Oak woodland: burn it or lose it?

The importance of prescribed fire for maintaining oak woodlands and oak forests is now so widely accepted in the forestry/fire ecology literature that it is no longer controversial. Probably the best up-to-date source of information on this topic is the Oak Woodlands and Forests Fire Consortium, an entity of the Joint Fire Science Program supported by the U.S. Forest Service. This Consortium presents monthly webinars which are openly available. Since they are archived, they are available at any time. Just go to the Oak Woodlands web site, choose  Activities, and then click on any webinar of interest. (The first time you will have to download a free application called Adobe Connect.)

On Dec 10 2013 the most recent webinar was held. This was presented by Dr. Gregory Nowacki, an ecologist with the U.S. Forest Service, and titled: Oak, Fire and Mesophication: Past, current and future trends of oak in the eastern United States. This was an illustrated review of an important paper that Dr. Nowacki had published four years ago with Marc Abrams with the title: The Demise of Fire and “Mesophication” of Forests in the Eastern United States. Although I summarized this paper in a blog post in 2009, I recommend watching the whole Webinar, which is well illustrated and easy to follow.

One of the most important points of the Nowacki/Abrams paper was the striking decrease in use of fire in eastern U.S.forests since the 1950s. This is illustrated in the graph below, which comes from the original paper.

Area burned in the eastern United States
(1938–1990) based on historic fire records held at the US
Forest Service, Fire and AviationManagement,Washington
Office, and compiled by ReginaWinkler (R6 Information
Technology Specialist). Area includesMinnesota, Iowa,
Missouri, Arkansas, Lousiana, and all states eastward.


This marked decline in burning since World War II is striking. For some reason, the "culture" of woods burning apparently changed after the war. Now the goal is to change this culture back. Private landowners are strongly encouraged to burn their oak woods, at least every three years, if not more often.

At Pleasant Valley Conservancy we first burned our oak woodlands in 1999, followed by burns in 2007, 2008, 2010, 2012. Although the first burn was rather spotty, it has improved with time and the 2012 burn was very good. The  plan is to continue burning at least every other year, either in spring or fall (depending on the weather).

The title of my blog post comes from a new brochure published by the Wildland Fire Laboratory at Humboldt State University (California). As it happens, California has lots of oak woodlands, mostly on private land. The brochure is intended to encourage landowners to burn. (I learned about this group because they used one of my oak woodland fire photos in the brochure.)

Survey of cool and warm season grasses

Most prairie and savanna restorationists are aware that there are two classes of grasses, distinguished by a fundamental property of photosynthesis (abbreviated C3 and C4). Many prairie grasses have the C4 pathway, and because of this are able to proliferate in warmer parts of the summer when other plants are suffering from moisture stress. For the most part, prairie grasses are warm season grasses, and savanna grasses are cool season grasses. The C3/C4 differentiation is a good example of evolution in action, and botanists have been able to explain at a fairly fundamental level how these two kinds of grasses differ.

I recently spent some time putting together a table that sorts out these two kinds of grasses, based on an extensive review that was published by S.S. Waller and J.K. Lewis in the Journal of Range Management (1979; vol. 32, pp. 12-28). Note that my table is just a subset of that of Waller and Lewis since they have a lot of grasses that we are not familiar with here in the Midwest.

As I indicated, the C3/C4 phenomenon is an interesting evolutionary development that relates to the ability of plants to tolerate drought. The explanation is fairly sophisticated, but the Wikipedia pages on this are very good. See this link. (What would be do without Wikipedia?)

Essentially, the C4 pathway permits plants to carry out photosynthesis during warm (or hot) summer conditions, making them able to outcompete C3 plants.

Note that in our temperate climate, the only C4 plants are grasses, but in the tropics many broadleaf plants have the C4 pathway.

It is interesting that although C4 plants represent only 5% of the Earth's plant biomass, and only 1% of the known plant species, they account for about 30% of terrestrial carbon fixation.

C3 and C4 grasses: abbreviated from Waller and Lewis

Genus	Common name	C3	C4
Agropyron	Wheat grass (not agricultural wheat, which is Triticum)	*
Agrostis	Bent grass	*
Andropogon	Big bluestem		*
Avena	Agricultural Oats	*
Bouteloua	Side oats		*
Bromus	Brome	*
Calamagrostis	Blue-joint	*
Dactylis	Orchard grass	*
Deschampsia	Hair grass	*
Dichanthelium (formerly Panicum) latifolium; leibergii; oligosanthes; praecocius	Panic grass	*
Digitaria	Crab grass		*
Echinochloa	Barnyard grass		*
Elymus	Wild rye	*
Eragrostis	Love grass		*
Festuca	Fescue	*
Glyceria	Manna grass	*
Hordeum	Barley	*
Koeleria	June grass	*
Leersia	Rice cut grass	*
Lolium	Perennial rye grass	*
Muhlenbergia	Muhly grass		*
Oryza	Rice	*
Panicum virgatum Panicum capillare Some other species of Panicum are C3 grasses	Switch grass Witch grass		*
Paspalum	Paspalum grass		*
Phalaris	Reed canary grass	*
Phleum	Timothy	*
Poa	Blue grass	*
Pragmites	Common reed grass	*
Schizachyrium	Little bluestem		*
Secale	Cereal rye	*
Setaria	Foxtail		*
Sorghastrum	Indian grass		*
Sorghum	Broom-corn		*
Spartina	Cord grass		*
Sphenopholis	Wedge grass	*
Sporobolus	Dropseed		*
Stipa	Needle grass	*
Triticum	Wheat	*
Zea	Corn		*

According to the Wikipedia article: "C₄ carbon fixation has evolved on up to 40 independent occasions in different families of plants, making it a prime example of convergent evolution. C₄ plants arose 25-32 million years ago (Oligocene)... and did not become ecologically significant until around 6-7 million years ago (Miocene). C₄ metabolism originated when grasses migrated from the shady forest undercanopy to more open environments, where the high sunlight gave it an advantage over the C₃ pathway."

Small invasion of Canada wild rye

Canada wild rye (Elymus canadensis) is often a substantial part of the seed mix for new prairie plantings, since it gets established quickly and acts as a native nurse plant for prairie seedlings. Since it is a short-lived perennial, it eventually is replaced by the permanent prairie grasses, such as Indian grass, and big and little bluestem.

We have never used this species in our prairie or savanna plantings, but even so, there are small patches here and there about the Conservancy. Where have they come from? Probably as a contaminant in the seeds of some other species. (It's difficult to guarantee complete seed purity.)

Small patch of Canada wild rye established in the Circle at the top of the ridge.
The long, curled awns are characteristic of this species.

The patch here consists of less than a dozen flower stalks, mixed in with Indian grass and little bluestem.