Why don't trees grow on the Great Plains? Herbivores

Cecil's Columns/Staff Reports
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I hate to question a trusted source but I’m going to anyway. In Cecil’s “Classic Column” reply to the question entitled “Why don’t trees grow on the Great Plains?”, dated July 8, 2005, he states that a 1935 paper identified the most likely reasons as being: drought, dry seasons, high ratio of evaporation to precipitation, flat terrain, lightning and fire (with explanations for each).

While watching a documentary recently on dinosaurs, specifically the large sauropods, (diplodocus, brachiosaurus etc.), these massive herbivores were shown pushing over trees at the edge of a forest in order to gain access the verdant vegetation within, as they were too large to enter the dense forest. If these outsize-grazers push down trees at the edge of a forest, much like today’s pachyderms, a clearing can eventually become a prairie. Aha, but the trees would regrow, wouldn’t they? No, because the vast herds of smaller dinosaur grass grazers, (the equivalents of today’s wildebeests) kept any grass and seedlings/saplings closely trimmed, so unlike the grass which can survive a regular buzz-cut, trees never had a chance to re-establish themselves.

It is possible that this deforestation then went on to have an impact on climate, but I am sure that the most likely reason for there being no trees on the plains is the combination of large and small herbivores’ feeding habits and about 150 million years. Could the 1935 survey on which Cecil’s reply column was based, be found wanting?
Yes, I know it’s an old column, but it’s SD’s fault for creating the random article button…

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We have a “random article” button and a front page? Why didn’t nobody tell me?

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Not very likely in this instance.

If the conditions are right for any fauna or flora then, provided they are not physically cut off from an area they will eventually colonise it.

Part of your supposition may well be correct in that animals are certainly capable of deforestation (think Mexico), even without power tools. But over the time periods in question the types of flora found on various areas of the Earth will have changed (possibly several times) because of normal cycles of climate change and, to a certain extent, climate change caused by continental drift. So reforestation would almost certainly have occurred at some point.

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Your theory is severely lacking in understanding climate and other variations on a geological time scale. For instance the Great Plains were sea bed for nearly half of your 150 million years:

http://library.ndsu.edu/exhibits/text/greatplains/text.html
http://library.ndsu.edu/exhibits/text/greatplains/fig06.jpg

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[quote=“naita, post:4, topic:548284”]

Your theory is severely lacking in understanding climate and other variations on a geological time scale. For instance the Great Plains were sea bed for nearly half of your 150 million years:

Naita, I stand totally corrected and have exposed my weak grasp of North American geographic terminology. I had understood “Great Plains” as being the great plains of the world (the temperate grasslands, savannah, and shrublands of Africa etc) not just the region covering the US and Canada. As you said, (and I have subsequently read up on) the area was covered by a shallow inland sea during the period I mentioned and apparently the largest mega-fauna discovered in palaeontological digs was mastodons.

However, if the region’s medium/large grazers were hunted to extinction just after the seas receded, and the Great Plains was (were?) not given a regular “mow”, would mega-floral colonization be completely unthinkable?

So once again, I am pointing an accusing finger at veggies…

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I don’t quite know how this fits into this current discussion, but here is an interesting article on the AAAS website that proposes that the extinction of the mammoth may have led to the growth of forests in the northern climes of Asia and America and as a consequence led to climate change.

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Hey Gordon, Thanks for that. It proposes that de/re forestation can have an effect on climate mainly due to darker colours of leaves absorbing light/heat that would otherwise be reflected back out to space, so the article holds with my loose theory on forests affecting climate. I likes the sentence “mammoths kept the temperate northern lands of North America, Europe, and Asia well trimmed and mostly free of forest” which seem to suggest that forests could have taken hold in the Great Plains. However, it doesn’t support my theory that once the tree-felling giant herbivores disappear, smaller grazers then stop saplings from taking hold and growing into mature trees and eventually a forest. Hmmmm, Curiouser and curiouser…

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There are several things at work here: Climate unfavorable for trees mean that not too many trees can grow. Quick growing grasses can out compete with trees. Trees need light, and tall grasses can cut off the light needed by young trees when the grasses re-sprout in the early spring. Grasses could also drain the resources in the soil a tree needs to grow. Finally, animals grazing could destroy any trees that did manage to grow.

An environment is the work of not just the climate, but the animals who inhabit that climate. In wetter climates, trees are able to over take the tall grasses and constant attack by grazing animals. Soon, the trees push out the prairie grasses and grazing animals and you end up with a forest environment.

At the edges of a forest/savanna environment is a constant battle between the two environments. If the climate is gradually growing wetter, the forests will encroach upon the grasslands. If the climate is gradually drying out, the forests will be in retreat and large creatures like elephants will help expand the grasslands.

So, yes. Large animals can help expand a savanna environment. They don’t need to push down trees either. The constant grazing and chomping of the vegetation favors fast growing grasses. However, if the climate gets too wet, trees have enough resources to reclaim the grasslands.

And, we’re not considering the effect that trees or grasses might have on the climate itself. Trees create a moister climate which helps support the trees growing in that environment. Grasses tend to allows water to evaporate faster and thus create a drier climate that’s preferred by grasses.

An environment has many inputs and feedback loops that will drive it one direction or another. Or, possibly stabilize the climate itself. Anything can cause the environment to change from forest to grassland to desert like conditions: Slight drying or wetting of the climate. Introduction of new animals into that environment. Or, introduction of new plant species.

The OP is partly right in that grazing animals do help maintain a grassland and keep out trees. And, larger animals like elephants and dinosaurs can push down trees and expand a grassland when the conditions are favorable.

But, that’s not the entire answer. Bison grazing kept the trees out of the great plains, but the plains didn’t turn back into temperate forests once the bisons were wiped out of an area. The Great Plains are simply too dry to support a forest environment.

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Thanks quazwart for the detailed post!

Interesting point about grasses out-competing trees. Not quite sure if I could imagine it in practice though. I know in India there is a lot of very tall grass (elephant grass?) but I have never seen vast expanses of it, only in lightly wooded areas. I imagine this to be so because tall grass requires some protection from the wind otherwise it risks being flattened (as can happen to wheat in open fields). Furthermore, even the very tall grass does not have the light blocking effect and therefore competition-suppressing ability that leaf canopies of the rainforests do, therefore I can’t really imagine grass hindering tree growth. I think there is some merit in the idea that they could compete for resources underground as tall grass’ roots form a dense spongy layer at the surface, but it is unlikely that tree roots would be unable to penetrate such systems and after having done so they would then go on to be far more extensive than the relatively shallow grass roots.

I liked your point about the “edges of a forest/savanna environment [being] a constant battle between the two environments”. This is really the crux of my argument. I never believed possible that a forest would spring up on a prairie, (even over a period of say 200 years), seed dispersal methods are not so efficient and a prairie just simply does not offer a sufficiently nurturing environment for trees to establish themselves quickly en masse. I believe territory gains at the forest/savannah battle front are incredibly slow, so slow in fact that it would be barely noticeable on a timescale such as that since the native bison were hunted to near extinction.

With trees left “un-chomped”, I am quite confident they generally win the war of attrition at the grass/tree front. Given aeons and no herbivore-ally ground-forces fighting for the grasses, forests would gradually take over grassland. I don’t think it is inconceivable that as the trees colonized the areas, they would have an effect on the local climate, by blocking winds and creating a more humid micro-climate that at the edges, would then favour further tree growth.

I am unable to predict however, whether trees’ effect on climate would be sufficient to really turn a prairie region into a forest. There might be areas that simply do not receive enough rainfall in a year to support even the least thirsty tree species, so it is possible that left to their own devices, trees would reach a limit, where like in the polar regions or on mountain slopes, the environment just does not provide adequate conditions to support tree life.

So, to sum up, battle at the edge, trees would win if not for herbivores, although time scales are huge. Tree colonization might affect micro-climate and permit further encroachment, but even if it does, there may be physical limits to their territory gains, after which only water-conserving grasses can survive.