More research on less tree growth after fire

(RJ Sangosti, The Denver Post)



This article summarizes some recent research on the topic:

Among Stevens-Rumann,’s work was a 2017 study of nearly 1,500 sites charred by 52 wildfires in the U.S. Rocky Mountains. Her research found that lower elevation trees had a tough time naturally regenerating in areas that burned between 2000 and 2015 compared with sites affected between 1985 and 1999, largely due to drier weather conditions.

More recently, a 2019 study written by her colleague Kerry Kemp found that both Douglas fir and Ponderosa pine seedlings in the Idaho’s Rocky Mountains — just south of B.C. — were also struggling in low-lying burned areas due to warmer temperatures, leading to lower tree densities.

Both studies attribute climate change to be the lead cause of why the trees are struggling to grow back in certain fire-scarred areas.

As a result, some ecosystems will no longer be able to support tree species. Instead they may convert to grasslands, she said.

We’ve talked about this before (for example, here).  But I would like to know how this kind of information is being incorporated into long-term planning for timber harvest levels. In accordance with the requirement for sustainability on national forests, we should be assuming forest growth consistent with the natural range of variation, which should reflect the effects of climate change on future forests.  What I would expect to be seeing based on this kind of research is reduced area suitable for timber production because it would become too dry, and reduced volume resulting from reduced density, slower growth rates and more frequent fires.  “Sustained yield” means that projections of lower future timber yields may lead to reduced near-term volume. I’ve looked at the timber volume documentation for a few forest plan revisions, and I haven’t found anything there about climate change (there’s usually an unconnected section on the effects of climate change somewhere).  (Projected timber harvest volumes are not tending to go down in revised forest plans.)  Maybe that just requires digging deeper than the public-facing documents or maybe it’s not happening.   Does anyone know more about this?

5 thoughts on “More research on less tree growth after fire”

  1. Jon, thanks for posting this! There have been many studies looking at natural regeneration in the Rockies, and they are interesting for a variety of reasons. Looking back from our humble regeneration efforts in south-Central Oregon in the 80’s, it is a great sociology of science exercise to think about how we thought about it then, and how scientists looked at regeneration, versus today. Similarities and differences.

    If the climate changes, it will have impacts on tree growth that we don’t know.
    Back in the 80’s we thought “there are many things that will affect tree growth (weather/climate, bugs, diseases, fires, management or not” and we don’t understand how that will impact them in the future, so the best we can do is try to calibrate our growth and yield models to fit the sites we’re working with today, and keep monitoring things so that if they change we will observe it and be able to react.”

    Today, we seem to think that we might be able to model those things based on a series of climate modelling assumptions (which include economic growth) followed by a set of vegetation modelling assumptions. What we continue to know: having a seed source is necessary but not sufficient, mineral soils and water availability are important, grass competition can be important, and so on, which were all learned by looking at regeneration.

    But one of the things that is most different about the approaches were that in the old days on dry site pp, if we didn’t get pp regen, we would assume that the area needed planting before the site was fully occupied in terms of other plants that use up water. Nowadays, we might think that due to climate change we shouldn’t plant, because the seedlings won’t grow, due to climate change, when we don’t actually know that. Because we don’t actually know 1) the microsite factors that are important 2) phenotypic plasticity of the trees we plant nor 3) how much climate change will impact either, nor mycorrhyzae, nor diseases nor insects, nor browsing herbivores, nor..

    My point being, if we had thought that in the 80’s for dry site ponderosa pine, we wouldn’t have had the major effort to learn to plant them (hiring reforestration specialists, buying tree coolers and so on).

    But back to modeling ASQ’s…we haven’t got a clue about the future so is the best practice assuming what is current, or using RCP 8.5? Or 4.5 and figuring halfway inbetween RCP 4.5 and what is current? I’d argue that this is not a useful exercise.

    Finally, it’s important to note that this study is about areas that may well be different from my observations and ppine has a great deal of genetic diversity. But for those who are out and about in pine and Doug-fir country, we can look around at the ground in our own areas and see what it looks like. For me “driving around” yields a plethora near some fires and hardly any pp around other fires. I’d guess soil and summer rain differences but it’s hard to tell past cone crops (pp cone crops where I worked are intermittent in the giant crops have enough to not be all eaten up by squirrels, etc.)

    On the west side of the Sierra Nevada, normal seed crops are borne on average every 2 to 3 years, while very heavy cone crops occur an average of 8 years apart. Trees may start to bear cones as early as seven years old and continue for 350 years, and those over 25 inches in diameter are the best producers. Cones may bear up to 70 seeds each. Seeds are damaged or eaten by insects, birds, and small mammals such as mice, chipmunks, and tree squirrels.


    Summer soil moisture most often determines whether ponderosa pine will grow in a particular area. The west slope of the northern Sierra Nevada, which receives an average rainfall of 69 inches, may be the wettest area in its natural range supporting ponderosa pine in large numbers.

    Ponderosa pine grows on a wide variety of soils, including those of igneous, metamorphic, and sedimentary parent materials. Its distribution on drier sites depends on soil moisture, which depends on soil texture and depth. The species grows better on soils which are medium in texture and, hence, release their moisture to plants readily.

    If Rocky Mountain ppines are as sensitive to rainfall, you may notice that according to this, rainfall may increase in the Northern Rockies.

  2. I would only argue that it would arbitrary to ignore research about the likelihood or effects of climate change. After a reasoned discussion, staying the course may still be the best answer, but I haven’t seen that discussion applied to forest plan timber harvest levels.

  3. The FS use to have many nurseries for propagating seedlings from local seed sources. It was not a bad idea. The FS doesn’t really plant much now compared to the amount of acreage burned. I think they should plant more trees taking into account which trees are most likely to survive.

  4. Incorporating this into timber harvest projections is a somewhat touchy subject, as I am sure you are well-aware! The process for determining timber harvest suitability and ASQ requires us to identify areas that are not suitable due to regeneration difficulty (among other things). Most of the forests that I work with did a fair amount of tracking of this (when most of the harvest was even-aged regeneration harvest) in the 1970s and 1980s. But when harvest shifted more to thinning, the need to keep track of that was lost. We continue to encounter areas in the west that we thought were forest, but with continued drought, and even fire, we find that insects are showing us where those areas that are really woodland or savannah are (after drought) and where those areas are that we cannot regenerate after fire (perhaps because they really should be woodland or savannah). Soils can tell us some of that, and so can some climate modelling. But, most of the forests that I work with have no tracking set up to identify these areas. Another issue is that we probably prescribed stocking on a lot of these areas that is far too high, particularly in places that experience periodic droughts. And many forests had over-optimistic timber harvest based on species composition changes that are proving to be ill-advised in drought-prone areas. A lot of our management has been based on a period of time that had a relatively stable climate.
    What we know right now and can do right now – recognize that yields are going to be more variable than we thought based on our prior management assumptions; do a better job of identifying areas that really should be woodland or savannah (and not included in timber projections) – they just appeared to be forest for a variety of reasons, and with more variable climate, they should be removed from the timber base.
    Beyond that, as Sharon notes, it’s difficult to incorporate other changes that might be expected from climate change because there is a lot of uncertainty. Our best bet in the mean time is to make sure we are restoring species composition, adjusting our stocking levels to incorporate drought and insects better, and carrying out fuels treatments to restore the role of fire.

    • Thanks for those insights A. Of course “touchy subjects” are exactly what the law expects full disclosure and rationales about. And making sure projected yields reflect changes like stocking, species and fire. Maybe you are in a position to help do that. Lack of “tracking” also suggests a monitoring need that should also be addressed through forest planning.


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