Savanna understory: factors influencing

This is a nice time for an overview of the understory vegetation in our bur oak savannas. The canopy cover varies quite a bit from one area to another, and this greatly affects the understory vegetation.

The best way to see the savannas right now is to take a stroll through the mowed trail that goes from the west end of Toby's Prairie to the Saddle Road at the west end of Unit 11A. The first part of the walk is through a savanna that is mostly closed (Unit 11B). About half way along you come out into a quite open savanna (Unit 11A). The transition is very easy to discern, because of the distinct difference in the understory vegetation. Unit 11A; open savanna: tallgrass. Unit 11B; closed savanna: sedges, milkweeds, purple Joe Pye, boneset, other forbs.

The two photos (11B; above) here show the difference.

The air photo/map below shows the approximate locations of the camera and the photo directions. The air photo, taken leaf-on by NAIP in the summer of 2013, clearly shows the locations of the oaks. The map was done by ArcGIS. The location of the gravel service road is shown by the orange dashed line.

Several items of management are relevant. The savannas were cleared of undesirable vegetation in 2000-2003 and were planted several times with open or closed savanna seed mixes. (The open savanna mix contained little bluestem, big bluestem, and Indian grass. The closed savanna mix did not have the prairie grasses.) The savannas have been burned annually since 2003 (11 years). It took about three growing seasons for the prairie grasses to get started in Unit 11A. Indian grass is the predominant prairie grass. Note that what we are seeing here is the late summer/early fall vegetation. The prairie grasses only become significant in late summer. During most of the summer a wide variety of forbs are present, including compass plant, pale purple coneflower, Canada milkvetch, etc.

Since the soil, topography, and weather are similar in both units, the most significant factor controlling the understory vegetation is light. Where the canopy is very open, prairie grasses dominate. In the more closed canopy, prairie grasses are only scattered.

Another matter of interest. Unit 11A had remnants of a prairie vegetation when we started restoration. Lead plant and New Jersey tea were present and have increased once the invasive woody plants were removed.