COLUMBIA — On one tree trunk, water bottle-sized patches of gray-blue fungus occupied the space where bark was supposed to be. A few feet away, another trunk was being attacked by a bleeding canker, a disease that looked like a burn mark. And on a third tree at the Baskett Wildlife Research and Education Center just east of Ashland, the leaves at the crown of the tree weren't quite dead, but their brown freckles showed they weren't completely healthy either.
White oak trees are dying at higher than expected rates in pockets of Missouri, but their symptoms aren't consistent. In 2011, foresters started getting increased reports of declining white oaks, a top timber product in Missouri, but the mystery of exactly what is killing the trees has left forest experts scratching their heads. Even after taking the effects of last year's severe drought into consideration, the rate of decline for white oaks is still abnormally high.
"White oaks would be one of the tree species we thought would be best adapted to hot and dry conditions," said Chris Lee, a doctoral student studying forestry at MU. "They are really tough. They are the best at conserving water, and they are really good at getting to the water other tree species can't get to. Yet we're seeing them die, and some of these other tree species seem to be not completely conking out the way some of these white oaks are."
To add to the mystery, trees in the same area aren't affected consistently. At Baskett Wildlife Research and Education Center, for example, a perfectly healthy white oak tree stood mere feet away from a dead one.
"Nothing is consistent," said Hank Stelzer, chair of the MU Forestry Department.
White oak trees are an important timber product in Missouri; the wood is used to produce pallets and barrels for whiskey and wine. The white oak industry hasn't been affected by the decline yet, and it is too early to tell when or if it will be.
Back in the patch of forest at the Baskett Center, Lee pointed out the crispy brown leaves at the crown of a newly deceased oak. The presence of the leaves showed this tree died quickly. But in many cases, Lee said, it's difficult to tell how long it took a tree to die by looking at it. And without being able to tell how quickly they are dying, it is hard to tell what is killing them.
"Many pathogens kill trees quickly," Lee said. "But if it's climate causing the issue, it can take decades for the tree to die."
If climate is to blame for the decline, the event that caused it could have happened several years ago, said Simeon Wright, forest pathologist for the Missouri Department of Conservation. Trees get stressed from extreme or unusual weather conditions, and Missouri has seen its fair share of extremes in the last few years. Conditions have bounced from too hot to too cold or from too wet to too dry.
The ping-pong back and forth can be tough on trees, even relatively resilient species like white oak.
"White oak grow in a fairly wide range of conditions," Wright said. "They grow in the central U.S. and are grown farther north and farther south. But extreme fluctuations in weather can cause a tree stress."
Before the drought, the trees were living a lush lifestyle. Water was readily available and conditions were suited for growing. Just like humans can react adversely to the loss of luxuries, trees can react adversely as well.
"The trees got used to good growing conditions," Stelzer said. "It's like taking candy from a spoiled kid."
Stressed trees become more susceptible to other problems, like cankers seen on the trees at the Baskett area, meaning any combination problems could be to blame.
"Oftentimes there is a stressor that starts or incites the problem and then something else contributes to it," said Rose-Marie Muzika, professor of forestry at MU. "Typically, several different things might finish off the tree."
Tree decline is common in Missouri, and red oaks in particular go through cycles of decline, Lee said, often linked to drought. But this decline of white oaks is a curious one. Since reports of heightened decline began in 2011, it is unlikely that the cause of the decline is directly related to the severe drought of the summer of 2012.
"With white oaks, it is not really known to be a problem in the same way it is with red oak," Lee said. "That's why we're puzzled."
MU, the Missouri Department of Conservation and the USDA Forest Service will be conducting research in the coming months to further investigate the causes of this decline. The researchers will also be identifying the areas where the decline is most severe, which is generally unknown at this point.
While it is too early to say anything for sure, Lee said this mysterious decline could tell part of the larger story of a changing environment.
"It could say something about changing environmental conditions, whether that is climate change or something else that is going on," Lee said. "When tough tree species like white oak start to die, we start to ask ourselves what it is about background conditions that is changing."
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