De-forking Quercus dentata

After taking charge of my father's oak collection in 2009, I noticed that several of the trees had forks in the trunks and hence co-dominant stems. Having no knowledge of these matters, I consulted Guy Sternberg, who I had visited in Starhill Arboretum in 2010, seeking guidance. I sent him a photograph of some of the oaks in question, and he replied with suggestions on how to tackle one of the worst cases: a Quercus dentata planted out in 1998. Guy described this tree as having a "Very Bad Fork" and prescribed a course of subordination or suppression of the weak secondary leader. Over several years, one of the codominant stems should be reduced via severe pruning, so that the other stem would outgrow the weaker one, and when eventually it was completely removed at the fork, the wound caused would be smaller in relation to the remaining trunk. (The technique is described in Guy's article "Pruning Oaks: Training the Young to Achieve Grandeur" in International Oaks No. 24; Ryan Russell also discusses the issue in his blog post "Pruning Young Oaks".) Guy sent back the photo of the Q. dentata with clear indications as to where the cut should be made. 

June 2011 photograph, with Guy's instructions for removing the fork. Closer look at the VBF (very bad fork), October 2012

This was one of the most attractive trees in the collection, so going at it with a chain saw was not to be embarked on lightly. However, eventually I decided that the danger of a split outweighed the damage I might do to the tree's appearance, and I began the process of pruning back one of the leaders. The photographs below show the progress over three years. I started higher up than Guy suggested, as I was in no hurry.

October 2012 (fall), pre-pruning The first cut

Lopping off a third of the crown did indeed have a shocking effect, and I had to wonder whether the decision had been the right one.

The aftermath

But after a year, it was apparent that the pruned stem was now smaller than the untouched one, and that the process seemed to be having the desired result.

May 2013 May 2013, another look at VBF

Two years ago the weaker stem was again cut down by half. Last year I decided that cutting at the fork would still leave a large scar, so I opted instead to reduce the section of the canopy borne by the weaker stem (see below). This winter I intend to cut at the fork. As can be seen in the picture bottom right, the canopy is beginning to adjust and to fill out into the space left by the removed leader.

The second cut (May 2013) March 2014: the remaining branch has been further reduced
Q. robur 'Pendula' having lost a large limb due to a split fork. Note the included bark at the top of the scar.

Coincidentally, last year we had a good example of what can happen if a fork like that, with included bark, is not corrected. Another 1998 planting, a Q. robur 'Pendula', lost a very large limb in a storm (not a particularly strong wind). As can be seen in the photo on the right, what broke was a fork with plenty of included bark. The effect on the canopy, which was reduced by about half, can be perceived in the photos below.

Before and after of Q. robur 'Pendula' (2010 on left, 2014 on right)

So I guess the moral of the story is: get those forks before they get you!