Yup, and I read (hope these link codes work...) Here and [url=http://www.psipog.net/activepsy/book2.html#_Toc472931487] here [/url] , even though the second website says it's nonsense to draw energy from the earth, I don't think it is but I haven't tried because I live in a high-rise and it's a bit of a limit of mind not to be able to draw energy from the earth from here. :p
It is possible to gain energy from any living being, including flora. I use the earth because I live In a vally. Other sources in space and such take longer because of the distance traveled. Crystals such as quartz help focus as well as energy build up.
Everything about us holds Magickal energy, the trick is to learn how to tap into that energy to use it for your workings.
Also it is very important to ask permission beforehand. Taking without asking is stealing, no matter what the source of the energy. The flora or fauna, you are wishing use as your source might not be able to spare that extra energy at that particular moment. And it's the polite thing to do.
If your source agrees, he/she or it may even give your magick an extra little boost which makes it more effective.
Aso Ley lines are a great source of magickal energy and they are almost always willing to lend their power to a spell.
Here are a couple of links to a meditation for drawing energy and a Tree meditation. If you live in a second story floor or higher, you can visualize the roots of the tree going thru floors, twining along the walls, thru the foundations of your building and sinking deep into the earth and bringing the energy up to you.