In particular, when a plant grows beside a relative, it directs fewer roots in its direction and grows less quickly, giving it a chance to settle in.
The presence of an unrelated plant in the area, however, even of the same species, causes the opposite reaction: it sends out more roots in its direction to take over as much soil as possible and grows more quickly, creating denser shade, apparently in an effort to out compete its neighbor.
At this point, no one is exactly sure how plants are able to recognize their kin, but some researchers speculate that it’s due to tiny chemical signatures present in the roots that are specific to closely related plants.