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

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D. <strong>Verbyla</strong> Table 2 Linear trends in annual maximum normalized difference vegetation index (NDVI) (1982–2003) by ecozone polygon within western <strong>and</strong> eastern boreal ecoregions. Slopes represent changes in NDVI (unitless) per year (n = 22). Mean temperature <strong>and</strong> precipitation values were extracted from each ecozone polygon <strong>based</strong> on climate data from Fleming et al. (2000). Climate <strong>and</strong> wildfire burn data were downloaded from http://agdc.usgs.gov/data/. Ecozone Including burns: Excluding burns: r 2 Slope P-value r 2 Slope P-value Mean May–Aug. temp. (°C) Total May–Aug. precip (mm) Davidson Mountains 0.14 –0.001 0.09 0.12 –0.001 0.11 4.2 129 Kobuk Ridges <strong>and</strong> Valleys 0.18 –0.002 0.05 0.18 –0.002 0.05 8.7 150 Lower Yukon Lowl<strong>and</strong>s 0.54 –0.004 < 0.01 0.54 –0.004 < 0.01 11.1 171 Kuskokwim Upl<strong>and</strong>s 0.36 –0.003 < 0.01 0.33 –0.003 < 0.01 10.5 207 Lime Hills 0.06 –0.001 0.27 0.06 –0.001 0.28 9.3 239 Kuskokwim Lowl<strong>and</strong>s 0.28 –0.003 0.01 0.30 –0.003 < 0.01 10.6 192 Cook Inlet 0.16 –0.002 0.06 0.16 –0.002 0.06 10.3 203 Copper River Basin 0.30 –0.002 < 0.01 0.30 –0.002 < 0.01 9.2 263 Tanana Lowl<strong>and</strong>s 0.51 –0.003 < 0.01 0.52 –0.003 < 0.01 11.1 196 Ray Mountains 0.41 –0.003 < 0.01 0.41 –0.003 < 0.01 10.1 192 Yukon–Tanana Upl<strong>and</strong>s 0.45 –0.003 < 0.01 0.45 –0.003 < 0.01 9.3 203 Yukon–Old Crow Basin 0.52 –0.002 < 0.01 0.53 –0.002 < 0.01 8.2 108 North Ogilvie Mountains 0.31 –0.002 < 0.01 0.30 –0.002 < 0.01 7.5 174 regression. Linear trends were computed for over 28,000 pixels <strong>and</strong> outputted as rasters <strong>of</strong> P-values <strong>and</strong> slope from each pixel’s regression. RESULTS <strong>The</strong>re were contrasting linear trends from 1982–2003 between the polar <strong>and</strong> boreal regions, with the polar region increasing (r 2 = 0.13, slope = +0.0011, P = 0.10), <strong>and</strong> the boreal region decreasing (r 2 = 0.41, slope = –0.0024, P = 0.002) in mean annual maximum NDVI. <strong>The</strong>re was no significant trend in the maritime region (r 2 < 0.01, slope = –0.0003, P = 0.817). All regions had a decrease in NDVI in 1992, presumably due to the stratospheric aerosols <strong>and</strong> subsequent cooling resulting from the 1991 Pinatubo eruption (Lucht et al., 2002). At the ecoregion polygon scale, the only significant (P > 0.05) positive trends in mean annual maximum NDVI were from the arctic tundra ecoregions north <strong>of</strong> the Brooks Range, with the strongest trend from the Arctic Coastal Plain (Table 1). <strong>The</strong>re were significant decreasing NDVI trends in boreal forest ecoregions, with the strongest trend from the Eastern Interior. Trends from all other ecoregions were not significant (Table 1). Within smaller ecozone polygons from boreal ecoregions, the strongest negative trends were from physiographic basins such as the Lower Yukon Lowl<strong>and</strong>s, the Tanana Lowl<strong>and</strong>s <strong>and</strong> the Yukon–Old Crow Basin (Table 2). Physiographic regions with the weakest negative trends were from areas with a maritime climate (Lime Hills <strong>and</strong> Cook Inlet) <strong>and</strong> colder mountainous regions (Davidson Mountains, Kobuk Ridges <strong>and</strong> Valleys). <strong>The</strong> 1982–2003 trends from climate station buffers were similar to regional trends from the ecoregion polygon analysis. <strong>The</strong>re were no significant trends (P > 0.05) in annual maximum NDVI among climate station buffers from the Bering tundra region <strong>of</strong> western <strong>Alaska</strong>. All arctic tundra buffers had significant positive trends <strong>and</strong> all boreal stations had significant negative trends in annual maximum NDVI (Table 3). In arctic <strong>Alaska</strong>, the interannual patterns <strong>of</strong> NDVI (both annual maximum <strong>and</strong> 1–15 June maximum NDVI) from the climate station buffers were strongly correlated (Pearson’s r > 0.80) with the interannual patterns NDVI from the Arctic Coastal Plain ecoregion. If the interannual NDVI from buffers <strong>and</strong> the ecoregion were not strongly correlated, then local factors such as variable cloud cover or differing disturbances among climate station might dominate the NDVI trend relative to the ecoregion NDVI trend. Over the 22-year period, the most rapid increase in NDVI in arctic tundra areas occurred during the 1–15 June composite period, probably due to snowmelt, budburst <strong>and</strong> leaf flush. <strong>The</strong> 1–15 June maximum NDVI within each climate station buffer was related to the tundra spring warmth index from each climate station (r 2 ranging from 0.33 to 0.66, P < 0.01). However, the relationship between 1–15 June maximum NDVI <strong>and</strong> annual maximum NDVI was weak <strong>and</strong> not significant (r 2 ranging from 0.05 to 0.03, P > 0.33), indicating that early growing season conditions may not control annual maximum NDVI in the <strong>Alaska</strong>n arctic tundra. Jia et al. (2003) reported a significant linear relationship in arctic <strong>Alaska</strong> between maximum NDVI <strong>and</strong> a summer warmth index, expressed as the sum <strong>of</strong> monthly mean temperatures above 0 °C. In this study, there was also a significant linear relationship (P < 0.01, r 2 = 0.58) between annual maximum NDVI <strong>and</strong> summer warmth index. However, the relationship between annual maximum NDVI values <strong>and</strong> the previous year’s summer warmth index values was stronger (P < 0.01, r 2 = 0.66). Among boreal climate station buffers, the early spring NDVI (1–15 May) was linearly related to the boreal spring warmth index (r 2 ranging from 0.48 to 0.58, P < 0.01). <strong>The</strong>re were no © <strong>2008</strong> <strong>The</strong> Author 550 Global Ecology <strong>and</strong> Biogeography, 17, 547–555, Journal compilation © <strong>2008</strong> Blackwell Publishing Ltd
<strong>Alaska</strong> NDVI trends Table 3 Linear trend in annual maximum normalized difference vegetation index (NDVI) (1982–2003) within climate station 100-km buffers. Slopes represent changes in NDVI (unitless) per year (n = 22). Climate station r 2 Slope P-value No. <strong>of</strong> pixels Mean May–Aug. temp. ( o C) Mean annual precipitation (mm) Bering tundra region Bethel 0.09 –0.002 0.16 455 10 453 Kotzebue 0.007 0.0003 0.72 219 8 282 Nome 0.01 +0.0005 0.59 145 8 438 King Salmon 0.01 –0.0005 0.63 291 11 497 Arctic tundra region Barrow 0.56 +0.005 < 0.01 419 1 111 Kuparuk 0.65 +0.005 < 0.01 235 3 97 Umiat 0.55 +0.004 < 0.01 278 6 126 Boreal forest region Bettles 0.33 –0.002 < 0.01 122 12 380 Delta 0.67 –0.004 < 0.01 118 13 309 Fairbanks 0.46 –0.003 < 0.01 180 14 278 Gulkana 0.30 –0.002 < 0.01 49 11 294 McGrath 0.43 –0.004 < 0.01 185 12 465 Talkeetna 0.19 –0.002 0.04 131 13 738 significant (P > 0.16) linear relationships between annual maximum NDVI as a function <strong>of</strong> early spring NDVI for boreal buffers (r 2 ranging from < 0.01 to 0.09) Unlike the arctic tundra stations, there were no significant linear relationships between annual maximum NDVI <strong>and</strong> current or lagged summer warmth index values (P > 0.30, r 2 < 0.10). Bunn et al. (2005) found that the previous spring minimum temperature was an important variable in predicting summer NDVI in coniferous <strong>and</strong> broadleaf boreal areas <strong>of</strong> Canada. In this study, the linear relationships <strong>of</strong> annual maximum NDVI as a function <strong>of</strong> previous spring temperature were weak (r 2 ranging from < 0.01 to 0.22). <strong>The</strong> linear relationships <strong>of</strong> annual maximum NDVI as a function <strong>of</strong> August through July precipitation were also weak (r 2 ranging from 0.06 to 0.24). <strong>The</strong> 1982–2003 pattern <strong>of</strong> an increasing NDVI trend in northern arctic <strong>Alaska</strong> <strong>and</strong> a decreasing trend in interior <strong>Alaska</strong> also was evident from the pixel-level linear regressions. <strong>The</strong> highest rate <strong>of</strong> increase occurred along the central <strong>and</strong> eastern Arctic coastal plain, while the highest rate <strong>of</strong> decrease occurred in basins <strong>of</strong> interior <strong>Alaska</strong> (Fig. 3). DISCUSSION NDVI trends across the <strong>Alaska</strong>n tundra <strong>The</strong> growing season <strong>of</strong> the Arctic is now at its warmest relative to at least the past 400 years (Overpeck et al., 1997). Arctic <strong>Alaska</strong> is undergoing a system-wide response to an altered climatic state (Hinzman et al., 2005). <strong>The</strong> summer warming in arctic <strong>Alaska</strong> may be due to a lengthening <strong>of</strong> the snow-free season, with early sensible heating <strong>of</strong> the lower atmosphere (Chapin et al., 2005). Vegetation responses have included delayed senescence (March<strong>and</strong> et al., 2004) <strong>and</strong> increased broadleaf shrub abundance across the <strong>Alaska</strong>n arctic tundra (Sturm et al., 2001; Tape et al., 2006). <strong>The</strong> increase in annual maximum NDVI may be due to an increase in height <strong>and</strong> cover <strong>of</strong> shrubs <strong>and</strong> graminoids (Walker et al., 2006). Within the arctic climate station buffers there was a significant linear relationship between annual maximum NDVI <strong>and</strong> annual summer warmth index values, consistent with the results <strong>of</strong> March<strong>and</strong> et al. (2004), who found infrared heating <strong>of</strong> tundra plots significantly increased NDVI within a few weeks <strong>of</strong> heating. A stronger linear relationship was observed between annual maximum NDVI <strong>and</strong> the previous year’s summer warmth index. For example, the highest summer warmth index at Barrow, Kuparuk <strong>and</strong> Umiat occurred in 1989 <strong>and</strong> the highest residual from the NDVI trend lines occurred the next year. Spring greenup, expressed as maximum NDVI from the 1–15 June composite period, was a poor predictor <strong>of</strong> annual maximum NDVI at the climate station buffer scale <strong>and</strong> at the ecoregion scale. Thus it appears that annual maximum NDVI was not dependent on whether spring was early or late for any given year. <strong>The</strong>re were significant positive temporal trends in NDVI from 1982 to 2003 at all spatial scales examined in the arctic tundra region. However, there were no significant temporal trends in NDVI at any spatial scale in the shrub tundra region <strong>of</strong> western <strong>Alaska</strong>. This may be due to the much colder climate <strong>of</strong> the arctic ecoregions relative to the Bering tundra ecoregion (Table 3), with warming occurring at a faster rate in the arctic region. In general, there has been less warming <strong>and</strong> drying in western <strong>Alaska</strong> relative to central <strong>and</strong> eastern arctic <strong>Alaska</strong>, which has been described as a steepened gradient in continentality over the past 20 years (Thompson et al., 2006). <strong>The</strong> pattern <strong>of</strong> increasing NDVI over the period 1982–2003 in tundra regions <strong>of</strong> <strong>Alaska</strong> followed this west–east trend, with the northern arctic trend strongest east <strong>of</strong> 160° W longitude (Fig. 3). © <strong>2008</strong> <strong>The</strong> Author Global Ecology <strong>and</strong> Biogeography, 17, 547–555, Journal compilation © <strong>2008</strong> Blackwell Publishing Ltd 551
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