In a high-density giant kelp (Macrocystis pyrifera) bed how many "holdfasts" or roots would there be per square meter of seafloor on average?
The density of Macrocystis pyrifera is pretty variable. According to Dayton et al. 1984 off of the Southern California coast it can range from less than 0.1 to to 1.0 $individuals/m^2$. The variation depends on the location, where there are different exposure levels to high energy waves, upwellings containing nutrients, and predation by sea urchin.
Dayton, P. K., Currie, V., Gerrodette, T., Keller, B. D., Rosenthal, R., & Tresca, D. V. (1984). Patch dynamics and stability of some California kelp communities. Ecological monographs, 54(3), 253-289. https://doi.org/10.2307/1942498