Drought, heat and an infestation of bark beetles has killed 26 million trees in the Sierra Nevada since October, an “unprecedented” die-off that heightens an already high wildfire risk. The new estimate by the U.S. Forest Service brings the loss of trees to at least 66 million since 2010 — a problem that grew worse despite an average winter of rain and snow that brought some relief. The latest estimates come from a May survey of six southern Sierra California counties, and the 26 million trees that apparently died between October 2015 and May continue a disturbing increase. Forest service officials found 40 million dead trees between 2010 and 2015 and say that nearly 30 million of those succumbed to drought and insect mortality between September 2014 and October 2015.