Evaluating the dendroclimatological potential of Blue Intensity on multiple conifer species from Australasia

Abstract:

We evaluate a range of Blue Intensity (BI) tree-ring parameters from 8 conifer species (12 sites) from Tasmania and New Zealand for their dendroclimatic potential and also as surrogate wood anatomical (WA) proxies. Using a limited dataset of ca. 10-15 trees per site, we measured earlywood maximum blue reflectance intensity (EWB), latewood minimum blue reflectance intensity (LWB) and the associated Delta Blue Intensity (DB) parameter for dendrochronological analysis. No resin extraction was performed which will likely impact the low frequency trends, so we have initially focused on the high frequency signal by detrending all tree-ring and climate data using a flexible 20-year spline. All BI parameters express low relative variance and weak signal strength compared to ring-width. Correlation analysis and principal component regression experiments identified a mixed response for most ring-width chronologies. However, despite the weak signal strength, strong calibrations with summer temperatures were noted for the EWB data for most sites. Significant correlations for LWB were also noted, but the sign of the relationship for most species (Pencil pine, low elevation Huon pine, Kauri, cedar, Silver pine and Pink pine) is opposite to that seen for all conifer species in the Northern Hemisphere. DB performed well for the Tasmanian sites but explained minimal temperature variance in New Zealand. Using the full multiple species/parameter network, excellent summer temperature calibration was identified for both Tasmania and New Zealand ranging from 52% to 78% explained variance with equally robust independent validation (Coefficient of Efficiency = 0.41 to 0.77). Comparison of the Tasmanian BI reconstruction with a WA reconstruction shows that these parameters record essentially the same strong high frequency summer temperature signal. Despite these excellent results, a substantial challenge exists with the capture of potential secular scale climate trends. Although DB, band-pass and other signal processing methods may help with this issue, it is likely that direct WA measurements may be the most robust way to minimise these colour bias issues. However, due to the high costs of WA measurement, BI data, at the very least, should be used to identify those species that could be targeted for WA measurement.