Abstract
Negative trends in the monthly MODerate resolution Imaging Spectroradiometer (MODIS) Enhanced Vegetation Index (EVI) time-series were found to be widespread in natural (non-cropland) ecosystems of the eastern United States from 2000 to 2010. Four sub-regions were detected with significant declines in summed growing season (May-September) EVI, namely the Upper Great Lakes, the Southern Appalachian, the Mid-Atlantic, and the southeastern Coastal Plain forests ecosystems. More than 20% of the undeveloped ecosystem areas in the four sub-regions with significant negative EVI growing season trends were classified as forested land cover over the entire study period. We detected relationships between annual temperature and precipitation patterns and negative forest EVI trends across these regions. Change patterns in both the climate moisture index (CMI) and growing degree days (GDD) were associated with declining forest EVI growing season trends. We conclude that temperature warming-induced change and variability of precipitation at local and regional scales may have altered the growth trends of large forested areas of the eastern United States over the past decade.
Highlights
There is considerable uncertainty about changing growth rates of forests in the eastern United States
The northwestern Great Lakes sub-region was detected as having a notable (1.6%) area coverage with significant positive Enhanced Vegetation Index (EVI) trends over the study period (Table 1), none of which could be attributed to regrowth following forest fires
The Upper Great Lakes region showed the strongest association between negative annual climate moisture index (CMI) and negative growing season EVI trends. These results suggest that dry years in this sub-region are closely related to declines in forest growth as detected by trends in summer-time EVI
Summary
There is considerable uncertainty about changing growth rates of forests in the eastern United States. Based on a small number of field site measurements, forest biomass accumulation was reported to be accelerating in the region at locations where nutrients and water were not limiting [1,2]. Reduction of wildfire throughout the eastern US has converted more lands to closed-canopy forests with more shade-tolerant, fire-sensitive plants [3]. The eastern US has become warmer, since 1970 at the rate of 0.45 ̊F per decade [4]. Outbreaks of insect pests and introduced pathogens have become dominant forces driving changes in forests of the eastern US [5]
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