Importance of Long-term Field Studies in Adaptation of Viticulture to Climate Change

Climate Observation Studies in Viticulture

There are various grapevine and climate change studies that are based on long-term climate observations related to viticultural data (Table 1) (Jones et al. 2005, Webb et al. 2007, Duchêne et al. 2010, Keller 2010, Hannah et al. 2013, Lereboullet et al. 2014, Biasi et al. 2019, Koufos et al. 2020, Santos et al. 2020). Temporal or spatial observations at a local, regional, or even global scale can provide important insights into the relations between vineyard ecosystem processes or pedoclimatic conditions. Such studies deal with the climatic impact on viticulture in general and the possible consequences of climate change, as well as provide potential adaptation options and mitigation strategies fundamental for the wine industry. Moreover, these long-term data sets are used to assess the impact of climate on the sustainability of viticulture. Many climatic factors influence viticulture (e.g., soil, geology, cultural aspects), but most studies focus on temperature, water availability, and solar radiation.

Most changes reported over recent decades are related to phenology (e.g., shifts in budbreak, flowering, veraison, and harvest dates), due to increasing mean temperatures during the growing season (Jones and Davis 2000, Duchêne and Schneider 2005, Schultz and Jones 2010, Tomasi et al. 2011). As for each variety, the length of the growing season is directly related to the growing season mean temperatures (Jones 2006). Consequently, a shift in the ripening period enables cultivation of other varieties with a different climatological background and in the long term, leads to a change of traditional varieties within single wine regions.

The phenological diversity provided by V. vinifera can be a useful mechanism to adapt grapevines to changing climates (Wolkovich et al. 2017). This was exemplified in 2019 with the addition of seven new disease- and heat-resistant varieties to the Bordeaux region (Bayar 2020), indicating that cultivar diversity may decrease the loss of agricultural areas (Morales-Castilla et al. 2020). In addition, the increase in potential evaporation demand as well as developments in frequency and intensity of precipitation lead to a higher risk of water stress, which inhibits plant growth. Furthermore, longer lasting drought results in lower vine performance and poor fruit development (Hardie and Considine 1976). Vine water deficit has been reported to affect berry size (Ojeda et al. 2001) and thus the composition of grapes and wine—due, for example, to an altered metabolism of phenols (Deloire et al. 2004). Wines have been described to be modified in sensory attributes such as fruitiness and vegetal aromas; e.g., Cabernet Sauvignon was characterized as fruitier and less vegetal under water deficit (Chapman et al. 2005). However, excessive humidity and precipitation during vegetative growth promote the development of dense canopies and compact bunches, limiting equipment accessibility to apply fungicides and thereby increasing the risk of fungal plant diseases. Under limited selection of plant protection products, e.g., due to regulatory restrictions, increased risk of fungal diseases has been considered a major challenge, especially with regard to the increasing share of organic viticulture worldwide, which reached 510,000 ha (7.5% of the global grape area) in 2021 (Willer et al. 2023). In any case, dense canopies, compact bunches, and fungal diseases can trigger undesirable fruit quality, leading to poor starting conditions for wine production with an increasing demand for enological improvements. Finally, through changes in radiation and sun exposure of the canopy and the bunch zone in particular, grapevine physiology becomes restricted and the risk of sunburn is reinforced. Described as an effect mechanism of high light intensities, temperature, and UV radiation (Rustioni et al. 2014), sunburn now occurs in all grapegrowing regions of the world and can result in decreased grape quality and yield (Gambetta et al. 2021). It has also been reported that sunburn has a negative effect on the perception of white wines (Rustioni et al. 2023). To avoid such risks, adaptation strategies such as modified canopy management practices or better adapted trellis systems aim to protect clusters by shading (Oliveira et al. 2014), improved water management of vineyards (Mirás-Avalos and Araujo 2021), and delayed fruit ripening (Previtali et al. 2022).

The collection of long-term sets of climate data in addition to longer experiments, and putting them into context with viticultural events, is of great importance in drawing attention to future shifts in the climatic conditions of grapegrowing regions. There is serious interest in better informing and eventually preparing the grapegrowing industry for future scenarios that have not yet occurred or been experienced in certain wine regions. However, such studies also entail drawbacks that are primarily structure-related and unavoidable, such as vineyard renewal due to uprooting of vines or relocation of weather stations within vineyard sites. Additionally, the climate-driven constraint of changing grapevine varieties (including rootstocks) is one example of inevitable adaptation strategies that will need to be addressed in viticultural research through long-term field trials. Ultimately, while observational studies are useful for identifying trends and long-term developments, they may be of limited use in determining specific plant mechanisms or functions of resilience.

Climate Experimental Studies in Viticulture

The study of predicted climate scenarios under field conditions is more complex than climate observation studies or time series studies. The necessary installations needed in the field to simulate, e.g., higher temperature and drought stress (Sadras et al. 2012, Treeby et al. 2018), elevated CO₂ (eCO₂) (Bindi et al. 2001, Treeby et al. 2018, Wohlfahrt et al. 2018), or radiation (Del-Castillo-Alonso et al. 2020), are costly and labor intensive but may assess direct effects of climate change on various timescales over several years.

When comparing existing field studies in viticulture regarding the impact of climate change and in particular the effects of eCO₂ (Table 2), it is evident that there is a lack of long-term studies. CO₂ enrichment experiments within field trials have so far lasted for a maximum of four seasons at two vineyard sites in the northern hemisphere (one site located in Italy [Bindi et al. 2001] and one located in Portugal [Moutinho-Pereira et al. 2009]) and at one site in the southern hemisphere in Australia (Edwards et al. 2016)—each study used red grape cultivars; studies on white cultivars were not available. The most recent study, VineyardFACE (Free Air Carbon dioxide Enrichment), started in 2014 in Germany (Wohlfahrt et al. 2018) using two cultivars (Riesling and Cabernet Sauvignon), and for the first time considers a white cultivar under an eCO₂ regime. VineyardFACE was planned as a long-term study for a period of at least 15 yr, for which the experimental vineyard was specially planted.

Due to the 2-yr reproductive cycle of grapevines, most studies resulted in little to no yield increase under eCO₂ in the first year. As fruit initiation happens in the year prior to the year of harvest, the treatment effect in terms of yield usually occurred from the second season of investigation onward, when initiation of fruit took place under eCO₂ concentration (Bindi et al. 2001, Kilmister et al. 2016, Wohlfahrt et al. 2018). Furthermore, eCO₂ was shown to modify bud fruitfulness due to an enhanced cross-sectional area of inflorescence primordia, which consequently influenced the potential crop load level at a very early stage of grapevine development (Wohlfahrt et al. 2019). This finding can be used as a key requirement to extending the duration of grapevine studies beyond the usual 3-yr investigation period in general, and more specifically, to categorize the first season’s results as an initial phase of a long-term field experiment.

Vine age varied widely among all studies (Table 2), as did vine spacing and training systems, each serving as examples of climatic and anthropogenic factors that highly influence vine performance. As an additional feature, VineyardFACE offers the possibility to follow grapevine response to an increased CO₂ concentration from 2 yr after planting, and thus from a very early developmental stage, which has not yet been considered in previous CO₂ enrichment trials. Contrary to existing eCO₂ studies on mature grapevines with lower or no water demand under eCO₂ and no signs of photosynthetic acclimation (Bindi et al. 2005, Tognetti et al. 2005, Moutinho-Pereira et al. 2009, Edwards et al. 2017, Treeby et al. 2018), young vines of VineyardFACE showed an increase in transpiration and stomatal conductance under eCO₂ in the first years of the experiment (Wohlfahrt et al. 2018) (Table 3), and thus had a higher water demand compared to the control vines. Ultimately, it must be clarified whether acclimation takes place after young vines have adapted to the field, becoming mature vines by establishing their vine root system, which happens during the 5 to 7 yr following planting (Champagnol 1984). Despite this, young vines grown under eCO₂ showed a higher water use efficiency similar to mature vines under higher CO₂ concentration (Table 3). This could be a result of the higher amount of carbon assimilated as leaf and fruit biomass (and presumably root biomass) produced per unit of water used by the vine (Wohlfahrt et al. 2018). Under rainfed conditions without supplemental irrigation, these results can be useful when considering planting concepts for new vineyards, especially when it comes to warm and dry periods such as the Rheingau Valley experienced in recent years, e.g., 2018, 2019, 2020, and 2022. Previous studies on vine age and its effect on plant performance have shown that young vines tend to be more sensitive to water deficit compared to old vines (Grigg et al. 2018, Bou Nader et al. 2019, Riffle et al. 2021), but results are also difficult to evaluate, as drivers of age-based differences are currently neither fully understood nor completely scientifically proven, due to a lack of long-term studies in this area comparable to the number of existing CO₂ studies.

Climatic Drivers and Terroir Context

Beyond individual studies, broader climatic dynamics and terroir interactions define how robust viticultural systems are predicted to change. Furthermore, cultivar, soil, and climate have been proposed as main factors of the terroir concept (van Leeuwen et al. 2004), also highly variable when comparing the four different CO₂ study sites (Table 2). Thus, long-term studies under various climatic conditions would help clarify season-to-season plant performance and the sustained processes through which grapevines pass within a changing climate. Choice of cultivar is important, particularly between white or red grapevine cultivars, regarding their yield-to-pruning weight ratio or a fruit maturity that depends highly on span of ripening. Additionally, existing cultivars must face seasonal extremes with increasing disease pressure and new plant pathogens, or the effects of heat and drought stress which also must be monitored to help further cultivar decision-making. Thus, a careful selection of cultivars is currently necessary, with the possibility of fitting not only in the present but also in future climate scenarios of a specific region. Vineyard biophysical characteristics such as soil or topography play an important role in terms of the definition of grape composition, and hence the final wine character (Bramley et al. 2011) (e.g., the type of soil in relation to the acidity profile of wines), and can help form individual wine styles. Here it is also useful to investigate long-term soil humus build-up and promotion of soil fertility in the long term, as these factors contribute to the final wine profile.

Finally, climate is the most unpredictable factor within the vine terroir concept, showing high annual variability in temperature and/or heavy rainfall events due to the intensified global hydrological cycle caused by global warming (Schimel 2018). Whereas the latter entails an increased risk of flooding and soil erosion, drought events are accompanied with increasing soil water repellency (hydrophobicity) and microbial activity losses, which in total lead to crop instability within viticultural areas. According to Cook and Wolkovich (2016), climate change has fundamentally modified large-scale drivers of harvest timing and wine quality—drought and moisture availability—across Switzerland and France. These authors proposed that the relation between drought and temperature has extenuated in recent decades, meaning that increased warming from greenhouse gases creates the high temperatures needed for an early harvest without a drought occurring. As one example of a greenhouse gas, CO₂ input to the atmosphere is higher than natural sinks can remove, thus the total amount of atmospheric CO₂ (aCO₂) increases every year. Between 2021 and 2022, the aCO₂ increase was 2.13 ppm, the eleventh year in a row in which the amount of CO₂ in the atmosphere increased by more than 2 ppm (as reported by https://www.noaa.gov/news-release/greenhouse-gases-continued-to-increase-rapidly-in-2022). Researching the effects of these future challenges—drought, elevated aCO₂, or heavy rainfall—on viticultural areas worldwide and in the long-term plays a significant role in understanding and classifying the functional dynamics behind them. This additional knowledge gained from long-term research enables multidecadal perspectives and will help elaborate and pass on instructions to grapegrowers about mitigating the effects of climate change.

Implications for Long-Term Study Design

The meaning of “long term” can be used variously when establishing a study design and could be proposed as follows. When phenology or climate observations are conducted, a period ≥30 yr is recommended to match WMO climate periods. Vine physiology, vine age, or yield trials are suggested to take ≥10 to 20 yr, covering the establishment, full production, and grapevine decline phases. eCO₂, warming, or water manipulation experiments should last at minimum ≥5 to 10+ yr to cover the 2-yr reproductive cycle of the grapevine and a possible acclimation phase. Additionally, minimum core measurements are recommended across sites regarding the climate (temperature, precipitation, radiation, humidity, reference evapotranspiration), the vine (phenology, pruning weight, yield, berry composition, disease incidence), and the vineyard site (soil characteristics, water status indicators, management practices, cultivar and rootstock information). Eventually, the value of meta-analysis and modeling would be greatly increased by the implementation of harmonized protocols.

CRediT Authorship Contributions

YW: Conceptualization, Data Curation, Writing – Review & Editing; YW and MS: Investigation, Writing – Original Draft; MS: Funding Acquisition

Data Availability

The data underlying this study are available on request from the corresponding author.