Since 1972, the U.S. Geological Survey's Landsat satellites have been the watchman that never sleeps with spectral bands capturing the subtle turning of green mountainsides into dying forests.
Links:
[1] http://admin.indiaenvironmentportal.org.in/news/mountain-pine-beetles-infest-and-ravage-thousands-acres-forest-lands-landsat-satellites-show
[2] http://admin.indiaenvironmentportal.org.in/category/newspaper/science-daily
[3] http://admin.indiaenvironmentportal.org.in/category/thesaurus/forests
[4] http://admin.indiaenvironmentportal.org.in/category/thesaurus/insects
[5] http://admin.indiaenvironmentportal.org.in/category/thesaurus/united-states-america-us
[6] http://admin.indiaenvironmentportal.org.in/category/thesaurus/forest-fires
[7] http://admin.indiaenvironmentportal.org.in/category/thesaurus/forest-management