Oaks and Other Hardwoods
While plant communities known as oak woodlands occur primarily in the Sierra Nevada foothills, the conifer forests of the Sierra also contain magnificent stands of oak and other hardwood species. Montane Hardwood is a type of forest plant community where oaks form a major component of the forest.
Oaks and other hardwoods form the foundation of the forest food web in the forests of the Sierra Nevada. They provide forage as well as abundant crops of acorns, berries, nuts, and other fruits that are essential for wildlife. They provide structural habitat for concealment, resting, nesting, denning, and birthing. Their flowers provide nectar and pollen that are essential for the survival of countless species of butterflies, bees, and beetles. Together they comprise the beautiful and diverse forest understory of the conifer forests of the Sierra. Many species of shrubs and trees are endemic to the forests of Northern California, and do not occur anywhere else on the planet.
Unfortunately, oaks and other hardwoods are declining throughout the Sierra Nevada, due to logging practices and mismanagement of natural fire cycles. The loss of hardwoods in the forests of the Sierra Nevada was identified in the 2001 Sierra Nevada Framework forest plan as one of five priority issues which must be addressed in order to sustain the viability of Sierra forest ecosystems.
The loss of oaks in particular was identified by Forest Service scientists as a serious threat. Oaks provide essential food and habitat for hundreds of species of animals which inhabit the Sierra Nevada. While most people don’t associate oaks with old-growth forests, black oaks (Quercus kelloggii) are found at elevations up to 2,400 meters or nearly 8,000 feet in elevation! Acorns are consumed by a variety of animals in every forest habitat type, from squirrels to acorn woodpeckers, black bear and black-tailed deer. They support the prey species that in turn are eaten by rare animals like the Pacific fisher, American marten, Northern goshawk, great gray owl, and California spotted owl—species which are associated with old-growth forest habitats. Many animals fatten up on acorns in the fall, providing them with the necessary fat reserves to survive the winter. Without oaks, animals throughout the Sierra would literally starve.
Oaks can become massive in size in the Sierra. The oldest giants, particularly black oak (Quercus kelloggii), are important denning and rest sites for the rare Pacific fisher. Black oaks don't start to produce a significant crop of acorns until they are at least fifty years old. The loss of black oak has resulted in long term, significant adverse impacts to Sierra Nevada forest ecosystems.
Forest management practices that favor the production of conifers for commercial lumber production, coupled with fire suppression, have severely impacted the quantity and quality of hardwood vegetation in the Sierra Nevada, especially on the western slope where hardwood diversity is highest. For many decades, foresters and loggers treated oaks and other hardwoods, as well as non-commercial conifers, as weed species that must be eliminated. Oaks are killed during clearcutting, thinning, and during plantation management that includes the use of herbicides.
A lack of understanding of native forest ecology has led to widespread conversion of natural oak-dominated landscapes to conifer tree plantations. Areas which normally would not support conifers are cleared of their oaks and diverse shrub communities, and densely planted with ponderosa pines. Potent chemical herbicides are used to suppress the natural regeneration of the native hardwoods and shrubs.
Scientific studies have shown that young ponderosa pine plantations are the most fire prone configuration—even more fire prone than shrub-dominated sites, and remain so for fifty years or more. Thus, such practices are not only devastating to wildlife which depend upon the rich food source found in hardwood communities, but the practice has greatly increased fire hazard throughout the Sierra.
Oaks are also cut for firewood in rural SN forest communities, but it is difficult to measure the impact this is having. Despite small improvements in forest management and policies on oak management, there is little oversight or enforcement of protective measures, and oak and other hardwoods continue to decline throughout the range.
Fire suppression is another factor resulting in the steady loss of hardwoods and native shrubs that are important for wildlife. Most hardwoods and native shrubs are dependent upon fire returning regularly to SN forests. All are adapted to survive under the conditions that occur in the fire-prone interior of the western slope of the Sierra Nevada. Seeds that can readily germinate after exposure to fire, and the ability to sprout from the base after fire has killed the tops of individuals, are two common adaptations. Fire also reduces disease and insect outbreaks. Most importantly, regular fire return keeps conifer species from shading out the hardwoods in the understory. This is significant for production of vital wildlife habitat.
In areas where oaks naturally predominate (west side lower elevation forests, where fire return intervals are short—between 5-30 years), fire suppression coupled with logging of all the old-growth has resulted in forests that are overly crowded. When fire does occur, as we know it will in these environments, the old oaks may be killed due to fire intensities that are too hot. Oaks are somewhat fire resilient, but can easily be killed if there is a build up of fuel at the base. While they may sprout from the base if the fire is not too hot, they will not produce abundant acorn crops again for decades. Thus it is essential to sustain wildlife viability to maintain and protect healthy oak populations.Conservation Measures
There are a number of steps that should be taken to ensure the survival of oaks and other hardwoods in the Sierra Nevada.
- Set limits on the amount of clearcutting permitted in each watershed on private timber lands as well as on public lands, as recommended by the California Department of Fish and Game in the State Wildlife Action Plan (2007).
- Oaks should also be protected during thinning operations on both public and private lands. The 2001 Sierra Nevada Forest Plan Amendment requirements for protection of westside hardwoods must be enforced.
- Restore fire resilient landscapes. Before fire can be returned, in places that are unnaturally overgrown with dense conifer cover in previously heavily logged environments, some mechanical thinning must be undertaken prior to allowing fire to return. Great care must be taken to ensure that fragile soils are not further harmed, and to protect legacy oaks and other important structural components of the forest during thinning.
- Introduce fire through prescribed/managed fire and allowing wildland fire use (letting fires proceed whenever possible, where property is not at risk), also known as appropriate management response (AMR).
Oak woodlands also are among the most biological diverse communities in the state, supporting 5,000 species of insects, more than 330 species of amphibians,
reptiles, birds and mammals, and several thousand plant species.
The oak woodlands of the western Sierra foothills are one of the most imperiled habitats in all of California. These oak woodlands, where approximately 70 percent of the region’s population lives, have been hardest hit by development. Less than 1 percent of the foothills are protected from development, and much of the area lies within commuting distance of rapidly growing cities in the Central Valley.
The Sierra Nevada Ecosystem Report of 1996 reported that "The oak woodland communities of the western Sierra Nevada foothills are the most vulnerable of the widespread vegetation types as a result of greater access by humans and of their continuing potential for urban development,”"(Vol I, page 18).
More than 65 percent of the oak woodlands in the Sierra Nevada region are privately owned and with continuing increases in land values these landscapes can be worth far more when converted to housing or intensive agriculture such as vineyards.
The clearing of oak woodlands for development has a tremendous ecological impact by degrading water quality and wildlife habitat. These habitats are among the most critical for California’s native species, including some 2,000 plants as well as 5,000 insects, 80 amphibians and reptiles, 160 birds and 80 mammals. Oak woodlands are also key to water quality concerns. As snowmelt flows down watersheds into streams and rivers, oaks help keep the soil in place, preventing erosion and stream sedimentation.
Oaks are most susceptible during the transition from seedling to sapling, and the main threats include overgrazing by cattle, and competition from the nonnative annual grasses that have replaced native perennial grasses. State researchers have determined that providing enclosures for young oak seedlings to prevent grazing by cattle goes a long way to ensuring their survival.
California's iconic oak woodlands have endured many assaults over the years. They've been cut for fuel, cleared for vineyards and housing developments, and their seedlings face intense grazing pressure and competition from invasive grasses. But the future also brings the new threat of climate change. Researchers have determined that the areas of the state where the climate is suitable for these oak species to grow will shift northward and could shrink to nearly half their current size as a result of global warming.
Camping, T. J., et.al. 2002. Change in Soil Quality Due to Grazing and Oak Tree Removal in California Blue Oak Woodlands. In: Standiford, Richard B., et al, tech. editor. Proceedings of the Fifth Symposium on Oak Woodlands: Oaks in California's Challenging Landscape. Gen. Tech. Rep. PSW-GTR-184, Albany, CA: Pacific Southwest Research Station, Forest Service, U.S. Department of Agriculture: 75-85. (552KB PDF)
Garrison, B.A., et.al. 2002. Age Structure and Growth of California Black Oak (Quercus kelloggii) in the Central Sierra Nevada, California In: Standiford, Richard B., et. al. Proceedings of the Fifth Symposium on Oak Woodlands: Oaks in California's Challenging Landscape. Gen. Tech. Rep. PSW-GTR-184, Albany, CA, Pacific Southwest Research Station, U.S. Forest Service, 665-679. (510KB PDF)
Jackson, R. D., and B. Allen-Diaz. 2002. Nitrogen Dynamics of Spring-fed Wetland Ecosystems of the Sierra Nevada Foothills Oak Woodland. In: Standiford, Richard B., et. al. Proceedings of the Fifth Symposium on Oak Woodlands: Oaks in California's Challenging Landscape. Gen. Tech. Rep. PSW-GTR-184, Albany, CA, Pacific Southwest Research Station, U.S. Forest Service, 119-129. (897KB PDF)
Keeley, J. E. 2002. Plant Diversity and Invasives in Blue Oak Savannas of the Southern Sierra Nevada. In: Standiford, Richard B., et. al. Proceedings of the Fifth Symposium on Oak Woodlands: Oaks in California's Challenging Landscape. Gen. Tech. Rep. PSW-GTR-184, Albany, CA, Pacific Southwest Research Station, U.S. Forest Service, 693-704. (560KB PDF)
Spero, James G. 2002. Development and Fire Trends in Oak Woodlands of the Northwestern Sierra Nevada Foothills. In: Standiford, Richard B., et. al. Proceedings of the Fifth Symposium on Oak Woodlands: Oaks in California's Challenging Landscape. Gen. Tech. Rep. PSW-GTR-184, Albany, CA: Pacific Southwest Research Station, U.S. Forest Service, 287-301. (773KB PDF)