Gabbro Island

Although not surrounded by water, for some of the plants growing on the Pine Hill gabbro “island” in El Dorado County, the surrounding landscape might as well be an uncrossable sea. Over 740 plant species have been recorded growing on the unique gabbro soils that occur on Pine Hill, three of which are found nowhere else in the world. Thus, this comparatively tiny 30,000-acre area just outside of Sacramento contains enormously significant species diversity.

Fascinating Features

  • Gabbro is a type of volcanic rock that is formed from magma rich in iron and magnesium and that is slowly cooled deep beneath the earth’s crust. The Pine Hill gabbro was lifted up to the surface during the formation of the Sierra Nevada mountains.
  • Soils formed from weathered gabbro rocks are generally red and mildly acidic. They contain more than 70 percent iron and magnesium minerals and may also be rich in other heavy metals, such as chromium.
  • The unusual characteristics of gabbro soils often prevent the establishment of plant species that are not adapted to the challenges of growing in soil with chemistry that may be toxic to plants or that can inhibit the uptake of major nutrients, such as nitrogen, phosphorus, and potassium.

Habitat Values

Plant populations on the gabbro soils at Pine Hill have evolved through natural selection to become species or subspecies that are uniquely adapted to the specific soil chemistry and climatic conditions present there. When a plant is limited to a single, defined geographic location and is found nowhere else, it is called an endemic. The Pine Hill gabbro island is just one of the many unique habitats within California, a state with more than 5,500 native plant species, 40 percent of which are endemic.

Biodiversity hotspot

The Pine Hill gabbro island is a biodiversity hotspot, supporting 583 plant species native to California and 158 introduced species. This unique habitat comprises only about 0.05 percent of the area of California but contains about 10 percent of California’s plant diversity.

Rare plant species

Eight rare plant species occur on the gabbro island at Pine Hill, including the Pine Hill Ceanothus (Ceanothus roderickii), Bisbee Peak rush-rose (Crocanthemum suffrutescens), Stebbins’ morning-glory (Calystegia stebbinsii), El Dorado mule’s-ears (Wyethia reticulata), El Dorado bedstraw (Galium californicum ssp. sierrae), Pine Hill flannelbush (Fremontodendron decumbens), Red Hills soaproot (Chlorogalum grandiflorum), and Layne’s butterweed (Packera layneae).

Pine Hill endemics

Three of the eight rare plant species, known as the Pine Hill endemics, are known only from Pine Hill gabbro soils and are not found anywhere else in the world. One of the most well-known species is the Pine Hill flannelbush, which is one of the rarest plants in California due to its extremely limited range and low number of individual plants.

Places to Visit

Pine Hill Preserve in El Dorado County is the closest incidence of gabbro island habitat near Sacramento. Much of the preserve has restricted access in order to protect the rare plants and their sensitive habitat, but here are our suggestions for accessing Pine Hill to see and learn about this unique habitat for yourself.