Here are a few off the top of my head:

Cactus Tree- Joni Mitchell
The Shining Birch Tree (aka Land of the Muskeg)- Wade Hemsworth
Tall trees- Dave Alvin
Tall Trees in Georgia- Buffy Sainte-Marie
The Trees They Do Grow Tall- Peter Bellamy
The Greenwood Tree- Norman & Nancy Blake
Pine Tree Woman- David Bromberg
The Pine- Tom Mitchell
Around the Corner Beneath the Berry Tree- The Weavers
The Rowan Tree- Margaret Christl
Tall Pine Trees- Peter, Paul & MaryPlum Tree- John Reischman
Apple Suckling tree- The Band
Shade of the Old Apple Tree- Ramblin' Jack Elliott
Cherry Tree- Kat Eggleston
Tanglewood Tree- Dave Carter & Tracy Grammer
Gum Tree Canoe- John Hartford
The Names of Trees- Tom Paxton
Requiem for the Giant Trees- Herdman, Hills & mangsen
Willow Tree- Roscoe Holcomb
Lone Tree Standing- Tim O'Brien
The Trees are Growing Bare- Brendan Nolan
A Row of Small trees- Garnet Rogers
Joshua Tree- Tom Russell
Forest For the Trees- Peter Rowan
Tree On A Hill- Peter Rowan
If A Tree Falls- Bruce Cockburn
Tree of Rhyme- Jack Hardy
The Merry Golden tree- Rick Lee