On a more immediate, less controversial time scale, changes in elevation and microclimatic variation yield differences in bloom and maturity for plants along a vertical gradient. About halfway down we observe the first blooming flowers; though most plant species have mature seed structures, a few are still blooming. Marcus observes that century plant capsules are less mature at the bottom of the canyon, though variations in plant growth are discontinuous; in addition to elevation, more locally variable factors such as shade, airflow, and moisture availability also shape the microclimates plants experience.
Without time to do a whole lot of research and writing on those plant communities, i shall simply present some images of them. Common in the canyon (but absent here, for lack of a good photo) is Quercus turbinella, which looks deceptively like a holly, until you find its acorns.