It’s been a dry spring in South Carolina, and as of mid-May the entire state is experiencing some level of drought, with over 70% of the state under extreme drought conditions (Figure 1). This, of course, isn’t great for the flora of our state – including our trees!
We are fortunate to have trees growing throughout our state, from the middle of urban areas to vast rural acreages. Trees in managed landscapes may benefit from supplemental irrigation during droughty times, while those in more rural areas are at the whim of Mother Nature. Regardless, trees typically respond the same when under drought stress and undergo the same general process.
When drought occurs, the lack of available water is sensed by the tree. Then two things happen: first, the tree essentially shuts down food production by closing stomata (small openings on the leaves which are essential for photosynthesis) to prevent water loss. This, unfortunately, also ceases the ability of that leaf to make food. Second, the tree pulls food reserves from the storage roots and stump and uses them to produce additional fine roots. Growing additional fine roots is a strategy to attempt to find water. If this doesn’t work, the tree more or less shifts into survival mode and begins to allow tissues to die in an attempt to save the tree as a whole. Typically, this starts with some foliage (e.g., tulip poplars are notorious for dropping leaves at the first hint of summer dryness), but smaller twigs and roots might also be “sacrificed.” If water is still not available or found, then larger parts of the tree begin to die and we see the typical tree decline that precedes tree death.
Taking a step back, understanding how trees work can help us understand why drought often leads to insect and fungal issues. Trees use the energy they make (through photosynthesis) in certain ways. There are 7 main things trees use energy for, and they are (in order of priority):
- Maintain respiration – this just means to “breathe” and stay alive.
- Produce fine roots and leaves – this means to make the tissues that gather the water (i.e., fine roots) and make the food (i.e., photosynthesize).
- Produce flowers and seeds (reproduce).
- Extend branches (get bigger and increase the ability to make leaves, which will increase the tree’s ability to make food).
- Store energy rich chemicals (adding carbohydrates to the large storage roots and/or stump to help the tree repair after an injury or reflush after losing foliage).
- Add wood to stems, roots and branches (grow larger).
- Create anti-pest chemicals for defense (i.e., protect itself from herbivores and fungi).
Did you notice where trees prioritize protecting themselves from pests? LAST! Only after all those other physiological things, like staying alive, growing and reproducing, are satisfied do trees allocate energy to protecting themselves from pests. This is the main reason why anytime a tree is not getting enough resources – whether that’s water, sunlight or nutrition – the tree becomes more susceptible to pests. Trees need resources to protect themselves, just like people need to eat a balanced, healthy diet to stay healthy.
Considering all of this, it is a reasonable expectation to see an increase in impacts from pests during droughty times. Hardwoods may simply lose leaves, while pines typically turn yellow/brown after a certain level of drought. There is not much we can do in natural areas, though managing (especially reducing) the basal area or competition can help – when there is less stem volume on a tract, there is more water for each stem. Trees in managed landscapes will benefit more from a thorough soaking once or twice a week rather than just a little water daily.
Proper management of your trees throughout their lifetime goes a long way towards creating a resilient forest. After all, we never know where, when or how long the next dry spell is going to be.
Additional resources include:
https://hgic.clemson.edu/factsheet/plants-that-tolerate-drought/
Author
Dave Coyle, Cooperative Extension, Forestry and Wildlife Specialist
Clemson University Cooperative Extension Service offers its programs to people of all ages, regardless of race, color, religion, national origin, disability, political beliefs, sex, marital or family status and is an equal opportunity employer.
