Southern California black walnuts (Juglans californica) are frequent in the coastal sage scrub, but on north-facing exposures on specific soil types, they form a woodland of which they are the only large woody species. Walnut wooland is located on the south side of campus, where it is used for cattle grazing; in the area around the Lyle Center for Regenerative Studies, and on north-facing slopes near Kellogg West and the Colllins College of Hospitality Management. This region is one of the few places in the world where walnuts dominate their own plant community.
Walnut woodlands in other parts of southern California often include other tree species, such as live oak and toyon, as well a shrubs, but the woodlands of Cal Poly and other parts of the San José Hills have few other trees or shrubs, and are dominated by grasses in the understory, in the valleys, and on south-facing slopes. These grasses are currently introduced Eurasian weedy grasses, and intentionally-planted, largely non-native pasture grasses. No one knows what grew between the walnuts before European settlement and grazing.
Grazing has been blamed for the low plant species diversity of the walnut woodlands, but diversity is low even in areas that have not been grazed for years or decades. It is likely that grazing has an impact on walnut woodlands, but there are no studies of what that impact might be.
A more likely explanation for the low diversity also explains the sharp contrast between the coastal sage scrub/oak woodland in the western part of campus and the walnut woodland in the south: these communities are on different soil types, and the soil that supports the walnuts is unusual for the area, being higher in calcium carbonate than many other regional soils.
Although southern California black walnut trees are not endangered, walnut woodland is one of the rarest and most endangered plant communities in southern Calfornia.