Tu BiShvat (Hebrew for the 15th day of the Hebrew month of Shvat) is the new year of the trees.
"Tu" (the Hebrew abbreviation of 15th) in the month of Shvat was set (Hillel's opinion generally overrules Shammai's) as the beginning of the tithable year for tree fruit: Calculating the tithe on fruit starts again for fruit that sets after that date.
The sukkah is a symbol of fragility. We build the temporary structure each year and know that it is only meant to last for the week-long holiday. It sways in the breeze. The raindrops land inside. The animals nibble at our decor. We know it could come crashing down on us.