tree:MoveTo(math.random(-2005,6100),4,math.random(-2005,6100))
Output: 19:45:43.847 - Unable to cast double to Vector3
I heard that doing
tree:MoveTo(Vector3.new(math.random(-2005,6100),4,math.random(-2005,6100)))
might work and I tried it, did not work. Did not give me an error either.

4 comments
Do you have the primary part set for that model? If you don't know what those are, here's a link that'll hopefully explain. ----------> http://wiki.roblox.com/index.php?title=API:Class/Model/PrimaryPart
if you tried tree:MoveTo(Vector3.new(math.random(-2005,6100),4,math.random(-2005,6100))) its suppose to work i just tried in my studio and its worked.
except if you have more than 1 tree named the same when you tried to take 1 with the variable and if your code is just this it will work but you'll can not see it cuz its do it to fast.
mmm that's not what the wiki says. Check your sources, specifically where you got this script from?