Ecological dynamics

Before European settlement, the natural plant communities for this ecological site ranged from a pre-fire ecology of dense island chaparral with a sparse understory of native grasses and forbs to a post-fire open, patchy island chaparral grassland dominated community. The reference state for this ecological site is similar to its pre-European state. However, the density of the island chaparral may be different due to more frequent fires and the invasion of non-native annual grasses and forbs. Primary species include island big-pod ceanothus (Ceanothus megacarpos spp. insularis), Channel Island scrub oak (Quercus pacifica), chamise (Adenostoma fasciculatum), island mountain mahogany (Cercocarpus montanus var. blancheae), island redberry (Rhamnus pirifolia), lemonade sumac (Rhus integrifolia) and manzanitas (Arctostaphylos spp.). In the central valley of Santa Cruz Island, this habitat is open and patchy, and is intermixed with the coastal sagebrush and grassland ecological sites.

Mixed chaparral is one of the most dominant and widely known plant communities throughout much of the Channel Islands. 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 variation of species within the mixed chaparral habitat will be different. 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).

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 fire adaptations include: sclerophyllous leaves, specialized stomates, extensive rooting systems which take advantage of water near the soil surface as well as water far into the soil profile, the shape of the canopies which dissipates radiation by convection, the height of the shrubs which can keep 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).

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. This clearing recharges 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. Very little research can be found on the fire adaptations of the island endemic chaparral species; however similar species that are found on the mainland are capable of re-sprouting and reseeding after fire, which suggests these species have similar adaptations. Post-fire chaparral on north-facing slopes exhibit higher species richness, higher species turnover rates, and faster vegetation recovery in terms of biomass accumulation, causing it to return to pre-fire species composition faster 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 was probably greater than 70 years, with lightning being the primary ignition source (Keeley and Fotheringham 2001). Fire intervals may have been up to 200 years, but have generally increased to 40 to 50 year intervals with the increase in human caused fires.

Chaparral is an 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. Others may 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 denser, it will become too crowded for many of these species to properly utilize the habitat. Consequently, fires are necessary to open up the canopy, which will provide 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.