Tree-ring analysis explains physiology behind drought intolerance

Public Release: 26-Dec-2018

Oregon State University

CORVALLIS, Ore. – Tree rings tell the story of what’s happening physiologically as fire suppression makes forests more dense and less tolerant of drought, pests and wildfires, new research shows.

Scientists at Oregon State University and Utah State University studied 2,800 hectares of mixed-conifer forest in central Oregon, with many of the ponderosa pines in the study area dating back hundreds of years prior to 1910, when putting out wildfires became federal policy.

Other trees in the area, where fire history has been studied extensively, were younger, comparatively fire- and drought-intolerant grand firs.

The findings, published in Global Change Biology, indicate that as stands of trees became thicker over the past century, trees were forced to use progressively more of the heavier stable isotope of carbon for photosynthesis, indicative of increasing drought stress as they restricted the passage of gases into their leaves.

The research also suggests that rising carbon dioxide levels – atmospheric CO2 has increased 40 percent since the dawn of the industrial age – can’t help trees overcome the effects of forests having become more dense without fires.

“We wanted to document the trajectory of sensitivity to drought stress in response to progressively increasing fire deficits, and the threshold level of stand occupancy where decreasing resistance and resilience to drought stress, bark beetles and wildfire set in,” said study co-author Christopher Still of the OSU College of Forestry. “This was an intersection of fire ecology and physiological ecology – two areas that don’t meet up as often as they should.”

Before 1910, frequent low-severity surface fires played a key role in maintaining the forests of the dry mountain regions of the western United States. In the decades since, the fire deficits that resulted from federal policy – in concert with grazing, logging and land-use changes – have caused major structural shifts in older forests as shade-tolerant and fire-intolerant species have moved in.

Over the same period of time, concentrations of carbon dioxide in the atmosphere have not only been rising but doing so at an increasing rate; the higher CO2 concentrations have an effect on leaf gas exchange – the processes through which trees obtain oxygen for respiration and carbon dioxide for photosynthesis.

“We’ve known for a long time that fire suppression has led to crowded forests, which means more competition for resources,” said College of Forestry graduate student Andrew Merschel, another study co-author. “We’ve known that because of that, trees are more prone to drought, which makes sense – there’s less water reaching deep below ground and more trees pumping it out. Our research shows in a physiological way what’s happening. We thought there would be signals in the annual tree rings, and there are.”

A tree adds a ring for each year of growth, and in a conifer the ring consists of a lighter-colored “early wood” portion and a darker-colored “late wood” component.

Ring-size variation reflects temperature and precipitation at the time the growth occurred. And the chemistry of each ring tells a tale too, including whether a tree can use more of the lighter carbon-12 isotope it prefers, or had to resort to using relatively more of the heavier carbon-13 isotope for photosynthesis as well.

Tree-ring carbon isotope records also have shown that trees respond to drought by becoming more efficient with their water use.

What hadn’t been known, though, was whether increases in water-use efficiency, bolstered by rising CO2 concentrations, were enough to overcome increased drought stress resulting from a recent in-growth pulse of younger, fire-intolerant trees.

“Increased tree density seems to outweigh the benefits of carbon dioxide ‘fertilization,'” Merschel said. “It’s not necessarily bad if younger, drought-sensitive trees that established since fire exclusion die because that’s consistent with way these forests used to look – more open, allowing ponderosa pine to live to be 600 years old. Fewer trees and reduced competition allow the old, fire-resistant trees that have survived centuries of drought, insects, and fire to persist as the structural backbone of dry forests.”

Lead author Steven Voelker of Utah State University notes that drought sensitivity of forests in the dry mountain regions of the Pacific Northwest may be compounded as climate change increases the length of the snow-free season.

“Recent research from OSU and other institutions also shows that denser forests reduce snowpack across the Northwest,” Voelker said. “We cannot predict exactly how all of these factors affecting forests will interact in the future, but it is certain that lower-density forests will have more snow and less drought stress.”

And without “major changes to policy and management aimed at reducing stand densities,” many forests in central Oregon will cross a threshold that makes them less resistant to drought and less resilient to wildfires and bark beetle outbreaks, he said.

Future work by this research group will expand tree-ring isotope analyses into different areas of Oregon and other parts of the western United States to assess the effect of increased competition across conifer forests where it might be dryer and hotter – and thereby provide a more comprehensive view of how forest resilience has changed because of fire suppression.



45 thoughts on “Tree-ring analysis explains physiology behind drought intolerance

  1. Bottom line in forestry is that you either manage everything, or you leave well alone. Anything in between will likely end in tears.

    • Here’s a link to an overview of forest management practices around the world.

      Different techniques are required for different circumstances. For instance, clear felling and then controlled burning can actually enhance biodiversity. Who knew? It seems likely to me that environmentalists wouldn’t understand that kind of thing and would try to enforce bad forest management practices.

  2. The findings, published in Global Change Biology, indicate that as stands of trees became thicker over the past century, trees were forced to use progressively more of the heavier stable isotope of carbon for photosynthesis, indicative of increasing drought stress as they restricted the passage of gases into their leaves.

    Were ‘forced’ to use the ‘heavier isotope’? What are they talking about? carbon-12?

    • Hmm that’s what I thought… how does a tree tell the difference between C12 and C13… that’s bloody clever biology if it can … what’s the mechanism cos I am sure we could use it in the nuclear industry somehow for separating isotopes.

      • Plants preferentially use the lighter CO2 molecules containing C12 isotopes. This is well-understood chemically, but the difference is quite slight. The fractionation is partly due to differences in diffusion rates (used in nuclear industry!) and partly because the heavier molecules are slightly less reactive.

        When plants are moisture-stressed they open their stomata less in order to limit evaporative water loss, but this also keeps down the amount of available CO2 forcing the trees to use more of the energetically less efficient C13 molecules.

        • Plants preferentially use the lighter CO2 molecules containing C12 isotopes.

          Maybe yes, maybe no.

          It would be important to know if the, per se, ….. “studied 2,800 hectares of mixed-conifer forest in central Oregon” ….. was located near the top of a mountain ….. or down in the lowlands in a valley, …. because, to wit:

          Differences in altitude are also known to affect terrestrial plant carbon isotopic signatures (δ13C) in mountain regions, since plant δ13C values at high altitudes are typically enriched (Körner et al. 1988; 1991) compared to the carbon signatures of plants from low altitudes. Soil organic matter also show enrichment in 13C with soil depth, which is suggested to be a consequence of humification and the loss of the lighter isotope (12C) via respiration, thus concentrating 13C in the soil organic matter (Kramer et al. 2003). This might be transitional to temperature and differences in decomposition. Moreover, the isotopic carbon signatures of autochthonous and allochthonous food-sources in aquatic ecosystems are generally separated, which is also reflected in the consumer community. Stable isotope analysis is therefore a useful method for determining the autotrophic or heterotrophic character of lake food webs (Karlsson et al. 2003; 2007).

      • continuing to comment myself…

        It is clear that when the stomata close down, CO2 can’t enter. However, when the amount of CO2 increases, it should be easier to get the CO2 without opening the stomata. So the proportions of isotopes will change due to change of atmospheric concentrations. What this tells about whether some level of drought stress was there or not, remains a bit elusive without reading the paper.

        The paper is paywalled, so how much ‘forced’, and how much interpretation by researchers, is left a mystery to me.

        This is science today. It is awesome, but that is not enough. You need to write a much-promising press release and leave the paper behind the paywall.

        • Plants frequently use certain isotopes preferentially. Bananas preferentially take up potassium 40 and are radioactive as a result.

          The conditions described in the stressed forest apparently cause a shift in the uptake from 12C to 13C. That is fascinating and obviously has potential for us to use this knowledge as an indicator of stress in analysis of tree rings.

          If it is an indicator of drought stress, then tree rings can be analyzed isotopically instead of by width. It would also make it possible to use trees that are not at the northern or southern growing limits.

          We now need an analysis of the influence on isotopic uptake caused by drought stress. This can be applied to existing collections (by species) to create a drought stress profile of the past.

    • That’s a good question, there are 15 isotopes of carbon one of which is less than 12g/mole and the other 13 are heavier than C-12. C-12 being ‘normal’ carbon.

  3. Tree-ring analysis explains

    may be

    might be

    Future work by this research group will require more money.

    • Could, should, according to the NYT, etc etc. Paying attention to these words can save you precious reading time.

  4. Scientists at Oregon State University and Utah State University studied 2,800 hectares of mixed-conifer forest in central Oregon, with many of the ponderosa pines in the study area dating back hundreds of years prior to 1910, when putting out wildfires…

    We used to call them “Forest Fires” Changing the language is a technique right out of Orwell’s “1984”.

  5. More govt regulations led to denser tree stands, which means more fuel for fires and fires could more easily jump from tree to tree. Pretty simple. Fewer fires should lead to fewer “fire-resistance genes” in the population. and more drought to more “drought resistant genes.” ..And tell us, how exactly does MotherNature provide for such a natural mass spectrometer and selection process for co2? How does denser forest lead to reduced snowpack? All the fallen snow must remain up in the canopy and sublime directly back to the atm from there?

  6. “dating back hundreds of years prior to 1910”

    In the late 1800’s, farmers and ranchers in Florida used to burn the forests and fields every year

  7. From the article: “And without “major changes to policy and management aimed at reducing stand densities,” many forests in central Oregon will cross a threshold that makes them less resistant to drought and less resilient to wildfires and bark beetle outbreaks, he said.”

    So Trump was correct to call for changes to forestry policy, including increased logging. Trump is one step ahead of everybody on just about everything.

    • Tom Abbott,

      I don’t think you understand the situation. It’s not that the Forestry Service has advocated against management to lower fire risk. The funds aren’t available. It’s very expensive to thin and conduct prescribed burns. What’s more, “For FY2017, the Administration is requesting $5.757 billion in discretionary appropriations for the FS (see Table 3). The FY2017 request is $40.0 million below the FY2016 request and $606.9 million below the FY2016 enacted discretionary appropriation” (the last column for fig. 4 doesn’t tally)

      Over time, appropriations have shifted proportionally from forest management (including infrastructure) to wildfire management – not prevention. In 2017 58% of the discretionary budget was for wildfire management.

      If Trump wants all his policies enacted, the USFS is going to need the funding to do it. Only a portion of timber sales goes back to the FS, and those go to the area that was cut, specifically for restoration purposes to ensure that trees grow back (or to recreational facilities, if there is a funding backlog).

      Furthermore, increased logging is not necessarily going to help decrease fire risk. The thinning that needs to be done is the small-diameter trees and underbrush, which may not be economical to loggers. Clear cutting often requires thinning as it grows – I suspect that some of the overgrown forest is due to past cutting as much as fire suppression. The Camp Fire traveled mostly over clear-cut forest on its way to Paradise. Even raising the limits of logging has a cost to it, since preparation for timber sales is involves its own field- and paperwork.

      Post-fire salvage operations don’t always do much in the way of fire prevention, and they have to be done within the following few years for the timber to be marketable. They may also decrease the ability of forest to regrow due to soil compaction and mortality of whatever seedlings have become established.

      Then there’s the fact that much of the forested land is in private hands.

      Without funding, the main effect of Trump’s EO is to benefit loggers. Without appropriate land management strategies (which take time and resources to develop), it may in some cases be counterproductive.

      We’ll see how much the FS gets this year.

      …Ah! Found it. Terrific. The budget has decreased by $486 million for 2019 vs. 2018. Well, so much for Trump’s being “one step ahead.” It’s all hot air.

      • Again, you are distracted by your own all-salvation-must-come-from-government stance. Allowing commercial loggers to harvest a so-called National Forest should require $0 federal appropriations.

        • Red94ViperRT10,

          You apparently don’t know much about forestry, and you certainly don’t know much about me.

      • The budget for 2018 was $4.73 Billion. The budget for 2019 is $4.77 Billion. And it will end up being higher than that, due to the cost of fighting wildfires, that is over budget most years. The budget for the forest service goes up every year, just like every other service in the USDA. They are built in, and anything less than the budget growth they have filed a “justification” for, is seen as a cut.
        The real problem is fire suppression is failing miserably to achieve its goals. It has interesting parallels with the “War on Poverty”. The unintended (or not) consequences of manipulating systems that sound good when the bill is written and signed. But allow problems to fester and escalate into systemic failures, because the problems are never addressed when they still manageable. From this document:

        In 1995, fire made up 16 percent of the Forest Service’s annual appropriated budget
        —this year, for the first time, more than 50 percent of the Forest Service’s annual budget will be dedicated to wildfire. Along with this shift in resources, there has also been a corresponding shift in staff, with a 39 percent reduction in all non-fire personnel. Left unchecked, the share of the budget devoted to fire in 2025 could exceed 67 percent, equating to reductions of nearly $700 mi
        llion from non-fire programs compared to today’s funding levels. That means that in just 10 years, two out of every three dollars the Forest Service gets from Congress as part of its appropriated budget will be spent on fire programs.

        • OK, so I don’t quite understand this. In my link, why is the current year’s budget compared to the 2018 annualized continuing resolution budget, which is $5,252,717,000, if the real budget was $4.73 billion? (That was before the $528,000,000 supplement for fire fighting.) The 2 previous years actual budgets were both around $5.6 billion; 2017 was $30 million LOWER than 2016 – and that 2017 actual budget is very close to the budget shown as the 2017 annualized CR in the 2018 budget overview.

          So, if I understand correctly, the 2018 budget would normally have been higher than Trump asked for? And the 2019 budget he’s seeking is now half a billion lower than what it would normally be if allowed to increase automatically (a billion less, if you include firefighting supplement)? Is that what you’re saying?

          Sure looks to me like the budgets have been decreasing in real terms, especially when you look at all the negative numbers comparing this year’s and last year’s annualized CR. The numbers are not adjusted for inflation, presumably.

          This is why I think the EO doesn’t carry much weight. He can try to move money around for more management, but to chop the budget for roads to $95 million makes it exceedingly difficult to manage the forest or open it up to logging. And he’s cutting thousands of full time jobs in favor of interns. I’ve talked to foresters who think interns are more bother than help. One thing he’s not cutting is administration, or paperwork. I understand there should be accountability, but that takes time, too.

          • It is complicated by the process. The Justification is what the FS wants. The budget is what is appropriated for them to spend. When they overspend, which they will, they get additional funds through riders in the CR’s (continuing resolutions). They “annualize” this and say it is what they should be budgeted, plus additional.
            Then you have the dynamic of the Administration asking for more than they will get, and the Congress saying it will give a certain budget, which they know will be spent on bureaucrats instead of workers, so there will be a “reckoning” when the real bills, exceed the budgeted amounts. It is a political dance where they always want more, they always get more, but if a Republican is in office they cry poverty, and if a Democrat is in office they trumpet how wisely the money was used to suppress forest fires. Which creates the fuel for the wildfires that will eventually happen, under the worst conditions.

  8. “whether a tree can use more of the lighter carbon-12 isotope it prefers

    I don’t find this idea of preference helpful, there is far to much hint here of a Maxwell’s demon sitting inside the needle of the pine tree, vetting the CO2 molecules and letting the C-12 pass but not the C-13.

    The real reason for the preference is because C-12 containing molecules are less massive than the C-13 ones.

    Re: Why do living organisms exhibit a preference for Carbon-12 over Carbon-13?

    The much more interesting observation of course is that we have a selective change in the amount of C-13 captured by these gymnosperm species during periods of drought stress.

    • Not a selective change. Plants open their stomata less in drought to conserve water by avoiding evaporative loss. This means less CO2, so the the CO2 gets used more intensively, including more of the heavier and less reactive C13.

      • It could also be more CO2 available, so less need to open stomata, so CO2 not diluted and more C13 ends up in use.

        I’m not buying to this paper easily.

  9. …as published in Global Change Biology. Given how many times the Global Warming folks have been wrong I am skeptical.

  10. Much ado about nothing it appears to me and I blame fossil fuels for giving men too much time on their hands to dither about with these increasingly marginal pursuits. Presumably this will all change when the fossil fuels are banned and they’re frogmarched off to the fields to chop trees for firewood or hoe taters or some such.

  11. – there’s less water reaching deep below ground and more trees pumping it out. Our research shows in a physiological way what’s happening. We thought there would be signals in the annual tree rings, and there are.”

    but but BUT – doesn’t temperature determine the size/number/density/thickness/whatever of tree rings?

    ….ecause that’s consistent with way these forests used to look – more open, allowing ponderosa….

    Errrrrr – what happened to the notion that ‘Nature abhors a vacuum’

    way these forests used to look

    Show me pictures please or, It Didn’t Happen…

    outweigh the benefits of carbon dioxide ‘fertilization

    There is no such thing out in The Real World.
    Something, anything, EVEN carbonoxide only becomes A Fertiliser when it was the Liebig Limiting Nutrient.
    (Something entirely overlooked even by skeptics)

    Thus we see how muddled these folks are, talking about drought (=water shortage) then dragging CO2 into the mix and I’m sorry, but I don’t see how they’ve explained anything.

    All we have here is a report on the observation of a small and insignificant ‘thing’ coupled to wild could possibly maybe and in the future speculations based on preconceived notions.

    Sorry folks. This is unreasonable behaviour on your part, any more of it and I demand a divorce.

  12. Un-ban wood stoves in CA. Most rural people are poor and would quickly gather up the dead wood. And ramp up logging and the construction of logging roads, of course.

  13. Until the last ice sheets retreated, there were Very Large Animals called mastodons and mammoths on the North American continent, as well as a few other large critters like giant sloths and the distant ancestors of today’s African giraffes, that grazed on woody plants like trees, and in some cases, ate the entire plant if it was small enough. There is a long, long list of megafauna on all the continents, if anyone is interested.

    That means they thinned out the woodlands and in Europe, the woolly rhino was large enough to do the same thing. Elephants, giraffes and rhinos are all that are left of the megafauna and they live on these woody plants. Those are only a few of the megafauna that engaged in forest management, which is what Mother Nature intended.

    There is nothing now other than humans to go on doing that, thin out the trees to let light and air into the forests, which are now prone to fires and lose their disease and drought resistance, among all those other things.

    Those spindly trees with skinny trunks and small tops are being deprived of nutrients. They should not look like that. A healthy tree that tall has lots of thick branches with a heavy load of leaves or needles, depending on the species. Without leaves or needles or room to spread the root system, they starve. Period.

    Any time you see trees that are all top and no trunk, it means they can’t get the nutrients they need to grow. No healthy tree is 20 feet tall on a trunk that I can close one fist around, with a few skinny branches and a thin load of leaves, all at the top.

    If the imbecilic ecohippies don’t understand anything about how Mother Nature really works, that’s their curse, not ours. We have to do what the giant mammals used to do, since those big lugs aren’t around any more.

    Otherwise, we end up with these massively destructive fires, which do the jobs that the magafauna aren’t around to do.

    It’s a real shame that the ecohippies have no understanding of how the natural world really works.

    • Forests capture sunlight and convert it to biomass energy. Don’t forget the lesser creatures, e.g. pine beetles are one form of life today that can and do use this energy and thin forests. Deer, elk and other animals will feed off small trees, and of course fires thin them also.

    • Yes, R Shearer, but that’s become the extreme now. That a forest should have to depend on such things seems out of kilter to me. This is why I thoroughly detest ecohippies.

      If you turned a herd of elephants loose in the forests of California, they’d have the job done in no time, and cost nothing other than taking them there. You prune fruit trees partly to make them easier to harvest, but mostly to produce better, tastier fruits.

      All these densely-packed forests need thinning, the same as a row of radishes in the garden. The DNR people went north of me to thin out a large stand of trees, various species, partly to reduce the fire hazard – always a priority – and partly because those trees are up to 40 feet tall on trunks that are barely 5 inches in diameter. That is not a healthy tree at all. The only part that gets sunshine and air is the top, where the very thin leaf mass is located. I was surprised when I saw it, but glad that DNR did it.

      If a forest is left to itself, with nothing to do the pruning for it, then fire, bugs, disease and finally just plain squashing out the younger plants will take place. In Siberia, there are forests so old and so thick that there is no room for humans to walk through them. The only route is through the treetops. When there were wildfires a few years ago in Russia, they jumped the roads to get to the fuel sources across the roads. When Yellowstone was having a problem with willows taking over land, someone brought in the elks and the elks started pruning them back. That’s how it’s supposed to be done. Since we can’t get the megafauna back, and they would do the job quickly, we have to do it now.

      Maybe if someone told the ecohippies that by NOT thinning out the forests, they will die out completely (it is a possibility), they’d get so agitated they’d evaporate into beneficial gases. Now there’s a thought.

  14. Trump is already on this case. Left alone, this guy would resolve all of the problems aching to be solved. Even this paper, which perhaps is a contribution to science, is linearly thinking in the biologist’s inimitable way. No proactive human solutions considered, even though they are hanging out there like a dog’s tongue. What Trump said for instance. They throw up their hands in the face of the task nature alone must solve.

    Anyway, this paper gives some hope. Iwas about to question the only thing I know about tree rings (age of the tree) after the t-ring readings put out by the clime syndicate. I was a logger in my younger days in the 50s in Jarvis Inlet, British Columbia and contributed to the largest clearcut in the world, apparently the only manmade feature visible from the moon, so like the thousands of Nobel Laureates of the IPCC, Im famous, too. We first sawed across the tree rings and at the mill we sawed along tree rings.

    • “It’s not necessarily bad if younger, drought-sensitive trees that established since fire exclusion die because that’s consistent with way these forests used to look – more open, allowing ponderosa pine to live to be 600 years old.”

      Ah, let young trees die – and hope that no fire happens before they rot on the ground. Forest thinning? What a horrible modern idea. Let Mother Nature develop all catastrophes she likes so much.

  15. Drought stress copeing is a function of the ramped up level of the plant molecule GABA (described in yesterdays WUWT post about wheat). GABA synthesis increases resistance to drought stress by making the plant capable of dealing with greater osmotic stress; it allows more internal water to be inside the plant & reduced wilting.

    Back in the 1990s experiments on young fir tree seedlings exposed to elevated CO2 (eCO2) plus drought found GABA was elevated in eCO2 when there was no excessive temperature stress at the same time. In that case (eCO2 + drought) the greatest rise in GABA was noted in the trees’ roots.

    To me the O.P.’s suggestion that: non-fire forest stand = high density, & that is why non-fire tree population = less drought tolerant trees is too simplistic. GABA synthesis (as explained in mentioned WUWT wheat thread) varies in different plants & even geno-types of the same kind of plant. I am suggesting it is not high tree density (in non-fire forests) responsible for increase of less drought tolerant trees (in the non-fire forest) but rather low GABA synthesizing trees that would normally have been held in check (by fires) gained a niche.

    As for the carbon isotope issue broached in O.P. , again (as pointed out elsewhere) rise in GABA beneficially impacts photosynthesis under plant stress so I see no reason why the heavier carbon isotope utilization is a negative factor. Authors also distinguish between 2 stages of growth represented in each year of a tree ring composite; we know that with age eCO2 reduces (the precursor molecule of GABA) glutamate in plants, so I can see another impact on the annual phased cycle in the tree ring components & which carbon isotope utilized due to this (glutamate availabiliy) as an upstream rate-limiting factor (of GABA as drought response).

  16. I didn’t read the report, even after made available without the paywall, but this sounds like it could be interesting sciencey stuff, with applicable findings related to forest management, without ever once referencing “Client Change(TM)”. That has become my test for is-this-science-or-is-this-bulls**t.

    • Red94ViperRT10 ,

      So any paper that says anything about climate change is BS? Don’t you realize the bias that reveals? That’s akin to saying, “Anything Trump does is wrong” or “Anything reported in the NYT is fake news” or “anything AGW skeptics say is not worth listening to.”

      You can’t justify any scientific stance if you are unwilling to look at evidence.

      Closed minds are the greatest danger we face, no matter what the issue is.

      • It has been virtually 100% reliable so far. How do I know that? Because I read beyond the first mention of “Climate Change(TM). That’s how I knew this article has merit. You missed my point… if it sounds like something that still should get published without mentioning climate change… I regularly read entire articles I disagree with. You should try it some time.

  17. So how does this effect carbon dating?
    I know that the study dealt with environmental conditions effecting the intake Carbon 12 vs Carbon 13 but it seems it would also point to environmental conditions effecting the intake of Carbon 14.
    Maybe even other isotopes used for dating?
    A Layman’s question.

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