Bristlecone Pine Maximum Latewood Density as a Superior Proxy for Millennial Temperature Reconstructions

Great Basin Bristlecone pine (Pinus longaeva) (PILO) trees are known for their old age. The longest tree-ring width (TRW) chronology covers a large part of the Holocene, and the temperature-sensitive upper treeline chronology extends back to 2575 BC. The TRW of upper treeline PILO trees is influenced by temperature variability, but the moderate strength and temporal instability of the signal is making it hard to use for reliable temperature reconstructions. Maximum Latewood Density (MXD) of conifers is known to be a good summer temperature proxy in the northern hemisphere. However, there are no PILO MXD records due to various reasons, including its location in semi-arid lower latitudes, as well as due to methodological difficulties such as narrow rings and a varying grain angle. Here, we used an X-ray Computed Tomography (X-ray CT) of 69 cross-dated cores to construct an MXD chronology of PILO from the upper treeline sites covering the last millennium, and to investigate its temperature signal. The chronology correlates significantly (r=0.63) with warm season (April to September) temperature for the period 1895-2005 and the signal is stable throughout the instrumental period. Our results demonstrate that MXD from the bristlecone pine archive can be used as a robust proxy for western North American warm season temperature variability at an unprecedented multi-millennial scale.