Cacti and other succulent plants are natutal sculptural forms like this 'bird cactus'.
But there are also other Sculptures from the land. Almost all made from materials of the land. An example is The Goodi Windi Monument; dedicated to the Aboriginal who went Down Under in Australia.

Goodi Windi very much enjoys his walkabout in the Klaas Voogds Maze nowadays.
The latest addition is the 'Pachycereus plaatjies'; a 4 metre high laminated wooden cactus made by Marthinus Plaatjies; a woodworker living in one of the 'less priviliged communities' nearby. Entirely made by hand; no machines, no screws or nails but with delves.
Besides these few items there are also 'the Observatory of the White South African Country Woman'; a 'Stone Age Cinema'; an Elephant Grave and a Scottish Caber; etc. etc.
See also
Land Art Project in South Africa