Abstract
Climate change models predict an increase in aridity, especially in the regions under Mediterranean-type climates such as the Mediterranean Basin. However, there is a lack of ecophysiological studies supporting the selection of the more drought-adapted ecotypes for reforestation programs. In this study, we analyzed the anatomical and functional adaptations of 18-month-old seedlings to drought on 16 Quercus suber L. populations from the southeastern rear edge of the species distribution in northern Tunisia growing in a common garden, in order to identify the most appropriate material to use in reforestations. The results evidenced that populations from more xeric habitats displayed the highest leaf dry mass per area (LMA) and lowest leaf area (LA) values, together with the largest increase in the bulk modulus of elasticity (Δε) in response to drought (i.e., elastic adjustment). On the other hand, some populations with intermediate values of aridity, LMA and LA displayed the sharpest increase in proline concentration (ΔPro), with a concomitant increase in osmotic potential at full turgor (Δπo) (i.e., osmotic adjustment). Therefore, two different strategies seem to drive the within-species variation of the studied Q. suber populations in response to water scarcity: (i) a water saver strategy for improving water stress tolerance through the maximization of the elastic adjustment; and (ii) a water spender strategy for maintaining water absorption and photosynthetic activity under moderate water stress through the maximization of the osmotic adjustment. We concluded that the higher elastic adjustment, together with reduced LA and increased LMA, implied a better performance under drought stress in the populations of Q. suber from more xeric habitats, which can be considered the most drought-adapted ecotypes and, consequently, the most appropriate for reforestation programs under an eventual increase in aridity.
Highlights
The Mediterranean-type climate is characterized by warm-to-hot summers, mild-to-cold winters, and a precipitation regime with a minimum in summer, which induces a dry period during the vegetative season [1,2,3]
Seedlings showing the highest values of leaf mass per unit area (LMA) and the smallest leaf area (LA) corresponded to populations inhabiting sites with the highest aridity (De Martonne aridity index, MAI, between 15 and 25), coinciding with the most southeastern populations of the species distribution (Figures 1 and 3)
The populations of cork oak (Q. suber) located at the southeastern distribution limit of the species showed a plastic response to drought, with a high degree of within-species variation
Summary
The Mediterranean-type climate is characterized by warm-to-hot summers, mild-to-cold winters, and a precipitation regime with a minimum in summer, which induces a dry period during the vegetative season [1,2,3]. The small and sclerophyllous leaves typical of the Mediterranean woody plants have been considered a functional adaptation to withstand water stress during these drought periods [8,9,10,11,12] This genuine Mediterranean climate is markedly influenced by geographic traits, such as the orography, the thermal dynamic behavior of the sea masses [13], and the transition to other climate types. This results in a diverse variety of Mediterranean sub-climates, from warmer and drier sub-climates in transition to arid climates, milder and more humid ones (e.g., in coastal areas) in transition to temperate climates, or drier and cooler variants in continental areas, closer to the typical steppe climates [3]. Peguero-Pina et al [18] compared seven provenances of the Mediterranean species Quercus ilex L. growing in a common garden and found an extreme reduction in leaf area (LA)
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