Bark of Rainbow Eucalyptus tree (Eucalyptus deglupta) in Costa Rica. Tree guru Michael Dirr calls the bark “one of the biological world’s great treasures.” The colors of the bark are due to the way patches of it shed at different times throughout the year, revealing a bright green inner bark that darkens to blue, purple, orange and maroon as it matures. The tree is harvested for pulpwood to make paper, especially in tropical areas. (Photo by Lynn Koch)