Attacks increase when temperatures climb and precipitation dwindles
By Summit Voice
SUMMIT COUNTY — Forests in the American West aren’t the only ones facing an increasing threat from tree-killing beetles. A European researcher recently studied the pattern and impact of outbreaks by the bark beetle in the southern Alps, measuring the size and distribution of the infested areas occurring along steep temperature gradients between 1994 and 2009 and matched the observations with climatic changes.
The results, published online in Springer’s Climatic Change, shows that there were more attacks by the spruce bark beetle on European Alpine spruce forests over a 16 year period, as temperatures rose and rainfall dropped, according to Lorenzo Marini, of the University of Padova in Italy.
The research suggests that spruce bark beetles (Ips typographys) will quickly respond to climate change. Forest damage due to this bark beetle has increased markedly during the last decades throughout the whole of Europe.
Two types of forests were identified: so-called ‘on-site’ forests where Norway spruce is within its limits of historical climate range, at higher altitudes where temperatures are generally cooler; and ‘off-site’ forests where spruce is growing outside of its natural climatic range, at lower altitudes and in a warmer climate.
No surprise — dry summers combined with warm temperatures were significant triggers for severe outbreaks. Forest damage per hectare was on average seven-fold higher where spruce was planted in sites warmer than those within its historical climate range.
Although trees planted off-site grew substantially faster, their defenses against the bark beetle were probably weakened by both low rainfall and warmer temperatures. In addition, dry conditions in the previous year encouraged the bark beetle to move to higher altitudes. The low rainfall at those higher altitudes gave the beetles new opportunities to find suitable, weaker host trees which, under normal rainfall conditions, would have been less susceptible to bark beetle attacks.
“Considering the increased susceptibility of spruce forests to bark beetle outbreaks, the most reliable and ecologically sound strategy to reduce outbreaks of I. typographus is sustainable forest management, including avoiding planting spruce outside its natural climatic range.”