Fine-Scale NDVI Reconstruction Back to 1906 from Tree-Rings in the Greater Yellowstone Ecosystem

Global warming and related disturbances, such as drought, water, and heat stress, are causing forest decline resulting in regime shifts. Conventional studies have combined tree-ring width (TRW) and the normalized difference vegetation index (NDVI) to reconstruct NDVI values and ignored the influences of mixed land covers. We built an integrated TRW-NDVI model and reconstructed the annual NDVI maps by using 622 Landsat satellite images and tree cores from 15 plots using point-by-point regression. Our model performed well in the study area, as demonstrated by significant reconstructions for 71.14% (p < 0.05) of the area with the exclusion of water and barren areas. The error rate between the reconstructed NDVI using the conventional approach and our approach could reach 10.36%. The 30 m resolution reconstructed NDVI images in the recent 100 years clearly displayed a decrease in vegetation density and detected decades-long regime shifts from 1906 to 2015. Our study site experienced five regime shifts, markedly the 1930s and 1950s, which were megadroughts across North America. With fine resolution maps, regime shifts could be observed annually at the centennial scale. They can also be used to understand how the Yellowstone ecosystem has gradually changed with its ecological legacies in the last century.

Greater Yellowstone climate assessment: past, present, and future climate change in greater Yellowstone watersheds

The Greater Yellowstone Area (GYA) is one of the last remaining large and nearly intact temperate ecosystems on Earth (Reese 1984; NPSa undated). GYA was originally defined in the 1970s as the Greater Yellowstone Ecosystem, which encompassed the minimum range of the grizzly bear (Schullery 1992). The boundary was enlarged through time and now includes about 22 million acres (8.9 million ha) in northwestern Wyoming, south central Montana, and eastern Idaho. Two national parks, five national forests, three wildlife refuges, 20 counties, and state and private lands lie within the GYA boundary. GYA also includes the Wind River Indian Reservation, but the region is the historical home to several Tribal Nations. Federal lands managed by the US Forest Service, the National Park Service, the Bureau of Land Management, and the US Fish and Wildlife Service amount to about 64% (15.5 million acres [6.27 million ha] or 24,200 square miles [62,700 km2]) of the land within the GYA. The federal lands and their associated wildlife, geologic wonders, and recreational opportunities are considered the GYA’s most valuable economic asset. GYA, and especially the national parks, have long been a place for important scientific discoveries, an inspiration for creativity, and an important national and international stage for fundamental discussions about the interactions of humans and nature (e.g., Keiter and Boyce 1991; Pritchard 1999; Schullery 2004; Quammen 2016). Yellowstone National Park, established in 1872 as the world’s first national park, is the heart of the GYA. Grand Teton National Park, created in 1929 and expanded to its present size in 1950, is located south of Yellowstone National Park1 and is dominated by the rugged Teton Range rising from the valley of Jackson Hole. The Gallatin-Custer, Shoshone, Bridger-Teton, Caribou-Targhee, and Beaverhead-Deerlodge national forests encircle the two national parks and include the highest mountain ranges in the region. The National Elk Refuge, Red Rock Lakes National Wildlife Refuge, and Grays Lake National Wildlife Refuge also lie within GYA.

Climate-induced treeline mortality during the termination of the Little Ice Age in the Greater Yellowstone Ecoregion, USA

Paleoclimate reconstructions for the western US show spatial variability in the timing, duration, and magnitude of climate changes within the Medieval Climate Anomaly (MCA, ca. 900–1350 CE) and Little Ice Age (LIA, ca. 1350–1850 CE), indicating that additional data are needed to more completely characterize late-Holocene climate change in the region. Here, we use dendrochronology to investigate how climate changes during the MCA and LIA affected a treeline, whitebark pine ( Pinus albicaulis Engelm.) ecosystem in the Greater Yellowstone Ecoregion (GYE). We present two new millennial-length tree-ring chronologies and multiple lines of tree-ring evidence from living and remnant whitebark pine and Engelmann spruce ( Picea engelmannii Parry ex. Engelm.) trees, including patterns of establishment and mortality; changes in tree growth; frost rings; and blue-intensity-based, reconstructed summer temperatures, to highlight the terminus of the LIA as one of the coldest periods of the last millennium for the GYE. Patterns of tree establishment and mortality indicate conditions favorable to recruitment during the latter half of the MCA and climate-induced mortality of trees during the middle-to-late LIA. These patterns correspond with decreased growth, frost damage, and reconstructed cooler temperature anomalies for the 1800–1850 CE period. Results provide important insight into how past climate change affected important GYE ecosystems and highlight the value of using multiple lines of proxy evidence, along with climate reconstructions of high spatial resolution, to better describe spatial and temporal variability in MCA and LIA climate and the ecological influence of climate change.