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Saturday, April 11, 2015

Does Rotting Wood Hurt Prairie Plantings?

Many years ago, restoration pioneer Ray Schulenberg of the Morton Arboretum told me that rotting wood acidifies the soil, that our kind of prairie likes limey soil, and thus rotting wood chips and branches would hurt a prairie or prairie planting.

But I have seen projects where fairly deep wood mulch is left after a brush clearing operation, and the  people in charge assured me that the prairie would grow fine from seed planted in the wood.

I've never tried a side-by-side comparison of seeding diverse prairie on woodless soil compared to wood-mulched soil. I wonder if others have.

I've seen a fallen tree in a savanna or prairie kill many of the species under the branches, even when the branches seem far enough apart to let plenty of light through. Is that "death by acid?" Indeed, the prairie there will often be replaced by a thistle patch.

We cut dense brush off a prairie wetland. Will thick beds of downed wood damage the planted prairie?
Last winter in deep snow we cleared dense buckthorn from a long-neglected area of Somme Prairie Grove. The area seemed like an opportunity, since an overstory of green ash had just died. As always, we burned as much of the brush as we could, but we didn't realize that some areas had dense rotting wood on the ground under the snow.
The area still has trees, but most of them are dead ash.
We had broadcast our seed in the fall, before we even cut the buckthorn, but I returned this spring to put out some extra seed. Now I noticed the dense wood. I suppose last fall I'd expected we'd clean it up and throw it on the bonfires over the winter.
Much of the ground had more wood than bare soil.
Now, standing there with our "prized" seed, I doubted the wisdom of investing our full diversity of  seed here. Our "wet prairie" and "wet-mesic prairie" mixes were rich with rare sedges, turtlehead, swamp betony, cardinal flower, prairie Indian plantain, gentians, gerardias, etc. etc. The warnings of Ray Schulenburg haunted me.
We hauled tarp-load after tarp-load of wood to last winter's bonfire scars. 
I suppose it might have been a valuable contribution to science for us to have left the wood in some plots and picked it up in other plots (after choosing which plot got what treatment by a truly random process). But - as is so often the case - we sacrificed the science because we didn't want to risk wasting the rare local seed. Perhaps someone else has done that science?
Brush piles of dead wood scavenged from the ground.

If anyone has experience with this kind of decision (or dare I ask for actual scientific data?), I'd certainly be interested.

This sort of thing is a long term problem for us who are trying to develop biodiversity restoration for conservation results. We sometimes do some science. But mostly we hope that other people would do it.

The problem is that a good academic scientist might want to control all the variables. Perhaps forty plots of each type - perhaps replicating the experiment on different soil types - and in different years under different weather conditions. And with those challenges, there probably wouldn't be resources to use the the very diverse mix of rare species that we seem to have such good results with. So perhaps that science wouldn't help me much? Instead, I talk with people who have a lot of experience.

That's kind of sad. "The discipline of restoration" is developing well. But "the science of restoration" - perhaps - not so much? I used to check the journal "Restoration Ecology" for data or conclusions that would help me in my on-the-ground conservation work. I never found much.

Have others had better experience?


  1. I have also seen downed trees kill many (all?) of the species under the trunk and larger branches. However, I typically see this in woodlands. I attribute the death of these species to rain being redirected or captured before reaching the soil. In other instances I have seen where prairie species have presumably moved into a wooded area after the death of large oaks has let in abundant light.

    In areas with sufficient moisture plants will begin to grow on downed wood after the logs have decomposed sufficiently. Sometimes these nurse logs are colonized by rare plants. One example of a rare plant that will colonize downed rotting logs is forked aster.

    I agree with Mr. Schulenburg that “rotting wood acidifies the soil” and that “our kind of prairies like limey soil.” However, I think Mr. Schulenburg is incorrect when stating that “rotting wood chips and branches would hurt a prairie planting.” Indeed, the most conservative species inhabit organic rich soils on the acidic side of the scale over a limey parent material. Less conservative species like Dogbane and Whorled Milkweed are able to tolerate the harsher conditions of high pH, less nutrient availability, and less water retention where abuse has stripped the rich black soil away.

    The only situation where I could imagine that “… wood chips and branches would hurt a prairie planting” is if the wood chips were so thick that the seedling’s roots could not reach the underlying soil. In this case the seedlings might die from being unable to access moisture.

    The above being said, there are some concerns with having too much slash left after clearing. When fire occurs through areas of dense slash the fire can burn for such a long period that the soil is sterilized. It does not appear this will be a problem in your situation since much of the woody material has been removed. In our region, wood chips and downed branches only tend to become fuel when fire conditions are very favorable or when the wood chips and branches have been consolidated.

    The other bigger concern, which is at the top of the mind of any burn boss in the region, is smoke. If the woody material does burn then it will smolder slowly releasing lots of smoke. Although this situation solves itself over time, it can cause a problem for an urban burn boss until the wood rots or is fully consumed by fire.

  2. Your following comment left me searching for a better explanation.

    “I've seen a fallen tree in a savanna or prairie kill many of the species under the branches, even when the branches seem far enough apart to let plenty of light through. Is that ‘death by acid?’ Indeed, the prairie there will often be replaced by a thistle patch.”

    In your paper, “Just a Few Oddball Species” you wrote the following.

    “In a few places we had actually sown prairie seed, choosing spots where we or the fires had cleared brush and left a hole in the canopy. Such planting had resulted in diverse, dense stands of prairie plants out in the open, but among the oaks we got at most a few wispy, flaccid shoots. In hospital nurseries doctors use the term “failure to thrive” to describe babies with a puzzling, sometimes fatal wasting syndrome. Our seeded “prairie understory” was failing to thrive.

    And a parallel ominous development gradually loomed up. During that second summer my dirty-kneed search for unfamiliar seedlings turned up growing numbers of weeds. Canada and bull thistle, dandelion, briars (Rubus sp), and burdock (Arctinum minus) were increasing exponentially. ... “

    I must wonder if your memory is correct regarding the first comment where you say a prairie was under a tree before it fell. Considering the excerpt from “Just a Few Oddball Species” it would seem more likely that any prairie in the shade of a tree was failing “to thrive” and the increasing thistles were more a response to the sudden change in light availability and lack of any existing strong competitors rather than the decomposition of the wood from the tree.

    1. An individual tree falling on rich "prairie" vegetation is very different from the heavily wooded oak areas we worked with on the edge of the little "prairies" in the North Branch forest preserves. Sometimes prairies have individual trees without lower limbs so the prairie vegetation grows fairly close.

      One of my memories is of planting rescued prairie lady's-slippers in quality prairie. This was before we realized we needed to cage them from the deer. But we worried about the deer, so, as an experiment, we planted one of them in a spot protected by the branches of a newly fallen dead tree. The other lady's-slippers did fine (at least for the first couple of years). The one under the rather sparce tree branches dwindled and died, and I noticed that much of the other prairie vegetation there died too. It reminded me of Ray Schulenberg's concerns.

    2. An expert grower of lady’s-slipper orchids told me they grow best in highly organic soil like that which could be found in woodlands. This has been my experience also. However, neither this grower nor I grow the prairie lady’s-slipper orchid. Few people grow this species. I have only seen them in specialized areas that are highly alkaline due to ground water flow or limestone deposits.

      I have noticed a similar dying of grasses from where a tree was once was located. However, in my example the tree was completely removed before I purchased my home and the grass is my lawn. The decomposing roots of the removed tree created a lot of mushrooms. I think the fungi, and not the wood, is killing the grass. I’ve tried in vain to establish more grass in that location. In contrast, I am always pulling up lawn grass that creeps into my garden beds which are mulched with wood chips. I think your question regarding wood killing prairie might be best answered by a mycologist.

      I plant lots of plugs in old agricultural restorations. I sometimes return to find the plants have not expanded beyond the radius of the plug. I think the old agricultural soil is just too unfavorable for some of my more conservative plants. I know you are afraid of restoration becoming too much like gardening. However, I think amending the soil should be considered in some cases if it helps get the ecosystem to a point where it can be self-sustaining. I read the below article in the Daily Herald and thought that restorationists could learn a lot from gardeners. Maybe we need to put some things back into the soil so we can get it to the point where the ecosystem can again take care of itself.

  3. Here is a paper that is relevant. Of the three prairies studied, two have soil with a pH near neutral and only one has soil with a pH around eight which is quite alkaline.

    It is interesting that the agricultural field sampled at Huntley has less organic matter and a much lower pH than adjacent virgin prairie. Usually less organic matter results in a higher pH. The low pH of the agricultural soils must be the result of added minerals like sulfur.

    Not all of the prairies in our region occur in limey soil. There are also prairies that occur on sand and sandstone. There are a whole different set of species that need these conditions. When planning a restoration it is very important that the soil is considered to improve the odds of success. In my garden I have found that feeding the things that live in the soil prior to establishing plants helps with their growth significantly. Maybe when preparing a site for restoration we should not only put effort into removing invasive species. We should also feed the things that live in the soil so our seed has good conditions for getting established.


  4. As a graduate student at Rutgers U. following WWII I was there at the time that Murray Buell joined the botany faculty. For a short while I even had an office/lab adjoining his office that was soon occupied by his first graduate student, John Cantlon. I made a point of going on all the field trips led by Buell through the rich ecosystems of New Jersey.

    Sometime about the late forties or early fifties Buell and Canton published an account that specified the role of fire in the maintenance of the Pine Barrens of southern N.J. Other reports followed including some by Silas Little at the N.J. Extension Service. ‘Prescribed burning’ was the mantra. When I read this first report of Buell and Cantlon, I had an epiphany that lay dormant for a quarter of a century when I assumed the stewardship of the James Woodworth Prairie.
    When I performed my first prerscibed burn on this sadly neglected prairie remnant, I had no qualms about doing so. I long remembered that Pine Barrens report and knew for certain that fire could be an important factor in maintaining a healthy prairie ecosystem.

    That research paper of long ago taught me a lesson. The problem is that it was never read, or at least given credence, by midwesterners. The lesson had to be relearned in our part of the the midwest much later than in N.J. I ask why ? I offer the early history of the Cook County Forest Preserve. It was touted as an international wonder, but it had one flaw. Its administration was politically motivated. In Chicago fashion this remarkable preserve was administered as a political fiefdom. It was administered from the top down, run with the precision of a fiefdom and with its ignorance of much earlier information that might have saved the degradation that you have mentioned.