Correction – Tree Invasion

Thank you to a couple people, particularly Dan Carter, for pointing out an inaccuracy in my last post about woody expansion in prairies.  In my second paragraph, I said that woody plants had expanded in Konza Prairie (Kansas) under annual fire.  That’s not correct.  Woody plant abundance has actually changed little in annually burned prairie units, but abundance has increased dramatically in units burned at 4 year intervals (and at longer intervals).  I’ve corrected that text now, but wanted to let everyone know about it.

I don’t think this changes anything else in my post.  Changes in fire frequency alone are unlikely to be the sole reason that woody plant expansion appears to be more rapid now than several decades ago.  I’m sure I’m not the only one who has observed smooth sumac (Rhus glabra) increasing under 3-year fire frequencies, for example.  In addition, I know of prairies that went through decades without any fire at all and didn’t appear to experience rapid woody plant expansion until the 1970’s or later.  It’s a big and complicated puzzle.

Regardless, the text in the post should now be correct!

If you haven’t done so, I’d encourage you to go back and read the many comments attached to the initial post.  There are some excellent responses and ideas from a number of readers.

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About Chris Helzer

Chris Helzer is an ecologist and Eastern Nebraska Program Director for The Nature Conservancy. He supervises the management and restoration of approximately 4,000 acres of land in central and eastern Nebraska - primarily along the central Platte River. Chris is also the author of "The Ecology and Management of Prairies in the Central United States", published by the University of Iowa Press.
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10 Responses to Correction – Tree Invasion

  1. James McGee says:

    Chris,

    If many of your readers are going to understand the reason increasing CO2 levels can select against C4 plants then they will need a more indepth description. The bottom line is C4 plants have a biochemical cycle, along with physiological differences, that allow them to photosynthesis better in hot dry conditions. The divisions of short grass prairie, mix grass prairie, tall grass prairie, savannah, and oak woodlands best follow patterns of increasing precipitation. As CO2 levels rise a plant’s leaf opennings, the stomata, can obtain more CO2 for photosynthesis while losing less H2O. The makes the C4 pathway less competitive.

    You can think of the factors leading to the survival of a species as being on either side of a scale. Some factors will sway survival toward a given species. Other factors will have the opposite effect. I think the really important measure is the tipping point. This might be what we are observing in many prairies.

    James

  2. Patrick Swanson says:

    I agree with other posters that the increase in woody shrub invasion is probably multifactorial. One poster commented on the timing of burns, with spring burns favoring expansion, and fall burns favoring suppression. I can see why this might be true because in the fall the woody plants are pulling back their resources into their root system and if they are burned at this point the have greater difficulty achieving this. In my experience I have had better luck keeping woody shrubs from resprouting if I cut and treat during the fall than if I cut and treat in the spring or summer. Maybe fire has the same effect.

    Another thing to consider with the timing of burns is the effects on caterpillars that feed on the leaves of woody plants. This year for example, I noticed a small copse of sumac that was completely defoliated by caterpillars. Now I wish I had learned what they were. Perhaps the timing of burns or burn frequency limits how well insects can exert their control on host shrubby plants. Just a thought.

  3. I’m not a scientist, biologist, or even an expert … I’m just a landowner trying to teach myself about retoring native prairie plants. We have noticed a major difference in old photos of our family land here in northern Oklahoma versus today. In the pictures from the 1950s through the 1970s, even early 1980s, there is far less “brush.”

    We could not figure out why. Is it because my grandfather’s horses are gone? There were only three of them with access to the hilltop, could they have trampled that much brush? I’m skeptical. I know my grandfather didn’t spend any time clearing brush — he was a farmer. All spare time was spent making a living, and clearing brush did not qualify as a mortgage-paying project. And it’s not a fire-related issue, because he only burned his wheat fields — not the woodland or open prairie areas.

    So, I’m fascinated to read that it’s not just an issue here at Hilltop House, but also in all prairies. Thanks for the informative posts, and keep up the good work!

  4. Bob says:

    Here’s a basic question – How we know the hostorical frequency of fire? I’ve heard of observing the frequency of fire scaring in trees. I’m a little skeptical that that would be relevant or accurate for prairie fires. What other physical evidence is there?

    • Chris Helzer says:

      Bob, it’s a great question. Historical fire frequency is tough. Fire scarring of trees along the edges of prairies is the go-to method, and while it has problems, good researchers can account for some of those and still come up with reasonably good estimates. There are some new methods of looking at carbon in soils that may help give us more exact estimates, but they’re still being developed.

      My stance is always the same. It’s nice to have that historical context, but it may or may not be anything like what we want to do today for management. Fire is an ecological process, but it’s also a tool, and we need to use the tool as it best fits our objectives.

    • Bob – I’m guessing your question reflects an assumption that since trees were, are, or should be scarce to absent in prairies, then there should be minimal to no opportunities for learning about fire history through fire scarring of trees. However, a lot of fire scar research has been done on the eastern edge of the tallgrass prairie where prairies and prairie-like communities intermingle and co-associate with eastern forests, resulting in communities characterized by herbaceous vegetation – savannas, glades, barrens, fens, etc.

      A quick search on Google Scholar using the keywords “fire”, “scar” and “prairie” indicates research of this type has been conducted in Ohio, Indiana, Illinois, Missouri, and Arkansas. Moving farther west, fire scar research has been conducted in the Flint Hills of Kansas and the Osage Hills of Oklahoma – the areas generally considered to represent the last locations where tallgrass prairie exists on a landscape scale. Moving even farther west (and south) into mixed-grass prairie, there has been fire scar research conducted in southwest Oklahoma. Going north but still within the mixed-grass region, a fair amount of fire scar research has been conducted in Black Hills region of southwest South Dakota and northeast Wyoming. All of these areas represent ecosystems where herbaceous communities intermingle with woody plant communities. The herbaceous vegetation drives the fire regime and the woody vegetation preserves the historical record of those fire regimes.

    • Beyond fire scars, a large body of evidence is available through historical accounts recorded by people (journals, diaries, narrative, letter, military records, etc.). Finally, research from around the world has demonstrated that a primary driver of fire regime is fuel amount and continuity. (Fire frequency is one of the components of fire regime, other components being fire intensity, fire season, etc.) A general rule is, when fuel amount and fuel continuity are high, the probability of fire ignition and fire spread is increased. When all else is considered equal (climate, ignition source), greater levels of herbaceous primary production (i.e. fuel) should result in a greater frequency of fire. Thus, a general trend in the Great Plains should be a positive relationship between the east-west gradient of primary production (high rainfall in the east = tallgrass prairies; low rainfall in the west = shortgrass prairies) and an east-west gradient of fire frequency (higher frequency in the east, lower frequency in the west). The fire scar research and the historical records support this model. A convergence of disparate sources of evidence, all leading to the same conclusions about fire frequency in North American prairies and grasslands.

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