Tom's Blog

Friday, September 8, 2017

Old-field thistle in an old field

Old-field thistle (Cirsium discolor) is a favorite plant of pollinators and birds. Goldfinches love them, and many wait until the thistle plant is mature (late August or early September) to nest.

At Pleasant Valley Conservancy, old-field thistle has been in our seed mix since restoration work began. However, since this thistle is a biennial, it moves around. Fortunately, seed viability is good, and colonization is generally successful. However, one hardly ever sees first year plants, which generally grow as low-lying rosettes.

The purpose of this post is to show the origin of this thistle's common name.

The east boundary of PVC is adjacent to a neighbor's field that has been used variously to raise cover crops for a near-by organic farming operation. Three years ago, cropping was abandoned, and this field turned almost immediately into an "old field." About three weeks ago, this field was almost a monoculture of Queen Anne's lace. Yesterday when I looked at it, a large number of old-field thistle plants had developed, more thistles than I had ever seen in one place.

This field is within yards of the PVC East Basin, a five acre restored prairie that has scattered old-field thistle plants.Although thistle seeds get around very well, I suspect that most of the "inoculum" for this field came from our East Basin next door.

According to Allen Stokes, who did a detailed study of goldfinch nesting in Lake Wingra marsh in 1944-1946, late-nesting goldfinches work very fast to complete their nest building and raise their young. This makes sense when one considers that in our area the weather could briefly "crash" at any time.

Incidentally, visitors to PVC generally miss completely the East Basin and the adjacent Ridge Prairie, because the main trail leads in the opposite direction. Now would be a good time for a detour in this direction.

The old field full of old-field thistle

Typical old-field thistle plant

Tuesday, September 5, 2017

Labor Day field trip: Now a Pleasant Valley Conservancy tradition

Kathie and I have been leading a Labor Day (1st Monday in September) field trip at Pleasant Valley Conservancy since at least 2002, possibly a year earlier. (I have sign-up lists going back to 2002.) Labor Day seemed to me then, and still does, an ideal time for a trip. Everyone is off work, the prairie grasses are in full bloom, and there are lots of great forbs in bloom.

The attendance that we have been getting seems to agree with me. In 2002 we had 15 attendees, and the number has been increasing ever since. Last year (2016) we had 35 and this year 45, which is the largest we have ever had.

I'm not really bragging about this. It just seems to me that a chance to see a restored oak savanna is something a lot of people are interested in. I'm just happy that Kathie and I are still able to lead this trip. (Although the last few years I have been leading from a Kawasaki Mule.)

Since this has been a wet summer, it is not surprising that the prairie grasses are really lush. The Indian grass and little bluestem on the South Slope were at their peak, and it was a joy to watch them blowing in the wind. Some people don't like a prairie dominated by warm-season grasses. I made the point that in mid-July the South Slope is a forbs-rich prairie, and forbs and prairie grasses get along very well together. See this link for a discussion of this point.

We generally get a few demon botanists on this trip, and this year was no exception. Scott Weber and Muffy Barrett, Chris Noll, and Raymond Roberts were all finding things that Kathie and I did not know we had. The prize this year was northern slender ladies' tresses (Spiranthes lacera), found by Raymond right off the trail. (At PVC last year, a week after Labor Day, Scott had found Spiranthes ovalis, another rarity.) These tiny orchids like disturbed grown, and most of our vegetation is too lush for successful habitation.

Labor Day 2005 trip pausing at the Far Overlook.
The large walnut visible in this photo has been removed and the prairie here is flourishingl

Part of the 2017 field trip group. This year we started and ended at the cabin.

Friday, September 1, 2017

Oak savanna restoration: when should the first burn be done?

When the first oak savanna burns were done at Pleasant Valley Conservancy in 1998 they were unsuccessful. The fire moved a few feet away from the drip torch line and then stopped. At the time we were told that “there aren’t enough oak leaves”, and this is why the fire did not carry.

With extensive experience doing oak savanna burns over the past 19 years, it is now possible to conclude that this explanation was incorrect. Take a look at the forest floor in the pictures of Kathie using a drip torch, both from 1998. There are plenty of oak leaves. After all, this is the first time in over 50 years that this oak savanna had been burned, and leaves had been falling ever since. Because of the slow decomposition rate of oak leaves, there was plenty of fuel, as the second photo especially shows. (Oak leaves are resistant to decomposition because of their high lignin content and rich array of tannins, which slow microbial activity.)

The reason the fire did not carry was because of all the coarse woody debris on the forest floor, and the dense tangle of living shrubs, mostly prickly ash, especially visible in the first photo. The fire movement is blocked by all the woody debris. Also, each standing woody stem blocks the movement of fire, and takes up space on the forest floor that would otherwise have a layer of oak leaves.

Trying to burn  the White Oak Savanna, April 1998. Note all the coarse woody debris and the tangle of brush in the background.
An area of the White Oak Savanna with good oak leaves, but the fire still did not carry well. Same burn as above

So why were we trying to burn?

We had taken our clue from prairie people, who knew that a simple burn could do wonders to rejuvenate a degraded prairie. (Think of the famous Muralt Prarie burn in 1975 in Green County!)

But oak savannas are not prairies. Degraded oak savannas have a heavy legacy of shrubs and coarse woody debris. We should never have burned anyway, because the fire would not get rid of the living shrubs. All it would do was top-kill them, and each plant would resprout from the base and send up new shoots, often a lot of them. Indeed, burning had the potential for doing more harm then good!

What we should have done was to clear the shrub layer (cut and treat with herbicide) and remove all the trees that were fire-sensitive and hence would not have been part of the original oak savanna. This was actually a central goal of the management plan, but the urge to burn was pretty strong. And this is what we did do, starting in these savanna areas in 1999.

Incidentally, the savanna shown in the photos is what we call the White Oak Savanna (Unit 12A/B). The photo shows what this savanna looks like in 2014, after clearing and annual burns for over 10 years. Now, when this savanna is burned, the fire carries very well!

White Oak Savanna in 2014, after over 10 years of annual burns.
These are the same trees that provided oak leaves in 1998.