Warning signals of tree mortality masked by extreme drought and bark beetles

A tree-ring study shows that when extreme drought occurs concurrently with bark beetle outbreaks, patterns of tree growth, physiology, and defense preceding tree mortality become less clear. Determining why some trees die while others survive both drought and insect outbreaks is valuable for forecasting tree mortality events, which are expected to become more frequent with further climate change. While some tree-level risk factors for both drought and beetle-related tree mortality have been identified, why some trees die while others survive these types of disturbances remains unclear. The 2012-2016 California extreme drought and concurrent bark beetle outbreaks killed an estimated 147 million trees and served as a unique opportunity to better understand tree mortality patterns with multiple concurrent disturbances.

Scientists collected stand and tree-level data on the Sierra and Los Padres National Forests in Central and Southern California, where tree mortality from the combination of drought and bark beetles was widespread. They compared trees that survived with those that died for ponderosa pine, pinyon pine, white fir, and incense cedar. They looked for differences in patterns of radial tree growth,  tree water use efficiency, and tree defenses and found that patterns preceding death varied by species. 

Ponderosa pine trees that died tended to have higher radial growth and lower relative defenses; both growth and defense seemed to be more sensitive to climate prior to death - for example, trees that died grew more in years with higher precipitation and less under drought compared to trees that survived. For white fir and pinyon pine, bark beetle dynamics seemed to play a more prominent role in mortality patterns. Risk factors for incense cedar mortality, for which the scientists did not see evidence of successful bark beetle attacks, included chronically slower growth and shorter heights, indicating that long-term growth suppression was a stronger driver of mortality for this species.

Risk factors for mortality differ across species with few generalizable patterns when bark beetle outbreaks occur in concert with extreme drought. Long-term physiological differences, shorter-term variability in defenses, and which trees bark beetles attack all influence susceptibility to mortality for ponderosa pine. Beetle dynamics alone seem to play a more prominent role in mortality patterns for white fir and pinyon pine. Incense cedar mortality appears attributable to long-term effects of growth suppression rather than bark beetle attacks.