An anticipated management solution to phenological shifts is planting later ripening and stress tolerant alternative varieties. Government response to climate change will determine the actions European growers are allowed to take to adapt to climate change, considering the current trials of alternative varieties planted in small diversity blocks in France as a positive example . Ancient varieties being tested in temperature gradient greenhouses in Spain for response to combination stresses of drought, heat, and elevated CO2 showed greater resiliency to stress and did not shift phenological timing, although this was a short-term experiment . In some cases, alternative varieties may be hybrid crosses between existing cultivars and later ripening varieties. However, hypothetical crosses between very late ripening varieties were modelled and still struggle to be late-ripening enough to endure the predicted 23-day shift and potential increase of 7°C expected by the end of this century for major wine grape growing areas . Alternative varieties can be identified by oenological and ecological principals that make them suitable candidates for replacing existing cultivars, such as flavor profile and ability to survive long term through stressful climate change conditions . The challenge of adapting new varieties is highlighted by current popular varieties struggling with increases in growing season temperatures , drainage collection pot however a combination of diversity block trials and greenhouse experiments will guide predictions of the best alternatives .
Our present knowledge of grapevine climate niches is limited relative to the vast diversity of cultivars . With California as an example, there are many potential late ripening varieties suitable as alternatives to early ripening Chardonnay that have yet to be tested in diversity blocks . Even clones can have a varied response to climate change variables . Varieties with heat and drought tolerance traits are a starting point for elevated CO2 studies, as we expand from understanding the mechanisms of change into exploring mitigation strategies. Exploring the vast diversity of grapevine using diversity plots is a straightforward ecological approach, which could be enhanced by evaluating the success of plants under several biotic and abiotic stresses predicted for the future. Many studies on the impacts of leaf removal suggest that manipulating canopy cover is an effective way to mitigate phenological shifts caused by climate change . Leaf removal at pre-bloom positively influences cell division in inflorescence, by reducing sugar transport and decreasing flower fertility, which mitigates cluster compactness . Not only can leaf removal aid in delaying phenology, but other positive impacts also include increasing acid to sugar ratio at harvest, increasing production of anthocyanins and flavonoids, and decreasing incidence of bunch rot disease . Ecologists generally study a system’s responses and interactions, and viticulturists need this system perspective for the challenges presented by climate change. Our understanding of the effects of elevated CO2 on the vineyard system is profoundly complicated by the interactive effects of other biotic and abiotic stressors. From an ecological perspective, long-term FACE studies are the most realistic predictors of response to elevated CO2.
Advocating for long-term agroecological studies is necessary to evaluate the top-down and bottom-up impacts of higher carbon availability on pest/disease interactions, grapevine growth and phenology dynamics, and the resulting quality of wine produced. Grapevine physiology will be impacted by elevated carbon dioxide, increasing temperatures, and extreme heat events during the growing season . FACE experiments highlight the necessity of water availability for grapevines to take advantage of increased carbon dioxide for productivity. Soil water availability impacts the opening of stomata, and in the case of Vineyard FACE, the vines had increased gs with more CO2 available . Grapevines may need more water under future climate conditions of elevated CO2 and temperature, while precipitation is expected to decrease in most of the wine growing regions of the world. Desiccation threatens vines through water loss from latent cooling under elevated temperature, resulting in higher cumulative water loss even when operating at higher water use efficiency. The modulating response of stomata documented across literature is dependent on the soil water availability and temperature regimes . In this synthesis, the varying levels of CO2, ambient temperatures, and duration of these experiments could have contributed to these contrasting results of stomatal behavior, as well as the conditions of the chambers and greenhouses, versus FACE infrastructure. Physiological response to abiotic stresses in future climate change conditions is likely to weaken grapevine, creating a vulnerability for biotic stresses such as pests. Overall, chewing pest pressure is anticipated to increase as carbon dioxide and temperature increase . It is unknown whether pest pressure can be compensated by the predicted increase in foliar growth and the effect of lower nutrient density on the populations of pests.
The growing season for grapes may require drastic changes in viticultural practices to manage pests, alleviate heat and drought stress, and predict harvest dates. Fungal infections are responsible for a majority of crop damage; therefore, it is critical to clarify if fungal infection will decrease in the future for predictions of grapevine yield. One of the biggest challenges for grape growers will be the shifts in phenological timing, with the potential for frost at early bud break, alterations in cluster formation and density, and compromising harvest with early maturation. Many of the short-term experiments described here did not find significant effects on phenology and yield, while long term studies account for acclimation and compounding effects of seasonal exposure to elevated carbon dioxide. Predictions of overall vineyard response to climate change are more accurate when experiments are field based, multi-seasonal, and combine the variables of water availability and temperature. Climate change is increasing the growing season temperatures in many of the world’s most important winegrape growing regions. According to the most recent IPCC Assessment Report, Climate Change 2021, global warming is expected to exceed 1.5°C – 2°C during this century . Warming caused by anthropogenic greenhouse gas emissions advances phenology in hundreds of plant species, with increased consequences for perennial crops . Climate warming has already altered the phenology of many plant species globally, including the phenology of valuable crop plants such as grapevine . Winegrapes, a globally important crop both economically and culturally, have become an important indicator of climate change, with well documented advancing phenology, shorter periods between phenological stages , and large inter-annual variability . Adapting to climate change has become a global priority, and the wine industry is likewise looking for more accurate predictive measures of phenology and strategies for future planting. Culturally and economically, grapevine is one of the most valuable crops in the world, evidenced by an annual production of 60 million tons of fruit , with varieties that have been cultivated for thousands of years, selected for color, flavor, and phenological timing . Grape growth and qualities are sensitive to growing season climate fluctuations, and there is a direct link between warming temperatures and early harvest dates . Earlier ripening forces farmers to harvest grapes at optimal sugar levels during warmer periods of the summer. Harvest should ideally occur later during a cooler period of the growing season after the berry has accumulated an appropriate balance of acids of sugars. Early harvesting decreases the quality of wine, round plastic pot evidenced by early ripening significantly altering berry chemical composition . Higher year-round temperatures impact varieties with chilling requirements, such as California’s premiere wine grape, Chardonnay . Globally, there have been shifts of 1-2 weeks for winegrape growing regions . In Europe, the growing season has lengthened by about 11 days over the last 30 years, which will impact grape berry and wine quality . Early bud burst threatens frost damage during volatile Spring temperatures . At present, the winegrape crop in Bordeaux has a month earlier harvest than it did 50 years ago . Models of warming indicate that increases in temperature are not uniform globally and that warming has increased in the major wine growing areas of California and Western Europe more than South America and Australia during the past 50 years . The phenological shifts resulting from growing season temperature increases are documented internationally, and models predicting phenology using temperature are becoming more precise . A multitude of studies both observational and experimental have identified an acceleration of phenology and decrease in periods between stages in response to warming growing seasons , but some show trends of the intervals between each stage widening . Previous grapevine modeling which quantified relative sensitivity of many varieties combined records of phenology across variable microclimates and conditions . Comparing phenological timing from different vineyards done does not capture the influence of the microclimate and microhabitat; elevation, management, soil type, and a multitude of other environmental factors can impact flowering time . The ampelography vineyard at University of California Davis allows for attributing the variation in phenology to the specific sensitivity of cultivars to changes in climate, rather than soil type, irrigation method, pruning, or other major sources of variability found when comparing multiple vineyards.
Temperature is the main driver of phenological development for grapes; heat accumulation impacts the biochemistry important for cell growth . A study of 15 cultivars in Australia documented a plateau in growth between 22-29°C . For many plant species, higher temperatures can stagnate growth, and we expect that some varieties of grapevine would be sensitive to temperatures greater than 40°C . In extreme cases, beyond inducing premature veraison, heat stress will cause loss of berries, inactivate enzymes, and reduce development of flavors critical for wine quality . We integrate into our models a measure of extreme heat to determine its effect on veraison, the stage most likely impacted by these events. In this study, we examined variability in the phenological responses of 137 varieties of Vitis vinifera over a 5-year period. We examined variability in the timing, in terms of growing degree days, of the three major phenological stages: budburst, flowering, and veraison. Our data provide an updated reference to the last major study of variety-level phenological responses in California, which examined 114 varieties nearly 40 years ago . We also compare traditional Vitis vinifera species with hybrids grown at the University of California Davis, originally cultivated by Harold Olmo. Overall, this study offers a comprehensive look at international varieties planted in California their relative phenological response to climate. This study aims to evaluate a wide range of cultivars to identify regions with lower sensitivity to climate change that may be used in adaptation, either through breeding or planting as alternatives. The UC Davis ampelography learning vineyard has been developed over the past decade to include approximately 300 international varieties planted adjacent to the Viticulture and Enology academic building. The vines are planted in groupings by geographic origin, for the purpose of teaching. The vines are trellised using vertical shoot position , with regular irrigation, and are treated throughout the growing season with sulfur sprays for pests and disease. The current study of phenology has been tracking over 130 varieties for over four years and measures the response of the varieties through three main phenological stages: budburst, flowering, and veraison. The phenological data has been collected from UC Davis starting in 2014, continued through 2019. For each of 137 varieties, we recorded the timing of three major phenological stages: Budburst, Flowering, and Veraison. The same individuals were monitored for 5 years. For each vine, three positions on the cordon were chosen at the start of each season before budburst, following the previous year’s recorded positions unless damage had occurred, in which case a nearby cordon was chosen . The primary buds from each two-bud spur were chosen at the most basal position. The three buds were tracked through each phase, treated as technical replicates averaged for an overall estimate for each individual vine. Each vine is a biological replicate, and two vines per cultivar were measured. The timing of budburst was recorded as stages 1-13 , based on the modified Eichhorn–Lorenz stage of the three positions monitored for each vine . The EL scale describes the phenological stages of grapevine and categorizes the stages as follows: budbreak, shoot development, flowering, fruit set, berries pea-sized, veraison, and harvest . Flowering was monitored from these same shoot positions, and once clusters started to develop, they were marked with flagging tape.