SMP servers run continuously. Does that mean that trees (and cacti, and sugar canes, and crops) grow always, even when no one is online? or do they only grow if their chunks are loaded?
I believe they actually have to be loaded in memory, going by this page from the Minecraft Wiki:
Trees will only grow if they are planted on a chunk of the map that is loaded into memory. If you plant saplings and then explore another more distant area of your world for a few Minecraft days, they will still be saplings when you return.