It appears the whole earth had lush vegetation before the flood. And higher air pressures were not entirely necessary to have gigantic plants and animals, but it would have helped.
This is the history of the heavens and the earth when they were created, in the day that the Lord God made the earth and the heavens,before any plant of the field was in the earth and before any herb of the field had grown. For the Lord God had not caused it to rain on the earth, and there was no man to till the ground;but a mist went up from the earth and watered the whole face of the ground. - Genesis 2:4-6 http://www.biblegateway.com/passage/?search=Genesis2:4-6&version=NKJV
There was lush vegetation because of the mist that went up from the ground every morning.
And it's referenced in Gen 7:11.
Except that that's not referring to a canopy.
I don't understand the large resistance to a canopy in the rest of the WB camp (since I'm in that camp). It's just not that big a deal.
Because it doesn't fit the evidence, nor does it fit the Bible.
Start here:
http://www.creationscience.com/onlinebook/FAQ3.html#wp1147115
(P.S. Don't forget to click to the next page a few times, as there's more than one page on that topic...)