Narayani and Desi, these are cut off bald cypress knees (with dogs for scale). Each of these circles should be a cone a couple of feet or more tall. Like a termite mound or giant ant hill. It's not clear what cypress knees do but they are all around the ground under the canopy and a bit beyond on the lakeshore side. This tree should naturally be thickly surrounded by cones of knees.
@narayani I figured they were called knees because when you see the trees in the swamps and bayous down South, they're in water and the knees kind of look a bit like someone sitting in the bathtub with their knees up. :D
