This might be slightly off-topic, but isn’t this cool? (And hey, anything medieval deserves a forum on the web.) Satelitte surveys of recently cleared land in the upper Amazon basin in Brazil have found evidence of a sophisticated pre-Columbian city that may date from the ninth century. Researchers believe this may be the fabled lost city of El Dorado, known to the early conquistadores.
Here’s a link to the basics: http://www.sphere.com/article/deforestation-reveals-ancient-amazonian-city/19309215?icid=main|main|dl1|link1|http%3A%2F%2Fwww.sphere.com%2Farticle%2Fdeforestation-reveals-ancient-amazonian-city%2F19309215
Better yet, check out the full study, published in the latest issue of the British journal of archaeology, Antiquity (vol. 83, no. 322, pp. 1084-95), an endlessly fascinating publication in its own right.
In Britain and Ireland, arial photography is a time-tested way of discovering previously unknown early medieval settlement sites, obscured by repeated plowing and nearly indiscernable except as crop marks visible from the air. Sadly, thanks to the rapid deforestation of ecosystems like the rain forest, even more lost civilizations may now be rediscovered.
Paula de Fougerolles' The Chronicles of Iona 
It’s nice to know something positive has come out of the deforestation!