Tom's Blog

Monday, November 24, 2014

The role of fire in a tallgrass prairie

It’s not too early to think about next spring’s controlled burns. One point that arises is burn frequency.

Restoration should be guided by the goals of the project. Thus, if the focus is tallgrass prairie, a fire-dependent ecosystem, then annual burns are essential, especially if it is a prairie remnant. (Prairies planted on former ag fields are a different topic.)

My recent post which analyzed the extensive research by the Konza Prairie group showed that for brush control in tallgrass prairies annual or biennial burns are essential.

Brush control is only one of the important reasons for burning a prairie or savanna remnant. The other major reason is because the forbs and warm-season grasses demand it. There is a vast amount of research that shows that without fire the species that we most want will have difficulty thriving. One of fire's main accomplishments is to remove the thick prairie thatch so that the delicate new prairie plants can reach the light.

The thickness of the prairie thatch varies from year to year depending on the summer rainfall. Sometimes it is so thick that delicate plant seedlings can not find their way through it.

A thick prairie thatch from a single year's growth. This is the prairie grass from the previous season, being mowed to widen a fire break before burning. Last year’s growing conditions were very good, so the grasses are lush. Also, snow was light so that the plants remain upright, guaranteeing a hot fire. 
Concerning the prairie thatch, pioneering papers by Weaver in Nebraska and Rice in Oklahoma are well worth consulting. Rice in particular showed that one role of fire was to remove last year’s thatch so that sun could reach the soil and encourage the growth of warm-season plants. The graph below makes this case very well.

Rice, E.L. and Parenti, R.L. 1978. Amer. Jour. Botany 65: 1091-1097. Rice’s burns were done in the spring dormant season.

In our own simplified work we divided a well established prairie into two halves, burning one and leaving the other unburned. The result with Toby’s Prairie is shown in the photo below. We obtained a similar result with the Pocket Prairie.

Toby’s Prairie divided into two blocks. The block to the right was burned and the block to the left was unburned. The difference in growth of the prairie grasses (Indian grass and little bluestem) is striking. The burn was done on 5 April 2006. The photo was taken on 15 Feb 2007.

How does fire act on a prairie? The photo below shows fire backburning through a prairie consisting of Indian grass and little bluestem. After the ashes are blown away, bare soil will be exposed. In this photo, because of the heavy snows the previous winter, the grasses lay flat, thus ensuring that the flame heights are low.

The action of fire on dormant prairie grasses. In such a thick mat, delicate prairie plants have a hard time finding the light.
Suppose you don't burn? The prairie plants will eventually find their way through the thatch, probably two weeks later. Eventually a new, somewhat impoverished, prairie will form which will form a new layer of thatch. Some of last season's thatch will decompose but not all, so the thatch gets thicker, and in the next year even thicker. Eventually the prairie will reach a point at which most prairie plants cannot survive. But the woody plants can, and eventually the prairie will become some sort of wood lot.

Tuesday, November 18, 2014

White-tailed Deer: Another reason to do an oak woods burn!

A neighbor scouting for deer in the North-facing oak woods that we had recently burned spotted nine female deer with their heads down, feeding. Eating on what? Acorns, of course. Everyone knows that acorns (hard mast in hunter terminology) are a favorite deer food: “the ecological equivalent of manna from heaven”. But you don’t usually see that many in a woods all together.

What is unusual here?
  • As I posted last month, this is a big mast year for red oaks, at least at Pleasant Valley Conservancy. 
  • The North Woods at Pleasant Valley consists primarily of red oaks, so right now the forest floor is full of acorns.
  • Two weeks ago we burned the whole North Woods.
  • Burning does two things for the acorns
    • It gets rid of the leaves so the ground is bare and the acorns are visible
    • The heat roasts the acorns and makes them more palatable.
  • Deer love acorns

Acorns in the red oak group taste bitter because of their high tannin content. One way to get rid of the tannins apparently is by roasting. “Acorns roasting on the open fire”?

My neighbor also managed to attract two fawns to him by a few of the right type of “clucks”. Pretty tame!

There is a question about the effect of fire on acorn viability. According to some U.S. Forest Service work, temperatures where oak leaves are burning can range from <79 to="">371 C. Viability tests on acorns collected after a prescribed burn showed that “patchy, low-intensity dormant season burns in oak forests reduced the viability of red oak acorns located on the leaf litter surface, but did not generally affect acorns in the duff or mineral soil.” Greenberg et al. Acorn viability following prescribed fire in upland hardwood forests. Forest Ecology and Management 275 (2012) 79-86.

In a good mast year lots more acorns are on the ground than needed for oak reproduction. The benefits of fire for oak growth must far outweigh any harmful effects.

It may still be possible to see a forest floor full of acorns. Unit 11B, the savanna just south of the upper road, has mostly red oaks, and a few weeks ago the ground was full of acorns. You should hear the ground crunch under your feet as you walk through these woods. Unless the squirrels have already packed them all away!

Sunday, November 16, 2014

Native invasive plants

Can a "native" species be called an invader? Whether an invader is native or not is irrelevant. If it spreads into an area, forms monocultures, and results in a marked decrease in diversity, it is undesirable and should be controlled.

Here are a few native invaders that under some conditions meet these criteria.

Latin name
Common name

Shrubs and trees

Celastrus scandens
American bittersweet
Twining; forms berries
Cornus racemosa
Gray dogwood
Prairies and open savannas
Cornus stolonifera
Red-osier dogwood
Wet areas
Corylus Americana
Under some conditions
Definitely invasive in Minnesota; probably in Wisconsin
Juniperus communis
Common juniper
Fire-sensitive; forms berries
Populus grandidentata
Big-toothed aspen
Root suckers
Populus tremuloides
Quacking aspen
Root suckers
Rhus glabra
Smooth sumac
Probably allelopathic; prefers sunny areas; root suckers; can dominate a site
Rhus hirta
Staghorn sumac
Probably allelopathic; prefers sunny areas; root suckers; can dominate a site
Rubus allegheniensis
Forms patches
Rubus flagellaris
Spreads close to ground
Rubus idaeus
Red raspberry
Forms very dense clones;
Rubus occidentalis
Black-cap, black raspberry
Tip roots
Salix exigua
Sandbar willow
Wetlands; root suckers; forms very dense clones; can dominate a site
Salix humilis
Prairie willow
Only rarely invasive
Vitis spp.
Zanthoxylum americanum
Prickly ash
Prairies and open savannas

Herbaceous plants

Arnoglossum atriplicifolia
Pale Indian plantain
Root suckers
Helianthus divaricatus
Woodland sunflower

H. grosseserratus
Sawtooth sunflower

H. tuberosa
Jersualem artichoke

Solidago canadense
Canada goldenrod

Woodland Sunflower can easily destroy the habitat for dozens of native species. So can sumac.

Here is a recent review with many examples: Carey, Michael P. et al. 2012. Native invaders--challenges for science, management, policy, and society. Frontiers in Ecology and the Environment. Volume 10(7): 373-381, doi:10.1890/11060 (published online 15 June 2012).

Monday, November 3, 2014

Woody encroachment on prairie and savanna remnants (and the importance of annual burns)

Woody encroachment is a major problem throughout prairie and savanna sites in Central North America, and the principal reason for this is insufficient fire. The purpose of this paper is to make a strong case for annual burns on prairie and savanna remnants.

The area being considered is that part of the U.S. where the original vegetation was primarily grassland and savanna. Historically, the vegetation was maintained by frequent fire, either from natural or anthropogenic causes. Native Americans burned, and pioneer settlers burned. When the fire-spewing railroads came, burns continued. When the land was cultivated, woodlots and savannas that remained uncultivated were burned to induce “green-up” for pasture.

Hill country is especially favorable for fire, because fire readily carries to the top. One match at the bottom may be all it takes. Elderly farmers confirm that they burned every year. A fire often got started as an escape from a burn pile. Burns were generally in spring because grass was well cured and burns well. A fire might even be started in January when there is still snow at the top of the hill.

According to a recent analysis by the U.S. Forest Service (Nowacki and Abrams, 2008), in the Eastern U.S. frequent fire continued through the end of the 1930s but gradually diminished in the post-World War II period (see graph). Thus our area had frequent fires until about the mid 1950s. Air photos from that period still show extensive open land, especially in hill country. It takes a while for woody vegetation to get established and take over. However, once enough shade is created, prairie grasses can no longer grow. A “tipping point” is reached after which there is explosive growth of woody plants. Virtually all at once the forest is continuous and grasses are gone. Air photos show that this occurred in our area in the 1960s and 1970s. By the early 1990s most of what had once been open land was now closed. Only scattered remnants were left to provide a poor clue to what the land must have looked like.
Areas burned in the Eastern United States (from Nowacki and Abrams, 2008)

Current burn programs are failing

Why? We are still burning, so what is the problem?

We have too many sites and some of them are not being burned.

Those that are being burned are often on a three-year burn cycle. If woody encroachment is occurring, this is not frequent enough. How often should we burn?

The most extensive research has been carried out by Kansas State University at Konza Prairie in eastern Kansas. Here a long-term (over 30 years) study has been comparing annual burning with other burn cycles. These fire frequency studies have shown that shrub expansion is prevented with annual or biennial fire intervals. A three-year fire interval sometimes maintains a grassland, but not always. And with fire intervals >3 years shrubs consistently become well established and grasslands disappears. The worst offenders are clonal shrubs such as sumac and dogwood. 
Figure from Ratajczak et al. 2014

But many prairies are routinely burned on a 3-year cycle without problems of woody encroachment. How can that be?

Most of these are planted prairies which have been established in former agricultural fields. There is no woody legacy. These fields formerly had corn or soy beans, were often held in crops for many years, the crops were probably Round-up Ready, and at the time the prairies were planted the fields were completely free of woody vegetation. They may have never seen a sumac or dogwood.

Let me emphasize that burning prairies planted on ag fields is quite different from burning prairie remnants.

In conclusion: prairie remnants should be burned every year, or (at the least) every other year. If not, the danger of woody encroachment is real, even on a 3-year burn cycle. There is always the possibility that due to bad weather or other circumstances, a year might be missed in a 3-year cycle. And maybe another year? The Kansas work showed that after 5 unburned years, it is virtually impossible to restore prairie to its former state by burns alone. You will be facing a long siege of cutting and herbicide work to get that site back to its previous state.

How about savannas? The legacy of woody plants is much more significant in savannas, meaning one must be even more careful. Annual burns must be used, and they must be carried out (probably) for the duration of the site.

References (should be found by an Internet Search and can probably be downloaded as PDFs):

Briggs, John M., Alan K. Knapp, John M. Blair, Jana L. Heisler, Greg A. Hoch, Michelle S. Lett, and James K. McCarron. 2005. An ecosystem in transition: Causes and consequences of the conversion of mesic grassland to shrubland. BioScience 55: 243-254.

Briggs, John M., Alan K. Knapp, and Brent J. Brock. 2002. Expansion of woody plants in tallgrass prairie: a fifteen year study of fire and fire-grazing interactions. American Midland Naturalist. 147: 287-294.

Ratajczak, Zak, Jesse B. Nippert, John M. Briggs, and John M. Blair. 2014. Fire dynamics distinguish grasslands, shrublands and woodlands as alternative attractors in the Central Great Plains of North America. Journal of Ecology 102: 1374-1385. [Figure]

Gregory J. Nowacki and Marc D. Abrams. 2008: The demise of fire and “mesophication” of forests in the Eastern United States. BioScience 58: 123-138. [Figure]

Friday, October 31, 2014

Great woods burn Wednesday 29 Oct 2014

Choosing the right day for a fall burn is always tricky. If you burn too early, most of the leaves (the principal fuel) aren’t down yet. And if you burn too late you might have snow---as we did last night!  
Getting 19 people there for a burn and then having to send them home because conditions are not right is dispiriting. 

This year everything gelled. The bur oak leaves had come down in mid October, the whites about a week before we burned, and the reds three days before. There had been no significant rain so the leaves were dry. A dry air mass had come in on a cold front and held for several days.

We burned the whole north woods, about 30 acres, with an average burn coverage of 93-95%.

Burn crew gathering for final words before travelling to the fire line

Amanda and Susan designed the burn, put in the fire breaks, and handled all the details: water, drip torch fuel, equipment, etc.

White board with burn plan

We have burned the north woods quite a few times and they had the technique down pat. The whole burn unit is ringed by fire breaks. The North Fire Break at the top of the hill is the longest. It had to be mowed and cleared of leaves. The fire break on the west end is potentially tricky since it is next to the very flammable South Slope. Kathie and Willis used our pumper unit, positioned on PV Road, to wet line the hand-mowed fire break next to the South Slope. Several backpack waters were also involved as the fire line moved down the hill here. The east fire break connected the burn unit by ending at County F.

Near the west end of the burn. The savanna on the left will be burned in the spring

The North Fire Break with the fire line backing down the hill. Because of the good fuel, the fire backburned very well.

The procedure is to start lighting in the middle of the North Fire Break, with two burn lines moving in opposite directions. Because this is a long fire break, and is near rather flammable savannas, it takes a long time to get blackened in. Once this is in, and the two lines start down the steep hill at the east and west ends, interior lighting can begin. This year three strippers started at the east end and spread out, moving most of the way to the west end, where two other strippers were working. It turned out that the fire line from the North Fire Break backed over the ridge and started down the hill, so that the upper stripper could skip some areas.

We started lighting at 11:30 AM and the burn unit was tied off at 2:00 PM. The next hour was spent finishing the stripping, and lighting the County F road cut.

Here are several photos of interior lighting, taken by Michael Vahldieck (one of the interior lighters).
Looking toward the N fire break from inside the burn unit (Vahldieck photo)

Fire lines coalescing (Vahldieck photo)

Typical fuel, mostly red oak leaves. Note the low flame heights (Vahldieck photo)

Note the green shrubs, which are about to be killed by the fire. (Vahldiect photo)

In earlier years, the north woods burn has involved lots of mop-up, but this year only a single smoking snag was a problem and had to be cut down. There were, of course, quite a few smoking logs, but these were well inside the burn unit and were allowed to burn. It is our standard practice to leave smokers unless they are close to the edge of the burn unit. With a burn of this size, most of the smokers are quite safe.

The next day Amanda and Susan surveyed the whole burn unit. The average burn coverage was 93-95%.

The day after the burn. Oaks are not damaged by the fire, although it is hot enough to top-kill all the brush.
There were two areas that did not burn well. One was an area at the west end that had apparently been logged a long time ago. By the time we arrived on the scene, it was a tangle of honeysuckles. Although we have been dealing with the honeysuckles, since there are no trees there are no oak leaves to provide fuel. The other that only burned about 75% was at the NE corner. This was another area that had been logged many years ago, leaving only a few large red oaks on the property line.

In all, a great burn!

Monday, October 27, 2014

Legacy effects in prairie restoration: part 2

To review: at the time the Nature Conservancy acquired Black Earth Rettenmund Prairie, the site had become about 50% wooded, including two major aspen clones, and lots of honeysuckle and other shrubs. Over the next 25 years, all of the major woody material was removed. Despite these efforts, woody vegetation has continued to appear.

In the research I am reporting here, I have used sumac as a surrogate for woody vegetation, since its distinctive fall coloration makes it very easy to locate. Using GPS, I recorded the coordinates of sumac in October of 2010 and 2011. The map below (based on GIS work) shows sumac locations overlayed on a map of the woody vegetation in 1987, before restoration had begun. The correspondence between the present-day sumac populations and the historic woody vegetation is striking. Recall that biennial burns have been carried out since restoration work began in 1987. Also, all visible woody vegetation was removed in the original restoration work in the period 1987-1998.

Another legacy effect is the difficulty of establishing prairie grass in former aspen areas. The photo below shows a comparison between a former aspen area (eradicated from trees in 1987-1988) and the same area in 2010. The dearth of grass in this former aspen area is striking. This area has been burned at least every other year since the aspen were removed. Also, the area  has been planted with grass seeds several times. Again, a legacy effect.

An important conclusion from my study is that despite the fact that the tree/shrub areas have been eliminated, the legacy of their former vegetation remains. Thus, what there is today is a result of what was there in the past. Despite the extensive restoration work that has been carried out, woody shrubs still arise, in locations that correspond to the original woody vegetation. The “land remembers”.

This conclusion has critical implications for restoration ecology, and emphasizes how important it is to understand the history of a site when devising restoration strategies. This concept has now been well accepted in restoration ecology. For instance: “site history is embedded in the structure and function of all ecosystems, environmental history is an integral part of ecological science, and historical perspectives inform policy development and the management of systems…” (Foster, David; Swanson, Frederick; Aber, John; Burke, Ingrid; Brokaw, Nicholas; Tilman, David; and Knapp, Alan. 2003. The importance of land-use legacies to ecology and conservation. BioScience 53: 77-88.)

My work could not have been done without the historic air photos, as well as the extensive written records found in the Nature Conservancy and Wisconsin Department of Natural Areas files. I strongly urge that restoration work be carefully documented and that written records as well as digital ones be archived for use by future generations.

Friday, October 24, 2014

Legacy effects in prairie restoration

In the past dozen or so years ecologists have begun to realize that the future of a prairie remnant is strongly influenced by its past history, and these “legacy effects” are important in restoration work.

I stumbled on these concepts of historical ecology while working on the restoration of Black Earth Rettenmund Prairie, a State Natural Area. Before it became a Prairie Enthusiasts’ site, this outstanding natural area was owned by the Nature Conservancy (TNC), and TNC had extensive files on its history. Also, there is an extensive sequence of air photos to follow the changes through the years.

In 1937, when the first air photo was taken by the U.S. Soil Conservation Service, the prairie was almost completely devoid of woody vegetation. During the four decades between the early period and 1980 there was a gradual increase in wooded area, and the rate accelerated between 1980 and 1987 (see graph below). The early shrub/tree development took place in the south part of the prairie, which is adjacent to a fairly extensive undisturbed woodlot. By 1949, the woody patches had increased, and some of the trees in the South unit were now quite large. (The stumps from some of these trees still remain today.)

Although only approximate, the rate of increase in woody vegetation has some resemblance to a growth curve. There is an extended lag phase in the early years, but beginning in the mid 1970s there seems to be a “tipping point” after which the site seems to “explode” with woody vegetation. (The terms “tipping point” and “explode” were first suggested to me by Randy Hoffman. According to him, left unchecked other prairie remnants in Wisconsin show this same phenomenon. The graph shows that at the peak, almost half of the site was wooded.

Upon acquisition in 1986, this preserve became a major project of the DNR and TNC. Tree and shrub removal and frequent prescribed burns by the DNR and TNC were used in attempts to restore the prairie.

Although these efforts led to substantial success, the legacy of the woody vegetation remains, considerably complicating restoration efforts. New shoots still appeared in former aspen areas. Sumac and gray dogwood were particularly troublesome, but brambles and grape were also a problem. One former wooded area had been invaded by willow and hazel, which have proved difficult to eradicate.

I gave a presentation on my work at the North American Prairie Conference in Winnipeg, Manitoba, in 2012.  My paper has now been published and can be downloaded as a PDF using this URLThis Blog Post is a brief summary.

A view looking SW from Fesenfeld Road, showing the extensive clone of aspen on the top and sides of the north unit. Note also the large clone of sumac on the lower slope (above the cropped field). 1986 photo from TNC archives taken by a real estate assessor.
A view of the “Saddle” area of Black Earth Rettenmund Prairie, probably taken in the late 1970s. In addition to the substantial brush, the edge of a large aspen clone can be seen in the lower right corner. Photo kindly provided by Cliff Germain, retired head of the Bureau of Endangered Resources, Wisconsin Department of Natural Resources.

Woody vegetation for different years as measured on air photos by GIS. (a) Thumbnails for 1937, 1987, and 2010. (b) Change in woody areas with time over the 73-year period.

The present work suggests some principles of importance for restoration ecology.

·       You can’t let things get out of hand. During the early stages, shrub establishment may seem benign and not worth worrying about. This is obviously wrong. At first, shrubs and trees are easy to remove. However, once the explosive phase of growth has set in, not only is the biomass needed to be removed much larger, but the whole population is growing at a vastly increased (quasi exponential) rate.
·       The importance of frequent fires. Fire is now a widely accepted component of prairie and savanna management systems. However, there is a tendency to assume that fire on a three- to four-year cycle may be sufficient. This is a mistake. Beginning at the time of TNC acquisition, burns have been conducted on the various units on the average every second year. Despite this frequency, shrub growth remains a problem. Sumac, prairie willow, brambles (Rubus spp.), and gray dogwood in particular continued to thrive. Aspen shoots continue to appear in former aspen areas. In those areas where the legacy of shrub growth remains, annual fire should be considered. The historical GIS data will help locate the areas where annual fires should be carried out.
·       However, burns alone are not enough. At Black Earth Rettenmund Prairie, with its biennial burn cycle, sumac, gray dogwood, prairie willow, etc. have remained or have developed into problems. There are strong reasons to believe that all clonal shrubs, even if they are native, should be eradicated by a multi-year program of herbicide use. If the prairie is burned in alternate years in the spring, then the shrubs should be treated with herbicide in the late fall of the second year after the burn (that is, before the upcoming spring burn).
·       Eradication of clonal shrubs is essential, even if native, because annual stewardship cannot be guaranteed to continue into the indefinite future.
·       Legacy effects must be taken into consideration when planning management strategies. Areas that were woody in 1986, when restoration began, continue to inhibit the established of true prairie species, especially graminoids. These areas not only need tree and brush removal, but planting with seed collected elsewhere on the site.
·       Significance of herbicide use. The prejudices against use of herbicides remain. The first seven years of restoration work at Black Earth Rettenmund Prairie were wasted by trying to control shrubs without the use of herbicides. (The State Natural Area Board had refused permission to use herbicides.) Although trees can often be controlled without herbicide (by girdling), shrubs, with their paramount ability to resprout, cannot be controlled without herbicide.
·       Seed collecting and overseeding are essential. A site that has been wooded for an extended period of time almost certainly has lost most or all of its warm season grasses, as well as some of the more light-demanding forbs. The seed bank for such a site is uncertain, and probably highly variable. Without overseeding, restoration of such a site may take years. Even if there is a good seed source near by, spontaneous overseeding will be random and, depending on the quality of the seed, the success will be highly variable. Even 25 years after the initial restoration work, formerly wooded areas at Black Earth Rettenmund Prairie still remained impoverished, even though they have been reseeded with native grasses and forbs. Although each restoration project must be analyzed individually, if the site was wooded it can safely be assumed that overseeding is advisable. In fact, why not plant? Unless this is a research site, it seems better to plant than to wait and see what happens. Overseeding is easy and relatively inexpensive, and the sooner it is begun the better. The only reason not to overseed may be if a local seed source is not available.

For some reason, Black Earth Rettenmund Prairie did not become infested with red cedar, as have all the other hillsides in the area. This was fortunate, because the shade imparted by red cedar is much worse than that of aspen or shrubs. Cedar-infested sites quickly lose all of their species diversity. In one study at Konza Prairie, herbaceous cover decreased over 99%. (See Briggs, John M., Knapp, Alan K. and Brock, Brent L. 2002. Expansion of woody plants in tallgrass prairie: a fifteen-year study of fire and fire-grazing interactions. American Midland Naturalist 147: 287-294.)

(I am grateful to Wayne and Sharon Gaskill, stewards for TNC, for sharing with me their detailed stewardship files.)