Via Slashdot, a recent study (Note: Actual study is in another link on the second site in PDF format) indicates that plants compete less with their siblings for resources like water. As someone in the Slashdot comments section noted, this may be due to plants not being able to sufficiently differentiate their siblings genetic material from their own but none the less, it is interesting.
An amusing observation about the comment in the last paragraph of the second link about scientists finally catching up to what gardeners have known for a long time made me think "Yeah, but did you know why it happens? No, so why brag?".