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Background: Understanding the mechanisms that explain the spatial distribution of conifers across biogeographical gradients is important for anticipating potential range shifts owing to global change. Classical explanations have involved trade-offs between shade and drought tolerances, but more recent studies observed that trade-offs between fire and drought tolerances could also be important. Aims: Here we propose that a contributing mechanism to explain how conifer species are distributed across productivity gradients - with marked variation in the incidence of fire - involves a trade-off between allocation to bark, which serves to protect against fire, or to embolism resistance, which serves to protect against drought. Methods: We compiled information from different datasets and performed regression analyses. Results: We observed a trade-off between bark thickness and embolism resistance in conifer species such that species show either large investments of carbon to the bark or have thinner barks but xylem resistant to embolism; we did not observe conifer species concomitantly showing high fire tolerance and embolism resistance. Conclusions: This study serves as a starting point for a novel framework on how fire and drought adaptations affect conifer biogeography. Additional studies will be necessary to discover the generality of our findings by including other species of conifers, for example those in the Southern Hemisphere. [ABSTRACT FROM AUTHOR]

Submediterranean forests are considered an ecotone between Mediterranean and Eurosiberian ecosystems, and are very sensitive to global change. A decline of Scots pine (Pinus sylvestris L.) and a related expansion of oak species (Quercus spp.) have been reported in the Spanish Pre-Pyrenees. Although this has been associated with increasing drought stress, the underlying mechanisms are not fully understood, and suitable monitoring protocols are lacking. The aim of this study is to bring insight into the physiological mechanisms anticipating selective decline of the pines, with particular focus on carbon and water relations. For this purpose, we performed a sampling campaign covering two growing seasons in a mixed stand of P. sylvestris and Quercus subpyrenaica E.H del Villar. We sampled seasonally twig xylem and soil for water isotope composition (δ18O and δ2H), leaves for carbon isotope composition (δ13C) and stems to quantify non-structural carbohydrates (NSC) concentration, and measured water potential and leaf gas exchange. The first summer drought was severe for both species, reaching low predawn water potential (-2.2MPa), very low stomatal conductance (12 ± 1.0mmolm-2 s-1) and near-zero or even negative net photosynthesis, particularly in P. sylvestris (-0.6 ± 0.34 μmolm-2 s-1 in oaks, -1.3 ± 0.16 μmolm-2 s-1 in pines). Hence, the tighter stomatal control and more isohydric strategy of P. sylvestris resulted in larger limitations on carbon assimilation, and this was also reflected in carbon storage, showing twofold larger total NSC concentration in oaks than in pines (7.8 ± 2.4% and 4.0 ± 1.3%, respectively). We observed a faster recovery of predawn water potential after summer drought in Q. subpyrenaica than in P. sylvestris (-0.8MPa and -1.1MPa, respectively). As supported by the isotopic data, this was probably associated with a deeper and more reliable water supply in Q. subpyrenaica. In line with these short-term observations, we found a more pronounced negative effect of steadily increasing drought stress on longterm growth in pines compared with oaks. All these observations confer evidence of early warning of P. sylvestris decline and indicate the adaptive advantage of Q. subpyrenaica in the area. [ABSTRACT FROM AUTHOR]

The main objective of this study was to apply the air-coupled broad-band ultrasonic spectroscopy in attached transpiring leaves of Vitis vinifera L. to monitor changes in leaf water potential (Ψ) through the measurements of the standardized value of the resonant frequency associated with the maximum transmitance (f/fo). With this purpose, the response of grapevine to a drought stress period was investigated in terms of leaf water status, ultrasounds, gas exchange and sugar accumulation. Two strong correlations were obtained between f/fo and Ψ measured at predawn (pd) and at midday (md) with different slopes. This fact implied the existence of two values of Ψ for a given value of f/fo, which was taken as a sign that the ultrasonic technique was not directly related to the overall Ψ, but only to one of its components: the turgor pressure (P). The difference in Ψ at constant f/fo (δ) was found to be dependent on net CO2 assimilation (A) and might be used as a rough estimator of photosynthetic activity. It was then, the other main component of Ψ, osmotic potential (π), the one that may have lowered the values of md Ψ with respect to pd Ψ by the accumulation of sugars associated to net CO2assimilation. This phenomenon suggests the existence of a diurnal osmotic adjustment in this species associated to sugars production in well-watered plants. [ABSTRACT FROM AUTHOR]

The turgor loss point (TLP), which is considered a threshold for many physiological processes, may be useful in plant-breeding programs or for the selection of reforestation species. Obtaining TLP through the standard pressure–volume (p–v) curve method in a large set of species is highly time-consuming and somewhat subjective. To solve this problem, we present an objective and a less time-consuming technique based on the leaf resonance able to calculate the relative water content (RWC) at TLP (RWCTLP). This method uses air-coupled broadband ultrasonic spectroscopy to obtain the sigmoidal relation between RWC and the standardized resonant frequency (f/fo). For the 13 species measured, the inflexion point of the RWC–f/fo relationship () was not statistically different from the value of RWC at the TLP obtained with the p–v curves (RWCTLP p–v). [ABSTRACT FROM PUBLISHER]

The implementation of non-destructive methods for the study of water changes within plant tissues and/or organs has been a target for some time in plant physiology. Recent advances in air-coupled ultrasonic spectroscopy have enabled ultrasonic waves to be applied to the on-line and real-time assessment of the water content of different materials. In this study, this technique has been applied as a non-destructive, non-invasive, non-contact, and repeatable method for the determination of water status in Populus×euramericana and Prunus laurocerasus leaves. Frequency spectra of the transmittance of ultrasounds through plant leaves reveal the presence of at least one resonance. At this resonant frequency, transmittance is at its maximum. This work demonstrates that changes in leaf relative water content (RWC) and water potential (Ψ) for both species can be accurately monitored by the corresponding changes in resonant frequency. The differential response found between both species may be due to the contrasting leaf structural features and the differences found in the parameters derived from the P–V curves. The turgor loss point has been precisely defined by this new technique, as it is derived from the lack of significant differences between the relative water content at the turgor loss point (RWCTLP) obtained from P–V curves and ultrasonic measurements. The measurement of the turgor gradient between two different points of a naturally transpiring leaf is easily carried out with the method introduced here. Therefore, such a procedure can be an accurate tool for the study of all processes where changes in leaf water status are involved. [ABSTRACT FROM PUBLISHER]

Forest ecosystems in the Mediterranean Basin are mostly situated in the north of the Basin (mesic). In the most southern and dry areas, the forest can only exist where topography and/or altitude favor a sufficient availability of water to sustain forest biomass. We have conducted a thorough review of recent literature (2000–2021) that clearly indicates large direct and indirect impacts of increasing drought conditions on the forests of the Mediterranean Basin, their changes in surface and distribution areas, and the main impacts they have suffered. We have focused on the main trends that emerge from the current literature and have highlighted the main threatens and management solution for the maintenance of these forests. The results clearly indicate large direct and indirect impacts of increasing drought conditions on the forests of the Mediterranean Basin. These increasing drought conditions together with over-exploitation, pest expansion, fire and soil degradation, are synergistically driving to forest regression and dieback in several areas of this Mediterranean Basin. These environmental changes have triggered responses in tree morphology, physiology, growth, reproduction, and mortality. We identified at least seven causes of the changes in the last three decades that have led to the current situation and that can provide clues for projecting the future of these forests: (i) The direct effect of increased aridity due to more frequent and prolonged droughts, which has driven Mediterranean forest communities to the limit of their capacity to respond to drought and escape to wetter sites, (ii) the indirect effects of drought, mainly by the spread of pests and fires, (iii) the direct and indirect effects of anthropogenic activity associated with general environmental degradation, including soil degradation and the impacts of fire, species invasion and pollution, (iv) human pressure and intense management of water resources, (v) agricultural land abandonment in the northern Mediterranean Basin without adequate management of new forests, (vi) very high pressure on forested areas of northern Africa coupled with the demographic enhancement, the expansion of crops and higher livestock pressure, and the more intense and overexploitation of water resources uses on the remaining forested areas, and (vii) scarcity and inequality of human management and policies, depending on the national and/or regional governments and agencies, being unable to counteract the previous changes. We identified appropriate measures of management intervention, using the most adequate techniques and processes to counteract these impacts and thus to conserve the health, service capacity, and biodiversity of Mediterranean forests. Future policies should, moreover, promote research to improve our knowledge of the mechanisms of, and the effects on, nutrient and carbon plant-soil status concurrent with the impacts of aridity and leaching due to the effects of current changes. Finally, we acknowledge the difficulty to obtain an accurate quantification of the impacts of increasing aridity rise that warrants an urgent investment in more focused research to further develop future tools in order to counteract the negative effects of climate change on Mediterranean forests. [ABSTRACT FROM AUTHOR]

Masson pine is an important afforestation species in southern China, where seasonal drought is common. The present study focused on the effects of Suillus placidus, an ectomycorrhizal fungus, inoculation on the growth and physiological and biochemical performance of masson pine seedlings under four different watering treatments (well-watered, mild drought, moderate drought, and severe drought) to evaluate the symbiotic relationship between S. placidus and masson pine seedlings. Ectomycorrhizal-inoculated (ECM) and non-inoculated (NM) seedlings were grown in pots and maintained for 60 days using the weighing method. Results showed that seedlings' growth, dry weight, RWC, chlorophyll content, PSII efficiency, and photosynthesis decreased as drought stress intensified in both ECM and NM plants. This suggests that drought stress significantly limits the growth and photosynthetic performance of masson pine seedlings. Nevertheless, increased An/gs and proline contents in both NM and ECM prevented oxidative damage caused by drought stress. In addition, increased peroxidase (POD) activity is an essential defense mechanism of ECM seedling under drought stress. Compared with NM, ECM seedlings showed faster growth, higher RWC, and photosynthetic performance, and lower lipid peroxidation in cell membranes under drought stress, as indicated by higher POD activity and lower proline and malondialdehyde (MDA). Our experiment found that S. placidus inoculation can enhance the drought resistance of masson pine seedlings by increasing antioxidant enzyme activity, water use efficiency, and proline content, thereby enhancing growth under water-deficiency conditions. S. placidus can be used to cultivate high-quality seedlings and improve their survival in regions that experience seasonal droughts. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

Holm oak (Quercus ilex L.) is a Mediterranean species that can withstand intense summer drought through a high resistance to cavitation far beyond the stomatal closure. Besides stomatal limitations, both mesophyll and biochemical limitations to CO2 uptake could increase in holm oak under drought. However, no studies have addressed how hydraulic and non-hydraulic factors may limit the recovery of photosynthesis when re-watering after inducing 50% loss of hydraulic conductivity. We measured photosynthetic traits, xylem embolism, and abscisic acid (ABA) in holm oak with increasing levels of drought stress and seven days after plant re-watering. Drought stress caused a sharp decrease in net CO2 assimilation (AN), stomatal and mesophyll conductance (gs and gm), and maximum velocity of carboxylation (Vcmax). The stomatal closure could be mediated by the rapid increase found in ABA. The high level of xylem embolism explained the strong down-regulation of gs even after re-watering. Therefore, only a partial recovery of AN was observed, in spite of non-hydraulic factors not limiting the recovery of AN, because i/ABA strongly decreased after re-watering, and ii/gm and Vcmax recovered their original values. Therefore, the hydraulic-stomatal limitation model would be involved in the partial recovery of AN, in order to prevent extensive xylem embolism under subsequent drought events that could compromise holm oak survival. [ABSTRACT FROM AUTHOR]

Extreme droughts and heat events, frequently produced in Mediterranean climates, induce anomalies in the ecosystem–atmosphere CO2 fluxes. In order to mitigate the consequences on forests and agriculture, managers must have a better knowledge of the ecosystem by monitoring plant status. Water status is commonly observed measuring water potential but when the extreme event is over, this parameter cannot show managers the recovery of other physiological processes such as photosynthesis. To address this problem, we have evaluated the Quercus ilex L. water status and photosynthetic capacity throughout an intense water scarcity event and a subsequent re-watering. Photosynthetic capacity was evaluated through chlorophyll fluorescence parameters and leaf reflectance indices. We found that all fluorescence parameters changed as water potential decreased and they did not completely recover after re-watering. Among the reflectance indices, the physiological reflectance index (PRI) varied similarly to fluorescence, obtaining a strong correlation with the non-photochemical quenching (NPQ). We proposed using PRI to detect the level of photosynthetic capacity in Q. ilex, as it is much easier-to-handle. We also concluded that intense droughts and heat stress not only might reduce photosynthetic capacity through changes in Chl fluorescence parameters during the stress period, but might also affect photosynthetic capacity once the plant water status is recovered. [ABSTRACT FROM AUTHOR]

• Holm oak is an evergreen and sclerophyllous tree closely associated to Mediterranean-type climates. • However, the origin of this species can be found in pre-Mediterranean lineages, under tropical or subtropical climates. • To survive in Mediterranean-type climate implies to withstand dry and hot summer, cold winters or both. • The overall response of holm oak to stress factors (drought, high or low temperatures) suggests the existence of a marked within-species variability. • The current taxonomical consideration of holm oak as a complex of two different subspecies might go beyond morphological traits or geographical distribution. Q. ilex is a circum-Mediterranean species which must deal with two stressful periods throughout the year: summer and winter. However, the intensity of these abiotic stresses is very variable depending on the specific area of its distribution range. Besides, two subspecies are usually distinguished, even sometimes recognised as two different true species. Nevertheless, differences in the response to both summer and winter stresses among populations overstep the purely taxonomic question. Regarding to temperature, this species has shown a high resistance to both extremes. On the one hand, critical temperature at leaf level has been estimated in 50.2 °C for a provenance in the Iberian Peninsula (Soria, Spain), a temperature which even in the hottest areas of its distribution range is hardly ever reached. On the other hand, minimal temperature for the survival of the leaves has been estimated in −19.8 °C for Italian provenances and -26.6 °C for one of the coldest provenances (Soria, Spain). Similarly, these temperatures are quite rarely reachable in their respective provenances. Concerning to summer-drought, this may compromise xylem stability in severe cases of scarcity of water in the soil, inducing cavitation when water soil potential is too negative. In this sense, Q. ilex has demonstrated to have a high resistance to cavitation, represented by quite negative values of P 50 and P 88 when compared to other species. Besides, Iberian provenances (i.e. the most continental ones) have shown a higher resistance to cavitation than French or Italian ones. Likewise, holm oak may suffer cavitation during winter, but in this case by a completely different factor: freezing-thawing cycles. Winter-cavitation seems to be related to vessel size, with wider vessels being more vulnerable to cavitate than narrower ones. In spite of its paleotropical origin, Q. ilex has achieved to develop multiple adaptations and physiological strategies that has enabled it not only to withstand the hard climatic conditions imposed by Mediterranean climate, but to colonize and even dominate great part of the Mediterranean landscape. [ABSTRACT FROM AUTHOR]