Fire is a natural element of ecosystems that has shaped world biomes as we know them today. With the exception of polar areas, fire affects all ecosystems with more or less periodicity. Fire appeared with vascular plants (420 million years ago) and started to be used by hominins 1.6 million years ago. The human use of fire produced unprecedented changes in ecosystems and the landscape.
Nowadays, some ecosystem functions cannot be understood without the presence of fire, such as the Mediterranean. Excessive fire suppression measures, land abandonment, the plantation of monospecific forests, and the increased frequency and intensity of summer droughts increase flammability and the risk of high-severity fires, such as the ones that affected California and Portugal during the summer of 2017 (Pereira et al. 2018).
Fire can change soil properties directly, by heat produced during the fire, and indirectly, by ash that is deposited after the fire. With the exception of smoldering fires and burning logs and piles, the direct impact of fire is only observed in the first few centimeters of soil. The degree of impact that any given fire has on soil depends on the soil’s properties, type of vegetation, topography, weather conditions during the fire and fire recurrence, intensity and severity.
Normally, low-severity fires (e.g. grassland and prescribed fires) can have temporary beneficial impacts on soils as a consequence of the deposition of ash and charred material rich in plant-available nutrients. The heating produced by these kinds of fires is low, and the direct impacts on soils are reduced. On the other hand, high-severity fires have detrimental impacts on soil as a consequence of the high temperatures reached, consuming a large quantity of organic matter and inducing soil degradation. These are the kinds of impacts that should be avoided, and they are becoming more frequent and intense as a consequence of the increasing frequency of high-severity fires.
There are several aspects that are important to understanding the degree of impacts of fires on soils, such as pre-fire land use and management. For example, an ecosystem with high biodiversity is more resilient to fire than a monospecific plantation. The most serious impacts on soil properties occur after high-severity fires; thus, these are the ones that should be considered for restoration management.
A high fire recurrence can also have negative impacts on soils, and the capacity to recover to pre-fire conditions is reduced. Ash properties can influence the degree of soil conservation after a fire and the nutrients that are incorporated into the soil profile. Topographic characteristics of the burnt area can facilitate erosion or promote accumulation of sediments and nutrients. Precipitation patterns after a fire influence soil degradation by increasing or reducing the transport of nutrients and sediments (e.g. storms after fire events increase ash and soil erosion). Rapid vegetation recuperation and restored canopy cover have the ability to substantially reduce soil degradation. The higher the resilience of an ecosystem to fires, the higher the capacity of the ecosystem to recover from fire disturbance. Finally, the ways that we intervene in burned areas helps determine the degree of soil degradation. Practices such as salvage logging can increase soil degradation, while mulching reduces it.
Despite the impacts of high-severity fires on soil properties, ecosystems have developed mechanisms to respond to fire disturbances. For this reason, we should question whether or not we should intervene in areas that are extremely vulnerable, especially in the period immediately after a fire. Many post-fire management measures are disastrous for soil (e.g salvage logging, site preparation), thus, in many cases, the best option is not to intervene. The “no intervention” option should be seriously considered. This does not mean that some areas should not receive some kind of restoration, but the interventions should be carried out with sustainable practices such as mulching. The important thing to remember is that post-fire management decisions can either reduce or trigger soil degradation.
These findings are described in the article entitled Post-fire soil management, recently published in the journal Current Opinion in Environmental Science and Health. This work was conducted by Paulo Pereira from Mykolas Romeris University, Marcos Francos and Xavier Ubeda from the University of Barcelona, Eric C. Brevik from Dickinson State University, and Igor Bogunovik from the University of Zagreb.
References:
- Pereira, P., Francos, M., Brevik, E.C., Ubeda, X., Bogunovic, I. (2018) Post-fire soil management. Current Opinion in Environmental Science and Health, 5, 26–32.