Tom's Blog

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.


Blogger FrankOnABike said...

So it sounds like the answer to the question you pose in the title is: As soon as the brush is cleared.

September 5, 2017 at 5:46 PM  
Blogger Tom's Blog said...

Right! But It's more efficient to clear the brush and undesirable trees at the same time.

October 8, 2017 at 5:26 PM  

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