Why are post oaks dying? The cause may be drought followed by deluge

Video: AgriLife Today

In the lush green landscape of a season with plentiful rain, memories of the record Texas drought of 2011 could fade. But hundreds of calls, emails and tree samples sent to the Texas Plant Diagnostic Lab in College Station tell a different story.

“Since the early spring to late summer, there have been many inquiries as to why post oaks have ‘suddenly’ died,” said Kevin Ong, director of the lab, which is operated by the Texas A&M AgriLife Extension Service, and an associate professor in the Department of Plant Pathology and Microbiology, College of Agriculture and Life Sciences.

“When you get a whole bunch of folks asking the same question, and they are from all over Texas — even in the Panhandle — you know that something widespread is up.”

What’s up is that leaves start yellowing, may develop spots and ultimately turn brown but still cling to the tree limbs, according to Sheila McBride, the lab’s lead diagnostician. At that point, the tree is already dead.

“We are seeing the symptoms everywhere. It’s in the urban environment. It’s in the rangeland environment. It’s in the woodland environment. It’s not just happening in one spot,” she said.

McBride said a few cases were reported in 2011 and again in 2013, mostly from arborists checking trees in their areas. But the flood of calls this year, beginning in the spring when post oaks should be bursting into a showy display of green leaves, was an alarm for plant pathologists who monitor diseases and other stresses that impact Texas flora.

McBride and David Appel, a Texas A&M AgriLife Research forest pathologist in College Station, as well as a professor in the Department of Plant Pathology and Microbiology, call the phenomenon rapid oak decline.

They agree that “rapid” is in the eye of the beholder and that by the time someone notices the symptoms – the yellowing, spotted or brown leaves on a tree — the tree has been declining over time.

“People will say the tree died overnight. But actually it has been developing these early symptoms for a long time before that. It collapses. It dies. It turns completely brown very quickly,” Appel said.

Especially hard hit is the Post Oak Savannah region of Texas, which is between the Blackland Prairies and the eastern Piney Woods. According to the Texas A&M Forest Service, the area extends from the Oklahoma border to south of San Antonio and was part of the historic oak belt that once ranged from near Canada to Central America.

Appel, who has researched oak wilt for decades, stressed that the current problem is not that disease. Oak wilt targets primarily live oaks whereas this situation is impacting post oaks, which are very resistant to the wilt.

To understand why post oaks are declining, he explained, one needs to understand their ecology. Post oaks are a type of white oaks, which are “very, very susceptible to site disruption and rapid environmental changes.

“They have particular areas where they like to grow, but as they get older and bigger, the stand gets more crowded. That means the competition for resources gets to be a problem,” Appel said. “So when we go through rapid environmental extremes from year to year, that makes for a tough time on the post oak physiology.”

That’s why plant pathologists peg the 2011 drought as the beginning of the decline of so many post oaks in Texas. Following the drought, several years of unusual weather patterns made it hard for the large, old post oaks to compete for the carbohydrates they need to live.

“Then this year included an extremely wet spring followed by a very hot, dry period,” Appel said. “Those two factors combined with the stress that was put on these trees in 2011 to cause the death of so many post oaks this year.