The Case for Structural Pruning

Here we can see the results of a tree allowed to mature with no previous thoughts of crown structure.  You can see how one side of the tree was completely ripped of exposing internal decay.

An over-extended co-dominate stem combined with some internal decay resulted in this trees demise.  If care had been taken earlier in the life of this tree to prune for one dominate leader, and well placed scaffold limbs, this tree would still be intact.

Compaction for the Sake of Grass?

One of my favorite calls is from a contractor, in this case a very large landscape company, that has been busted by the city for failing to do any tree protection.

So our assignment is to decompact areas leveled for the sake of sod installation.  Our medium is compacted red clay.  If you have used an AirKife or Airspade in this scenario then you know attacking glazed clay is futile.  I go about it by finding an area where I can penetrate the soil as close to the severe compaction as possible.  Once deep in the soil,  I angle the air tool under the glazed area.of compaction.  By repeating these steps around the area, it eventually begins to break up in to large peds.

In the past I would try to break up peds into finer soil, but after having gone to Jim Urban's 'Up by Roots,' I now try to preserve peds.  These peds are an essential part of soil structure, especially in heavy clay soils.  Of course, I then mix in composted organic material to help maintain soil structure and improve nutrient holding and water holding capacity.

An argument may by made that August is not the best time to be using an AirKnife in such a large area, as it may dessicate fine roots.  In the face of progress, there are only so many options.

Roots and Backhoes

We were called out because the developer wanted to save this tree... after,of course, they already prepped the area for the new retaining wall.

The minimum safe distance for trenching on one side of this 40 inch plus Southern red oak according to industry standards should be around 12ft.  This excavation is about 7ft away from the trunk.

Tree health is no longer a concern, instead this tree has been made a potential hazard.  This picture is a good view of the amount of root mass under the soil.

Irrigation and Poor Drainage

Last week I wrote about the importance of water management, and the difficulty of getting it right in poorly drained soils.
This picture illustrates that point perfectly.  The arborvitae that looks the worst is sitting in the lowest spot of the planting area.  Drip irrigation is present, and run diligently a few times a week in a heavy clay soil.

Wax Scale

Here a picture of an Indian wax scale on a white mulberry.  Wax scales are interesting, as they are all female. That's right no males. It's kinda weird.

In mass, wax scales can stress out a tree or shrub, and produce copious amounts of honeydew, though I have seen neither.

The best management strategy is to simply pick them off in winter.

Vertical Root

We have been working with a local builder on some tree preservation.  This picture is of a vertical sinker root and parent root about 15 feet from the base a 40 inch+ willow oak.  The top of the horizontal root is about 2 feet below grade.
Maybe not the greatest picture, but a rare glimpse of tree structure within the soil.

Structural Pruning

I went to the NC Urban Forestry Council's annual conference yesterday, and had the opportunity to here Dr. Ed Gilman speak about one of my favorite topics... Structural pruning.  Now I must admit, while I enjoy listening to Dr. Gilman and I love the topic, I felt like sitting through another lecture on structural pruning was a bit below me. I was pleasantly proven wrong.  Here are some of the highlights:

- Pictures presented displayed a tree just before being thinned, just after being thinned, and the same tree 12 months later.  Twelve months after thinning the tree looked almost identical to just before it was thinned.  The conclusion is, thinning trees may not be the best practice when attempting to abate the risk of branch/whole tree failure.

- Don't be afraid to remove over 50% of a tree's canopy when structural pruning at planting to promote a "hyper" central lead.  Young trees recover fast from pruning.  Removing or subordinating all competing branches at planting will guide good tree structure for some time.  Competing branches can actually shade out the central leader, weakening it, and this may contribute to branch failure in the future.

- Don't be afraid to make big cuts to remove or subordinate competing branches on medium size trees.  Again, Dr. Gilman presented pictures of aggressive cuts, and how the tree recovered with much improved structure a few years down the road.

- Dr. Gilman introduced the idea of aspect ratio when it comes to branch size.  Lateral/secondary branches should be smaller in diameter (smaller in aspect ratio) then their parent stem, and the smaller the better.  Branches with larger aspect ratios are at greater chance of failure.  Ideal aspect ratio was not specified, but smaller the better was the conclusion.

- Finally, we where shown video of a tree before canopy reduction being blown by a dynamic wind load machine.  There was about 12 inches of play at about 1/2 way up the stem.  The same tree was then pruned to reduce about 30% of the branches from the crown.  The tree then had about 3-4 inches of play at about 1/2 way up the stem.  We may relate this data to individual large branches in trees with stem and root defects.  If we reduced all large branches by 30% on tall trees with defects that may predispose the tree to failure, then we may be able to retain more trees vs. removing them.  Awesome!

As with everything, we must take in to consideration the tree's species, condition, and the site when applying this data in the field.  Never the less, a great presentation, and I can't wait to get out there to get my structural prune on. 

Compacted Soils

There is no question that trees growing in compacted soils generally won't grow as well as trees in a lower bulk density soil.  Compacted soils adversely affect root tip elongation, affect gas exchange, and disrupt water availability.

In the latest edition of Arboriculture and Urban Forestry a study performed by Barbara A. Fair, James D. Metzger, and James Vent  concludes  water availability in compacted soils may be the limiting factor for tree growth.  They found carbon dioxide levels were 5-18 times higher in compacted soils compared to atmospheric concentrations, while O2 concentrations were similar to atmospheric levels despite density.

This tells us that water management is increasingly important in our urban landscapes.  Often tree owners tell me they irrigate, but the irrigation is usually too little or way too much based upon the site.  Getting the correct amount of water in compacted soils can be a tricky code to break.  Think of a line of plants along a slight grade change.  Many times you will see the trees on the high side parched for water, while the trees in the low area are sitting in saturated soils well above field capacity.

To Stub or not to Stub?

For decades we were taught to prune all limbs, big or small, at the branch collar.  However, over the past few years a debate has been growing about what to do when pruning large limbs.  Some would argue that when removing large branches or stems it is better practice to leave a stub.  The thought is by leaving a stub, decay organisms would take more time to enter the parent stem, thus offsetting internal decay and structural weakness for some time.

I have debated this in my head for years, but recently while reading Fungal Strategies of Wood Decay in Trees by Schwartze, Engels, and Mattheck, I came across these lines.  "After removal of a branch, this cut surface is like a chemical battlefield, as not only spores of wood decay fungi germinate there but also the spores of many other fungi, e.g. the mold fungi.  Because of the competitive pressure and interplay of different fungi, it is much more difficult for wood decay fungi to become established on such a substraight..."  It goes on to say that leaving a  large stub allows more potential for wood decay to become established, and once established, wood decay is difficult to slow down.

Now of course, tree species and specific wood decay organisms all play a part. From the conservation biology point of view, large declining stubs are also home for many species of arthropods and fungi that would otherwise not have a home, so there may be some intrinsic value.  With that in mind, site use and targets may come in to play.