By now, most of North America has thawed out from a brutal winter that introduced unhappy phrases such as “polar vortex” into the lexicon. But some effects of the long-lasting, subfreezing temperatures are only now becoming apparent. One surprise was the discovery that starving rats in New York City had attacked the trees in urban parks for sustenance. “With the deep snow and the cold winter, probably they didn’t have access to the normal food supply and it was a lot colder this winter,” Rich Hallett, a research ecologist with the U.S. Forest Service, told WNYC. “So they went after the trees.” The trees — which even in winter have some carbohydrates (mostly sucrose) in the vascular tissue beneath their bark — had been gnawed by rats all the way around the base of the trunk, a practice called “girdling” that usually kills a tree. More