I'm talking about trees. Tree scientists tell us that over the past 30 years, three-quarters of eastern US tree species have been shifting to the west. And they are doing so at what the scientists consider a fast rate - 9.5 miles per decade. No, trees don't migrate by hitching up their roots and trudging through the forest; they gradually shift their population centers over many years to places that offer better conditions. But the scientists don't quite know exactly why this happening. Some of the factors causing the migration are: changes in how we're using the land, new pest infestations, and conservation and planting projects.