SATELLITE REMOTE SENSING OF TERRESTRIAL NET ... - CiteSeerX

Mitigation and Adaptation Strategies for Global Change (2006) DOI: 10.1007/s11027-005-9014-5

C

Springer 2006

SATELLITE REMOTE SENSING OF TERRESTRIAL NET PRIMARY PRODUCTION FOR THE PAN-ARCTIC BASIN AND ALASKA J.S. KIMBALL1,2 , M. ZHAO2 , K.C. MCDONALD3 and S.W. RUNNING2 1 Flathead

Lake Biological Station, Division of Biological Sciences, The University of Montana, 311 Bio Station Lane, Polson, MT 59860-9659, USA 2 Numerical Terradynamic Simulation Group, Department of Ecosystem and Conservation Sciences, The University of Montana, 32 Campus Drive #1224, Missoula, MT 59812-1224, USA 3 Jet Propulsion Laboratory, California Institute of Technology, 4800 Oak Grove Drive, Pasadena, CA 91109-8001, USA (Received 27 October 2005; accepted in final form 24 June 2005)

Abstract. We applied a terrestrial net primary production (NPP) model driven by satellite remote sensing observations of vegetation properties and daily surface meteorology from a regional weather forecast model to assess NPP spatial and temporal variability for the pan-Arctic basin and Alaska from 1982 to 2000. Our results show a general decadal trend of increasing NPP for the region of approximately 2.7%, with respective higher (3.4%) and lower (2.2%) rates for North America and Eurasia. NPP is both spatially and temporally dynamic for the region, driven largely by differences in productivity rates among major biomes and temporal changes in photosynthetic canopy structure and spring and summer air temperatures. Mean annual NPP for boreal forests was approximately 3 times greater than for Arctic tundra on a unit area basis and accounted for approximately 55% of total annual carbon sequestration for the region. The timing of growing season onset inferred from regional network measurements of atmospheric CO2 drawdown in spring was inversely proportional to annual NPP calculations. Our findings indicate that recent regional warming trends in spring and summer and associated advances in the growing season are stimulating net photosynthesis and annual carbon sequestration by vegetation at high latitudes, partially mitigating anthropogenic increases in atmospheric CO2 . These results also imply that regional sequestration and storage of atmospheric CO2 is being altered, with potentially greater instability and acceleration of the carbon cycle at high latitudes. Keywords: AVHRR, arctic tundra, boreal forest, carbon cycle, climate change, NPP

1. Introduction Boreal forest and Arctic tundra biomes of the pan-Arctic basin and Alaska encompass approximately 25 million km2 of the northern high (>50◦ N) latitudes and 30% of the combined North American and Eurasian landmass. This region is currently undergoing dramatic environmental changes largely in response to recent and persistent climatic warming, including permafrost melt and increases in soil active layer depths, advances in the timing and length of seasonal growing seasons, increased vegetation growth and alteration of land-atmosphere net CO2 exchange (Serreze et al. 2000; Myneni et al. 1997; Oechel et al. 1995; McDonald et al. 2004). These environmental changes are of critical importance to the global carbon

J.S. KIMBALL ET AL.

cycle and the potential for further global warming because of the large reservoir of carbon currently stored in seasonally frozen and permafrost boreal and tundra soils. Historically, this region has been a large sink for atmospheric CO2 , a major greenhouse gas, and is currently estimated to contain up to 40% (775 Pg C) of the global soil organic carbon reservoir (Lal and Kimble 2000). A growing body of evidence indicates the boreal-Arctic carbon reservoir is becoming increasingly unstable through continued warming and drying of the region, with the potential for rapid carbon losses from regional disturbances such as fire, enhanced soil organic matter decomposition and heterotrophic respiration (Comiso 2003; Oechel et al. 1995; Stocks et al. 2002). These changes could result in large net CO2 emissions to the atmosphere, potentially reinforcing regional and global warming (IPCC 2001; Chapin et al. 2000). Alternatively, recent warming trends may be enhancing regional atmospheric carbon sequestration through increased vegetation growth, potentially offsetting regional terrestrial carbon losses from enhanced decomposition and respiration processes. Net primary production (NPP) is the primary conduit of carbon transfer from the atmosphere to the land surface and is thus a fundamental component of the global carbon cycle. The NPP process involves plant sequestration of atmospheric CO2 through photosynthesis and carbon storage in vegetation biomass and soils. The status and relative strength of the landscape as a net source or sink for atmospheric CO2 is dependent on the balance between terrestrial carbon uptake through NPP and carbon release from volatilization and respiration processes. Terrestrial carbon storage can vary from an order of months to decades in vegetation biomass pools and up to several millennia in relatively stable soil carbon pools consisting of cool, wet, seasonally-frozen soils and permafrost. Long-term satellite and aerial remote sensing observations of a greening arctic indicate a regional response of increasing NPP and accelerated sequestration and storage of atmospheric CO2 by vegetation at temperate and high northern latitudes (Myneni et al. 1997; Sturm et al. 2001; Keyser et al. 2000). These changes also appear to be having a significant impact on atmospheric greenhouse gas concentrations, resulting in earlier seasonal drawdown and larger seasonal amplitude of atmospheric CO2 (Keeling et al. 1996; Randerson et al. 1999). Increased NPP may be a direct response to a warming climate (e.g., more favorable temperatures for photosynthesis), as well as an indirect response to increased soil nutrient (primarily nitrogen, N) availability (Jarvis and Linder 2000; Schimel et al. 1996). Long-term manipulative chamber studies of Arctic tundra under controlled environmental conditions indicate generally increased nutrient availability and positive NPP response to warming for vascular plants, deciduous shrubs and graminoids, but decreases in productivity for nonvascular plants (Van Wijk et al. 2003). The overall effect of these changes on regional NPP is uncertain and appears to vary by community type and species composition. Repeat aerial photography since the 1950s indicates some areas of Alaskan tundra are getting shrubbier (Sturm et al. 2001), consistent with a general increase in NPP. Other studies show a general increase in NPP

SATELLITE REMOTE SENSING OF BOREAL-ARCTIC NPP

for boreal stands in Alaska, Canada and the northern boreal tree line, primarily in response to regional warming and associated lengthening of the growing season at high latitudes (Keyser et al. 2000; D’Arrigo and Jacoby 1993). However, other evidence from tree ring data indicates a negative boreal NPP response to a warming climate due to temperature-induced drought stress (Barber et al. 2000). Regional disturbances from fire and insect defoliations in boreal high latitudes also appear to be accelerating in response to regional warming and are likely to strongly influence regional patterns, interannual variability and trends in NPP for the boreal biome (Stocks et al. 2002; Fleming 2000). Much research has been conducted on the NPP of northern latitude environments, though most studies have involved intensive measurements of single plots or surveys of networks of individual stands across large regions. Regional assessment of spatial and temporal patterns of NPP from these data is problematic because of the extreme spatial and temporal complexity of environmental and biophysical processes for northern latitudes and the limited aerial and temporal coverage and inconsistent protocols of various field based observations. Alternatively, global satellite remote sensing at optical and near-infrared wavelengths provides repeat monitoring capabilities, consistent sampling and spatially contiguous spectral reflectance information that are sensitive to vegetation structure and photosynthetic biomass. This information has been used as a major driver of simple production efficiency models for spatial mapping and monitoring of gross primary productivity (GPP) and NPP across large regions (Prince and Goward 1995; Potter et al. 1999; Running et al. 2000). Previous studies have documented NPP spatial and temporal patterns over regional to global extents and various time periods using a variety of satellite remote sensing based approaches (e.g., Goetz et al. 1996; Nemani et al. 2003). For this investigation, we apply a biome-specific production efficiency model driven by daily surface meteorological inputs from an atmospheric GCM and satellite remote sensing observations of vegetation structure to quantify NPP spatial patterns, annual variability and recent trends for the pan-Arctic basin and Alaska from 1982 to 2000. We also conduct statistical and model sensitivity analyses to determine the primary physical and environmental drivers of NPP variability. The implications of these results are discussed in terms of the regional carbon cycle and its potential to partially mitigate or reinforce the effects of global change.

2. Methods The study domain encompasses Alaska and the pan-Arctic basin including all land areas draining into the Arctic Ocean, Hudson Bay, James Bay, Hudson Strait and the Bering Sea (Figure 1). The region is defined in terms of nodes of the National Snow and Ice Data Center (NSIDC) north polar Equal-Area Scalable Earth (EASE) grid (Armstrong and Brodzik 1995). The study domain spans a latitudinal range

J.S. KIMBALL ET AL.

Figure 1. The study domain encompasses the terrestrial pan-Arctic drainage basin and Alaska. The region spans a latitudinal range from 43.4◦ N to 86.6◦ N and represents 39,926 EASE-Grid cells with a 25 km × 25 km spatial resolution. Regional elevations are also presented and range from sea level to approximately 5,000 m; sub-basin outlines and major rivers are shown in red and blue, respectively.

from 45.35◦ N to 83.62◦ N, while land areas within the region comprise 39,926 grid cells with nominal 25 km × 25 km resolution and a total representative area of approximately 25 million km2 . We used a NOAA AVHRR based global land cover classification to define major biomes within the study region (Myneni et al. 1997; DeFries et al. 1998). Boreal forest and Arctic tundra represented approximately 37% (9.2 × 106 km2 ) and 29% (7.3 × 106 km2 ) of the region, respectively, while grassland-agricultural systems represented 8% (2.0 × 106 km2 ) of the study domain. The rest of the study domain was composed of permanent ice and snow, barren land and inland water bodies (26% or 6.6 × 106 km2 ). These nonvegetated areas were masked from further analysis to isolate relationships between terrestrial vegetation productivity and environmental parameters. A biome-specific Production Efficiency Model (PEM) was used to calculate GPP and NPP for unmasked pixels within the 25 km resolution EASE-grid domain. The PEM logic is described and verified in detail elsewhere (Running et al. 2000; 2004; Nemani et al. 2003) and summarized below. GPP (g C m−2 ) was derived on a daily


basis as: GPP = ε · FPAR · PAR

(1)

ε = εmax · T f · VPD f

(2)

where ε is a light use efficiency parameter (g C MJ−1 ) for the conversion of photosynthetically active radiation (PAR, MJ m−2 ) to GPP, where PAR is assumed to represent 45% of incident solar radiation; FPAR is the fraction of absorbed PAR; εmax is the potential maximum ε under optimal conditions (i.e., no environmental stress); T f is a scalar that defines reductions in photosynthesis under low temperature conditions, while VPD f is a scalar that defines similar reductions under suboptimal surface air Vapor Pressure Deficit (VPD) and associated daytime water stress conditions. Both T f and VPD f are dimensionless parameters ranging from 1 for optimal conditions to 0 under complete canopy stomatal closure and minimal photosynthetic activity. Both T f and VPD f are defined from daily minimum air temperature (Tmin ) and daylight average VPD using simple photosynthetic response curves. These response curves and εmax are prescribed for different biome types defined from the global land cover classification. Net primary production (g C m−2 ) is derived on an annual basis as the difference between the annual summation of daily net photosynthesis and autotrophic growth and maintenance respiration: NPP =

365

(GPP − Rm

lr )

− (Rm

w

+ Rg )

(3)

1

where Rm lr is the daily maintenance respiration of leaves and fine roots as derived from allometric relationships to canopy leaf-area index (LAI) and an exponential relationship between respiration and temperature. The Rm w parameter represents the annual maintenance respiration from live wood, while Rg represents the annual growth respiration; both Rm w and Rg are derived from allometric relationships between vegetation biomass and maximum annual LAI, and exponential respiration response curves to air temperature. The characteristic response curves for these calculations vary according to major biomes as defined by a Biome Properties Look-Up Table (BPLUT) and the global land cover classification. The BPLUT used for this investigation defines response characteristics for 11 major biomes including evergreen and deciduous needleleaf and broadleaf forests, mixed deciduous and evergreen forests, grasslands, shrublands and croplands. The PEM used for this investigation is currently being used for operational global assessment and monitoring of GPP and NPP using LAI and FPAR data from the MODIS sensor onboard the NASA EOS Terra satellite from 2000 onward (Running et al. 2004; Zhao et al. 2005). A detailed description of these algorithms and associated BPLUT properties can be found in the MODIS MOD17 User’s Guide (Heinsch et al. 2003).

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For this investigation we applied the PEM described above to assess spatial and temporal variability in annual vegetation productivity for the pan-Arctic study region over a 19-year period from 1982 to 2000. The PEM requires spatially explicit and temporally contiguous inputs of daily surface meteorology, LAI and FPAR to derive GPP and NPP. Monthly LAI and FPAR data were obtained from the NOAA AVHRR Pathfinder data set (Myneni et al. 1997), which has an approximate 16 km2 spatial resolution and extends over the entire domain and 19-year study period. The LAI and FPAR data are based on a monthly maximum value compositing of AVHRR spectral reflectance data to mitigate cloud cover, smoke and other atmospheric aerosol contamination effects (Myneni et al. 1997). These data were reprojected to the 25 km polar EASE-grid format using a nearest-neighbor resampling scheme. The monthly LAI and FPAR data were then resampled to a daily time step by temporal linear interpolation of adjacent monthly values. Surface air temperature, incident solar radiation and VPD inputs were provided by daily reanalysis data from the National Center for Environmental Prediction (NCEP; Kistler et al. 2001). The NCEP meteorological data are available globally in approximately 1.875 degree (∼208 km) spatial resolution and were reprojected into the 25 km resolution polar EASE-grid format using a bi-Lagrange interpolation approach (Serreze et al. 2002). PEM calculations were conducted for vegetated cells within the study region from 1982 to 2000, and spatial patterns and annual variability and trends in GPP and NPP were evaluated accordingly. Spatial and temporal variability in vegetation productivity was assessed for the entire study region, as well as within major biomes, and Eurasian and North American portions of the region. A linear trend analysis was conducted on mean annual results to evaluate temporal trends in vegetation productivity. The statistical significance of these trends was assessed at the 90% probability level. We conducted a statistical correlation analysis to assess correspondence between annual productivity calculations and mean annual surface meteorological parameters from the NCEP reanalysis. To isolate the relative contributions of meteorological and vegetation structural parameters to observed temporal behavior in NPP, we conducted a series of PEM simulations over the 1982 to 2000 period by: (1) constraining daily meteorological inputs to 1982 conditions while allowing satellite remote sensing based LAI and FPAR inputs to vary; (2) constraining LAI and FPAR inputs to 1982 conditions while allowing daily meteorological inputs to vary. We then conducted a statistical correlation analysis of these results relative to PEM calculations derived from historical variability in both meteorological and LAI and FPAR time series inputs. These simulations provided a mechanism for isolating climate impacts to vegetation productivity from other potential impacts such as atmospheric nitrogen and CO2 fertilization, disturbance and forest regrowth; and residual sensor calibration and aerosol effects on LAI and FPAR data. We compared regional mean annual NPP results to atmospheric CO2 concentration records from NOAA CMDL arctic and subarctic monitoring stations to assess the relationship between annual vegetation productivity results and growing season


dynamics inferred from regional atmospheric CO2 concentration network measurements. Mean monthly CO2 records were extracted for Point Barrow (71◦ N), Ocean Station (66◦ N), Cold Bay (55◦ N), Alert (82◦ N) and Mould Bay (76◦ N) CMDL stations (Conway et al. 1994). Previous studies have shown that the seasonal atmospheric CO2 cycle at high northern latitudes is dominated largely by northern terrestrial ecosystems, with minimal impacts from ocean exchange, fossil fuel emissions and tropical biomass burning (Randerson et al. 1997; Heimann et al. 1998; Erickson et al. 1996). The average of the five stations’ mean monthly records were normalized as the difference between monthly and mean annual CO2 concentrations for each year. The timing of the spring and fall 0-ppm crossing dates represent the X-intercepts of the normalized monthly CO2 transition between positive (winter) and negative (summer) conditions and are an indication of the timing and duration of vegetation photosynthetic activity and the net uptake and sequestration of atmospheric CO2 over land (Keeling et al. 1996; Randerson et al. 1997). We calculated the dates of seasonal downward (spring) and upward (fall) 0-ppm crossings of the normalized seasonal cycle of atmospheric CO2 , and the period between these respective crossings, and used this information as respective surrogates for the initiation, termination and duration of annual growing seasons from 1982 to 2000. These parameters were derived from individual station records, averaged on a yearly basis across all stations and compared with pan-Arctic averages of annual GPP and NPP results. Annual anomalies of PEM and CMDL station results were calculated relative to long-term means or linear least-squares regression results where significant (P ≤ 0.1) secular trends were observed. Relationships between annual anomalies in GPP, NPP and atmospheric CO2 based growing season parameters were then evaluated accordingly.

3. Results Satellite derived NPP results for the study domain are summarized in Table I, while a map of mean annual NPP for the 1982–2000 period is presented in Figure 2. Mean annual NPP for the region was approximately 354 ± 151.5 [s] g C m−2 yr−1 , with substantial spatial and temporal variability. Boreal forests cover approximately 39% (9.7 × 106 km2 ) of the study area and account for 47% of the total average annual NPP for the region, while Arctic tundra represents 33% (8.3 × 106 km2 ) of the region and 21% of total NPP. Boreal broadleaf deciduous forests were the most productive land cover class on a unit area basis (596 ± 138.6 [s] g C m−2 yr−1 ), followed by boreal coniferous forest (441 ± 150.9 [s] g C m−2 yr−1 ), grassland (340 ± 104.8 [s] g C m−2 yr−1 ), agricultural (269 ± 74.0 [s] g C m−2 yr−1 ) and tundra (247 ± 84.2 [s] g C m−2 yr−1 ) classes. Average annual NPP was approximately 9% greater for North America relative to Eurasia, but constituted a lower, 36% of the total carbon sequestered annually for the region due to the smaller relative land area. Annual NPP was also inversely proportional to latitude above approximately 60◦ N

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TABLE I Summary of PEM derived NPP results for the study domain Classification region

1 IGBP Class

2 Area

Pan-Arctic North America Eurasia Boreal Arctic Grassland-Agric.

N/A N/A N/A 1,3,4,5,8,9 6,7,16 10,12

100.0 33.7 66.3 39.0 33.2 10.2

(%)

3 NPP av (g C m−2 yr−1 )

NPP/GPP (%)

Raut /GPP (%)

354 (151.5) 374 (177.4) 342 (132.7) 444 (145.1) 247 (84.2) 329 (97.8)

51.3 46.4 50.1 50.9 50.4 54.8

48.7 46.3 50.0 44.8 41.3 45.2

1 International

Geosphere-Biosphere Program (IGBP) defined global land cover classes (IGBP, 1992). 2 Proportional area represented within the entire 25 million km2 study domain. 3 Mean annual NPP (standard deviations in parentheses) for 1982–2000 period.

due to the general decrease in growing season length, light intensity and seasonal temperatures at higher latitudes (Figure 3). Below 60◦ N the relationship between NPP and latitude was indeterminate due to increasing land cover diversity and the greater influence of available water limitations on annual productivity (Churkina and Running, 1998). Annual variability in NPP for the domain ranged from 6% to 93% and was generally larger near the northern and southern boundaries of the region due to greater topographic and associated climatic diversity, and smaller representative land area along basin margins. Spatial variability in the NPP results show distinctions among major regions and vegetation classes that are generally consistent with other boreal and Arctic NPP estimates derived from intensive field measurements and stand inventory based approaches within the domain. Relative magnitudes of the PEM results, however, generally fall within the upper ranges of these reported values. Annual NPP for Arctic tundra has been reported to range from approximately 70 g C m−2 yr−1 for low tundra shrub communities of the high Arctic up to 500 g C m−2 yr−1 for tall shrub communities of the low Arctic, with an average level of approximately 90 g C m−2 yr−1 for the global tundra biome (Saugier et al. 2001; Shaver and Jonasson 2001). Boreal NPP has been reported to range from 123 to 460 g C m−2 yr−1 for Siberian and European forests, respectively, based on chronosequence studies and national forestry statistics (Schulze et al. 1999). NPP values for mature spruce forests in central Alaska were reported to be approximately 225 g C m−2 yr−1 (Ruess et al. 1996), while values reported for boreal forests in central Canada range from 226 to 478 g C m−2 yr−1 and are generally higher for broadleaf deciduous forests relative to needleleaf evergreen forests (Gower et al. 1997). NPP observations for grasslands within the region range from 137 to 477 g C m−2 yr−1 and generally show intermediate levels of productivity between tundra and boreal


Figure 2. Mean annual NPP (g C m−2 yr−1 ) derived from PEM calculations over the pan-Arctic domain and 19-year (1982–2000) study period; masked areas are shown in gray and represent unvegetated surfaces including permanent ice and snow, open water and barren land.

forest biomes (Olson et al. 2001). Regional variations in these values largely reflect seasonal temperature, light availability, soil moisture and soil nutrient limitations to vegetation growth and respiration processes at high latitudes and upper elevations (Bonan and Shugart 1989; Churkina and Running 1998). NPP represented approximately 51% of annual GPP, while Raut accounted for 49% of GPP. Respiration costs relative to GPP were generally lower for tundra compared to boreal and grassland-agricultural areas due to a positive Raut response to temperature, and characteristically cooler average temperatures of Arctic regions relative to lower latitude areas occupied by other vegetation classes. The average proportion of GPP devoted to respiration costs was also relatively high for Eurasia due to lower productivity across this region relative to North America. A map of the long-term (1982–2000) trends in satellite derived annual NPP is presented in Figure 4 and summarized in Table II. NPP trends were predominantly positive across 62% of the vegetated study region, while a smaller 38% of the region

J.S. KIMBALL ET AL.

Figure 3. The latitudinal mean (shown in black), maximum and minimum (shown in grey) annual NPP for the pan-Arctic domain and 19-year study period (top); the latitudinal distribution of boreal forest, Arctic tundra and other major land cover categories represented within the study domain as derived from the global land cover classification is also presented (bottom).

showed negative NPP trends. The largest positive trends occur in boreal northwest Canada, eastern Alaska and southern Siberia. The largest negative trends occur in central Siberia, northern Europe, northern Mongolia and the boreal-grassland transition area of southern Alberta, Canada. For the entire region, a positive trend was observed in mean annual NPP of approximately 2.7% per decade from 1982 to 2000. Positive NPP temporal trends were also observed for the major continental portions of the study region that were higher for North America (3.4% 10 yr−1 )


Figure 4. Map of the multi-year trend in PEM based annual NPP (g C m−2 yr−1 ) across the study domain excluding permanent ice and snow, open water and barren areas (indicated in grey).

relative to Eurasia (2.2% 10 yr−1 ). However, these trends were not statistically significant (P > 0.1). There were also positive NPP trends for boreal (3.5% 10 yr−1 ), Arctic (1.3% 10 yr−1 ) and grassland-agriculture (3.0% 10 yr−1 ) biomes, though these trends were only significant for boreal and grassland-agriculture areas. TABLE II Summary of PEM based NPP decadal trends (%) within the study domain. 1982–2000

1982–1991

Classification region

Trend

r-value

P-value

Trend

r-value

P-value

Pan-Arctic North America Eurasia Boreal Arctic Grassland-Agric.

2.7 3.4 2.2 3.5 1.3 3.0

0.32 0.30 0.24 0.40 0.12 0.41

0.19 0.22 0.33 0.09 0.63 0.078

13.2 15.6 11.7 14.2 10.2 11.3

0.80 0.87 0.66 0.82 0.55 0.68

0.005 0.0009 0.040 0.004 0.10 0.03

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Figure 5. Mean annual NPP anomalies (g C m−2 yr−1 ) for the pan-Arctic domain and Eurasian and North American components of the study region relative to long-term (1982–2000) conditions.

Increasing NPP trends for the period were tempered by an approximate 2-year cooling event and associated lower productivity rate following the June 1991 volcanic eruption of Mt. Pinatubo (Figure 5). The 1992 NPP anomaly was 7.5% lower than the long-term mean for the region. Prior to 1992 the pan-Arctic NPP trend was a positive 13.2% per decade (P = 0.005). Temporal NPP trends for continental and biome level components of the study region were also significant and substantially larger prior to 1992. Previous studies have also documented the apparent shortterm effects of the Pinatubo eruption on LAI, NPP and net CO2 exchange at high latitudes (Nemani et al. 2003; Lucht et al. 2002). Large continental differences in NPP anomalies are also apparent; a large positive NPP anomaly in 1998 for the North America portion of the study region corresponds to warm air temperature anomalies in spring and early summer over North America, in contrast to cooler temperatures and an associated lower NPP anomaly over Eurasia for the same period. A large negative NPP anomaly for North America in 2000 corresponds to relatively cool (3–4 ◦ C below mean, 1992–2000, summer conditions) summer air temperatures across Canada and Alaska; annual precipitation for much of Canada in 2000 was also 10% to 40% below normal conditions (IPCC 2001). In contrast, the Eurasian portion of the study region showed similar, though relatively less extensive summer cooling over portions of central Siberia and near normal conditions elsewhere, while annual precipitation was above normal for much of the region. A map of the statistical significance of long-term NPP trends is presented in Figure 6. These results indicate that NPP trends were statistically significant at a


Figure 6. Map of statistical significance of the 19-year trends in annual NPP across the study domain excluding permanent ice and snow, open water and barren lands (indicated in gray). Levels of significance of the least-squares linear regression relationships between annual NPP and year (independent variable) for the 1982–2000 time series were determined for each 25 km grid cell. Excluding the masked areas (gray), 4.7% of the region showed significance levels above 99% (blue); 6.6% with significance levels between 95 and 99% (yellow); 6.1% between 90 and 95% (red) and 82.6% of the study domain (dark green) are those areas for which trends had less than 90% significance.

minimum 90% probability level over approximately 17% (3.2 × 106 km2 ) of the vegetated study region, while NPP trends were significant at a higher 95% minimum probability level over approximately 11% (2.1 × 106 km2 ) of the vegetated region. The most significant trends occurred in regions showing the largest positive changes in annual NPP. In North America, the most significant positive NPP trends occurred in boreal forests of northwest Canada and eastern Alaska. In Eurasia, the most significant positive NPP trends occurred in boreal forests and grasslands of southcentral Siberia in the headwater regions of the Lena and Yenisei rivers. Average summer air temperature anomalies were positively correlated to satellite derived GPP (r = 0.700; P < 0.001) and NPP (r = 0.634; P = 0.004) annual fluxes. Years having relatively warm summers appear to promote greater net photosynthesis

J.S. KIMBALL ET AL.

relative to Raut , resulting in greater NPP; alternatively, relatively cool summers promote the opposite response. These results are consistent with previous studies indicating that GPP and NPP at high latitudes are predominantly cold temperature limited (Churkina and Running 1998; Nemani et al. 2003). Results of the model sensitivity analysis indicate that annual NPP variability for the study region is primarily driven by changes in terrestrial vegetation cover as detected by satellite LAI and FPAR measurements (r = 0.886; P < 0.0001), while variability in the climatic drivers of NPP (i.e., daily air temperature, humidity, solar radiation) is of secondary importance (r = 0.689; P = 0.001). Of the climatic variables, however, air temperature is the primary driver of annual variability in NPP (r = 0.686; P = 0.001). Thus while surface air temperature records show a relatively consistent global warming trend since the early 1980s, annual NPP for the high latitude study region appears to be largely benefiting from warmer temperatures, particularly during the late spring and early summer growing season period, when solar radiation levels are near seasonal maxima and plant available moisture is relatively abundant. Mean annual vegetation productivity for the region was inversely proportional to the timing of growing season initiation inferred from atmospheric CO2 concentration records as shown in Figure 7. The timing of growing season initiation from atmospheric CO2 records accounted for 33% and 34% (P ≤ 0.010) of annual

Figure 7. Correspondence (r = −0.583; P = 0.010) between annual anomalies of regional average NPP (g C m−2 yr−1 ) for the study domain and growing season onset (days) defined by the timing of the spring drawdown of atmospheric CO2 from NOAA CMDL high latitude (>50◦ N) monitoring sites; positive NPP anomalies are associated with increased vegetation productivity and earlier growing season onset (i.e., CO2 drawdown) while negative NPP anomalies are associated with decreased productivity and a delay in growing season onset relative to the long-term (1982–2000) record.


variability in GPP and NPP, respectively. Years with relatively early growing seasons were associated with increased annual productivity, while years with delayed growing seasons showed lower annual productivity rates. No significant relationship was found between PEM based GPP and NPP results and atmospheric CO2 derived measures of the timing of growing season termination or duration, however. These results are consistent with other regional studies showing strong linkages between the timing of seasonal thawing, growing season onset and annual vegetation productivity in boreal landscapes (Jarvis and Linder 2000; Kimball et al. 2004). At high (>50◦ N) northern latitudes cold temperatures and limited photoperiod largely constrain the growing season length to a relatively brief period during spring and summer months relative to temperate latitudes where growing seasons begin earlier and can extend well into the fall (Nemani et al. 2003). In the spring, incident solar irradiance in these regions is near seasonal maxima and GPP and NPP proceed at relatively high rates following seasonal thawing and the arrival of warmer air temperatures (Kimball et al. 2004; Suni et al. 2003). Boreal evergreen forests can accumulate as much as 1% of their annual NPP on a daily basis immediately following seasonal thawing in the spring, while potential accumulation rates are substantially reduced by decreasing photoperiod during later stages of the growing season. Thus variability in the timing of growing season onset, rather than growing season offset or duration, has a relatively large impact on annual vegetation productivity for the region. Total estimated annual carbon sequestration by NPP for the region averaged 6.5 ± 0.303 (s) Pg C yr−1 over the 19-year period. This value represents 12% of the estimated global average annual NPP derived over a similar period (Nemani et al. 2003). It is also sufficient to account for the 1.0 to 3.5 Pg C annual terrestrial sink for atmospheric CO2 estimated to occur at mid to high latitudes (Ciais et al. 1995; Fan et al. 1998), assuming terrestrial carbon emissions from soil respiration and regional disturbances such as fire are less than approximately 50% of annual NPP. Mean annual NPP for the period is equivalent to 3.1 ppm of atmospheric CO2 (1 ppm per 2.12 Pg C per year) and represents approximately 31% of the mean seasonal drawdown of atmospheric CO2 at high (>50◦ N) northern latitudes (Conway et al. 1994; McDonald et al. 2004). Total cumulative NPP over the 19-year study period was approximately 123 Pg C and is also equivalent to 58 ppm, or more than twice (210%) the total secular CO2 increase (27.7 ppm) for the period. However, the observed NPP trend yields a net increase of 0.31 Pg C over the 19-year period, which is equivalent to 0.15 ppm, or only 0.5% of the total secular CO2 increase for the period. These results indicate that any additional terrestrial carbon sequestration attained through increasing NPP at high latitudes is unlikely to significantly offset the observed secular increase in atmospheric CO2 . Additionally, increasing disturbance trends from fire and insect defoliations in boreal regions and the potential for large positive soil respiration responses under a warmer climate are likely to more than offset any additional terrestrial carbon sequestration gains from enhanced NPP at high latitudes.

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4. Discussion Boreal and Arctic biomes are important components of the global carbon cycle because they currently store from 400 to 800 Pg C and represent 20% to 40% of the total global terrestrial biospheric carbon pool (Saugier et al. 2001; Lal and Kimble, 2000). The majority of this carbon is currently stored in cold soils that have been relatively stable throughout the Holocene and thus a major atmospheric carbon sink. Vegetation productivity for the region is characteristically low compared to temperate forests, due to low temperature constraints on plant metabolic activity. Seasonal cold temperatures, permafrost and high soil active layer water levels also inhibit soil decomposition processes, providing an additional, indirect constraint to NPP through soil nutrient (N) limitations to vegetation growth (Bonan and Van Cleve 1992; Schimel et al. 1996; Shaver and Jonasson 2001). The results of this study indicate a recent trend of increasing NPP for the region. Enhanced vegetation productivity appears to be a function of increased photosynthetic leaf area, earlier onset of seasonal growing seasons and warming of spring and summer air temperatures. These results also imply an increase in soil N availability required to support additional vegetation growth. Observations of recent soil and permafrost warming and increasing soil active layer depths for the region indicate increasing soil decomposition processes and associated increases in plant available N to support additional NPP (Serreze et al. 2000; Oelke et al. 2004; Schimel et al. 1994). Other observations from long-term satellite microwave remote sensing records show a recent advance in the onset of the growing season at high latitudes, promoting a more favorable environment for increased plant growth (McDonald et al. 2004; Kimball et al. 2004). The cumulative evidence from these studies imply a shift in carbon storage at high latitudes from relatively stable soils toward less stable above-ground biomass and an associated acceleration of the boreal-Arctic carbon cycle. The continuation of declining permafrost and increasing soil active layer depths, combined with increasing vegetation NPP could enhance regional drying in the absence of adequate compensation from a positive precipitation trend (Oechel et al. 2000). These effects would tend to promote heterotrophic respiration and additional soil carbon losses to the atmosphere. Large-scale disturbance from fire and insect defoliations is an important component of boreal ecosystems. Fires consume more than approximately 2 million ha annually in boreal Canada and Alaska (Amiro et al. 2001). Observational records over the last 50 years indicate that fire activity increased substantially during the 1970s and 1980s for this region in association with a warming climate (Stocks et al. 2002; Kasischke et al. 2002; McGuire et al. 2004). Long-term regional fire observation records are comparatively sparse for Eurasia, though current records indicate that annual fire activity ranges from 2 to 13 million ha, and that Eurasian fires are predominantly ground fires, while North America is characterized by more stand replacing crown fires (Isaev et al. 2002; Conard et al. 2002). The


successional pattern following stand replacing fires in North America is generally one of herbaceous vegetation and deciduous shrubs followed by relatively productive mixed deciduous and coniferous forests dominated by aspen, birch and white spruce replacing lower productivity stands dominated by more fire prone black spruce and moss vegetation (Van Cleve and Viereck 1980). The most significant increases in vegetation NPP during the study period occur in regions of northwestern Canada and central Alaska that have also experienced increased fire activity. Spring and summer warming trends that appear to be enhancing regional NPP may also be enhancing regional fire activity. Satellite observations of increased NPP also indicate a potential positive feedback to increased fire activity through additional vegetation biomass accumulation and associated fuel loading for fires. Increased fire activity may also be a mechanism driving enhanced NPP for the region, since fires increase soil nutrient availability to plants by promoting warmer soils and deeper soil active layer depths, as well as replacing older and less productive stands with younger, more vigorous vegetation. Thus satellite evidence of increasing NPP may reflect both a causal mechanism and response to increasing fire activity. In the boreal forest biome regional disturbance from insect defoliations can impact even more extensive areas on an annual basis than fire (Fleming et al. 2002). Insect defoliations reduce NPP in the short-term, while severe outbreaks involving successive defoliations can result in tree mortality and the replacement of entire stands with younger and more productive vegetation in the long-term (McGuire et al. 2004). As with both NPP and fire, the intensity and frequency of defoliations also appear to be positively correlated with regional warming trends (Fleming 2000). Thus satellite observations indicating an increasing NPP trend may not necessarily correspond to increasing carbon sequestration and boreal sink strength for atmospheric CO2 since increased vegetation carbon sequestration may be offset by carbon losses from increased decomposition, respiration and combustion processes. Further research is needed to clarify these relationships. For this investigation we utilized daily meteorological simulations from an atmospheric General Circulation Model (GCM), satellite optical-infrared remote sensing based measures of regional vegetation parameters and general assumptions of plant structure and physiological responses to environmental processes within major biomes to simulate the relative magnitude and variability in annual NPP for the pan-Arctic basin and Alaska. The regional land cover classification information used for this study and associated model NPP results largely reflect dominant, overstory vegetation types and do not explicitly account for additional or subdominant vegetation categories within individual land cover classes. Spatially explicit meteorological data sets for the pan-Arctic region available from relatively coarse spatial resolution GCM simulations can differ substantially depending upon the particular model and methods employed in the simulation. The reliability of these data sets are particularly uncertain for the high latitudes where regional monitoring networks useful for calibration and reanalysis of GCM simulations are extremely sparse and largely confined to coastal areas and lower elevations.

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Satellite monitoring of high latitude regions from optical-infrared remote sensing is also problematic due to low solar illumination and image degradation from frequent cloud cover and aerosol impacts. Seasonal patterns and long-term trends in vegetation activity derived from NOAA AVHRR Pathfinder records are also uncertain because of issues related to the coarse monthly temporal compositing of the data, and navigational drift and cross-calibration of instrument series. The daily interpolation of monthly AVHRR records used for this study can impose an artificial phenology on growing season NPP calculations if the interpolated FPAR values deviate from actual canopy conditions. However, the potential effects of these deviations on vegetation productivity simulations are minimized during the margins of the growing season when FPAR is relatively low and NPP is primarily constrained by low temperatures. Similar uncertainties arise from the use of mean monthly atmospheric CO2 records to predict the timing of the growing season. Despite these limitations, these monthly data records have been used extensively to assess seasonal patterns and trends in vegetation activity at high latitudes because of a general lack of long-term global observations at finer temporal scales. All of these factors have the potential to adversely impact the relative accuracy of satellite based NPP simulations for the region. Nevertheless, the results of this study are consistent with a growing body of evidence indicating large-scale changes in vegetation structure and productivity at high latitudes associated with regional warming.

Acknowledgments This work was supported by grants from the National Science Foundation Office of Polar Programs and from the National Aeronautics and Space Administration (NASA) Office of Earth Science Enterprise. Portions of the research described in this paper were conducted with data infrastructure support from the Arctic Regional Integrated Hydrological Monitoring System (ArcticRIMS) at the University of New Hampshire, and carried out at the Jet Propulsion Laboratory, California Institute of Technology, under contract with NASA.

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