NASA's Mars Global Explorer has sent back images of peculiar formations that resemble earthly trees and bushes. Sprouting in the vicinity of the Martian south pole, these "growths" are easily hundreds of yards wide. Since temperatures fall below -200°F in these Martian "forests" this is certainly not "life as we know it"---it is probably not life at all!
Current thinking is that these Martian formations are indeed growths, though lifeless ones, much like those mineral spires that sprout around the edges of mineral-rich lakes. The tufa mounds and towers at Mono Lake, California, are good examples, but on a much smaller scale. On Mars, the growths are probably frozen carbon dioxide.
The Martian "ghost forests" are probably
frost ferns or mineral growths.
(Gravitz, Lauren; "Ghost Forests of Mars," Discover, 22:18, November 2001.)
Reference. See ETM12, "Curious Columnar Structures" in Carolina Bays, Mima Mounds, etc.