Every fall Patrick McMillan takes his Plant Taxonomy class on a field trip to Roan Mountain, TN, to study island biogeography and the ecology of high grassy balds. He talks about the importance of ecosystem engineers in keeping things stable - in this case, keeping the grassy balds grassy.
Grass won't just stay grassy on its own. Left to its own devices, it often will go back to forest, probably through an ugly transitional stage involving lots of brambly bushes. What keeps the bushes and trees from growing? Large animals. In the case of our grassy balds, it was the bison and Indians that used to roam the Appalachians. (Before that - maybe mammoths? But that's a very long time ago.)
There were bison up in the South Carolina mountains in Catesby's time. He writes about them:
"I then went to the upper uninhabited parts of the country, and continued at and about Fort Moore, a small fortress on the banks of the River Savanna, which runs from thence a course of 300 miles down to the sea, and is about the same distance from its source, in the mountains. I was much delighted to see nature differ in these upper parts, and to find here abundance of things not to be seen in the lower parts of the country; this encouraged me to take several journeys with the Indians higher up the rivers, toward the mountains, which afforded not only a succession of new vegetable appearances, but most delightful prospects imaginable, besides the diversion of hunting buffelo's, bears, panthers, and other wild beasts."
In his entry on the Pseudo Acacia, he also mentions the bufello:
Vol. 1, Appendix p. 20, Bison americanus and Pseudo Acacia hispida floridus roseis
"I never saw any of these trees but at one place near the Appalachian mountains, where Bufellos had left their dung; and some of the trees had their branches pulled down, from which I conjecture they had been browsing on the leaves.... I visited them again at the proper time to get some seeds, but the ravaging Indians had burn'd the woods many miles around, and totally destroyed them, to my great disappointment; so that all I was able to procure of this specious tree was some Specimens of is which remain in the Hortus siccus of Sir. H. Sloane, and that of Professor Dillenius at Oxford."
As it happens, there are no Pseudo acacia in the Sloane specimens today, but there are two (00087396X and 00087400J) in the Oxford collections. Patrick has identified them as Robinia hartwigii Koehne, Granite dome locust.
There are no bison roaming the grassy balds today (no Indians burning the locust trees either), though Patrick would love to see them restored. Instead, there are cattle grazing in some areas. The grazed parts are empty and grassy. Just beside those grazed bits are areas with the same basic ecological components but no cattle. Those parts are overgrown with brambles, bushes, trees - no grassy balds there.
Charles Darwin described this phenomenon in On the Origin of Species, in Chapter 4, "Struggle for Existence:"
"How important an element enclosure is, I plainly saw near Farnham, in Surrey. Here there are extensive heaths, with a few clumps of old Scotch firs on the hill-tops; within the last ten years large spaces have been enclosed, and self-sown firs are now springing up in multitudes, so close together that all cannot live. When I ascertained that these young trees had not been planted, I was so surprised at their numbers that I examined hundreds of acres of the unenclosed heath, and literally I could not see a single Scotch fir, except the old planted stumps. But on looking closely between the stems of the heath, I found a multitude of seedlings which had been perpetually browsed down by the cattle. In one square yard I counted thirty-two little trees; and one of them, with twenty-six rings of growth, had during many years, tried to raise its head above the stems of the heath, and had failed. No wonder that, as soon as the land was enclosed, it became thickly clothed with vigorously growing young firs."
"Here we see that cattle absolutely determine the existence of the Scotch fir...."