A typical farmer must wait a year or two to see how a new crop variety works out. Imagine if the crop was trees.
An experiment with Ponderosa pine trees has run more than a generation and turned up some unplanned results. Saplings planted for their potential to grow the fastest, straightest timber turned out poorly prepared to survive a mountain pine beetle epidemic.
“Few people have 40 years of data, or opportunity to have a strong event like the pine beetle outbreak show up in the middle,” said University of Montana biology professor Anna Sala, lead author of a study in the Proceedings of the National Academy of Sciences. “That gave us the opportunity to ask a general question. Do faster-growing families suffer more mortality from bark beetle? The answer is yes.”
Sala specializes in physiological ecology, examining the features of plants and trees and evaluating the cost and benefit of each feature for survival. The idea that strength in one area may accompany weakness in another has been around biology ever since humans started selective breeding. With cash crops, bigger is better and time is money. But breeding trees presents the problem that those who start the study might be dead before it produces results.
Sala was fortunate to have access to a research plot in UM’s Lubrecht Experimental Forest planted by the Inland Empire Tree Improvement Cooperative in 1974. The group of industry foresters, academics and government scientists planted about 4,000 seedlings from 204 vigorous-growing Ponderosas and waited to see which produced the best wood.
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What they didn’t count on was the local mountain pine bark beetle shifting from endemic, or background levels, to a forest-wrecking outbreak. The BB-sized beetles attack pine trees in swarms. If the tree can’t produce enough sap to pitch them out, the beetles lay millions of eggs under the bark. When they hatch, the larva consume much of the tree’s nutrients while introducing a fungus that also blocks its ability to defend itself. Deprived of food, the pine’s needles turn red and fall, resulting in stands of “gray ghost” dead trees.
“All organisms have different growth rates — some people are taller and some are shorter,” Sala said. “And in biology there’s a well-known pattern: Faster-growing organisms have shorter lives. Bacteria live and die very fast. Elephants are slow-growing. We were initially looking at trade-offs between growth and drought resistance. But with the outbreak, we changed the goal and shifted to bark beetle resistance.”
And it turns out trees selected for their early fast-growing properties have poorer insect defenses as they near maturity. Sala still doesn’t know the reason. It could be that the speedier trees can’t alter their sap chemistry to repel the beetles as well as their slower-growing cousins. Additional research might get closer to the answer.
“In nature, these fluctuations occur a lot,” Sala said. “That helps maintain genetic diversity. No trait is best all the time.
“So if you are a tree breeder, what do you do? You want to maintain genetic diversity, particularly in long-lived organisms. The Irish potato famine was a case where lack of genetic diversity caused a catastrophe.”