Individual-based dendrogenomic analysis of forest dieback driven by extreme droughts

Droughts driven by global change are triggering worldwide forest dieback, a phenomenon that is predicted to worsen. We combined genome-wide single nucleotide polymorphisms (SNPs) and dendrochronological approaches to assess genetically-based individual tree vulnerability to past extreme droughts that caused massive mortality of coihue (Nothofagus dombeyi (Mirb.) Blume) forests in northern Patagonia, Argentina. We collected fresh leaves and wood cores from pairs of trees, one with a healthy crown (HC) and another with a partially affected crown (PA), at four sites impacted by droughts in 1998, 2008, and 2014. We used dendrochronological techniques to estimate parameters in terms of growth trends due to drought and genomic analysis to assess the relationship of genomic variation with water stress. While 5155 neutral loci did not differentiate PA from HC trees, a set of 33 adaptive SNPs did, 8 of which were related to water stress. Association analysis between genomic variants and dendrophenotypic traits yielded 6 SNPs that were associated with a growth measure as resilience to cope with drought. Our preliminary results indicate that susceptibility to drought in N. dombeyi could be determined at the genomic level. The combination of these approaches provides a framework for the identification and analysis of candidate genes for stress response in non-model species.