摘要：Trends towards earlier greenup and increased average greenness have been widely reported
in both humid and dry ecosystems. By analyzing NOAA (National Oceanic and
Atmospheric Administration) AVHRR (Advanced Very High Resolution Radiometer)
data from 1982 to 2007, we report complex trends in both the growing season
amplitude and seasonally integrated vegetation greenness in southwestern North
America and further highlight regions consistently experiencing drought stress. In
particular, greenness measurements from 1982 to 2007 show an increasing trend in
grasslands but a decreasing trend in shrublands. However, vegetation greenness in this
period has experienced a strong cycle, increasing from 1982 to 1993 but decreasing
from 1993 to 2007. The significant decrease during the last decade has reduced
vegetation greenness by 6% in shrublands and 13% in grasslands (16% and 21%,
respectively, in the severe drought years). The greenness cycle correlates to both
annual precipitation and dry season length derived from NOAA North America
Regional Reanalysis data. If drought events continue as predicted by climate
models, they will exacerbate ecosystem degradation and reduce carbon uptake.
