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SPATIAL AND TEMPORAL VARIABILITY IN THE GROWTH AND CLIMATE RESPONSE OF TREELINE TREES IN ALASKA ANDREA H. LLOYD and CHRISTOPHER L. FASTIE Department of Biology, 372 Bicentennial Hall, Middlebury College, Middlebury, VT 05753, U.S.A. E-mail: [email protected]

Abstract. In this study, we investigated the response of trees growing at the cold margins of the boreal forest to climate variation in the 20th century. Working at eight sites at and near alpine and arctic treeline in three regions in Alaska, we compared tree growth (from measured tree ring-widths) to historical climate data to document how growth has responded to climate variation in the 20th century. We found that there was substantial regional variability in response to climate variation. Contrary to our expectations, we found that after 1950 warmer temperatures were associated with decreased tree growth in all but the wettest region, the Alaska Range. Although tree growth increased from 1900–1950 at almost all sites, significant declines in tree growth were common after 1950 in all but the Alaska Range sites. We also found that there was substantial variability in response to climate variation according to distance to treeline. Inverse growth responses to temperature were more common at sites below the forest margin than at sites at the forest margin. Together, these results suggest that inverse responses to temperature are widespread, affecting even the coldest parts of the boreal forest. Even in such close proximity to treeline, warm temperatures after 1950 have been associated with reduced tree growth. Growth declines were most common in the warmer and drier sites, and thus support the hypothesis that drought-stress may accompany increased warming in the boreal forest.

1. Introduction Climate models predict that the effects of anthropogenic climate change will be greatest over land masses at high latitudes in the northern hemisphere (Houghton et al., 1996). Changes in temperature and growing-season length are likely to have pronounced effects on temperature-limited ecosystems in the arctic and subarctic. Increases in temperature may alter community distribution (Suarez et al., 1999), composition (e.g., by favoring shrubs over herbaceous taxa; Chapin et al., 1995), and ecosystem carbon and nutrient fluxes (e.g., Nadelhoffer et al., 1992; Keeling et al., 1996; Jonasson et al., 1999; Gulledge and Schimel, 2000). Ecosystem responses to rising temperatures may, in turn, feedback on climate. Both positive feedbacks (those that will enhance the warming trend) and negative feedbacks (those that will moderate the warming trend) on climate may occur as ecosystems respond to climate change. Positive feedbacks may arise in response to processes that reduce surface albedo (e.g., expansion of boreal forest into tundra; Bonan et al., 1992; Chapin et al., 2000; White et al., 2000) and alter ecosystem CO2 fluxes (e.g., release of soil carbon from tundra soils; Oechel et al., 1993). Increased productivity Climatic Change 52: 481–509, 2002. © 2002 Kluwer Academic Publishers. Printed in the Netherlands.

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ANDREA H. LLOYD AND CHRISTOPHER L. FASTIE

of subarctic and arctic ecosystems, in contrast, may strengthen the existing highlatitude forest carbon sink (Dixon et al., 1994) and thus act as a negative feedback on warming (White et al., 2000). The response of particular species and ecosystems to climate warming may therefore have implications beyond the boundaries of that particular ecosystem. Observational studies indicate that the arctic has warmed substantially during the 20th century (Chapman and Walsh, 1993; Houghton et al., 1996; Serreze et al., 2000) and paleoclimatological data indicate that the 20th century has been warmer than previous centuries (Jacoby et al., 1985; D’Arrigo and Jacoby, 1993; Jacoby et al., 1996; Overpeck et al., 1997; Mann et al., 1998; Jacoby et al., 2000). Hemispheric records of 20th century temperature indicate that temperatures in the northern hemisphere increased sharply from 1900 until approximately 1940, remained constant or decreased from 1940 to 1970, and then increased again after 1970 (Houghton et al., 1996). Estimates of the rate of warming in recent decades vary, but Stone (1997) estimates that temperatures along the western Arctic coast of Alaska rose 1.41 ◦ C from 1965 to 1995, and Chapman and Walsh (1993) estimate an increase, in the arctic as a whole, of 0.5–0.75 ◦ C per decade from 1960 to 1990. Evidence for recent warming in the Arctic also comes from records of sea ice extent and thickness, which indicate a small (2% or less) decline in the areal extent of Arctic sea ice and a reduction in ice thickness in recent decades (Chapman and Walsh, 1993; Wadhams, 1995; Serreze et al., 2000), and from measurements of permafrost warming and thawing (Osterkamp and Romanovsky, 1999). The causes of the observed warming trend remain uncertain. The temperature increase over high-latitude continents is associated with a positive phase in the Arctic Oscillation (AO) index, an important natural source of variability in the arctic climate system (Thompson and Wallace, 1998; Shindell et al., 1999; Thompson et al., 2000). Although variation in the AO is likely to be at least partially the result of internal variability in the Arctic climate system, modeling analyses suggest that this change in atmospheric circulation, and the warming associated with it, may also be a manifestation of anthropogenic climate change (Fyfe et al., 1999; Shindell et al., 1999; Gillett et al., 2000). Changes in precipitation during the same time period remain much more uncertain. There is some observational evidence for an increase in precipitation in the 20th century between 55◦ and 85◦ N latitude (Houghton et al., 1996), but considerable uncertainty exists in the data (Kattsov and Walsh, 2000). Precipitation may be a critical mediator of ecosystem response to rising temperature, particularly in the drier areas of the arctic and subarctic. Lynch and Wu (2000), for example, have shown that soil moisture limitation in late summer can have large effects on ecosystem carbon uptake in the boreal forest, and Barber et al. (2000) have concluded that temperature-induced drought stress is reducing growth of boreal forest trees. Winter precipitation may also be important in determining the onset of the growing season: Vaganov et al. (1999) have proposed that increases in winter precipitation during the late 20th century may have delayed the onset of the growing season

GROWTH AND CLIMATE RESPONSE OF TREELINE TREES IN ALASKA

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and thus reduced overall tree growth and altered the response of growth to summer temperatures. There are some indications that climate changes observed during the 20th century have already begun to affect community composition and ecosystem processes. Some of these changes have been consistent with expectations (derived both from theory and from paleoecological data on ecological responses to past climate changes, e.g., MacDonald et al., 1993; Kullman, 1995; MacDonald et al., 2000) about the effects that warming should have on cold-environment ecosystems. For example, boreal forest expansion into tundra has been observed in northwestern Alaska (Suarez et al., 1999), and Chapin et al. (1995) observed an increase in shrub birch biomass and a decrease in sedge biomass in unmanipulated study plots in tussock tundra in northern Alaska. Increases in plant growth in northern high latitudes have been inferred from AVHRR data (Myneni et al., 1997) and from analyses of a 40% increase in the annual amplitude of atmospheric CO2 concentrations (Keeling et al., 1996). Despite the finding that warming has been temporally associated with enhanced productivity, analyses of tree growth responses to climate variability have suggested that warming may not consistently lead to increased plant growth, even in temperature-limited ecosystems. Barber et al. (2000) found that the growth of low-elevation white spruce in central Alaska had declined during the 20th century, and attributed the decline to temperature-induced drought stress. Other studies have found that the sensitivity of high-latitude tree growth to temperature has declined in recent decades: non-climatic factors or factors other than temperature (e.g., moisture stress) may thus have become increasingly important limits on tree growth in high-latitude forests (Jacoby and D’Arrigo, 1995; Briffa et al., 1998a,b; Jacoby et al., 2000). These results suggest that, even in the coldest areas of high-latitude forests, there may be diminishing benefits to further increases in temperature. The response of boreal forest trees to temperature has important implications for high latitude climates (e.g., White et al., 2000): an increase in tree growth and biomass will strengthen the boreal forest C sink and may thus act as a negative feedback on climate warming. The importance of this potential feedback will depend on the degree to which rising temperatures stimulate tree growth, and although positive responses of tree growth to temperature remain the most commonly reported mode of climate response among boreal forest trees (Garfinkel and Brubaker, 1980; Jacoby et al., 1996; Jacoby and D’Arrigo, 1997; Briffa et al., 1998; Jacoby et al., 2000), findings of inverse temperature responses (Barber et al., 2000) and reduced sensitivity to climate among trees with a positive temperature response (e.g., Jacoby and D’Arrigo, 1995; Briffa et al., 1998; Jacoby et al., 2000) suggest that assumptions that boreal forest productivity will continue to increase as temperatures rise may be unwarranted. The goal of this study was to examine spatial and temporal variability in the growth and climate response of trees at and near treeline in three different regions of Alaska. In particular, we address three questions. First, is there variation among

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regions in growth trends or climate response of trees growing at treeline? We compare the growth of treeline trees in three climatic regions: the White Mountains in central Alaska, the Alaska Range in south-central Alaska, and the Seward Peninsula in western Alaska. The regions vary in mean temperature and precipitation, and may thus provide insight into the prevalence of temperature-induced drought stress in boreal forest trees. Second, is there variation in growth trends or climate response between trees growing at the forest limit (treeline) and trees growing immediately downslope from treeline? Trees at the very margin of the boreal forest, where temperature is presumably most limiting to growth, should have the strongest response to warming temperatures. Comparison of tree growth response to climate across a relatively steep environmental gradient like that found at treeline may indicate the degree of fine-scale spatial variability in tree growth response to climate. Finally, does the relationship between tree growth and climate change during the 20th century? We compare the correlations between tree growth and climate during three time periods in the 20th century in order to describe how the relationship between growth and climate has or has not changed over time.

2. Materials and Methods 2.1. DESCRIPTION OF STUDY SITES We sampled trees at eight sites in three regions of Alaska (Figure 1; Table I). Climate data from stations in or near each region are summarized in Table II. The Yukon River valley, which is just north of the White Mountains sites, is the driest region, with the most extreme temperatures (warmest summers, coldest winters). Average annual precipitation at Circle, on the Yukon River, is 2/3 of that in the other regions. Climate stations for this region, however, are at lower elevations (100 years old) trees at each study site. At four of the sites (Usibelli, Twelvemile Summit, Nome Creek, and Eagle Summit) we also cored trees immediately downslope from tree-

Station name

McKinley NP Nome Circle City Fairbanks

Region

Alaska Range Seward Peninsula White Mts

39.37 37.80 22.07 32.18

Annual precipitation (cm)

20.49 15.24 8.45 15.64

Summer precipitation (cm)

52% 40% 40% 49%

Summer precipitation as % of annual

1.1 –13.8 –27.3 –21.6

Mean January temperature (◦ C)

12.5 10.8 16.3 16.2

Mean July temperature (◦ C)

–3.17 –3.22 –6.19 –2.50

Mean annual temperature (◦ C)

Table II Climatic conditions within three study regions in Alaska. Data for Fairbanks (University Experiment Station), McKinley Park, and Nome are 1961–1990 climate normals computed by the National Climatic Data Center. Data for Circle City are uncorrected for missing values or changes in observation time. Summer is defined as June, July, and August


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line. These trees are referred to as ‘below treeline’ but all are within 75 m elevation of treeline. Two different sampling strategies were used to obtain tree cores. All trees, regardless of age, were cored within 2 to 5 study plots at treeline at Canyon Creek, Monahan Flats, Usibelli, Twelvemile Summit, Nome Creek, and Eagle Summit. Plot size ranged from 400 to 2500 m2 , depending on tree density. Chronologies were developed from the oldest of those trees. We developed additional treeline chronologies at two sites (Bank, Grasshopper Hill) and below treeline at four sites (Usibelli, Twelvemile Summit, Nome Creek, and Eagle Summit) by systematically coring only the oldest trees, which we identified by morphological traits (thick branches, flat top, large trunk). Trees were cored just above the root swell, at an average height of 13.2±18.4 cm (mean ± standard deviation). Each tree was cored repeatedly until a core through (or as near as possible to) the pith was obtained. 2.3. TREE RING METHODS Cores were mounted in wooden strips and sanded to 400 grit with a belt sander. Additional hand-sanding to 600 grit was applied when necessary to resolve latewood boundaries. We measured the width of rings in each core on a sliding bench micrometer (Velmex, Inc.) to a precision of 0.001 mm. Measured rings were crossdated using the computer program Cofecha (Holmes, 2000) and using visual crossdating (Stokes and Smiley, 1968). We subsampled from the complete data set of crossdated trees at each site in order to create data sets that were homogenous with respect to tree age. Only trees >105 years old were used, in order to maintain constant sample depth during the 20th century, and to exclude young trees whose climate response might differ from that of old trees (e.g., Sceicz and MacDonald, 1994). The characteristics of each chronology are summarized in Table III. We were unable to homogenize chronology length at one site (Eagle Summit treeline) where a dearth of old trees prevented us from achieving a chronology length similar to that at other sites. With the exception of that site, however, an analysis of variance indicated that the mean number of rings per tree did not differ significantly (P > 0.05) between chronologies at a site (e.g., treeline and below treeline) or among chronologies from different regions. A chronology was developed for each site. The measured and crossdated ringwidth series from each tree was detrended and standardized. We used conservative detrending methods to remove age-related growth trends while preserving as much low-frequency variation as possible. Each tree’s ring widths were therefore fit with a negative exponential curve, a line of negative slope, or a horizontal line. A dimensionless index of tree growth, from which age-related growth trends have been removed, is produced by dividing the ring width value in year t by the value of the fitted line or curve in year t. This method removes relative differences in growth rate among trees, but preserves the year to year variation in growth that is the focus

Usibelli

Alaska Range

Eagle Summit

White Mountains

Nome Creek

Twelvemile Summit

Bank Grasshopper Hill

Seward Peninsula

Canyon Creek Monahan Flats

Site

Region

Treeline Below treeline Treeline Below treeline Treeline Below treeline

Treeline Treeline

Treeline Below treeline Treeline Treeline

Elevation

113 127 100 119 135 109 131 100

156 ± 23 161 ± 25 118 ± 21 161 ± 38 187 ± 58 181 ± 42 175 ± 25 141 ± 30

164 264 284 258 223 214

203 210

8 19 12 12 23 21

29 24

15 15 35 51

154 131 109 110

189 ± 24 171 ± 29 182 ± 76 144 ± 35 223 213 356 301

Number of trees

Number of rings/tree Mean Min Max

1833 1734 1713 1739 1775 1785

1808 1790

1774 1784 1642 1697

Start year

1997 1997 1997 1997 1997 1999

1997 1999

1997 1997 1997 1997

End year

Table III Description of chronologies. See methods for distinction between ‘treeline’ and ‘below treeline’ chronologies. The number of rings is not a precise estimate of the tree age. Tree age is likely to be two or more decades older than the number of rings


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of this study. The chronology value for each year was obtained by averaging the ring-width index value for each year across all trees. Comparisons of tree growth patterns among sites were made by calculating Pearson’s product-moment correlation coefficients among all pairs of chronologies for the period 1800–present. Long-term growth trends were compared by using linear regression to estimate growth trend over the last 150 years (the time period during which sample depth was greatest). We divided this time period into three periods of equal length: 1851–1900, 1901–1950, and 1951–2000. Because the choice of time periods may influence the results, we repeated the analysis using three 31-year time periods (1901–1932, 1933–1964, 1965–1996) and two 30-year time periods (1940–1969, 1970–1999). The overall pattern of results did not differ substantially for these analyses, so we have presented results only for the three 50-year time periods. 2.4. ANALYSES OF TREE GROWTH AND CLIMATE All tree-ring chronologies were more highly correlated with climate data from the University Experiment Station in Fairbanks (64◦ 51 N, 147◦ 52 W, 146 m a.s.l.) than with other stations, so analyses were conducted using only those climate data. Climate data (total monthly precipitation, monthly mean, minimum, and maximum temperature) were obtained from the National Climatic Data Center for the time period 1923–present. We estimated missing values (which amounted to 1% or fewer of the possible observations) by multiplying the mean value for that month by the normalized value for that month and year at the nearby Fairbanks International Airport station (with which the University Experiment Station is highly correlated for all months). We created a number of seasonalized variables: summer temperature (average of June–August) and precipitation (sum of June–August), fall temperature (average of September–October), winter temperature (average of November–March), winter precipitation (sum of September through April), spring temperature (average of April–May), and growing year temperature and precipitation (average or total of September through August). Analyses were conducted for a 15-month temporal window, from June in the calendar year prior to the growing season to August of the current year. This window was chosen because all of the chronologies had highly significant first-order autocorrelation, but the majority had weak or insignificant second and higher-order autocorrelation (data not shown). Lagged climate responses were therefore assumed to be largely restricted to the previous year. We used Pearson’s product-moment correlation coefficients to describe the relationship between tree growth and seasonal and monthly values of temperature (mean, minimum, and maximum) and precipitation. Correlations between tree growth and seasonalized variables are not presented in the results because they always mirrored the significance of correlations with component months (i.e., tree growth was never


491

correlated with a seasonal variable and not correlated with one of the component monthly values). We used stepwise regression analysis to determine the % variance in tree growth explained by climate. Monthly and seasonal climate data (mean, minimum, maximum temperature, total precipitation) were used as candidate predictor variables and standardized chronology values were used as dependent variables. We used a true stepping procedure in which any variable could be entered or removed at each step in the procedure. We estimated variance explained by the adjusted R 2 (adjusted for loss of degrees of freedom) of the model that explained the most variance in growth. The end date of our chronologies ranged from 1996 to 1999, but we conducted all climate-growth analyses only through 1996 so as to have a comparable data set for all chronologies. We conducted these analyses for the full time period for which climate data were available (1923–1996) and for three 25-year periods which were selected to evenly divide the full period into three equal-sized shorter time periods: 1923–47, 1948–72, and 1973–96. 3. Results 3.1. CORRELATIONS AMONG CHRONOLOGIES Significant correlations among chronologies indicate that year to year variation in growth and longer-term growth trends are held in common by many sites/regions (Table IV). Chronologies were, in general, more highly correlated with chronologies from sites in the same region, but most chronologies were significantly and positively correlated with all other chronologies. The Eagle Summit treeline chronology was the exception to this pattern, showing no correlation with any Seward Peninsula trees, and relatively weak correlations with some of the other chronologies in its region. 3.2. ALASKA RANGE 3.2.1. Treeline Although the three Alaska Range chronologies are significantly positively correlated with one another, low frequency growth trends at Canyon Creek and Monahan Flats in the southern Alaska Range (Figures 2a,b) differ from those at Usibelli in the northern Alaska Range (Figure 2c, solid line). These differences are reflected in higher correlation coefficients between Canyon Creek and Monahan Flats (r = 0.890) than between those sites and Usibelli (r = 0.499 and 0.603, respectively) (Table IV). At Usibelli, in the northern Alaska Range, treeline tree growth exhibited a significant positive trend from 1900 until 1950. Growth remained high (relative to the 1800s) but with no significant trend from 1950 until present (Table V). In the southern Alaska Range, in contrast, growth declined during the last

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Table IV Correlations among chronologies. Values are Pearson’s product-moment correlation coefficients. Numbers in bold type are significant at P < 0.05. Column headings are abbreviated site names, which are shown in full as row headings B

G.H.

U-t

U-b

C.C.

M.F.

E.-t

E.-b

12-t

12-b

N.C.-t

N.C.-b

Seward Peninsula (arctic treeline) Bank Grasshopper Hill

1.000 0.598

1.000

Alaska Range (alpine treeline) Usibelli (treeline) Usibelli (below) Canyon Creek Monahan Flats

0.266

0.577

1.000

0.236

0.464

0.684

1.000

0.161

0.224

0.499

0.540

1.000

0.277

0.39

0.603

0.550

0.890

1.000

White Mountains (alpine treeline) Eagle (treeline) Eagle (below) 12 mile (treeline) 12 mile (below) Nome Creek (treeline) Nome Creek (below)

–0.062

0.022

0.615

0.379

0.618

0.609

1.000

0.336

0.524

0.223

0.235

0.097

0.194

–0.176

1.000

0.324

0.670

0.810

0.617

0.430

0.536

0.517

0.481

1.000

0.256

0.552

0.326

0.413

0.248

0.282

–0.083

0.723

0.616

1.000

0.301

0.493

0.585

0.588

0.499

0.569

0.393

0.523

0.699

0.569

1.000

0.198

0.337

0.365

0.329

0.130

0.199

0.082

0.636

0.468

0.525

0.719

1.000

half of the 1800s and then positive growth trends persisted throughout the 20th century (Table V). The growth of treeline trees at all three sites in the Alaska Range was positively correlated with summer temperature for the full analysis period (1923–1996; Table VI). At the two southern Alaska Range sites (Canyon Creek, Monahan Flats), growth was inversely correlated with autumn temperatures during the full analysis period (Table VI). Fewer significant correlations with temperature occurred during the three shorter analysis periods (Table VI). There were no significant correlations between treeline growth and temperature from 1923 to 1947. Strong inverse correlations with fall temperatures occurred at Canyon Creek and Monahan Flats during the period 1948–1972, mirroring the pattern seen for the full analysis period. Trees at Monahan Flats were also inversely correlated with prior summer temperature


493

Figure 2. Ring-width indices from trees at three study sites in the Alaska Range. Ring-width index is a dimensionless index of tree growth (see Methods). The darker line is a 25-year smoothing spline fit to the data to emphasize low-frequency growth trends. The solid black line (with solid black spline) is the chronology developed from trees at treeline. The dashed black line (with solid grey spline) is the chronology developed from trees just below treeline (see Methods).

during this period. During the most recent analysis period, tree growth at Canyon Creek and Monahan Flats was inversely correlated with summer maximum temperature (r = −0.479 and r = −0.399, respectively; data not shown). Trees at Monahan Flats, despite the significant inverse correlation with maximum summer temperature, were also positively correlated with mean summer temperature between 1973 and 1996. During this same time period Usibelli treeline tree growth was inversely correlated with spring temperatures.

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Table V Growth trends from 1851 to present. Standardized regression coefficient (β) is shown for each site for each of the three time periods. Significant values (P < 0.05) are highlighted in bold type Site

1851–1900

1901–1950

1951–present

Alaska Range Canyon Creek Monahan Flats Usibelli (treeline) Usibelli (below)

0.220 0.067 0.230 0.595

0.521 0.731 0.798 0.145

0.718 0.401 –0.002 0.481

0.230 0.343

0.264 0.577

–0.753 –0.672

–0.608 0.000 –0.417 –0.499 –0.666 0.009

0.743 0.706 0.620 0.691 0.797 –0.011

0.804 –0.904 –0.108 –0.528 –0.494 –0.638

Seward Peninsula Bank Grasshopper Hill White Mountains Eagle Summit (treeline) Eagle Summit (below) Nome Creek (treeline) Nome Creek (below) Twelvemile Summit (treeline) Twelvemile Summit (below)

Treeline tree growth in the Alaska Range was uncorrelated with growing season precipitation except during the middle analysis period (1948–1972), when trees at Monahan Flats and Usibelli were positively correlated with summer precipitation. Canyon Creek trees were inversely correlated with summer precipitation during that time. Significant correlations with non-growing season precipitation also occurred at these sites. Treeline trees at Usibelli were inversely correlated with winter precipitation for the full analysis period (1923–1996). Trees at Canyon Creek and Monahan Flats were positively correlated with fall precipitation between 1948 and 1972 and Canyon Creek trees were also positively correlated with winter precipitation from 1973–1996. The amount of variance in tree growth explained by all tested climate variables (see Methods for complete list) increased over time at Canyon Creek and Monahan Flats (Figure 5a). At Usibelli, in contrast, climate explained less of the variance in growth during the most recent analysis period (1973–1996) than during either of the two previous analysis periods (Figure 5a).

495


Table VI Number of months in each season in which Alaska Range tree growth is significantly correlated with mean Fairbanks temperature. Seasons, for the purposes of this table, are defined as 3-month quarters: prior summer (prior June, July, August), fall (September, October, November), winter (December, January, February), spring (March, April, May), and summer (June, July, August). A maximum of three significant correlations (one for each month) is therefore possible within each season. Significant correlations are those for which P < 0.05. The columns labeled ‘+’ and ‘–’ indicate whether the correlation is direct (+) or inverse (–) Season

Site

Early period 1923–1947) + –

Middle period (1948–1972) + –

Late period (1973–1996) + –

Entire period (1923–1996) + –

Previous summer

Usibelli (treeline) Usibelli (below treeline) Canyon Creek Monahan Flats

0 0 0 0

0 1 0 0

0 0 0 0

0 0 0 2

0 0 0 0

0 1 0 0

0 0 1 0

0 0 0 0

Fall


0 0 0 0

0 0 0 0

0 0 0 0

0 1 1 1

0 0 0 0

0 0 0 0

0 0 0 0

0 1 1 1

Winter


0 0 0 0

0 0 0 0

0 0 0 0

0 0 0 0

0 0 0 0

0 0 0 0

0 0 0 0

0 0 0 0

Spring


0 0 0 0

0 0 0 0

0 0 0 0

1 0 0 0

0 0 0 0

1 1 0 0

0 0 0 0

0 0 0 0

Summer


0 0 0 0

0 0 0 0

0 0 1 0

0 0 0 0

0 0 0 1

0 0 0 0

1 1 2 2

0 0 0 0

3.2.2. Below Treeline Growth of trees growing below treeline at Usibelli in the northern Alaska range (Figure 2a, dashed line) increased from 1851 to 1900, remained steady from 1901 to 1950, and increased again from 1951 until the late 1990s (Table V). Growth was significantly correlated with tree growth at treeline at the same site (r = 0.684, Table IV). The relationship between growth and climate was similar for these trees as for the treeline trees described in the previous section. Growth was inversely correlated with fall temperatures for the full analysis period and between 1948 and

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1972 (Table VI). Growth was inversely correlated with prior summer temperature from 1923 to 1947 and from 1973–1996 (Table VI). Growth was also inversely correlated with winter temperature during this most recent analysis period (Table VI). Below-treeline tree growth at Usibelli was inversely correlated with winter precipitation during the full analysis period and was positively correlated with fall and winter precipitation from 1948–1972 and with spring precipitation from 1923–1947. The amount of variance in the growth of below-treeline trees at Usibelli that was explained by climate was lower from 1973–1996 than during the previous two time periods (Figure 5a). 3.3. SEWARD PENINSULA The two chronologies from the Seward Peninsula (Bank and Grasshopper Hill) have similar low-frequency growth trends (Figures 3a,b). Growth at both sites declined from 1875 to 1900, then increased until the mid-1900s and declined again until the 1990s. Tree growth at Grasshopper Hill had a significant positive trend during the period 1901–1950, and growth at both sites had significant downward trends after 1950 (Table V). There is some suggestion of an increase in growth beginning in the 1990s, but it is unclear whether the upturn indicates a sustained increase in growth. Climate response of Seward Peninsula trees changed over the course of the 20th century. Tree growth at both sites was correlated inversely with some aspect of temperature during the full analysis period (1923–1996; Table VII). Tree growth at both was inversely correlated with spring temperature and at the Bank site, tree growth was also inversely correlated with prior summer temperature. Tree growth was positively correlated with summer (Bank) or winter (Grasshopper Hill) temperature between 1923 and 1947 (Table VII). Growth was uncorrelated with temperature at Grasshopper Hill during the period 1948–1972, and was positively correlated with winter temperature at the Bank site (Table VII). In the most recent time period, tree growth at the Bank site was correlated inversely with previous summer temperature, while growth at Grasshopper Hill was correlated positively to summer temperature (Table VII). Tree growth was not strongly correlated with precipitation at these sites. Inverse correlations between previous summer precipitation occurred at the Bank site during the full analysis period (1923–1996) and occurred at both sites during the most recent short analysis period (1973–1996). Tree growth at these sites was largely uncorrelated with precipitation for the two earlier analysis periods, with the exception of a single significant positive correlation between tree growth at the Bank site and spring precipitation during the earliest analysis period. The amount of variance in growth explained by climate tended to be higher during the three shorter analysis periods than during the full analysis period at both Seward Peninsula sites, a pattern that is consistent with the observation that climate


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Figure 3. Ring-width indices from trees at two study sites on the Seward Peninsula. See Figure 2 caption for explanation of lines.

response has changed over the 20th century (Figure 5b). Although the amount of variance explained by climate differed among analysis periods, there was no consistent directional trend over time. Climate explained the most variance in tree growth during the early analysis periods and the least amount during the middle analysis period. 3.4. WHITE MOUNTAINS 3.4.1. Treeline Intra-regional variability in the growth of treeline trees was more pronounced in the White Mountains than in the other regions. Treeline chronologies exhibited three patterns of long-term growth. The treeline chronology from Eagle Summit (Figure 4a, solid line), like those of the southern Alaska Range, exhibited a significant positive trend in growth throughout the 20th century (Table V). In contrast, at treeline at Twelvemile Summit (Figure 4b, solid line), like on the Seward Peninsula, a significant positive trend in growth in the first half of the 1900s gave way to a significant negative trend in growth after 1950 (Table V). Finally, at treeline at Nome Creek (Figure 4c, solid line), as in the northern Alaska Range, there was a significant positive growth trend from 1901–1950, but no significant trend in growth after that (Table V).

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Table VII Number of months within each 3-month season in which Seward Peninsula tree growth is significantly correlated with Fairbanks temperature. See Table VI for description of column headings Season

Site



Late period (1973–1996) + –


Previous Bank 0 summer Grasshopper Hill 0

0 0

0 0

0 0

0 0

1 0

0 0

1 0

Fall

Bank 0 Grasshopper Hill 0

0 0

0 0

0 0

0 0

0 0

0 0

0 0

Winter


0 0

1 0

0 0

0 0

0 0

0 0

0 0

Spring


0 0

0 0

0 0

0 0

0 0

0 0

1 1

Summer Bank 1 Grasshopper Hill 0

0 0

0 0

0 0

0 1

0 0

0 0

0 0

Climate response in the White Mountains was similarly variable among sites. During the full analysis period, for example, tree growth at treeline at Eagle Summit was positively correlated with temperature, the growth of treeline trees at Nome Creek was inversely correlated with temperature, and the growth of treeline trees at Twelvemile Summit was uncorrelated with temperature (Table VIII). From 1923 to 1947, treeline tree growth was uncorrelated with temperature. From 1948 to 1972, only Eagle Summit trees were correlated with temperature, showing a positive correlation with summer temperature (Table VIII). In the most recent analysis period, inverse correlations with spring temperatures occurred at treeline at Nome Creek and Twelvemile Summit, and inverse correlations with prior summer temperature occurred at Nome Creek. At Eagle Summit, growth was positively correlated with both winter and summer temperature during the most recent analysis period (Table VIII). Unlike Seward Peninsula trees or Alaska Range trees, which tended not to be highly correlated with precipitation, the growth of treeline trees in the White Mountains was positively correlated with spring and/or growing season precipitation. Positive correlations with summer (or previous summer) precipitation occurred at Eagle Summit and Twelvemile Summit during the full analysis period and

499


Table VIII Number of months within each 3-month season in which White Mountains tree growth is significantly correlated with mean Fairbanks temperature. See Table VI for description of column headings Season

Site



Late period (1973–1996) + –


Previous Summer

Eagle Summit (treeline) Eagle summit (below) 12 mile Summit (treeline) 12 mile Summit (below) Nome Creek (treeline) Nome Creek (below)

0 0 0 0 0 0

0 0 0 0 0 0

0 0 0 0 0 0

0 1 0 0 0 1

0 0 0 0 0 0

0 1 0 2 1 2

2 0 0 0 0 0

0 1 0 3 2 3

Fall

Eagle Summit (treeline) Eagle Summit (below) 12 mile Summit (treeline) 12 mile Summit (below) Nome Creek (treeline) Nome Creek (below)

0 0 0 0 0 0

0 0 0 0 0 0

0 0 0 0 0 0

0 0 0 0 0 0

0 0 0 0 0 0

0 0 0 0 0 0

0 0 0 0 1 0

1 0 0 0 0 0

Winter


0 0 0 0 0 0

0 0 0 0 0 0

0 1 0 0 0 0

0 0 0 0 0 0

1 0 0 0 0 0

0 0 0 0 0 0

0 0 0 0 0 0

0 0 0 1 0 0

Spring


0 0 0 0 0 0

0 0 0 0 0 0

0 0 0 0 0 0

0 0 0 0 0 0

0 0 0 0 0 0

0 1 2 1 1 1

0 0 0 0 0 0

0 1 0 2 1 2

Summer


0 0 0 0 0 0

0 0 0 0 0 0

1 0 0 0 0 0

0 0 0 0 0 0

1 0 0 0 0 0

0 0 0 0 0 0

1 0 0 0 0 0

0 2 0 1 0 1

500


Figure 4. Ring-width indices from trees at three study sites in the White Mountains. See Figure 2 caption for explanation of lines.

during the early analysis period. Growth was significantly correlated with spring precipitation at Nome Creek during the most recent analysis period. At treeline at Twelvemile Summit and Nome Creek, the two sites at which growth was inversely related to temperature, the amount of variance in growth explained by climate was lower during the most recent analysis period than during either previous analysis period (Figure 5c). At Eagle Summit, in contrast, the amount of variance in growth was highest during the most recent analysis period (Figure 5c).


501

Figure 5. Variance explained by climate at each site during four analysis periods: 1923–1947, 1948–1972, 1973–1996, and 1923–1996. We used stepwise regression to determine the amount of variance in tree growth explained by mean, minimum, maximum temperatures and total precipitation during each analysis period. Values are R 2 (multiplied by 100), adjusted for loss of degrees of freedom. All regression models for which an R 2 value is shown were significant at P < 0.05.

502


3.4.2. Below Treeline Growth of below-treeline trees declined in the last half of the 20th century at all three sites in the White Mountains (Table V, Figures 4a–c, dashed lines). At Nome Creek and Eagle Summit the growth declines followed a 50-year period in which there was a significant positive trend in growth. The late-20th century growth decline was absent from adjacent treeline trees at these same two sites. Inverse correlations with temperature predominated in the below-treeline trees (Table VIII). Tree growth at all three sites was inversely correlated with prior and current summer temperatures for the full analysis period and was inversely correlated with prior summer temperature and spring temperature between 1973 and 1996 (Table VIII). The growth of below treeline trees at Eagle Summit and Nome Creek was inversely correlated with prior summer temperature between 1948 and 1972 (Table VIII). The sole positive correlation between growth of below-treeline trees and temperature occurred at Eagle Summit between 1948 and 1972 (Table VIII). The growth of below-treeline trees at Eagle Summit and Twelvemile Summit was positively correlated with summer precipitation during the full analysis period. Positive correlations with summer precipitation occurred during the early analysis period at Eagle Summit. Scattered significant correlations with spring precipitation also occurred at Twelvemile Summit and Nome Creek. At two of the three sites, the amount of variance in below-treeline tree growth explained by climate was lowest during the most recent analysis period (Figure 5c). At the third site, Twelvemile Summit, the amount of variance explained by climate was lowest during the early analysis period and intermediate during the most recent analysis period.

4. Discussion Patterns of tree growth and the relationship between growth and climate varied among regions, among elevations, and over time. Although our analytic methods were not designed to detect the precise timing of changes in tree growth trends, they do allow us to compare the overall trend among different time periods. Trends in tree growth during the early half of the 20th century (1901–1950) were highly similar among sites and regions. During that period, tree growth increased at 9 of 12 sites. Of the other three, one site (Bank) had a positive trend that was only marginally insignificant (P = 0.064). The rapid warming observed throughout the Northern Hemisphere during this period (Houghton et al., 1996; Overpeck et al., 1997; Mann et al., 1998) therefore seems to have been favorable for the growth of trees at the limit of the boreal forest. Reports of increasing tree growth during this time are, in fact, widespread at treeline in Alaska, Siberia, and the Yukon Territory (D’Arrigo and Jacoby, 1993; Jacoby and D’Arrigo, 1995; Jacoby et al., 2000).


503

Continued warming after 1950 (Chapman and Walsh, 1993; Houghton et al., 1996; Overpeck et al., 1997; Serreze et al., 2000) was associated with two distinct effects on tree growth. In the Alaska Range, tree growth increased in the latter half of the 20th century. Three of four chronologies developed in the Alaska Range had significant positive trends during this period; the fourth chronology (Usibelli treeline) had no significant trend. On the Seward Peninsula, growth at both sites declined after 1950. In the White Mountains, a range of responses were seen. Most chronologies (4 of 6) experienced significant declines in growth during the period from 1951 to present. In contrast, the growth of treeline trees at Eagle Summit continued to increase during this period. This site was poorly correlated with other nearby sites overall. Mean tree age at this site was significantly younger than at other sites, and this may be an indication that tree age or forest history can influence tree climate response. The role of these factors certainly warrants further study. In addition to these regional differences in late 20th century growth trends, elevation relative to treeline had an apparent effect on the prevalence of growth declines after 1950. Trees growing below treeline were more likely than those growing at treeline to experience declining growth and inverse correlations with temperature after 1950. In the White Mountains, warming after 1950 was associated with decreased growth at all three below-treeline sites. In contrast, warming was associated with decreased growth at only one of the three corresponding treeline sites. Sites below treeline are likely, by virtue of their lower elevation, to be warmer than sites at treeline. The greater prevalence of growth declines in chronologies from trees below treeline compared with treeline trees at the same sites therefore suggests that slight differences in geography are sufficient to produce large differences in climate response. This difference between trees at treeline and those just below treeline therefore mirrors on a fine scale the broad-scale pattern identified in previous studies, in which trees at warmer sites within the center of the boreal forest have responded negatively to increasing warmth (Barber et al., 2000) while those at cooler sites at the margin of the boreal forest have responded more positively (e.g., Jacoby et al., 2000). The widespread occurrence of late-20th century growth declines at our below treeline sites, which are in close proximity to treeline, further suggests that negative responses to increasing warmth like those identified by Barber et al. (2000) may be widespread. In addition to varying among regions and elevations, the relationship between tree growth and climate also varied over time during the 20th century; that temporal variation is consistent with the hypothesis that increasing warmth has ceased to benefit treeline trees on the Seward Peninsula and in the White Mountains. Trees at both sites on the Seward Peninsula responded positively to temperature in the early decades of the 1900s. In recent decades, however, the two sites differed in their response to climate (despite sharing similar growth trends). Growth was inversely correlated with the previous summer’s temperatures at one site (Bank) and positively correlated with the current summer’s temperature at the other (Grasshopper Hill). This difference in climate response suggests that temperature stress alone

504


can not explain the observed growth declines at both sites on the Seward Peninsula. In the White Mountains, inverse responses to summer temperature were most common in the period from 1973 to 1996. In general, inverse correlations with mean temperature were more common during the most recent period analyzed (1973–1996) than during earlier time periods. At sites in the White Mountains and on the Seward Peninsula, inverse correlations with temperature were absent or rare in the early and middle analysis periods. This pattern does not simply reflect an increase in the amount of variance explained by climate (Table IX) and thus seems to be a real change in climate response. This supports the conclusion that increasing warmth has ceased to benefit some treeline trees. The results of this study thus suggest that, despite abundant evidence that temperature is an important limit on tree growth and reproduction at treeline (e.g., Griggs, 1934; Tranquillini, 1979; Black and Bliss, 1980; Wardle, 1981; Kullman, 1987; Lloyd, 1997; Lloyd and Graumlich, 1997), warming in the late 20th century has been associated with a reduction in the growth of treeline trees in two of the three regions studied. This reduction in growth may represent the effects of non-climatic stressors, or may be a consequence of changing climate. Briffa et al. (1998a,b) attribute declining sensitivity of Northern Hemisphere tree-growth to climate in the latter part of the 20th century to the cumulative impacts of non-climatic stressors. They propose that a range of factors, including CO2 fertilization, N deposition, ozone depletion, and other forms of air pollution might affect tree growth and thus alter the response of trees to climate. These non-climatic stressors leave a particular ‘fingerprint’ in the data: as the non-climatic stressors have become increasingly important since the mid-1900s, the correlation between tree growth and climate, or the amount of variance in tree growth explained by climate, has declined. In this study, a non-climatic explanation for post-1950 growth declines would be seen in a consistent reduction in the amount of variance explained by climate. Three of the seven chronologies in which tree growth declines after 1950 do, indeed, conform to this prediction. However, four of the seven chronologies contradict this prediction. Non-climatic stressors are therefore not a sufficient explanation for the patterns described here. Vaganov et al. (1999) have suggested that declining tree growth at treeline in Eurasia may reflect the effects of precipitation on growing season length. In particular, they propose that increases in winter precipitation may lead to a delay in the onset of the growing season and thus lead to a reduction in tree growth despite increases in average air temperature. Although a similar mechanism is certainly plausible in Alaska, two features of our data argue against this hypothesis. First, the strong inverse correlations with temperature, especially in recent decades, and the absence of widespread correlations with winter/spring precipitation suggests strongly that trees are responding to temperature and not to precipitation. Second, Vaganov et al. (1999) suggest that one effect of the inferred change in snowmelt was a decline in the strength of tree growth response to temperature. We


505

found changes in climate response, but no evidence for a systematic decline in the strength of tree growth response to climate. The spatial pattern of late 20th century reductions in tree growth, in which declining growth is less prevalent in the Alaska Range (particularly the southern Alaska Range) than in the White Mountains or the Seward Peninsula and less prevalent in treeline trees than in below-treeline trees, suggests a second climatic explanation for the unexpected finding that warmth has ceased to benefit trees growing at the temperature-determined limit of their distribution. Because evapotranspiration rates are positively related to temperature, increased temperatures are likely to be associated with increased rates of water loss. If temperature warms without a compensating increase in precipitation (which seems to be the case here; Serreze et al., 2000), plants may become increasingly water-stressed. Rising temperatures are thus likely to lead to decreases in growth rate in water-limited ecosystems. One would expect that these decreases in growth should appear first in the driest and warmest sites, and should spread, as temperatures continue to rise, to moister and cooler sites. Barber et al. (2000) present data that suggest that rising temperatures have contributed to tree drought stress in low-elevation sites since at least 1906. As temperatures have continued to rise since that time, temperature-induced drought stress may have spread to cooler and moister sites like those described here. Two features of the spatial pattern of growth declines in our study sites are consistent with this hypothesis. First, in the White Mountains growth declines were observed more frequently in trees below treeline than in trees at treeline. The elevational difference between these two groups of trees is slight: the difference in growth across such short gradients is therefore surprising and suggests that there are steep thresholds of climate sensitivity at treeline. The higher prevalence of growth declines in the presumably warmer below-treeline sites may reflect the direct effects of warmer growing season temperatures. Temperature-induced water stress may also be the indirect consequence of differences in competitive environment between forests at the two elevations. Tree stand density is higher in forests below treeline than in similar forests at treeline. Competition for belowground resources (like water) may therefore be more intense, leaving trees more vulnerable to water stress. This hypothesis awaits an experimental test. Second, the differences among regions in the prevalence of late-20th century growth declines are also consistent with the water-stress hypothesis. Climate data summarized in Table II indicate that our study sites differ climatically. Alaska Range and Seward Peninsula sites experience cooler growing seasons than the White Mountain sites, and the Seward Peninsula and White Mountain sites receive lower annual and summer precipitation than the Alaska Range sites. Alaska Range trees would thus be least likely to have begun to experience temperature-induced water stress, and trees in the White Mountains, the warmest and driest region, would be most likely. And, indeed, our results are consistent with this prediction: inverse correlations with temperature were most prevalent, in terms of the number

506


of sites affected, in the White Mountains and were least prevalent in the Alaska Range sites. Our results thus indicate that even at the extreme elevational and longitudinal limits of the boreal forest, increasing warmth has been associated with declining tree growth. The spatial patterns in our data suggest that temperature-induced drought stress may explain the negative response to warming temperatures, but the mechanism underlying that response remains unknown. We must, for example, consider the possibility that the effects of temperature are indirect (e.g., Chapin, 1983; Jacoby and D’Arrigo, 1995). Warming can lead to an increase in shrubbiness (Chapin et al., 1995), warming of the permafrost (Osterkamp and Romanovsky, 1999), and changes in nutrient cycling rates (e.g., Nadelhoffer et al., 1992). All of these factors might be expected to affect tree growth. For example, if shrub productivity increases as temperature warms, belowground competition for nutrients and water between trees and shrubs might become more intense. Although competition between mature trees and shrubs for aboveground resources (light) is likely to be highly asymmetric, belowground competition may be much more size-symmetric (Weiner, 1990; Casper and Jackson, 1997). Shrubs growing with the trees at the forest-tundra boundary might therefore be able to compete successfully for belowground resources with the taller and larger trees. Competition with shrubs might result in increased tree water stress or in nutrient limitation, both of which could cause decreases in growth and changes in response to climate. Analysis of a time series of aerial photographs has, in fact, suggested that shrub biomass may have increased in the late 20th century at our Seward Peninsula sites (Silapaswan, 2000). Indirect effects like these could be responsible for the observed pattern of growth declines, as competition for water and other resources would be expected to have a greater effect on growth in the drier study sites. Manipulative field experiments will be necessary to distinguish between the direct and indirect effects of climate change on trees in these sites. In conclusion, our analyses of the growth of white spruce trees at and near treeline in three regions in Alaska indicate that continued warming can not be expected to lead to increases in tree growth in the colder areas of the boreal forest. Widespread growth increases were observed during the early half of the 20th century, suggesting that post-Little Ice Age warming did initially lead to widespread increases in tree growth. After 1950, however, the growth of trees in two of our study regions began to decline, despite continued warming. Woody biomass in the boreal forest represents a substantial terrestrial C sink (Post et al., 1982; Dixon et al., 1994; White et al., 2000). Temperature-induced increases in tree growth and forest productivity therefore have the potential to act as a negative feedback on climate. Our results indicate that even at the coolest margins of the boreal forest, however, assumptions that rising temperature have stimulated or will continue to stimulate tree growth are unwarranted.


507

Acknowledgements This research was supported by the National Science Foundation (OPP-9731717), by the Bonanza Creek LTER at the University of Alaska Fairbanks, and by Middlebury College’s Faculty Professional Development Fund. Sarah Childs created the map shown in Figure 1. We thank Larry Hinzman and Kenji Yoshikawa for providing unpublished precipitation data from Caribou Peak. The manuscript was greatly improved by the comments of three anonymous reviewers.

References Barber, V., Juday, G. P., and Finney, B.: 2000, ‘Reduced Growth of Alaskan White Spruce in the Twentieth Century from Temperature-Induced Drought Stress’, Nature 405, 668–673. Black, R. A. and Bliss, L. C.: 1980, ‘Reproductive Ecology of Picea mariana (Mill.) B.S.P. at Tree Line near Inuvik, Northwest Territories, Canada’, Ecol. Monogr. 50, 331–354. Bonan, G., Pollard, D., and Thompson, S. L.: 1992, ‘Effects of Boreal Forest Vegetation on Global Climate’, Nature 359, 716–718. Briffa, K. F., Schweingruber, F., Jones, P., Osborn, T., Harris, I., Shiyatov, S., Vaganov, E., and Grudd, H.: 1998a, ‘Trees Tell of Past Climates: But Are They Speaking Less Clearly Today?’ Phil. Trans. Roy. Soc. London B 353, 65–73. Briffa, K., Schweingruber, F., Jones, P., Osborn, T., Shiyatov, S., and Vaganov, E.: 1998b, ‘Reduced Sensitivity of Recent Tree-Growth to Temperature at High Northern Latitudes’, Nature 391, 678– 682. Casper, B. B. and Jackson, R. B.: 1997, ‘Plant Competition Underground’, Ann. Rev. Ecol. Syst. 28, 545–570. Chapin, F. S.: 1983, ‘Direct and Indirect Effects of Temperature on Arctic Plants’, Polar Biol. 2, 47–52. Chapin, F. S., Eugster, W., McFadden, J. P., Lynch, A. H., and Walker, D. A.: 2000, ‘Summer Differences among Arctic Ecosystems in Regional Climate Forcing’, J. Climate 13, 2002–2010. Chapin, F. S., Shaver, G., Giblin, A., Nadelhoffer, K., and Laundre, J.: 1995, ‘Responses of Arctic Tundra to Experimental and Observed Changes in Climate’, Ecology 76, 694–711. Chapman, W. L. and Walsh, J. E.: 1993, ‘Recent Variations of Sea Ice and Air Temperature in High Latitudes’, Bull. Amer. Meteorol. Soc. 74, 33–47. D’Arrigo, R. D. and Jacoby, G. C.: 1993, ‘Secular Trends in High Northern Latitude Temperature Reconstructions Based on Tree Rings’, Clim. Change 25, 163–177. Dixon, R. K., Brown, S., Houghton, R. A., Solomon, A. M., Trexler, M. C., and Wisniewski, J.: 1994, ‘Carbon Pools and Flux of Global Forest Ecosystems’, Science 263, 185–189. Fyfe, J. C., Boer, G. J., and Flato, G. M.: 1999, ‘The Arctic and Antarctic Oscillations and their Projected Changes under Global Warming’, Geophys. Res. Lett. 26, 1601–1604. Garfinkel, H. L. and Brubaker, L. B.: 1980, ‘Modern Climate-Tree Growth Relationships and Climatic Reconstruction in Sub-Arctic Alaska’, Nature 286, 872–874. Gillett, N. P., Hegerl, G. C., Allen, M., Stott, P. A.: 2000, ‘Implications of Changes in the Northern Hemisphere Circulation for the Detection of Anthropogenic Climate Change’, Geophys. Res. Lett. 27, 993–996. Griggs, R. F.: 1934, ‘The Edge of the Forest in Alaska and the Reasons for its Position’, Ecology 15, 80–96. Gulledge, J. and Schimel, J. P.: 2000, ‘Controls on Soil Carbon Dioxide and Methane Fluxes in a Variety of Taiga Forest Stands in Interior Alaska’, Ecosystems 3, 269–282.

508


Hammond, T. and Yarie, J.: 1996, ‘Spatial Prediction of Climatic State Factor Regions in Alaska’, Ecoscience 3, 490–501. Holmes, R. L.: 2000, The Dendrochronological Program Library, Laboratory of Tree-Ring Research, The University of Arizona, Tucson, AZ. Houghton, J., Meira Filho, L., Callander, B., Harris, N., Kattenberg, A., and Maskell, K. (eds.): 1996, Climate Change 1995: The Science of Climate Change, Cambridge University Press. Jacoby, G. C., Cook, E. R., and Ulan, L. D.: 1985, ‘Reconstructed Summer Degree Days in Central Alaska and Northwestern Canada since 1524’, Quat. Res. 23, 18–26. Jacoby, G. and D’Arrigo, R.: 1995, ‘Tree Ring Width and Density Evidence of Climatic and Potential Forest Change in Alaska’, Global Biogeochem. Cycles 9, 227–234. Jacoby, G. C. and D’Arrigo, R.: 1997, ‘Tree Rings, Carbon Dioxide, and Climatic Change’, Proc. Nat. Acad. Sci. 94, 8350–8353. Jacoby, G. C., D’Arrigo, R., and Davaajamts, T.: 1996, ‘Mongolian Tree Rings and 20th-Century Warming’, Science 273, 771–773. Jacoby, G., Lovelius, N., Shumilov, O., Raspopov, O., Kabainov, J., and Frank, D.: 2000: ‘Long-Term Temperature Trends and Tree Growth in the Taymir Region of Northern Siberia’, Quat. Res. 53, 312–318. Jonasson, S., Michelsen, A., Schmidt, I., and Nielsen, E. V.: 1999, ‘Responses in Microbes and Plants to Changed Temperature, Nutrient, and Light Regimes in the Arctic’, Ecology 80, 1828–1843. Kattsov, V. M. and Walsh, J. E.: 2000, ‘Twentieth Century Trends of Arctic Precipitation from Observational Data and a Climate Model Simulation’, J. Climate 13, 1362–1370. Keeling, C., Chin, J., and Whorf, T.: 1996, ‘Increased Activity of Northern Vegetation Inferred from Atmospheric CO2 Measurements’, Nature 382, 146–149. Kullman, L.: 1987, ‘Long-Term Dynamics of High-Altitude Populations of Pinus sylvestris in the Swedish Scandes’, J. Biogeogr. 14, 1–8. Kullman, L.: 1995, ‘Holocene Tree-Limit and Climate History from the Scandes Mountains, Sweden’, Ecology 76, 2490–2502. Lloyd, A. H.: 1997, ‘Response of Treeline Populations of Foxtail Pine (Pinus balfouriana) to Climate Variation over the last 1,000 Years’, Can. J. Forest Res. 27, 936–942. Lloyd, A. H. and Graumlich, L. J.: 1997, ‘A 3,500 Year Record of Changes in the Structure and Distribution of Forests at Treeline in the Sierra Nevada, California, U.S.A’, Ecology 78, 1199– 1210. Lynch, A. and Wu, W.: 2000, ‘Impacts of Fire and Warming on Ecosystem Uptake in the Boreal Forest’, J. Climate 13, 2334–2338. MacDonald, G. M., Edwards, T. W. D., Moser, K. A., Pienitz, R., and Smol, J. P.: 1993, ‘Rapid Response of Treeline Vegetation and Lakes to Past Climate Warming’, Nature 361, 243–246. MacDonald, G. M., Velichko, A. A., Kremenetski, C. V., Borisova, O. K., Goleva, A. A., Andreev, A. A., Cwynar, L. C., Riding, R. T., Forman, S. L., Edwards, T. W. D., Aravena, R., Hammarlund, D., Szeicz, J. M., and Gattaulin, V. N.: 2000, ‘Holocene Treeline History and Climate Change across Northern Eurasia’, Quat. Res. 53, 302–311. Mann, M. E., Bradley, R. S., and Hughes, M. K.: 1998, ‘Global-Scale Temperature Patterns and Climate Forcing over the Past Six Centuries’, Nature 392, 779–787. Myneni, R., Keeling, C., Tucker, C., Asrar, G., and Nemani, R.: 1997, ‘Increased Plant Growth in the Northern High Latitudes from 1981 to 1991’, Nature 386, 698–702. Nadelhoffer, K. J., Giblin, A. E., Shaver, G. R., and Linkins, A. E.: 1992, ‘Microbial Processes and Plant Nutrient Availability in Arctic Soils’, in Chapin, F. S., Jefferies, R. L., Reynolds, J. F., Shaver, G. R., and Svoboda, J. (eds.), Arctic Ecosystems in a Changing Climate: An Ecophysiological Perspective, Academic Press, San Diego, pp. 281–300. Oechel, W., Hastings, S., Vourlitis, G., Jenkins, M., Riechers, G., and Grulke, N.: 1993, ‘Recent Change of Arctic Tundra Ecosystems from Net Carbon Dioxide Sink to a Source’, Nature 361, 520–523.


509

Osterkamp, T. E. and Romanovsky, V. E.: 1999, ‘Evidence for Warming and Thawing of Discontinuous Permafrost in Alaska’, Permafrost Periglacial Processes 10, 17–37. Overpeck, J., Hughen, K., Hardy, D., Bradley, R., Case, R., Douglas, M., Finney, B., Gajewski, K., Jacoby, G., Jennings, A., Lamoureux, S., Lasca, A., McDonald, G., Moore, J., Retelle, M., Smith, S., Wolfe, A., and Zielinski, G.: 1997, ‘Arctic Environmental Change of the Last Four Centuries’, Science 278, 1251–1256. Post, W. M., Emanuel, W. R., Zinke, P. J., Stangenberger, A. G.: 1982, ‘Soil Carbon Pools and World Life Zones’, Nature 298, 156–159. Serreze, M. C., Walsh, J., Chapin, F. S., Osterkamp, T., Dyurgerov, M., Romanovsky, V., Oechel, W., Morison, J., Zhang, T., and Barry, R.: 2000, ‘Observational Evidence of Recent Change in the Northern High-Latitude Environment’, Clim. Change 46, 159–207. Shindell, D. T., Miller, R. L., Schmidt, G. A., and Pandolfo, L.: 1999, ‘Simulation of Recent Northern Winter Climate Trends by Greenhouse-Gas Forcing’, Nature 399, 452–455. Silapaswan, C.: 2000, Land Cover Change on the Seward Peninsula: The Use of Remote Sensing to Evaluate the Potential Influences of Climate Change on Historical Vegetation Dynamics, M.S. Thesis, University of Alaska, Fairbanks, Alaska. Stokes, M. A. and Smiley, T. L.: 1968, An Introduction to Tree-Ring Dating, University of Chicago Press, Chicago, Illinois. Stone, R. S.: 1997, ‘Variations in Western Arctic Temperatures in Response to Cloud Radiative and Synoptic-Scale Influences’, J. Geophys. Res. 102, 21769–21776. Suarez, F., Binkley, D., Kaye, M. W., and Stottlemyer, R.: 1999, ‘Expansion of Forest Stands into Tundra in the Noatak National Preserve, Northwest Alaska’, Ecoscience 6, 465–470. Szeicz, J. M. and MacDonald, G. M.: 1994, ‘Age-Dependent Tree-Ring Growth Responses of Subarctic White Spruce to Climate’, Can. J. Forest Res. 24, 120–132. Thompson, D. W. J. and Wallace, J. M.: 1998, ‘The Arctic Oscillation Signature in the Wintertime Geopotential Height and Temperature Fields’, Geophys. Res. Lett. 25, 1297–1300. Thompson, D. W. J., Wallace, J. M., and Hegerl, G. C.: 2000, ‘Annular Modes in the Extratropical Circulation. Part II: Trends’, J. Climate 13, 1018–1036. Tranquillini, W.: 1979, The Physiological Ecology of the Alpine Timberline, Springer-Verlag, New York. Weiner, J.: 1990, ‘Asymmetric Competition in Plant Populations’, Trends Ecol. Evol. 5, 360–364. Wadhams, P.: 1995, ‘Arctic Sea Ice Extent and Thickness’, Phil. Trans. Roy. Soc. London A 352, 301–319. Wardle, P.: 1981, ‘Is the Alpine Timberline Set by Physiological Tolerance, Reproductive Capacity, or Biological Interactions?’ Proceedings of the Ecological Society of Australia 11, 53–66. White, A., Cannell, G., and Friend, A.: 2000, ‘The High-Latitude Terrestrial Carbon Sink: A Model Analysis’, Global Change Biol. 6, 227–245. (Received 18 January 2001; in revised form 12 July 2001)