Drought-scorched trees not necessarily dead
Matt Bonham, Texas AgriLife Extension Service assistant, Overton, checks out a red oak tree whose leaves are browning and dropping off early due to the drought. Though the tree appears to be in trouble, it's not necessarily dead, according to Bonham. (Texas AgriLife Extension Service photo by Robert Burns)
COLLEGE STATION – “Our trees are dying!”
Reports like this are coming from throughout the state, but don’t write off that shade tree or loblolly pine just yet, said a Texas AgriLife Extension Service expert.
“It’s really a wait-and-see kind of a game,” said Dr. Eric Taylor, AgriLife Extension forestry specialist, Overton. “Just because a tree’s leaves have turned brown – or that the leaves have shed from the tree – doesn’t mean the tree is dead.”
Trees respond to moisture stress and heat with natural defense mechanisms, and these responses vary from species to species, Taylor said.
Southern red oaks, for example, will senesce – draw back the nutrients from their leaves – early in response to stress.
“It’s the pulling back of nutrients that causes the leaves to turn brown,” Taylor said. “That leaf has actually been abscised from the tree and is no longer functioning, but it doesn’t mean the tree is dead.”
Other species don’t have that defense mechanism. They will simply drop their leaves while they are still green, Taylor said.
“You see a lot of that with the trees with thicker, waxier leaves,” he said.
Some trees can’t just drop leaves. All they can do is wilt, according to Taylor.
“Now those leaves will be wilted past the point of recovery, but none of that means the tree is dead,” he said. “We’ll have to see what happens next spring. If the tree was happy and healthy and somewhat vigorous, then it will likely have enough carbohydrates and energy reserves to come back next spring.”
There are some trees that are more drought-tolerant, particularly species that have smaller leaves with a waxy covering, such as live oak.
Pines are different from hardwood trees in how they respond to drought and heat, he said. They don’t have the built-in senescent defense mechanisms of pulling back lots of nutrients from leaves.
“They have two years of needle growth, and they’ll tend to shed the oldest needles and hold onto the current needles, which is why many pines have both brown and green needles,” he said. “If those current needles begin to shed, then that’s a good indication perhaps that the tree is beyond the point of recovery.”
Taylor said newer needles are found on the tips of the branches, while the older needles will be farther inward.
Limb drop is another matter, he said. It has to do with the “water status” within the tree. When trees transpire out more water than they receive, the lignin or glue that binds cells together breaks down.
“The sudden collapse of spaces between cells will cause limbs to drop, even though they may still appear to be alive and functioning.”
Taylor noted that trees in over-crowded stands are at higher risk of dying.
“You’ve got to look back a the last decade, because trees are very slow to respond. So it’s the last decade of extremely intense temperatures and drought that the trees today have to work their way through.”
