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Non-Native Annual Plants- Acorn Challenge Number Two
When
Europeans arrived, they brought with them first to Mexico, and then to
California del Norte, a suite of aggressive, annual plants from the barnyards
of the northern and eastern Mediterranean. Only a few places in the world
have climates matching the Mediterranean, and California is one of them.
Finding a very similar environment, these grasses lucked out and just
took off across the landscape. We know that adobe bricks in the first
missions had very little of these annual, Mediterranean plants. By the
time the last missions were built, the bricks were packed with these weedy,
annual species. Maybe 300 years ago, and certainly now, most of the "golden,
rolling hills of California" are covered with barnyard weeds from
Spain.
These weedy annuals have a loose and fast life. As soon as it rains, often in the fall or early winter, leaves of filaree (Erodium spp.) start to cover the land. Once the soil warms, the annual grasses (Bromus diandrus, B. hordaceous, Avena fatua, A. barbada, Vulpia myuros) bolt up through the soil, fed by large seeds, full of carbohydrates. By May, the hills turn green with these annuals. With shallow roots, they rapidly exhaust the water in the top 6 inches of soil, move all nutrients from roots, stems and leaves into many large seeds, which fall by the millions. A square foot of California annual grassland can have 2,000 seeds of annual grasses and broad-leafed weeds. When things turn brown on the hillsides, the nutrient value of the annual grass parts that remain is so poor that cattle will starve without feed supplements.
It
is into this thicket that acorns of blue oak and valley oak are planted.
The incredibly fine, abundant roots of annual grasses rapidly remove the
soil water from the top few inches of soil, just where the tiny oak seedling
is starting. Even if you put the acorn in a pot, in a green house, with
high densities of annual grasses, blue oak acorns fail to grow and by
the second year, wither away and die.
All is not lost – there are places where the non-native grasses do not dominate so entirely; shaded places. If blue oaks are planted under shade cloth, they thrive. In three different environments at Hastings, researchers planted blue oaks into gopher-free environments with shade cloth, and the oaks grew to seedlings. Without gophers, and without shade, the blue oaks withered and died among the weedy grasses, just as they did in the greenhouse experiments.
Yet even, there may be a potential natural, rare event that might get the acorns through both gophers and weedy grasses. A late spring snowstorm arriving after the oak leaves have unfolded can be heavy enough to knock down many parental oak branches. Shade under the branches would minimize food for the gophers and provide a slightly cooler, wetter place for young oaks to grow. Then if you had a few years without ground fires, this might do the trick and suddenly we see a multitude of seedling or even sapling oak trees.