First measurements of Blue intensity from Pinus peuce and Pinus heldreichii tree rings and potential for climate reconstructions

Climate changes, their regional patterns, origin, and prediction are currently one of the most important scientific challenges. Tree-rings are among the most widely used proxies for past climate variation. However tree-ring width (TRW) from certain tree species and regions often do not contain robust climate signal. Other parameters such as Maximum latewood density (MXD) of conifer tree rings are more sensitive to summer temperatures, but the measurements have high costs. A potential surrogate for MXD is blue intensity (BI), which is based on the measurements of high-resolution images of the wood. The method has been tested and applied for several species. However it has not been tested up to now for Pinus heldreichii Christ (PIHE) and Pinus peuce Griseb. (PIPE). Those species are with limited distribution on the Balkan Peninsula and due to their longevity (frequently more than 500 years with potential to more than 1000 years) may serve as one of the best proxies for past climate variation in SE Europe. We composed BI chronologies following standard procedures from PIHE and PIPE trees from subalpine locations in the Pirin Mts, Bulgaria. The correlation analysis with climate parameters revealed strong and significant positive correlations of PIHE BI (Delta BI and Latewood BI (LwBI) series) and summer temperatures. The highest were 0.74 (p < 0.05) with July-August average temperatures for the period 1933-1983. The correlation values of.BI remained higher than 0.6 for the whole period (1933-2014) and sub-periods both for average monthly and average maximum temperatures. LwBI values of the PIHE chronology had correlation coefficients above 0.52 (highest 0.64) with August and August-July temperatures. The PIPE BI chronologies were also positively correlated with summer temperatures, but showed lower values than PIHE with lower temporal stability. The most stable were the correlations with LwBI average August temperatures, which were above 0.52. Delta BI series displayed high correlations for the 1933-1983 period, but then decreased and this caused overall lower correlations with August temperatures. Our data shows that there is potential to develop long BI chronologies and proxy climate reconstructions from the studied species and in this way complement the knowledge of the past climate of SE Europe.