Drought: Killing trees slowly

Nov 5, 2013

This tree on the MU campus survived the drought of 2012, but died this year, possibly due to the long-term effects drought can have on a tree.
Credit Gary Grigsby / KBIA


In much of mid-Missouri during June, July and August, rainfall was well below normal. 

It wasn't as dry as last year though when Missouri's worst drought in more than 30 years killed or damaged everything that needs water to grow, including trees.  Most trees survived last year's drought and with more rain this year, especially in the spring, that might lead you to think trees are out of danger.  But that's not necessarily the case.  That's because drought is complex and you need to understand a bit of the science involved to know how it's effecting trees.

You can't help but hear the drone of a motor as you approach a small building on some heavily wooded land at MU's Baskett Wildlife Research site five miles east of Ashland.  The motor is powering a pump that gets air from various sample locations on site to measuring devices.  "Those are the water vapor and CO2 contents being measured in real time," said Steve Pallardy.  CO2 or carbon.  That's why Pallardy has spent a good deal of time here during the past 10 years.  The recently retired MU Forestry professor, who one of his colleagues calls the drought guy, said he is working on a project for the federal government designed to measure the rate at which trees absorb water from the atmosphere.  "As a system gets drier under drought, its ability to take CO2 out of the atmosphere is greatly reduced.  And the historic drought we had last year reduced it to about 50% compared to its best year of taking it out in the early 2000s."  

And you might wonder why that matters.  Well, Pallardy has been doing research on trees and drought at MU for more than 30 years.  He said it is not the lack of water that usually kills a fairly large tree.  "What happens is the amount of carbon that the tree stores will be reduced.  And as that accumulates over the years you can get a deficiency of carbon.  And carbon is the currency of life for trees.  It provides the ability to grow roots for the uptake of water and minerals.  It can also help in fending off pathogens.  So a lot of mortality you see in a forest like this won't occur for several years down the road."

In the woods where Pallardy is doing his research the white oak is the dominant tree.  It is also a common tree in the Ozarks of southern Missouri where Dr. Rose Marie Muzika does much of her research.  Muzika is a conservation biologist  who is on the Forestry faculty at MU.  Her area of interest is natural processes and disturbance in forests.  She knows a lot about what causes trees to die.  "Trees usually die slowly.  So a given drought might stress a tree but that might be followed by an insect attacking that tree.  Or a disease because it is stressed."

She says one tree that is definitely stressed right now is the white oak.  "Within the last year quite a bit of mortality.  Fungal pathogens taking hold in white oaks.  And it is tempting to consider that last years  drought may have predisposed these trees to be vulnerable to these other agents.  But we're really not sure."  In any case, Muzika said it is an unusual event to see such mortality in the white oak since it is generally more drought resistant than other types of oak trees found in the state. 

And while Muzika and Pallardy do much of their research in deep woods, they both said the trees they see affected by drought are often the same kind of trees you are likely to have in your back yard.