Treeline displacement may affect lake dissolved organic matter processing at high latitudes and altitudes

Abstract Climate change is causing a rapid shift in treeline position, both towards higher altitudes and latitudes 1 inducing changes in soil properties such as organic matter content and composition 2 . Eventually, soil-derived organic matter is transported to alpine and subarctic lakes with yet unknown consequences for dissolved organic matter diversity and processing. Here, we experimentally investigated the consequences of treeline shifts by amending subarctic and temperate alpine lake water with soil-derived dissolved organic matter (DOM) from above and below the treeline. We used ultra-high resolution mass spectrometry (FT-IR MS) to track molecular DOM diversity (i.e., chemodiversity), estimated DOM decay and measured bacterial growth efficiency. In both lakes, soil-derived DOM from below the treeline increased DOM chemodiversity mainly through the enrichment with novel polyphenolic and highly unsaturated compounds. These compositional changes were associated with reduced overall and compound-level DOM reactivity and reduced bacterial growth efficiency. Our results suggest that treeline advancement has the potential to enrich a large number of lake ecosystems with less biodegradable DOM, affecting bacterial community function and potentially altering the biogeochemical cycling of carbon in lakes at high latitudes and altitudes.