How can a winegrower harness vineyard spatial variability to make better wines?
Numerous studies carried out since 2000, on irrigated and non-irrigated vineyards have shown that vineyards exhibit great spatial variability for many factors such as yield or berry quality components (Santesteban et al, 2013). This spatial variability can be particularly pronounced in non-irrigated vineyards when seasonal water deficit is very important and can not be modulated via irrigation (Ojeda et al., 2005).
Aerial view from Vineyards in Andalucia, Spain (source)
A temporal stability exists in vineyard spatial pattern
The NDVI mapping can reveal vine vigor heterogeneities between blocks, within block or even between plants when the picture resolution is sufficient (here).
The time stability for the shape and size of uniform areas within the vineyard is a consequence of 1) the perennial nature of the vine and 2) the stability of soil characteristics. Studies analyzing the stability of vineyard spatial heterogeneity over multiple seasons have been published for some quantitative parameters such as vigor and yield (Tisseyre et al., 2008).
In 2011, Kazmierski et al., conducted a study to explore whether vineyard spatial pattern, using NDVI as an index, was stable across multiple vintages. Interestingly, the study demonstrated a relative stability of vineyard spatial structures from one vintage to the next. However, over several vintages, – due to green operations and changes in cultivation practices- the shape and size of uniform structures within the vineyard change gradually. Thus, the footprint from man-made practices can effectively modify vineyard spatial structure and which directly affects vineyard performances. It simply is going to take several seasons before those spatial changes become long term changes.
To keep track of gradual changes in vineyard spatial structure over time and to monitor their effect on the leaf area development, NDVI is a useful tool as reported by various authors (Bramley, 2005, Tisseyre et al., 2008). Early season (ie. before veraison), NDVI can discriminate blocks which keep a “stable” spatial pattern from one vintage to another from blocks where the spatial structure varies strongly from one season to the next. For instance, vineyards whose spatial structure is less stable may simply be reflective of a strong seasonal contrast in the amount of rain received during the early stage of vine growth… and future climatic trends precisely predict more extreme variations in the weather.
Spatial heterogeneity of a vineyard plot in the Gironde (map NDVI Avion Jaune / Fruition Sciences)
#1: NDVI and fruit sampling before harvest
The vineyard spatial heterogeneity is essential to take into account when sampling the fruit from any vineyard. Some recent studies have suggested that a relationship exists between berry weight and NDVI (Carrillo et al, 2016) which justifies the implementation of a sampling strategy based on vineyard spatial structure as revealed from the NDVI map obtained at veraison, particularly if you are interested in tracking berry volume variations.
The goal of vineyard stratification before sampling is to carve out uniform areas within the vineyard. This preliminary step will remove the effect of block spatial variability on fruit chemical parameters analyzed after berry sampling. By performing fruit sampling within the most homogeneous zones, you eliminate analytical biases caused by spatial variability. For example, you can delineate areas where uniform fruit composition dynamics have occurred before harvesting for sugars and berry volume. This knowledge, obtained prior to harvest, will guide the winemaker into the vineyard areas where it is most informative to perform berry tasting prior to picking. From there, the winemaker increases his ability to validate harvest date decision or fruit allocation to different winemaking programs. Another benefit attached to the implementation of stratified sampling technique is to generate data which directly reveal the link between climate, soil and plant. In that regard, because it reveals local effects on fruit maturity, the stratified sampling technique will contribute to express the unique potential of each terroir.
#2: NDVI and harvest date
If you are interested in zoning your vineyard to delineate uniform fruit composition area, it is more complex. In 2012, Fiorillo et al. studied the relationship between uniform NDVI areas within a non irrigated vineyard and the uniformity of fruit composition coming from those same areas. For 4 consecutive vintages, authors investigated the relationship between NDVI and the values of grape parameters obtained at two different harvest dates. The first and early harvest date was triggered at technological maturity (ie. around the time when sugar loading ends); The second and late harvest date was triggered two weeks later. Interestingly, correlations with NDVI were higher with the earlier harvest date and decreased with later harvest date, particularly for anthocyanins and polyphenols. In practical terms, Fiorillo’s results suggest that you can rely on a NDVI to delineate uniform fruit composition areas if you are going to harvest early. However, as harvest decision is more and more delayed after sugar loading ends, the correlation between fruit chemical composition and NDVI areas is degraded. From a winemaker perspective, it means that the informative value of a NDVI map to delineate harvesting area becomes less and less relevant as picking date is more and more delayed. (for more info on sugar loading read our previous blog here)