Wednesday, July 25, 2012

Signs of drought stress on pecan trees

    The entire midwest is suffering from the driest summer since the mid 1950's. Pecan trees are pretty good at handling dry weather conditions but weeks of temperatures in the 100's (F) and very little rainfall have started to take their toll.
    The other day, I drove past some native pecan trees that were growing along a small creek south of Chetopa. Among this small stand of natives, one tree showed the definite signs of drought damage (photo at right). The tree of the left has started to yellow with many leaves turning brown. This tree is shutting down just like it would in the fall in preparation for winter. Going dormant early is the pecan tree's last defense against severe drought.
    Just like every other tree characteristic, reaction to drought varies widely among seedling trees in a native grove. The photo above demonstrates that fact. But soil depth is also a factor. Pecan trees growing along creek banks far removed from major river flood plains will be the first to suffer drought stress. Soils in these locations are relatively shallow (5-7 ft.) compared to the extremely deep soils found in major river bottom flood plains (20-25 ft.). Deep soils have a larger capacity to store water and trees growing in these soils seem to weather drought far better.

    Regardless of soil depth, young trees are not as well equipped as mature trees to weather a drought (see Tree age and drought stress). The photo at left shows a young tree suffering from lack of rainfall.  By just looking at the tree, you can tell it is having a hard time. Leaves are light green with many internal leaves yellow or browning. To protect itself from drought this tree is starting to shed leaves.
    By taking a close look at a single limb on the young tree, you can see see how a tree starts to shut down (photo at right). Trees shed the oldest leaves first (at the base of this year's new growth) while shoot terminals remain green. Looking at a single compound leaf, basal leaflets are shed before the terminal leaflet.
   Note that, in all the photos of this post,  the ground cover is already crispy brown. Like I said before, pecan trees are pretty good at handling drought. Unfortunately, this summer looks to be among the top 5 driest on record and our trees are already suffering for it.