It is interesting thinking about what fire can do to our soil and surrounding forest floor, but what does it really do to it? It is to my understanding that depending on the objective, low intensity ground fires help control woody understory, influence healthier regeneration of diverse plant species, controls certain insects/diseases and improves ungulate/livestock foraging. Prescribed burning requires understanding fire-regime to better predict fire intensities for future ecosystem renewal. Forest fires that are either natural or man-made, tend to be rougher on ecosystems, causing much disturbance. It is concerning that fire causes a chain reaction in soils physical properties, by disturbing/disrupting the ecosystem, exposing the soil to the sun. Raised soil temperatures cause further dehydration, and impermeability, typically affecting the microbes in the soil, and future plant regeneration. So, based on the degree of applied temperatures and lengths of time being burned, top-soil can potentially be damaged to the point of water-repellency and root system failure. Tis is why it is important to implicate Fire ecology, and TEK, into a modifiable, manageable tool to sustain environments in many specific ways. Importantly, by evaluating and managing the fire effects on soil properties in our forests properly, and how it affects plant regeneration.
Fire-regime includes data collected over many years, allowing us to describe patterns, frequency, and intensity of fires that reoccur in sizable areas over periods of times. In fire ecology, it is important to understand fire-regime as a management tool for the renewal of certain types of ecosystems and impacts on environments and landscapes. Considering variations of topography/relief, climate, and fuel; allows plausible evaluation and predictions for future interactions of fire. (Fire Ecology, 2020) (Fire Regime)
Some classifications of fire-regime to consider are as such,
After fire, the surviving species become further adapted to their fire regimes, benefitting from removal of insects/diseases, invasive species and having the ability to regenerate and repopulate fuller. But if fire-intervals are too frequent, and fire-severity is too high, damage may occur to the environment leading to long recovery periods, and/or soil repellency and root system failure. If fire doesn’t take place for a long period of time in an area, there could be over-growth of fuel, or species outgrown by invasive ones.
Ecological succession is defined as occurring levels of changes in a biological community, referred to as primary and secondary succession (Tibbits, 2017). Ecologists usually characterize succession processes by the changes in vegetation that successfully arise since diverse plant/animal species specialize in different stages depending on living conditions and requirements (Fire Ecology, 2020). Primary succession is defined when new ground is formed from volcanic eruption flow or from glacial retreat, where there is new bare rock or glacial till, (Tibbits, 2017). Secondary succession is defined when a habitat regenerates after a disturbance/disruption such as fire. It is greatly influenced by fires mosaic creation of different habitual patches. The surviving and first species to regrow during secondary succession (after fire), are known as pioneer species (red cedar, alder, most pines and larches, yellow poplar, aspen, lichens/mosses, fireweed, lignotubers). Conifer forests bear serotinous cones/seeds that are usually found among the organic matter mixed within top-soil layer. Conifer forests depend on recurring fire, (preferably prescribed, but natural ignitors like lightning occur too), in order to release seedlings and regenerate. All fire surviving species benefit from removal of insects/diseases, invasive species and having the space made available to regenerate and repopulate fuller. Important factors to consider for ecological succession are characteristics of soil properties, in order to identifying specific nature of fire-adaptation/adapted ecosystems, along with climate and topography influences.
Did you know that fire speeds up the normal process of mineralization of organic matter which would take microbes years to do with dead foliage, and decades for stumps? Did you also know that organic matter is a reservoir for nutrients and is the source for most available anions (phosphorus, sulfur, and nitrogen)? Stored nutrients in the organic matter layer release slowly during decomposition, providing a steady source of nutrients that keep leeching levels low. As a reservoir, organic matter and humus provide chemically active cation exchange sites that retain important cations (like potassium, calcium, magnesium). Estimates are stated that 50% of cation exchange capacity is provided through organic matter soil in some forests. Its also know as an active chelating agent that retains many metals and helps aggregate the soil. Aggregate soils have higher infiltration rates that non (DeBano, 1990). When fire oxidizes organic compounds, elements that form anions are lost at greater numbers than elements that form cations due to their temperature thresholds. Leftover ashes consist of soluble oxides of the alkali elements. The oxides are quickly changed to carbonates, which have alkaline reaction tending to neutralize acidity in the soil. Thus, soil pH generally increases after fire, but amount is based on cation exchange capacity of the soil. PH levels eventually return to normal from growth of herbs and shrubs which reduce leaching of cations and enable active circulations of nutrients (Livingnaturenow, 2017).
As we know, soil properties such as soil structure and porosity, moisture, temperature, PH, nutrient loss/availability may be altered after fire occurrence damages the organic matter layer (LFH). Alteration of properties are based on fire-intensity, severity, and frequency. It is understood low-intensity fires do not damage the soil as much because of only removing the L-Horizon (litter) layer or sometimes L and F (fauna) layers. Intense temperatures, however, consume organic material within the topsoil, (LFH layer), which is needed to hold sand, silt, and clay particles into aggregates. Porosity reduction is followed when mineral soil is revealed, allowing impact of water dispersal upon soil aggregates, and hydrophobic substances, thus clogging soil pores. Dispersal of hydrophobic organic compounds coat the soil making a discrete water repellant layer, which can cause further dehydrated soil. Loss of macropores (>0.6mm diameter) in the soil, leads to loss of microbes and invertebrates that tunnel through it causing even less root system infiltration rates. Plants will have difficulty up-taking nutrients and water, and eventually die out, sealing the soil up thus increasing surface runoff areas and erosion. Of course we must consider that hydrophobicity is variable since it changes according to the type of fuel, fire, soil… (Livingnaturenow, 2017) (DeBano, 1990)
Elder’s from various places throughout the globe practised/passed along this knowledge, because ancestors relied on specific biota to survive and they learned that from natural occurring fire/added heat, the earth replenishes itself. Since we have all this information forged into our brains, which prescribed burning tactics, and temperatures are we going to need to apply for improved plant regeneration without damaging the soil? Which species have heating resistance/tolerance, what kind of root systems and depths are there, and how much damage they can handle? The answer is never going to be the same due to different influences on different conditions and resources, but we do know low-severity, ground fires aren’t too worrisome.
Using fire as a management tool for restoration takes a whole new level of understanding. Especially considering our fuels types, weather indexes, seasonal timing, and forest biodiversity. In order to maintain proper plant regeneration, it is our duty to look after our fires around and within us, so we don’t suffocate the soil. We cannot let the flames get too high or burn for too long or else our planted roots will be lost and take longer to recover. All our habitats are influenced by resources and conditions and all evolve differently. But when habitats are left for too long, not being taken care of, they combust with much force. Naturally occurring ignitors have no remorse doing the dirty deed of starting fire. Forests re-start from the bottom eventually.