By Christina Procopiou

In the 20th century, chestnut blight and Dutch elm disease decimated billions of U.S. trees growing in forests and lining urban and suburban streets. The tree diseases, caused by invasive pests—a wind borne fungus spore from Japan and a beetle from the Netherlands, respectively—changed the face of one American city landscape after another and cost local governments and homeowners a fortune.

Groves of ash trees lost to beetles. Stately alders under assault by moths. And fungus-like organisms attacking oaks.

In the battle to halt the march of tree-killing pests across the country, it's easy to get discouraged.

Even so, researchers at the University of Tennessee and an environmental group warned in a recent report that without action, forests during the next few decades will fundamentally change as species die and take with them entire ecosystems.