Scots pines (Pinus sylvestris) as sources of biological ice-nucleating macromolecules (INMs)

Abstract

Scots pine (Pinus sylvestris) is the most widespread pine species in the world. It is a major tree species found in the northern hemisphere and in large parts of the boreal forest. In recent studies, birch trees, another notable species in the boreal forest, have been identified as an important emission source of ice-nucleating macromolecules (INMs). The INMs were found in pollen1 and all over the tree’s tissue (e.g., branch wood, bark, petioles, and leaves)2,3. Similarly, Scots pine pollen were found to be ice nucleation active, but until now, no further investigation of INMs from other tissue types has been conducted.In this laboratory and field study, we collected samples from six different Scots pine trees in urban parks in Vienna, Austria. We investigated the distribution of INMs among three different tissue types, namely bark, branch wood, and needles, by extracting them from the milled sample (as a bulk sample) and the surface of the intact tissue and measuring ice nucleation activity in immersion freezing mode. We aimed to quantify the overall INM content of this species and assume it is independent of the growing region, as previously reported for birch trees, allowing us to extrapolate our results to the vast locations where Scots pines are found. In addition, we investigated the ability of rain events to wash the INMs off trees in a field study. We found INMs in all samples with freezing onset temperatures ranging from -16°C to -29°C. The bulk samples showed INM concentrations ranging from 105 to 109 per mg dry weight active at -25°C and higher. In surface extracts from the intact tissue, we found concentrations from 105 to 108 INMs per cm2 of the extracted surface. Most importantly, we found all rain samples to contain INMs with similar freezing onset temperatures to the lab extracts.On the basis of our results, we estimate that one square meter of Scots pine stand has the potential to release about 109 to 1012 INMs active at -25°C or higher. This estimation reveals pine trees as a massive reservoir of INMs. Boreal forests containing large numbers of birch and pine trees must be considered an essential source of atmospheric INMs. We propose Scots pine as an important emission source of INMs, nucleating ice in immersion freezing mode at moderate supercooled temperatures and thereby impacting the microphysics of mixed-phase clouds.  (1)          Pummer, B. G.; Bauer, H.; Bernardi, J.; Bleicher, S.; Grothe, H. Suspendable Macromolecules Are Responsible for Ice Nucleation Activity of Birch and Conifer Pollen. Atmospheric Chemistry and Physics 2012, 12 (5). https://doi.org/10.5194/acp-12-2541-2012.(2)          Felgitsch, L.; Baloh, P.; Burkart, J.; Mayr, M.; Momken, M. E.; Seifried, T. M.; Winkler, P.; Schmale, D. G.; Grothe, H. Birch Leaves and Branches as a Source of Ice-Nucleating Macromolecules. Atmospheric Chemistry and Physics 2018, 18 (21). https://doi.org/10.5194/acp-18-16063-2018.(3)          Seifried, T. M.; Bieber, P.; Felgitsch, L.; Vlasich, J.; Reyzek, F.; Schmale, D. G.; Grothe, H. Surfaces of Silver Birch (Betula Pendula) Are Sources of Biological Ice Nuclei: In Vivo and in Situ Investigations. Biogeosciences 2020, 17 (22). https://doi.org/10.5194/bg-17-5655-2020.