Environmental Drought


There are four basic types of drought.  1. Agricultural   2. Climatical  3. Biological   4. Meteorological.  The biggest problem for trees during dry times is over watering.  Also the lack of Coarse Woody Debris (CWD)in forest, reduces the potential for water reservoirs depleting the required moisture regime. Hemlocks are prime example of this.

It sounds like a paradox - but it is the truth!  The major problem during a drought is OVER WATERING.  To avoid it, remember that one important aspect of tree biology is the difference in "plumbing" i.e., the way water is transported (loaded), stored and used.  For instance, Oak, Elm, Chestnut, Robinia (named after first arborist.) - Black Locust are ring porous trees.  In these, on cross section, you see the vessels quite large in the early spring, laid out in concentric circles, and copiously absorbing water in the early growth season after which they close down.  On the other hand, Maple, Sycamore, Honey Locust, Tulip Tree are diffuse porous trees with the vessels equally sized and distributed over the cross section. They take up water during the entire growth period (moderate amounts).  The Birch also belongs in this group since it requires water and keeps its stomatas open throughout. Pine, Spruce, Ginkgo, Fir are coniferous trees with tracheids that become thicker walled as the growth period continues. These type of trees are similar to ring porous trees when it comes to water requirements.  Thus, one will choose the appropriate location for planting either of these groups according to the water supply. But there are also trees, such as Ash and Mulberry, which are "semi-diffuse", not quite belonging to either category and developing tyloses, plugging of the vessels, irregularly.  Thus, while ring porous and coniferous trees may develop root rot because of their inability to handle water during drought in August and September if watered heavily. The list given above should serve as a guide line. Again, just water enough to moisten the soil to the depth of the roots (upper four inches of soil) and out until the drip line of the diffuse porous trees during droughts in August months. Ring porous and coniferous trees do not require very much, if any, water at that time. Trees with proper mulch without hostile grass under them, as well as native and non native trees planted properly, do better than others. Do not let the hose lie around with the water running.

Mechanical Drought

This is the result of mechanical soil compaction by heavy equipment, by repeated traffic (cars, people, heavy lawnmowers, lack of mulch).  In the spring this silent killer often goes unnoticed.  And too often we deny our part in creating a dysfunctional system and use a number of excuses i.e., "water stress, frost injury, blight etc.".

Another thing to consider: In some areas of Israel they stopped the use of insecticides.  The incidence of breast cancer went down.

If, during a drought, you don't water blueberries in August and September, you won't have the berries next year.


John A. Keslick Jr.

Tree Biologist

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