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Home » News/Events » The Journal » Vol. 13 Issue 3 » Lessons from Fresno: Bringing Back the Native Forest Understory

Lessons from Fresno: Bringing Back the Native Forest Understory

  • Heyo Tjarks, Central Valley Restoration Ecologist

Non-native annual weeds dominated the restoration site prior to the start of our project.

The neighboring non-native grassland (to the left in this photo) is brown and lifeless while the recovering native understory on our project site (to the right in this photo) supplies food for the base of the riparian forest food web. (Photo from June 6th 2016.)

The ecological function of forests across the globe is founded in the base of its food web – the herbs and grasses that provide food and cover for insects and small wildlife. We call this critically important forest component the “understory” but perhaps it should be called the “foundation”.

Over the past 150 years, California has lost almost all of its native riparian forests due to the conversion into agricultural operations including orchards, row cropping, and cattle grazing. In orchards and row cropping, the native forests were typically clear cut and routine repeated weed control (e.g. tilling and herbicide applications) was used to eliminate the foundational grasses and herbs. In areas that are grazed however, even if the trees were harvested and seedlings were destroyed by grazing, the understory may not have been completely destroyed. Although most grazed areas in California are now dominated by non-native annual grasses, we’ve been finding the relics of the historical understories at a few of our projects.

One in particular is the Riverbottom Park and Schneider Property restoration site along the San Joaquin River in Fresno and Madera Counties. The project area straddles the ailing San Joaquin River in the northwest corner of Fresno and is surrounded by urban development and orchards. Early gravel mining activities have altered the topography of the lowest portion of the flood plain closest to the river, however the majority of the project site has never been tilled. The site has a history of cattle grazing, including up until last year when we began the 100+ acre restoration project. As we began the project, the area was dominated by a mixture of non-native, annual weeds including ripgut brome (Bromus diandrus), Italian rye (Festuca perennis), Wild oat (Avena fatua), Bermuda grass (Cynodon dactylon), yellow starthistle (Centaurea solstitialis), and black mustard (Brassica nigra). Although a handful of native herbaceous species were sparsely found throughout the site, the annual non-natives were outcompeting the natives by absorbing soil moisture quickly in early spring, then dying back as thatch that shades native seedlings and keeps them from growing.

In order to create optimal conditions for the relic native understory, River Partners lightly tilled the project area in September 2015. Turning the thatch over into the first few inches of soil at the appropriate time of year created a bare soil surface that sped up the decomposition of the thatch. Over the next three months as the drip-irrigation system was being installed, California finally started receiving some much-need winter rains. However, with the rains came the germination of all the annual non-native vegetation. In order to keep these plants from reasserting their strangle hold on the project site, an herbicide application was used to control the non-natives shortly after germination.

Doveweed (Croton setigerus) is one of the native plants that provides support for invertebrates during the late summer when all of the non-native annual grasses are long-gone. Over time, the planted forest will grow up around this plant, and the doveweed will give way to more shade-tolerant natives.

By aggressively controlling the first flush of non-native plant growth, we gave the native understory an opportunity to become established. Little did we expect that it would establish as well as it did. By early spring, the site was dominated by a handful of native perennial plants including dove weed (Croton setigerus), vinegar weed (Trichostema lanceolatum), American wild carrot (Daucus pusillus), Kellog’s tarweed (Diandra kelloggii), popcorn flower (Plagiobothyrs leptocladus), and jimson weed (Datura wrightii), to name a few. Although a few herbicide spot treatments were need to control a handful of later germinating perennial weeds, the native understory effectively took over. After the neighboring property’s non-native annual grasses were dead and dry in May, the native understory remained green and flowered throughout the summer – even through the weeks on end that were over 100 degrees.

Understories serve people as well as the threatened wildlife that depends on riparian forests across California. Well-established native understories can protect the soil from erosion especially during floods, and can resist the re-invasion of non-native weeds which can lower maintenance costs for land managers. The recognized ecological importance that the native understory plays for wildlife became abundantly clear this summer. Although few signs of life were seen in the neighboring non-native grasslands, the native understory at our project site was providing food and habitat structure to numerous wildlife species throughout the summer. Pollinators were plentiful throughout the site; seed eating birds such as dove, finches, and sparrows foraged in numbers; and native lizards and snakes were able to bask in the sunny open spaces and forage for small mammals that were also taking advantage of the copious amount of seeds.

Over the years, we here at River Partners have been pioneering different approaches to installing native understories, often marrying agricultural techniques with ecological restoration approaches to gain efficiency and performance. We have noticed that lands which have only been grazed, and never tilled for crop, tend to still harbor relic understory vegetation. Rather than undertaking the timely and more expensive approach of seeding a new understory plant community across the entire site, well-timed and targeted weed control efforts can control the non-native annuals, giving room for the natives to thrive again.

The above article originally appeared in the Fall 2016 issue of the River Partners Journal.