What happens to a burnt forest floor?
November 17, 2024 12:12 PM Subscribe
A ground fire burnt through a forest floor of dead leaves near us.
All the ground cover is gone, and there are many charred logs, but presumably it was not hot enough to climb the trees; they seem fine.
What happens now? Erosion will be an issue till growth recurs. In the soil, microbes, bacteria and fungi, worms and bugs were burnt at the surface level, or do they retreat? Weren’t they food for ground eating birds, other animals?
But what will grow back, on the ground, and on the burnt logs, logs that had been full of life? Native plants vs. invasives. Will they ‘battle’ for the nutrients, sun? How can the fire help?
All the ground cover is gone, and there are many charred logs, but presumably it was not hot enough to climb the trees; they seem fine.
What happens now? Erosion will be an issue till growth recurs. In the soil, microbes, bacteria and fungi, worms and bugs were burnt at the surface level, or do they retreat? Weren’t they food for ground eating birds, other animals?
But what will grow back, on the ground, and on the burnt logs, logs that had been full of life? Native plants vs. invasives. Will they ‘battle’ for the nutrients, sun? How can the fire help?
presumably it was not hot enough to climb the trees; they seem fine.
From the point of view of the trees, among the evolved functions of leaf litter are to catch fire and increase the probability that seedlings will survive and prosper by reducing the underbrush that would otherwise have shaded them out, and reducing the population of seed and seedling predators, as well as making the elemental nutrients that were sequestered in the dead leaves available to seedlings and mature trees alike.
Which means that the trees have most likely been through this multiple times and that everything will restore itself eventually.
posted by jamjam at 1:26 PM on November 17, 2024 [3 favorites]
From the point of view of the trees, among the evolved functions of leaf litter are to catch fire and increase the probability that seedlings will survive and prosper by reducing the underbrush that would otherwise have shaded them out, and reducing the population of seed and seedling predators, as well as making the elemental nutrients that were sequestered in the dead leaves available to seedlings and mature trees alike.
Which means that the trees have most likely been through this multiple times and that everything will restore itself eventually.
posted by jamjam at 1:26 PM on November 17, 2024 [3 favorites]
The Wikipedia article on fire adaptations has some good general info on how plants and animals survive or recover from fires.
In my area we had a couple major fires in the 2000s roll through - most of the area burned was scrub brush that makes up a lot of the regional biome here. What looked like a blackened wasteland has pretty much recovered to what it was prior to the fires in the following 20 or so years, just because the plants are kind of adapted to it and with our normal rainfall they tend to have very deep roots to be able to bounce back. At higher elevations where it was more like a typical forest (a mix of oaks and pines), it seemed like the destruction was total but a pretty decent number of oaks actually survived and the area has been filled in by pioneer species in the normal ecological progression from a grassland to a forest. Unfortunately, hardly any pines survived, so I don't know that I'll see anything like the forests of my youth up there in the course of my lifetime.
posted by LionIndex at 1:38 PM on November 17, 2024 [2 favorites]
In my area we had a couple major fires in the 2000s roll through - most of the area burned was scrub brush that makes up a lot of the regional biome here. What looked like a blackened wasteland has pretty much recovered to what it was prior to the fires in the following 20 or so years, just because the plants are kind of adapted to it and with our normal rainfall they tend to have very deep roots to be able to bounce back. At higher elevations where it was more like a typical forest (a mix of oaks and pines), it seemed like the destruction was total but a pretty decent number of oaks actually survived and the area has been filled in by pioneer species in the normal ecological progression from a grassland to a forest. Unfortunately, hardly any pines survived, so I don't know that I'll see anything like the forests of my youth up there in the course of my lifetime.
posted by LionIndex at 1:38 PM on November 17, 2024 [2 favorites]
Some seeds are specifically adapted to fire - they have seed coats that actually require fire damage in order to germinate. In places where fires are not a common natural occurrence, or where fires are more intense than usual because of a buildup of flammable materials (ironically this can happen because of inappropriate fire suppression practices), the recovery won’t be as fast. But the above point about fires removing competition for new plants is still true. The first new growth will be either seeds that were present in the soil already and were either too deep to be damaged by the fire or could withstand it, plus plants that specialize in disturbed environments and can distribute their seeds widely. “Plants that specialize in disturbed areas” largely includes species that in horticultural settings we’d call weeds, precisely because of their ability to pop up in bare soil, but that doesn’t equate to invasive or bad, and over time those things get replaced by other species as the ecosystem recovers.
posted by showbiz_liz at 2:08 PM on November 17, 2024 [3 favorites]
posted by showbiz_liz at 2:08 PM on November 17, 2024 [3 favorites]
Heat rises, so things only a few inches below the surface are going to survive. This means an awful lot of roots and bacteria and worms and nematodes and even small mammals, are going to have been okay because they were not burnt, and as soon as things are cool enough they can go back to their normal routines. Just as earthworms go underground when it gets sunny and come out when the ground is saturated so they won't drown, the earthworms will have gone deeper to find cooler soil underground during the fire.
Grass thrives when it is grazed, or cut, or burned. The roots die back to allow for the lack of green grass blades providing the plant with oxygen, just as the roots die back when the grass withers in winter. Each time this happens the roots that die turn into topsoil - that's the process by which topsoil is made. The roots will grow back more thickly than before the burn.
Ash from the burning will change the ph of the soil to be more acidic. This will encourage certain species that like highly acid soil to grow, such as blueberries. To create a good blueberry bush you need to burn off the area every few years.
With the undergrowth burned off, more tree seedlings will survive than do during a year without fire, as they will be able to get enough sun and not have to compete with shrubs and forest floor greenery. You'll seem more new young trees in the next few years, especially hardwoods.
The growth of the new hardwood shoots will bring in the deer, whose staple food is hardwood browse.
Many animals that seek for new territory the same year that they are born will be on the move next summer, such as squirrels, woodchucks and birds of various sorts. The burned over terrain will probably be turning green again by midsummer just when they are looking for new territory to live and the forest will suit them just fine.
If you had any Japanese knotweed growing in the area it will increase massively. Japanese knotweed is capable of surviving after it has been buried in lava. The roots survive and come right back up between the cracks in the pumice. If the knotweed was invasive before the fire it will be triply so afterwards.
If the standing trees are fine, the chances are it was a very quick burn and the insides of the fallen logs are not charred. Insects will probably have survived inside under the char. The charred wood will start to crumble in the rain and the snow. The same decomposition will continue going on in the fallen trees as it did before the fire.
Softwood trees are much more flammable than hardwood trees. You'll see fewer of those trees and more hardwoods. Ordinarily the hardwood trees stands form partially firebreaks during a forest fire. Someone above mentioned a forest fire in their area that took out the pine but left surviving oaks - that's because the pine resin made the pine trees much more vulnerable than the oaks. Acorns even sometimes survive being charred on the forest floor and can sprout afterwards. But pine cones, which are frequently collected as tinder for starting campfires, are likely to have been completely burned.
Much depends on the individual species in your forest and the extent of the fire. To know what would have survived and what will bounce back stronger than ever after your fire we'd have to know the location and what species grew and lived there.
It's amazing how fast and how far a panicked squirrel fleeing a fire can run - many species like that will survive a small forest fire, but end up at greater risk after the fire when they find themselves unable to find new territory and new food supplies and be without time to prepare nests before winter.
posted by Jane the Brown at 7:28 PM on November 17, 2024 [5 favorites]
Grass thrives when it is grazed, or cut, or burned. The roots die back to allow for the lack of green grass blades providing the plant with oxygen, just as the roots die back when the grass withers in winter. Each time this happens the roots that die turn into topsoil - that's the process by which topsoil is made. The roots will grow back more thickly than before the burn.
Ash from the burning will change the ph of the soil to be more acidic. This will encourage certain species that like highly acid soil to grow, such as blueberries. To create a good blueberry bush you need to burn off the area every few years.
With the undergrowth burned off, more tree seedlings will survive than do during a year without fire, as they will be able to get enough sun and not have to compete with shrubs and forest floor greenery. You'll seem more new young trees in the next few years, especially hardwoods.
The growth of the new hardwood shoots will bring in the deer, whose staple food is hardwood browse.
Many animals that seek for new territory the same year that they are born will be on the move next summer, such as squirrels, woodchucks and birds of various sorts. The burned over terrain will probably be turning green again by midsummer just when they are looking for new territory to live and the forest will suit them just fine.
If you had any Japanese knotweed growing in the area it will increase massively. Japanese knotweed is capable of surviving after it has been buried in lava. The roots survive and come right back up between the cracks in the pumice. If the knotweed was invasive before the fire it will be triply so afterwards.
If the standing trees are fine, the chances are it was a very quick burn and the insides of the fallen logs are not charred. Insects will probably have survived inside under the char. The charred wood will start to crumble in the rain and the snow. The same decomposition will continue going on in the fallen trees as it did before the fire.
Softwood trees are much more flammable than hardwood trees. You'll see fewer of those trees and more hardwoods. Ordinarily the hardwood trees stands form partially firebreaks during a forest fire. Someone above mentioned a forest fire in their area that took out the pine but left surviving oaks - that's because the pine resin made the pine trees much more vulnerable than the oaks. Acorns even sometimes survive being charred on the forest floor and can sprout afterwards. But pine cones, which are frequently collected as tinder for starting campfires, are likely to have been completely burned.
Much depends on the individual species in your forest and the extent of the fire. To know what would have survived and what will bounce back stronger than ever after your fire we'd have to know the location and what species grew and lived there.
It's amazing how fast and how far a panicked squirrel fleeing a fire can run - many species like that will survive a small forest fire, but end up at greater risk after the fire when they find themselves unable to find new territory and new food supplies and be without time to prepare nests before winter.
posted by Jane the Brown at 7:28 PM on November 17, 2024 [5 favorites]
What happens depends on fire severity, surviving canopy density, soil, precipitation frequency and intensity, and topographic aspect and slope. Here we have areas of reduced understory beneath lush canopy, oak sprouts, cypress regeneration, thickets of invasive broadleaf brush, baked-clay wastelands, and landslides.
I infer that this question relates to Sterling Forest, which is part of the Appalachian oak forest where I lived most of my life. Historically, before chestnut blight and aggressive fire suppression altered the forest structure, surface fire every decade or two was part of the regional fire regime. Older trees in your forest are acclimated to occasional combustion of 1000-hour and smaller fuels and may even benefit.
Broadleaf seedlings and shrubs draw moisture from the soil and release it into the air. Hotter temperatures during the leaf-on season increase the rate of transpiration, intensifying competition between the overstory and understory. Reducing understory leaf area for the next few seasons will make more moisture available to the larger trees, improving their health and resilience through drought. Burning off the surface fuels may also improve percolation as oak leaves often form impervious plaques that prevent rainfall from entering the soil. Forest managers often prescribe fire to benefit the overall stand in this way; a piece just up the road from me will be burned after the atmospheric river passes, while the larger fuels remain moist yet the finer fuels are dry enough to consume.
In areas where the overstory remains healthy, I would not expect soil loss or invasive propagation. Where canopy loss suddenly makes more sunlight and water available is where changes will occur, including saturated soils sliding downslope. Here there was little effort to guide the post-fire succession -- one planting of 150 seedlings per acre where 700+ were required was outcompeted by shrubs both native and invasive. In other areas, unguided succession led to the resurgence of a culturally significant fruiting shrub that had been suppressed by less fire-tolerant competitors. What will regrow in Sterling Forest is a question for someone who knows that area from before the elimination of fire, local historians or the Munsee.
posted by backwoods at 8:30 PM on November 17, 2024 [5 favorites]
I infer that this question relates to Sterling Forest, which is part of the Appalachian oak forest where I lived most of my life. Historically, before chestnut blight and aggressive fire suppression altered the forest structure, surface fire every decade or two was part of the regional fire regime. Older trees in your forest are acclimated to occasional combustion of 1000-hour and smaller fuels and may even benefit.
Broadleaf seedlings and shrubs draw moisture from the soil and release it into the air. Hotter temperatures during the leaf-on season increase the rate of transpiration, intensifying competition between the overstory and understory. Reducing understory leaf area for the next few seasons will make more moisture available to the larger trees, improving their health and resilience through drought. Burning off the surface fuels may also improve percolation as oak leaves often form impervious plaques that prevent rainfall from entering the soil. Forest managers often prescribe fire to benefit the overall stand in this way; a piece just up the road from me will be burned after the atmospheric river passes, while the larger fuels remain moist yet the finer fuels are dry enough to consume.
In areas where the overstory remains healthy, I would not expect soil loss or invasive propagation. Where canopy loss suddenly makes more sunlight and water available is where changes will occur, including saturated soils sliding downslope. Here there was little effort to guide the post-fire succession -- one planting of 150 seedlings per acre where 700+ were required was outcompeted by shrubs both native and invasive. In other areas, unguided succession led to the resurgence of a culturally significant fruiting shrub that had been suppressed by less fire-tolerant competitors. What will regrow in Sterling Forest is a question for someone who knows that area from before the elimination of fire, local historians or the Munsee.
posted by backwoods at 8:30 PM on November 17, 2024 [5 favorites]
Speaking from a US Pacific Northwest perspective... that's the best kind of forest fire. Clears out the undergrowth, gets all the fire-friendly seeds going, and generally just "resets" in the way it's supposed to work.
It's the too-hot fires that come from too much undergrowth from too much fire suppression that end up damaging the older trees and messing everything up.
As long as it was a quick run through, it *should* even leave a lot of the root structures that help hold the soil together. It's not foolproof, but really, this is what is generally considered the best-case scenario.
posted by stormyteal at 12:21 AM on November 18, 2024 [2 favorites]
It's the too-hot fires that come from too much undergrowth from too much fire suppression that end up damaging the older trees and messing everything up.
As long as it was a quick run through, it *should* even leave a lot of the root structures that help hold the soil together. It's not foolproof, but really, this is what is generally considered the best-case scenario.
posted by stormyteal at 12:21 AM on November 18, 2024 [2 favorites]
(Chiming in as someone who does prescribed burns in North Carolina) What happens during and after a burn is very, very site- and context-specific. Variables include the history of the site, its slope and orientation, sunny vs shaded, green vs dry plant tissue, flammability of plant tissue, are the plant fuels patchy or contiguous, depth and kind of leaf litter, weather variables like temperature, humidity, wind speed and direction, time of day and year, etc. There are whole areas of research into this as people like fire ecologists try to understand and conserve fire-adapted ecosystems, and as firefighters and land managers try to use prescribed fire techniques to reduce the severity of bad wildfires.
I like this YT video put out by The Nature Conservancy discussing what happened to the Sycan Marsh Preserve in Oregon when the Bootleg Fire burned through their forest treatment plots in 2021. In some forest stands, only above-ground plant tissue was consumed by the fire, but below-ground, plant roots and microbes and probably lots of other stuff remained alive in the soil -- the fire moved through fast enough that the soil level and below was largely untouched. In another stand with a lot of deadfall and undergrowth (aka fire fuel), the fire burned much hotter and for longer, burning down through the soil and consuming much more of the life below the soil surface.
posted by Drosera at 11:02 AM on November 19, 2024 [1 favorite]
I like this YT video put out by The Nature Conservancy discussing what happened to the Sycan Marsh Preserve in Oregon when the Bootleg Fire burned through their forest treatment plots in 2021. In some forest stands, only above-ground plant tissue was consumed by the fire, but below-ground, plant roots and microbes and probably lots of other stuff remained alive in the soil -- the fire moved through fast enough that the soil level and below was largely untouched. In another stand with a lot of deadfall and undergrowth (aka fire fuel), the fire burned much hotter and for longer, burning down through the soil and consuming much more of the life below the soil surface.
posted by Drosera at 11:02 AM on November 19, 2024 [1 favorite]
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This one topic is in my experience sort of laughed off as a “ha, how innocent we were!” byline, at least in comparison to Covid. But for that ecosystem, it really was a massive scale fire and particularly ecologically, given that so many affected flora and fauna are not found outside of Australia.
This video does a good job detailing some of your questions 18 months post-fire.
posted by seemoorglass at 1:07 PM on November 17, 2024 [1 favorite]