Fire and riparian forest management: implications of Arundo donax on community resiliencE
A work crew takes on a dense stand of giant reed along San Antonio Creek.
Fire has consistently shaped and driven ecological processes for millions of years. We observe this first in charcoalified plants in the late Silurian, about 425 million years ago, which record some of the first forest fires on the planet. Fast forward to the early Cretaceous, 145 million years ago, continuous fire aided in the diversification of early angiosperms, which quickly adapted. More recently in the late Pliocene, around 3 million years ago, uplift in California established a mediterranean climate, with wet winters and dry summers. Oak woodlands and associated sclerophyllous vegetation began to dominate Southern California, including the establishment of chaparral and coastal sage scrub ecosystems following the last glacial maximum, 20,000 years ago.
Around the same time, stewardship of lands by indigenous peoples across the state, utilizing fire for proliferation of native plants, drastically increased the levels of diversity throughout the state. When we talk about fire, we talk about ecological succession. Native ecosystems are well prepared to follow fire, as they always have. It is the introduction of non-native species, combined with fire suppression, which has drastically impacted the cycle of disturbance in the landscape. Indigenous peoples use cultural fire to burn in mosaics, creating varied fire intensities—high, mixed, and low—each resulting in a unique post-fire landscape. This is where the most diversity occurs. After the Thomas Fire, the hillsides around Ojai revealed parts of its character that had been buried for so long. The soil was awakened and expressed in multitudes of Humboldt lily (Lilium humboldtii ssp. ocellatum), fire following campion (Silene multinervia), large-flowered phacelia (Phacelia grandiflora), chaparral snapdragon (Antirrhinum thompsonii), golden eardrops (Ehrendorferia chrysantha), and deerweed (Acmispon glaber).
As the charred hillsides lit up with color, riparian ecosystems painted a vastly different story. Columns of matchsticks, giant reed (Arundo donax), choked riparian zones and lined edges of the floodplains, encouraging the spread of fire. Did you know that giant reed’s flammability and height can facilitate the spread of embers for up to two miles in large wind events? As the plant matures and stems desiccate, stands of giant reed become riddled with dry material, and the fuel builds up fast. Moisture content within giant reed, especially during its dormancy through late fall and winter, increases fire susceptibility dramatically.
Strong scouring storm flows can dislodge and create large mats of giant reed. This can lead to flooding by damming channels and blocking flows at bridges and culverts. Giant reed can also colonize new areas when it washes downstream, spreading its negative impacts. Just north of Highway 33, flooding from piled-up arundo caused severe erosion on San Antonio Creek’s north bank.
Giant reed was the first species to regrow after the Thomas Fire in this stretch of San Antonio Creek, outcompeting native species, hindering their natural recovery, and forming a dense monoculture.
In comparison to an arroyo willow thicket, which has a relatively high moisture content and lower surface to volume ratio, giant reed acts as kindling. Not only does giant reed alter fuel loads and increase the intensity and spread of fires through the riparian zone, but the consequences of these fires are also a concern.
Traditionally, fire moves through these ecosystems at low intensity, allowing riparian vegetation to survive and recolonize post-fire. During high intensity fires encouraged by giant reed, surrounding plant communities suffer, resulting in destabilization of banks and upland areas.
Timing of these fire events are often followed by rainfall, which can completely wipe out large portions within the channel. Stands of giant reed, in areas that are supposed to be firebreaks, actively pose a major threat to the wildfire resilience of the Ojai Valley. Our work in collaboration with CAL FIRE, Pax Environmental and Ventura County Resource Conservation District addresses the urgent necessity to restore our watershed’s corridors, and replace stands of giant reed with lush riparian forest.
Martin Schenker, Restoration Field Crew Manager