You can lager (age, cold condition) both ales and lagers. The language can be confusing because we use the German word "lager" (storage) to describe both the cold storage conditioning and the product typically produced this way. Keep in mind though, lager yeast strains are a relatively new discovery. Most of your medieval beers were ales which they lagered in caves packed with ice and straw so they remained cold all year long. Specific lager strains were isolated later and were better suited to performing in colder temperatures.
I've even read that on some early batches, the brewers didn't really know if they would get a lager or an ale out of a batch. The pitch was likely a mixture of all sorts of yeasts. When the fermentation was done and the weather was cold, the ale yeasts were suppressed, but the lager yeast in the pitch would take off, so cold weather produced cleaner lagers. If it was warmer, the ale yeasts dominated and they would end up producing an ale. If the weather was hot they wouldn't brew, there were laws against Summer brewing in Germany. Most all were lagered - not just for taste, but to extend the beer supply all year long... Prost!
Not to mention that the yeast morph and mutate over time. Is
s. pastorianus (lager yeast) an evolutionary off-spring of
s.cereviase (ale yeast)? Did ale yeasts travel from sub-equatorial regions through the Balkin states (i.e., "Bohemia") into Germany where centuries of long, cold storage (lagering) caused the yeast to mutate into a completely different beast (bottom fermenter)? Many microbiologists believe this is exactly what happened. Some genetic science as well as more recent gene sequencing has shown that a popular 'lager' yeast, WLP-838, is in fact
s. cereviase and NOT
s. pastorianus. The popular "Southern German Lager" yeast is in reality "Southern German
Ale" yeast. After hundreds of years it didn't become a true lager, but it did adapt to cold temperature fermentation. Who knew?
Likewise, the famous ales of Northern Germany, the Alts in Dusseldorf and the Koelsches of Koln, have evolved into hybrids that ferment at mid-range temperatures for shorter periods of time without longterm storage, yet exhibit lager-like characteristics, even though they are true top-cropping ales. Now granted the climate in Dusseldorf is cooler than Munich, much like the difference between Detroit and Houston in the U.S. In fact the term "Alt Bier" differentiates the "old" style of brewing with the more "modern" beers brewed in Bavaria and stored in underground caves during the summer months. The Reinheitsgebot (German purity laws of 1513) dictated the prohibition of brewing during summer months, thus the necessity for underground storage of 'stockpiled' beer for the summer. They were strictly observed and enforced in the South, but not so much in the North. Thus, the Old Style (Alt) in Koln and Dusseldorf, and the New Style (lagers, pils) in Munich and Bavaria.
It's a chicken/egg conundrum. Did different yeasts develop into different styles of beer or did different styles of fermentation cause an adaptation into different types of yeasts? Either way, both have become delightful and enjoyable beverages that I have had the extremely good fortune to have experienced numerous times in their native settings during my life of travels. The journey continues.