Tom's Blog

Tuesday, March 20, 2018

Prescribed burning in cold weather

This season we have been doing prescribed burns in fairly cold weather. The dew points have been quite low, so that there was no dew on the grass even when the temperature dropped to the mid 20s (F).

Leading up to each burn day we had several days of clear, sunny weather with temperatures reaching the upper 40s. But for the two successful prairie burns we did on Friday March 16, 2018 the temperature never got above 32 F! We burned the East Basin (5 acres, with complicated topography) and the Valley Prairie (4 acres adjacent to the PVC wetland).

I had no success finding any information in the fire literature about burns in cold weather. Perhaps most people think you can’t burn when it is cold.

One of the most important parameters for a prescribed burn is dead fuel moisture content. This time of year prairie grass is fully cured, which means that its moisture content is completely controlled by external conditions, most importantly rainfall (or lack of it) and relative humidity (R.H.). Although temperature does have an effect on the rate at which moisture of the grass is taken up or lost, the most important influence of temperature on dead fuel moisture is the effect on R.H. When we burned Friday, the R.H. was below 50%.

Although the fire literature has tables for estimating dead fuel moisture, these are only approximations and don’t take local conditions into consideration. But there is a simple way of estimating dead fuel moisture of oak leaves, which are a fairly good surrogate for grass.

This is McCarthy’s test for assessing the moisture content of hardwood leaves by bending.

Moisture content
Behavior during bending
Leaves crack if creased but do not break entirely
Leaves crack if folded more than a right angle
Leaves crack if bent at a right angle but do not break freely, especially at the veins
Leaves break entirely apart if bent at right angles
below 10%
Leaf crumbles when you pick it up

Source: McCarthy, E.F. 1927. Weather and forest inflammability in the southern Appalachians.
Monthly Weather Review, March 1927.

 Here is a guide to using the McCarthy test: When using the bend test, take several samples in both sunlight and shade and if on a slope, at the top, middle, and near the bottom of the unit. If the leaves have been wet from precipitation, wait at least one day of full sunshine with medium winds and RH below 50%.

Just before we started the East Basin burn, I used the oak leaf bend test. It gave a reading of 14%, which is a good dead fuel moisture content for a prairie burn

The photos show the fire behavior in the East Basin Prairie, a restored prairie. The fuel here, mainly Indian grass and little bluestem, was in its 8th growing season and obviously well established.

According to NOAA weather radio, the early afternoon temperature was 32 or 33 F over all of southern Wisconsin.

Successful prairie burn under cold-weather conditions

Keeping a straight burn line on  the east side of the unit

Doing some interior lighting on a later stage of the burn
This prairie has a complicated shape, with level areas surrounding
a steep south-facing basin

Two fire lines meeting on the final stage of the burn

Almost complete burn coverage with just a few unburned patches.
The white ash indicates complete combustion.

The south fire line (facing north) still had piles of snow.
Note also the snow on the distant hill.

Final check of the burn.
Most of the small smokers are well inside the unit and will be left to burn up

6146 6157 6163 6165 6168

Tuesday, March 6, 2018

A very successful early-season oak savanna burn

The Weather Bureau made no mention of it, but March 2-4 2018 provided unusually good weather for controlled burns, especially for oak savannas. This is quite a bit earlier than we usually do savanna burns, but one element of burn planning is to burn whenever the weather is right.

Carrying out controlled burns of natural areas require lots of attention to detail.

We had already done a major burn of our South Slope prairie and bur oak savanna remnants on February 27, with the same kind of weather conditions. After a few days of uncertain weather, the good weather returned. A cold air mass moved in with low dew point (21 F) and moderately warm temperatures (45-50 F). Especially important was that the sky was clear and the sun was strong. (In early March the angle of the sun is already high enough to bring on significant warming of the fuel;  grass and oak leaves.)

As long as you have low R.H. and sun, you don’t really need warm weather, provided it is above freezing. An additional important weather element was a good wind (8-10 mph) from the south. (Because oak savannas often have downed woody debris, a good wind is an important element for a savanna burn, since it helps to keep the fire moving along.)

It took a day to get everything ready, so the burn was scheduled for March 4, a Sunday, giving us the opportunity of getting more than usual volunteers. These together with the Integrated Restorations crew gave us 16 people, all experienced burners. (The IR folks understand about burn scheduling and are willing to work on Sunday!)
The crew assembling at the anchor point. The vehicles were parked on a black area from a previous burn.
Each person had a two-way radio, essential on such a burn.
We burned all the area shown in light tan on the map, 36 acres. Because of logistics, those units north of the motor vehicle road (Unit 19B,C,D, and E) were burned separately after lunch along with several units from a future planned burn (Toby’s Prairie, Annex, Triangle; see map); 6 acres.

We call the units we burned the Basin Savanna, since they essentially wrap around the Pocket Prairie, which acts like a “basin”. The oaks in the upper part of the basin are burs and those lower down predominantly white, with some Hill’s oak and black oak intermixed.

The location of the start of the burn, which was also our anchor point, was the junction on the map where 19E, 12B, and Toby’s Prairie come together. One crew moved west from there along the gravel road, backburning down through 11B and 11A until they reached Unit 18. The other crew moved east, backburning through 12A and  20 until they reached the Pocket Prairie.

Timing was critical and it was essential that the two crews coordinate. We did not want the 2nd crew to get too far ahead because once the 12A fire line reached the Pocket Prairie it would touch off a head fire in the prairie that could sweep up into 18 and 11D. Thus, the 1st crew had to get the 11A backburn finished first. Fortunately, every crew member had a two-way radio, so that coordination was good. Amanda, the burn boss, monitored the two crews carefully.

The Basin Savanna burned very well, benefiting from the RH of 35-40% and a strong drying wind gusting from the south or east.

We had twelve drip torches, and used every one!

One of the important requirements of the savanna burn was to protect the lower branches on the large trees, especially those attractive “savanna oaks”. The way to do this was to avoid head fires in the savanna.

Another goal was to protect as much as possible the living birches, since they are greatly used as wildlife habitat (woodpeckers, bluebirds). The litter at the bases of the birches had been removed with a leaf blower.

Another goal was to protect snags (great wildlife habitat) by clearing the litter around them.

Another reason for backburning is that it keeps the flame height low.

However, because of the gusty wind, random change from backfire to headfire sometimes occurred. Although the wind helped to carry the fire in the savanna, it had potential danger for special trees, birches, snags.

Tree-centered spot firing One procedure for protecting special trees, birches, and snags, was tree-centered spot firing. The drip torch operators had been instructed in how to use this technique. See my blog post for details.

The savanna units back-burned well but gusts occasionally turned the fire into flank or head fire in some areas. Fortunately, these were relatively infrequent.
Sometimes all one had to do was watch the savanna burn. This is a late stage of the burn. Ron Endres photo
A view across the Pocket Prairie to Unit 11D of the Basin Savanna. The oaks at the top are burs, those below are whites.
The fire line moving down through the Basin Savanna eventually reached the Pocket Prairie, jumped the firebreak, and started the Pocket Prairie on fire. The Pocket Prairie mostly burned that way but was also lit from Pleasant Valley Road. Fire also started in the SW corner of the Pocket Prairie area by a spot fire from Unit 18. 

One of the goals was to get the fire into  Unit 11A and Unit 18 before the Pocket Prairie caught on fire, because when the Pocket Prairie burned there was a possibility of it igniting Unit 18 from the bottom and hence turning Unit 18 burn into a head fire.

Note that with the lush fuel of the Pocket Prairie and the low RH, the Pocket Prairie burns very rapidly. It takes no more than 10 minutes for this whole 4.5 acre prairie to burn. However, this year there was an area of the Pocket Prairie which burned but the tall dead stems, mostly forbs, did not burn up. This area is in the outflow of storm water events from the ravine, in a wide line between the ravine and the outflow into the culvert under Pleasant Valley Road at the south end of the Pocket Prairie. This is an area that we had recognized as wet-mesic and had planted with species suitable for that sort of soil. The unburned area shows in one of the photos.

Recap on the weather: Dry air mass came in Friday and held for weekend. Sunny. Dewpoint 21. Temp mid 30’s at night to low 50’s by noon. South wind around 10 mph.

The sun was an important factor There were no clouds during night or day as well as for the two days leading up to the burn. Because of the low dewpoint, no dew on the grass or leaves. The wind was also important in helping the fire “carry” through the savanna. The temperature is not as critical as long as above 45.

Nice comment from volunteer Ron Endres: Will and I were talking about how every single burn is different. Both the Tuesday and Sunday burns had fire behaviors we had never seen before. I think one reason we like being on these crews is that there is an element of "observe and then react" and often problem solving on the spot. There are very few "boring" fires. We also like burning with you [Amanda] and Craig and Chris. Even though you are the Burn Boss and Line Bosses, you draw on our experience. We always feel like we are needed. We work on a number of different crews and we don't always feel this way. And when the weather is like it's a wonderful day.

Wednesday, February 28, 2018

An interesting phenomenon of grassland fire behavior

Yesterday (February 27, 2018) we did the annual burn of the tallgrass prairie on the south slope of Pleasant Valley Conservancy State Natural Area.

Because of the extensive bur oak savanna on the upper ridge, most of the burn was conducted as a backfire. However, there are no trees on most of the lower slope so to speed up the process, a headfire was used.

The principle fuel is little bluestem and Indian grass, with a few patches of big bluestem.

Yesterday the wind was gusty and variable, so that the grass burned either quietly or in quick bursts as the wind intensity varied. The photo below is typical of the headfire.

Headfire behavior. The flame has just been caught by a sudden gust of east wind (0648.jpg)

After the burn was over, Kathie noticed that on the lower slope the fire residue was in narrow linear strips, with white ash alternating with black ash (black char).

According to the fire literature, white ash is a product of complete combustion and black ash is the result of incomplete combustion. I assume that when a strong gust of wind passed over the burning grass, the burn temperature rose and complete combustion occurred, leading to white ash. When the wind was less strong, the temperature was less, and incomplete combustion produced black ash. The interesting thing is that there were several alternating strips of white and black.

The smoke in the photo below shows the linearity of the wind.

Fire behavior showing possible origin of the linear strips. The flames have just been caught by the wind. (0640.jpg)

Unfortunately, I did not get a photo of the long linear strips, but the photo below shows short strips of white and black ash from the same burn.
Bank of grass that has just been burned showing short strips of white and black ash (0620.jpg)
White ash is primarily inorganic substances such as potassium hydroxide, potassium carbonate, and non-alkali substances such as chloride and sulfate salts. The fragile leaf structure may be maintained temporarily but quickly disintegrates. Black ash (char) is primarily amorphous carbon.

Per NOAA, Feb 27 noon was 55 F and 55% RH. Wind 8-10 mph from south. However, at PVC, the wind often shifted to the east.

Saturday, February 17, 2018

Measuring the canopy coverage of an oak savanna

Typical open canopy of a savanna. This is a July view of the northwest side of Unit 10.

Quite a few methods for measuring a forest canopy cover exist, but on-the-ground methods are very time-consuming. However, with a good air photo, the analysis can be done very easily with GIS.

At Pleasant Valley Conservancy we have over 40 acres of high-quality oak savanna and a similar amount of oak woodland. Because of the heterogeneity of PVC, the canopy cover varies widely. I was interested in determining the coverage of some different sites and I was also interested in observing the changes in canopy coverage with time.

The general procedure in ArcMap was to overlay a digital grid of dots over an air photo whose sites are delineated, and count the percentage of dots that are over trees. However, because dots are difficult to see against the tree background, I used triangles, with the lower right corner of the triangle used as the “dot”. I created the grid in Corel Draw, saved it as a GIF file, and imported it into ArcMap as a layer.

I used air photos from various years to give a wide time series. The oldest air photo was from 1937. I adjusted the contrast and brightness so that the tree images were sharp. Color photos were converted to B/W.

Previously I had created a GIS layer that had the outlines of each management unit. Thus, I could determine the canopy cover for any PVC area. The photo shows the digital grid and management  units on top of the 1937 air photo.

A portion of PVC showing the dot overlay of the 1937 air photo. The lower right corner of the triangle was considered to be the "dot".
The Unit labels can be turned off when doing the measurements.

The accuracy of this technique depends on the density of the digital grid, and the ability to recognize the edge of the tree canopy. Some air photos were better than others, but all were of sufficient quality for my purposes.

% canopy coverage


Prairie remnant
Bur oak savanna
Bur oak savanna
Bur oak savanna
Bur oak savanna
Bur oak savanna
White oak savanna

The drastic change from 1937 to 1990 shows what happens in the absence of fire. We started clearing the bad stuff in 1998 and by 2005 we had most of PVC cleared. In most units, the canopy cover is lower in 2005 than it was in 1937, probably reflecting the extent of our restoration work. As expected, there is little change between 2005 and 2010.

Another procedure for canopy analysis is on-the-ground vertical photography with a fish-eye lens. This makes it possible to determine the canopy coverage at any site. I used this procedure when I was studying the distribution of purple milkweed, a classic savanna species. Most of the sites where purples were established had about 50% coverage. 

An image of a purple milkweed site with a superimposed grid is shown in the photo below.

Vertical image taken with a fish-eye lens.
Each grid corner represents a dot".
I estimated 55% coverage for this image.

Friday, January 19, 2018

Unusual fall weather for a savanna burn November 6, 2004

What's happened to those great fall days that were so perfect for savanna and oak woodland burns?

The last several years it has been difficult to find a good day for a woods or savanna burn.

Since my memory may be faulty, I've been reviewing my notes from burns from earlier years. How about this:

On November 6, 2004 the temperature at 2 PM was 75-78 F and the R.H. was 18-20% (measured by Kestrel). I question whether the Kestrel is accurate at that low R.H., but there were no clouds and the air was clear and sunny. Ideal conditions for a savanna burn. And we didn't need a permit!

 The map shows what we burned. The blue dashed line along the south side of the burn unit represents a new trail we call the Mid Savanna Trail. We put that in along the middle of Unit 11 savanna in order to provide access to this area. That trail ends at the East Overlook and provides a short-cut for people who want a smaller hike. 

According to my pencil marks, the burn coverage was very good. The few small open circles are areas that had brush patches. Not brambles, because the fire burned right through them.

We had 10 people, part paid and part volunteers. We started lighting in the far east end of the burn unit. There were two burn lines, one moving south and west along the blue dotted fire break, and the other moving north and west along the north side of Unit 19 and then east along the South Fire Break. Once the whole periphery had been blackened, extensive interior lighting was done. The burn boss surveyed the area and directed drip torches to areas needing fire.

Kathie lighting along the Mid Savanna Trail

We had two pumper units. One was a high-power rig from the Prairie Enthusiasts, and the other was an electrically operated pump on the back of our Kubota tractor that Paul Michler had made.

With the low humidity it was perhaps not surprising that we had a flaming tree, a large bur oak in Unit 10. We used the high-power pump to put this fire out. The Kubota rig was used to put out smokers that were too near the fire line.

This flaming tree was mostly alive; only the dead side branch was on fire

Using TPE's high-pressure pumper unit to extinguish the fire.
An extension ladder is being used.

We started lighting at 10:45 AM, and finished at 1:15 PM. Interior lighting and mop-up took until 3 PM.

 A great burn! Will we ever see November conditions like this again?

Paul Michler made this pumper unit.
The electrical pump works off the tractor battery.
This whole unit was put together for less than $400.

Monday, January 15, 2018

Brush and tree cutting at Pleasant Valley Conservancy: 20 years of progress

As the graph and map show, once we got started clearing the prairie and savanna remnants we kept up a fairly regular pace. Although it isn’t shown on the graph, most of the biomass cut consisted of trees. There was lots of brush, but it was less than 10 feet tall, and only constituted a small part of the biomass. (It did, however, provide a good base for a burn pile.)

By the end of 2006 we had all the important areas cleared. From then on, our main woody plant work was the new flush of brush and herbaceous weeds brought on by the great increase in light to the ground.

Also, once the brush and trees were clear from a unit, two other activities became major focuses: 1) controlled burns in that unit, both spring and fall; 2) seed collecting and planting. Both activities were critical to increase the diversity of the understory vegetation in the newly cleared unit.

I am rather fond of the map. It gives a good view of where we worked, and when. I made it with ESRI’s ArcGIS, once I had learned how to use that software (quite a learning curve!). The areas we tackled first (1997-98) were the most visible, and provided the easiest access. Later, we moved to areas where access was not as good. Fortunately, our ridge-top road held up fairly well to half-ton pick-up trucks in winter.

However, we soon realized that the road needed to be graveled, and this turned out to be worth the expense. Only when the ground was frozen hard did we allow vehicles off-road. It was important to keep vehicles from ruining the delicate savanna and prairie sod.

Gravelling the service road between Toby's Prairie and the White Oak Savanna: 2003
This was a substantial expense but well worth it
A major reason our restoration work has been so successful is that we had an outstanding restoration crew in Michler & Brown LLC. Paul Michler and Willis Brown complemented each other very well, and their other employees, especially Todd Shumate, Craig Annen, and Chris Knief, have been outstanding. During the major periods of clearing, especially from 2001 too 2005, M & B had a crew of 6, which was about optimum. My other post hasWillis' comments on how the crew worked. 

Another reason our work was so successful is that Kathie and I worked very closely with the crew. We monitored the work carefully, often helped with herbicide treatment of cut stumps, and monitored burn piles. We opened  up our field station/cabin to the crew for lunch, and generally ate with them. Most of the clearing crew also participated in burns, and sometimes even volunteered for seed collecting and planting.

I should emphasize that PVC could have been cleared much faster, but that would have been undesirable. Once a unit is opened, it must be “tended” carefully or it will quickly become destroyed. This means controlled burns, seed collecting and planting to create a diverse understory, and continuous weed and brush control. My advice always is: if you can’t burn, don’t clear. But burns alone are not enough.

Although we did not have a time schedule when we began restoration, I think our rate was just about optimal for the resources that we had available.

This unit had over 25 large walnut trees. Many were big enough for lumber and were skidded
to the Town road and off to the saw mill. Photo from a snow-free winter (2004)

We only permitted truck access when the ground was frozen hard

Black and red oak logs waiting to be converted to fire wood. Lots of wood generated in savanna clearing work

A well-built brush pile. The nearby standing trees will later be cut and thrown on the fire .

Friday, January 12, 2018

How things have changed! Photos from the North American Prairie Conference field trip of August 12, 2004

Almost 14 years ago Kathie and I led a field trip to Pleasant Valley Conservancy for the North American Prairie Conference that was held at UW-Madison. Photographer Dennis Connor was there, snapping pictures with his film Nikon. (It's necessary to use "film" as an adjective these days.)

After the trip, Denny gave me a fat envelope full of prints, which I dutifully filed.

I am now working on the restoration work done at PVC in 2004, and was delighted to find all these photos still intact. Although we thought PVC looked pretty good in 2004, I am surprised at how brushy it looks in these photos. 

There are other changes, also. For instance, that large bur oak that is hanging  over the trail came down about 10 years ago. And we no longer use the South Fire Break, since we have larger burn units these days. And lots of those savanna grasses that the group is admiring have been replaced in most open areas with warm-season grasses. (And Kathie and I look a "little" different also.)

Kathie is standing at the north edge of the Pocket Prairie, with the White Oak Savanna in the background

This is the Side Road separating Unit 10 from 11A. It was a sad day when the big bur oak came down 10 years ago.

The group is on the South Fire Break. Unit 8 , with its venerable bur oaks, is on the right.

Elymus riparius