Regional climate change with warming at about 0.3 to 0.4 per decade in the last three decades are killing trees in western parts of the U.S. This increase in temperature over the last few decades caused widespread hydrological changes such as declining fraction of precipitation falling as snow, declining snowpack water content, earlier spring snowmelt and a consequent lengthening of summer drought.
These changes have lead to doubled mortality rate compared to levels in 1950s changing from 1 % to 2 % over the last 50 years, and there is also the fear that warmer temperatures would increase the number and prevalence of insects and diseases that attack trees resulting in even higher tree-killing. Warmer temperatures have for instance given boost to bark beetles population that are killing trees in many parts of the United States.
Tree mortality rate is growing and although the rise of mortality from 1 % a year to 2 % a year may not seem that big to some people it is really a matter of small numbers that are adding up over the time. What scientists fear the most is the fact that this could be only a beginning of a trend that could cause much bigger negative impact on our forests.