Bogs...

Yesterday, during a presentation about wetlands, I was trying to explain what a bog is and how they form.  A student asked if I have any pictures of bogs - I didn't have any in my presentation.  I do however have a few pictures that I have taken of bogs.

The first photograph was taken at the Alma College Biological Station near Vestaburg, MI.  This bog shows the typical features of a bog that formed in a kettle lake.  A floating mat of living and dead sphagnum moss rings the bog.  Other plants, including trees, have taken root in the floating moss layer.   

This photo shows a view across the bog early in late April 2010.  In the foreground can be seen plants growing atop the floating mat of sphagnum moss.  The evergreen trees (mostly spruce) in the background are also growing atop the moss.  The leafless deciduous trees beyond the spruce trees are growing in the uplands that surround the bog.

Vestaburg Bog - Evergreen trees on the far side of the bog are growing on the floating mat of sphagnum moss.

The next two photographs come from the Chippewa Watershed Conservancy's Schaftenaar Preserve at the Hall's Lake Natural Area.  This bog also formed from a small kettle lake.  Unlike the Vestaburg Bog, this bog has filled in almost completely and there is no central area of open water. 

The first photo shows volunteers and staff from the Chippewa Watershed Conservancy crossing an area of this forested bog during a wetland plant survey in 2014.

Hall's Lake Natural Area - traversing a forested bog

This photograph shows a mushroom growing up in the carpet of sphagnum moss.  Sphagnum moss is the characteristic plant of bog.

Sphagnum moss

So how do bogs like these form?

When the glaciers that covered Michigan during the last Ice Age began to retreat northward they did not melt evenly.  In many places large blocks of ice were left behind when the rest of the nearby glacier melted.  These blocks of ice would be surrounded by a thick layer of glacial till.  Glacial till is the name for all of the sediments such as rock, gravel and sand that were transported under and within the glacier as it advanced southward.  When the glaciers retreated, the till was left lying over the landscape.

When the block of ice melted, they left a depression surrounded by hills formed from glacial till.  These depressions are known as kettle lakes.  Many kettle lakes are deep and steep sided.  Most kettle lakes lack input or output from streams.  They drain slowly through the soil surrounding them and are recharged by rainwater.  Because the lakes lack input from streams, they often lack many of the minerals that plants need to thrive.

While the surrounding upland areas are colonized by a wide variety of plants, the mineral-poor water of the kettle lakes restricts the number of species that can grow within them.   One species that does well in this habitat is sphagnum moss.  The moss will begin growing from the shoreline and extend out over the surface of the water as a floating mat.  At this point we can begin to refer to the kettle as a bog.

Over time this floating mat will thicken to the point that other plants will begin to take root within the moss, including trees.  At the same time, organic matter will slowly begin to accumulate on the bottom of the bog.

Over time the mat will continue to thicken and grow toward the center of the bog.  Organic matter will also continue to fill the bottom of the bog.

Eventually the kettle will fill in completely with live and dead moss and other organic matter.  The organic matter will continue to hold large amounts of moisture and may continue to support species that cannot be found in the nearby uplands.