Litter Accumulation

The Driftless Area is known for its dissimilar ecosystems, soil types, and topography. On one side of the road is a low-diversity planting on mesic soil exploding with Indian grass and Canada goldenrod. On the other side is a linear, moderately-diverse remnant on dry, rocky, steep, infertile soil. A mile down the road is a remnant wetland bursting with a diversity of forbs and sedges. Would we manage these sites the same? Would we use the same management techniques and the same management schedules?

There is science available for us to create solutions and develop a measured approach for our land management. Using a calendar-based guideline such as “burn every 3 years” has no value when dealing with the vagaries of nature, different soil types, and different climate and weather. The optimal fire management strategy removes litter when it accumulates to a certain amount rather than removing it on a calendar-dictated schedule. Nature doesn’t operate on a rote basis, neither should our management activities.

Fire’s main purpose is to remove the accumulation of senesced plants to provide light and space to the ecosystem. Some litter buildup maintains moisture and provides nutrients but an over accumulation can suffocate plants. How do we know when the litter is too much?

Let’s dig in and figure it out. We’ll review the science of litter affects on plant growth, the pros and cons of litter accumulation, and how we can use science-based research to determine when litter needs removing. Knowledge is power. Knowledge allows us to create personalized management plans and gives us confidence that we’re working our plan and not someone else’s.

In restoration practice, we are bombarded with statements of fire’s value and how often we should burn. Rarely do those statements accompany an adequate explanation (if any) of fire’s purpose or effects. Yet, without this critical information, we’re applying management techniques which may not fit our goals or be advantageous for our land.

What Influences Plant Growth?

Thatch accumulates when soil conditions don’t sustain high populations of thatch-decomposing organisms.

Fire has a modest direct influence on plant production; its main purpose is to remove thatch. Yet, thatch (aka litter) has benefits when left to decompose and it has benefits to its occasional removal.

The most significant impact on plant growth and health is biotic (e.g. soil microbes, fungi) and abiotic (e.g. drought, rainfall, and topography) interactions. Litter feeds the biotic environment and sets up a positive feedback loop whereby the duff provides nutrients to the soil fauna which, in turn, decomposes the litter (Whelan 115; Majer 46) and feeds the plants. Soil fauna diversity is related to litter quantity and a slow decomposition rate. Yet, these decomposition rates are complex and vary from site to site and vary within a site. Soil type, ecosystem composition, and weather (to mention a few) will affect the rate of decomposition and hence your management decisions. Burning can reduce the diversity and quantity of soil arthropods because it removes the litter, which is their shelter and their food (Anderson 1973).

Digging deeper into what influences plant growth, we find productivity isn’t improved with the  nitrogen released by fire as other studies suggest but rather from physical and biological changes to the site and the plants due to litter removal (Ojima et al. 1997). In other words, litter removal by fire, the subsequent soil warming which activates the soil microbes, and adequate soil moisture are the contributing factors to enhanced plant production (Hulbert 1988). Understanding the importance of soil fauna makes it clear that removing the litter too often can be detrimental to forb production.

Grasses and legumes can benefit from this air-born nitrogen (see grass/fire paper) but forbs need the soil fauna to intervene and convert it into a form the plant can use.

Soil microbes diagram

Source: Mycorrhizal Planet and The World Beneath our Feet, James Nardi

Pros and Cons of Litter Accumulation

 Litter accumulation (aka detritus, thatch, duff) maintains a consistent daily temperature buffering against heat and cold fluctuations that hinder plant growth. Litter is moisture’s friend. It aids snow accumulation, lengthens snow melting time, extends rainfall infiltration time, and reduces evaporation (Ehrenreich and Aikman 1963). It is particularly beneficial in drought years (Redmann 1978) because soil moisture is essential to plant health. Burning away the detritus increases water stress and decreases production (Abrams et al. 1986; Redmann 1978; Daubenmire 1968).

Litter removal has advantages as well. Removing it warms the soil earlier in spring by as much as 10°F (Ehrenreich and Aikman 1963). This can expand the growing season by 2-3 weeks before the temperature benefits have disappeared (Ehrenreich and Aikman 1963, Ojima et al. 1994). Periodic litter removal is important to ensure desired plants aren’t suffocated and diversity is decreased.

Litter removal changes the all important soil moisture — rain absorption decreases, evaporation increases, and transpiration increases. Soil with litter absorbs rainfall on average 1” per hour while absorption on bare soil averages ¼” per hour (Ojima et al. 1994). Soil litter minimizes erosion and maintains soil moisture necessary for germination and seedling survival; this is essential for plant health in drought years. We often don’t think about transpiration affecting soil moisture. Yet, the plants benefiting from the extra growing weeks after a burn will transpire for a longer period reducing the soil moisture (Ehrenreich and Aikman 1963).

 

Like everything in restoration, litter removal has trade-outs. One must be aware of these to decide the most favorable management decision for each ecosystem. Species composition will impact litter effects as well as individual species’ responses. Litter is one of the “most important reservoirs for soil diversity” (Nekola 2002) and it’s this diversity of microbes, bacteria, and fungi that create healthy soil. 

Litter accumulations are a good way to store carbon in the soil, mitigating climate change. We just need to know when too much of a good thing is no longer a good thing.

The video describes the process of litter converting into Soil Organic Matter.

Burn or Mow and How to Know

 How does one know when to burn – in other words, when does the litter need removing? Carson and Peterson (1990) studied the effects of litter accumulation and provided these management guidelines. Litter accumulation of:

  • 5-10.5oz/1 square yard increases yield and diversity;
  • 5oz – 32oz/1 square yard has little effect on community production or biomass and variable effects on plant diversity and density; and
  • Above 32 oz/1 square yard decreases productivity and diversity as well as plant reproduction. Knapp and Seastedt (1986) agreed with this research.

Curiosity took hold of me! What does the landscape look like when litter is accumulated to these levels? Will I be able to tell when a burn is needed without measuring?

I randomly marked off 1 square yard sections in 4 differing ecosystems, clipped the vegetation, sifted out soil and sticks, and weighed it. I took close up photos of the square yard and a wider view of the entire area. All these photos were taken in late March 2019. 

The litter weighed 12oz. This prairie was planted in 2010 with 10lbs/acre and a 60/40 forb/grass mix with no tallgrasses in the mix although a few have found their way into the planting. It was burned in 2012 and 2018. Lots of bare dirt is visible.
The litter weighed 12oz. This prairie was planted in 2010 with 10lbs/acre and a 60/40 forb/grass mix with no tallgrasses in the mix although a few have found their way into the planting. It was burned in 2012 and 2018. Lots of bare dirt is visible.

Bag 4: The litter weighed 12oz. This prairie was planted in 2010 with 10lbs/acre and a 60/40 forb/grass mix with no tallgrasses in the mix although a few have found their way into the planting. It was burned in 2012 and 2018. Lots of bare dirt is visible.

Bag 1 – The litter weighed 10 ¼ oz. This came from a restored prairie entering its 5th growing season. It has never been burned. It was mowed in 2015, the first year after it was planted and again in 2017. It was planted with a 60/40 mix of forbs/grasses, 10lbs to the acre, and no tall grasses. Lots of bare dirt is visible.
Bag 1 – The litter weighed 10 ¼ oz. This came from a restored prairie entering its 5th growing season. It has never been burned. It was mowed in 2015, the first year after it was planted and again in 2017. It was planted with a 60/40 mix of forbs/grasses, 10lbs to the acre, and no tall grasses. Lots of bare dirt is visible.

Bag 1 – The litter weighed 10 ¼ oz. This came from a restored prairie entering its 5th growing season. It has never been burned. It was mowed in 2015, the first year after it was planted and again in 2017. It was planted with a 60/40 mix of forbs/grasses, 10lbs to the acre, and no tall grasses. Lots of bare dirt is visible.

In the above examples, it is interesting that the mowed one weighed less than the burned one!

Bag 2 – The litter weighed 16¼ oz. This is the edge of a planting between the mowed trail and a oak/hickory woods. It was planted with 10 forbs and 5 native cool season grasses at a rate of 10lbs per acre in 2012. This area has never been burned but was mowed twice; once in 2013 and again in 2015.
Bag 2 – The litter weighed 16¼ oz. This is the edge of a planting between the mowed trail and a oak/hickory woods. It was planted with 10 forbs and 5 native cool season grasses at a rate of 10lbs per acre in 2012. This area has never been burned but was mowed twice; once in 2013 and again in 2015.

Bag 2 – The litter weighed 16¼ oz. This is the edge of a planting between the mowed trail and a oak/hickory woods. It was planted with 10 forbs and 5 native cool season grasses at a rate of 10lbs per acre in 2012. This area has never been burned but was mowed twice; once in 2013 and again in 2015.

Bag 3 – The litter weighed 26oz. This prairie was planted in 2008. It was burned in 2010 and 2012. This section is heavy Sorgastrum nutans (Indiangrass). It was planted 10lbs to the acre with a 60/40 forb/grass mix, then overseeded with Indiangrass. 

This prairie was mowed in 2019. In March 2020, a sample was taken that weighed 28oz. While it did not appear that this prairie lost diversity in 2019, the 2020 photos do show a mulching effect from the litter.
Bag 3 – The litter weighed 26oz. This prairie was planted in 2008. It was burned in 2010 and 2012. This section is heavy Sorgastrum nutans (Indiangrass). It was planted 10lbs to the acre with a 60/40 forb/grass mix, then overseeded with Indiangrass. 

This prairie was mowed in 2019. In March 2020, a sample was taken that weighed 28oz. While it did not appear that this prairie lost diversity in 2019, the 2020 photos do show a mulching effect from the litter.

Bag 3 – The litter weighed 26oz. This prairie was planted in 2008. It was burned in 2010 and 2012. This section is heavy Sorgastrum nutans (Indiangrass). It was planted 10lbs to the acre with a 60/40 forb/grass mix, then overseeded with Indiangrass. This prairie was mowed in 2019. In March 2020, a sample was taken that weighed 28oz. While this prairie did not lose diversity in 2019, the 2020 photos show a mulching effect from the litter.

 

I learned from my simple experiment that one has to look closely into the ecosystem. Based on Carson and Peterson’s (1990) study, none of these ecosystems would need burning. My assessment is that if one can see bare soil and seedlings, it would fall into the “not needing to burn” category although mowing would be an option. The one heavily planted with Indiangrass appears to have accumulated enough litter to create a mulching effect on the plants because no bare soil was visible. This could be due to the recent high winds and last winter’s heavy snow that laid the plants down, matting them to the ground. This was the first winter this prairie had been affected in this way. Since it wasn’t up to the 32oz level, we opted to mow it this year (2020) rather than burn it. We are curious what the effects will be.

Mowing has many advantages and should be included in your toolbox of management techniques. It is favorable to the soil ecosystem by maintaining moisture and providing organic nutrients to the soil, which supports the soil fauna and the plants. Mowing prevents wholesale mortality of insects overwintering in the stems and litter. Mowing can be done when burning isn’t possible and mowing requires only one person. Haying is another option to be considered as it would cut and remove some of the vegetation. Mixing up management techniques can maintain diversity. Burning too often can erode biodiversity and disrupt the ecosystem’s nutrient cycles for the forbs (Scott et al. 2014:125, Blair et al. 1998:238). It also increases grasses at the expense of forbs, creating a loss of biodiversity. (see grass paper for myriad of references)

When we plant a new prairie or are working on a degraded remnant, we mow it the first few years instead of burning it. We want to maintain soil moisture and organic matter needed for seed germination and seedling development. Our first time to experiment with the mowing produced astonishing results. We seeded at a rate of 10 lbs per acre with 92 species; the 2nd growing season we identified 54 species. Other plantings and the remnant areas have produced equally positive results with our mowing regime. While this is our story, it’s the science provided from the above that explains these results.

Conclusion

We all want the simple answer but there isn’t one. The one-size-fits-all mantra of burning every 3 years could be expending resources unwisely and working against your restoration goals. Our native ecosystems are ever changing. Each year brings different weather patterns and sometimes different plant species.

Use the science-based guidelines for litter removal and couple them to meet your goals and your lands’ unique needs based on topography, soil type, and annual changes. We can quit the guessing game. We can quit over scheduling unnecessary management. We can quit relying on what others tell us. Resources are limited and valuable.

References

Abrams, Marc D., Alan K. Knapp, and Lloyd C. Hulbert. 1986. A ten-year record of aboveground biomass in a Kansas tallgrass prairie: effects of fire and topographic position. American Journal of Botany 73(10): 1509-1515. 

Anderson, R. C. 1973. The use of fire as a management tool on the Curtis Prairie, Proceedings Annual [12th] Tall Timbers Fire Ecology Conference: a quest for ecological understanding. Lubbock, TX. Tall Timbers Research, Inc., Tallahassee, FL. p. 23-35.

Blair, John M., Timothy R. Seastedt, Charles W. Rice, and Rosemary A. Ramundo. 1998. “Terrestrial Nutrient Cycling in Tallgrass Prairie.” In Grassland Dynamics: Long-term Ecological Research in Tallgrass Prairie, edited by Alan K. Knapp, John M. Briggs, David C. Hartnett, and Scott L. Collins. New York: Oxford University Press.

Carson, Walter P. and Chris J. Peterson. 1990. The role of litter in an old-field community: impact of litter quantity in different seasons on plant species richness and abundance. Oecologia 85: 8-13.

Daubenmire, Rexford. 1968. Ecology of fire in grasslands. Advances in Ecological Restoration, v5. New York: Academic Press: 209-273.

Ehrenreich, John H. and John M. Aikman. 1963. An ecological study of the effect of certain management practices on native prairie in Iowa. Ecological Monographs 33(2): 113-130.

Hulbert, Lloyd C. 1988. Causes of fire effects in tallgrass prairie. Ecology 69(1):46-58.

Knapp, A.K and T.R. Seastedt. 1986. Detritus accumulation limits productivity of tallgrass prairie. BioScience 36(10): 662-668.

Nekola, J.C. 2002. Effects of fire management on the richness and abundance of central North America grassland land snail fauna. Animal Biodiversity and Conservation 25(2): 53-66.

Ojima, Dennis S., D.S. Schimel, W.J. Parton, and C.E. Owensby. 1994. Long-and short-term effects of fire on nitrogen cycling in tallgrass prairie. Biogeochemistry 24: 67-84.

Redmann, R.E. 1978. Plant and soil water potentials following fire in a northern mixed grassland. Journal of Range Management 31: 443-445.

Scott, Andrew C., David M.J.S. Bowman, William J. Bond, Stephen J. Pyne, and Martin E. Alexander. 2014. Fire on Earth: An Introduction. West Sussex: John Wiley & Sons, Ltd.

Whelan, Robert J. 1995. The Ecology of Fire. Cambridge, NY: Cambridge University Press.