Drought Stress or Browning of Trees

Author: Dr. Jeff Stringer, UK Department of Forestry
Kentucky had the hottest, driest June on record followed by the second wettest July on record.  Many homeowners have observed trees "going dormant" or turning brown and dropping their leaves.  What does this mean and should I be concerned about whether the tree is alive or dead or injured?  Dr. Stringer has written the following regarding drought stress on trees in Kentucky.

Trees don’t actually go into dormancy (in a true sense), they just rest for a while during the winter. Trees track night length to tell them when they should start their leaf abscission process (usual in later August) that results in an October leaf drop. If all of a sudden in December you increased day length you could fool them into leaf out, so therefore it is not a true dormancy.  Regardless, the leaf browning that is going on now is not due to dormancy it is due to desiccation or drying out. If you have a wet fall there will be some species that will try to refoliate (which is unfortunately not a good thing for them). Many of the understory trees can be really effected by droughts and show more symptoms than overstory trees. Some because they have naturally shallow root systems such as dogwoods. Some overstory species (not all) can tap deep water reserves and thus show less evidence of the drought than understory trees. Losing leaves is a mechanism that helps avoid total tree desiccation. You see this in species like river birch that losses leaves when it gets a little droughty (these are drought avoiders). Some species keep leaves on and tolerate desiccation of their tissues – these are drought tolerators (many oaks fall into this category). However, the severe nature of the drought makes it hard to tell which trees will actually succumb and which will pull out of it. My guess is that there will be a large number of the understory trees that will refoliate next year. Not to say they will be highly vigorous, but they will not be total dead either. Some will lose some branches etc. due to the internal desiccation. Regardless the woods and the trees in it will not be in good shape next year. If we have another bad year (drought, late spring frost or freeze, insect outbreak) you will definitely see mortality effects.