Vegetation and Habitats
Plant Communities
Plant communities of the Blue Oak Ranch include blue oak woodland, valley oak woodland, black oak woodland, coast live oak woodland, riparian forest, chamise chaparral, Diablan sage scrub, nonnative annual grassland, wildflower field, and native perennial grassland (Bainbridge and Kan, 1997). Four of these are threatened plant communities; valley oak woodlands, blue oak woodlands, wildflower field, and native perennial grasslands. Streams on the ranch support healthy stands of riparian vegetation in addition to aquatic species. They are important habitat for migratory birds, and may be migratory corridors for numerous aquatic and terrestrial animal species. The steep slopes of the Arroyo Hondo canyon provide an excellent contrast of vegetation, with the north and east-facing slopes supporting vegetation adapted to relatively cool, moist environments, and the west and south-facing slope vegetation (including chamise chaparral) adapted to hot, dry conditions.
Grasses
Patches of native grasses still occur in many places on the ranch. They are dominated by purple needle grass (Nasella pulchra), but perennial species of barley (Hordeum), bluegrass (Poa spp.), three-awn (Aristida ssp.), melic (Melica ssp.), and wildrye (Elymus and Leymus ssp.) are sometimes dominant or codominant (Bainbridge and Kan, 1997). The native grasses are most abundant on mesic north-facing slopes, in the understory of mixed evergreen woodland (Arroyo Hondo), under trees in oak savannas, and in open areas on ridge tops and slopes.
Oak Woodland
Blue and valley oak woodlands have become quite rare in California, and few are protected from grazing and the encroachment of suburban development. Oak woodland covers a large extent of BORR, much of this is composed of blue and valley oaks. Compounding the possibly bleak future for oak woodlands is the lack of recruitment of young oaks. In the Mt. Hamilton area, valley oaks seem to be suffering most from this problem. The exact mechanism or mechanisms preventing oak regeneration is not known, but protection from pigs, cattle, and ground squirrels has been shown to increase survivorship of existing seedlings. Prescribed fire may also help by opening canopies and reducing competition between grasses and seedlings. However, fires must be carefully controlled for low heat, and conducted at frequency intervals that allow seedlings to become established. Physiological research suggests that ozone may extend stomata opening in blue oak, forcing nighttime transpiration during periods of drought and influencing shorter leaf retention. Oaks, as the most widespread and representative forest and woodland species in California, offer excellent research opportunities for climate change and pollution-effects research.
Non-native Plant Infestations
Among introduced species, yellow starthistle (Centauria solstitialis) and medusahead grass (Taeniatherum caput-medusae) are highly invasive. At present, C. solstitialis occupies only about 30 acres of the ranch, but it is spreading quickly. Taeniatherum, probably introduced from bales of hay, covers most of the flat areas of the ranch.
Other introduced species of concern are Italian thistle (Carduus pycnocephalus), tocalote (Centauria melitensis), Canada thistle (Cirsium arvense), and bull thistle (Cirsium vulgare). Italian thistle in particular has become widespread in dense stands, and is a spring and summer feature under the drip lines of many oaks on the ranch.