Illustrated below are eight reasons why prairies are so different from place to place, or even within a single prairie.
1. Slope
2. Aspect
3. Soil texture
4. Soil Depth
5. Distance above water table
6. pH
7. Nutrient Availability
8. History
These differences apply as well to oak woodlands and savannas.
Pure loess soils can be deep on bluffs where post-glacial winds piled them up. These soils can be challenging for some species of plants.
Where glaciers left behind deposits of sand - and especially gravel - plants have a harder time of it and tend to be shorter. On the other hand, sand areas support species of plants (and animals) not found in the the rich black soils.
In most of the tallgrass region, soils are deep. But in some places "glacial torrents" washed away entire moraines - down to the bedrock. Some of the best examples are along the DesPlaines and Kankakee Rivers. After long post-glacial millennia, only a few inches of soil have built up. Plants need to be adapted to the chemistry of the dolomite bedrock as well as to severe drying of the shallow soils. One plant species that grows nowhere else is the leafy prairie clover, at least in Illinois. This species also occurs in a few populations in Tennessee and Alabama, where they also grow on shallow soils.
Plants may be tallest where it's wettest - until they start to get shorter where the water's deep. Plants may be most diverse per square foot or square meter where the moisture is moderate (mesic).
Here again, the "average" or the "moderate" is richest. Most prairies and woods are "neutral" in this sense.
Especially high or low amounts of certain nutrients may stunt some plants and favor others, promoting specially adapted plant communities. These situations are most common in unglaciated (or long ago glaciated) areas, where nutrients have been leaching from the soils for long times.
Even though all other factors may be pretty much the same, sites can be quite different simply because of differing histories. This panel cites just some of those differences.
Heavy clay soils have the finest textures, with the smallest pores between particles. Plants there are somewhat limited by the tendency to get "waterlogged," as moisture moves more slowly through the soil. A special example is found in the ecology of flatwoods. In these communities, an impenetrable clay layer forms a few feet down. Snow melt and spring rains turn these areas into ponds. But in summer the shallow functioning soil dries out rapidly, so the plants and animals here need adaptations that allow them to survive long periods of standing water and long periods of severe drought.
Sandy and boggy soils tend to be acidic. When the acidity is extreme, most species can't survive, and only specially adapted (often rare) species grow.
Fen soils are the opposite, basic, high pH. They form where calcium-rich water emerges from limestone-filled moraines. As with bogs, the extreme pH slows decay of each year's dead herbs, and a saturated layer of peat forms. The most extreme fen areas don't support typical prairie plants but do have many rare plants that are found only there. Some specialized plants, oddly, are found in both bog and fens. They seem to be adapted to low levels of available nutrients and are able to protect themselves from pH extremes.
You don't have to know all this to appreciate and marvel at the richness of the tallgrass region's prairies, savannas, and woodlands.
The Illinois Natural Areas Inventory lists twenty-three types of prairies, all surviving today with different dominant and characteristic species of animals and plants. That number expands, of course, when you consider the savannas, woodlands, wetlands, and forests. And these basic types blend into each other in various gradations, so that no two spots are the same. What a blessing! that so many natural areas survive, with beauty and intricacy to appreciate! and their biodiversity so important for the future of this planet!
Therefore this year we celebrate the 60th anniversary of the Illinois Nature Preserve System ... and rededicate ourselves to care and protect, into the future, with optimism and commitment.
Backstory Note:
I made these drawings long ago when I was a Field Rep for the Illinois Nature Preserves Commission - for public education. Wise people helped me with them, and I'm sorry I can't remember quite who - so I could acknowledge them, as they deserve. So many people have contributed over the years! Bless them all!
Please offer improvements in Comments (below). Or send to: sommepreserve@gmail.com. I can incorporate them into the text of this post as they arrive? Or they can remain as Comments if they are more personal experiences and insights. Thanks for any input.
Thanks to Christos Economou, Amy Doll, and Eriko Kojima for edits and proofing.
That is a good graduate student level overview of what makes prairies/savannas/woodlands different. People with doctorates might have more nuances to add on the chemistry of nutrient availability.
ReplyDeleteThere are several things I would like to mention.
Trees tend to grow on the downwind side of water courses. I have been told this is because water courses act as fire breaks. This is a reason forests along rivers historically grew on even the southwest facing slopes in otherwise grassland landscapes.
Temperature is important. Species adapted to cooler environments more common to the north, like Platanthera huronensis, may live in the same area as species more commonly found further south, like the Spring Water Dancer damselfly, where springs maintain a relatively constant temperature. Similarly, the massive size of Lake Michigan has a moderating effect on temperature allowing both bearberries, a northern species, and bluehearts, a southern species, to grow in areas near this lake’s shore. Bogs maintain temperature by evaporative cooling in summer and the heat of fusion of water, that is to form ice, during winter thereby hosting many northern species but also southern species at the northern limit of their ranges, for example Platanthera ciliaris. There are many more examples I did not list.
There are also emergent properties. For example, as savannas dotted through out prairies expand in size, the gaps between them become narrower. Narrower gaps funnel the wind making it blow stronger. Stronger wind means hotter fire. In the transition zone between savanna and grassland this funneling effect of the wind appears to have historically limited the size of savanna patches in prairie. This phenomenon can still be seen when driving to Nachusa Grasslands on Route 64 during a windy day in winter. The highway crossing historical prairie, farm fields, has little snow covering it because the wind blows the snow away. When driving down wind of a historical savanna, with their still surviving burr oaks, the wind is slowed and the snow drifts, covering the highway. Such differences in wind speed have significant impacts on fire intensity causing trees to be eliminated from prairie areas but protecting trees within, and downwind of, the savanna areas.
Another habitat that I would consider to be an emergent property is marl flats. As carbon dioxide dissolves into ground water, it changes to carbonic acid. Carbonic acid is what dissolves the limestone. When the water seeps from the ground, the carbonic acid changes back to carbon dioxide and is released into the air. As the carbonic acid in the water decreases, the lime it has dissolved becomes supersaturated. The supersaturated lime is what makes the pH high. The supersaturated lime then precipitates out of solution as as marl. The entire cycle is driven by carbon dioxide which is the result of plants breaking down. The opposite extreme are bogs where saturated conditions prevent plant material from fully oxidizing leading to the buildup of acidic peat.
Ecosystems are dynamic. They are much more complicated than the pond to bog to forest narrative that is still being presented.
Each natural area is unique. You have presented a nice exposition of the major factors that impact the species of plants that grow in a place. What has been neglected is the role of animals in changing the properties of soil. A number of mammals dug into the soil to make dens resulting in spoil piles that are sites for colonization. Moles and pocket gophers are other major soil changers that are absent from glacial moraine (high clay) soil. Ant are other major players in the top soil as are many other insects. Crayfish bring subsoil material (sometimes >2m deep) to the surface. Paul Orlando studied crayfish at JWP in Glenview and estimated crayfish could bring 20cm of clay to the surface in 10,000 years.
ReplyDelete