Landsat Reveals Continued Tree Dieback in the Sierra Nevada Mountains after Drought

Scientists in Code SGE are generating new maps of the 2013-2015 drought impacts on all forests of the Sierra Nevaga range based on yearly changes in the summer time Landsat moisture index.  Ames scientists began by examining Landsat satellite imagery at nearly all of the high tree die-back sites in the southern Sierra region flown by the U. S. Forest Service in 2015.  The first outcome confirmed that changes in drought-sensitive satellite indices from the years 2011 to 2015 closely matched patterns of tree die-back across the USFS aerial survey locations in the southern Sierra region.  But there was much more to learn; the satellite data record was analyzed further to put the 2013-2015 drought into historical context, particularly with respect to noticeable increases in forest canopy browning due to summer water limitations, progression warming, and widespread bark beetle attacks.  According to the satellite data from the summers of 2015 and 2016, areas most severely impacted by constant water shortages were located just southwest of Yosemite, Kings Canyon, and Sequoia National Parks, and in the Sierra and Sequoia National Forests.  Changes in the Landsat data at dying tree locations nearly resembled those from charred forest stands burned in the 2013 Rim Fire near Yosemite and the 2015 Rough Fire near Kings Canyon National Park.  The higher numbers of dead trees detected by Landsat on north-facing slopes can be explained by increased competition among dense stands of white fir and incense cedar for scarce soil moisture.   Continued analysis of this unique NASA satellite data over the next several years at these same locations will help inform forest and park managers about the potential for further, delayed tree die-back.