A walk in the Sierra foothills will take you through some of the loveliest native habitat in California - oak woodland. Here you will see huge stately trees, some of them hundreds of years old. Beneath their spreading canopies, you may find countless acorns scattered on the ground, or tiny oak seedlings poking through the soil. What you won't see many of are slender saplings.
These woodlands have adult oaks and baby oaks, but relatively few toddlers or teenagers. Translated into oak years, there is a paucity of trees between the ages of 5 and 60. And that means trouble for Californiaís native oaks. If seedlings are not surviving into adulthood to replace old oaks as they die, then we could lose our woodlands.
To deal with this alarming trend, ecologists at UC Davis are trying to understand why young oaks are dying. We tend to think that being eaten is a plantís biggest danger, and it's certainly true that plenty of things consume acorns and seedlings, including grasshoppers, rodents, deer, and in the past, humans. Indeed, acorns pounded into flour were the staple food of Native Americans in the region.
But along with the threat of becoming a meal, seedlings must also compete with nearby plants for resources in the soil, such as water and nitrogen. Focusing on one species of native tree, the blue oak, Kevin Rice, Doria Gordon and their colleagues in UCD's Ecology Graduate Group and the Department of Agronomy and Range Science have examined the ways in which an oak seedlingís neighbors might influence its survival and growth.
Professor Rice and his co-workers followed the fates of blue oak seedlings that were planted with two different species of neighbors. The two plants were similar in that neither was native to California, and both were annuals, but they differed in their root systems and therefore in how "thirsty" they were.
In boxes filled with soil from the Sierra foothills, researchers planted one batch of acorns with an annual grass that has a fibrous, spreading root system, good at soaking up moisture from the soil. They varied the amount of grass in the boxes, so that some neighborhoods were much more crowded than others.
They planted another batch of acorns with a herbaceous, low growing plant, known as a forb, with a long tap root that takes up soil water relatively slowly.
The resulting differences among the boxes in how fast the soil dried out (faster with grasses than with forbs) were reflected in differences among the oak seedlings in how well they fared. For example, in forb neighborhoods, both the roots and the shoots of young oaks grew faster and for a longer period into the summer, than they did in grass neighborhoods.
While the type of neighbor did not seem to affect how many acorns produced shoots (about half for both), the density of neighbors did. In the crowded grass neighborhood, only 20% of acorns sprouted.
Among the many factors that might influence the survival of oak seedlings, this study shows that competition for moisture from nearby plants is clearly important. The oak youth of today do not thrive in the crowded, hard-drinking neighborhoods of annual grasses; to be more specific, alien annual grasses.
These findings support the suggestion that the dying of our young blue oaks is related to the near total replacement of Californiaís native grasslands with foreign invaders.
Back in the days before the Gold Rush, the floors of our oak woodlands were covered with native perennial bunch grasses such as deer grass and blue wild rye. These perennial plants, with whom oak trees evolved, extract water from the soil far more gradually than do annuals, maintaining a relatively moist environment for oak seedlings. Then, in the mid 1800ís, the Europeans arrived and, well, there went the neighborhood. The annual grasses they brought with them had a much faster system of reproducing than the perennials, and the native vegetation lost its foothold. The neighborhoods of young growing oak trees changed forever.
It is too late for us to solve the oak problem by restoring Californiaís
native grasses. However, ecological studies like those from UCD,
can help us understand what makes oak seedlings thrive or die
in their present habitat, and enable us to create conditions in
which these uniquely Californian trees may start regenerating
once again.
Dept. of Anthropology, University of California, Davis, CA 95616
e-mail: kjstewart@ucdavis.edu