For centuries, tree lovers mighty and humble have planted and nurtured trees—elms, oaks, ginkgoes, magnolias, apples, and spruces (to name but a handful of America’s 600-some species). "I never before knew the full value of trees," wrote Thomas Jefferson in 1793. "Under them I breakfast, dine, write, read, and receive my company. What would I not give that the trees planted nearest the house at Monticello were full grown." But trees were often taken for granted in a new nation that seemed to have a limitless supply.