in chocobo woods there is a tree and when i click on the tree my item screen comes up- what do i need to use on the tree

you use gyshal greens on it or however its spelt

If you use the Gyshal Greens (which can be found or bought in Gyshal) you summon Fat Chocobo! inside of which you can store items. Fascinating.

where is Gyshal and where can i find gyshal greens?

Run around the world on a chocobo, you get a gnomish bread and you'll find Gyshal, on the west end of the rim of the floating continent. It can only be reached by ship. Don't dismount your chocobo to get there. It won't work.

Edit: Woops ^^ There's a shop in Gyshal with Gyshal greens, and another with Mystic Keys. Get at least 20 of those, they're important if you don't have a thief in the front line.

DoubleEdit: Getting the GyshalGreens is completely optional, this is only if you want to buy like one of EVERYTHING that ever exists. I'm at Crystal Tower and not using it.