Mann–Kendall test [91,92], while the slope of the linear trends was estimated by means of the nonparametric Sen’s method [93]. The data were processed in the Excel template MAKESENS, developed by the researchers of the Finnish Meteorological Institute [94]. Mann–Kendall test is a nonparametric (distribution-free) test widely used in detecting trends, mainly in precipitation and temperature data series, as it makes no assumptions about the underlying distribution of the data, it allows missing values, and it is not influenced by extreme values. The Mann–Kendall test verifies the null hypothesis of no trend H₀ (the data are independent and randomly ordered) against the alternative hypothesis H₁, which assumes that there is an increasing/decreasing monotonic trend [95]. Sen’s slope is determined as the mean of all pair-wise slopes for any pair of points in the dataset. The trends are evaluated using the Z test value; positive Z values indicate an increasing/upward trend, while negative Z values indicate a decreasing/downward trend. The significance levels (α) are 0.001 (***), 0.01 (**), 0.05 (*), and 0.1 (+).

2.3.3. Interpolation

While particular parameters are measured or computed for meteorological stations, the effective determination of the spatial distribution of mean climate variables represents a necessary first step towards the development of stochastic models for more refined assessments of the climate impact [96].

For the present study, interpolation was used in order to reduce the drawback triggered by the limited number of meteorological stations and to provide a spatially comprehensive image of environmental variables. Nevertheless, potential limitations are related to the fact that the resulting datasets might lack the highly detailed spatial resolution required by the representation of crop microclimatic conditions, especially in areas characterized by significant climatic gradients. Starting from the findings of the present study, further local-scale assessments or even dependable prediction models for Oltenia will have to make use of high-resolution downscaled data and to consider climate reanalysis.

The parameters related to temperature, rainfall, and actual sunshine duration, as well as the derived indices relevant for grapevine cultivation suitability, were primarily associated to the georeferenced point dataset comprising the 14 meteorological stations in the study area. Although these points display a rather homogenous coverage within Oltenia (with a slightly better distribution in the eastern half), we considered the further addition of eight support stations located in the nearest Romanian, Serbian, and Bulgarian areas to be beneficial to a better representation in a GIS environment. Subsequently, starting from the spatial and numerical attributes of our area and dataset, the spline regularized method, integrated in the ArcGIS 10.6.1 Spatial analyst tools, proved to be a well-suited interpolation method for the estimation of the meteorological characteristics of the entire area under study. The choice of method for these parameters came after prior comparisons with various interpolation techniques; it is also supported by other studies that showed regularized spline to have minimum mean absolute error and root mean square error values in precipitation and minimum root mean square error in actual sunshine duration, as well as to perform well for other climatic parameters, such as temperature or solar radiation [97].

3. Results

3.1. Spatial Distribution of the Climatic Parameters and Suitability for Quality Wine Production (1961–2020)

The mean values (1961–2020) of the considered parameters and of the two applied indices reveal a good suitability of Oltenia to obtain QRW, especially in its southern half. Thus, the average values of GST exceed 19 °C in the southern and eastern parts of the plain, which correspond to Swr (19.2 °C). The average for MOHwr is lower, 17.9 °C, as this wine-growing region has a greater altitudinal range (about 300 m) compared to Swr (70 m) and it includes plain, piedmont, and Subcarpathian areas. The highest value is registered in the southwestern extremity, at Dr.-T. Severin (19.4 °C), and the lowest in the north, at Polovragi (16.3 °C) (Figure 2a).

All the stations register GST values suitable for high-quality wines, either QRW or QWW. In the case of ∑Ta, the average value for Swr is 3521.5 °C, while MOHwr registers 3276.7 °C (

Table 3. Growing season average temperature (GST) and the sum of active temperatures (∑Ta) for Oltenia during the time periods of 1961–1990 (′61/′90) and 1991–2020 (′91/2020).

Meteorological Station	GST (°C)			Difference	$\sum \mathit{T}\mathit{a}(°\mathbf{C})$			Difference
	Mean	1961–1990	1991–2020		Mean	1961–1990	1991–2020
Dr.-T. Severin	19.4	18.8	20.0	+1.2	3553.5	3446.9	3660.1	+213.2
Calafat⁕	19.4	18.9	20.0	+1.1	3564.2	3461.3	3667.1	+205.8
Bechet⁕	19.2	18.8	19.6	+0.8	3514.2	3440.7	3587.6	+147.0
Băilești⁕	19.2	18.6	19.7	+1.1	3512.5	3418.2	3606.8	+188.6
Craiova	18.6	18.1	19.2	+1.1	3414.1	3309.6	3518.7	+209.1
Caracal⁕	19.1	18.6	19.6	+1.0	3495.2	3403.0	3587.3	+184.3
Slatina	18.8	-	19.1	-	3441.2	-	3498.2	-
Bâcleș	17.7	17.0	18.4	+1.4	3242.0	3112.0	3372.0	+260.0
Tg. Logreşti	17.2	16.9	17.6	+0.7	3158.9	3095.1	3222.7	+127.7
Drăgășani	18.1	17.5	18.6	+1.1	3315.6	3212.8	3418.4	+205.6
Apa Neagră	17.0	16.5	17.5	+1.0	3110.1	3017.0	3203.1	+186.0
Târgu Jiu	17.9	17.3	18.5	+1.2	3272.5	3167.6	3377.5	+209.9
Polovragi	16.3	15.9	16.7	+0.7	2985.8	2917.2	3054.3	+137.1
Râmnicu Vâlcea	17.9	17.3	18.4	+1.1	3273.4	3173.6	3373.1	+199.4
MOHwr	17.9	17.3	18.4	+1.1	3276.7	3161.3	3339.8	+194.2
Swr⁕	19.2	18.7	19.7	+1.0	3521.5	3430.8	3612.2	+181.4

). ∑Ta > 3180 °C indicates suitability for QRW in both wine-growing regions (Figure 2b), except for the Subcarpathian area located in the immediate vicinity of the mountains and the central–northern part of the Getic Piedmont. At the station level, there are only three sites that register less than 3180 °C, two of them north of the present limits of the wine-growing region (Apa Neagră and Polovragi) and one within MOHwr (Tg. Logrești), due to its higher altitude; nevertheless, they present good suitability for QWW + RTW.

With reference to PP, Swr is clearly located in the area suitable for QRW (305.6 mm), while MOHwr displays a higher amount, namely 412.2 mm. The lowest amount, below 300 mm, corresponds to the southernmost area, which is located along the Danube Valley, this drier area having a larger extension in the southwestern part of the plain, in the proximity of the Calafat station. There are six stations with values above the 390 mm threshold (the upper limit of QRW), three of them situated north of the present limit of MOHwr. The entire area located south of the latitude of 44°50′ (plain and southern piedmont area) is favorable for QRW, while, north of it, the area is suitable for WTW, SW, and WD (Figure 2c and

Table 4. Precipitation amounts (PP) and actual sunshine duration (ASD) throughout the growing season for Oltenia during the time periods of 1961–1990 (′61/′90) and 1991–2020 (′91/2020).

Meteorological Station	PP (mm)			Difference	ASD (Hours)			Difference
	Mean	1961–1990	1991–2020		Mean	1961–1990	1991–2020
Dr.-T. Severin	358.8	358.7	358.9	+0.2	1566.6	1531.0	1602.3	+71.3
Calafat⁕	297.0	283.5	310.4	+27.0	1594.1	1563.2	1624.9	+61.7
Bechet⁕	301.5	292.6	310.5	+17.9	1572.4	1536.8	1608.0	+71.2
Băilești⁕	307.4	299.1	315.6	+16.5	1622.4	1526.6	1718.2	+191.5
Craiova	348.9	328.2	369.6	+41.4	1555.3	1564.4	1546.1	−18.3
Caracal⁕	316.5	323.3	309.6	−13.6	1594.0	1553.2	1634.8	+81.7
Slatina	345.2	-	365.4	-	1498.8	-	1494.6	-
Bâcleș	351.4	351.9	351.0	−0.9	1553.1	1538.7	1567.6	+28.9
Tg. Logreşti	401.7	397.7	405.7	+8.0	1459.0	1438.3	1479.7	+41.4
Drăgășani	393.0	383.8	402.2	+18.4	1551.0	1523.9	1578.1	+54.2
Apa Neagră	488.6	465.0	512.3	+47.3	1412.2	1375.6	1448.7	+73.1
Târgu Jiu	451.5	445.4	457.6	+12.2	1404.5	1377.4	1431.6	+54.2
Polovragi	535.5	525.8	545.2	+19.3	1289.9	1287.5	1292.3	+4.8
Râmnicu Vâlcea	447.5	435.4	459.6	+24.2	1410.8	1380.1	1441.5	+61.4
MOHwr	412.2	410.2	422.8	+18.9	1470.1	1446.3	1488.3	+41.2
Swr⁕	305.6	299.6	311.5	+12.0	1595.7	1545.0	1646.5	+101.5

), as precipitation amounts gradually increase northwards.

In terms of ASD, the mean values indicate that Swr is adequate for QRW (1551–1610 h) both at the regional level and in the case of all considered meteorological stations, while MOHwr generally displays a good suitability for QWW + RTW (1451–1550 h) (Figure 2d). However, territorial differences between the landforms included in this wine-growing region emerge; most of the plain and the southwestern part of the piedmont are suitable for QRW, while the largest part of the piedmont and the Subcarpathian present good suitability for QWW + RTW. In the eastern part of the piedmont, around Drăgășani, due to the specific relief conditions (slope orientation mostly) and to a particular topoclimate, an area suitable for QRW (ASD between 1551 and 1610 h) emerges.

The viticultural potential, as emphasized by the two applied indices, is among the highest in the country, taking into account its position and the values of the climatic parameters important for grapevine cultivation. Based on the mean values of Ibcv and IAOe, Swr is suitable for QRW, while MOHwr is suitable for QWW, with 11.4 units and 5061.6 units for the first region and with 7.4 units and 4573.2 units for the second region, respectively.

Despite the fact that the study area is not extensive in surface, the mean values of the two bioclimatic indices reveal certain spatial differences. Both Ibcv and IAOe indicate that the southern half of the Oltenia region has a good suitability to obtain QRW, except for the northern piedmont area (Figure 2e,f). Thus, based on Ibcv, the piedmont area offers better conditions for white wines, while IAOe indicates this area as suitable for QWW + RTW (class II). In the Subcarpathian, the average values of Ibcv are correlated with high suitability for QWW, while IAOe highlights a more detailed perspective, namely that the areas with lower temperatures and higher precipitation amounts, such as the western and northern extremities, are suitable for WTW + SW + WD (class III), while the central and eastern parts, where the heliothermal resources are greater, are suitable for QWW + RTW (class II).

3.2. Spatial Distribution of the Climatic Parameters and Suitability for Quality Wine Production: 1961–1990 Versus 1991–2020

Overall, the climatic parameters that recorded a significant increase at the national level in the past decades are those related to temperature and actual sunshine duration. When comparing the two periods, i.e., 1961/1990 and 1991/2020, there is an obvious positive difference in the case of GST, ∑Ta, and ASD (Figure 3; Table 3 and Table 4), which means the heliothermal potential of the region increased even in the northern extremity, indicating important shifts from one interval to another.

GST reached 18.4 °C for MOHwr and 19.7 °C for Swr (Figure 3a), namely a +1.1 and +1.0 °C difference, respectively, compared to the period of 1961/1990. In the southwestern extremity of the plain, GST exceeded 20 °C for most of this landform and the southern part of the piedmont recording values above 19 °C. A reduction can be noticed in the area with GST values below 17 °C, which presently characterizes only the central northern part of the Subcarpathian, around Polovragi. At the station level, most of the differences are within the regional range (+1.0/+1.1 °C), with few exceptions, i.e., Bâcleş +1.4 °C and Tg. Logreşti +0.7 °C, both in the piedmont area of MOHwr. It should be mentioned that the western part of the piedmont registered greater temperature increases compared to the central and eastern sectors of the same landform even if the altitude is higher, with topography (slope exposure) playing an important role in this case.

The ∑Ta emphasizes an average increase of +181.4 °C and +194.2 °C for Swr and MOHwr, respectively. Consequently, for the period 1991/2020, the entire analyzed region presents suitability for QRW (∑Ta > 3180 °C), including the Subcarpathian area at the foot of the mountains, with the exception of Polovragi, which is located at a higher altitude (Figure 3b). In the case of Swr, there is no change in terms of suitability class, whereas, for the second wine-growing region under study, a shift from QWW + RTW to QRW is noticed.

PP is the parameter characterized by a certain stability during the analyzed period. Generally, the amounts registered at the station level indicate a slight increase, namely less than 20 mm in most cases (Table 4 and Figure 3c). There are two stations, i.e., Caracal (the eastern part of the plain) and Bâcleş (western part of the piedmont), where PP slightly decreased (−13.6 mm, respectively −0.9 mm). During the period of 1991/2020, the southern half of Oltenia registered precipitation values theoretically adequate for QRW and above the irrigation threshold, while the northern half is more favorable for WTW, SW, and WD, the amounts increasing northwards. However, in the south, mainly in the case of Swr, where the increase is lower (+12 mm) and accompanied by higher temperatures as well, evapotranspiration is intense and efficient irrigation is necessary, particularly in drier years.

With regard to ASD, the increases are not as significant as the temperature ones, the differences between the two periods being lower than 100 h for all of the analyzed stations except for Băilești, situated in the plain area. There is one station, Craiova, located at the contact between the plain and the piedmont, where the ASD decreased by about 18 h, concomitantly with PP increase. In the plain area of both wine-growing regions, ASD exceeds 1600 h during the growing season, which means good suitability for QRW, as sunshine duration positively influences sugar/acidity content, sugar–acid ratio, and pH value. Thus, based on ASD, there is an emerging change in the suitability class for both regions in the second period (Figure 3d); in the case of Swr, the +101.5 h at the regional level means a shift from QWW + RTW to QRW, while the +41.2 h increase registered for MOHwr indicates a shift from WTW, SW, and WD to QWW + RTW.

Taking into account that ∑Ta and ASD augmented, while PP remained rather stable, it is quite normal to have an increase in both applied bioclimatic indices within the study area when the two periods (1961/1990 and 1991/2020) are compared in terms of mean values. Thus, Ibcv indicates +0.6 units for MOHwr and +0.9 units for Swr (

Table 5. Mean, maximum, and minimum values of the bioclimatic index (Ibcv) during the periods of 1961–1990 (′61/′90) and 1991–2020 (′91/2020) in the Oltenia wine-growing regions.

		Mean		Max.		Min.
Period/Wine-Growing Region		1961–1990	1991–2020	1961–1990	1991–2020	1961–1990	1991–2020
Ibcv (Units)
Dr.-T. Severin		9.3	10.4	21.2	21.5	4.6	3.6
Calafat⁕		11.9	11.8	24.6	21.2	6.4	5.5
Bechet⁕		11.1	11.5	20.4	24.3	5.6	4.7
Băilești⁕		10.8	12.3	23.2	23.4	5.4	4.6
Craiova		9.5	9.4	18.6	22.4	6	3.5
Caracal⁕		10	12.1	17.5	30.5	5.6	4.4
Slatina		-	8.9	-	19.8	-	3.3
Bâcleș		8.2	9.5	13.8	16.6	5.2	3.6
Tg. Logreşti		7	7.3	11.4	14.4	3.9	2.8
Drăgășani		7.9	8.3	13.5	17.2	4.4	3.5
Apa Neagră		5.6	5.9	10.9	14.6	3	2.6
Târgu Jiu		5.6	6.1	12.9	14.8	3	2.9
Polovragi		4.3	4.5	7.8	11.5	2.4	2.3
Râmnicu Vâlcea		6.1	6.6	11.1	13.4	3.5	2.9
Average	MOHwr	7.1	7.7	13.5	16.6	4.0	3.1
	Swr⁕	11.0	11.9	21.4	24.9	5.8	4.8
Difference	MOHwr	+0.6		+3.1		−0.9
	Swr⁕	+0.9		+3.5		−1.0

and Figure 3e) in the second period compared to the first one, but there is no shift in terms of suitability at the regional level (QWW in the case of MOHwr and QRW in the case of Swr). However, the regional MOHwr mean value for the 1991/2020 interval, namely 7.7 units, indicates that the wine-growing region gets closer to the 8 units threshold, which means suitability for QRW. The IAOe increase of +237.9 units in the second period compared to the first one also marks a shift in suitability for MOHwr, from class II to class I, namely from QWW + RTW to QRW. It has to be mentioned that the mean value for Swr exceeded 5000 units; more specifically, it reached 5197.1 units, which means an increase of +271 units compared to the period of 1961/1990 (

Table 6. Mean, maximum, and minimum values of the oenoclimate aptitude index (IAOe) during the periods of 1961–1990 (′61/′90) and 1991–2020 (′91/2020) in the Oltenia wine-growing regions.

		Mean		Max.		Min.
Period/Wine-Growing Region		1961–1990	1991–2020	1961–1990	1991–2020	1961–1990	1991–2020
IAOe (Units)
Dr.-T. Severin		4869.2	5153.5	5368.2	5722.4	4422.1	4381.4
Calafat⁕		4991	5231.5	5376.8	5790.3	4498.7	4664.1
Bechet⁕		4934.9	5135.2	5389.5	5638.9	4512.3	4504.7
Băilești⁕		4895.7	5259.3	5317.3	5842.1	4547.3	4594.7
Craiova		4795.8	4945.2	5260.7	5590.8	4338.1	4227.2
Caracal⁕		4882.9	5162.5	5365.8	5841.4	4434.1	4500.6
Slatina		-	4877.4	-	5464.2	-	4164.3
Bâcleș		4548.7	4838.6	4932.2	5585.2	4179.8	4235.7
Tg. Logreşti		4385.7	4546.7	4767.4	5273.7	3992.1	3763.7
Drăgășani		4602.9	4844.3	5111.4	5346.9	4180	4164.6
Apa Neagră		4177.7	4389.5	4756.6	4995.2	3765.4	3755.1
Târgu Jiu		4349.6	4601.5	5020.1	5256.2	3927.4	4002.2
Polovragi		3928.9	4051.5	4400.8	4869.1	3558.1	3376.4
Râmnicu Vâlcea		4368.4	4604.9	4798.4	5164	4000.7	3961.9
Average	MOHwr	4447.4	4685.3	4935.1	5326.8	4040.4	4003.3
	Swr⁕	4926.1	5197.1	5362.4	5778.2	4498.1	4566.0
Difference	MOHwr	+237.9		+391.4		−37.2
	Swr⁕	+271.0		+415.8		+67.9

and Figure 3f).

Locally, at the station level, shifts occurred in the case of MOHwr, particularly in the northern area. Thus, in the Subcarpathian, the central and eastern sectors became suitable for QRW (class II became class I), while, in the western part, at Apa Neagră, class III became class II, indicating suitability for QWW + RTW. Only in the northern part, at Polovragi, suitability remained unchanged, namely class III (WTW + SW + WD). If taking into account Ibcv, there are no changes except for the Drăgășani area, which is now more noticeably suited for red wines, the rest of the piedmont and Subcarpathian being classified as favorable for white wines.

The maximum and minimum values of the indices for the two periods also revealed differences between Swr and MOHwr. Ibcv maximum values show a notable increase of +3.1 units for MOHwr and +3.5 units for Swr. In the first region, the suitability threshold of 15 units is exceeded only in the period of 1991/2020, while, in Swr, maximum values are above the threshold in both periods (21.4 and 24.9 units, respectively). As opposed to the maximum values, minimum values are characterized by a decrease (−0.9 units for MOHwr and −1.0 units for Swr), which indicates a rise in Ibcv variability over the past three decades. The years with minimum values display suitability for white wines in the southern part of the region but, in the case of MOHwr, the conditions are unsuitable for grapevine growing (Table 5 and Table 6).

IAOe maximum values indicate an increase of +391.4 units for MOHwr and +415.8 units for Swr (Table 6), both regions displaying suitable conditions for QRW. About 5300 units in MOHwr and 5800 units in Swr are reached over 1991/2020, the difference between the two periods being 130–150 units greater than that indicated by Irimia et al. [19] when comparing 1961/1990 and 1991/2013 (ROCADA database). However, the situation is different when the minimum values are taken into account, as MOHwr registered a decrease (−37.2 units), the suitability class being modest, i.e., class III (suitable for WTW + SW + WD) in both periods. There is an increase in the minimum values (+67.9 units) in the case of Swr, but the suitability class did not change (i.e., class II, suitable for QWW + RTW).

Taking into account the climatic parameters introduced in the equations of the two indices, it results in a higher dependence of the annual values of Ibcv on the precipitation amounts and, to a lesser extent, on temperature and actual sunshine duration. Thus, maximum values correspond to the years characterized by high precipitation deficit during the growing season, while minimum values to the years with great precipitation excess. Considering both maximum and minimum values, the period of 1991/2020 is much more homogenous in terms of common years as compared to the 1961/1990 interval; thus, significant examples are those of the years 2000 (southwestern plain area, eastern piedmont area, and Subcarpathian) and 1993 (the rest of the region) in the case of the maximum Ibcv and 2014 (southwestern plain area, western and central piedmont area, and western and northern Subcarpathian area) and 2005 (the rest of the region) in the case of the minimum Ibcv. During 1961/1990, the maximum Ibcv was registered in different years, without an obvious territorial pattern—1962 (four stations), 1965, 1985, 1988 (two stations each year), and 1990 (three stations). The same situation is highlighted by minimum Ibcv values as well—1967, 1969, 1970, 1979 (one station for each year), 1975 (five stations), 1976, and 1980 (two stations each year).

Compared to Ibcv, IAOe is similarly dependent on the three aforementioned variables. Usually, PP is correlated with ASD and, in some cases, even with ∑Ta, namely in years characterized by higher PP during the growing season, ASD is lower and so is the ∑Ta. The relation between the three variables is illustrated by the maximum and minimum values of IAOe (Table 6), especially in the period of 1991/2020. Maximum values correspond to either 2000 (eight stations), which is the year with the driest growing season in most cases, or to 2012 (six stations), which registered the hottest growing season (11 stations out of 14). Minimum values generally correspond to the years with great precipitation excess, namely 2014 and 2005. The same homogeneity is specific to the period of 1961/1991, the maximum IAOe corresponding to 1962 (seven stations) and 1985 (five stations), years with the lowest precipitation amounts during the growing season, while the minimum IAOe corresponds to 1976 (nine stations) and 1980 (three stations), years with the lowest ∑Ta. The aforementioned values of the bioclimatic indices and their homogeneity suggest that the influence of local factors tends to become less important, high/low temperatures or precipitation amounts being triggered by particular synoptic situations that affect large surfaces.

3.3. Trends in the Main Climatic Parameters and Bioclimatic Indices

The determination of the trends is equally important in order to establish an evolution pattern and to take adequate adaptation measures in due time. The linear trendline reveals no significant changes in the case of Ibcv and no differences between the two wine-growing regions: 0.21 units for Swr and 0.20 units for MOHwr (13 units (60yr)⁻¹ and 12 units (60yr)⁻¹, respectively). The increase in IAOe values within both wine-growing regions is far more significant but higher in the case of Swr—3.98 units yr⁻¹ (239 units (60yr)⁻¹)—compared to MOHwr—3.18 units yr⁻¹ (191 units (60yr)⁻¹) (Figure 4a,b). These differences between the two applied indices result from the greater dependence on the precipitation amount of Ibcv (no positive or negative trend of this parameter) compared to IAOe, which is mainly determined by ∑Ta—5.88 units yr⁻¹ for Swr and 7.43 units yr⁻¹ for MOHwr (353 units and 446 units (60yr)⁻¹, respectively)—and less by ASD (3.73 units yr⁻¹ for Swr and 0.03 units yr⁻¹ for MOHwr (224 units and 22 units (60yr)⁻¹, respectively)) and PP.

Based on the nonparametric Mann–Kendall test, statistically significant positive trends of IAOe were registered for all the analyzed meteorological stations, namely p < 0.001 (two stations within Swr and four stations within MOHwr), p < 0.01 (two stations within each wine-growing region), p < 0.05 (three stations within MOHwr), and p < 0.1 (one station within MOHwr) (

Table 7. Test Z, statistical significances (SS), and Sen’s slope estimate (Q) for the applied bioclimatic indices (1961–2020).

Meteorological Station	Ibcv			IAOe
	Z	SS	Q	Z	SS	Q
Dr.-T. Severin	1.58		0.044	3.64	***	9.323
Calafat⁕	0.04		0.001	3.23	**	7.366
Bechet⁕	0.85		0.020	3.25	**	6.588
Băileşti⁕	1.93	+	0.046	4.76	***	10.349
Craiova	−0.66		−0.015	2.03	*	4.423
Caracal⁕	2.15	*	0.059	3.77	***	8.346
Slatina	−0.96		−0.035	1.85	+	5.038
Bâcleş	1.84	+	0.037	3.94	***	8.353
Tg. Logreşti	0.57		0.009	2.21	*	4.557
Drăgăşani	1.47		0.023	3.55	***	8.375
Apa Neagră	0.92		0.013	2.67	**	6.791
Târgu Jiu	0.98		0.011	2.77	**	7.375
Polovragi	0.40		0.004	2.29	*	5.257
Râmnicu Vâlcea	1.27		0.018	3.36	***	7.553

Viticultural region: Swr⁕, MOHwr.

and Figure 5).

The trends registered in the case of Ibcv are more heterogenous and generally not statistically significant. Thus, within Swr, trends are positive (significant in the eastern and central parts of the wine-growing region, p < 0.05 at Caracal and p < 0.1 at Băilești). Within MOHwr, positive but not statistically significant trends are registered in the piedmont and Subcarpathian area, while, in the plain, at the contact with the piedmont, trends are negative (Craiova and Slatina). With reference to the climatic parameters introduced in the equations of the two bioclimatic indices, ∑Ta obviously increased (p < 0.001 for 13 stations and p < 0.01 for 1—Târgu Logrești), the same evolution pattern being noticed for GST. In the case of PP, the trends, either positive or negative, are not significant, while ASD presents a more heterogenous situation. For Swr, all trends are positive and significant, highlighting a more consistent increase in the heliothermal resource compared to MOHwr (the northern sector of the plain registers a significant negative trend, p < 0.1, and the northern Subcarpathian area a negative trend, while the rest of the stations display positive but not statistically significant trends, except for the western extremity of the Subcarpathian, p < 0.05) (Figure 5). The increase in actual sunshine duration can be correlated with the change in the precipitation type (more showers) and the increasing trend in the frequency of rain showers, both in spring and summer [35], especially in the south–southwestern plain area.

4. Discussion

The geographical distribution of vineyards greatly depends on the climatic conditions of a region. Thus, climate is often considered the main limiting factor, as well as an important element when experts assess the ‘terroir’ [98]. Consequently, any short- or long-term change in the climate variables will trigger modifications in the plant phenology and vegetative growth (bud break, flowering, veraison, and harvest), quality of grapes, and, of course, wine quality and typicity [3]. Temperate regions face not only the substantial modification of the grapevine climate suitability but also a great variability in the considered climatic parameters, which determine significant fluctuations in the quality of grapes and wine vintage from one year to another.

Besides the immediate impact of the meteorological variables on grapevine development, yield, and quality of grapes, there are also long-term effects. A longer growing season and the shifts in the phenological cycles of plants are among the consequences of gradually higher temperatures. Bandoc et al. [99] pointed out that the rise in temperature has already triggered the increase in the growing season’s length, on average, by 14 days between 1961 and 2010 for 1 °C in southern and southeastern Romania, which overlaps with the most important agricultural area of the country. A longer growing season and greater thermal resources are indicated as the factors triggering the northwards displacement of the area suitable for grapevine cultivation together with a greater suitability for red grape varieties within regions that were previously appropriate only for white varieties [17,18,57,100,101]. Patriche and Irimia [102] emphasized that the potential for quality red wines increased significantly from 21,217 km² (8.9%) in the period of 1961/1990 to 73,385 km² (30.8%) in the period of 1991/2013, while the potential for quality white wines decreased from 45.4% to 38.2%.

The average values of both applied indices reveal an improvement in climate suitability for the wine production in the case of MOHwr, while Swr, one of the areas traditionally suitable for quality red wines, kept its potential. Different spatial changes emerge in Oltenia during the analyzed period, the latitudinal shift of climate suitability being mainly determined by the obvious and statistically significant increase in temperature and actual sunshine duration, as precipitation amounts are rather stable within the entire region, as also emphasized in different climate studies [28,39,41].

This shift is characteristic to MOHwr and is better emphasized by IAOe compared to Ibcv. Thus, based on IAOe, the area suitable for quality red wines extended northwards with about 30′ of latitude on average (55.5 km), with certain differences between the eastern and western sectors of the region, namely 26′ in the east (about 48 km, from Drăgășani to Râmnicu Vâlcea) and 33′ in the west (about 61 km, from Bâcleș to Târgu Jiu). In the case of Oltenia, no altitudinal shift is registered, as the new suitable areas from the Subcarpathian, which are in the north of the region, are generally below the altitudes of the southern piedmont area. Ibcv also illustrated a northward but less extended shift, about 8–11′ (15 to 20 km), the spatial pattern being similar to the one highlighted by IAOe, namely a greater extension in the western sector.

In terms of quality white wines, a northward shift of the suitable area is also noticeable. For the moment, we cannot notice an obvious shrinkage of the area favorable to their production overall, as new areas became suitable. However, taking into account the general configuration of the relief (the presence of the Carpathians in the north of the region), we can point out the narrowing of the suitability space in the future, once the quality red-wine-suitable area expands northwards. Higher altitudes are unsuitable for grapevine cultivation not only because of more restrictive climate conditions but also because of the soil characteristics.

In Romania, shifts in climate suitability for wine production are mentioned by different authors. Thus, based on IAOe, Irimia et al. [4,19,51] pointed out the significant expansion of the area climatically suitable for grapevine growing at the country level, as well as changes in the suitability structure, most notably in Transylvania. In Oltenia, the aforementioned studies indicated a shift from quality white wines to quality red wines for the piedmont area and along the valleys of the rivers in the Subcarpathian, while a shift from white table wines to quality white wines characterized the rest of this unit. Compared to these results, the present study revealed a greater expansion of the area suitable for quality red wines in the Subcarpathian space.

At the regional level, there are much more numerous studies indicating shifts in suitability. In Moldavia (eastern Romania), Irimia et al. [17,18] indicated an increase in the surface suitable for quality white wines in the case of the Avereşti wine-growing region, together with the introduction of red varieties (based on different indices, among which were Ibvc and IAOe); in the case of the Cotnari wine-growing region, the area suitable for quality white wines was documented to reduce in size, as opposed to the area suitable for red wines (based on IAOe). Considering the past 20 years, Pușcalău et al. [58] applied various bioclimatic indices, including Ibvc and IAOe, and highlighted the suitability change (quality red wines) of the Odobești wine-growing region located in the proximity of the Curvature Carpathians. Based on different bioclimatic indices (HI, Ibvc, IAOe, etc.), Zaldea et al. [59] also pointed out a moderate suitability for red wines in Moldavia. For Oltenia, this northward shift of the area suitable for quality red wines and quality white wines was mentioned by Vlăduț et al. [101], based on HI and WI.

The altitudinal and latitudinal shift of the area suitable for obtaining quality wines is also consistent with other studies from different European areas. Irimia et al. [103] pointed out significant changes in climate suitability in important wine-growing regions, such as Bordeaux (France), Loire Valley (France), Rhine-Main-Nahe (Germany), La Rioja (Spain), Cotnari (Romania), and Sussex (UK). The authors based their conclusions on three indices, namely GST, HI, and IAOe, also emphasizing that the wine regions with low altitudes, especially those located in the south, are more vulnerable to climate change. An increase in the suitability was mentioned by Kryza et al. [15] for the Poland–Germany–Czech Republic transboundary area (based on the sum of active temperatures and growing degree days), by Koźmiński et al. [104] in Poland (based on the sum of active temperatures—a 100 to 150 km northward shift), Koch and Oehl [16] in Palatinate, Germany (based on HI), Vrsic et al. [20] in Slovenia (based on HI and the growing degree days), and Charalampopoulos et al. [105] for the Balkan area (based on the growing degree days). The extension of the areas cultivated with warmth-loving cultivars was also mentioned in Hungary [106].

However, on the general background of an increasing suitability for quality red wines at the regional level, southern Oltenia presently faces two challenges: the potential loss of specific climate, as well as the potential loss of wine type production in the Swr traditional viticultural area, mainly because of temperature increases. The findings of other studies pointed out that, if the optimum thermal conditions for the presently cultivated varieties are exceeded, a potential decrease in wine quality may occur because of lower acidity [7], increase in the sugar content of berries [107], and reduction in anthocyanin content [108,109].

Such situations were mainly reported in southern Europe [9,11,110,111,112] but also in southern and western Romania [49,56,60,61,113]. In Oltenia, Băducă Câmpeanu et al. [114] noted the advancement of all phenological phases due to temperature increases, while also pointing out a higher accumulation of sugars and anthocyanins and lower acidity at absolute maturity, which clearly indicates better red wines but unbalanced white wines. Bucur and Babeș [49], for example, indicated +14 days with T_max > 35 °C and +35 days with T_max > 30 °C for Craiova during the interval of 1977–2015 compared to the reference period of 1961–1990, which significantly correlated with a higher accumulation of sugar, an acidity reduction, and a decrease in grape yield. The same situation was mentioned by Bucur et al. [61] for the Bucharest area and Buciumeanu et al. [115] for the Ștefănești viticultural center, both in southern Romania.

Most of Swr already exceeded 19.5 °C GST, which means daily maximum temperatures often reach 35–40 °C and daily minimum temperature remains above 20 °C in summer, especially in July and August [41], which have a greater contribution to the growing season temperature compared to the other months. July and August coincide with the veraison period, extremely high temperatures thus affecting the optimum development of grapevine and berries. The summer of 2024 was relevant from this viewpoint. Calafat and Dr.-T. Severin registered the highest mean annual temperature in the country, i.e., 14.9 °C (+3 °C, respectively, +2.8 °C above the mean value of 1961–2020) [116]; there were more than 15 days with maximum temperature ≥35 °C in southern Oltenia and along the Jiu and the Olt Valleys up to the Subcarpathian in July [117] and between 16 and 20 such days in southwestern Oltenia in August [118]; there were 24 cases of daily maximum temperatures ≥40 °C. In the interval of 1991–2020, ∑Ta exceeded 3600 °C in the proximity of the Danube River, which means +213.2 °C at Dr.-T. Severin and +205.8 °C at Calafat compared to the previous interval. This increase in the sum of active temperatures at the regional level (+194.2 °C in case of MOHwr and +181.4 °C for Swr) is consistent with the one identified by Irimia et al. [51] for the periods of 1961/1990 and 1990/2013 (+187 °C for the MOHwr and 190 °C for the Swr), based on ROCADA database.

Consequently, thermal stress emerges as the most probable threat for viticulture within Swr. However, hydric stress should also be considered, as southern Oltenia, especially in the second part of summer, is frequently affected by precipitation deficit and drought, which may also increase in frequency and intensity in the near future. For this area, previous studies also indicated an increase in the water deficit and aridity conditions, which will amplify in the future, especially during the summer months, on the background of temperature rise [45,48].

For the moment, the potentially negative impact of climatic conditions on the development of the grapevine can be mitigated through different interventions at the vineyard level during the growing season. However, if changes become persistent and gain in intensity and/or frequency, medium- and long-term adaption measures will become a necessity. Some of the significant adaptive measures that the present winegrowers could implement to enhance the resilience of this sector to climate change involve changing vineyard design or using shading nets in order to reduce water consumption and evapotranspiration, implementing sustainable soil management practices, such as appropriate tillage or cover crops, and using supplementary irrigation when water resources are available [119]. Nevertheless, the complex implications of such adaption measures should be further studied trough tailored assessments, as they would increase competition for water resources and might raise production costs to levels that are not economically sustainable for some winegrowers. On the longer term, adaptive strategies to temperature rise and declining water availability should involve the use of drought-resistant or late-ripening grapevine cultivars, as well as the adoption of resilient training systems that protect grapes from overexposure to direct sunlight [88,120].

As underscored by the present study, the changing viticultural suitability that comes with climate changes is also an opportunity for vineyard relocation in new areas. Nonetheless, this could also imply a risk that further research must assess because the land use changes induced by vineyard expansion involve loss of existing cropland or wild habitats and an enhanced stress on natural resources [119].

The strategic planning of the winemaking sector and, in a broader sense, of the winescape management and capitalization in the climate change context should be a priority for any traditional and presently important viticultural region. This study could support winegrowers and policymakers in identifying and prioritizing adaption measures. The structural changes of the current wine-growing patterns will require resources (mainly financial and informational), which must be taken into account by all stakeholders in Oltenia. The perception of climate change represents a significant factor in the adoption of such measures, so it is essential for the vignerons in Oltenia to become aware of the impact of climate change on their vineyards and to optimize their decisions by considering the findings of climate change research.

5. Conclusions

Oltenia is the third region in Romania in terms of vineyard surface, which highlights the importance of the assessment of current climate conditions, their potential change, and impact in relation to grapevine. We have analyzed climate data (registered at 14 meteorological stations during the growing season) for a 60-year period, split into two equal intervals (1961–1990 and 1991–2020) in order to better understand the changes that occurred at the regional level. The comparative analysis of the two intervals revealed substantial changes in both climatic variables taken into consideration, as well as in the two applied indices. we note a significant increase in temperature values, a moderate increase in the actual sunshine duration, and a relative stability of the precipitation amounts, especially in the last 30 years of the analyzed period. Based on the two used indices, the bioclimatic index and the oenoclimate aptitude index, grapevine climate suitability improved, especially within MOHwr. The area suitable for quality red wines reached the southern Subcarpathian in the period of 1991/2020, which means 15 to 20 km and about 55 km expansion northwards, respectively, according to the values of the two aforementioned indices. With regard to the suitability for quality white wines, new areas emerged in northern Oltenia at the foot of the mountains. However, taking into account the relief configuration (the Carpathian Mountains), we presume that the climatically suitable area for wine grape growing will not extend much northwards and at higher elevations in spite of the improvement of the thermal regime. There are other climatic variables, such as frost, hail, winds, precipitations, etc., which limit the suitability of the region.

Despite the auspicious context—emergence of new areas suitable for grapevine growing in the north of the region and increase in the surface suitable for obtaining high quality red wines—there are certain concerns, especially for southern Oltenia. Heat stress, accompanied by hydric stress tends to increase and affect the proper development of grapevine. If temperature increase continues at the same rate, as emphasized by different studies or estimated by the climatic models, the respective area will first acquire characteristics suitable for the production of table grapes and then lose its viticultural potential. Consequently, local producers should take into account not only a potential change in the traditional types of wine production but even the necessity to introduce new varieties that proved to be more vigorous and productive in hotter climates.

The present findings add to the Romanian and European specialized literature by highlighting the temporal and spatial dynamics of the suitability for wine production in Oltenia under a changing climate and, thus, represent an important tool for stakeholders in the winemaking industry. However, the research needs to be further refined by adding other elements that are important for the assessment of present and future viticultural potential of an area (topography, soils, increased CO₂ levels, weather-related extreme events, etc.). Although the present study analyzes a significant time interval, we admit that it may not grasp longer-term climate variability and trends. Future research should extend this assessment to include new data, which could enhance our understanding of the ongoing impacts of climate change on viticulture. Furthermore, new studies that integrate high-resolution spatial datasets and advanced modeling techniques would be an important instrument for stakeholders in viticulture, enabling them to plan and adapt by implementing new practices and, thus, boosting their resilience in the face of climate variability. Nevertheless, as pointed out above, it is important that future research should carefully consider the broader economic and natural implications of climate change adaptation measures in order to achieve viticultural strategies that are adapted to the particular climatic challenges and distinctive economic context of the analyzed vineyards.