[Avodah] Tu?bishvat is the Rosh Hashanah (new year) for trees. What does this mean?

[Alexander Seinfeld]

Date: Wed, 27 Jan 2021 23:01:57 -0500
From: Zev Sero <zev at sero.name>
> As a matter of metzius, are there any fruits that, in Eretz Yisrael's
> climate, reach onas hama'asros by the middle of Shevat, so that a hard
> date is required to separate them?

> I would have thought the logical place to separate the years would be in
> mid-winter when there are no fruit growing anyway, so that it doesn't
> matter whether the exact date is the 1st or the 15th....

The Gemara (Rosh Hashanah 14ab) tells of R' Akiva picking an Esrog at that
time and having a suffik about it's maaser.