Tom's Blog

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

Monday, January 1, 2018

Controlled burns: Increase efficiency by doing two burns at once

I am reviewing the 20-year history of burns at Pleasant Valley Conservancy. Every year I have kept careful notes of each burn, and have written a narrative afterward, with a map showing burn coverage. We have always tried to do large burns, especially for the oak savannas. However, the topography of PVC is complicated, and we have two savanna areas that are only connected in the middle (by what we call the Side Road). There is a Ridge-top Savanna, consisting of units 8, 10, and 19A,B,C,D  (about 13 acres). And a Basin Savanna, consisting of three units that form a basin surrounding the Pocket Prairie (Units 11, 12, and 18), about 24 acres. See map


When we first started burning these savannas, we often did them on different days. However, good burn weather doesn’t come along every day. My philosophy has always been: “When the weather is right, and the crew is available, burn, burn, burn!

The first time we did two burns at the same time was November 14, 2003. Because it was a fall burn, there were lots of people available (we had 15) and all were experienced. When we met at the cabin, I realized that we did not need so many people for a single burn, and suggested to the two most experienced people that we run two separate burns simultaneously. They agreed, and each chose his crew members.

The wind was out of the south west, about 5-7 mph gusting to 11-12 mph. We liked the wind speed and direction, since it helped carry the fire up through the units. Getting the fire to carry well was often difficult in those early years of restoration, since the warm-season grass was just getting started, so we used head fires a lot. The temperature was 50 F and the R.H. in the upper 30s.

We had everything set the day before: fire breaks, water jugs spotted near the burn units, drip torches full, etc. We took the whole crew and all the equipment to the top of the hill in trucks, dropping the Basin Savanna crew off on the way to the farther Ridge-top Savanna.

Backburning from the Service Road

The crew doing the Ridge-top Savanna burned Unit 19 first, thus establishing a solid black line at the downwind side of the unit. They had two drip torches, one went along the north side of Unit 19 and the other went along the service road. Once Unit 19 had been burned (about 1:30 PM), this crew used a head fire by lighting along the fire break at the south side of Units 8 and 10. The SW wind carried the fire up through the units, getting an almost 100% coverage. This was the best these two units had burned. In past years we had needed a lot of interior lighting to get good coverage of these units, but not this year.

Simultaneously, the Basin Savanna crew started at the top (NE) corner of Unit 12A, near the steep ravine. One drip torch went west. The main problem here was because of the SW wind, they had to ensure that Toby’s Prairie did not ignite. Spotters were placed inside Toby’s Prairie, but fortunately spotting was not a problem. Once the drip torch reached the west end of Unit 12A there was less problem. This drip torch continued lighting the savanna south of the service road (shown in red on the map), backburning Units 12B and 11B. It was 1:30 PM when the drip torch reached the top of the hill where the unit 19 burn had begun. This drip torch continued west along the service road to the side road, lighting Unit 11A.

The other drip torch of the Basin Savanna burn went south down the edge of the steep ravine. Although there was little fuel in the ravine, this line made sure fire did not burn down into the ravine. Once this line reached the bottom of the ravine (adjacent to the Pocket Prairie) the drip torch moved along the south edge of Unit 12A, 11D, and 18. The southerly wind carried the fire uphill, so there was little problem with the Pocket Prairie burning. By the time the bottom crew reached Pleasant Valley Road the Ridge-top crew was finished and one of its drip torches and two waters burned the line separating Units 7 and 18.

Two drip torches were used for interior lighting in Unit 11, starting below the crest of the hill. There was a good leaf pack at the west end of Unit 12 and the east end of Unit 11, and these areas burned very well. The lower part of these units had less leaf pack (fewer trees) and did not burn as well so extensive interior lighting was done.

We finished interior lighting at 3 PM, at which time the wind had mostly died down and a bit of rain started to fall. Great timing!

There was very little mop-up. We had cleared around snags and some special trees the day before. One large snag burned and fell, but well inside the burn unit. Also, there were a few piles of cut logs or brush piles scattered here and there. Most of these burned, a few for quite a while, but since none of these were near the edges, we let them burn out (our standard practice).

In all, a great burn!


There are several advantages to this two-burn approach. All crew members were used extensively. Once there was a black line around the perimeter, novices could be used for interior lighting. And with changeable weather, more fire on-the-ground can be obtained before conditions worsen.






Typical forest floor in the early years of a savanna restoration
A head fire is often the only way to get the fire to carry




One of the problems in savanna burns is getting the fire to
carry through downed timber and coarse woody debris


Sunday, December 3, 2017

Oak Savanna Restoration: Opening Up the Woods

Oak savanna restoration involves a lot of delicate removal of invasive trees and shrubs. (I refrain from using the word “logging”, which has connotations of habitat destruction.)

The long-term goal of the restoration process is an unimpaired functioning ecosystem in which viable populations of native species are maintained. This includes not only oaks, but associated tree species, as well as an understory of native herbaceous savanna plants with occasional native shrubs. Also, a range of ages in the oaks is desirable, thus ensuring continued integrity of the savanna ecosystem.

Much of the initial tree removal work can be carried out in parallel with brush removal. The cut brush provides an excellent base for the burn pile that is an essential part of the tree removal process. All cut stumps (both trees and brush) must be treated with herbicide, generally Garlon 3A 15% in water) or Garlon 4 20% in oil, to prevent resprouts. Glyphosate can also be used, at 50% dilution in water. Always add a dye to the herbicide mixture so that crew members will know that cut stumps have been treated. It is essential to treat the stumps soon after cutting to be certain that they are not missed.

Tree removal is a slow and expensive operation. A lot depends on the initial quality of the site, the terrain, and its access. The time required for the complete cutting and cleaning a savanna site in southern Wisconsin on fairly difficult terrain varied between 100 and 200 person-hours per acre.

Here are some thoughts about the removal process:

·         The ideal time to clear an area is winter, when the ground is frozen hard and there is (at least a light) snow on the ground. Under these conditions, the soil is not disturbed and (depending on slope) a truck (1/2 to 3/4 ton) can be driven off-road to pick up cut wood. Also, burn piles can be used without danger of fire getting out of control. Depending on the location, there may be two or three months of suitable conditions. Clean-up and some other less impactful parts of the job could be done at other times of the year.

·         Select a site (5 acres or less) to be restored. This is a site with large open-grown oaks. It should also be a site with good access (so that firewood or saw logs can be brought out).

·         Ideally, the land manager (who does not do any cutting) should be on site, at least at the beginning of the work, and preferably daily when major tree cutting is in progress.

·         Survey the site and mark trees, either those to be saved or those to be cut. Marking trees to be saved is preferable. Be sure everyone on the crew understands the markings!

·         One or more burn piles are needed. It is most efficient to burn at the time of cutting, with the burn pile centered in the area being cut. If it is not possible to burn at the time of cutting, keep the number of burn piles at a minimum by building large piles. (A 10-12 foot high burn pile is fairly common in savanna restoration.)

·         Select sites for burn piles. These should be in open areas, so that flames do not affect nearby branches of important trees. Make sure there is enough open space for the anticipated size of the pile(s). If there is no snow on the ground, use a leaf blower to clear a wide fuel-free zone around the pile.

·         Start a burn pile. To do this, collect downed timber and standing dead trees or shrubs, getting enough fuel to start the fire. Start the fire with drip torch fuel (diesel or kerosene could also be used). The fire will be maintained by further cutting. Keep the burn pile compact by trimming side branches before throwing them on the pile. The branches and logs should be laid on the burn pile in parallel fashion to ensure a good burn. In a well-built burn pile, the fire will be easy to start and to maintain. In a large site, more than one burn pile should be made. Burn piles should be preferably 100 feet apart.

·         While the burn pile is being established, other crew members can start on the trees. Here is the procedure used by an experienced crew leader.


“We had only one person cutting down trees.  It is important to cut down the trees one at a time. Cut down one tree and put it on the burn pile before cutting down the next. This is actually quicker than cutting down several trees at once. This is because cleaning up several trees on the ground is much slower.

The downed tree is cut into pieces (chunks) that are large but small enough to be dragged by a single person to the burn pile.  Once at the burn pile another person with a chain saw cuts the tree chunks into pieces of appropriate size to be placed on the burn pile.  A third person puts the pieces on the pile.

     This description calls for 4 people. If the trees are small, two people can be cutting trees, as long as the two crews are far enough apart so as to not interfere with one another. Two people to drag each tree chunk and one or two people to cut the chunks into pieces for the pile. It is important that the pieces are placed in parallel on the pile so that they burn quickly.” Willis Brown, CEO of Michler & Brown LLC.

·         Make sure that when the tree falls it does not damage any protected oaks. Cut the small branches into size to throw on the burn pile. If the tree is to become firewood, cut the bole into segments short enough to lift into a truck or leave for cutting by a cooperator who has agreed to remove the fire wood. For saw logs, cut in 8 foot or larger segments. Mark these saw logs and leave in place for later skidding to a nearby road.

·         Crew size: Four is the minimum. Six is preferable.

·         Important! Do not try to drag large cut trees or logs long distances! Bring the crew to the trees. If the work is done when the ground is frozen, the soil will not be disturbed.

Typical view around an actively burning brush pile
All of the small logs and twigs will be burned up by the end of the day


Getting rid of the wood
The goal of savanna restoration is to create an open tree canopy (10-50% cover). The goal of tree removal is not to generate trees for market, but to restore the land to its former state.

Although it may be possible to have some economic return from the cut trees, this should only be done if it can be ensured that damage to the habitat will not occur. Loggers operate with a different rationale than restoration ecologists. Slash that a logger might leave is often not compatible with the restoration process and logging trucks may damage fragile topsoil layers.

Removal of woody plants is best done in the winter, preferably when there is snow on the ground. Selection of personnel is important. The workers should be able to identify the various tree species in winter and should be careful when felling trees to be certain that they will not damage nearby trees that are to remain.

Although wheeled vehicles are essential for tree bole removal, they should only be permitted when the ground is frozen solid (preferably with some snow cover). If firewood removal is not to occur, the logs must be cut in lengths short enough for a single person to place them on the fire.


Unless the crew is very experienced, the land manager should be on site when tree cutting is being carried out to monitor the process. Once a tree is cut it is gone forever!

Nicely cleared slope with scattered brush piles. They will be burned on snow.
Although this is a mainly a prairie remnant, the same principles apply.



Large brush pile being tended. Note that for stability it has been built adjacent
to a tree that is also to be cut down.



Felling a large tree. This whole unit is being cleared, and when the work is
done, only scattered open-grown oaks will be left.


Low impact skidding of large walnut saw logs. This tractor can haul three logs at a time.
Much faster than a horse or oxen!





Tuesday, November 21, 2017

Buckthorn eradication: a 20-year story

This post is based on 20-years work restoring oak savannas and tallgrass prairie remnants at PVC. During that period, we were able to obtain a lot of experience on the eradication of invasive woody plants, of which buckthorn was the most difficult. See the table at the end for an index to my blog posts that dealt with buckthorn.

Common buckthorn (Rhamnus cathartica) is the #1 invasive shrub on the IPAW list. When we started serious restoration work at Pleasant Valley Conservancy in 1997, it was present in very dense patches throughout most of the savanna areas. Later I discovered that buckthorn is a calciphile, and was growing mainly in high-calcium high-pH soils derived from or influenced by dolomitic rocks. In many of the calcium-rich oak savanna areas, the  understory was almost a buckthorn monoculture.

During our major restoration work, buckthorn was removed at the same time as all the other invasive woody plants. Buckthorn removal mainly involved cutting the buckthorn thickets with a brush cutter or chain saw, and treating all the cut stems with herbicide (generally Garlon 3A at 15-20%). Buckthorn was not the only “bad” plant in our degraded oak savannas, but in the calcium-rich areas it was the principal one. The mature open-grown savanna oaks were also being crowded by invasive trees such as walnut, elm, cherry. There were even some “bad” oaks, such as the more rapid-growing oaks of the black oak group, and these were removed also.

According to my notes, all of the large patches of buckthorn were removed during the major clearing that took place in 1998 through 2004.

Once the savanna areas were cleared, fire was introduced as a principal tool. Although the use of fire was important, it only top-kills woody plants. The underground rootstock remains alive, and quickly resprouts. In fact, there is a strong “legacy” of woody plants in any formerly wooded area. If a cleared area is abandoned, it will quickly become overgrown again.

Although this “legacy” is important for most woody species, it is especially severe with buckthorn. It took me some years to discover that with buckthorn there are three legacies:

  • 1.      Buckthorn produces a toxic chemical called emodin which inhibits the growth of other plants and prevents “good” plants from becoming established. It creates what we have called a “buckthorn desert”. It generally takes about three years for the toxic chemical to dissipate.
  • 2.      An extensive seed bank from the large number of berries produced. (Although only the female buckthorn plants produce berries, the ratio of female to male plants can be as high as 6:1.) The seed bank only lasts a few years.
  • 3.      An extensive rootstock (sometimes called a caudex) from which new shoots can arise. It took me some years to realize that the rootstock of the buckthorn can remain dormant from many years.

In the earlier years of our restoration work I did not realize the seriousness of the buckthorn problem. (I hardly mentioned it in the new management plan I wrote in June 2005.) It was not until 2006-2007 when we did our major “all-out” push to clear brambles and other woody shrubs that had arisen since the initial restoration work, that I became aware that we also had a buckthorn problem.

By this time I had started my first Blog (Tom’s Journal), and buckthorn was a major topic. The table at the end of this document gives links to the various Blog posts. 

There is a lot of internet material on buckthorn, mostly focusing on getting rid of it. Most of this work is based on short-term studies, or on anecdotal evidence. Buckthorn has a complicated ecology, which enters into a discussion of control methods.

Through the 20-year period that I have been following buckthorn I determined that buckthorn rootstock can be long-lived, and remain dormant for many years. Thus, an area that once had had a large buckthorn infestation can have numerous invisible and dormant rootstocks.

In order to get some quantitative data, in 2010 I chose a site of about 100 X 100 feet where buckthorn had been removed around 2000 but kept coming back. I canvassed the site carefully and sprayed each buckthorn shoot with Garlon 3A (foliar at 4% aqueous). Most of the plants had only single shoots, but some had more than one. I sprayed all the shoots I could find at each survey.

In the three early years, when there were lots of shoots, I returned over and over again at 2-3 week intervals, and sprayed all the “new” buckthorn shoots that had arisen. At the end of each season, I made sure that there were no live buckthorn shoots left.

The table shows the results. My conclusion was that in this small, buckthorn-rich area, there were many dormant rootstocks, but not all of them left dormancy the same year.

Year
Buckthorn shoots sprayed
Year total
2010
not recorded
>300
2011
12 + 30 + 71 + 35 + 27
175
2012
17 +18 + 55 + 5 + 44 + 20 + 82
241
2013
6 + 7 + 8
21
2014
0
0
 
Buckthorn resprouts, waiting to be sprayed! These could be either from
fire-killed plants or from dormant rootstock




I should emphasize that the original removal of buckthorn was in either 1999 or 2000. Unanswered is what stimulates these dormant rootstocks to start growing after many years?

My conclusion is that buckthorn might be eradicated from a site, but it might take many years.

Note that the area I studied had been heavily “infected” by buckthorn, and that there was plenty of time for the plants to make extensive underground rootstocks. Sites recently colonized by buckthorn might be eradicated much easier. However, I suspect that most sites in southern Wisconsin that are full of old-growth buckthorn would be similar to my site, especially if they had dolomitic soils.

A single buckthorn plant still green after the native vegetation has senesced.
Can be sprayed without causing any peripheral damage,
with either glyphosate or triclopyr

Today Pleasant Valley Conservancy is virtually free of buckthorn! This is because we have worked very hard to kill the plant when we see it. Every year, we canvass all sites where buckthorn has been a problem.

We are especially helped by the fact that all of the savanna areas, where buckthorn had been the biggest problem, are burned each year. Fire keeps buckthorn at a juvenile status, and prevents any extensive spread by underground roots or rhizomes. Fire makes it possible for us to spray buckthorn shoots when they are still small (foliar spraying is done in the spring).


Any buckthorn plants not killed in the spring can be killed when they are found in summer, autumn, or winter by basal bark treatment. The dark shiny leaves of buckthorn plants make them easy to spot, especially in the fall of the year when native vegetation has senesced.

Single buckthorn stem that had been basal-barked 
with 20% Garlon 4 in oil. The leaves are curled
and dying.


Blog posts through the years on buckthorn. The ones before 2008 are no longer available
Date published
Title
Link
26-Sept 2006
Buckthorn
Tom’s Journal (Blog) no longer available
20-March 2007
Hundreds of small buckthorns
Tom’s Journal (Blog) no longer available
3-Apt 2007
What does it cost to cut brush?
Tom’s Journal (Blog) no longer available
7-May 2007
Successful results of brush clearing
Tom’s Journal (Blog) no longer available
1-July 2007
Buckthorn seedlings
Tom’s Journal (Blog) no longer available
16-Oct 2007
The underground life of buckthonr
Tom’s Journal (Blog) no longer available
24-Oct 2007
Attacking small buckthorn
Tom’s Journal (Blog) no longer available
9 Nov- 2007
A few joys of herbicide spraying
Tom’s Journal (Blog) no longer available
12-Nov-2007
More spraying [buckthorn]
Tom’s Journal (Blog) no longer available
13-Nov 2007
Like a needle in a haystack
Tom’s Journal (Blog) no longer available
15-Jun-2008
Buckthorn and the oak savanna
http://pvcblog.blogspot.com/2008/06/buckthorn-and-oak-savanna.html
6-Oct-2008
Good time for foliar spraying of small buckthorn
http://pvcblog.blogspot.com/2008/10/good-time-for-foliar-spraying-of-small.html
20-Oct-2008
Buckthorn spraying continues
http://pvcblog.blogspot.com/2008/10/buckthorn-spraying-continues.html
23-Oct-2008
All out attack on buckthorn
http://pvcblog.blogspot.com/2008/10/all-out-attack-on-buckthorn.html
3-Nov-2008
Buckthorn out of control
http://pvcblog.blogspot.com/2008/11/buckthorn-out-of-control.html
25-May-2009
From brambles to buckthorn
http://pvcblog.blogspot.com/2009/05/from-brambles-to-buckthorn.html
3-Sep-2009
Dealing with small buckthorn: a two-fisted approach
http://pvcblog.blogspot.com/2009/09/dealing-with-small-buckthorn-two-fisted.html
2-Oct-2009
Controlling invasives around the big bur oak
http://pvcblog.blogspot.com/2009/10/controlling-invasives-around-big-bur.html
26-Nov-2009
Soil pH/Calcium and buckthorn distribution
http://pvcblog.blogspot.com/2009/11/soil-phcalcium-and-buckthorn.html
1-Dec-2009
Buckthorn control by basal bark treatment
http://pvcblog.blogspot.com/2009/12/buckthorn-control-by-basal-bark.html
10-Apr-2010
Response of buckthorn to basal bark herbicide treatment
http://pvcblog.blogspot.com/2010/04/response-of-buckthorn-to-basal-bark.html
6-May-2010
Eliminating small buckthorn
http://pvcblog.blogspot.com/2010/05/eliminating-small-buckthorn.html
17-May-2010
Spraying resprouts; big weekend
http://pvcblog.blogspot.com/2010/05/spraying-resprouts-big-weekend.html
21-Aug-2010
Buckthorn underground
http://pvcblog.blogspot.com/2010/08/buckthorn-underground.html
29-Oct-2010
Mazo Bluff buckthorn
http://pvcblog.blogspot.com/2010/11/mazo-bluff-buckthorn.html
12-Nov-2010
Final days for buckthorn control?
http://pvcblog.blogspot.com/2010/11/final-days-for-buckthorn-control.html
27-May-2011
Story of a small buckthorn area
http://pvcblog.blogspot.com/2011/05/story-of-small-buckthorn-area.html
10-Aug-2011
Anatomy and significance of a buckthorn root collar
http://pvcblog.blogspot.com/2011/08/anatomy-and-significance-of-buckthorn.html
21-Oct-2011
Buckthorn: late fall reprise
http://pvcblog.blogspot.com/2011/10/buckthorn-late-fall-reprise.html
11-Apr-2012
Power Point on buckthorn eradication
http://pvcblog.blogspot.com/2012/04/power-point-on-buckthorn-eradication.html
11-Oct-2012
Brown thrasher flushed from nest while spraying buckthorn
http://pvcblog.blogspot.com/2009/06/brown-thrasher-flushed-from-nest-while.html
15-May-2013
Selective spraying of leafed-out invasive shrubs
http://pvcblog.blogspot.com/2013/05/selective-spraying-of-leafed-out.html
21-May-2013
Buckthorn: what will mowing do?
http://pvcblog.blogspot.com/2013/05/buckthorn-what-will-mowing-do_21.html
16-Nov-2013
Good time for major buckthorn control work
http://pvcblog.blogspot.com/2013/11/good-time-for-major-buckthorn-control.html
18-Sep-2014
Buckthorn eradication: results of a five-year study
http://pvcblog.blogspot.com/2014/09/buckthorn-eradication-results-of-five.html