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THE KEYSTONE ROLE OF OAKS IN PNW ECOSYSTEMS

Published September 10th, 2022 in Restoration

Oregon white oaks provide food and habitat for herbivores, acorn consumers, parasitic wasps, cavity nesters, epiphytes and a diverse suite of decomposers (modified from blm.gov/or/districts/salem/files/white_oak_guide.pdf).

By Ken Carloni

A couple of decades ago, a timber company clearcut all the conifers in a forest adjacent to a historic property in Yoncalla. While this was a shocking event for the stewards of that land, they were relieved to see that at least the centuries-old oaks on the property had been left standing. That small bit of solace evaporated when workers later arrived and began hacking the oak’s trunks with “hypo-hatchets”. With each slash of this highly efficient tool, a powerful silvicide was injected into the cambium below the bark. Within a few days, centuries of biological legacy were reduced to lifeless hulks.

Why did the timber company do this? To get rid of anything that would compete with the new Douglas fir seedlings destined to be the next harvest of 2X4s. Oaks were simply thought of as “trash trees” to be gotten rid of to make way for the next cash crop.

Oak-dominated habitats in the interior valleys of the Pacific Northwest are in steep decline caused by 175 years of conversion to housing subdivisions, timber plantations, grazing land, orchards, and vineyards. Estimates of remaining oak ecosystems in our region range from 5-15% of their pre-colonization extent with less than 1% of that land protected in parks and reserves. There is little monetary value in oak trees and there is always a more profitable use for their habitat.

But over the last few decades, ecologists have discovered that oaks are the linchpins that hold together a vast array of organisms in highly complex ecosystems. Far beyond their value to commerce and industry, the habitats dominated by these majestic giants support an astonishing number of organisms that would not survive otherwise. In the remainder of this article, we will explore the outsized role oaks play in maintaining the rich biodiversity that contributes significantly to the quality of life in our region, the landscape history that has brought us to this point, and ways you can help reverse the decline of oaks in Douglas County.

Makers, Takers, and Rakers: The Web of Life

All ecosystems are composed of interacting populations of diverse organisms that can be divided into three functional groups: Producers, Consumers, and Decomposers.

Producers include photosynthetic plants and bacteria – they are responsible for capturing light energy and converting it to the chemical bonds of complex organic molecules. Consumers comprise the herbivores that eat the producers and the predators that eat those plant eaters or other smaller predators. Decomposers include a myriad of organisms that digest the waste products and dead bodies of producers and consumers. This group is largely made up of bacteria, fungi, and small invertebrates such as earthworms, sow bugs, springtails, and millipedes that feed on decaying biomass.

Populations of producers, consumers, and decomposers interact in complex Food Webs where they compete for resources, avoid enemies, and cooperate with friends. Producers can be thought of as the “makers” in a food web, capturing energy and creating the resources that the whole system ultimately depends on. Consumers are the “takers”, exploiting other organisms as food to maintain their populations. Decomposers are the “rakers”, sifting through the waste and debris from other organisms to find bits of leftovers that they can use to complete their life histories.

Oaks Drive Biodiversity

This pencil sketch from Sept. 21, 1841 of the South Umpqua River by Henry Eld of the Emmons expedition documents a landscape fire in the background, an “orchard of oak trees” in the middle ground, and an Umpqua Indian in the foreground. Notice that the landscape is dominated by meadow and savanna unlike current conditions.

Most ecosystems contain a few species that have an inordinately large effect on the biological diversity found in their habitats. These are referred to as keystone species. For example, top predators such as wolves control the populations of large grazing animals that if left unchecked would overgraze the plants that provide food and habitat for birds and smaller mammals. These plants also shade streams, maintaining the cold water essential for the survival of salmon, trout, and the myriad small invertebrates that they feed on. Beavers serve a keystone role as ecosystem engineers, building dams that create wetlands which regulate stream flows and support numerous plant and animal species that would not otherwise survive in that habitat.

Oaks have a keystone role in enriching the biodiversity of their habitats by acting as a foundation species. This type of habitat-forming organism creates the structural complexity in an ecosystem that leads directly to biological complexity. It is estimated that oak trees provide food and habitat for as many as 4,000 different types of insects worldwide – more than any other genus of tree. The Oregon white oak (Quercus garryana) is the most abundant oak species in the Umpqua Valley and provides food and habitat for several hundred insect species and over 100 wildlife species, including birds, small mammals, reptiles, and amphibians. They maintain this enormous diversity by providing many resources to the other members of their ecosystems including:

ACORNS. The highly nutritious nuts of oak trees are produced in great abundance and are an important and often essential food source for birds, rodents, deer, and insects. These animals are in turn prey for higher-level consumers, so the energy and nutrients derived from acorns will eventually make their way into the bodies of top predators. Acorns were also an important staple for many indigenous peoples in the Pacific Northwest and beyond.

PHOTOSYNTHESIS. The leaves and twigs of oaks provide food for many species of leaf-eating and sap-tapping insects. Oaks provide food for more species of moths and butterflies than any other native plant. At least six species of gall wasps make homes for their developing larvae in specialized growths (galls) they induce on leaves and twigs.  Many of these insects are important pollinators that other flowering plants in the ecosystem depend on.

HABITAT STRUCTURE. Oaks have a highly branched architecture and the ability to heal damage from broken limbs resulting in the creation of cavities within living trees. These are an essential resource for cavity-nesting animals such as woodpeckers that feed on insects attacking other parts of the tree, and rodents that disperse acorns. Deep fissures in the bark provide cover for lizards and predatory insects, and roosting habitat for small birds and bats. All of these animals help defend the tree against high levels of insect damage. Large branches and rough bark also provide substrate for dozens of species of mosses and lichens, providing cover for even more spiders and insect predators. The dense canopy casts deep shade, helping to maintain soil moisture and cooling nearby streams.

NUTRIENT CYCLING. The deep layers of leaf litter produced from leaves that drop every fall create microhabitats for a wide variety of arthropods, worms, small mammals, reptiles, and amphibians. The decomposers among these organisms break down this decaying biomass, releasing nutrients to surrounding soils to be recycled back into living organisms. Oak roots also provide nutrients to the mycorrhizal fungi that “infect” them. In return, the filaments of these fungi greatly expand the absorptive surface of this mutualistic partnership and bring large amounts of water and dissolved minerals into the root system. The fruiting bodies (mushrooms) of these fungi provide food for mammals that then disperse fungal spores to other trees in their droppings.

LONG-TERM CARBON STORAGE. Although not a direct benefit to the immediate habitat, oaks take carbon dioxide (the main greenhouse gas) out of the atmosphere and store it in their roots, trunks, and branches for centuries. Preserving existing oaks and restoring degraded oak habitats can have a significant mitigating impact on the climate crisis.

Restoring Oak Habitats: Humans, Oaks, and Fire

Over the last two decades, the impact of Native American burning practices on pre-European fire regimes has been well documented in many regions of North America, and southwestern Oregon is no exception. The Rogue, Umpqua, and Kalapuya peoples systematically burned large expanses of grasslands in our region. The fire-resistant bark of oaks allowed more of them to survive annual grass fires set by indigenous people compared to most conifers (the pines being an exception here). In fact, early settlers to the savannas of western Oregon described coming upon “orchards of oak trees”, and the open lands were full of edible bulbs, fruits, and game.

The late Cow Creek tribal elder Chuck Jackson was once asked: With so many oaks around, why are there so few (none?) bedrock mortars in southwest Oregon compared to California? His response was that shelling, grinding, and leaching the tannins out of acorns took a lot of work, and after all that, they didn’t taste very good. But the deer ate a lot of acorns to fatten up for the winter, and they tasted a lot better. So no matter how the energy of acorns made it up the food chain and into the stomachs of local tribespeople, oaks played an important role in their lives.

In an ecological sense, Native Americans were keystone ecosystem engineers in their habitats — by controlling local fire regimes, indigenous managers had as robust an impact on dryland biodiversity as beavers did on wetlands through their control of hydrological regimes.

But in the absence of landscape fire, the more fire-intolerant conifer species will nearly always create dense stands that eventually overtop and shade out legacy oaks and inhibit the survival of their seedlings. Those of you who have been reading this column in the 100 Valleys for the last few years will recall that a number of our members and board members are actively involved in oak habitat restoration in Douglas County.

We Need Oaks and Oaks Need Us

The growing appreciation of the keystone role oaks play in maintaining native biodiversity, and alarm over their rapid decline, has led a coalition of state and federal agencies along with landowners and NGOs (including Umpqua Watersheds) to come together to form the Umpqua Oak Partnership (UOP). This dedicated group of professionals has been working to educate the public on the value of preserving and maintaining our remaining oak habitats for the last three years.

Do you own or manage acreage with a significant oak component? If so, the UOP can help with grants and technical assistance for restoring these critically important habitats. For more information, visit pacificbirds.org/umpqua-oaks-partnership/ or contact UOP coordinator Steve Denney (stevedenn@mydfn.net) or OSU Extension Forester Alicia Christiansen (Alicia.Christiansen@oregonstate.edu).

 

 

 

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