Plant communities are the base of support for myriad forms of wildlife. Each animal species has its own requirements for food, cover, and nesting materials. The loss of a plant that may seem insignificant in itself can disrupt the life cycles of other species and even put their continued existence at risk.
For this reason, vegetation management is a cornerstone in maintaining species diversity, a primary goal for the Open Space Authority. By using geographic information system (GIS) technology staff members can organize, display and interpret large amounts of data about each property OSA manages. This knowledge becomes a powerful tool in dealing with the forces that put plant communities and especially rare, threatened and endangered species at risk.
The ecosystems of the Santa Clara Valley began to change with the introduction of ranching and agriculture in the nineteenth century. Logging in the mountains, sedimentation in the county’s creeks and fill in the baylands all contributed to the disruption of natural communities. But it was the surge in urbanization beginning in the 1950s that had the largest impact.
The transformation of the valley from agriculture to industry took only a few decades. The orchard economy reached its peak in the early 1940s when 132,000 acres supported 8 million fruit trees. The city of San Jose had around 75,000 residents and the population of the county was less than 200,000.
By 1970 there were fewer than 20,000 acres of tree crops. Five of the nation’s seven largest semiconductor firms were located in the valley. The City of San Jose had grown to 135 square miles and a population of 460,000. There were over a million people living in the county.
The soil constituting much of the land converted to urban uses was unusually fertile and well watered. Whether the acreage supported natural plant communities or agricultural crops, burying it under concrete and asphalt represented a significant resource loss.
Because some native plants have a limited distribution or narrow soil and climate requirements, they are particularly vulnerable to destruction of their environment. According to the U.S. Fish & Wildlife Service, Santa Clara County is home to eight endangered plant species, four of which may have already been eliminated from the area, and 48 species of concern to the Service or of local conservation concern.
Invasive plants are a threat to every ecosystem in Santa Clara County. As they spread to natural areas they dominate and often displace native species. They can also affect native landscapes at the ecosystem level, altering soil chemistry, water availability, the frequency and intensity of fires and the natural patterns of plant succession.
Nonnative plants have become so well established in California that total eradication is unlikely. Just one pest species, yellow starthistle (Centaurea solstitialis), has overrun an estimated 14 million acres statewide. Addressing this problem is recognized as a long-term challenge. Strategies are focused on preserving biodiversity and ecosystem balance rather than simply eliminating noxious plants. The Open Space Authority employs a number of tools in its program of integrated weed management.
One of the most fundamental is the same tactic gardeners use: pulling weeds by hand. In the case of larger, well-rooted specimens, the use of a wrench comes in handy. Each year field staff work with volunteers to uproot French broom (Genista monspessulana) from riparian habitats along Uvas and Llagas creeks. Mowing and cutting are mechanical means of removing unwanted plants.
Managed grazing is successful on grasslands OSA manages in the east foothills, on Coyote Ridge and on Rancho Cañada del Oro Open Space Preserve. Because cattle prefer nitrogen-rich invasive grasses like oat grass and brome, their grazing behavior creates an opening for native species to mature. The number of cattle on a property, the length of their stay and the timing of their presence are all carefully planned and monitored.
Prescribed fire is another method that is successful in removing invasive plants from grasslands. Fires can be timed to prevent the seeds of pest species from maturing. Burning also removes accumulated dried plant material which stimulates native plant regeneration, particularly of native bunch grasses. The diversity of wildflowers and their foothold among the other plants is also improved by prescribed fire.
The application of pesticides and the use of biological controls, typically insects, are other means of removing invasive plants. Weevils, six species of beetles and flies have been used in the western United States for several decades to contain weed infestations.
One strategy for restoring a native plant mix in a given location is to combine replanting with removal techniques. For example, a grassland that has been overrun by nonnative species can be thinned of its weeds through a process of cutting, burning and pesticides. This opens the way for reseeding with native bunch grasses such as purple needlegrass (Nassella pulchra). It also creates an environment more favorable for native wildflowers.
Planting native seedlings such as coast live oak (Quercus agrifolia) and blue oak (Quercus douglasii) is another way to tilt the vegetation mix of an ecosystem toward its natural order. In oak savannas naturally occurring seedlings can be protected by fencing or other guards from damage by grazing animals. Creating a more age-diverse stand of oaks extends the life and health of the entire ecosystem.
Human activity can lead to a number of negative impacts on plant communities, sometime with the best intent in mind. Hikers, campers and others who enjoy the outdoors can, by their sheer numbers, wear down a habitat. Cattle can have the same effect, especially if they are on a property too long, at the wrong time of the year or are free to range among sensitive plants or wetlands.
Other forces beyond human causes and human control also affect the composition and health of grasslands, forests and wetlands. Some of these conditions, such as prolonged drought and severe flooding, appear to be made more intense and more unpredictable by climate change. The timing of plant life cycles and the elevations at which plants occur also seem to be changing due to patterns of global warming.