Verbyla, D.. 2008 The greening and browning of Alaska based on ...

<strong>Alaska</strong> NDVI trends
woolly sawfly/stem cankers in alders (Ruess et al., 2006). <strong>The</strong> area
affected by insect <strong>and</strong> disease infestations exceeds wildfire in
boreal <strong>Alaska</strong> (Malmstrom & Raffa, 2000) <strong>and</strong> would probably
cause a decrease in NDVI.
<strong>The</strong> lack <strong>of</strong> simple correlations between interannual maximum
NDVI <strong>and</strong> precipitation among boreal climate station buffers is
not surprising. Spatial variation <strong>of</strong> precipitation is high relative
to temperature (Simpson et al., 2002). For example, the Fairbanks
<strong>and</strong> Delta climate stations both occur on the Tanana River
floodplain with overlapping 100-km climate buffers (Fig. 2),
have strongly correlated annual maximum NDVI values (Pearson’s
r = 0.91, 1982–2003), <strong>and</strong> mean monthly temperature (Pearson’s
r = 0.99, 1982–2003). However, the correlation <strong>of</strong> monthly total
precipitation between these two stations is substantially lower
(Pearson’s r = 0.61, 1982–2003), <strong>and</strong> the actual mean precipitation
within 100 km <strong>of</strong> each climate buffer is likely to vary
substantially relative to precipitation at a climate station.
<strong>The</strong> short-term response <strong>of</strong> boreal trees to precipitation may
also be species- <strong>and</strong> site-specific. For example, Yarie & Van Cleve
(2006) found reduced diameter growth <strong>of</strong> floodplain white
spruce <strong>and</strong> balsam poplar trees, but not in most upl<strong>and</strong> species
in a rain-exclusion experiment. Barr et al. (2004) found a decline
in aspen maximum leaf area index during a 4-year drought, but
no concomitant decline from understorey hazelnut canopies.
<strong>The</strong> long-term decreasing trend in NDVI in boreal <strong>Alaska</strong> is in
contrast to an increasing trend in the boreal forest <strong>of</strong> the
Komi Republic <strong>of</strong> north-west Russia (Lopatin et al., 2006). Both
regions have experienced warming since the early 1980s.
However, much <strong>of</strong> boreal <strong>Alaska</strong> is semi-arid with potential
evapotranspiration exceeding precipitation (Barber et al., 2000;
Gower et al., 2001), while precipitation exceeds potential
evapotranspiration in the Komi Republic (Lopatin et al., 2006).
In this study, the NDVI regression slopes were most negative <strong>and</strong>
the R 2 values were greatest from the warmest <strong>and</strong> driest boreal
ecozones (Table 2). <strong>The</strong> strongest negative trends among boreal
climate station buffers were also from relatively warm/dry areas
with an annual precipitation <strong>of</strong> less than 500 mm <strong>and</strong> mean
summer temperature above 12 °C (Table 3).
Although the GIMMS-NDVI data have been corrected for
major volcanic eruptions, sensor calibration <strong>and</strong> orbital drift
over time, confounding factors leading to residual error are
possible, including reduction <strong>of</strong> NDVI due to cloud <strong>and</strong> cloud
shadow, extreme viewing angles, low solar elevation early or late
in the growing season <strong>and</strong> atmospheric effects. <strong>The</strong> strategy <strong>of</strong>
selecting the maximum NDVI during a composite period lessens
the effect <strong>of</strong> these confounding factors, but does not completely
eliminate these problems. However, the NDVI trends in this
study consistently occurred at a variety <strong>of</strong> spatial scales. For
example, cold artic tundra had significant increasing trends in
annual maximum NDVI at the ecoregion <strong>and</strong> 100-km buffer
scale, while warmer shrub tundra from western <strong>Alaska</strong> had no
significant trends at these spatial scales. Boreal forest had significant
decreasing trends at all spatial scales examined <strong>and</strong> the
trends were consistently strongest in the eastern interior where
growing seasons are the warmest <strong>and</strong> driest relative to other areas
in this study.
ACKNOWLEDGEMENTS
I thank Scott Goetz, Martha Raynolds, John Yarie <strong>and</strong> the
anonymous referees for their useful suggestions that helped
improved the manuscript. This research was supported by the
Bonanza Creek LTER (Long-Term Ecological Research) program
(funded jointly by NSF grant DEB-0423442 <strong>and</strong> USDA Forest
Service, Pacific Northwest Research Station grant PNW01-
JV11261952-231).
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© <strong>2008</strong> <strong>The</strong> Author
Global Ecology <strong>and</strong> Biogeography, 17, 547–555, Journal compilation © <strong>2008</strong> Blackwell Publishing Ltd 553