Of course, the short answer is “none”; there is no one particular style emerging that fills the role in the way that Bocks have. American brewers brew what strikes their fancy for the season, if they focus on the season at all.
But there are trends. Empirically I would point to a range of beers that are pale-ish (up to red and brown), crisp and refreshing, probably hoppy, but with a bigger malt bill and strength that what you would expect from a summer ale—something that drinks equally well on a hot day as a cool one.
Pretty loose! But then I’m on the West Coast of the U.S., where beer drinkers’ tastes have been spoiled by bold beer with lots of hops. As a point of reference, I would look toward American/Northwest Red Ales like Deschutes Cinder Cone Red (which I’ll have a review of soon), or paler-but-hoppy beers like Sierra Nevada’s ESB (Early Spring Beer).
Bière de Mars is another spring style, a French-Belgian style of beer brewed in (you guessed it) March. I like Jolly Pumpkin‘s description which could fit the style pretty well:
A sustaining beer that is brewed to comfort in the gusty ides of March and welcome in a wealth of warmer weather.
Besides Jolly Pumpkin, the best-known examples probably come from New Belgium Brewing, Brewery Ommegang, and Flying Dog. These are robust Bières de Garde that are earthy, spicy, fruity, and complex—perfect beers for cool spring evenings.
But remember what I said about there truly being no particular tradition being adhered to? Take a look at BeerAdvocate’s Top Spring Seasonals to really bring this home. Dark Lord Imperial Stout? Live Oak Hefeweizen? Melt Down Double IPA? All I can say is, “Wow.”