Continuing with the 2017 Napa Vintage Report presentation summaries, today’s article is focused on winegrape phenology with Elizabeth M. Wolkovich, and ripening dynamics with Nicolas Bernard.
Modeling the future of winegrape phenology
As the second scientific speaker following the keynote address at the Napa Vintage Report, Elizabeth M. Wolkovich, Assistant Professor, Department of Organismic and Evolutionary Biology, Harvard University, presented findings from her ongoing research on the modeling of winegrape phenology from dormancy to budbreak, flowering, veraison, and maturity.
Modeling budbreak is difficult, since the date relies upon dormancy. Vines only exit the dormancy stage when they’ve accumulated enough “chilling hours”, or amount of time spent within a cool winter stage. However, vines do not accumulate any chilling hours below freezing, and temperatures that are “too warm” observed during the winter may offset some already-accumulated chilling hours. Once vines accumulate enough chilling hours they enter “ecodormancy”, where the accumulation of heat drives the plant transition out of dormancy and into budbreak.
For modeling budbreak and each stage individually thereafter, Dr. Wolkovich and her team have discovered that each stage has a specific “rate of development” curve, with a peak optimal temperature for development. This means that the same temperatures will have different effects on the rate of development and rate of transition between phenological stages for different varietals. Additionally, above the optimal temperature for a varietal, the rate of development drops significantly with higher temperatures. This likely played a role in 2017 in Napa, with many extreme heat events during early summer and the months leading up to harvest. So far the models that Dr. Wolkovich has developed work well for modeling Cabernet Sauvignon and Merlot in the Napa Valley. In an era of warming climate, we may continue to see delayed development due to high temperatures both during dormancy, and during the growing season.
2017 ripening dynamics in Napa: Berry active sugar loading, berry volume evolution, harvest position
Moving further into the growing season, the Vintage Report’s analysis of the 2017 fruit maturity evolution in Napa began after lunch with a presentation by Nicolas Bernard, the Research and Development Manager for Vivelys. He presented the Vivelys grape maturation measurement tools, the sugar loading period, and results from Napa and around California.
The active sugar loading period is the period from veraison to physiological ripeness, when sugar is no longer actively added to the berry. One of the most important factors to check at this physiological ripeness, or sugar loading stop, is the brix level. The brix level at sugar loading stop is an indicator of the quality of the ripening. The sugar loading period depends both on photosynthetic activity (climate, vine water status, and nutrition) and vine equilibrium (yield and leaf area).
In Napa in 2017, the average sugar loading stop in Cabernet Sauvignon occurred on the 22nd of August at 20.3 brix, with an average of 160 mg of sugar per berry. This observed 20.3 brix at sugar loading stop was lower than what was observed in 2016. Both the duration of the sugar loading period and final sugar amount per berry were both lower than the previous two vintage, 2015 and 2016. The shorter period of sugar loading was likely caused by extreme heat wave events in August and early September, right during the key period of active sugar loading for Napa Cabernet Sauvignon. This hypothesis aligns with a sharp dip/stall in the berry volume evolution at the same end of August/beginning of September timeframe. During those heat waves a 10% loss of berry volume was observed. Finally, the 2017 harvest window was closer to the end of the active sugar loading period than both of the previous vintages. In other words, there was less time between physiological maturity and harvest than in 2015 and 2016.