Not really my field of expertise, and Baz and Yogi have already got most of the anagrams anyway, but here's my list for the last 10 (along with the unused letters for completeness). Still struggling on #16: I'm convinced it's an oak, but I can't make the rest fit, and am concerned that I'm *ahem* barking up the wrong tree. (I did that with #20, thinking it was some sort of pine...) 
11) Canadian Redwood - S
12) Weeping Willow - N
13) Flowering Dogwood - D
14) Douglas Fir - R
15) Western Red Cedar - A
16) ?
17) Horse Chestnut - T
18) Common Juniper - N
19) Slippery Elm - S
20) Mountain Maple - A