The Thomas fire burned 440 square miles - roughly the size of the City of Los Angeles – and broke the record as the largest California wildfire in recorded history. In its wake it left scorched and barren hillsides. Seemingly before any-one could react, disaster struck again, and heavy rainfall sent floods and mudslides down the exposed slopes.
The question on everyone’s mind seemed to be: what can we do to help keep this from happening? Botanists and ecologists have been seeking answers to this question over the past 50 years, and our understanding of fire ecology has come a long way. The short answer is to let nature take its course and allow for native plants to re-sprout while weeding out any arising invasive plants and – if necessary – control erosion through physical means such as net-ting and wattles.
This isn’t always an adequate answer for people with a strong desire to actively help. Since the fire and mudslides many groups have sprouted up to encourage and empower people to spread seeds throughout the burn areas. This practice is unnecessary and potentially harmful to the ecosystem. In the past, agencies would rush in after a fire to spread seeds along burnt hillsides to revegetate slopes in an attempt to prevent runoff and erosion.
We learned the hard way that seeding with non-natives (most often ryegrass) caused more problems than it solved. Ryegrass seeds grow incredibly quickly after they have been spread, and it seemed like a great solution to the problem. The plants would put down roots to stabilize the soil and grow into thick mats of grass to slow floodwater.
Yet the benefits of spreading ryegrass were temporary and largely superficial. Ryegrass – especially annual ryegrass – dries out quickly after the rainy season ends, the stabilizing roots wither and the dry grass becomes great fuel for wildfires. We have since learned that our native ecosystem is adapt-ed to occasional wildfires and many native plants lie in wait to sprout up after a fire. When fire clears dense, old-growth chaparral, space opens up and seed banks that have been waiting for many years to be stimulated by fire can sprout and flourish.
Many seeds need to be charred by fire to break the outer coating or change its chemical composition and al-low the seed to germinate. Other plants have large root systems that retain nutrients and energy and are able to ‘stump sprout’ after a fire. The regrowth of a variety of native annual and perennial plants results in a complex interweaving of different root types that can stabilize the soil better than any artificial seeding mix that could be devised.
Tossing rye-grass seeds on top of the slopes would smother this recovery process and result in an ecosystem that supported fewer animals and was less resilient to future wildfires. Yet seeding even with natives is not often a good approach for restoring habitat or preventing erosion simply because it is redundant: most natural areas already have a substantial seed bank with a variety of local species that respond quickly to fire.
Counterintuitively, it may even cause more harm than good.
“There is unanimity among ecologists and botanists that re-seeding with wildflowers is unnecessary and could be very damaging. California owes its incredible diversity of native plants and wildflowers in part because natural barriers to plant migration isolated local populations of plants, facilitating the development of locally adapted species over millennia. Reseeding with non-local plants breaks the geographic barriers down and leads to homogenization of our flora. I enjoy poppies and arroyo lupines as much as the next person but not everywhere. Who wants that?” ~ Thomas W. Mulroy Ph.D. | Ecological Society of America. Restoration and Erosion Control Following Wildfire Seeding with wildflowers is unnecessary and could be very damaging. Adapted from a story by Patt Wilson McDaniel at the California Native Plant Society7
Flooding and landslides remain a chief concern post-fire and the determining factor in their severity is the nature of rain we receive. If we get gentle and inter-mitten rains, the water will soak into the soil and the plants will grow and provide the protection we need from future rains. If the rains come hard and heavy, no amount of seeding could have prevented run-off. The only effective way to prevent landslides and flooding on bare hillsides is with physical barriers such as matting or erosion control waddles. For those that are concerned about flooding and landslides near their homes, there are a couple things you can do: Our recommendation is to use biodegradable matting (like coconut fiber) with a wide weave so plants can sprout through it. Farm supply stores and hardware stores carry it, although a lot of people will be trying to buy it after a fire, and it may need to be ordered. The matting should be placed on steep, bare slopes and held in place with fabric staples which are also sold with the matting. On very steep slopes the fabric can be rolled into ‘wattles’ and held in place with wooden stakes to make check dams. Be very careful when choosing the material to use for erosion control matting.
As mentioned above, using bide-gradable materials such as coconut fiber is a good idea be-because it is temporary and biodegradable. Alternatively, there are a number of types of plastic mats that are meant to stay in place and not degrade. Depending on the slope profile and conditions, the benefits of leaving the plastic mesh in place may outweigh the costs.
Also, if you decide to use the premade wattles we strongly encourage you to do your research on the filling of the wattle as they are made from a vast variety of materials and they can introduce weeds into native habitat. When considering your options for erosion control methods and materials, make sure you know your specific goals of the areas targeted for erosion control and pick the products that will work best for each area. Depending on the conditions, contours and over-all goals of your specific restoration sites, erosion control ma-trials may not be necessary. The spreading of locally sourced seed and possibly the installation of vegetative cuttings/con-trainer stock may be all that is needed. Of course, the plant palette should be as site-specific as possible and could transition between multiple zones such as riparian, chaparral, etc.