Vernal Pools in Oak Woodlands

Oaks ’n’ Folks – Volume 11, Issue 2 – September, 1996

Puddles or Unique Habitats?

Vernal pools are seasonal bodies of water that form in shallow depressions following winter rains. Because of an underlying hard pan layer, water may persist for several months providing a wetland oasis for many uniquely adapted species of plants and animals. More prevalent taxa such as waterfowl, frogs, salamanders, dragonflies, and numerous aquatic insects readily use the pools for feeding, breeding, and juvenile development.

Over tens of thousands of years, a number of highly specialized plants and macro-invertebrates such as fairy shrimp, have evolved the ability to exploit the flood/drought environment associated with vernal pools. Often,species abundance and composition in one pool is quite different from that of a neighboring pool. The environmental and biological factors responsible for these differences have helped to produce a highly specialized group of obligate vernal pool species found nowhere else in the world.

For over three decades scientists have been intensively studying vernal pools and have gained insights into the structure, function, and composition of these unique ecosystems. Spatially well-defined, vernal pools have contributed greatly to our understanding of the effects of habitat fragmentation on small, isolated populations. Long thought of as mud puddles, mosquito breeding sites, or simply ignored all together, vernal pools are now recognized as unique natural resources worthy of preservation.

Although most common on terrace soils bordering the east side of the Central Valley at the base of the Sierran foothills, vernal pools are scattered throughout the state on a variety of soils and land forms. Historically,vast stands of valley oak dominated the better developed vernal pool landscapes of the Central Valley, but most of these oaks and the associated pools were lost early on due to agricultural development and then later by urban expansion.Today, many intact pools are located throughout the oak woodlands of the North Coast Ranges and the lower elevation foothills of the Sierra Nevada,in sites traditionally grazed by livestock.

Located in the rugged terrain of the Mayacamas Mountains, the U.C. Hopland Research and Extension Center (HREC) has many vernal pools within its 5,300 acres of oak woodland, grassland, and chaparral. Twelve of the larger vernal pools at HREC were studied and found to vary considerably in land form, soil type, mean area and volume, and species composition (table 1). Researchers here are currently collecting baseline data on plant and aquatic invertebrates,as well as on soil characteristics and water chemistry. This information is essential for future experiments involving treatments and manipulations.For example, one current research project is evaluating the consequences of sheep grazing on plant and macro-invertebrate composition, and the potential benefits of grazing for controlling exotic annual grass encroachment. HoplandResearch and Extension Center provides a unique research opportunity to study vernal pool ecology and implement science-based management necessary to preserve the biological diversity of North Coast vernal pools. This type of applied ecological research will hopefully provide direction and guidance to landowners, managers and policy makers who are trying to balance human population growth with conservation goals and objectives.

Table 1. Summary data for HREC vernal pools
Number of pools > 100 square meters 12
Elevation 1,080 – 2,960 ft.
Area 180 – 3,069 square meters
Water Depth (5-30-96) 0 – 120 cm.
Percent Shade 0 – 40
Total Plant Species 105
Introduced Species 33
Vernal Pool Obligates 8

Kerry Heise
Adina Merenlender
Gregory A. Giusti

Integrated Hardwood Range Management Program

prepared and edited by Richard B. Standiford and Pamela Tinnin