As Bordeaux region is entering into summer, overall flowering stages went smoothly and the leaf area operations are undergoing under good weather conditions.
At this stage of the season, we can take a step back and review how such an atypical spring has called into question usual cultivation practices.
With level of rain being high in March, low in April and above average in May, 2017 remains a dry year. The importance of rainfall in May indicates that water will not be a limiting factor for vine growth. Moreover, the first measurements of water deficit using pressure chamber before sunrise (predawn leaf potential) up until last week further confirmed the lack of early water deficit. If we take into account the stormy rains observed at the end of June, the vineyard has enough water supply to ensure proper veraison and on certain terroir, proper maturation processes.
Figure 1 : Average monthly rain in Bordeaux
Frost and heat wave during spring time
Overall in France, meteorological observations reveal a strong imbalance between the first 15 days of April, which displayed higher temperatures (+2 to + 2.5 ° C) than seasonal average and the last 15 days (-1.5 ° C compared with seasonal averages). An exceptional frost event hit the region late April (reminiscent of 1991). Then over May 25-29, an unusual heat wave was observed and such extreme heat had not been recorded since 1900. In June, the trend towards higher temperatures remained with extreme above 35°C. This is the first time we have experienced heat waves so early.
According to our experience and the scientific literature we call “heat wave” a climatic event that generates a water vapor deficit (VPD) greater than 4kPa. As a reminder, the VPD corresponds to the pressure to be exerted on a volume of gaseous air to make appear the first drop of liquid water. The hotter and dryer the air, the higher the VPD. When the water vapor deficit exceeds 4 kPa, it causes vine water loss to the atmosphere even under stomatal closure.
Figure 2 : Vapor Pressure Deficit in Bordeaux (Fruition sciences Bordeaux)
Heat wave, soil moisture and plant response
When soil is still rich in water, this a heat wave event is not threatening for plant water use. The vine will simply use more water to compensate. On the other hand, under limiting soil moisture conditions, leaf burning symptoms; green berries or maturity disruption may be observed.
This being said under more arid conditions, like in California, when air temperature reaches highs above 37C, we can also observe leaf burning symptoms even under well watered conditions. At this point it is really important to distinguish that the cooling effect provided by enhanced vine transpiration is not sufficient to lower leaf temperature enough. Heat stress caused leaf burn symptoms and irrigation can not help. When lack of soil moisture is not an issue, the best practical answer is to fight the atmospheric stress caused by the high temperature. In practical terms lowering air temperature is the only cure and irrigation will not help (for instance by misting water or limiting ground heating effect)
Faster Degrees days accumulation leading to earlier bloom
Overall, spring mean averages are higher than averages. This translates into a fast degrees days (DD) accumulation ( from March 1st ) considerably higher than that of 2016.
By the end of May 2017, the current season is 125 DD ahead of last year, which corresponds to a plant development stage 15 days ahead compared to average (source Fruition Sciences).
In response to the more rapid temperature accumulation, an early flowering stage was observed and full bloom was reported everywhere early June. As a reminder, thermal time, expressed in degrees day, sums temperatures above a certain threshold (generally 10 ° C for a vine) over the course of one full day. It is a useful bioclimatic index because it is correlated with vine phenology.
Slower shoot growth rate
Despite the high rain supply and the fast accumulation of degrees days, shoot growth rate is slower than last year (- 0.5 cm / TT / m²) even on site with little or no frost exposure.
We speculate that such low speeds are linked to mineral deficiencies (in particular nitrogen deficiency) induced by the winter conditions. Indeed the low winter temperatures combined with the lack water have led to a slowing down of organic matter mineralization processes by the bacteria. On the other hand, the hot weather condition after the rainy May season resulted in a revival of soil mineralization. However as we discussed in our previous blogs on Nitrogen uptake, the intensity of Nitrogen uptake is slowing down at this point of the season so that this ‘late May boost” in soil mineralization may not always benefit the vine Nitrogen uptake
Picture 1 : Cab cluster during nouaison (week of June 15th 2015)
Regarding potential returns in 2017, it is important to remember that bud fertility and yield potential is built over two consecutive years.
Indeed it was around flowering in 2016 that bud floral initiation (affecting bud fertility in 2017) took place. Furthermore, inflorescence differentiation processes (ie. the stage which determines the number of clusters per productive branch) is sensitive to water and nitrogen deficits (Guilpart, 2014). Since no water or nitrogen deficits were reported, cluster development in 2017 is generally generous. On top of this, researchers have reported that a correlation exists between shoot growth rate during the year and shoot yield that same year. This is explained by a very active cell activity in the meristems resulting in elongated rachis with more berries (Keller et al., 2015). Consequence of the lower shoot growth rate, we can expect lower yields in 2017 on unfrozen site, particularly lower cluster weights than in 2016. In addition, symptoms of millerandage, visible this year, confirm this trend leading to the production of less compacted clusters.
Following the frost wave, this season start is really intense and challenging for winegrowers since we can observe the juxtaposition of two vegetative cycles next to each between frozen vineyards (under delayed development) and unfrozen vineyards (under rapid development). Fortunately the contrast between those 2 stages tends to diminish thanks to the intense heat of June and a wet soil in the deep horizons which accelerate the regrowth of frozen vines.