The plant communities are typical of the Coastal Douglas-fir Biogeoclimatic Zone. The drier western and southern slopes in the upper parts of Observatory Hill are open rock outcrops with a mosaic of mossy/grassy vegetation and several stands of Garry oak (Quercus garryana). The western slopes with Quercus garryana represent younger stages of the Garry oak (=White Oregon Oak) vegetation after the 1934 fire.
The mixed forest of Observatory Hill can be classified as mid-successional (45-80 years). Forest age is a factor in determining species composition of macrofungi, especially of the mycorrhizal fungi. However, the greatest richness in species was found in more complex, late- successional mixed forests.
Stands dominated by Douglas-fir (Pseudotsuga menziesii) are the major vegetation forest types in the study area. In the lower, colluvial slopes the forests belong to the Pseudotsuga menziesii – Gaultheria shallon and Pseudotsuga menziesii – Mahonia nervosa site series; whereas on the upper parts of the top plateau the forests belong to the drier Pseudotsuga menziesii – Melica subulata site series. Large area of this forest has never been logged and is a good example of the drier Douglas-fir forest.
The area at the eastern base of the hill below the access road is covered with wetter Douglas-fir forest (Thuja plicata – Eurhynchium oreganum site series) with scattered big-leaf maple (Acer macrophyllum). Big-leaf maple forms a small stand along a small ephemeral stream at the bottom of this area.