Digging Deep: Fire suppression may not help savannahs as much as previously thought

How fires alter carbon stocks (ie the carbon stored in the biomass and soils) in different biomes has been a longstanding question in ecology. A recent study examines the effect of different fire regimes in Kruger National Park, South Africa, which constitutes a tropical savanna ecosystem. Savannas are fire-dependent ecosystems where fire durbance has played a key horical role in their evolution and maintenance.
It is important to note the difference between ‘forest’ fires – a term that is usually loosely used – and savanna fires. ‘Forest fires,’ as the name suggests, occur largely in temperate and boreal forests and burn trees. On the other hand, savanna fires only burn herbaceous plants that are close to the ground and, therefore, might release lesser CO₂ into the atmosphere.
While a biome’s response to fire is largely dependent on the vegetation type, savannas -and-large exhibit a loss in carbon and nitrogen in surface soils, albeit to varying degrees. contrast, soil C and N in temperate and boreal needle-leaf forests have been observed to be immune to fire. Fire also alters the composition and quantity of biomass. Biomass could refer either to the trees and shrubs (terrestrial aboveground biomass) or root networks under the soil (belowground/root biomass). Aboveground biomass also constitutes grasses and even dead leaves and shoots that fall off the vegetation (litter).

Here, however, Zhou et al. (2022) focus on both above- and below-ground biomass. The study employed a LiDAR sensor (light detection and ranging) on a drone to ascertain aboveground woody biomass (in terms of tree density), and a ground penetrating radar to determine belowground woody coarse lateral root biomass. The measurement of belowground biomass obtained via the aforementioned non-invasive method were supplemented with field-based measurements of woody taproot biomass.
The Kruger National Park (KNP) offers an excellent template to study the effects of varying fire regimes, their interplay with rainfall, and their ultimate impact on vegetation. For one, the park contains a wide climatic gradient in terms of precipitation and soil types. Secondly, KNP has been subject to a large experiment in fire manipulation that has been running since 1954.
The study examined plots in a high-rainfall region of the KNP with different fire regimes. One set of plots mimicked the natural fire frequency at KNP ie occurring once in three years. Another group of plots exhibited complete fire suppression, with no fire occurring at all. A third group consed of plots that displayed an increased fire frequency, where fires occurred every year.
The study found that fire suppression, compared to triennial fires, increased tree height, tree cover and aboveground woody biomass. Fire suppression or less frequent fires also contributed to an increase in belowground woody biomass, but not in the same proportion as aboveground biomass as usually believed. The reason behind this is that trees in savannas that burn quite frequently invest heavily in belowground root biomass in order to be able to recover after fire. ‘This result contradicts the assumption of constant root-to-shoot ratio applied elsewhere to estimate belowground carbon.’

Further, contrary to previous observations, the study observed that soil organic carbon remained largely unaffected changes in fire frequencies. In fact, a previous study in KNP even reported no significant loss in C and N post fire. Zhou et al. (2022) argue that this is because most studies sample soil only up to a very shallow depth, and effects of fire suppression are limited to the top horizons of the soil column. Additionally, the paper argues, that substantial carbon is stored in the soil in even treeless areas, owing in large part to the input C4-derived carbon into soils (grasses fix atmospheric CO₂ as four carbon molecules, hence.
But, the moot question is: does total fire suppression do anything to meaningfully mitigate CO₂ release and sequestration? According to the present study, whatever improvements in carbon sequestration were observed were largely due to increase in the aboveground woody biomass. However, the improvement is not very significant. When juxtaposed with triennial burning, a complete absence of fire for six decades ‘increased whole ecosystem carbon storage only 35.4%, even though tree cover increased 78.9%.’
Ecologs have repeatedly argued that fire suppression and tree planting can actually reduce biodiversity in savanna and grassland ecosystems. Tree planting programmes also tend of fail because seed germination rates are quite low in savanna and grassland biomes. Not only that, they may actually be counterproductive as an altogether absence of fire allows woody vegetation to develop for long periods of time, and leads to a massive fire when fire eventually occurs.
Calling to question studies that preceded them and advocated afforestation in grasslands and savannas or fire suppression, the paper asserts that ‘the benefits of trees-for-carbon and fire-suppression schemes for climate mitigation have been exaggerated.’
The author is a research fellow at the Indian Institute of Science (IISc), Bengaluru, and a freelance science communicator. He tweets at @critvik