Ecological dynamics

The valley oak (Quercus lobata) woodlands are a rapidly disappearing ecosystem in California. Valley oaks are common to higher floodplains along rivers and streams, often associated with riparian forests dominated by Fremont cottonwood (Populus fremontii). These areas were and are known to be extremely fertile land for agriculture crops and grazing lands, thus, much of the land suitable for valley oak regeneration has been lost to both habitatdestruction and soil moisture availability (Meyer 2002). The valley oak is one of the largest oaks in North America that can grow trunks up to 6 or 7 feet in diameter. They are deciduous trees that require large amounts of water at rooting depth and rich soil created by spring flooding (Pavlick et al 1993). Natural regeneration of these trees has been steadily declining, due primarily, to the loss of habitat and the removal of natural yearly flooding events (Sork et al 2002). Extensive research has been done to determine what can be done to restore valley oaks to riparian woodlands, however many have found that without major changes to current land uses, there is little chance of valley oak restoration success. Why the valley oaks are disappearing from Pinnacles National Monument is similar, if not the same, as the other valley oak habitats across the state.

Soil moisture, and in turn, soil texture, seem to be the major components in successful seedling germination and establishment for valley oaks (Adams et al 1992). The valley oak needs deep soil that is able to retain soil moisture from spring flood events, into the growing season, in order to ensure survival through the hot temperatures of summer. Flooding is especially important for valley oaks, since they prefer the higher parts of the floodplain, where the water table is further down so as not to saturate the roots, but still offers the soil moisture during the key part of the growing season (Griggs and Golet 2002). This also explains why soil texture is an important component as well. Loamy soil textures have a higher water holding capacity, and will hold water later into the year, whereas sandy soil textures have larger pore spaces, and allow water to flow through rather quickly, making the water-holding capacity very low. Thus, making floodplains with sandy soil textures a more difficult growing medium for new valley oak seedlings than loamy soil textures (Meyer 2002).

There are other factors that could play additional roles in the lack of regeneration of valley oaks in Pinnacles National Monument including; rodent and deer predation of seedlings, lack of open niche space for new seedlings to utilize, and shading created by the older valley oak canopy. Valley oak woodlands that are highly utilized by deer and rodents, and in Pinnacles in particular, feral pigs, will have a much more difficult time regenerating due to predation of their seedlings in the first year. It has been speculated in the research that this is also a factor that can be solved by replacing the natural flooding regimes because the rodents drown, thus reducing the number of seedling predators significantly (Meyer 2002).

Most valley oak woodlands that still remain throughout California are filled with tall, old trees, some 150 years old with an annual grassland understory. These annual grasses, such as bromes and wild oats, create problems for new oak seedlings because they tie up all the available nutrients and water, leaving nothing left for the oak seedlings to begin germination (Adams et al 1992). With the addition of the shade created by the large canopy of the oaks, the seedlings have lost virtually any chance for germination or survival (Griggs and Golet 2002).

The valley oaks in Pinnacles National Monument are more than likely being affected by all of these circumstances, which might explain why there is currently no regeneration of the valley oaks. The soils found throughout most of the park, and in particular, the valley oak woodlands along Chalone Creek, are from a granitic parent material, and thus are primarily course sandy loams and course sands. Because water-holding capacity in these soils is low, the spring flooding of Chalone Creek is necessary to assist in adding water to the system every year for the riparian species. Chalone Creek was probably a much larger body of water a hundred years ago, before the dam on Bear Creek (which flows into Chalone Creek) was constructed and other human land management occurred. Without the higher levels of flow every year, the floodplain for Chalone Creek in the valley oak woodlands has gotten more and more narrow, restricting the width of the riparian vegetation, and thus much of the area where the valley oaks still persist has become more upland influenced with annual grasses and buckwheat (Eriogonum fasiculatum). Unfortunately without restoring some of the natural flows to Chalone Creek, it is likely there will continue to be little to no regeneration of valley oaks in this park. With the rapid drainage of water through the sandy soils, in addition to a decreased floodplain, an increase in annual grass understory, and the increased shade created by the older remaining valley oaks, this valley oak woodland has become significantly altered.

Restoration of this ecosystem would be possible; however it would be costly and require fairly labor-intensive strategies. Without replacing the natural disturbance regime, it would be necessary to manually plant valley oak seedlings with an auger and protect them from predation either with wire-mesh cages or tubing, although tubing does constrict growth (Adams et al 1992). They would also require some sort of irrigation or additional watering, due to the sandy soil textures in the Park, in order to ensure they live through the hot summer season in the Central Valley (Griggs and Golet 2002).