The Dendroclimatological Potential of Willamette Valley Quercus garryana
ABSTRACT
We develop a 341-year Oregon white oak (Quercus garryana Dougl.) tree-ring chronology in Oregon's Willamette Valley to evaluate climate-growth relationships and determine the species' dendroclimatological potential at our site and in the surrounding region. The standardized and residual chronologies exhibit significant positive correlations with previous-year April and May temperatures, inverse correlations with previous-year spring precipitation and summer PDSI, a positive correlation with current-year July precipitation and summer PDSI, and inverse correlations with current-year June temperatures. The strength of these relationships varies over time. Significant shifts in the chronologies' mean and variance align with phase changes in the Pacific Decadal Oscillation (PDO), with lower and more variable growth during the warmer, drier positive phase of the PDO over the instrumental record. The absence of similar shifts prior to the 1900s, suggests a lack of temporal consistency in the expression of PDO variability at our site. The strong crossdating at our site reflects a cohesive climate signal, and the climate analysis illustrates the potential to develop proxy data over multiple centuries. Together, these results indicate a potential to expand the network of currently available climate proxy data by utilizing Q. garryana in dendroclimatological research.
Study area. (a) The native range of Quercus garryana in North America (after USGS 2007), (b) the location of our study area within the Eola Hills of the Willamette Valley of western Oregon, (c) Zena Forest and Farm property boundaries, topography as depicted by LiDAR data for the property, and the locations of sampled trees, (d) average monthly temperature (black line) and precipitation (grey bars) at our site (NOAA 2013), and (e) temperature, precipitation (NOAA 2013) and Palmer Drought Severity Index (PDSI) (Palmer 1965) values (gray bars) and means (black bars) stratified by the cool (−) and warm (+) phases of the Pacific Decadal Oscillation (PDO) (Mantua and Hare 1997). T-tests show statistically significant differences between warm and cool phase climate variables during the winter months when the impact of PDO on PNW climate is most pronounced: mean March temperature (0.9°C, p < 0.0005), mean January precipitation (20%, p = 0.0041), and January PDSI (18%, p = 0.0547).
Standard (STD) and residual (RES) ring-width-index chronologies, sample depth (n), expressed population signal (EPS), and mean correlation (r) among all measurement series calculated over sequential 50-year windows, overlapped 25 years.
Correlation (r) between the residual ring-width-index chronology and monthly temperature, precipitation, and Palmer Drought Severity Index (PDSI) data over (a) the entire instrumental record (1896–2012) and (b) moving 20-year windows. Correlations were calculated from the previous January (pJan) through the current December, and only those variables showing the strongest relationships in (a) were examined over time (b). PDO phase changes are as defined by Mantua and Hare (2002).
(a) The residual ring-width-index (RWI) chronology (black line) with standard deviation (±1 standard deviation shown as gray shading) to illustrate changes in variability by PDO phases, (b) correlations (r) between the residual chronology and monthly instrumental PDSI values during positive (warm) PDO (light gray) and negative (cool) PDO (dark gray) phases.
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