Results from the Red Cedar Chronologies
The five long red cedar chronologies produced by this project are shown in Fig. 3, plotted from north (top) to south (bottom). See Fig. 1 for their geographic locations. All are at least 500 years long, with some notably longer. In particular, the long composite Cedar Knob/Bible Knob/Moser Knob red cedar chronology (CKJ/BKJ/MKJ) from West Virginia provides the rare opportunity in the eastern US to look at tree-growth/climate relationships over the past 1500 years, though only the past 1000 years is well replicated (i.e., more than 10 time series used for estimation of the mean value function). Notably, there are periods of low frequency variations that are suggestive of Medieval Warm Period/Little Ice Age-like climate variability. These are broadly highlighted in the plot in Fig. 3d by the 50-year low-pass filtered version in red, along with the plots for the other four red cedar chronologies from Pawtuckaway Mountain, NH, Middleburgh, NY, Tunkhannock, PA, and Norris Basin, TN.
Figure 3. Five red cedar chronologies developed by this project. Note that the Pawtuckaway and Cedar/Bible/Moser chronologies are composites of nearby sites. See the Fig. 1 map for general details of where these sites located..
Climate response functions (not shown) for these chronologies revealed significant relationships with local and regional rainfall and temperature in the early spring and summer, suggestive of drought sensitivity on these well-drained sites. Significant correlations were also found with regional Palmer Drought Severity Indices (PDSI), which has contributed to the new set of North American drought reconstructions described in Cook et al. (2004) and used in the North American Drought Atlas described earlier.
Results from Eastern White Cedar
Buckley et al. (2004) used the northern white cedar tree-ring data from the Niagara Escarpment to produce inferred estimates of precipitation over the Bruce Peninsula for the past 1400 years. This reconstruction is shown in Fig. 4. Problems with distorted growth and site disturbances in the 20th century required somewhat severe detrending to be applied to the raw ring-width series. As a consequence, some loss of low-frequency climate variability has undoubtedly occurred. Regardless, there is strong evidence for multi-decadal variability in this record. Noteworthy periods of inferred drought are indicated along the bottom axis of the plot. This reveals a period of unusually frequent drought in the AD 1470-1650 interval, which was followed by a period with little drought up to 1850.
Figure 4. An inferred precipitation record for the past 1400 years from Ontario’s Bruce Peninsula (from Buckley et al., 2004). Noteworthy periods of inferred drought are indicated along the bottom axis.
A demonstration of the drought sensitivity of the Door Peninsula northern white cedar chronologies is illustrated in Fig. 5. In this case, simple correlations between monthly temperature, precipitation, and PDSI are shown to reveal the seasonal structure of these trees’ response to climate.
Figure 5. Response to temperature, rainfall and PDSI for the Door Peninsula northern white cedar tree-ring chronologies with Wisconsin Climate Division 3 and 6 instrumental data. Note the especially strong (filled black) association between tree growth and the PDSI during the summer growing season months.).
Our research has conclusively demonstrated the potential of eastern red cedar and northern white cedar for the reconstruction of past climate, especially that related to drought. The series developed by this project have already made important contributions to the North American Drought Atlas, and other cedar chronologies still under development will contribute as well. The eastern United States has few old-growth stands of living trees that can provide records of past climate even as far back as 400 years in the past. So, the availability of cedar chronologies that cover the past 500-1000 years is extraordinarily important for studies of past climate there..