The most endangered tree in the contiguous United States is most likely a battered old oak hidden deep in a Texas mountain range. …Scientists only realized the species still existed after stumbling upon the ailing specimen during an expedition this spring. And without swift action, researchers warn, Quercus tardifolia could truly disappear. The species is among some 100 U.S. trees staring down the barrel of extinction, according to a sweeping new assessment published Tuesday in the journal Plants People Planet. Amid an onslaught of invasive insects, a surge in deadly diseases and the all-encompassing peril of climate change, as many as 1 in 6 trees native to the Lower 48 states are in danger of being wiped out, the scientists say. The threatened list includes soaring coast redwoods, capacious American chestnuts, elegant black ash and gnarled whitebark pine. Yet only eight tree species are federally recognized as endangered or threatened.