Guest essay by Eric Worrall
According to the study authors, frequent controlled burns reduce the ability of some forests to sequester CO2.
More frequent fires reduce soil carbon and fertility, slowing the regrowth of plants
Long-term effects of repeated fires on soils found to have significant impacts on carbon storage not previously considered in global greenhouse gas estimates.
BY SARAH DEROUIN
DECEMBER 11, 2017
Frequent burning over decades reduces the amount of carbon and nitrogen stored in soils of savanna grasslands and broadleaf forests, in part because reduced plant growth means less carbon being drawn out of the atmosphere and stored in plant matter. These findings by a Stanford-led team are important for worldwide understanding of fire impacts on the carbon cycle and for modeling the future of global carbon and climate change.
“Almost all the synthesis studies done to date conclude that fire has relatively little effect on soils, but in large part, researchers focused on a single fire event,” said Adam Pellegrini, a post-doctoral scholar at Stanford’s School of Earth, Energy & Environmental Sciences and lead author on the study.
Focusing on three different types of landscapes – savanna grasslands, broadleaf forests, and needleleaf forests – from 48 sites covering multiple continents, the researchers compiled records of soil fertility after fires over up to 65 years. Comparing the changes in soil nutrients over time, they found that in frequently burned areas in savannas and broadleaf forests, there was a 36 percent reduction in soil carbon and a 38 percent reduction in nitrogen compared to areas that were protected from fire. Conifer forests did not show this reduction in soil carbon and nitrogen after fires.
The researchers stressed that they are not advocating fire suppression. “Fires often increase the diversity of plants and reduce the risk that a landscape will have a high-intensity fire,” said Pellegrini.
Instead, in a time where climate change creates drier and warmer conditions that favor fire, fire managers and conservationists may have to shift their management strategies.
“Managers may need to take a longer view of how much and how often they choose to burn systems,” said Jackson.
Adam Pellegrini is also a NOAA Climate and Global Change Postdoctoral Fellow. Rob Jackson is also a senior fellow at the Precourt Institute for Energy. Additional Stanford co-authors include postdoctoral scholar Anders Ahlström. The paper also includes authors from Lund University, University of Minnesota, Western Sydney University, Yale University, University of Wisconsin–Stevens Point, Kansas State University, and the University of Utah, University of California, Irvine.
The study was funded by the National Oceanic and Atmospheric Administration, the Gordon and Betty Moore Foundation and the Department of Energy.Read more: https://news.stanford.edu/2017/12/11/decades-increased-burning-depletes-soil-carbon/
The abstract of the study;
Fire frequency drives decadal changes in soil carbon and nitrogen and ecosystem productivity
Adam F. A. Pellegrini, Anders Ahlström, Sarah E. Hobbie, Peter B. Reich, Lars P. Nieradzik, A. Carla Staver, Bryant C. Scharenbroch, Ari Jumpponen, William R. L. Anderegg, James T. Randerson
& Robert B. JacksonNature volume 553, pages 194–198 (11 January 2018)
Fire frequency is changing globally and is projected to affect the global carbon cycle and climate1,2,3. However, uncertainty about how ecosystems respond to decadal changes in fire frequency makes it difficult to predict the effects of altered fire regimes on the carbon cycle; for instance, we do not fully understand the long-term effects of fire on soil carbon and nutrient storage, or whether fire-driven nutrient losses limit plant productivity4,5. Here we analyse data from 48 sites in savanna grasslands, broadleaf forests and needleleaf forests spanning up to 65 years, during which time the frequency of fires was altered at each site. We find that frequently burned plots experienced a decline in surface soil carbon and nitrogen that was non-saturating through time, having 36 per cent (±13 per cent) less carbon and 38 per cent (±16 per cent) less nitrogen after 64 years than plots that were protected from fire. Fire-driven carbon and nitrogen losses were substantial in savanna grasslands and broadleaf forests, but not in temperate and boreal needleleaf forests. We also observe comparable soil carbon and nitrogen losses in an independent field dataset and in dynamic model simulations of global vegetation. The model study predicts that the long-term losses of soil nitrogen that result from more frequent burning may in turn decrease the carbon that is sequestered by net primary productivity by about 20 per cent of the total carbon that is emitted from burning biomass over the same period. Furthermore, we estimate that the effects of changes in fire frequency on ecosystem carbon storage may be 30 per cent too low if they do not include multidecadal changes in soil carbon, especially in drier savanna grasslands. Future changes in fire frequency may shift ecosystem carbon storage by changing soil carbon pools and nitrogen limitations on plant growth, altering the carbon sink capacity of frequently burning savanna grasslands and broadleaf forests.Read more: https://www.nature.com/articles/nature24668
Sadly the full study is paywalled, but I think we get the idea.
The model used in the 2017 NOAA study may have been defective. A 2018 NSF study made the surprise discovery that 26% of bio-available nitrogen in soil comes from rocks, so model based estimates of fire driven nitrogen depletion based on theories prevalent in 2017 were likely based on incorrect assumptions. The 2018 study authors explicitly mentioned the impact of their discovery on carbon sequestration and soil nitrogen models.
Frequent controlled burns improve human safety and reduce the intensity of fires, by reducing available fuel loads.