The Eastern Forests and Plant Succession

This was going to be another Q and A, but I ran into this question that deserved an extended response:

Q: It really is hard to imagine prairies existing so far east.  Are you sure that some of them are not just abandoned fields?  I mean I know you explained that we once got a lot of fires and such here, but we also have a lot of rain and water in general, and most land that I have seen go to seed again turns into a forest in a bit of time.  I think you just get excited when you find wildflowers.

A: More of a comment than a question.  Maybe both?  Cuomment?  Prairies and more so savannas existed well into areas that we imagine are only forests.  Central Mississippi and Alabama, both notable for being quite humid and rainy places, once had an extensive band of tallgrass prairie known as the Black Belt.  The pine flats stretching from Long Island to Miami were a mosaic of thicker forests merged with savannas, which southward also include wonderful shrubs in the underbrush like the Dwarf Palmetto (Sabal Minor):

Google Earth Streetview at above coordinates, or just south of Vero Beach, Florida, on I-95, east side, northbound.  

All of these areas were much more prone to regular rainfall than the interior plains, but what they shared in common, indeed, what makes grasslands possible is a combination of moisture, fire, soil, and sun exposure.   In earlier posts about how savannas and eastern grasslands work, I noted that the forest and grass were locked into a never-ending natural battle against one another that depended greatly on the specific combinations of the above ingredients.  In places a bit further off of the front lines, the ingredients would favor one over the other.  The suppression of natural fire regimes from the nineteenth century onward have skewed such patterns, but in other ways have showed us accelerated models of what a transition from a seemingly barren prairie into a lush forest can look like, often in a period of a few decades or even less.  While the above picture is somewhat natural and "ordinary", it is also somewhat indicative of what happens when the ingredients have been altered.  The shrubs are starting to take over far more than they otherwise would, and the forest is, well, turning into a forest rather than a savanna or "pine barrens".

Was the above stretch of land ever a thick forest?  It might have been, and it might be again, which is part of the wonderful process known as plant succession that defines the shifting mosaic of landscapes that is natural eastern North America.  Fortunately, there are ways to advance beyond conjecture to figure out what used to be "open" and what used to be "closed", ways which also do a lovely job illustrating what succession is all about.

First of all, we can check records from those who came before us.  When Europeans landed on these shores, they were often amazed at the world that stretched before them, leaving us journals and other accounts of what they found as they pressed into the interior.  While these days we get excited over things like skyscrapers and deluxe shopping malls, people then noticed things like big trees, rocks, and even landscapes that surprised them, like the "park like lands of pleasant Michigan".  We tend to think of our ancestors as hacking their way through a primeval forest that was untouched by the native folk, when the truth of the matter is that they often had a fascinating voyage of discovery and enjoyed the variation in the landscape.

Secondly, we can take a look at what is actually in patches of land like the above shot.  If this was once a "solid forest", there would be a lot of stumps in the palmettos, either from the passing of lumber men or a raging inferno, the latter of which would have taken out much more of the canopy and left us with a different forest from what we see, namely one with either fewer or no trees, or one far more arboreal, as in here:

Streetview, the special friend takes us to I-95 northbound again, this time just south of Jacksonville, Florida.

Where we do have more of a forest, we can also take notice of isolated elements in the canopy.  Where we do have either a really broad tree or one that stands out from the rest of a dominate stand of just a few species, this can indicate that at one point this area did have ground that was exposed.  Why is this?  Well, a canopy, while not exactly perfectly uniform, does more or less expand up and out at a rate that favors quicker growing species which tend to dominate everything else, such as maples.  Very rarely does an opening stay exposed long enough for a massive oak or pine to out-compete a bunch of sun-struck maples.  A really broad oak, for example, might be a sign that the area was once an oak savanna.

Taken in Island Lake State Recreation Area, Michigan.

A lone trooper like this Eastern Red Cedar (Juniperus Virginiana) almost certainly means that instead of growing so well in the shade of the canopy, the tree is a survivor of something far more exposed.

Taken at Brighton State Recreation Area, Michigan.  Yes, I know it looks like an Eastern Hemlock.

In fact, it would not be too out of place to suggest that the above scene may have once looked like this:

At an edge of Island Lake State Recreation Area, Michigan.  

Now I don't know the history of the site pictured well enough to claim it is an actual grassland, either emerging or remnant, but the soil is rather sandy (the hills around there, in fact, are stabilized dunes).  In any case, Eastern Red Cedar is an opportunist that can really only get started and prosper under sunny conditions, and often pioneers tree cover on abandoned fields.  Here it is a passing feature of changing landscape.  In more greatly open areas, such as parts of Oklahoma, it is native but actually considered a noxious weed!  I have seen some on oak savannas, which have always been a bit of a misnomer to me anyway.  Yes, oaks are resistant to fire and can handle the drier conditions offered by exposure, but so can these things, junipers, and pines.  


Regardless of what tree deserves the crown of the landscape, our isolated friends are but another way of showing us that nature is not as compartmentalized as we suspect her to be, but always in motion.  This is how we can get from this: 

Taken at Island Lake State Recreation Area, Michigan.  Prairie Remnant converging into Oak Savanna.

To this: 

Taken at Brighton State Recreation Area, Michigan.  Prairie Remnant with edges dominated by savanna converging into mature Oak-Hickory forest.

To this: 

Taken at Brighton State Recreation Area, Michigan.  A little bit of everything, but mostly becoming a mature Maple-Beech forest, a half mile from the above photo.

Basically, nature provides new conditions that the flora of the area then move in to take advantage of.  At first, sun-loving plants that can tolerate not only a bake but also some rather dry soil dominate the situation.  They can be maintained by fires even if they only occur once every few years to a decade, as competing shrubs and trees will be burned out.  Sometimes, however, the fires will not come around, animals such as deer will not graze on certain patches of ground during years of abundance, and the soil quality might have been good enough to allow some species to make a rebound quicker than others.  Given the chance, many areas of eastern North America will indeed progress from grassland to savanna to a dry forest to a more moist forest.  Again, it all depends on the ingredients available, how they are used, and how much they are used.  


Nature is not static.  Yes, we have prairies so far east of the drier rain shadow of the Rockies not because we share general climate conditions with the plains states, but because nature has provided opportunities for these ecosystems to flourish, but they are hardly always passing elements of the landscape.  This also means that not everything was a forest before we came by and plowed it all into farms and cities.  The forest, as much as the grassland, was dependent on conditions to make it possible in the first place.  Was there a lot of forest?  Yes.  Was there also enough open and park-like land to be noticeable?  Yes.  Very often we do have areas that have been forest for a very long time, often hundreds of years.  Sometimes we even get lucky enough to see a rather lovely marriage of the different worlds.

Taken at Brighton State Recreation Area, Michigan.  In the middle of a mature Oak-Hickory forest we have a shaft of sunlight penetrating to the forest floor and letting a Black-Eyed Susan (Rudbekia Hurta) grow amidst its much taller friends.