Blue intensity of Swiss stone pine as a high-frequency temperature proxy in the Alps

Abstract

Tree rings are widely used for climatic reconstructions and for improving our understanding of ongoing climate change in high-altitude sensitive areas. X-ray maximum latewood density is a very powerful parameter to reconstruct past climatic variations, especially if compared to tree-ring width, but this method is neither inexpensive nor timesaving. However, blue intensity (BI) has resulted in an excellent maximum wood density surrogate that measures the intensity of reflected light from latewood in the blue spectra. This methodology is still considered a prototype parameter, and more data are needed for validation of the method. We present the first BI values coming from Swiss stone pine (Pinus cembra L.) collected on the southern margin of the Alps. Analyses were performed by testing different solvents and polishing techniques, as well as different CooRecorder pixel percentage settings. The results demonstrate that solvents and software parameters have little influence on the final chronologies. Dendroclimatic analyses demonstrate that Swiss stone pine BI can be a useful tool to extract at least the high-frequency variations in July–August temperatures with a correlation coefficient of up to 0.6 (over the 1800–2017 time period). The immunity of Swiss stone pine to insect defoliator outbreaks further enhances the reliability of the BI values of this species in reconstructing past high-frequency temperature variations in high-altitude sensitive areas.

Multi-centuries mean summer temperature variations in the Southern Rhaetian Alps reconstructed from Larix decidua blue-intensity data

“…However, the highlighted slight influence of the considered percentage of pixels on the correlation with climate is corroborated by a previous study performed using Swiss stone pine (Pinus cembra L.), which attributed greater importance to the sampling polishing process than to the considered pixel amount or extractives removal process (Cerrato et al, 2023).…”

“…The data presented here represent, to our knowledge, the first attempt to use European larch BI data for dendroclimatic purposes in the European Alps. In fact, only one other study used BI data from this species but for dendroentomological purposes (Arbellay et al, 2018), whereas only a few studies with dendroclimatic purposes exist for the area but focused on other alpine species (Cerrato et al, 2023;Frank and Nicolussi, 2020;Österreicher et al, 2014;Trachsel et al, 2012;Babst et al, 2009).…”

“…S1 in the Supplementary Material). For both the LWBI and EWBI, we considered the mean values of the 25 %, 50 %, 75 % and 100 % of the darkest (and lightest when considering the EWBI) pixels in the frame (Cerrato et al, 2023). For easier comparison with climate data, BI values were inverted following standard procedures (Rydval et al, 2014;Wilson et al, 2014).…”

“…Blue intensity, albeit a relatively new methodology, has already been tested on several coniferous species (e.g., Scots Pine (Pinus sylvestris L.) in Fennoscandia and Scotland, various Picea ssp., Pinus ssp., and Tsuga ssp. in North America and Europe and other coniferous species in Asia and Oceania (see Reid and Wilson, 2020;Cerrato et al, 2023 for more information).…”

“…Nevertheless, in the European Alps, only a few studies have been performed using BI data (Cerrato et al, 2023;Arbellay et al, 2018;Nicolussi et al, 2015;Österreicher et al, 2014), and even fewer have been performed on European larch. Thus, additional tests are needed (Reid and Wilson, 2020).…”