Rooting for resilience: transcriptomic, epigenomic and physiologic responses of silver fir ( <i>Abies alba</i> Mill.) to experimental drought

Understanding the biological mechanisms underlying tree responses to drought is critical for preserving forest biodiversity, as current global climate change is challenging the ability of drought-sensitive trees to cope with water shortage. In this study, we investigate how silver fir (Abies alba Mill.) responds to experimental drought stress, more specifically, atmospheric drought caused by high vapor pressure deficit (VPD), by analyzing the gene expression and DNA methylation profiles of different organs alongside physiological variables under well-watered, drought and recovery conditions. Roots exhibited a stronger transcriptomic response than leaves, with 50 times more altered transcripts, revealing their value for assessing water stress in this species through the expression of genes involved in water transport. In addition, brassinosteroid-related genes can serve as stress markers both in roots and leaves. VPD-induced drought also affected DNA methylation, which, like transcriptomic and physiological variables, begins to normalize once the stress is over, suggesting some resilience to drought. However, A. alba struggles to improve intrinsic water-use efficiency, which raises its vulnerability to VPD-induced drought. Our results suggest that silver fir forests might be able to cope with short drought events, but prolonged periods of water shortage, which are likely to increase with climate change, may surpass their resilience thresholds, increasing the likelihood of hydraulic failure and carbon starvation.