Why is new york called the "big apple"?

"Big Apple" is a nickname for New York City. It was first popularized in the 1920s by John J. Fitz Gerald, a sports writer for the New York Morning Telegraph. Its popularity since the 1970s is due in part to a promotional campaign by the New York tourist authorities.

What's the origin of the phrase 'The Big Apple'? ... He reports hearing the phrase used by stable lads while on a visit to New Orleans in 1920, although there's nothing in that report to link the phrase to New York.

they use tree rings to figure out weather patters.