Introduction

The grapevine (Vitis vinifera L.) is a perennial plant with an annual cycle that is highly dependent on environmental conditions (Parisi et al., 2014). Among them, air temperature represents a central factor for the cultivation of grapevines. In cases where the water, nutrient and radiation requirements of the plants are fulfilled (Nendel, 2010, Webb et al., 2007), thermal conditions control “with only minor other influences” (Gladstones, 2011) grapevine phenology and final grape maturity. Consequently, changes in thermal conditions are impacting grape phenology, grape maturity, wine typicity and, as a last consequence, the economic sustainability of many traditional grapegrowing regions such as Luxembourg, where the wine industry traditionally represents an economically important sector.

On the east- to south-faced vineyards along the Moselle River grapevines have been cultivated since the Roman times. The quality and the quantity of annual wine production in Luxembourg have been documented in a wine chronicle since the beginning of the 9^th century A.C. (Molitor et al., 2016b). Nowadays (2017), the total area of the Luxembourgish grapegrowing regions covers 1303 ha between Schengen and Rosport over approximately 42 km along the Moselle River (Anonymous, 2018). The position of the viticultural fields in the Luxembourgish grapegrowing region is depicted in Figure 1. In 2017, most cultivated cultivars were Müller-Thurgau (syn. Rivaner) (23.3% of the total acreage), Pinot gris (15.2%), Auxerrois (14.8%), Pinot blanc (12.6%), Riesling (12.5%), Pinot noir (9.7%) and Elbling (6.1%) (Anonymous, 2018). Total wine production of the region on average of the vintages 2008 to 2017 reached 109 092 hl (Anonymous, 2018).

Worldwide, thermal conditions have significantly changed during recent decades at both global and regional scale. According to the Intergovernmental Panel on Climate Change (IPCC), human influence, primarily the burning of fossil fuels, has been the dominant cause of global warming for several decades (IPCC, 2013). Based on regional climate change projections taken from the ENSEMBLES and the CORDEX projects, an air temperature increase of up to 4 °C – depending on the emission scenario – is projected for Luxembourg by the end of this century (Goergen et al., 2013; Junk et al., 2016). Also, changes in precipitation patterns towards drier summers and wetter winters are projected (Goergen et al., 2013).

Figure 1. The Luxembourgish grapegrowing region. Vineyards are depicted in red, main rivers in blue and borders between countries in black. The weather station used for present investigations is located in Remich.

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Since grape phenological development is predominantly air temperature-driven (e.g., Duchene and Schneider, 2005; Gladstones, 2011; Keller, 2015; Moncur et al., 1989), the projected temperature increase due to climate change will have significant impacts on viticulture, particularly close to the climatic frontiers of viticulture where the dependence of grape phenology and maturity on climatic conditions is most pronounced (Brazdil et al., 2008). Phenology represents a major factor in the distribution of the viticultural areas (Garcia de Cortazar-Atauri et al., 2017). With ongoing climatic change, northern regions are expected to become more suitable (in terms of climatic conditions) for ripening grapes (Jones and Schultz, 2016). In contrast, regions where temperatures are already close to optimum for best wine quality might become too hot to produce high quality wines with balanced fruit in the future (Jones and Schultz, 2016). To assess the viticultural opportunities, risks and challenges related to climate change and furthermore to support the development of adequate adaptation strategies, numerical models simulating the effects of temperature conditions on plant development are helpful tools.

Recently, Molitor et al. (2014b) developed a high-resolution phenology model covering all 27 BBCH (Biologische Bundesanstalt, Bundessortenamt und Chemische Industrie) plant phenological growth stages defined by Lorenz et al. (1995) from budburst to grape harvest (i.e., BBCH stage 89 describes “grapes ripe for harvest”). The incorporation of (i) an upper threshold temperature, above which a further increase of the temperature will not accelerate plant development, and (ii) a heat threshold, above which a further increase of the temperature will slow down plant development, in this model have been demonstrated to improve the precision of the model compared to commonly used un-capped cumulative degree day-based phenology models (e.g., Amerine and Winkler, 1944; Duchene et al., 2010; Hoppmann, 2010; Nendel, 2010; Oliveira, 1998; Parker et al., 2011; Schultz, 1992; Zapata et al., 2015). Under increased air temperature, the improvement in precision gained through the incorporation of additional thresholds is expected to be even more pronounced (Molitor et al., 2014b).

Since (i) air temperatures are expected to increase in the future due to climate change and (ii) the ripening period will likely take place earlier, usually in the warmer parts of the season due to faster phenological development driven by higher temperatures, a two-fold impact on air temperature increase in the ripening period could be expected (Duchene et al., 2010), while little is known about the influence of climate change on the temperature conditions in other phenophases. In addition, a temperature increase in the ripening period is expected to alter the typicity of the wine (Jackson and Lombard, 1993).

Consequently, the aim of the present study was to investigate the future phenological development of the cultivars Müller-Thurgau, Riesling and Pinot noir, as well as the consequences on the air temperature conditions in specific phenophases, especially in the ripening period, based on (i) the budburst model of Molitor et al. (2014a), (ii) the high-resolution phenological model of Molitor et al. (2014b) and (iii) a multi-model ensemble of ten regional climate change projections under the conditions of the Luxembourgish grapegrowing region.

Materials and methods

Results

1. Annual temperature evolution

Figure 2 shows the observed annual mean air temperatures for the weather station at Remich as well as the multi-model mean of the ten ensemble members (bias corrected). A significant increase in the annual air temperatures compared to the reference period is projected (Table 2).

Figure 2. Observed annual average temperatures in Remich (red line) and projected (A1B emission scenario; ten ensemble-based regional climate change projections; multi-model mean) annual average temperatures (blue line) in the period 1970 to 2100. Ensemble spread (+/- 1 standard deviation) is indicated in grey.

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Table 2. Average (ten ensemble-based regional climate change projections) annual air temperatures, monthly air temperatures, as well as pre-bloom (BBCH 09-61), bloom (BBCH 61-69), post-bloom (BBCH 69-81) and ripening (BBCH 81-89) air temperatures in the Müller-Thurgau, Riesling and Pinot noir cultivars in the different 30-year time spans. Δ (°C) = temperature difference (in Kelvin) compared to the reference period (past; 1971-2000).

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	Past (1971-2000)	Present (2001-2030)		Near future (2031-2060)		Far future (2061-2090)
	T (°C)	T (°C)	Δ (°C)	T (°C)	Δ (°C)	T (°C)	Δ (°C)
Year (01/01-31/12)	10.3	10.8	0.5	11.8	1.5	12.9	2.6
January	2.3	2.9	0.6	4.3	2.0	5.4	3.1
February	3.0	3.7	0.7	4.9	1.9	5.8	2.8
March	5.9	6.6	0.7	7.4	1.5	8.4	2.5
April	9.6	10.2	0.6	11.1	1.5	11.4	1.8
May	13.8	14.2	0.4	15.0	1.2	15.6	1.8
June	16.8	17.3	0.5	18.1	1.3	19.4	2.6
July	18.9	19.5	0.7	20.2	1.3	21.4	2.5
August	18.3	19.0	0.7	19.9	1.7	21.1	2.8
September	15.2	15.7	0.5	16.6	1.4	17.8	2.6
October	10.8	11.2	0.4	12.2	1.4	13.3	2.5
November	5.6	6.1	0.6	7.3	1.8	8.7	3.2
December	2.7	2.9	0.2	4.3	1.5	5.9	3.1
Müller-Thurgau
Pre-bloom (BBCH 09-61)	14.4	14.4	0.0	14.6	0.2	14.5	0.1
Bloom (BBCH 61-69)	17.6	17.8	0.1	18.2	0.5	18.3	0.6
Post-bloom (BBCH 69-81)	18.8	19.3	0.5	19.7	0.8	20.7	1.8
Ripening (BBCH 81-89)	16.3	17.5	1.2	19.2	2.9	20.9	4.6
Riesling
Pre-bloom (BBCH 09-61)	14.5	14.6	0.0	14.7	0.2	14.7	0.1
Bloom (BBCH 61-69)	17.7	17.9	0.2	18.4	0.7	18.4	0.7
Post-bloom (BBCH 69-81)	18.7	19.3	0.6	19.9	1.2	20.9	2.2
Ripening (BBCH 81-89)	15.2	16.6	1.3	18.4	3.2	20.3	5.1
Pinot noir
Pre-bloom (BBCH 09-61)	14.4	14.4	0.0	14.5	0.2	14.5	0.1
Bloom (BBCH 61-69)	17.5	17.6	0.1	18.0	0.5	18.2	0.6
Post-bloom (BBCH 69-81)	18.7	19.3	0.5	19.8	1.0	20.7	2.0
Ripening (BBCH 81-89)	15.0	16.5	1.5	18.4	3.3	20.3	5.3

Temperatures of the same phases that differed significantly according to the non-parametric Mann-Whitney U-test (p= 0.001) compared to the reference period (1971-2000) are marked in bold.

Discussion

Present analyses revealed a constant increase of the annual air temperature in the Luxembourgish grapegrowing region in the future confirming previous studies (Junk et al., 2015a, Junk et al., 2015b, Junk et al., 2016, Lokys et al., 2015, Molitor et al., 2014b).

The simulation of the future phenological development took place based on the high-resolution phenological model as proposed by Molitor et al. (2014b). Here, optimized threshold temperature triplets for phenology simulation are defined statistically per cultivar based on long-observation data. Even though these optimized thresholds are purely statistical, model validation for Müller-Thurgau over a broad range of locations in Europe showed a high accuracy (Molitor et al., 2014). Using this cumulative degree day approach, in all three cultivars of investigation, all 27 phenological stages were computed to be reached significantly earlier in the future time spans than in the reference period confirming studies in other viticultural regions in recent years (e.g., Caffarra and Eccel, 2011; Duchene and Schneider, 2005; Duchene et al., 2010; Fraga et al., 2016; Garcia de Cortazar-Atauri et al., 2017; Ramos, 2017; Sadras and Moran, 2013; Trought et al., 2015). Differences in the extent of earlier appearance of phenological stages as well as in the length of phenophases observed by the different authors are supposed to be caused by differences in (i) the phenological models used (e.g., taking into account the effect above optimum temperatures or not), (ii) cultivars of investigation as well as (iii) the climatic conditions in the studied regions.

In the past, BBCH stage 89 (“berries ripe for harvest”) was simulated not to be reached in all years in all three cultivars. This was especially the case for the (compared to Müller-Thurgau) relatively late ripening cultivars Pinot noir and Riesling. In the near as well as in the far future, heat summation will – according to present analyses – not be a limiting factor for full grape maturity – not even in Riesling or Pinot noir.

Due to generally increasing air temperatures, budburst (BBCH 09) is modelled to be reached significantly earlier in the future than in the past confirming previous analyses (e.g., Caffarra and Eccel 2011, Fila et al., 2012, Molitor et al., 2014a). The consequences of the earlier budburst on the future late frost risk are controversially discussed in scientific literature (Kartschall et al., 2015, Kotremba et al., 2014, Molitor et al., 2014a, Mosedale et al., 2015, Sgubin et al., 2018). While the analyses of Molitor et al. (2014a) indicate that the late frost risk might decrease in the future, other analyses revealed inconsistent or even increasing late frost risks. These contrary results might be explained by the respective underlying budburst models used.

The total length of the period between BBCH 09 and BBCH 89 (season duration) is projected to be shortened in the future in all cultivars confirming projected data of Webb et al. (2007) for the Australian grapegrowing regions. Interestingly, the length of the different phenophases prior to veraison is projected not to change significantly in the future compared to the reference period. That is, the shift of the phenological development until veraison towards the beginning of the year is mainly the result of the earlier budburst. This is in accordance with observations of Duchene and Schneider (2005) in the Alsace region where the time span from budburst to flowering was constant between 1965 and 2003. In contrast, the phenophase 85 representing the period between BBCH 85 and BBCH 89 is projected to be up to 13 days shorter in the far future in the case of Pinot noir. The explanation for adverse effects on phenophase lengths in different phases of development can be attributed to the course of the daily average air temperatures in the four time spans. Figure 7 demonstrates that this increase is relatively constant in the course of the year (left) compared to the reference period. In contrast, when plotting the daily average air temperatures relatively to the date of budburst, comparable temperature conditions are projected in the period around BBCH 09 in all four time spans (Figure 7; right). As consequence of the relatively constant temperature conditions, the length of the phenophases (mainly determined by temperature conditions) is not significantly affected in the early stages but shifted towards the beginning of the year.

Figure 7. Course of daily average temperatures in the four time spans (past: 1971-2000; present: 2001-2030; near future: 2031-2060; far future: 2061-2090) (i) relative to 01/01 (day of the year – DOY; left) or (ii) relative to the date of budburst (BBCH 09; right).

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As a consequence, the computed pre-bloom air temperatures do not show any significant differences between the reference period and the future time spans. This is the case because this phase is shifted towards the earlier (usually colder) part of the year. In consequence, mainly temperature-dependent steps of grape physiology between budburst and bloom (such as inflorescence differentiation and flower initiation (Keller 2015; Molitor and Keller, 2016)) might not be systematically affected (even though there is an increase in spring temperatures).

While, according to present results, air temperatures are already increasing significantly in the post-bloom to veraison period, the length of the phenophase stays relatively constant until veraison. This apparent contradiction can be explained by the fact that the air temperature conditions in this period are situated in all time spans predominately in the optimum range for further phenological development (e.g., in the case of Riesling between 18 °C and 24 °C daily mean air temperature). Here, a further slight increase of daily average air temperatures does not influence the pace of the phenological development, if the heat threshold is not exceeded.

For practical viticulture under given climatic conditions, the relatively stable length of the phenophases prior to veraison means that consequences on the temporal distribution of the pre-veraison workload in the vineyard are expected to be relatively slight. That is, since phenological development is not accelerated in this period, spray intervals and time frames for canopy management measures, for example, might not be affected.

However, higher air temperatures in the time frame between bloom and veraison might have an impact on annual yield. Recent studies have demonstrated negative correlations between yield and temperatures, especially maximum temperatures, over the first three weeks after bloom for the Riesling cultivar (Molitor and Keller, 2016). Hence, higher air temperatures in this period might lead to a decrease in annual yield in the future while in other periods the effect of higher temperatures on the annual yield was observed to be mainly positive (Molitor and Keller, 2016). To clarify the overall effect of climate change on yield formation, in a further step, the present data on future temperature and phenological conditions will be combined with the temperature- and precipitation-driven yield models for the Müller-Thurgau and Riesling cultivars developed based on the multi-annual yield records for the Luxembourgish grapegrowing region (Molitor and Keller, 2016).

The most distinct influences on the temperature conditions as well as on the length of the phenophases are modelled for the ripening period between veraison and harvest. The increase in the ripening temperature is most pronounced in late ripening cultivars such as Riesling and Pinot noir. Here, the grapes are ripening at a later stage of the year where temperature differences compared to the reference period temperatures are most distinct (temperature decrease in the autumn), while in case of Müller-Thurgau, the ripening period is closer to the summer plateau of air temperatures. The projected increase in the ripening temperatures in the near future is 1.9 °C (Müller-Thurgau; 2.9 °C), 2.1  °C (Riesling; 3.2 °C) and 2.2 °C times higher (Pinot noir; 3.3 °C) than the temperature increase in the month of September (1.5 °C). This phenomenon is the result of two additional effects: (i) the general temperature increase and (ii) the shift of the phenology towards the earlier, generally warmer period of the year.

In fact, this two-fold effect demonstrates that changes in temperature conditions in calendar-based (=anthropogenic) time frames (such as in the “Cool Night Index” (Tonietto and Carbonneau, 2004), taking into account minimum temperatures in the month of September (northern hemisphere)), might not completely reveal the real temperature changes in specific developmental stages.

More generally, this fact is indicating a general limitation of calendar-based climatic indices used in viticulture such as the Winkler index (Amerine and Winkler 1944), the heliothermic index of Huglin (1978) or the average growing season temperature according to Jones (2007) since they do not take into account the plant response to climate (Caubel et al., 2015) and, hence, might not perfectly reflect the real vegetation period of specific cultivars (Holzkämper et al., 2010).

Present results furthermore demonstrate that the length of the ripening period (BBCH 81-89) is projected to decrease in the future (e.g., Riesling, past: 44.6 days; far future: 33.9 days). This is confirming the results of the analyses of Tomasi et al. (2011) in Northern Italy. Authors observed a significant decrease of the length of the period between veraison and harvest in the period 1964 to 2009 caused by increasing temperatures (Tomasi et al., 2011). Due to the fact that the harvest date might under practical conditions be influenced by several factors (Garcia de Cortazar-Atauri et al., 2010) the impact of temperature on the length of the ripening period is controversially discussed in the literature. For example, van Leeuwen and Destrac Irvine (2017) found opposite trends in the length of the period between veraison and harvest. However, based on present results we assume that the temperature-sum approach proposed here might, at least under the climatic conditions of the Luxembourgish grapegrowing region, better reveal the real average temperature conditions in the simulated ripening period (since its length is assumed to be influenced by temperature conditions) than approaches which calculate the temperature conditions in this period as the average temperatures between veraison and a temporally fixed date such as (i) 35 days thereafter (Duchene et al., 2010) or (ii) 60 days thereafter (Schultz and Hofmann, 2017).

The strong air temperature increase in the ripening period as described above is presumably linked to distinct changes in the wine typicity of a specific region.

In fact, higher temperatures are leading to altering fruit ripening rates (Martinez-Lüscher et al., 2015), changes in flavour and aroma profiles (Trought et al., 2015) as well as decoupled anthocyanin and sugar syntheses in berries (Sadras and Moran, 2013). Expected higher sugar concentrations are leading to higher alcohol contents (Jackson and Lombard, 1993) in the wines, and an acceleration of the degradation of organic acids (Duchene et al., 2010), threatening both the freshness and lightness that is especially exemplary for white wines in (former) cool climate grapegrowing regions, such as Luxembourg. Furthermore, the fruitiness and aroma of grapes and wines is expected to be negatively affected by high ripening temperatures (Duchene et al., 2010). To maintain the wine typicity of the region, potential adaptation strategies to mitigate the consequences of climatic change in general and higher ripening temperatures in particular might consist of measures leading to a temporal delay of the maturation period. This might be achieved by a shift towards cooler sites (e.g., with higher elevations or lower exposition) or regions (e.g., at higher latitudes), cultivars or clones with a later ripening characteristic, maturity-retarding rootstocks, the application of antitranspirants (Gatti et al., 2016) or specific crop cultural measures including training systems (Molitor et al., 2019), delayed winter pruning (Friend and Trought, 2007) and adapted canopy management (Parker et al., 2016, Stoll et al., 2013, Trought et al., 2015).

Conclusions

Present analyses demonstrated that under the climatic conditions in the Luxembourgish grapegrowing region each of the 27 phenological stages according to BBCH code are projected to be reached significantly earlier in the future than in the reference period. While significant changes in the phenophase lengths are absent in early stages, the ripening period length is significantly shortened in the future according to these projections. Since (i) air temperatures are generally projected to increase in the future and (ii) the ripening period will take place earlier (usually in the warmer parts of the season), climate change is implicating a two-fold impact on ripening period air temperature increase. Consequently, the air temperature increase in the ripening period (far future compared to reference period: + 4.6 °C to + 5.3 °C) is projected to be markedly higher than in the annual averages (+ 2.6 °C). This significant increase of the ripening period air temperatures potentially threatens the wine typicity of the traditional grapegrowing regions and therefore calls for specific adaptation strategies.

Acknowledgements: The authors thank B. Fuchs (Weinbauamt Eltville, Germany), O. Baus (Hochschule Geisenheim University), S. Fischer, R. Mannes and M. Schultz (Institut Viti-Vinicole, Remich, Luxembourg) for providing parts of the historical meteorological and phenological data sets, F.K. Ronellenfitsch (LIST) for GIS maps, M. Sulis (LIST) for critical proof-reading, L. Auguin (LIST) for language editing, B. Augenstein and R. Krause (Geosens Ingenieurpartnerschaft, Schallstadt, Germany) for running the phenological models on the VitiMeteo platform, J. Niewind (LIST) and P. Sinigoj (former CRP – Gabriel Lippmann) for their support in data management, the Institut Viti-Vinicole for financial support in the framework of the research project “TerroirFuture – Impact of climate change on viticulture in Luxemburg: risk-assessment and potential adaptation strategies” as well as the European Union in the framework of the “Clim4Vitis” research project (Horizon 2020 research and innovation programme; grant agreement No. 810176).

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