Anomalous 20th century tree growth, Mackenzie Delta, Northwest Territories, Canada

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1 Click Here for Full Article GEOPHYSICAL RESEARCH LETTERS, VOL. 34, L05714, doi: /2006gl029139, 2007 Anomalous 20th century tree growth, Mackenzie Delta, Northwest Territories, Canada Michael F. J. Pisaric, 1 Sean K. Carey, 1 Steven V. Kokelj, 2 and Donald Youngblut 1 Received 19 December 2006; revised 1 February 2007; accepted 7 February 2007; published 13 March [1] A number of contemporary dendroecological studies from northwestern North America have highlighted a divergence in growth trends during recent decades. These studies suggest that warmer temperatures are now exceeding the physiological threshold of some northern tree species, or perhaps are contributing to increased drought stress as current precipitation is insufficient to offset increasing water demands under warmer conditions. Here we document additional evidence of these diverging growth trends from the Mackenzie Delta region of Canada and show they are anomalous to the twentieth century. Using wavelet coherency analyses we demonstrate that our white spruce tree ring chronologies exhibit little divergence from one another during the past four centuries, but coherency of the data-sets rapidly break down after the 1930s. Citation: Pisaric, M. F. J., S. K. Carey, S. V. Kokelj, and D. Youngblut (2007), Anomalous 20th century tree growth, Mackenzie Delta, Northwest Territories, Canada, Geophys. Res. Lett., 34, L05714, doi: /2006gl Introduction [2] Dendroclimatological studies carried out in northern Canada and Alaska during the s [Garfinkel and Brubaker, 1980; Jacoby and Cook, 1981; D Arrigo and Jacoby, 1992; Szeicz and MacDonald, 1994] identified predominantly positive relations between ring-width and growing season temperatures. More recent studies from some of these same areas indicate that warming during the past few decades may be causing divergent growth responses amongst white spruce (Picea glauca [Moench] Voss) as tree growth within sample populations has become inversely correlated with summer temperature [Jacoby and D Arrigo, 1995; Barber et al., 2000; Lloyd and Fastie, 2002; Davi et al., 2003; D Arrigo et al., 2004; Wilmking et al., 2004, 2005; Driscoll et al., 2005]. Recent studies have termed these diverging growth trends positive responders (POS) and negative responders (NEG) [Wilmking et al., 2004; Driscoll et al., 2005]. While diverging growth trends are apparent at some sites in northwestern North America, it is important to note that a vast portion of northern forests have been greening in response to recent warming trends [Szeicz and MacDonald, 1995; Myneni et al., 1997; Zhou et al., 2001; D Arrigo et al., 2006]. This partial browning 1 Department of Geography and Environmental Studies, Carleton University, Ottawa, Ontario, Canada. 2 Water Resources Division, Department of Indian Affairs and Northern Development, Yellowknife, Northwest Territories, Canada. Copyright 2007 by the American Geophysical Union /07/2006GL of northern forests suggests that their ability to act as carbon sinks may be more complicated than previously thought. In this paper we utilize wavelet coherency analysis [Grinsted et al., 2004] to investigate diverging growth trends in several populations of white spruce growing in the Mackenzie Delta region of northwestern Canada. 2. Methods [3] Tree ring chronologies were developed from nine sites in the eastern Mackenzie Delta (Figure 1). All sites were sampled in the late summer of 2004 along East Channel of the Mackenzie Delta (except Campbell Dolomite Upland (CDU); J. M. Szeicz et al. (contributors) International Tree Ring Databank CANA138.RWL; available at During preliminary chronology development it was recognized that individuals within a site have very different growth trends in the 20th century. Some series show a decreasing growth trend after the late 1930s that continues until approximately the 1980s and later in some instances. The apparent divergence of growth in the Mackenzie Delta is earlier than reported in previous studies and may reflect the more northern location compared to the other sites. A smaller subset of individuals follows a general increasing growth trend throughout the 20th century. These observations prompted us to separate the 654 individual ring-width series into two groups (positive and negative responders) defined by their 20th century growth. Two regional ringwidth chronologies were then developed for these respective groups. Wavelet and wavelet squared coherency analyses were all carried out using the standard chronology produced using Program ARSTAN [Cook, 1985]. [4] Annual tree growth for the Mackenzie Delta chronologies was compared with temperature and precipitation records from the Inuvik airport (available from Environment Canada at The temperature record covers the period The precipitation record extends from 1957 to 2003, but missing data limits the useable portion to During the period , growing season temperature (June Sept) increased from 9.3 to 10.5 C while growing season precipitation remained effectively unchanged. To assess the relations between climate and annual tree growth in the Mackenzie Delta, Pearson s correlation coefficients were computed between the standardized tree-ring data and monthly/ seasonal values of total precipitation and mean temperature. [5] To explore relations among the tree-ring time-series, wavelet analyses were performed [Farge, 1992; Foufoula- Georgiou and Kumar, 1995; Torrence and Compo, 1998]. We applied the commonly used Morlet wavelet after Grinsted et al. [2004, equation 1] with w = 6 used to L of5

2 Figure 1. Map of the Mackenzie Delta region in Northwest Territories, Canada. Numbers correspond to the nine tree ring sites: (1) Campbell Dolomite Uplands, (2) Timber, (3) Deadwood, (4) Blueberry, (5) MP, (6) M, (7) Fallen Tree, (8) MS, and (9) Hidden Lake. YEV, Inuvik Airport. provide a balance between time and frequency localization and to satisfy the admissibility condition [Farge, 1992]. Following this, the wavelet squared coherency was computed to compare responder and negative responder chronologies [Grinsted et al., 2004, equation 8]. The significance test for the local wavelet spectrum, crosswavelet spectrum and squared wavelet coherency is performed against Gaussian red noise using Monte Carlo methods described by Torrence and Compo [1998] and Grinsted et al. [2004]. 3. Results [6] The nine ring-width chronologies developed for the eastern Mackenzie Delta show common patterns of variability as indicated by the high mean correlation coefficient of (range = to 0.934; common period, ). These relations between sites are maintained when individual series from each site are separated into POS and NEG site chronologies [POS (mean = 0.614; min = 0.406; max = 0.893; ), NEG (mean = 0.618; min = 0.424; max = 0.897; ); all significant at p < 0.05]. Based on the strong common signal between sites we constructed two regional chronologies by examining linear growth trends during the 20th century and separating individual series from each site into responders and negative responders. The two chronologies are very similar to one another during much of the last 400 years (Figures 2a and 2b). In the 20th century, agreement between the two chronologies diminishes. The POS chronology follows an increasing growth trend in association with warming temperatures, while the NEG chronology shows an increasing growth trend only up to the late 1930s. This is followed by a decline in growth that persists to the year of sampling for some individual series. These trends are not likely spurious; the high sample depth suggests the differences in growth response reflect sub-population differences and not the effect of a few individuals (Figure 2c). On average, 25% of the series from an individual site exhibited the POS 20th century growth trend. However, the proportion of POS series at a given site varied from 0 to 51% of the trees sampled. There appears to be no age-related bias in the proportion of POS and NEG series, as trees of various age classes comprise these two groups. [7] The POS and NEG chronologies exhibit very different responses to climate in the Inuvik area (Table 1). For the POS responders, significant and positive relations occur with June temperature during the current growing season; no significant relations with precipitation are noted. The NEG responders exhibit significant, inverse relations with summer season temperatures during both the previous and current growing seasons. The relations during the previous year are particularly strong, especially for June and July. The NEG chronology also shows a strong positive relation with April precipitation (not shown). Similar trends between temperature and our POS tree-ring chronology exist with Northern Hemisphere summer (June September) temperature anomalies [Brohan et al., 2006] (Figure 2b). The low frequency trends in the POS tree ring record match the Northern Hemisphere temperature anomalies remarkably well, especially from the early 1900s onwards. [8] To further explore the relation between POS and NEG chronologies, the local wavelet spectra of the time series were computed (Figures 3a and 3b). The chronologies show high variances (Figures 3a and 3b): (1) intermittently at short temporal scales (1 4 years) from 1600 reflecting interannual variation in tree growth; (2) at approximately 16-year scale centered around 1665 and 1775 reflecting longer cycles intermittent in the time series; and (3) at longer time scales from approximately 1750 in both series. Some of these variations are statistically different from that of red noise, but the area of each significant variation (contour with solid black line) is less than 5% of the total area, or outside the cone of influence (COI), and therefore may be spurious. Outside of the COI, the spectra must be viewed with caution due to edge effects [Torrence and Compo, 1998], yet the sharp nature of the Morlet wavelet warrants examination of these regions. For the NEG chronology, a low frequency (between 64 and 128 year scale) high power variance begins in approximately 1750 and continues to the end of the data set (Figure 3b). This reflects the gradual oscillations of the NEG chronology observed in Figure 2b during this period. Conversely, outside the COI, the POS chronology exhibits high variance at the 16 to 64 year scale beginning in approximately 1900, representing 2of5

3 Figure 2. (a) First differences between the POS and NEG chronologies. Solid horizontal line denotes mean for the period , with 1s and 2s as dotted and dashed lines. (b) POS and NEG chronologies plotted with Northern Hemisphere summer (June September) temperature anomalies ( ) (Climate Research Unit, University of East Anglia CRUTEM3, available at [Brohan et al., 2006]. Correlation between the tree ring and temperature series [POS chronology (r = 0.650, p <.001); NEG chronology (r = 0.002, p = 0.061)]. Correlation coefficients indicated on the figure are for the two tree ring chronologies, (p <.001) and (p = 0.268). (c) Chronology sample depth through time. the rise, fall and rapid rise beginning at approximately 1850 in this series (Figure 2b). [9] The wavelet coherency was calculated for the two series as it is analogous to a traditional correlation coefficient and Figure 3c can be viewed as a localized correlation coefficient in time frequency space [Grinsted et al., 2004]. Significant coherency exists between the POS and NEG chronologies at multiple temporal scales from However, beginning at approximately 1800, the coherence at longer time scales begins to slowly diminish. From the late 1930s coherency of the data-sets rapidly breaks down from the 32-year scale to the 4-year scale by 2000, although a 16-year cycle reappears in Interestingly, both data sets have similar high-frequency responses (<4-year cycle) from 1600, indicating that despite low-frequency divergence in the time series, high frequency variability remains similar throughout. Further, the coherency of the two data sets are typically positive (arrows in the right direction) indicating that changes in ring width index are generally in phase. 4. Discussion [10] Analysis of white spruce tree ring series in the Mackenzie Delta region of northwestern Canada indicates two distinct growth patterns during the 20th century. Approximately 25% of the trees exhibit significant and positive Table 1. Significant Pearson s Correlation Coefficients for Tree Ring and Temperature Data From the Inuvik Airport ( ) a Year Preceding Growth May June July Aug. Sep. Oct. Nov. Dec. POS n.s. n.s. n.s. n.s. n.s. n.s. n.s..274 NEG n.s n.s..224 n.s. n.s. n.s. Year Preceding Growth Jan. Feb. Mar. Apr. May June July Aug. Sep. POS n.s..247 n.s. n.s. n.s..315 n.s. n.s. n.s. NEG n.s. n.s. n.s. n.s. n.s n.s..246 a Coefficients in bold are significant at p <.05; n.s., not significant. 3of5

4 Figure 3. Wavelet and wavelet squared coherency analysis of the Mackenzie Delta POS and NEG chronologies. Continuous power wavelet spectrum of the standardized (a) POS and (b) NEG chronologies. Legend for Figures 3a and 3b indicates power. (c) Squared wavelet coherence between the standardized POS and NEG chronologies. The relative phase relationship is shown as arrows (in-phase pointing right, anti-phase pointing left) and legend indicates squared coherence. All significant sections show in-phase behavior. Significant periodicities (p <.05) against red noise are outlined in black on the wavelet and wavelet squared coherency spectra. The cone of influence (COI), where edge effects might distort the picture, is shown as a lighter shade on all the figures. relations with growing season temperatures in the Mackenzie Delta region and with Northern Hemisphere summer temperature anomalies (Table 1 and Figure 2b). The NEG chronology, characterizing 75% of the trees, shows significant inverse relations with growing season temperature during both the current and previous years (Table 1). The relations between the POS chronology and Northern Hemisphere temperature anomalies are remarkably strong, especially since warming in the late 20th century is often underestimated by tree ring proxies [e.g., Briffa et al., 1998]. Furthermore, these relations appear to be time stable [r = 0.65, p < ( ); r = 0.62, p < ( )], but this is not true of the NEG chronology and Northern Hemisphere temperature anomalies [r = 0.46, p < ( ); r = 0.06, p = 0.720) ( )]. Identification of diverging growth trends and the construction of sub-chronologies may advance standard methods of analyzing tree ring data and possibly contribute in the resolution of temperature underestimation or reduced temperature sensitivity during the 20th century. [11] Negative relations between tree growth in northern regions and growing season temperature have been attributed to a number of factors, including physiological thresholds related to temperature [D Arrigo et al., 2004] and temperature-induced drought stress [Barber et al., 2000; Wilmking et al., 2004; Driscoll et al., 2005]. The NEG chronology from the Mackenzie Delta also exhibits positive relations with April precipitation during the current year, highlighting the importance of late winter snowfall on growth throughout the Mackenzie Delta and not just in the adjacent upland region [cf. Szeicz and MacDonald, 1996]. However, for the NEG chronology, an inverse relationship with temperature during the current and prior growing seasons and positive relations with precipitation during the late winter suggests that the combination of temperature and precipitation (i.e., temperature-induced drought stress) have been concomitant factors controlling tree growth in this region during the mid and late 20th century. The importance of late-winter precipitation may be two-fold: (1) abundant late winter precipitation can be an important source of moisture during early wood development, and (2) a deep snow pack can protect the shallow root systems of white spruce from cold temperatures in the late winter and early spring. Injury to sensitive tissues from spring frost is a frequent occurrence in boreal regions [Sutinen et al., 2001]. Further, while most cold-adapted conifers can tolerate severe freezing temperatures, winter desiccation and decreased cold hardiness due to unusual warm spells can result in injury to conifer tissues as the growing season begins [Sutinen et al., 2001]. Thus, a protective late season snow pack may reduce tissue damage that could inhibit plant-growth processes. [12] Our analyses corroborate observations from elsewhere in northwestern North America that relations between climate and white spruce growth may not be stable through time. The POS and NEG chronologies from the Delta region (Figure 2) and the wavelet analyses (Figure 3) show that prior to the 20th century, growth trends were similar for these two subsets of trees. Beginning at approximately AD 1800 there is some loss of coherency between the two series at long temporal scales while coherency at shorter time frequencies remains. The loss of coherency at 1800 may be related to differing growth responses following the Little Ice Age (LIA). However, the loss of coherency at this time is relatively minor and remains low until the 20th century when the two populations begin to diverge significantly from one another. Following the 1930s the coherency of the two data-sets diminishes at most temporal scales. A 16-year cycle does reemerge in approximately 1970 as growth rates of the NEG responders gradually increase. [13] The analyses of our chronologies show that the growth of treeline white spruce populations were synchronous for at least three centuries preceding the 1900s. Prior to the 20th century, these chronologies also exhibit common low-frequency variation with other proxy records with a demonstrated temperature signal [cf. Mann et al., 1999; Esper et al., 2002], suggesting the Mackenzie Delta chronologies also represent a proxy record of summer temperature. The loss of coherency between the POS and NEG chronologies during the last century suggests that the dominant growth response of white spruce to summer 4of5

5 temperature has undergone profound change in association with 20th century warming. 5. Conclusions [14] In the Mackenzie Delta region, climate-growth relations in white spruce appear to be undergoing profound changes during the 20th century. These trends are part of a large scale pattern that is emerging across subarctic tree line in the Northern Hemisphere. Wavelet coherency analysis indicates that during the past four centuries our subsets of trees had very similar growth trends until the early part of the 20th century. During the last century tree growth has become convoluted with respect to summer temperature across large regions of the northern hemisphere. Identifying these anomalous growth trends is critical for developing robust reconstructions of past climate and to understand the potential responses of boreal ecosystems to future climate warming. [15] Acknowledgments. 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