This is a such small change (of something that's bugged me for years) that I'm sneaking it onto the Xmas wish list:

"Your skin becomes covered in bark."
"Someone's skin becomes covered in bark."
"The bark on your skin flakes off."
"Your skin starts to toughen up but then turns normal again."

WTF? I'd imagine even in Thera rangers suddenly sprouting bark on their skin would be considered magical, so I propose you change the echo(es) to something else that doesn't imply mystical transformation. I haven't come up with an echo I'm really happy with, but I'd make it incorporate willpower/ignoring pain, something to the effect of:

"You harden yourself to shrug off cuts and wounds as if your skin were bark."
"Your skin feels normal / less like bark again." (wear off)
"You fail to toughen up your skin." (fail)

If you think the skill should (still) have a room echo, it could simply be:

"Someone concentrates intensely for a moment."