Evidence of drought- induced decline in anatomical traits of Scots pine and Silver fir in Iberian Peninsula

The evident increase of air temperatures and associated drier conditions of the last few decades are linked to episodes of drought-induced forest dieback, particularly near the southernmost limit of distribution of tree species (rear edge) where xeric conditions prevail. It is still unclear the role played by the two main mechanisms proposed to explain forest dieback, namely hydraulic failure and carbon starvation. Here we reconstruct wood anatomical features in two tree species experiencing recent diebacks in their rear edges located in NE Spain: silver fir (Abies alba) and scots pine (Pinus sylvestris). We use this retrospective approach to infer the causes of recent dieback in response to the severe 2012 drought. We classified trees based on their current defoliation degree in response to the drought. Then, we sampled 5 declining (highly defoliated) and 5 non-declining (scarcely defoliated) trees per species and cutting both transversal and tangential sections, we analyzed the following anatomical traits: lumen transversal area, cell wall thickness, cell number, theoretical hydraulic conductivity and ray parenchyma cell dimensions. Linear mixed-effects models were used to quantify how climate and wood anatomy are related depending on the defoliation class. We found that lumen area differed between trees showing contrasting defoliation many decades before the onset of the dieback. On the other side, the amount ray parenchyma did not differ between defoliation classes. We suggest that the previous low hydraulic conductivity observed in declining trees predisposed them to dieback. Our findings provide support to dieback mechanisms based on hydraulic deterioration in conifers rather than to carbon starvation