Title: A Systematic Review of Eco-Physiological Responses to Drought Stress in Mediterranean Woody Plants: Implications for Climate Change Adaptation

Drought stress represents a primary environmental constraint limiting plant growth and survival in Mediterranean-type ecosystems. As global climate models project increased aridity and more frequent extreme drought events, understanding the eco-physiological mechanisms that govern plant responses to water scarcity becomes critical for predicting ecosystem resilience and guiding conservation strategies. This systematic review synthesizes current literature on the physiological, biochemical, and molecular adaptations of Mediterranean woody plants to drought conditions, with a focus on water-use efficiency, hydraulic failure, and carbon allocation trade-offs. The review highlights key mechanisms including osmotic adjustment, stomatal regulation, and xylem cavitation resistance, while identifying knowledge gaps regarding long-term acclimation and interspecific variability. Findings underscore the necessity of integrating eco-physiological traits into predictive models for climate change adaptation.

1. Introduction

Mediterranean regions are biodiversity hotspots characterized by seasonal drought and high interannual precipitation variability. Climate change scenarios predict a 20-30% reduction in precipitation by 2100, coupled with rising temperatures, intensifying drought stress for native woody species. Plants in these ecosystems have evolved diverse adaptive strategies, ranging from drought avoidance (e.g., deep rooting systems) to tolerance mechanisms (e.g., osmotic adjustment and antioxidant defense). However, the capacity of these strategies to buffer against extreme droughts remains uncertain. This review aims to systematically examine eco-physiological responses across species and functional types, with the goal of identifying traits that confer resilience under future climate regimes. Understanding these responses is essential for developing effective restoration and management practices in Mediterranean ecosystems under changing environmental conditions.

2. Water-Use Efficiency and Stomatal Regulation

Water-use efficiency (WUE), defined as the ratio of carbon assimilation to transpiration, is a key physiological trait under drought stress. Mediterranean woody species typically exhibit conservative water-use strategies, with isohydric behavior maintaining leaf water potential via tight stomatal regulation. For instance, *Quercus ilex* and *Pinus halepensis* show significant reductions in stomatal conductance (gs) under moderate drought, leading to decreased photosynthesis but enhanced intrinsic WUE. Recent studies using carbon isotope discrimination (δ13C) indicate that long-term WUE has increased in some populations over recent decades, possibly as a plastic response to rising atmospheric CO2 and aridity. However, prolonged stomatal closure may lead to carbon starvation and reduced growth, highlighting a trade-off between short-term water conservation and long-term viability. Stomatal sensitivity to vapor pressure deficit (VPD) appears to be a critical determinant of drought tolerance, with species showing higher sensitivity potentially better equipped to avoid hydraulic failure.

3. Hydraulic Failure and Xylem Cavitation

Hydraulic failure, caused by embolism in xylem conduits, is a major mechanism of drought-induced mortality in woody plants. Under severe water deficit, tension in the xylem sap exceeds the threshold for cavitation, leading to air seed infiltration and loss of hydraulic conductivity. Mediterranean species display a wide range of resistance to cavitation, often correlated with wood density and vessel anatomy. For example, *Arbutus unedo* exhibits high resistance due to narrow vessels and thick pit membranes, whereas *Cistus* species are more vulnerable. The safety margin—the difference between minimum water potential and the potential causing 50% loss of conductivity—varies among species, with those having narrower safety margins being at greater risk under extreme drought. Recovery from embolism is possible through refilling mechanisms, but the efficiency of this process under repeated drought cycles remains poorly understood. Integrating hydraulic traits into vegetation models is crucial for predicting mortality risk under future climate scenarios.

4. Osmotic Adjustment and Carbon Allocation

Osmotic adjustment, the accumulation of solutes such as proline, sugars, and inorganic ions, helps maintain cell turgor and physiological function under water deficit. This mechanism allows plants to sustain growth and photosynthesis at lower water potentials, thereby delaying the onset of hydraulic failure. In Mediterranean shrubs like *Rosmarinus officinalis* and *Lavandula stoechas*, osmotic adjustment contributes significantly to drought tolerance, with solute concentrations increasing up to 30% under severe stress. However, the energy cost of synthesizing compatible solutes may divert resources from growth and reproduction. Carbon allocation patterns shift under drought, with increased investment in root systems for water acquisition at the expense of shoot biomass. This trade-off affects competitive ability and ecosystem dynamics, particularly in mixed-species stands. Long-term studies indicate that repeated drought events can lead to depletion of carbon reserves, exacerbating vulnerability to pathogens and subsequent mortality.

5. Biochemical and Molecular Responses

At the cellular level, drought stress triggers the production of reactive oxygen species (ROS), leading to oxidative damage unless countered by antioxidant enzymes such as superoxide dismutase, catalase, and ascorbate peroxidase. Mediterranean plants often exhibit constitutive high antioxidant activity, which may confer a pre-adaptation to periodic drought. Gene expression analyses reveal upregulation of dehydration-responsive element-binding (DREB) proteins and late embryogenesis abundant (LEA) proteins, which stabilize membranes and protect cellular structures. Epigenetic modifications, including DNA methylation and histone modifications, may provide a mechanism for stress memory, allowing plants to respond more efficiently to recurrent drought. Nonetheless, the translation of molecular findings to field-based predictions remains challenging due to the complexity of signaling networks and interactions with other stressors such as heat and nutrient limitation.

6. Interspecific Variability and Functional Types

Not all Mediterranean woody species respond identically to drought. Functional classifications based on leaf habit (evergreen vs. deciduous), root depth, and wood anatomy help predict species-specific vulnerabilities. Evergreen sclerophylls like *Quercus suber* generally exhibit higher WUE and cavitation resistance compared to deciduous species such as *Populus alba*, which rely on deep roots and rapid growth. However, within functional groups, substantial variability exists, often linked to local adaptation and genetic diversity. For instance, populations of *Pinus pinea* from drier sites demonstrate greater drought tolerance than those from mesic habitats, suggesting evolutionary potential. This intraspecific variation is critical for conservation planning, as it may buffer against future climate change. Restoration efforts should prioritize provenances with demonstrated drought resilience to enhance ecosystem stability.

7. Implications for Climate Change Adaptation and Conservation

The reviewed evidence indicates that while Mediterranean woody plants possess a range of adaptive mechanisms, the projected intensification of drought may exceed their physiological thresholds. Assisted migration, selection of drought-tolerant genotypes, and modification of stand density are potential management strategies to mitigate impacts. However, the effectiveness of these interventions relies on accurate predictions of species’ responses under novel climatic conditions. Incorporating eco-physiological traits, such as hydraulic safety margins and WUE, into species distribution models can improve projections of range shifts and extinction risk. Moreover, maintaining genetic diversity within populations is essential for evolutionary adaptation. Future research should focus on multi-stressor interactions (e.g., drought combined with heat or fire) and long-term field experiments to validate laboratory findings. Such knowledge will be pivotal for sustaining Mediterranean ecosystems in the face of global environmental change.