This discussion is a follow up with our last blog on the 2017 maturation
What is the situation end of september?
A general disorder in sugar accumulation is observed in northern California. The trend is widespread and shows sluggish sugar accumulation rate across many different vineyards. The extend of the phenomenon indicates that slow fruit ripening profile is a seasonal tendency of 2017. This observation has been echoed by many winemakers, perplexed not only by the sluggish sugar accumulation rate but also by the slow evolution of other fruit compounds such as aromas and tannins.
In our last blog (here) we shared a general observation of low berry volume by the last week of August 2017. We formulated 2 hypothesis: either berry volume was lower than usual, possibly because of a reduction in seed number per berry or berry volume was lower than usual because of a delay in berry mass accumulation. 4 weeks later, the slowly moving amount of sugar per berry observed across many contrasted locations in Napa, Sonoma, Livermore and beyond confirms that a significant delay in sugar loading is observed and this trait can be considered a specific trait characterizing the 2017 vintage in Northern California. Of course sugar concentration may increase rapidly, but it is mainly due to berry losing water, not because berry is loading sugar.
Nitrogen or Water can not be culprits
To accumulate sugar plant photosynthesis activity and sap flow rates must be sufficient to convert CO2 in sucrose and transport sucrose from the leaves to the fruit. Can any of those 2 processes explain why we observe such a generalized slow ripening in 2017?
Photosynthesis activity can be lowered in years when nitrogen uptake is low. However, our statistical analysis of weekly nitrogen accumulation during the period of nitrogen uptake did not reflect any large scale reduction in Nitrogen level. In this context, it is difficult to characterize 2017 as a low nitrogen year, unlike 2014 for instance. If photosynthesis activity is low, nitrogen deficit can not be used as a common culprit since it does not match the 2017 “seasonal trend” …the culprit is elsewhere.
Sap flow and vine water use have been relatively high during the season. In some situations, the development of large transpiring leaf area sometimes lead to the apparition of excess water deficit. However, analysis of vine water use profiles with our sap flow sensors does not show a greater severity in water deficit in 2017. In fact many vineyards have been successfully dry farmed. If sucrose transport from the leaves to the fruit is low, severe water deficit can not be used as a culprit neither.
High temperature most likely the culprit
In fact, we may simply observe a plant physiological response to global warming affecting particularly fruit temperature. A very recent article, published by Lecourieux et al, in 2017 investigated in depth the effects of pre-veraison warming on berry ripening dynamics. The researcher team imposed a 2 weeks heat period pre-veraison (8 degree Celsius above the non heated treatment) and tracked the effects of warmer temperatures on berry sugar and color accumulation.
Figure 1 illustrates the consequences of heat exposure early season: a 2 weeks delay in sugar accumulation and color accumulation.
Figure 1 : Heat exposure early season induces a 2 weeks delay in berry ripening (adapted from Lecourieux et al, 2017).
What is there to learn about the 2017 high temperature effect?
Reducing berry temperature all along the season (and not only the last 2 weeks before harvest) is the challenge that must drive new vineyard planting decisions as well as fruit zone management decision in the context of growing conditions getting warmer in Northern California. There are a few practical consequences to that:
- It may not be a good idea to keep developing vineyards in a “Bordeaux style”. As the fruit zone gets close to the ground (<50 cm), berry temperatures gets exponentially higher. During the day, ground soil temperature frequently increases above 60 Celsius (140 Fahrenheit) and radiative heat add to the effect of warmer air temperature,
- Enhance trellis design and canopy manipulation strategy to favor air movement within the canopy,
- Soil management limiting ground radiative impact (modifying the albedo with a dry “whitish” cover crop),
- Other vineyard operations mitigating high temperature in the cluster zone, such as water “pulse misting” every time air temperature reaches a threshold can effectively reduce berry temperature by more than 10 Celsius.