Climate and Soil Microsite Conditions Determine Local Adaptation in Declining Silver Fir Forests

Abstract

Ongoing climatic change is threatening the survival of drought-sensitive tree species, such as silver fir (Abies alba). Drought-induced dieback had been previously explored in this conifer, although the role played by tree-level genetic diversity and its relationship with growth patterns and soil microsite conditions remained elusive. We used double digest restriction-site-associated DNA sequencing (ddRADseq) to describe different genetic characteristics of five silver fir forests in the Spanish Pyrenees, including declining and non-declining trees. Single nucleotide polymorphisms (SNPs) were used to investigate the relationships between genetics, dieback, intraspecific trait variation (functional dendrophenotypic traits and leaf traits), local bioclimatic conditions, and rhizosphere soil properties. While there were no noticeable genetic differences between declining and non-declining trees, genome-environment associations with selection signatures were abundant, suggesting a strong influence of climate, soil physicochemical properties, and soil microbial diversity on local adaptation. These results provide novel insights into how genetics and diverse environmental factors are interrelated and highlight the need to incorporate genetic data into silver fir forest dieback studies to gain a better understanding of local adaptation.

Keywords: Abies alba; PLFAs; SNPs; climate warming; ddRADseq; dendroecology; forest die-off; soil microbiome; soil nutrients.

Figure 1

Principal component analysis (PCA) of the 3958 A. alba SNPs showing the first and second principal components. Each color represents a different population (CO, GA, PE, PM, SO).

Figure 2

Allele frequencies of SNPs showing selection signs (SNP 333, SNP 1034, SNP 2697) in the studied A. alba populations (CO, GA, PE, PM, SO). Dark gray represents the frequency of allele 1; light gray represents the frequency of allele 2.

Figure 3

Allele frequencies of SNP 681, associated with a precipitation variable (BIO15, precipitation seasonality, represented in blue) (a), and SNP 2794, associated with a temperature variable (BIO2, mean diurnal range, represented in orange) (b). Dark gray represents the frequency of allele 1 in each population; light gray represents the frequency of allele 2 in each population.

Figure 4

Venn diagram showing the number of SNPs associated with different environmental variables (bioclimatic variables, soils—abiotic, soils—biotic) or showing selection signs, and the coincidences among them.

Figure 5

Mean values of resistance to the 2005–2006 drought (Rt0506), resilience to the 2005–2006 drought (Rs0506), and response to the 1998 drought (Rs98) with respect to genotypes (allele 1/allele 2) of SNP 887 (a), SNP 1769 (b), and SNP 3878 (c), respectively.

Figure 6

Silver fir, Abies alba, distribution in purple, as described by EUFORGEN, and study sites in the Pyrenees (GA, CO, PE, PM, SO), marked with black circles.