What can tree rings can tell about forest fires?

At school, I’m sure every child was once fascinated by the knowledge that the rings on a tree trunk can tell it’s age. The very fact that the age of a tree can be recorded this way used to be unreal, and to actually count the rings of a cut tree and figuring out the age of a tree in front of us was an inexplicable discovery.

Trees can be dated in a variety of ways; dendrochronology (the study of tree rings) is but one of them. In recent times, this method has been criticized by many people because of the contradiction it provides. Why would you cut and kill a tree, just so you can figure out it’s age? One such mistake in 1964 had lead to the cutting of the world’s oldest tree at that time. An accident that could have been avoided, if the method of dating trees were different.

However, tree rings are very valuable for various other reasons. That is why, the study of dendrochronology still continues with an altered methodology. Scientists do not cut the entire tree to look at the trunk as a slab, but use drills to cut out a small, thin core of the tree that fully captures the information embedded in the core. Not only do they record the age, but can also serve as an important in climate studies and fire predictions. This characteristic is now being used to recontruct climate record pertaining to forest fires, and can be helpful in discerning patterns of fire in a particular region.

Why is this important? As I have mentioned before, forest fires have massive ecological benefits if controlled and used in the right way. But it is key that forest managers do not let it get out of hand.

The connection between climate and tree rings

During periods of dry weather, there is competition for food between different parts of the tree. Consequently, the base of the stem is sacrificed for more important parts of a tree. The tree rings created during this time at the base of the stem are usually narrower and wider than during moist periods. This of course, depends from species to species and the climatic zone in question (Note- Many other factors also control the growth of tree rings; we simply infer possible rainfall/temperature information from the rings present on the core. It’s not an exact method, probabilities are involved).

This information can be used to determine long periods of dry weather in the climatic record. Can this be linked to possible fires in the past? Yes. In fact, scientists have gone on to link large scale climatic fluctuation with occurrence of forest fires in the western US, as shown in the picture below-

burn scars in tree rings के लिए इमेज परिणाम
La Nina is part of an ocean circulation phenomenon that results in warmer than usual climates across the western coast of the US.

How forest fires are recorded in a tree

burn scars in tree rings के लिए इमेज परिणाम
Source: Google Images.

This is obviously subject to the fact that the tree survives the forest fire. If it does, the fire leaves burn marks in the tree and the tree trunk, that can be embedded amongst the tree rings. So when a dry period is superimposed by a fire mark, it is fairly safe to assume that that year saw a forest fire in the region. The intensity of a fire can also be guessed using these fire marks. Such studies are now being carried out in western US to figure out how droughts and dry periods have affected forest fires in that region.

What do these studies tell us?

Even in the past, periods of extended dry weather and droughts have caused forest fires that are significantly larger than normal fires experienced during normal years. Also, the very intense fires that we have seen all over the world, especially in the US, are not outliers in the historic sense. If the tree rings and burn scars are any indication, such fires have occurred in the past several times.

Given that many parts of the world are now experiencing droughts, along with the danger of human-induced warming, it is likely that the dry periods we are seeing today can extend for many years to come. And if that is the case, the fires will only burn bigger and brighter and hotter.

This is a very grave observation that needs to be taken into account in fire-management and land management plans all over the world, not just the US.


References-

  1. https://www.nature.com/articles/d41586-018-01686-y
  2. https://www.nature.com/news/future-us-megadroughts-set-to-be-the-worst-in-1-000-years-1.16916
  3. https://www.nature.com/news/prepare-for-larger-longer-wildfires-1.22821
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4 thoughts on “What can tree rings can tell about forest fires?”

  1. 1) While cookies (whole slabs) are useful in studying fire scars and culturally modified trees, it is not necessary to cut down a tree to count the rings. We use increment core borers to take out a pencil size core form the tree without harming it.

    2) Trees are not thermometers nor rain gauges. Ring patterns are caused by a variety of interacting environmental factors, not temperature and precipitation alone.

    3) Recent extreme fires are the result of more than a century of fire suppression resulting in abnormally high fuel loads. This is a self-correcting problem.

    Liked by 1 person

    1. 1) of course, I know that tree cores are used today to count the rings. But how useful is a pencil core in discerning fire scars? Will they even be visible?

      2) I agree that a variety of factors affect the growth of rings in trees. Temperature and precipitation are just two of the factors, but for the purpose of this particular study that is our focus. I realize that the inferences we draw about past climate from tree rings is far from absolutely accurate. But in many cases, they ARE useful.

      3) I did not know that, thank you. So simply the presence of high fuel load is causing abnormally large fires? While fuel does play a significant role, isn’t the dry and hot conditions contributing to more frequent fires?

      P.S- I sometimes oversimplify my articles to get the point across to my readers, most of whom are very new to the concepts of environmental science. But, thank you for correcting these things. I shall alter the post accordingly 🙂

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  2. Again, forest fire frequency is a function of many factors, including weather, aridity, wind, precipitation, lightening, increasing human interface and fuel loads. There is no clearly discernible global trend in forest fire frequency that can be attributed to “climate change” alone.

    When we look at historic photographs of the American west, it’s clear that naturally occurring fires were frequent, keeping the forests thinned and limited in extent. This was, in part, due to indigenous practices of habitat modification by fire, but it was largely a natural phenomenon, in which North America forests have evolved over millennia.

    Since 1910, a heavy forest fire year, government proscribed fire suppression, coupled with genocide of the Native American population, has resulted in massive overgrowth of remaining North American forests. This has resulted in “dog-hair” tree stands that are far more susceptible to extreme fire damage than forests where fires have taken their natural course. Fire suppression has also resulted in large and extended tree borer infestations, leaving large swaths of dead and dying trees.

    Economic pressure by timber companies, and population pressures pushing into the forest interface, continue official fire suppression in areas other than designated wilderness. These factors alone have increased the frequency of destructive forest fires, regardless of any weather or climate influences.

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    1. Aren’t weather, acidity, precipitation all a function of climate? And wouldn’t climate change then affect these factors?
      Climate change alone is not a cause of forest fires, but certainly it is a factor that could make the fires worse. Do you agree?

      Of course, human interference are a huge factor in the start of forest fires in the recent years. I never explored the possibility of fire suppression as a cause of exacerbating forest fires. Thank you for this, I shall try and write a post exploring this phenomenon.

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