Correlations between tree rings and climate

Figure 6.2 summarises the correlations between the 115 SPL10 tree-ring chronologies and monthly 10-year high-pass filtered moisture and temperature variability over the period 1950–

2000. Correlation patterns suggest that despite the northerly location, annual tree growth anomalies through most of the region are strongly associated with moisture variability during the current spring and the previous summer as indicated by highly significant negative corre-lations with temperature and positive correcorre-lations with surface soil moisture estimates based on GLDAS-2 Noah. The patterns of correlation with monthly scPDSI are less significant and differ substantially from those of simulated soil moisture, except for sites south of 65^oN. A sim-ple comparison between the annual cycles of soil moisture simulated by Noah and Snow Water Equivalent (SWE) from Globsnow shows that the seasonality of simulated moisture is highly consistent with snowpack dynamics at the grid boxes corresponding to the tree-ring sites (Sup-plementary FigureE1). This suggests that the difference in the correlation patterns between the two products and interannual tree growth might be related to limitations in the ability of the scPDSI water balance model to correctly represent soil moisture dynamics at the northernmost sites, most of which are strongly clustered along the latitudinal treeline (Figure 6.1).

Figure 6.2: Correlation between each SPL10 tree-ring chronology in the network (rows) and monthly surface soil moisture (0-10 cm), scPDSI and mean air temperature (columns) between 1950 and 2000. Correlations are given for an 18-month window from April of the previous growing season (*) to September of the current growing season. The stippling indicates months with significant correlations (p < 0.05). The elevation of the chronologies is shown in the right panel.

Dynamic regression with the Kalman filter indicates that the spring and summer moisture responses of tree growth revealed by the correlations with temperature and simulated soil mois-ture are generally stable through time on a year-to-year basis (Figure 6.3). However, some sites do display a moderated degree of time-dependence in their responses to current May and previous July temperatures and moisture.

The regional clustering of tree-ring sites with similar climate responses based on

correla-6.3. Results 129

Figure 6.3: Additional variance (R²) explained by the dynamic Kalman filter regression model between the SPL10 chronologies and monthly air temperature and soil moisture compared with a standard time-invariant linear regression model. R² values are shown for months when the dynamic regression model was selected over the standard model based on the minimum Akaike Information Criteria (AIC). Higher R² indicates stronger time-dependence in the associations.

tions with temperature and the scPDSI is shown in Figure 6.4a–b. The first cluster includes sites located at relatively high elevations and along the latitudinal treeline. Trees at these sites show a pattern of growth response to moisture over a short period during the previous summer and current spring (i.e., concurrent negative correlations with temperature and positive corre-lations with the scPDSI), followed by a positive response to temperature during the current summer (Figure 6.4b). The second cluster, representing most of the tree-ring sites (N=75), shows a stronger and more sustained growth response to moisture conditions between the previ-ous summer and the current spring. Similar temporal patterns of short versus extended period of moisture response are obtained when using Noah soil moisture instead of the scPDSI for cluster analysis (Supplementary Figure E2). However, the growth response to spring moisture availability is more clearly represented and the spatial clustering of sites changes in the southern part of the network in Canada.

Negative growth responses to July temperatures during the previous season and the current spring are more common and significant in Alaska and northwestern Canada (Figure 6.4c–d), whilst significant positive responses to current summer temperature tend to occur in colder lo-cations at high elevations and along the latitudinal treeline (Figure6.4e). The negative growth response to previous July temperature across most of Alaska is consistent with even stronger negative correlations with maximum thaw depth (Figure6.5). Similarly, growth at some high-elevation sites on the west coast that respond positively to current summer temperatures show comparatively stronger positive correlations with maximum thaw depth during the current grow-ing season. The apparently stronger association with thaw depth than with simple air tempera-tures suggests that soil thermal regime has an important influence on modulating tree physiology and growth, particularly in the discontinuous permafrost region.

Positive responses to soil moisture availability during the previous summer and current spring inferred from both Noah and the scPDSI are widespread along the western interior of Canada and interior Alaska (Figure6.4f, Supplementary FigureE2). The positive response of tree growth to

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Chapter 6. Re-assessing moisture limitation on tree-ring growth in the boreal forest:

a case study in northwestern North America

spring soil moisture availability in this region suggests an influence of winter snowpack, however tree growth is poorly correlated with estimates of maximum SWE based on either the snow model of the scPDSI (Figure6.4g) or Globsnow data (not shown).

Figure 6.4: Clustering of tree-ring sites based on their patterns of monthly correlations with temperature and the scPDSI. (a) Map of dominant k-means clusters of sites with similar climate response. (b) Individual and cluster-averaged monthly correlations with temperature (T) and the scPDSI (M). (d–g) Spatial distribution of the strongest correlations between tree growth (SPL10) and temperature, scPDSI, and maximum SWE estimated by the snow model of the scPDSI. (h) Map of moisture and temperature constraints of summer NDVI as explained in Figure5.5. The dark gray shading in the maps denotes the boreal forests. The purple and cyan lines indicate the location of the treeline and the southern edge of the discontinuous permafrost, respectively.

6.3. Results 131

Overall, linear correlations between the SPL10 tree-ring chronologies and monthly tempera-ture and estimated moistempera-ture availability suggest a dominant moistempera-ture limitation of white spruce growth across the study domain, predominantly reflecting conditions during both previous and current growing seasons. This pattern of response is broadly consistent with the dominant mois-ture limitation of peak summer vegetation greenness in the same regions that were shown in Chapter 5 for correlations between summer NDVI and summer temperature and scPDSI (Figure 6.4h).

Figure 6.5: Correlations between year-to-year variations in tree growth (SPL10) and maximum thaw depth dur-ing the previous and current growdur-ing seasons. The small black crosses indicate significant (p < 0.05) correlations.

6.3.2 Relationships between tree-ring width and growing season parameters