Extension by Rachel Mealor
A Look at New Literature: Postfire Soil Water Repellency
By Rachel Mealor, UW Extension Range Specialsit
Although the warm temperatures have been a nice reprieve from bitter cold and falling snow, many of us are most likely wary of the lack of moisture on our landscapes. With a lack of moisture the thought of wildfires is never far from the mind, especially with vast acreages of timber killed by pine beetles. Looking through the literature, particularly Rangeland Ecology and Management, an article struck me as pertinent to this situation.
How a site recovers from wildfire depends on the extent that the ecological processes are changed, both before and after a fire occurs. We can all probably point to different fires in which the ecological processes have either changed a lot or very little. One alteration in particular that can impact how a site recovers is the soil’s ability to wet and retain water through the development or enhancement of a postfire water-repellent layer.
Soil water repellency is common in arid and semiarid climates (a majority of Wyoming) where woody plants with oily or waxy leaves are present in thick litter layers. The secretion of waxes and oils from plants, insects, and microorganisms can induce water repellency. A water-repellant soil layer may develop when the heat from the fire volatilizes organic substances in the litter and upper soil layers. The volatilized compounds then move downward into the soil, concentrating around soil particles in the cool soil layers below, leading to a shallow wettable layer at the soil surface but an intensified water-repellent zone below.
With a negative consequence of postfire, water repellency is when this situation leads to decreased site stability by allowing wind and water erosion and an impairment of revegetation success. As one can imagine, steep slopes deserve more attention as extensive soil erosion is probable. If seedling establishment fails, there is a high probability of weedy species invading exposed soils. Because of the large investment that is put toward postfire rehabilitation, it becomes important that treatment strategies to mitigate impacts from soil water repellency be recognized and implemented.
Broadcast seeding followed by one-way anchor chaining has shown to improve the establishment of an aerial-seeded species in burned pinyon-juniper ecosystems. The tilling action of the chain is thought to improve seedling establishment by enhancing seed-to-soil contact while breaking up the water repellant layer of the soil. Applying wetting agents or surfactants after fire has also been shown to reduce soil erosion and improve vegetation establishment on water-repellent soils in the chapparal ecosystem. Although wetting agents have been largely applied in urban landscapes, they have had limited use in wildland systems. Water-repellent soil that was treated with wetting agents had significantly higher infiltration rates, soil water content, plant density and biomass than without the wetting agents.
Researchers established a greenhouse experiment in Utah to evaluate the mechanisms by which anchor chaining and wetting agents influence restoration efforts in water-repellent soils. The study evaluated seedling emergence, survival and growth of bluebunch wheatgrass compared to that of cheatgrass in the presence of water-repellent soil. They also evaluated the effectiveness of wetting agents and soil tillage for ameliorating soil water repellency and improving water content, seedling density, plant survival and biomass. The soil that was used was collected from a subcanopy of burned Utah juniper trees that experienced a wildfire in 2007.
The results indicated that wetting agents can decrease the effects of postfire water repellency by helping to restore ecohydrologic function in conjunction with reseeding efforts (supporting earlier observations). Using the wetting agent, plant density more than doubled and biomass production tripled regarding bluebunch wheatgrass, mainly due to higher soil water content during emergence and early growth periods.
Soil tillage that was intended to simulate anchor chaining showed mixed results. Tillage showed lower peak plant density values compared to the control and wetting agent treatments, but fewer seedlings dried out throughout the study, resulting in higher seedling densities (on average) compared to the control.
There was a decrease in seedling emergence that was assumed to be a result of soil water availability being limited for seeds in contact with the water-repellent soil near the surface. However, within the tillage treatment, some seeds may have been associated with breaks in the water-repellent zone from the tilling of the soil. This may have created favorable conditions for seedling emergence and survival creating a zone where the seedlings could be connected with the underlying soil moisture reserves.
This study also shows that mechanically tilling water-repellent soil can enhance plant growth. The increased above-ground biomass in the tillage treatment over the control suggests the growth of surviving seedlings was increased by higher root zone water availability associated with tilling. Overall, there was not any significant benefit to applying both tillage and wetting agent treatments together over just adding a wetting agent alone.
In this particular study, applying the wetting agent appeared to be superior for an increase in seedling emergence and plant density when compared to tilling the soil. However, an important caveat is that this study was conducted in a greenhouse, which could differ greatly from a field situation. The results of this study should be interpreted with caution because of the greenhouse setting.
For example, tilling was designed to simulate soil disturbance from an anchor chain, but methods in the greenhouse study failed to capture other benefits associated with anchor chain treatments like the chain covering the seeds that are broadcast and producing “safe sites” (i.e., microtopographic locations with increased duration and amount of soil moisture). Therefore the full potential of anchor chaining is not showcased in the study and a field study would be useful in comparing anchor chaining, wetting agents, and a combination of the two for improving seedling emergence and plant survival.
Another point to consider from this study is the suggestion that water repellency limits the establishment success of bluebunch wheatgrass in a similar way as cheatgrass. Consequently, by treating the soil water repellency issue there becomes a potential to also promote the establishment of cheatgrass or other invasive weeds. The presence of weed seeds will determine whether wetting agents or anchor chaining will assist in the promotion of a weed invasion.
There is a real concern that catastrophic wildfires provide invasive species the opportunity to establish and spread by increasing resource availability. Therefore, seeding is used to establish desirable plant species before invasive species can gain a foothold. Successful seeding may be effective at reducing cheatgrass invasion, but if postfire water repellency decreases seeding success, cheatgrass could become the dominant species. The potential negative effect of water repellency on seeding survival right after a wildfire could provide an opportunity for weed invasion after soil water repellency has broken up one or more years after the fire. Therefore there may be a chance to indirectly decrease a potential weed invasion if wetting agents or anchor chaining can increase the establishment of desirable plants in water-repellent soil.
This study indicates that soil water repellency can hinder seedling survival and plant growth of both bluebunch wheatgrass and cheatgrass. Results support the use of wetting agents as an effective way to mitigate the effects of soil water repellency and promote the establishment of bluebunch wheatgrass and, perhaps, other seeded species. This particular study did not illustrate that soil tillage (through anchor chaining) would improve seedling density, but showed that tillage could enhance survival and biomass of seedlings that do emerge. However, future research is needed to repeat this study in the field and evaluate long-term effects of both treatments on reseeding success.
As we come into the peak fire season, it is interesting and useful to understand the dynamics of a system and the importance of sound restoration efforts.
Information was drawn from Madsen, M.D., S.L Petersen, B.A. Roundy, B.G. Hopkins, and A.G. Taylor. 2012. Comparison of postfire soil water repellency amelioration strategies on bluebunch wheatgrass and cheatgrass survival. Rangeland Ecology and Management. 65:182-188. For more information, contact Rachel Mealor at 307-766-4139 or rdmealor@uwyo.edu.
