Oak Woodlands in the Elder Creek Watershed
The majority of our county was once dense blue oak woodland, which accounts for so much of the rural aesthetic and also serves as habitat and food source for most animal species prized for hunting or observing. Tehama County still has more blue oaks than any other county in the state, but much of the former woodlands have been clear-cut or severely thinned, as the aerial photo below shows.
Valley oaks, which live in the moister soils along creeks and rivers, have been reduced to only about 2% of their former population and area[i], making the future for this ancient species threatened. Likewise, all other native riparian trees have seen similar levels of reduction, as most of the valley oak woodlands have been converted to mono-crop agriculture or hay fields. For more on the former richness of this ecosystem, see our Indigenous Sacramento Valley page in our History section.
While there are still significant stands of blue oak woodlands with normal stand density (see below) in our watershed, there are also plenty of areas where the density is poor or the trees have been cleared altogether. In fact, two large ranches that border Elder Creek, totaling several thousand acres, have been removing these trees at an alarming rate in recent years. Locally, and statewide, regeneration of blue oaks has been poor to abysmal since the early 1900’s. The land is not recovering from the removal of its keystone species, and cutting most or all oaks from one area only accelerates soil loss, aridification, nutrient loss and diminishing wildlife.
Some of our neighbors have begun working to bring the issue before the Tehama County voters and want to ban the cutting of native oaks in the Upland Ag zoned lands. For more information, and to support this effort, contact us to add your name to the list.
California Oaks did a survey of oak woodlands in the state and used aerial photography to classify stand “density.” Density is simply the percentage of the land area, as seen from above, that is covered with live tree crowns. It is very important for determining habitat value of forest and woodland areas, so each stand observed was classified when possible.
Most of these forests are in sparse to medium density stands: 32.8% had sparse (10-25% by area) crown cover, 30.4% had poor (25-39%) crown cover, 29.7% had normal (40-59%), and only 6.9% had “good” (60% crown cover or greater). Relatedly, tree size is important. Larger creatures require larger cavities and larger nesting cavities require larger trees. Where possible woodlands were classified as to consisting of “Large” or “Small” trees. Of the 10.4 million acres so classified 11.2 percent were “Large” trees while 88.2% percent were “Small” (as are most blue oaks).[ii]
Threat of Development
California Oaks’ study also states that more than 300,000 acres of oak woodland are threatened with development by 2040 in a region including sixteen inland counties from Tehama to Sacramento. Statewide, 1 million acres of oak woodland is slated to succumb to development over the next 30 years and 750,000 more acres will be threatened. That’s on top of the one million acres of oak woodland already lost to development since 1950.
While logging oak woodlands for firewood has been fairly common practice since the arrival of Euro-Americans, it throws a wrench into ecological integrity, the result of eons of evolution of a certain place. A complex, old-growth biological community is removed and the land tries to recover from the trauma. Non-native pioneer species like medusa head and yellow star thistle proliferate for decades, making the land rather unproductive on all counts. In many instances, subdivisions or ranchette development follows the clear-cutting, eliminating the possibility of unfragmented woodlands.
We see that a wise culture would maintain its oak woodlands and match its own population to the carrying capacity of these lands.
Oaks and Food
Finally, oaks provide us with acorns, which are a fantastic staple food source that requires no fertilizer, no irrigation, no industrial processing, no refrigeration and no intensive management. See our Water-wise Food Choices page for more on this. These acorns also are a significant food for the animals that provide the foundation of a sustainable human diet in this region: feral pigs, deer, and possibly domesticated sheep, cattle and goats.
The Many Benefits of Woodlands
Trees, and plants in general, affect the water cycle significantly:
• their canopies intercept a proportion of precipitation, which is then evaporated back to the atmosphere (canopy interception)
• their litter, stems and trunks slow down surface runoff
• their roots create macropores – large conduits – in the soil that increase infiltration of water and increases soil carbon, the foundation of humus
• they contribute to terrestrial evaporation and reduce excessive soil moisture via transpiration
• their litter and other organic residue change soil properties that increase the capacity of soil to store water
As a result, the presence or absence of trees can change the quantity of water on the surface, in the soil or groundwater, or in the atmosphere in that locale. This in turn changes erosion rates and the availability of water for the biological communities they support.[i] With very few if any exceptions, the more trees, the denser the canopy, the more benefits to the hydrologic cycle.
Trees, particularly dense woodlands and forests, provide immeasurable benefits to all other life, of which we humans are a part, and on which we depend. Aside from the hydrologic benefits noted above and detailed below, oaks provide food and habitat for more than 300 species.[ii] Some of those species, like deer, quail, feral pigs and turkeys, are high quality protein sources for a truly sustainable human diet in this region. And as we look at the smaller members of the food web, we find that an astonishing 5000 species of invertebrates and ten to one hundred million individual invertebrates may be present in a single acre of healthy oak woodland. [Source: Oak Woodland Invertebrates: The Little Things Matter/UC ANR publication 21598] As the food web’s foundation disappears, so too does complexity, which results in loss of diversity, which eventually leads to desertification if the assaults to the community don’t end.[iii]
Trees and the Water Cycle: Details
The role of trees in the hydrological cycle is perhaps the most significant reason to preserve and enhance the remaining oak woodlands in this region. Forests serve in the water cycle in two critical ways: transpiration and water storage. Plants and trees possess stomata, cells on the underside of their leaves that release water vapor from the plant into the atmosphere. The transpiration process begins when trees draw water from the soil through their roots and transport it upward in vessels—xylem carries water and nutrients upward, phloem distributes water and nutrients throughout the plant. Trees, depending on size and species, transpire from 5,000 gallons (18,921 l) to almost 50,000 gallons (189,210 l) of water per year. Warm weather, low relative humidity and wind increase the transpiration rate. Soil also affects water transpiration. When soil moisture levels are low, trees slow their transpiration rate to conserve water. Global warming has exerted a critical effect on forest health because it increases the incidence and severity of droughts worldwide, and in doing so it accelerates the death of drought-vulnerable trees.
Trees’ second major hydrologic role involves their capacity to store water. Trees act as a watershed by absorbing water during floods and storing and slowly releasing water in times of low rainfall. Surface waters, groundwaters (or aquifers), and plant life comprise the earth’s total watershed. Part of trees’ role in the watershed involves regulation of the water table, the area underground where water has completely filled the spaces between rocks and soil particles.[iv] When roots are in contact with soils that vary in moisture content, they may act as conduits for water redistribution throughout the soil. The process of water transport from deep to shallow soil layers through roots, termed hydraulic lift, has been demonstrated in a large number plant species, including grasses and cacti as well as shrubs and trees. It occurs horizontally as well as vertically, so the more general term ‘hydraulic redistribution’ (HR) is now used. Downward HR can enhance the rate of recharge of deeper soil layers following rainfall events that are not sufficient to saturate the rooting zone. HR is most common among deeply rooted species, but can occur even in shallow-rooted species. The process generally occurs only at night or during periods of heavy cloud cover when stomata are closed, and when there are great differences in soil moisture content. Roots then provide a low-resistance pathway for water flow from areas of high to low soil moisture. When the water potential in the shallow roots reaches a certain threshold above the water potential in the surrounding soil (this threshold varies among species), water begins to exude from the roots and into the soil.
The amount of water moved by HR is relatively small but may have a significant impact on the rate of soil drying. HR can replace 40–80 % of the daily water used from those upper layers. One research team found that HR delayed soils in coniferous forests from reaching the point at which no water can be obtained from that soil layer by an additional 16–21 days, depending on the local ecology. [Source: How Trees Influence the Hydrological Cycle in Forest Ecosystems, B.J. Bond, F.C. Meinzer and J.R. Brooks]
The details above mean that trees, particularly forests and dense woodlands, increase the relative air humidity around and downwind of themselves, thereby increasing the likelihood of precipitation. Much of the Middle East and parts of north Africa that were once forests are now deserts. Clearing the land of trees is the primary culprit of this desertification and decline in rainfall.
Since every living being needs water and cannot exist without it for more than a few days, maintaining a clean supply is paramount to maintaining life as we know it. Trees play a vital role in maintaining a continuous supply of available water, and therefore are critical to maintaining biodiversity, the foundation of a stable landbase.[v] This means that a human culture that doesn’t prioritize a healthy water cycle and intact woodlands and forests can never be sustainable.
So when it comes to native trees, the more the better. While many of us were taught that trees suck water from the ground, causing waterways to get drier, a more accurate view is that–with a few rare exceptions–they benefit every facet of a local water cycle.
Another common misperception is that the majority of tree roots are deep in the ground and, therefore, that it is okay to disturb the roots just outside a tree’s drip line (outer edge of its canopy). In reality, the majority of a tree’s roots are in the topsoil, where most of the nutrients reside. And the roots extend outward at least twice the radius of the canopy, meaning all significant disturbances and compaction just outside the drip line actually hurt the tree’s ability to feed itself and to share the water it has pulled up from the subsoil. Beyond that, science now demonstrates that trees take oxygen not only from the air, but also through their roots. Compacting soil then compromises a tree’s ability to breathe.
Trees and Air
Trees and forests also help produce the air we breathe and their depopulation is a significant cause of global warming. While they grow, trees absorb carbon dioxide (CO2) and produce oxygen. Even as trees reach old age, they continue to store the CO2 they have absorbed during the growth process. The microorganisms living in forest soil also collect and store carbon. An acre of trees can grow about 2 tons of wood per year. This same acre removes 3 tons of carbon dioxide and produces 2.5 tons of oxygen per year. A person uses less than one pound of oxygen per day, or about 278 pounds per year. So every acre of forest produces enough oxygen for 18 people. At the same time, an acre of forest removes 13 tons of dust from the surrounding environment and releases 8,000 gallons of water into the air. This results in forest areas being cooler and cleaner than surrounding areas. [vi] It is no wonder forests often make people feel better!