Ecological dynamics

Before European settlement, the natural plant communities for this ecological site ranged from manzanita-dominated chaparral to scrub oak-dominated chaparral with sparse understories of native perennial and annual grasses and forbs (also commonly known as mixed chaparral) to a mixed chaparral-grassland dominated community (post-fire). The reference state for this ecological site is similar to its pre-European state; however the density of the chaparral species may be different due to fire suppression and the understory species are now commonly dominated by annual grasses and forbs. Primary species include oats (Avena spp.), bromes (Bromus spp.), annual fescues (Vulpia spp.), miner’s lettuce (Claytonia spp.), shooting-star (Dodecethon spp.), catsear (Hypochaeris spp.), clarkia (Clarkia spp.) and lotus (Lotus spp.). These two plant communities can be found all through this ecological site, however the manzanita-dominated plant community is more prevalent and visible throughout Pinnacles National Monument.

The reference state for this ecological site is a numerous mixture of common chaparral species, however the dominant species include bigberry manzanita (Arctostaphylos glauca), pointleaf manzanita (Arctostaphylos pungens), buckbrush (Ceanothus cuneatus), toyon (Heteromeles arbutifolia), scrub oak (Quercus berberdifolia), and the shrubby interior live oak (Quercus wislizeni var. frutescens), with lesser dominants including birchleaf mountain mahogany (Cercocarpus betuloides var. betuloides), coffeeberry (Rhamnus ilicifolia), hollyleaf cherry (Prunus ilicifolia), California ash (Fraxinus dipetala), gooseberry and current (Ribes spp.), chamise (Adenostoma fasciculatum), California buckwheat (Eriogonum fasciculatum), man-root (Marah fabaceus), foothill pine (Pinus sabiniana) and an understory composed of native and annual grasses and forbs. Other species that can be found throughout this ecological site include; poison oak (Toxicodendron diversilobum), California buckeye (Aesculus californica), monkeyflower (Mimulus spp.), penstemon (Penstemon spp.), honeysuckle (Lonicera spp.), dodder (Cuscuta spp.), phacelias (Phacelia spp.), wooly bluecurls (Trichostema lanatum), larkspur (Delphinium spp.), bedstraw (Galium spp.), catsears (Hypochaeris spp.), bluegrass (Poa spp.), foothill needlegrass (Nassella lepida), silver hairgrass (Aira caryophyllea), ripgut brome (Bromus diandrus), soft brome (Bromus hordeaceus), oats (Avena spp.), rabbitfootgrass (Polypogon monspeliensis), rat-tail fescue (Vulpia myuros), and goldback fern (Pentagramma triangularis).

Next to chamise, mixed chaparral is the most dominant and widely known plant community throughout much of California’s mid-elevational ranges. It is comprised of dense stands of shrubs that are all structurally similar with deep roots and evergreen, sclerophyllous leaves. Depending on the combined influences of disturbance, elevation, aspect and soils, the mix of species within the mixed chaparral habitat will vary. Although soil and parent material have some influence on the variety of species, microclimatic differences in soil moisture, temperature, and aspect often dictate what species will be most dominant in a given location (England 2006 and Fried et al 2004).

At Pinnacles National Monument the mixed chaparral varies primarily due to the strongly sloping north-aspects, which allow for better moisture conservation and cooler temperatures. The upper-most portions of the hills, which are generally more exposed to the sun, are dominated primarily by manzanitas, buckbrush, and chamise. 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. Whereas at mid-slope, there is a much greater diversity of many of the species listed for this site, and in the concave depressions where water drains and temperatures stay cooler, there is a much higher dominance of toyon, birchleaf mountain mahogany, and hollyleaf cherry. As the mixed chaparral nears the bottom of the slope it grades into the hollyleaf cherry-dominated ecological site (R015XI105CA-Footslopes and backslopes 17-19” p.z.). 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.). Foothill pine is found scattered throughout this site, most often in the drainageways or fractured bedrock among the Pinnacles and rock outcroppings where water is readily available.

The primary elements that maintain this ecological site are drought and fire. 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.