Big native pecan tree weakened by wood rot

    Last harvest season, I came across a tree (photo at right) that I was scared to shake. From a distance it just looks like is a big old native tree that bears the scars of the 2007 ice storm (numerous sprouts from chopped-off limbs). It was only when I went to clamp onto the tree that I noticed a large crack in the trunk. I carefully shook the tree in December  but made a mental note to remove the tree come late winter.

    The trunk of this native tree had a large crack running over eight feet in length (photo at left). I looked inside the crack and discovered a rotten cavity inside the core of the tree. This tree was a potential disaster waiting to happen so I decided to cut it down.

    Cutting down a hollow tree can be tricky. The structure of the tree is so weak that you can't predict when or how the tree will fall once you start cutting. I had cut only one quarter around the trunk, when I heard the sound of wood splintering and cracking. I pulled out my chainsaw and moved away from the tree to watch it come crashing down. It was not the prettiest way to fell a tree but at least this potentially hazardous tree was on the ground (photo at right).

    The way the tree split apart upon falling allowed me to photograph the internal structure of the wood. I've labeled the key portions of the tree trunk in the photo above. Starting at the far left is a very normal looking layer of bark. Inside the bark are multiple layers of white sapwood. The heartwood of this pecan tree is reddish brown. As I looked to the center of the tree I came across a portion of the heartwood that was riddled with the white hyphae of a wood-rotting fungus. Fungal decay of the heartwood caused it to become soft and lighter in color. The hollow core of the tree represents the portion of the wood that has been completely broken down by fungal activity.    

    Inside the hollow core of the tree, I found the fruiting body of the wood-rotting fungus (photo at right). This fungus is a member of a mushroom family commonly referred to as shelf fungi. The fruiting body of this mushroom is hard and feels woody. Rather than gills, the underside of the shelf fungus has numerous pores that release spores. The appearance of shelf fungi on a tree is sure sign of significant internal wood rot.

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. 

Snap! A young pecan tree breaks in the wind.

     Last week we had a few summer thunderstorms that packed some pretty good winds. Following the storm, I found a young trees snapped in half (photo above). How frustrating!

     My first thought was to see if I could discover why this tree, out of all the trees on the farm, broke in two.  I took a close look at the fractured stump (photo at right). I noted that much of the break was splintered except for two areas of smooth breaks on the southwest side of the tree (yellow arrows). From the appearance of the wood, these smooth breaks were probably present inside the tree even before the tree broke over in the wind. This observation was cause for further investigation.

     I used a chain saw to cut through the trunk of the tree just below the break (photo at left). I found discolored wood on the south by southwest side of the tree. The tree became prone to wind breakage because nearly 1/3 of the tree's supporting wood was dead and starting to rot. What caused this wood decay? The sun.
     This tree was a victim of  overzealous tree pruning. To get lower limbs out of the way of mowing or other tractor operations, too many low limbs were removed from this tree all at one time. This exposed a previously shaded trunk to intense heating by the midday and afternoon summer sun. This heat build up on the south and southwestern portions of the trunk killed the cambium halting all wood and bark growth. The resulting dead tissues attracted wood rotting fungi and the tree was unable to heal over the sun-scalded trunk.
   Now that we've lost the entire top of this young tree and the remaining stump is severely damaged by wood rot, I've decided to cut this tree off at ground level and allow a stump sprout to form a new trunk. In a couple of years, we'll need to re-graft the tree. With all the root energy this tree will have to push new growth, we should be back to nut production in no time.