When ashes stain forest rivers: post-fire effects on soil and water ecosystems

Water pollution and soil disturbance are the most devastating effects of large uncontrolled fires, but in carefully managed and controlled fire-prone ecosystems, wildfires can also have positive implications.

Source: USDA Forest Service

Uncontrolled, extreme forest fires can have a devastating impact on the ecosystems where they occur: one of the most visible of these consequences is the destruction of landscapes. Forest loss due to fire incidents is often lamented – as clearly seen in the case of recent fires in the Amazon – for its destruction of vegetation that provides oxygen and acts as a carbon sink by capturing and storing CO2. However, the impacts of fire on forest ecosystems can enact less recognised, long-term effects, including damage to soil and river basins that provide shelter and food to a wide variety of organisms.  

In the Mediterranean, one of the most serious post-fire consequences in forest ecosystems is the loss of soil due to erosion; surface destruction and human intervention can lead to soil compaction that hinders water penetration and reduces soil moisture. The strong storms and torrential rains typical of fire-prone, Mediterranean climatic regions, combined with slopes newly stripped of vegetation and thus unable to inhibit water flow, wash ashes and their toxic particles into the flows of rivers and streams, increasing the murkiness and pollution of the water.

The soil of a forest, geologically speaking, is the most superficial and mobile layer of the Earth’s crust, being the result of weathering and the activities of living beings over millions of years. Therefore, after a fire and the corresponding loss of vegetation cover and soil, changes in the physical features of the soil leave landscapes prone to the forces of erosion: gravity, water and wind.

Soil erosion, run-off and sealing

Extreme fire events contribute to immediate increased erosion, but the long-term prediction of the amount of post-fire erosion depends on a combination of factors linked to landscape conditions, fire characteristics and rainfall patterns in a particular forest biome.

Surface erosion already begins when the forest is in flames, due to the exposure of the soil and rocky components that shape the forest landscape. After this initial phase, erosion acts on the steepest slopes and territories, especially during heavy rain, when increased runoff can leave slopes completely bare.

Faced with a post-fire landscape with little vegetation to quell rain flow and absorb surface water, the amount of water available for runoff increases. Changes in soil porosity, a measure of the spaces in soil that can fill up with air or water, must also be taken into account. Extreme fires can cause the ground surface to be sealed with fine sediments or ashes that restrict the soil’s capacity to absorb water after a fire, causing hydrological problems such as water repellency.

Water repellency depends on the properties of the soil and the intensity of the fire heat, which vary in different ecosystems and times of year. Although each fire incidence differs, the reduced ability of the soil to absorb water can be fairly short-lived. In contrast, erosion on slopes after a fire can be significantly higher than the previous dynamics, and can take three years or more to return to pre-fire vegetation and erosion levels.

Source: Pexels

Ash spills and water contamination risk

Material from eroded hillsides initially fills small streams, until storms then cause large flows of debris after the fire: with water sludge, ashes, metals, organic compounds and sediments depositing downstream into larger water bodies, including rivers and lakes. At the same time, the lack of water absorption by the soil as a result of fire-related impacts leads to higher surface water flows and a slight increase in reservoir water levels.

But while these side effects might have positive implications, others emerge with harmful results for hydrological systems. For example, concentrations of polycyclic aromatic hydrocarbons (PAHs), a highly toxic organic compound produced when biomass is burned, have been shown to increase in the water slightly after a forest fire. This aspect indicates a potential impact on water supply management of forest watersheds, which may also involve public authorities foreseeing the public health effects of fires as an additional element of water quality deterioration.

Some historical examples demonstrate the catastrophic potential of fires in freshwater ecosystems, with some of the most salient examples in Europe being the forest fires in Portugal and Galicia, Spain. Precisely in these areas of the northwest of the Iberian Peninsula, villagers commonly refer to the “rivers of ash” that result from forest fires as chapapote do monte (mountain oil) or lava negra (black lava), especially after the disastrous fires that occurred there in 2006 and 2017. The fires devastated more than 80,000 and 40,000 hectares in the region respectively.

Example of forest fire ash spills in Mediterranean streams after runoff. Source: Moisés Cruz Ballesta

Do all fires damage these ecosystems?

As we have seen, the consequences of large, uncontrolled fires can be disastrous for soil and water ecosystems, but this does not imply that fire itself has only negative impacts. In the Mediterranean basin, fire-prone ecosystems need fire as a way of clearing dead vegetation, reducing biomass, enriching the soil and allowing the growth of new plant species.

While megafires induce long-term effects on geomorphological and hydrological processes, which may involve other natural hazards such as floods or mass soil movements, forest areas under controlled conditions can be set on fire, burning fuel that could potentially feed a future wildfire.

Prescribed burning and “backfire” are two strategies that can play a crucial role in mitigating the undesirable effects of potential high intensity fires: both approaches use fire to create a manmade firebreak, or gap, in combustible material to prevent or contain the spread of extreme wildfires. These techniques, together with forest management and restoration measures to prevent erosion, such as building dikes and planting specific trees and vegetation species, help reduce further damage to soil and water ecosystems.

Even with all this knowledge, the impact of fires on these ecosystems still requires deeper study in Mediterranean countries, and calls for a multidisciplinary approach involving hydrology, soil science, chemistry, biology, public health, and even socioeconomic studies to understand the role of people and infrastructure in fire-prone ecosystems.

There is significant variability in the impact of pollution depending on the region’s climate system, forest fuels, forest and landscape management as well as existing water supply systems. What is known with certainty, though, is that changes in climatic patterns in the Mediterranean, resulting from global warming and increasingly heavy torrential rains, worsen this problem and increase the risks of post-fire contamination.

Source: Pixabay

Sources & additional information

VIASarah Feder
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Gerard Fernández
Communications Officer at the European Forest Institute. Responsible for the dissemination of research, activities and projects developed at the EFI's Mediterranean Facility (EFIMED) and its regional network. Based in Barcelona.