There may be times when you need to secure the end of a rope around a large tree. Provided the tree is not oozing with sap, here's a great way to do it.
First off, have a good look at the tree. The American Alpine Institute blog has a good memory jog for this, "Five-and-Alive." The tree should be at least 5 inches in diameter, 5 feet tall, and alive with a good root base. Be wary of trees that could have a shallow root-base, in dirt, sand and/or on top of a rock. Thanks AAI, good advice!
Starting with one end of the rope, take a few coils in your hand, and pass them around the tree trunk three or four times. This may well take more rope than you think, so start with a bit extra. Tie a loop in the end of the rope, like a figure 8 on a bight or overhand, and clip this bight onto the loaded strand to close the rope system. If you tie this correctly, the friction alone from the rope on the tree bark will support the load, and this carabiner at the end should never see any force at all.
If you REALLY want to go with elegant minimalism, you can skip the carabiner entirely, and tie a bowline around the load strand. That's pretty cool, an anchor you can hang a truck off of that does not use a single piece of metal.
This is actually one of the strongest anchors you can build, for two reasons: one, you’re using the entire strength of your dynamic climbing rope, and two, there is no knot under tension (tensionless, get it?) which weakens the rope.
If you're doing this on a conifer tree, there's a good chance you're going to get sap onto your rope. YUCK! This is something you want to avoid if at all possible, so try not to use conifer trees.
(If you do get tree sap on your rope, hands or anywhere else, you can use mayonnaise, believe it or not, to clean it off quite effectively.)