Abstract
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.
Highlights
Forests cover 30% of the world’s land surface and support ecosystem functioning and human societies [1]
From the 1906 simulated image (Figure 2A), we found that most areas still had comparatively high vegetation density whose overall normalized difference vegetation index (NDVI) reached 0.32 (Figure 2E)
From the reconstructed NDVI spatial distributions in 1906 through 2015, we found the tendency of mean NDVI was increasing from 1906 to 2015
Summary
Forests cover 30% of the world’s land surface and support ecosystem functioning and human societies [1]. Vegetation plays a vital role in stabilizing the ecosystem, yet its interaction with climate, human activities, and related disturbances trigger ecosystems to drift from the standard thresholds resulting in regime shifts [2]. The ecological regime shifts definition, according to Biggs et al [3], emphasized three characteristics: large, abrupt, and long-lasting changes in the ecosystems, which often have considerable impacts on human economies and societies. “Abrupt” refers to the instant variation possibly caused by short-term disturbances, such as fire, insect outbreak, or drought. These ecological disturbances break down the balances in the previous ecological system.
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