There was a famous case in Los Angeles in 2006 mirroring these same circumstances.
About 300 families were using property that didn't belong to them for community gardening.
The new owner evicted them to build a warehouse. I believe the landowner ultimately won his right to claim use of his land but as of last year had yet to build his warehouse and the land was vacant.
http://en.wikipedia.org/wiki/South_Central_Farm
Cowperson