This is quite impressive. It should be very much doable in ICE, but the author doesn't go into a lot of detail, particularly about how he made the plant shapes.
I'm low on time these days, but if something is being put together I'll try to help.

I image they will be built with some kind of L-System in houdini and the answer would most probably be somewhere in here http://algorithmicbotany.org/ also, isn't treegen an L-system generator? and there has been several other L system variants in XSI before.

I forgot to mention Fabricio Chamon ICETree compound.

http://iceduous.blogspot.ca/

I have uploaded the various compounds. The various compounds (so far) can be downloaded from the ICE Compound area at:

http://www.si-community.com/community/viewtopic.php?f=19&t=2269

In time for 2013 release,

Cheers!

Dan