Why is it so hard to keep trees out of prairies? (and why is it getting harder?)

The standard explanation for why historical prairies had so few trees is pretty simple – frequent fires kept them out.  Yes, there were other constraints such as frequent droughts, particularly in western prairies, and there were large browsers like elk and pronghorn that helped, but it was fire – set by both people and lightning – that was the major control on tree expansion. 

Historically, frequent fires would have helped keep small trees out of prairies and savannas, but larger oak trees were pretty tolerant of fire.

However, if that explanation is true, why are there so many prairies, managed with frequent fire, being invaded by native and non-native trees alike, for which the only feasible control method seems to be herbicide treatment? 

I think there are a couple possible explanations.  First, we’re in a relatively wet climatic cycle (geologically speaking) that probably favors tree establishment.  Second, we’re still missing those elk and pronghorn – although you’d think whitetail deer and cottontails would make up for that.  Third, we’ve added some novel species like Siberian elm, autumn olive, and a host of others that may not have the natural controls on establishment that some of the native trees have.  However, could it be that in the western tallgrass prairie and the mixed grass prairies, the biggest explanation is simply that there are so many more trees in the surrounding landscape than there used to be?  I just wonder if the seed rain from those trees is so heavy that it overrides any natural controls prairies have for outcompeting them. 

I don’t have any good data to support this other than observation and logic, so I’d love to hear from others who have other ideas (or data!).  But I have seen several cases where aerial photography shows that prairies, even without much fire, experienced only small increases in tree encroachment for decades.  Then, in the last couple of decades, those prairies have become nearly closed in by trees.  It seems to me that continuous seed rain from the surrounding landscape, supported even more by the few “pioneer” trees and shrubs that made it into those prairies early on, eventually overwhelmed those prairies.  Browsers – including little ones like voles – may only be able to keep up with a certain number of tree seedlings.  In addition, little pioneer trees that got started in the middle of those prairies grew large enough to support perching birds.  Those birds brought in more seeds (along with free fertilizer), and those isolated trees became ever-expanding patches of trees and shrubs.  The result was an exponential curve of tree encroachment.

Sometimes the reasons for tree encroachment are not hard to figure out.

Of course, while tree encroachment may be happening at a faster rate than it did a century or two ago, the other side of the equation is that our smaller, more fragmented prairies are more vulnerable to that encroachment than the historic sea of grass would have been.  A couple acres of sumac shrubland in the middle of a huge grassland is not a big deal, but it’s hard to put up with a couple acres of sumac in a 5 or 10 acre prairie remnant that represents the only high quality prairie in its county.

In some ways the explanations of why tree encroachment is a bigger problem than it used to be are beside the point.  Regardless of why it’s happening, it’s clear that in many (most?) prairies today, keeping trees out has become a bigger challenge than simply determining an appropriate fire frequency.  It requires vigilance and consistent patrolling with herbicide applicators, in addition to management with fire and/or grazing. 

The good news is that there are some tools and techniques available to make herbicide application easier and less harmful to surrounding prairie vegetation.  One of the best techniques I’m aware of is the use of a PVC herbicide wand, which is essentially a long piece of PVC pipe  filled with a sponge stuck in one end.  The wand was originally developed to apply herbicide to small cut stumps to prevent regrowth.  But if you fill it with a 3:1 ratio of crop oil to herbicide (Remedy, Garlon, or other Triclopyr-based herbicide) you can use it as an applicator for the “basal bark” method of herbicide application.  The basal bark technique works by applying that oil/herbicide mixture to the base of young trees, and the oil helps penetrate the thin bark and kill the tree.  It works any time of year – even winter – and is very effective.  We used to use hand and backpack sprayers to apply the herbicide until our clever land steward, Chris Rundstrom, thought of putting the herbicide wand to use instead.  Using the wand saves a lot of repair and replacement of sprayers because the plastic and rubber gaskets and seals in those sprayers get eaten up by the oil.

Basic Instructions for Basal Bark Treatment of Small Trees (see herbicide label for specifics)

- 3:1 ratio of Crop oil: Remedy© (or other herbicides with the chemical triclopyr – check label for instructions).

- Dye if needed.

 Apply to the lower 12” of small trees (less than 3” diameter) with small sprayer or PVC wand.  Just need to get a 2” band wet on one side of smaller trees or 2” ring around larger ones.

 Works on any deciduous tree young enough to have thin smooth bark, including willows, dogwood, sumac, cottonwood, mulberry, ash, Siberian elm, Russian olive, etc.

 Apply any time of year – including winter.  When applying in the dormant season, trees may green up for a week or two in the spring before they die.

Instructions for making a PVC herbicide wand:    http://www.invasive.org/gist/tools/wand.html

Another trick that can work well in some situations is the use of a wick applicator.  An old rope wick – like farmers used to use on soybeans – can do the trick, but there are numerous “roller wicks” available today that can get more herbicide on the plants with less dripping.  A wick application can work well when the trees are fairly dense, taller than the surrounding vegetation, but still flexible enough that a tractor or atv can get through them without tearing up the vehicle or wick applicator.

One example of a roller wick: http://www.agriweld.com/pages/weedwipers.html

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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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11 Responses to Why is it so hard to keep trees out of prairies? (and why is it getting harder?)

  1. I’ve been thinking about this puzzle too – in my case, why the bluffs in my area stayed so bare for so long, and then seemed to get overgrown with trees in such a short time. And, how there would be frequent enough fires – in this very hilly, rather wet area – to really keep the brush and trees out.

    I had come up with some thoughts like yours – but not as well defined or expressed. Thanks for talking about this!

    • Chris Helzer says:

      Thanks for replying. I’d be curious to hear from others in Wisconsin about this – you’re not a lot wetter than a lot of the areas I’m familiar with in eastern Nebraska (30-33 inches/year zone). Any idea what the historic fire frequency was in your area?

      Of course another aspect to consider was tree/brush clearing that might have happened during early European settlement to make areas for grazing livestock. I’m guessing that kind of activity is very hard to trace, though, and probably bluff tops wouldn’t have been the first choice for that kind of work?

      • The yearly rainfall around her is 34-35 inches – so just slightly wetter than where you are. I wish I knew about fire frequency in the past, but I have no idea.

        Because it’s so hilly, and the hills are so steep and the valleys so narrow, I would think that if a fire got started it would quickly burn up to the top and then stop. There just aren’t the big expanses of flat land to keep it moving. So I would think the fires would have been fairly small ones.

        I have aerial photos back to the 1930s, which show that our land was much more open. I’m sure the flatter areas were that way because they were heavily grazed and farmed all the way to the edges of what was possible. But hillsides are very steep, and must have been very difficult for cows to walk on, and not easy for people to clear.

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  3. James McGee says:

    Chris, I have a few thoughts on this topic. First, have you ever seen what Buffalo do to trees? I think a few Buffalo could easily move the ecosystem into another “bowl” ecologically speaking. Secondly, I have been told by a ecology professor that certain fungi in the soil prevent the development of woody vegetation. However, I have no idea why this might have changed. My third thought is increasing leaves of atmospheric CO2 maybe allowing trees to out compete grasslands. Water stress would not be as much of an issue for plants since they would be able to keep stomata less open in a higher CO2 environment.

    James

  4. James McGee says:

    Chris, In my area we use a different method to apply herbicide. I cannot take credit for this idea since it originally was not mine. However, I no longer remember who originally thought of this method. We use small 4-6″ paint rollers with long handles. The longer the handle the better, since this prevents the need to bend down. I soak the herbicide on the paint roller in a paint bucket. I originally had a problem with the paint roller not rolling on the bottom of the paint bucket. I solved this by cutting out the textured surface on a disposable paint roller pan liner and placing it in the bottom of the bucket. I often only dip the tip of the paint roller in the herbicide. I can then tilt the tip of the paint roller up so the herbicide slowly runs down the dry part of the paint roller. This gives me time to get the roller to the bark before it begins dripping. The result is the ability to apply a lot of herbicide quickly in a very targeted manner.

    I usually only basal bark really small diameter woody species. I often use the above technique on resprouts. Our local public land owners require that trees be cut and the resulting piles get burned asap. This requirement is to maintain a positive public perception. Additionally, much less herbicide must be used for cut stump application than for basal bark application. I also have girdled trees and applied herbicide to the girdle. I have found this to be an effective way to minimize herbicide use on larger single trunk species. My experience with this has been on my own property because of local public landowner requirements. The problem with leaving dead trees standing is they become a trellis for vines that then cast shade. If vines take over they can quickly cast as much shade as the original tree. This can effectively undo any benefit of woody species control efforts.

    James

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  6. Jim Gorman says:

    Hi, Chris:

    I just bought all the materials for my PVC herbicide wand which I will be building over the weekend. [http://www.invasive.org/gist/tools/wand.html]. The hardest parts to find were the four 1.25″ lavatory gaskets. I had to go to a plumbing supplier and even then I had to buy gaskets I will have to cut down and modify. Our local CPI carries Remedy and crop oil and our county weed control has the herbicide dye, so I should be good to go on Monday. I can’t wait to go for several nice long walks and doctor up a whole bunch of young honey locusts. This beats the heck out of using lopping shears and then treating the cut stumps with Tordon on my hands and knees! Thanks for your invaluable info on this project and technique.

    Happy St. Patrick’s Day!

    Regards,
    Jim

  7. Pingback: Killing Small Trees in Prairies – A Helpful Tool | The Prairie Ecologist

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