Was chatting about these features on another forum dedicated to this kind of discussion. We don't think they are trees, but they certainly are strange and interesting anomalies.
A while ago I found something similar to these, but looked like spiders. I think these are natural geological formations native to Mars, but since scientists are comparing to all they know (ie; Earth geological features), they haven't a clue what these are geologically. The area is engulfed with frost, or so they say, but its probably not trees anyway, because I remember when Joseph P. Skipper of Mars Anomaly Research discovered similar features with the MOC imagery, he and his team claimed it were trees and were later proven wrong with the more advanced generational imagery, HiRise I think.
From another thread, again including these links Ive compiled of almost all Mars imagery directories, for those interested.
http://hiroc.lpl.arizona.edu/images/ (HiRISE Imaging)
http://marsoweb.nas.nasa.gov/hirise/ (HiRISE Imaging Science Experiment)
http://marsoweb.nas.nasa.gov/HiRISE/hirise_images/ (HiRISE Online Image Viewer)
http://hirise.lpl.arizona.edu/ (HiRISE Mars Reconnaisaince Orbiter)
http://photojournal.jpl.nasa.gov/targetFamily/Mars (Assorted .TIFF HiRes Images)
http://www.msss.com/moc_gallery/ (Mars Global Surveyor)
http://marsrovers.jpl.nasa.gov/gallery/images.html (Spirit/Opportunity)
http://marsprogram.jpl.nasa.gov/gallery/images.html (Spirit/Opportunity)
http://mars.jpl.nasa.gov/odyssey/gallery/images.html (Mars Odyssey)
http://mars.jpl.nasa.gov/mgs/gallery/images.html (Mars Global Surveyor)
http://themis.asu.edu/ (Mars Odyssey Themis)
http://marswatch.astro.cornell.edu/mars.html (Cornell Mars Image Gallery)
http://www.mentallandscape.com/C_CatalogMars.htm (Soviet Mars Images)
http://nssdc.gsfc.nasa.gov/planetary...th_images.html (Mars Pathfinder Images)
http://marsprogram.jpl.nasa.gov/MPF/mpf/image-arc.html (Mars Pathfinder Directory)
http://mars.esa.int/ (ESA - Mars Express)
http://phoenix.lpl.arizona.edu/gallery.php (Phoenix Mars Gallery)
http://ida.wr.usgs.gov/ (PDS MGS/MOC Image Collection)
http://crism.jhuapl.edu/index.php (Compact Reconnaissance Imaging Spectrometer for Mars)
Cheers