What are these strange protrusions encircling the Tembusu tree near CELC?

Find out from the students taking AR2101 Design 3, who have been blogging about their PLYground projects.

Since the semester began, Dr Patrick Janssen's students have been documenting their work, starting with conceptual drawings, progressing to scale model prototypes in the design studio, finally reaching the current phase - building the actual model.

From what I understand, these things are supposed to be stepped on...