The following post recently appeared on the American Association of Wine Economists' Facebook page. The question posed in the post is an interesting one.
These same data were published by the AAWE back in June 2017, at which time I worked out what the yields are, in fact, related to, since it apparently isn't quality. It turns out to be latitude, with increasing yields as one moves away from the equator, as shown in my first graph. Each point represents one of the French regions, with its latitude plotted horizontally and wine yield vertically.
What the line on the graph indicates is that three-quarters of the variation in yield is exponentially related to latitude. The areas named are the main governmental regions of France, so they are very large, and encompass several wine regions — I simply chose the degree of latitude that runs through the region. Obviously, it would be better to have data for each wine region individually, associated with more precise latitudes.
Interestingly, in another AAWE post we do have such data, for the 2016 winegrape yields in the wine regions of Italy. These data are shown in the next graph, where each point represents one of the Italian wine regions. The data reveal four regions (in pink) whose yield is much greater than anywhere else in Italy, or France for that matter — from the top, they are Emilia-Romagna, Veneto, Abruzzo and Puglia.
However, as indicated by the line on the graph, the remainder of the Italian winegrape regions do show the expected relationship between yield and latitude — however, this time only a bit more than a third of the variation in yield is exponentially related to latitude. This difference (35% vs 74%) might be the result of using smaller geographical areas, or it might be related to the use of winegrape yield instead of wine yield (the latter must be less than the former).
Finally, yet another AAWE post shows the average wine yields for selected countries in Europe in 2008. Note that Germany, which is north, has higher yields than France, while Spain and Portugal, which are south, have lower yields than France. So, the trend between yield and latitude is repeated even at the geographical level of country.
If you want a more global perspective on yields by country, then you can check out this other AAWE post: Average vineyard yields 2000-2015.
In western Europe, wine yields are strongly related to latitude, rather than necessarily being related to quality. However, it is well known that "correlation does not equal causation" — latitude is unlikely to be having a direct effect. Whether the proximal effect of latitude is via temperature, or some other factor such as day length or seasonal water availability, is not clear.