Steph,
I would be more concerned about natural items drying out and becoming so brittle that they disintegrated in the frame package. Probably not all items but quite a few things found in nature aren't supposed to last very long by design. Leaves, cones, flowers, etc. must be treated and carefully handled to condition for any length of time.
Feathers are also an item that is very temporary, contrary to some people's belief. Little critters and such will find their way into the frame package and devour your feathers in a short period of time (a few years rather than a century).
You may be inadvertedly carrying in damaging things with your nature items and not know it. Mold spores, little critters, moisture, etc., may all be accidently placed in the frame package along with the nature items and you won't realize it until it is too late.
Just a few points to ponder today.
Framerguy