Hardwoods Need Our Protection
We Californians love our oaks. Oak woodlands include some of the most beautiful forestlands in the state. There are 20 species of native oaks found throughout California on approximately 20 million acres in widely different areas: the central valley, lower foothills, mixed coniferous zone and coastal mountains.
Not only are oak woodlands beautiful, they are surprisingly productive communities. More than 330 species of animals use oak habitats for some part of the year. Oaks are found in extremely diverse habitats; about 50 habitat types have been identified.
As with almost all natural habitats in California, oak woodlands currently face a number of threats and uncertainties. These include concerns about poor regeneration, competition from invasive species, native and introduced pests, habitat loss, changes in land use, threats from fire, and climate changes.
Rule #1: Protect the Roots
Young oaks are more flexible than older ones. While young oaks can generally adapt and survive under a variety of conditions, mature oaks are extremely sensitive to change and can be weakened or killed by any number of activities.
This is because of their elaborate root system, developed over decades, that transports moisture and nutrients and provides structural support for the growing tree. Any activity that damages the roots can compromise a mature tree.
The root system begins in the acorn. Most of an acorn’s energy goes into the fast-growing tap root that probes deep underground to seek reliable moisture. Tap root growth continues for the first few years after which the tree’s resources can finally go into above-ground and leaf growth.
Lateral roots have a different job. They spread out horizontally in the top 2 or 3 feet of soil and provide structural support for the tree. They also have fine roots that absorb moisture and nutrients. As the oak matures, it sends out deep vertical roots from the laterals which find deeper soil moisture as well as add stability.
With all of these roots in place the mature oak becomes quite set in its ways. Any activity (e.g., grading, filling, trenching, paving) that removes roots, compacts the soil, or changes moisture availability may affect the permeability of the soil and the tree’s ability to exchange gas and moisture, and thus harm the tree. Poor drainage can smother roots and promote fungi that cause crown and root rot.
When choosing species to plant near oaks, remember that oaks are adapted to California’s hot dry summers and cannot tolerate excess moisture during the dry season. Plant only drought-tolerant plants that require no summer water, and even those should be no closer than 6 feet from the base of the tree. Do not plant any vegetation that needs summer irrigation—those plants have thick roots that can inhibit the oak’s air and water exchange. Any irrigation should be done outside of the Root Protection Zone (RPZ), an area about 1.5 times larger than the dripline.
Many of the precautions to protect oaks are actually ways to protect the root system. Keep this in mind as you make decisions to care for your property and trees.