Ecological dynamics

The reference state for this ecological site ranges from a hollyleaf cherry-dominated chaparral with a sparse understory of native perennial and annual grasses and forbs to a California sycamore-California live oak-California buckeye-dominated woodland with an understory dense with native perennials, annuals and ferns. Primary species include hollyleaf cherry (Prunus ilicifolia), California sycamore (Platanus racemosa), California live oak (Quercus agrifolia), and California buckeye (Aesculus californica). Lesser dominants will often include foothill pine (Pinus sabiniana), buckbrush (Ceanothus cuneatus), coffeeberry (Rhamnus ilicifolia), toyon (Heteromeles arbutifolia), birchleaf mountain mahogany (Cercocarpus betuloides var. betuloides), California ash (Fraxinus dipleta), poison oak (Toxicodendron diversilobum), creeping snowberry (Symphoricarpus mollis), scrub oak (Quercus berberidifolia), monkeyflowers (Mimulus spp.), penstemons (Penstemon spp.), fringed redmaids (????), California maidenhair (????), and pine bluegrass (Poa secunda).
These two plant communities can be found all through this ecological site; however the hollyleaf cherry-dominated plant community is more prevalent and visible throughout Pinnacles National Monument. The hollyleaf cherry plant community is comprised almost entirely of dense, uniform stands of hollyleaf cherry and little to no understory. It is found on the relatively moist, cool, lower toe- and foot-slopes of the Monument, often times grading into the riparian plant communities. The density of hollyleaf cherry is varied primarily due to how strongly sloping the north-aspects are, which allow for better moisture conservation and cooler temperatures. The more exposed toe- and foot-slopes are more exposed to the sun and have a mixture of both hollyleaf cherry and buckbrush. These portions of the slope have higher evapotranspiration rates, lower soil moisture, and often times erode more readily, making the soils more conducive to species that are drought tolerant. On the more shaded north-facing slopes, the dominance of hollyleaf cherry becomes almost uniform, with lesser species including California buckeye and foothill pine. These sites stay cooler and hold water longer, allowing hollyleaf cherry to competitively dominate the site. As the hollyleaf cherry nears the bottom of the slope it grades into the riparian areas ecological site (R015XI103CA-Riparian areas 17-19” p.z.).

The California sycamore-California live oak-California buckeye-dominated woodland plant community is found exclusively in the cool, shaded canyons where evaporation is minimal. These sites are collecting lots of topsoil from upslope, creating soils that are very deep and high in soil moisture. Due to it’s proximity to the creeks running through the Monument, there is a numerous mixture of species; however the most consistent species that dominate the site are California sycamore, California live oak and California buckeye.
In the canyons, the mixed chaparral will be dominated more heavily by scrub oak and the shrubby interior live oak and will begin to grade into the blue oak woodlands (F015XI200CA-Quercus douglasii/Nassella pulchra-Koelaria marcrantha 17-19” p.z.).

The primary elements that maintain this ecological site are drought and fire. Hollyleaf cherry is long-lived and shade-tolerant and like most chaparral species, it has deep roots and evergreen, sclerophyllous leaves. Chaparral species are physiologically adapted to droughty conditions and many are also well-adapted to fire. Some of these adaptations include: extensive rooting systems (taking advantage of water near the soil surface as well as water far into the soil profile); sclerophyllous leaves; specialized stomates; the shape of the canopies (dissipates radiation by convection); the height of the shrubs (keeping the leaves away from the hot soil surface); post-fire crown sprouting; and seeds that often require high temperatures before germination (Fried et al 2004).

Along with fires and the droughty conditions, erosion also plays a significant role in maintaining this site. The site is located primarily on steep slopes with soils that are highly susceptible to surface erosion. Chaparral provides these slopes with substantial ground cover, helping dissipate raindrop impact on the soil surface as well as protecting the surface from strong winds. The extensive rooting structures of chaparral shrubs also help anchor the soils, binding soil particles and slowing the flow of subsurface water. Due to the properties of the soil, the shrubs are only able to minimize the erosion and therefore the top surface is constantly being removed and deposited downslope. This constant disturbance keeps the surface horizon thin, and weakly developed, which is most likely one of the reasons higher seral species, such as blue oak, are not found in high densities in this site.

The successional patterns of chaparral begin and end with fire. Chaparral can develop into extremely dense, sometimes impenetrable stands and many of the shrubs become senescent, requiring a fire to clear out the dead branches and heavy litter, recharge the soil’s available nitrogen and carbon, and stimulate new growth and seed germination. Most of the dominant chaparral species are able to rapidly recover and grow after a fire. Bigberry manzanita, for example, is killed by fire but is an obligate post-fire seeder, requiring high-intensity fires to crack the seedcoats; whereas toyon is relatively non-flammable and will sprout vigorously from dormant buds located on the root crown after a fire (Howard 1993 and McMurray 1990). Post-fire chaparral on north-facing slopes exhibit higher species richness, higher species turnover rates, and faster vegetation recovery in terms of biomass accumulation and return to pre-fire species composition than chaparral communities found on the drier, south-facing slopes (Guo 2001).

The historical fire regime for these mixed chaparral communities is difficult to determine prior to European settlers arrival, however there are indications of a fire-return interval somewhere between 50-100 years, with lightning being the primary ignition source (Fried et al 2004 and Keeley et al 1986).

Chaparral is important habitat for many species of birds, reptiles, rodents and mammals. Some deep-habitat dwellers rely almost entirely on chaparral for food, shelter, and reproduction sites, whereas others occupy the ecotonal zones between the chaparral and woodland, chaparral and grassland or chaparral and riparian habitats to satisfy their various needs. As the chaparral becomes more and more dense, it becomes too dense for many of these species to properly utilize the habitat and thus fires are necessary to open up the canopy, which provides better movement through the site. It also stimulates new growth of the shrubs as well as the understory herbaceous species, providing more food and shelter for a greater variety of species.