Sunday, July 28, 2013

The anatomy of natural limb pruning

   I was walking through the grove checking for scab lesions when I came across a butchered off limb on one of our Giles trees (photo at right). This now-dead limb was the product of the pruning job we were forced to perform in response to massive limb breakage caused by an ice storm in Dec. 2007.  You can see that the limb tried to resprout a few new shoots, but the entire branch stub soon became shaded and died from lack of sunshine.
    While I walked back to the barn to grab a chainsaw, I though-- "why don't I take a look inside this limb to see how pecan trees naturally shed large limbs broken by storms".

    First, lets take a closer look at this branch stub at the point it connects with the trunk (photo at left).  At a point around 2.5 inches out from the trunk, I spotted a line in the bark marked by a slight change in bark color and texture. The yellow arrows point to the top and bottom of this line which actually marks the boundary between living and dead bark tissue.
   Using my chain saw, I pruned off the limb just outside the branch collar revealing nice healthy wood tissue inside (photo at right). Now that this limb has been pruned off, the tree can start callusing over the wound.
    I picked up the pruned-off branch stub to take a closer look. Except for a small portion of the branch that had been closest to the trunk, most of the branch stub was in various stages of wood decay. A nice, white shelf fungus was growing out from cracks in the bark (photo above). The wood was also riddled with insect holes that seemed to be serving as a home for a colony of ants. With all these agents of wood decay and destruction so close to the main trunk, how does the tree prevent additional wood damage? 

   To find out,  I used the chainsaw to dice open the branch stub so I could look at the wood inside. The photo at right shows about 8 inches of the original 30 inch branch stub. However, this is where the action is--where live wood tissue meets decaying wood. The white wood on the left is living tissue that was removed from the tree when I made the pruning cut (described above).
    I mentioned earlier that I could see a difference in bark color and texture between areas of living and dead tissues. Well, that line also shows up in the cross section of the bark. The orange arrow at the top of the photo points to a dark black line in the bark that marks the boundary between living and dead tissues.  And remember those pretty white fungi?  Fungal growth inside the wood leaves a series of fine black lines in unique patterns that wood-workers would recognize as a condition known as "spalting" (see yellow arrow). All the holes in the wood are the result of insect activity; wood borers at first, then a colony of ants. 
    However, the most important thing to see in the wood of this mostly rotten branch stub is the very prominent boundary layer (blue arrow) between healthy white wood and decaying branch. By filling all the pores in the wood with water repelling organic compounds, the tree prevents wood rotting organisms from advancing into the trunk. This plugging of the pores has stained the wood a chestnut brown in a layer about 1/4 inch wide. The outside limit of the boundary zone is marked by a fine black line indicating the limit of fungal activity.  
    If I had let nature take its course, the rotten portion of the branch stub would eventually fall off the tree (years from now). Only then, could the tree start to callus over the wound. However, with proper pruning the callusing process can begin immediately.