Throughout Sonoma County, homes and streets are graced by many different species of trees. City streets and homes are shaded by large London planes and decorated with crape myrtle and beautiful fruit trees.
Now that the drought is officially accepted in California, what will happen to the trees if we stop watering? What are the best trees to plant for drought conditions? How can we best manage trees in the landscape and orchard?
When I am asked these questions, as a certified arborist, I consider the kind of tree (species), the soil and root conditions and location.
Trees naturally store water. They retain water in all the cells of their structures: roots, trunk, branches and leaves. In dry periods, trees reveal many survival mechanisms. The baobab tree in Africa has almost no branches and a trunk that swells and stores water. Some trees such as the California buckeye drop their leaves when soils become dry.
Water requirements for trees vary by the species, dependent on where the specific tree evolved. The tree’s requirements do not change when the tree is transported out of its area and planted in a different climate. Trees that originated on the east coast of the United States where the water comes from the sky in the summer, like red maple, still need water in the summer in Sonoma County. Trees that originated in the desert such as acacia and African sumac naturally require less water. Other trees evolving in the Mediterranean basin, such as olive trees, require little or no water.
A curious fact is that coast redwood trees harvest water from fog. In their natural habitat, fog supplies one third of their water.
Roots of trees are roughly divided into surface “feeder” roots and tap roots that penetrate deeply into the earth. Roots grow horizontally out beyond the “drip line,” the imaginary line descending from the edge of the leaf canopy. Trees planted in sidewalk strips grow their roots under the sidewalk and into the garden.
California natives that do well in the summer, such as coast live oak, are able to survive because they have deep tap roots. In the summer, their surface roots become dormant and they depend on their deep roots.
While some California natives are adapted to summer dry conditions by going dormant in extended dry weather, other California natives need water. Trees such as sycamore and poplar naturally grow near streams and lakes where the roots are in wet soil.
Water lies close to the surface in parts of Sonoma County. Parts of the valleys were originally marshlands. Hills surrounding the valleys are watersheds for streams and rivers. Trees in Petaluma and Santa Rosa might have their roots in water either from streams or from a water table not far beneath the surface.
Trees are part of an ecosystem; they provide living creatures with many benefits. They cool our streets and houses. They provide food and shelter for animals and insects as well as an environment for life in the soil. When it does rain, leaves and branches reduce the force of the water and allow it to infiltrate gradually into the soil, reducing runoff and erosion, and recharging the water table.
Watering wisely and saving trees are not mutually exclusive. But to do so, one needs to consider the species, soil conditions and location.