So many of our beautiful and precious trees are lost all over the world to storms, floods, fuel, development, farming, logging, bug kill, disease, industry, pollution and forest fires.
We all know the terrible impacts these wildfires have on humans and critters, but how do they affect the soil and plants?
Recent ecological research has shown that fire is an integral component in the function and biodiversity of many natural habitats, and that some organisms within these communities have adapted to withstand and even exploit wildfires. For example, some plants and trees cannot release their seeds or cones won’t open without the heat and flames, so fires are now regarded as a ‘natural disturbance’ that drives the evolution of species and controls characteristics, rather than destructive. Because of this, land managers are in a constant dilemma whether to let nature takes its course or allow controlled burns when there are humans and infrastructure pretty well all over the place these days.
A ‘fire regime’ describes the characteristics of fire and how it interacts or impacts an ecosystem. For example, low intensity ground (or understory) fires will burn through soil that is rich in organic matter, surface fires will burn through dead plant material that is lying on the ground, crown fires will burn in the tops of shrubs and trees and high intensity fires can destroy everything.
Fire intolerant plants tend to be highly flammable and are completely destroyed, sometimes never returning to that community. Fire-tolerant species – or resprouters – are able to withstand a degree of burning and can continue growing despite the damage by storing extra energy in their roots. Fire-resistant plants, such as a mature Ponderosa Pine, generally suffer little damage in understory fires because their branches are high and their bark is very thick and designed to withstand fires.
Ground fires benefit the forests in many ways and in some cases are vital to the survival of several species. For example, they remove low-growing underbrush to open it up to sunlight, reduce competition by adding nutrients to the soil, prevent large damaging wildfires that can completely destroy forests by eliminating fuel sources, and restore health to the forest by clearing out the weaker trees and debris. They also allow for new grasses, herbs and shrubs to regenerate, which provides new food and habitat, increases the water supply and raises stream levels.
There are fewer plants to compete over it and kill tree diseases and insect infestations such as the pine beetle that prey on them.
Fires can also affect the soil by burning the overhead vegetation, leading to increased sunlight on the surface by day and greater cooling through the loss of that heat at night.
Fewer leaves will allow more rain to reach the soil surface and plant transpiration will be reduced, allowing the soil to retain more moisture. However, exposure to sunlight, wind and evaporation will dry the soil and an impermeable crust on the soil surface may be created if organic matter on the ground was heated by the fire into a waxy residue, which can lead to increased erosion through run-off.
Microbes vary in their heat tolerance, but are more likely to survive deeper down.
Nutrient loss happens through oxidation, volatilization, erosion and leaching by water, (temperatures must be very high to cause a significant loss of them), but they’re often replaced by the organic matter left behind in the fire. The minerals that were formerly part of the decaying plant matter become more soluble and available in the ash, and charcoal counteracts some of the nutrient and water loss due to its absorptive properties.
Soils tend to have a higher pH after a fire because of acid combustion and fires can also alter the texture and structure of soils by affecting the clay content and the soil’s properties.
So it’s not all bad when it burns. It just depends on how bad and how close they are to us!