Search

Highlights • Reductions in stand density improve post drought growth resilience. • Drought resistance is higher in P. sylvestris than in P. nigra in heavily thinned stands. • Water-use efficiency is negatively related to thinning intensity. • Thinning makes plantations less vulnerable to drought. Abstract Previous research has provided insights into the potential response of growth and water use efficiency to thinning in Mediterranean forests, but little is known about the potential benefits of silviculture for plantations under severe drought stress. We selected two stands of Pinus sylvestris L. and P. nigra Arnold., and used dendrochronology and carbon isotopes (δ13C) to understand the growth and functional responses of high-density planted pine forests to thinning in drought-prone areas. Resistance, recovery, and resilience indices were calculated for each species using BAI data. We expected heavy thinning to produce stands that were more resilient to drought, particularly for the more drought-tolerant P. nigra. Differences in the basal area increments (BAI) and intrinsic water-use efficiency (iWUE) were found between the unthinned stand and the thinned stands, for both species. After thinning, BAI decreased as iWUE increased but this relationship was only marginally significant in P. sylvestris. Thinning increased growth resistance, recovery, and resilience following the severe drought of 2012. Our findings suggest that water shortage, linked to recurrent droughts, together with high tree competition, negatively affected tree growth and increased iWUE, which explains the dieback of some of these pine plantations. We evidence the vulnerability of densely planted Mediterranean pines to the forecasted warmer and drier conditions. These results show that a heavy thinning treatment (60% of basal area removed) provides a promising silvicultural framework for the adaptation of these drought-sensitive Mediterranean mountain pine forests to the potential risks of climate change. [ABSTRACT FROM AUTHOR]

Drought is likely to increase in intensity and frequency across most of the Mediterranean areas due to climate change. There is thus an urgent need to assess differences in the ability of plants to withstand water stress, especially when selecting appropriate species for ecological intervention. This study focuses on Mediterranean-type ecosystems (MTEs) and identifies plant traits associated with drought resistance that are key in differentiating plant functional types in relation to water use. We further discuss how this knowledge can be used by restoration practitioners. The study was conducted in the Banksia woodlands, Southwestern Australia, and six areas across a gradient of water availability were selected. We measured twelve functional traits associated with water use in fifteen plant species. Next, we applied multivariate analyses to examine how traits varied in relation to each other, grouping species based on these traits and investigating similarities within and between functional groups and sites. Functional trait correlations were consistent with the worldwide leaf and wood economic spectra. Among the twelve traits measured, six explained most trait variation: mean xylem vessel diameter (D ave ), number of xylem vessels per mm −2 (D s ), leaf mass per area (LMA), stem density (WD), foliar carbon isotope composition (δ 13 C), and leaf water potential at turgor loss point (π TLP ). Species were clustered into five different functional groups. Differences within and between functional groups and sites are reported through their Euclidean distances. Analyses of these traits provided insights into the water-use strategies of native plants, revealing those species with greatest potential to resist water deficits. Such knowledge enables the formation of a more functionally diverse assembly of species bearing complementary traits, which in turn can be used to strengthen resistance to invasion in restored communities. This functional ecological approach is transferable to other and for application by restoration practitioners since the traits selected are relatively easy and cheap to measure and require only simple analytical approaches. [ABSTRACT FROM AUTHOR]

Past and present environmental changes cause significant changes in tree growth in many parts of the world, where both decreasing and increasing growth trends have been detected over the last decades. The Mediterranean basin is especially sensitive to climate change and subsequent tree growth declines. In this article, we present the first study on recent tree growth trends in Turkey. Pinus nigra is a drought-sensitive species and one of the most common and economically important native conifers to Turkey. Tree-ring cores were taken from 61 Pinus nigra plots spread over the entire Lakes District (Southwest Turkey), near the species’ southern range limit. The samples cover the 1839–2013 time period. We apply the Regional Curve Standardization technique and statistical modeling to the tree ring width data to investigate long-term growth trends. Both methods show remarkably similar results: a slowly increasing growth trend until the 1970s, followed by a decreasing trend. This recent negative trend is highly correlated with increased temperature and drought in summer, which suggests that it is likely caused by climate change. [ABSTRACT FROM AUTHOR]

Spatial association between pine recruitment and shrubs is widely observed in dry environments and often associated with physical facilitation provided by the shrub, alleviating seedling drought stress. However, shrubs may also facilitate recruitment by sheltering seeds from granivores. In this study we investigated the influence of shrub-related microsites on post-dispersal pine seed survival in dry Mediterranean shrubland. We present a novel approach investigating the survival of single seeds in the context of long-distance wind dispersal. In four dry shrubland sites of central Israel, we placed seeds of Pinus halepensis in different microsites associated with Sarcopoterium spinosum shrubs: under canopy, at two margins and in between shrubs. We monitored the seed persistence monthly, replacing seeds removed by granivores. We also compared seed survival in burned vs. unburned areas. Logistic regression showed that seeds placed under shrubs had significantly higher survival rates than elsewhere. In unburned sites survival was higher than in burned sites and remained higher under shrub canopies. We show that these effects were substantial by considering the length of the season through which seeds need to persist. We conclude that shrubs constitute an important facilitator for Pinus halepensis colonization in dry shrubland through their positive effect on seed survival. [ABSTRACT FROM AUTHOR]

Increased forest vulnerability is being reflected as more widespread and severe drought-induced decline episodes. In particular, the Mediterranean area is revealing a high susceptibility to phenomena of loss in tree vitality across species. Within tree species, oaks (Quercus spp.) are experiencing extensive decline in many countries. However, in the wake of the so-called "oak decline phenomenon", the attention on these species has generally been limited. In this paper, we review the current available literature on oakdecline cases reported within the Mediterranean Basin, with particular remark for those occurred in Italy and Spain. More specifically our main aims were to: (i) provide an update on the patterns and mechanisms of decline by focusing on tree-ring and wood-anatomical variables; (ii) provide some hints for improving the resistance and resilience of oak stands experiencing decline. Our review reveals that drought is reported as the main driver triggering oak decline within the Mediterranean Basin, although other causes (i.e., increasing temperature, pathogens attack or excessive stand density) could exacerbate decline. In most reported cases, drought induced a substantial reduction of growth and changes in some wood anatomical properties. Indeed, growth decline prior death is also indicated as an early-warning signal of impending death. In ring-porous oak species, declining trees were often characterized by a very low production of latewood and a decrease in lumen area of the widest earlywood vessels, suggesting a potential reduction of hydraulic conductivity. Moreover, hydraulic dysfunction is reported as the main cause of decline. Finally, regarding management actions that should be considered for improving the resilience of declining stands and preserve the species-specific stand composition, it could be useful to shorten the rotation period of coppice stands or promoting their gradual conversion towards high forests, and favoring more drought-resistant species should also be considered. In addition, regeneration prior to regeneration cuts should be improved by anticipating seed dispersal or by planting oak seedlings obtained from local germoplasm. [ABSTRACT FROM AUTHOR]

The differential degree by which paraheliotropism may counterbalance the deleterious impact of high irradiance between congeneric species in relation to different water availabilities has been poorly investigated. We followed the evolution of gas exchange, quenching analysis and OJIP parameters in restrained (R) and free (F) to move leaves of Cistus monspeliensis ( CM ) and Cistus salvifolius ( CS ) under drought stress (WS) and well-watered conditions (WW). Concerning gas exchange parameters, leaf restriction effect was overall not significant in CM except in apparent carboxylation efficiency (C e ) under WS, while CS showed a significant sensitivity of maximum net photosynthetic rate (A max ), stomatal conductance (gs) and C e even under WW. The recovery analysis highlighted also a faster gs recovery in F leaves. Furthermore, in both the species, restriction affected photon allocation pathways especially in terms of light-regulated and light-independent constitutive non-photochemical energy dissipation under WW, ultimately affecting electron transport rate (ETR). Nevertheless, the OJIP analysis provided us evidences that CM was characterized by a down-regulation of ETR while an impairment occurs in CS . In CM this was due to its ability to modify a certain fraction of reaction centers thus resulting in a higher capability for dissipation of excess light energy under well-watered conditions, not affecting electron transport efficiency. This response was not observed in CS . Overall, we demonstrated that congeneric species, even mostly sharing the same physiological targets, differ in the degree by which leaf movements help to counterbalance the negative effect of the high irradiance in relation with the amount of water available. [ABSTRACT FROM AUTHOR]

Performance and survival of Mediterranean trees depend on their capacity to cope with water stress. In harsh environments, xylem growth strategies are critical to ensure efficient water transport, though these are largely unknown for many species, including most Mediterranean oaks. We investigated xylem anatomical traits in tree-ring series of deciduous Quercus ithaburensis and Q. boissieri in the South-Eastern Mediterranean Basin to assess xylem response to climate variability, with special attention to dry and warm years and to temperature increase over the last decades. Abundant precipitation and low temperature during the wet season (November–April) benefited xylem formation of both species. However, intra-ring analysis revealed different patterns, as the size of early-formed vessels in Q. boissieri was related to previous-year ring, while in Q. ithaburensis it was strongly affected by current wet season conditions. During dry years, vessel size and number in Q. ithaburensis were strongly reduced, but they fully recovered in the following year (high resilience). Conversely, Q. boissieri vessels were barely reduced during dry or warm years (high resistance). Different strategies allow both species to overcome dry and warm years, but their capacity to withstand long-term warming seems to differ. Q. ithaburensis xylem traits did not evidence any trend over the last few decades, while Q. boissieri was facing a decline in some traits. Our analysis suggests that the growth strategies of Q. ithaburensis would be effective under future drier conditions, while temperature increase could induce a hydraulic deterioration and a potential decline in the less fit Q. boissieri . [ABSTRACT FROM AUTHOR]

Mediterranean pine forests are at risk of experiencing a decline in tree growth in response to climate warming if rising temperatures amplify drought stress. In mountain areas, tree growth could be enhanced in temperature-limited high elevations, whilst it might decline at water-constrained low elevations. Species differential responses could, however, modulate the impact of drought on forests along altitudinal gradients. To test for evidence of species differential drought impacts along an altitudinal gradient, we studied the growth responses of two Iberian pine species ( Pinus sylvestris and Pinus nigra ) subjected to Mediterranean conditions in Eastern Spain. We analysed the stability of growth (basal area increment) responses to climate and drought during the 1950–2014 period by using resistance and resilience indices. Pinus sylvestris growth was enhanced by warm spring temperatures, while Pinus nigra growth was improved by a positive spring water balance. Pinus sylvestris growth decreased temporally at the lower end of its altitudinal range, whereas Pinus nigra growth decreased at the upper end. Pinus sylvestris exhibited low growth stability at its low-elevation limit. Pinus nigra resistance also decreased along its altitudinal range, but this effect was compensated by a high resilience. In mixed stands the results were contrasting, with Pinus sylvestris (at the lower altitudinal range) being more vulnerable to drought-induced growth decline than Pinus nigra (at the upper altitudinal range). Under more severe and frequent dry spells, Pinus sylvestris at low altitude may be highly vulnerable to water shortage while Pinus nigra performs better. These growth responses are in line with the expectation of a contraction in the realized niche of Pinus sylvestris with extirpation of low-elevation stands if warmer and drier conditions continue. Moreover, we show that analysing growth resilience in response to drought is a useful method to anticipate likely changes in species dominance and thus to assist managers in designing forest adaptation strategies. [ABSTRACT FROM AUTHOR]

Worldwide, extensive agricultural losses are attributed to drought, often in combination with heat in Mediterranean climate regions, where grapevine traditionally grows. The available scenarios for climate change suggest increases in aridity in these regions. Under natural conditions plants are affected by a combination of stresses, triggering synergistic or antagonistic physiological, metabolic or transcriptomic responses unique to the combination. However the study of such stresses in a controlled environment can elucidate important mechanisms by allowing the separation of the effects of individual stresses. To gather those effects, cuttings of two grapevine varieties, Touriga Nacional ( TN) and Trincadeira ( TR), were grown under controlled conditions and subjected to three abiotic stresses (drought - WS, heat - HS and high light - LS) individually and in combination two-by-two ( WSHS, WSLS, HSLS) or all three ( WSHSLS). Photosynthesis, water status, contents of H2O2, abscisic acid and metabolites of the ascorbate-glutathione cycle were measured in the leaves. Common and distinct response features were identified in the different stress combinations. Photosynthesis was not hindered in TN by LS, while even individual stresses severely affect photosynthesis in TR. Abscisic acid may be implicated in grapevine osmotic responses since it is correlated with tolerance parameters, especially in combined stresses involving drought. Overall, the responses to drought-including treatments were clearly distinct to those without drought. From the specific behaviours of the varieties, it can be concluded that TN shows a higher capacity for heat dissipation and for withstanding high light intensities, indicating better adjustment to warm conditions, provided that water supply is plentiful. [ABSTRACT FROM AUTHOR]

Changes in climate trends and extreme climatic events are increasingly impacting on forests around the world. In order to better understand how and where major ecological and climatic changes will affect our forested ecosystems, tools based on landscape sensitivity analysis need to be developed to help inform sustainable forest management. This study was undertaken in the Northern Jarrah Forest (NJF) region in the Mediterranean climate of southwest Western Australia. Extreme drought and multiple heatwaves in 2010/2011 resulted in large-scale tree canopy dieback in the NJF. In this study, we used Landsat satellite imagery to (1) accurately map the damaged areas, (2) produce a damage probability model and (3) compare the model with a probability model derived from data collected through an airborne flight survey. We found that the Landsat-derived Normalized Difference Vegetation Indexwas a good indicator of drought/heat induced damagein the NJFregion. Both probability models identified the same set of topography and climate-related factors for determining the probability of drought/heat damage within the landscape. Extrapolation of the Landsat satellite method-based model, however, produced a more deterministic and useful drought/heat damage sensitivity map for the NJF region. The techniques and tools developed, and applied, in this study can readily be transferred to other regions around the world and can assist in the sustainable management and timely climate adaptation efforts to accommodate our future forests. [ABSTRACT FROM AUTHOR]

Differences in water-acquisition strategies of tree root systems can determine the capacity to survive under severe drought. We evaluate the effects of field water shortage on early survival, growth and root morphological variables of two South American Mediterranean tree species with different rooting strategies during two growing seasons. One year-old Quillaja saponaria (deep-rooted) and Cryptocarya alba (shallow-rooted) seedlings were established under two watering treatments (2 L·week-1·plant-1 and no water) in a complete randomized design. Watering improved the final survival of both species, but the increase was only significantly higher for the shallow-rooted species. The survival rates of deep- and shallow-rooted species was 100% and 71% with watering treatment, and 96% and 10% for the unwatered treatment, respectively. Root morphological variables of deep-rooted species such as surface area, volume, and diameter were higher under unwatered treatment. On the other hand, shallow-rooted species had a higher total root dry mass, length, surface area with watering treatments. Our findings suggest that deep-rooted species are highly recommended for reforestation in dry conditions, even under low soil water availability. Water supplements during the summer season can attenuate the differences between deep- and shallow-rooted species in their ability to survive drought during the early stage. [ABSTRACT FROM AUTHOR]

Studies have shown that some microorganisms autochthonous from stressful environments are beneficial when used with autochthonous plants, but these microorganisms rarely have been tested with allochthonous plants of agronomic interest. This study investigates the effectiveness of drought-adapted autochthonous microorganisms [ Bacillus thuringiensis (Bt) and a consortium of arbuscular mycorrhizal (AM) fungi] from a degraded Mediterranean area to improve plant growth and physiology in Zea mays under drought stress. Maize plants were inoculated or not with B. thuringiensis , a consortium of AM fungi or a combination of both microorganisms. Plants were cultivated under well-watered conditions or subjected to drought stress. Several physiological parameters were measured, including among others, plant growth, photosynthetic efficiency, nutrients content, oxidative damage to lipids, accumulation of proline and antioxidant compounds, root hydraulic conductivity and the expression of plant aquaporin genes. Under drought conditions, the inoculation of Bt increased significantly the accumulation of nutrients. The combined inoculation of both microorganisms decreased the oxidative damage to lipids and accumulation of proline induced by drought. Several maize aquaporins able to transport water, CO 2 and other compounds were regulated by the microbial inoculants. The impact of these microorganisms on plant drought tolerance was complementary, since Bt increased mainly plant nutrition and AM fungi were more active improving stress tolerance/homeostatic mechanisms, including regulation of plant aquaporins with several putative physiological functions. Thus, the use of autochthonous beneficial microorganisms from a degraded Mediterranean area is useful to protect not only native plants against drought, but also an agronomically important plant such as maize. [ABSTRACT FROM AUTHOR]

This work investigated the variability in drought tolerance under arid conditions of Mediterranean alfalfa genotypes with the overall aim to assess the main criteria that are associated with the relative tolerance and to discover the most tolerant ecotypes. For this, 16 alfalfa genotypes originating from seven countries of the Mediterranean basin were tested in an experimental station in south of Tunisia. The trial was conducted under two irrigation treatments. The first was normally irrigated by providing an amount of water corresponding to the potential evapotranspiration of the crop, and in the second with water deficit which was applied by stopping the irrigation during 8 weeks in summer. A significant decrease was observed under water deficit for biomass production, leaf stem ratio and water use efficiency. The sensitive index, in stress treatment, varies between 13.8% and 46.2% for dry yield. Results showed that some genotypes exhibited high-forage yield potential even in the presence of stress, mainly Amerist, Sardi10 and Siriver. Proline accumulation in leaves was greater in water-stressed plants, while the K+osmo-regulatory role was not definite. High biomass production, accumulation of proline and constancy of K+in leaves are the most important criteria for tolerant alfalfa. [ABSTRACT FROM AUTHOR]

Temporary ponds (TPs) are shallow water bodies characterized by alternating phases of drought and flooding. They exhibit a small-scale zonation with a central belt (CB), an intermediate belt, and an outer belt (OB). Starting from the hypothesis that plant assemblage composition is affected by the position within TPs, our aims were to define the small-scale patterns of plant functional types, soil evolution, and soil physicochemical properties and to analyze their relationships and how they were affected by pond characteristics (i.e., elevation, substratum, total size, belt size, maximum water depth, and hydroperiod). Our results pointed out that patterns of plant functional types and soil evolution followed the position within the pond, soil physicochemical properties were weakly affected, and there was a relationship between plant functional types, soil small-scale patterns, and pond characteristics. Principal component analysis showed a positive correlation of Alfisols with CB, aquatic and amphibious species, maximum water depth, and hydroperiod, and of Entisols with OB and terrestrial species. A combined microtopography–hydrology effect was the primary factor controlling soil evolution and plant functional type patterns. As a consequence, modification in topographic shape and/or hydrologic parameters could alter TP environments. The results provide information for management strategies as an answer to the concerns surrounding continued worldwide TP decline. [ABSTRACT FROM AUTHOR]

The cultivation of giant reed lignocellulosic feedstock on marginal land, appears of great interest in the Mediterranean basin, due to its high production potential under drought conditions. Starting from a three years field survey, in this paper a LCA was applied to the overall 15 years life cycle of a Mediterranean giant reed cultivation in Southern Italy, including Direct Field Emissions (DFE) and non-productive phases of cultivation. Agricultural practices performed yearly for field maintenance (FM-urea fertilization) and harvest operation (HO) shared the largest part of total burdens, nonetheless, the impact of crop establishment and final removal appeared significant. FM was strongly affected by DFE of soil biogenic N 2 O and fossil CO 2 (about 23% and 9% of total Climate change, respectively) and volatilized NH 3 (about 69%, 42% and 35% of total impact for Terrestrial acidification, Particulate matter formation and Marine eutrophication, respectively). For Marine eutrophication, also nitrate leaching in the plantation year, affected significantly the overall life cycle. Differently DFE linked to field application of K-P fertilizers for seedbed preparation (P and heavy metal emissions) and pest for final eradication (glyphosate emission), appeared more circumscribed. The study highlighted the need to carefully approach to soil carbon storage, and disclosed to be sensitive to uncertainty linked to DFE and crop yield pattern estimation. [ABSTRACT FROM AUTHOR]

Context: Silver fir is declining and dying at its southern margin on the Mediterranean area, where climate is expected to be warmer and drier. At a regional scale, silver fir seems to be vulnerable to drought, and at a forest stand scale, tree death seems to be distributed according to soil water availability. Aim: To understand the vulnerability of silver fir to drought, factors involved in the regulation of transpiration were assessed with respect to soil properties in order to document the spatial distribution of death rates. Methods: Soil properties were characterized by electric resistivity measurements. Sap flow density and predawn needle water potential were recorded on sampled trees during several years, and crown specific transpiration was estimated. In addition, the vulnerability of coarse roots and branches to cavitation was quantified. Results and conclusion: Trees growing on soils with a large water storage capacity were the most vulnerable to drought induced soil water deficits. Transpiration was down-regulated as soon as predawn water potential decreased. The vulnerability to cavitation was low, which protected the trees from run-away xylem embolism. Severe soil water deficits led to a rapid decrease of transpiration, which was still visible the following year. The drop-off in transpiration was mainly due to inner sap flow that almost ceased after the drought on all monitored trees. Our results suggest that root dynamic and the ability of roots to take up water were modified by soil water deficit over several years. Such a regulation scheme needs to be better documented and included in models to address silver fir forest responses to drought. [ABSTRACT FROM AUTHOR]

Highlights: [•] This review focuses on the significance of volatile isoprenoids for plant protection. [•] Volatile isoprenoids have antioxidant and membrane stabilizing properties. [•] Volatile isoprenoids are beneficial for plants, especially under Mediterranean climate constraints. [•] Isoprene is phylogenetically determined, and is replaced by less volatile isoprenoids under prolonged and severe drought. [ABSTRACT FROM AUTHOR]

Highlights: [•] Mediterranean plants are exposed to prolonged summer drought stress, requiring specific adaptations to survive. [•] Embolism repair or ion-mediated compensation might help Mediterranean plants to cope with xylem cavitation. [•] Small leaves in Mediterranean plants may guarantee high vein density, protecting from drought-induced hydraulic dysfunction. [•] Climate change might surpass the capacity of Mediterranean plants to survive more intense or prolonged drought events. [ABSTRACT FROM AUTHOR]

Highlights: [•] Rising CO2 will exert beneficial effects on Mediterranean sclerophylls, which are slow in acclimation. [•] Warming may counteract the effects of elevated CO2 on Mediterranean vegetation. [•] Prolonged and severe drought events may favor drought-deciduous over sclerophyll species. [•] If drought-induced metabolic limitations to photosynthesis prevail, the impact of rising CO2 is little. [•] Species at the Southern limit of their distribution range will be the most affected by climate change. [ABSTRACT FROM AUTHOR]

Increased drought length and intensity is expected in the Mediterranean basin under anthropogenic increase in atmospheric CO, leading to extreme events not yet encountered in the present climate variability. Understanding ecosystems responses and capturing peculiar ecophysiological processes related to these events have been investigated in the field by rainfall manipulation experiments. Quantifying the actual drought faced by the ecosystem under control and dry plots, or among experiments remain a key challenge for explaining functional impacts on plant growth. Full-profile soil water content can be tricky to assess in rocky soils, and time-consuming plant water potential measurements remain a discrete information unable to capture short rainfall pulses. We propose here to fully investigate the water budget of a total rainfall interception manipulation on a Mediterranean shrubland, coupled with a plant-soil water balance model. We could accurately simulate the seasonal course of plant water status, including small rainfall pulses. We then derived yearly estimates of water stress integral for each water treatment, leading to an estimate of 66-86 % increase of drought intensity for the dry treatment compared to the control. Comparing actual and expected plant water budget from simulations in the dry plots allowed to identify and quantify the impact of methodological issues related to rainfall interception experiments as side effects for intrusive rain drops and subsurface lateral water flow. [ABSTRACT FROM AUTHOR]

Extreme climatic events such as intense droughts are becoming more frequent in Mediterranean regions, but our understanding of their impact on tree performance is still fragmentary. We analyzed growth and sap flow responses for a 3-year period including the most stressful drought over the last half century in the evergreen Pinus nigra and the deciduous Quercus faginea, two dominant tree species in the continental plateau of the Iberian Peninsula. Our aim was to quantify the differential impacts of this event on the performance of both species and their modulation by local microclimate. Growth was registered with digital dendrometers, and water use was assessed by continuously recording sap flow in 8-9 coexisting adult individuals of each species in two sites. Q. faginea spring growth rate decreased by 60 % during the dry year at the dry site, while the decrease in P. nigra was around 36 %. P. nigra exhibited larger sap flow reductions during the dry season and also larger decreases during the extreme year, but in contrast to Q. faginea, it was able to recover growth and sap flow values after the extreme drought. Minor microclimatic differences between sites had significant effects on growth and water use, with slightly more mesic conditions significantly attenuating the impact of drought on both species. Findings suggest that the study species were near to their tolerance thresholds, so that even moderate increases in the intensity and frequency of unusual droughts have important consequences for individual tree performance, and eventually species coexistence and ecosystem processes. [ABSTRACT FROM AUTHOR]

Highlights: [•] Predictive mortality models of one Mediterranean oak and one pine species. [•] Describe variability in the pattern of mortality and decline along climatic gradients. [•] Pine mortality and decline are partly related to enhanced drought. [•] Climate and management interact to cause oak-pine decline. [•] Pines at their xeric ecotone are threatened by global change. [ABSTRACT FROM AUTHOR]

Abstract: Water availability is one of the key environmental factors that affect plant establishment and distribution. In many regions water availability will decline with climate change, exposing small seedlings to a greater likelihood of drought. In this study, 17 leaves, stem, root, and whole-plant traits of ten woody Mediterranean species were measured under favourable growing conditions and seedling drought survival was evaluated during a simulated dry-down episode. The aims of this study were: i) to assess drought survival of different species, ii) to analyse which functional traits predict drought survival time, and iii) to explain species distribution in the field, based on species drought survival and drought strategies. Drought survival time varied ten-fold across species, from 19 to 192 days. Across species, drought survival was positively related to the rooting depth per leaf area, i.e., the ability to acquire water from deeper soil layers while reducing transpiring leaf area. Drought survival time was negatively related to species ability to grow quickly, as indicated by high relative growth and net assimilation rates. Drought survival also explained species distribution in the field. It was found that species were sorted along a continuum, ranging between two contrasting species functional extremes based on functional traits and drought performance. One extreme consisted of acquisitive fast-growing deciduous species, with thin, soft metabolically active leaves, with high resource use and vulnerability to drought. The opposite extreme consisted of conservative slow-growing evergreen species with sclerophyllous leaves, deep roots, a low transpiring area, and low water use, resulting in high drought survival and drought tolerance. The results show that these drought strategies shape species distribution in this Mediterranean area. [Copyright &y& Elsevier]

Highlights: [•] Thermo- and medio-Mediterranean vegetation belt areas suffer from wild fires. [•] We instigate climate effects on fire linked to features of phreato-, thero-phytes. [•] Rainfall, not temperature, determines fire spread, both explaining fire numbers. [•] A rainy wet season suppresses burnt area as deep rooted plants become moister. [•] A rainy spring promotes fire outbreaks and spread as therophytic fine fuel increases. [ABSTRACT FROM AUTHOR]

Future changes in tree growth, associated with a warmer and drier climate, are predicted for many species and locations across the European Mediterranean Basin. However, quantification of the intensity and severity of related consequences for forest ecosystem functioning and productivity remains challenging. Species-specific distribution limits that are particularly sensitive to small changes in the ambient climate may provide an ideal test bed to assess the nature of past growth trends and extremes and their responsible controls. Here, we seek to understand how twentieth century climate change affected the growth of European beech ( Fagus sylvatica L.) nearby its south-eastern distribution limit in Albania and Macedonia on the Balkan Peninsula. We sampled 93 living trees from undisturbed mixed forest stands at ~1,450 m a.s.l. and 29 timbers from nearby historical buildings. Application of different tree-ring detrending techniques allowed robust composite chronologies with varying degrees of high- to low-frequency variability to be developed back to 1648 ad. Comparison with local meteorological station measurements and continental grid-box climate indices revealed spatiotemporal instability in growth-climate response patterns. Nevertheless, year-to-year and decadal-long fluctuations in radial beech growth were significantly ( P < 0.001) negatively correlated at −0.61 with June-September temperature over the 1951-1995 period. This (inverse) relationship between increased beech growth and decreased summer temperature is somewhat indicative for the importance of plant-available soil moisture, which likely controls ring width formation near the species-specific south-eastern distribution limit. Significant positive correlations between beech growth and drought (scPDSI; r = 0.57) confirm metabolistic drought constraints. However, an unexpected late twentieth century growth increase not only contradicts the previously observed growth dependency to summer soil moisture, but also denies any putative drought-induced forest ecosystem suppression in this part of the Mediterranean Basin. [ABSTRACT FROM AUTHOR]

Forest dynamics will depend upon the physiological performance of individual tree species under more stressful conditions caused by climate change. In order to compare the idiosyncratic responses of Mediterranean tree species ( Quercus faginea, Pinus nigra, Juniperus thurifera) coexisting in forests of central Spain, we evaluated the temporal changes in secondary growth (basal area increment; BAI) and intrinsic water-use efficiency (iWUE) during the last four decades, determined how coexisting species are responding to increases in atmospheric CO concentrations ( C) and drought stress, and assessed the relationship among iWUE and growth during climatically contrasting years. All species increased their iWUE (ca. +15 to +21 %) between the 1970s and the 2000s. This increase was positively related to C for J. thurifera and to higher C and drought for Q. faginea and P. nigra. During climatically favourable years the study species either increased or maintained their growth at rising iWUE, suggesting a higher CO uptake. However, during unfavourable climatic years Q. faginea and especially P. nigra showed sharp declines in growth at enhanced iWUE, likely caused by a reduced stomatal conductance to save water under stressful dry conditions. In contrast, J. thurifera showed enhanced growth also during unfavourable years at increased iWUE, denoting a beneficial effect of C even under climatically harsh conditions. Our results reveal significant inter-specific differences in growth driven by alternative physiological responses to increasing drought stress. Thus, forest composition in the Mediterranean region might be altered due to contrasting capacities of coexisting tree species to withstand increasingly stressful conditions. [ABSTRACT FROM AUTHOR]

Abstract: The effects of climate change on plant reproductive performance affects the sequence of different plant reproductive stages from flowering to seed production and viability, as well as the network of relationships between them. These effects are expected to respond to different components of climate change, such as temperature and water availability, and may be sensitive to differences in species phenology. We used long-term experimental drought and warming treatments to study the effect of climate change on flower production, fruit and seed-set, seed size and seed germination rate (proportion of germinating seeds) in three Mediterranean shrubs coexisting in a coastal shrubland. Larger plants produced significantly more flowers in all three species, and higher fruit-set in Dorycnium pentaphyllum. Flower production was reduced in drought and warming treatments in the spring-flowering species D. pentaphyllum and Helianthemum syriacum, but not in the autumn–winter species Erica multiflora, which increased flowering in the warming treatment. However, the drought treatment eventually resulted in a decreased seed-set in E. multiflora. Structural equation modelling revealed strong correlations between the sequential reproductive stages. Specifically, flower density in inflorescences determined seed-set in H. syriacum, and seed size and germination rate in E. multiflora. Nevertheless, the relevance of relationships between reproductive traits changed between climatic treatments: in D. pentaphyllum a direct relationship between plant size and seed size only arised in the drought treatment, while in H. syriacum climate treatments resulted in a stronger relationship between the number of flowers and seed-set. This experimental study shows the ability of changing climatic variables to determine the reproductive sequential process of woody species. We show that several parameters of the reproductive performance of some Mediterranean species are affected by drought and warming treatments simulating climate change, highlighting the importance of changes in both water availability and temperature, and the sequential relationship between reproductive stages. Phenological patterns also contribute to species’ differential responses to climatic change, due to the relationship of these patterns with resource availability, environmental conditions and plant–pollinator interactions. [Copyright &y& Elsevier]

Abstract: The influence of different green roof layering types on the amount of water available to plants was investigated in the specific climatic context of the Mediterranean region. Water status, productivity and survival rate of Salvia officinalis L. plants growing in experimental green roof modules were monitored between early spring and late summer. Experimental data showed that: (a) substrate and water retention layer retained respectively 34% and 90% in volume of water potentially available to plants; (b) water retention layer had a positive effect on plant water status and survival; (c) the design of the overall green roof system, and in particular the characteristics of the drainage layer, influenced the amount of water transferred between different green roof elements, thus significantly influencing the amount of water available to plants. In particular, significant amounts of water were shown to be transferred from the retention layer into the cavities of the plastic drainage layer in response to temperature fluctuations, leading to day/night cycles of water evaporation/condensation. Targeted modifications of the geometrical and technical features of drainage elements were shown to be potentially useful to improve plant survival during intense and/or prolonged drought events. [Copyright &y& Elsevier]

Highlights: [•] We assessed yield and persistence of elite grass cultivars across Mediterranean sites. [•] Cultivar adaptive responses were modelled as a function of relevant climatic variables. [•] Our results support crop recommendation and breeding, also in view of climate change. [•] Tall fescue has general interest for Mediterranean drought-prone areas. [•] Completely summer-dormant cocksfoot with improved yield can be useful in warmer sites. [ABSTRACT FROM AUTHOR]

Abstract: The ability of an invasive species to establish and spread to new areas may depend on its ability to tolerate a broad range of environmental conditions. Due to climate change, increasing occurrences of extreme events such as droughts are expected in the Mediterranean region and invasive species may expand if they cope with water stress. Limited information is available on the responses of Acacia longifolia, one of the most aggressive plant species in Portuguese coastal sand dune ecosystems, to prolonged water stress. In this study, we exposed A. longifolia plants from two distinct populations, one from the wet (northern) and another from the dry (southern) climate regions of Portugal, to drought conditions, and monitored morphological, physiological and biochemical responses. One-month-old seedlings were submitted to three different water treatments which involved watering twice a week, every 7 days and every 10 days, respectively, for three months, under controlled conditions. Overall, the progressive drought stress significantly affected most of the growth parameters considered, except the root:shoot ratio. Water stress also increased the uptake of ions (Ca2+, Mg2+, K+ and Na+) and N concentration. On the contrary, the C/N ratio decreased under water stress conditions. Isotopic analysis did not reveal significant differences in δ13C with water treatments but the same pattern was not observed in δ15N values. Compared with the wet climate population, the dry climate population showed somewhat differing responses to water stress, indicating a genetic difference between populations. These results provide insights into limitations and opportunities for establishment of A. longifolia in a drought-prone scenario. [Copyright &y& Elsevier]

Copyright of Ecoscience (Ecoscience) is the property of Taylor & Francis Ltd and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

Abstract: We aimed to investigate the effects of UV radiation and drought on the leaf morphology and anatomy of six native Mediterranean species with different degrees of sclerophylly (Phillyrea angustifolia, Pistacia lentiscus, Daphne gnidium, Ilex aquifolium, Laurus nobilis, Rosa sempervirens). One-year-old seedlings of these species were grown in a glasshouse under three different UV conditions: UVB plus UVA radiation (UVBA), UVA radiation (UVA) and without UV radiation (UV0), and under two watering regimes (low- and well-watered). We observed a significant reduction in leaf area and thickness in response to drier conditions in all the species. The combination of UVB and UVA radiation resulted in leaves with a higher leaf mass area (LMA) and thickness, basically as a consequence of an increase in the thickness of the palisade parenchyma. UVA radiation specifically affected the adaxial epidermal cells, which were thicker and longer than those grown without UV. However, when UV radiation effects were analysed within each watering treatment, well-watered plants showed a higher sensitivity to UV whilst UV did not affect significantly the leaf parameters measured in low-watered plants. Lastly, the more sclerophyllous plants were the least sensitive to UV radiation which suggests that leaf sclerophylly would influence the UV plant response. [Copyright &y& Elsevier]

1. Some plant functional traits evolved with high temporal resource variability and disturbance in ecosystems where these factors are prevalent. Persistence of characteristics of these functional traits in ecosystems may depend on continued resource variability and disturbance, which in turn may promote functional diversity. In Mediterranean ecosystems, experiments that eliminate temporal resource variability and disturbance are needed to detect functional trait dependence on these factors. 2. The purpose of this study was to experimentally assess how interannual rainfall variability, summer drought and seasonal grazing modify the characteristics of functional traits (life span, flowering time, seed size and plant size) in old-field (6-15 years) Mediterranean herbaceous communities. 3. We designed a 9-year factorial field experiment that manipulated Mediterranean rainfall variability in three ways: (i) constant water availability with no summer drought; (ii) autumn and spring water availability but with summer drought; and (iii) no water supplied to rainfall; and grazing regimes: (i) autumn grazing; (ii) spring grazing; and (iii) non-grazing, in each of the three scenarios of water availability. At a community scale, we measured abundance of different categories within four plant functional traits: plant life span (annual and perennial), flowering time of annuals (spring and summer) and seed and plant sizes of spring annuals (small and large). 4. Interannual rainfall variability in autumn and spring (IRVAS), summer drought and grazing reduced perennial cover. IRVAS was necessary for the persistence of small-seeded and small-size spring annuals. IRVAS and summer drought increased spring annuals in grazed treatments. 5. Results suggest that IRVAS, summer drought and grazing favour the coexistence of species, through improved functional diversity in seed and plant sizes and increased abundance of spring annuals, the most species-rich functional group. Both effects may be the reason for the high species richness in grazed Mediterranean herbaceous communities. [ABSTRACT FROM AUTHOR]

Extreme climate events, such as severe drought episodes, may induce changes in vegetation if they induce species-specific adult mortality and changes in the seedling recruitment pattern. In 2005 a severe drought occurred in Doñana National Park (south Spain) causing extensive shrubland mortality. Over the following years we monitored the soil seed bank and seedling emergence via a gradient of canopy dieback induced by the drought episode. The canopy dieback corresponded to an increase in emergence of seedlings of woody species in 2007, probably because of the reduced competition induced by canopy loss. The soil seed bank of woody species sampled in 2008 was less abundant on plots with a higher proportion of dead vegetation, probably because of depletion of the seed bank as a result of the increased germination in the previous year and also as a result of a reduction in seed supply in these sites. Accordingly, in 2009 we detected reduced emergence of woody species on plots that had suffered the greatest shrub mortality. We failed to find any significant changes in patterns of the soil seed bank and seedling emergence of short-lived herbaceous species, indicating greater resilience in these types of species. This study highlights the resilience of Mediterranean shrublands to climate fluctuations at one extreme of the variability characteristic of these ecosystems. An increase in the frequency of severe drought episodes - increasingly probable under the new climate conditions - does have the potential, however, to induce changes in vegetation, especially in woody communities that need more time to replenish their seed banks. [ABSTRACT FROM AUTHOR]

Abstract: Perennial ryegrass (Lolium perenne) is a cool-season grass species that is often infected by the leaf-inhabiting endophyte Neotyphodium lolii. This particular endophyte is asexual and has the potential to impact host survival, growth and reproduction. The objective was to assess the potential costs or benefits of endophyte infection on drought stress tolerance of native perennial ryegrass accessions originally collected from Italy, Morocco, Tunisia, and Turkey. Sixty infected (E+) individuals from each accession were planted in a greenhouse. Half of these individuals were treated with a systemic fungicide to eliminate the endophyte (E−). For two drought periods water was withheld for 10–14 days and then allowed a one week recovery period following each. In some accessions under drought, E+ plants had more tillers, greater tiller lengths, total dry mass and green shoot mass than E− plants, suggesting a positive effect of endophyte infection on host growth. Total tiller length and the number of tillers showed significant population×treatment×infection interactions for 4 of 6 populations. This work is one of the few that documents the effects of endophyte infection for a common forage grass species from wild populations native to its distributional range. The results demonstrate that endophyte infection can help ameliorate abiotic stress such as drought and there may be a selective advantage for grasses from certain Mediterranean regions. [Copyright &y& Elsevier]

Current climate projections predict drier and warmer conditions in the Mediterranean basin over the next century. While advanced spring growth due to warming has been described in the literature, few data are available on the effects of drought on phenology. Hence, the phenology and growth of two Mediterranean shrubs, Erica multiflora and Globularia alypum, was studied in a rainfall exclusion field experiment to simulate spring drought in a natural shrubland. We estimated the onset of growth in spring by monitoring the appearance of new stems, and the end of growth in summer by following the elongation of stems. Drought treatment caused earlier onset of the spring growing season in E. multiflora, whereas no advance was observed in G. alypum. However, growth cessation was not affected in E. multiflora. Drought reduced the growth of both shrubs, as reflected in less stem elongation. The results show that a drier climate might affect not only growth but also spring phenology of some Mediterranean species. We suggest that a reduction in the cooling effect of transpiration may have analogous effects to warming and might advance the start of growth in E. multiflora, a species whose phenology has been described as warming-sensitive. The lengthening of the growing season resulting from advanced growth did not imply higher productivity, as growth was restricted by drought. [ABSTRACT FROM AUTHOR]

Extensive livestock is a basic socio-economic feature of the Mediterranean region whose environmental and economic sustainability depends on the ability of forage resources to withstand climatically stressful conditions. Perennial forages such as tall fescue can be a valuable alternative to annuals, if they can survive across successive summer droughts. Three-year dry matter yield and plant survival of five cultivars of Mediterranean-type tall fescue were evaluated in six sites of Algeria, France, Italy, Morocco and Portugal, with the following objectives: (i) modelling adaptive responses and targeting cultivars as a function of environmental factors associated with genotype × location interaction; and (ii) defining plant ideotypes, adaptation strategies and opportunities for international co-operation for regional breeding programmes. Site mean yield and winter temperatures were positively correlated, whereas sward persistence was positively correlated to lower site heat and drought stress. Cultivar adaptation was adequately modelled by factorial regression as a function of site spring-summer (April-September) drought stress (long-term potential evapotranspiration minus actual water available) for yield, and annual drought stress for final persistence. Specific-adaptation responses to high- or low-stress environments emerged which were consistent with drought-stress levels of cultivar selection environments. However, the wide-adaptation response of cultivar Flecha suggested that breeding for wide adaptation can be feasible. [ABSTRACT FROM AUTHOR]

In order to explore the effects of climate change on Mediterranean regenerating forests, we experimentally assessed the effects of increased drought on the reproductive attributes of Quercus ilex over a 4-year period (2005–2008). We also investigated whether traditional thinning (selection of one to a few stems per stump) could mitigate the consequences of increased drought in oak coppices. Increased drought reduced the number of reproductive trees, mean number of female flowers produced and acorn crop size, although most of these effects appeared only in the last 2 years of the experiment. In a different way, thinning enhanced all reproductive attributes, but its main effects were transient and covered only 1 or 2 years after the application of the treatments. Our results indicate that a moderate reduction in rainfall (15 per cent) reduces the reproductive ability of Q. ilex. This may have long-term negative consequences for recruitment as well as for the fauna feeding on acorns. Although traditional thinning may mitigate the consequences of increased drought, it has a remarkably short-term effect. This highlights the need to re-examine traditional forestry practices as potential adaptive strategies for coping with climate change in Mediterranean regenerating forests. [ABSTRACT FROM AUTHOR]

The species×location interaction was of great importance in explaining the behaviour of genetic material. The study presented here shows, for the first time, the performance, under field conditions of the new tritordeum species, compared to wheat and triticale in a wide range of Mediterranean countries (Spain, Lebanon and Tunisia). The results obtained revealed that despite the diversity of environmental conditions, the main differences in yield were due to genotypes, especially to differences between species. The multi-local study with different growth conditions revealed important information about the water availability effect on yield. In the lowest yielding environments (Tunisia rainfed), Tritordeum and triticale yields were equivalent. However under better growth conditions (Spain), tritordeum yield was shown to be lower than wheat and triticale. Interestingly, when water limitation was extended during the pre-anthesis period, differences in tritordeum versus wheat-triticale yield rate were larger than when water stress occurred during anthesis. These variations were explained by the fact that kernel weight has been found as the limiting factor for yield determination in tritordeum, and a delay in the anthesis date may have been the cause for the low kernel weight and low yield under Mediterranean drought conditions. Such differences in yield between tritordeum and wheat or triticale could be explained by the fact that tritordeum is a relatively new species and far fewer resources have been devoted to its improvement when compared to wheat and triticale. Our results suggest that breeding efforts should be directed to an earlier anthesis date and a longer grain filling period. Tritordeum proved to have possibilities to be grown under drought environments as a new crop, since its performance was quite close to wheat and triticale. Besides, it has qualitative added values that may improve farmers’ income per unit land. [Copyright &y& Elsevier]

Perennial grasslands provide numerous agroenvironmental benefits due to continuous soil cover. In Mediterranean areas, chronic summer drought is expected to increase as a result of climate changes. Plant adaptations that protect meristematic tissues include dehydration delay, dehydration tolerance, and summer dormancy. Summer dormancy can only be reliably tested in plants not subjected to water deficit. Under summer irrigation, complete dormancy is manifested by cessation of growth in association with full senescence of foliage and induced dehydration of leaf bases. Incomplete dormancy occurs when leaf growth is partially constrained and associated with moderate levels of foliage senescence. Summer dormancy is under hormonal control and is induced under increasing photoperiod and temperature. Recent results show that drought cannot induce summer dormancy under early-spring short days, although a water deficit under late-spring long days reinforces it and could enhance drought survival. Dehydration tolerance and dormancy are independent phenomena. Summer dormancy has been correlated with superior survival after severe summer droughts in many perennial grass species. This trait has potential for improving cultivars able to meet agronomic and environmental goals. [ABSTRACT FROM AUTHOR]

Abstract: The Mediterranean region suffers of drought which affects plant behaviour regarding biochemical responses. Accordingly, the effects of water deficit on growth, essential oil and fatty acid composition of caraway (Carum carvi L.) seeds were investigated. Plants were treated with different levels of water deficit: control, moderate water deficit and severe water deficit. Plant growth (height, fresh and dry matter weight) was significantly reduced by severe water deficit. This last caused also important reductions of the seed yield and yield components. Drought decreased significantly seed total fatty acid contents and particularly the petroselinic ones whose proportions decreased significantly by 12.17 and 18.47%, in comparison with the control, under moderate water deficit and severe water deficit, respectively. Besides, moderate water deficit increased the essential oil yield (expressed as g/100g on the basis of dry matter weight). The main essential oil constituents were carvone and limonene which showed an increasing of their contents under water deficit levels. Thus, water deficit induced a significant reduction in growth parameters and fatty acid content, and an increase in the essential oil compounds. These bioactive compounds have been required in many industrial products. [Copyright &y& Elsevier]

Abstract: Under climate change, severe and recurrent droughts can reduce forest production and cause widespread tree dieback. The response of different vegetation types to climate change can vary greatly and, therefore, must be individually assessed. This study was carried out in a Mediterranean oak forest (Quercus pyrenaica) subject to seasonal summer drought. To examine the response of the forest to the climate conditions predicted under climate change, a Soil–Vegetation–Atmosphere Transfer model [SPA, Williams, M., Rastetter, E.B., Fernandes, D.N., Goulden, M.L., Wofsy, S.C., Shaver, G.R., Melillo, J.M., Munger, J.W., Fan, S.M., Nadelhoffer, K.J. 1996. Modelling the soil-plant-atmosphere continuum in a Quercus–Acer stand at Harvard Forest: the regulation of stomatal conductance by light, nitrogen and soil/plant hydraulic properties. Plant, Cell, Environment 19, 911–927] was used. The model was parameterized using mostly local measurements (independent of the verification data) and tested against in situ sap flow measurements obtained during year 2007. The predictions of the model were broadly consistent with the observed dynamics of sap flow (the model explained 71% of the variance in daily transpiration and 75% of half-hourly sap flow), leaf water potentials and soil water content. Once the model had been validated, simulations were carried out under warmer and dryer conditions. Predicted warmer conditions (4°C) caused a moderate increase in total simulated transpiration. Less frequent precipitation (40% longer dry periods between rainfall events) had very little effect on transpiration. In contrast, transpiration was reduced by 17% when the soil water reserves at the beginning of the summer were lower than in 2007, corresponding to those measured in a very dry year (2005). The reduction was exacerbated when changes in temperature and rainfall were also considered (up to 28% decline in transpiration). The higher atmospheric CO2 concentrations (712ppm) simulated together with climate change, did not prevent the decline in tree water use or soil water storage at the end of the summer. All scenarios caused the soil water storage to reach extremely low values at the end of the dry season (a minimum of 25mm). It is concluded that climate change is likely to have a negative impact on tree water use and soil water resources in the study area, increasing the water deficit by as much as 30%. [Copyright &y& Elsevier]

Abstract: Olive tree (Olea europaea L.) is commonly grown under environmental conditions characterised by water deficit, high temperatures and irradiance levels typical of Mediterranean semi-arid regions. Measurement of gas exchange, chlorophyll content, chlorophyll fluorescence and photoinhibition was carried out on two-year-old olive trees (cv. ‘Coratina’) subjected to a 21-day period of water deficit followed by 23 days of rewatering. At the beginning of the experiment, plants were divided in to two groups and subjected to different light regimes: exposed plants (EP) under a mean photosynthetically active radiation (PAR) at mid-day of 1800μmolm−2 s−1 and shaded plants (SP) under a mean PAR of 1200μmolm−2 s−1. The effect of drought and high irradiance levels caused a reduction of gas exchange and photosystem 2 (PSII) efficiency, in terms of quantum yield of PSII (Φ PSII) both in EP and SP. Shading conditions allowed plants to maintain a high photosynthetic activity at low values of stomatal conductance, whereas in EP the reductions in photosynthetic efficiency and intrinsic water efficiency were due to non-stomatal components of photosynthesis. The decrease in photosynthetic activity and the increase of photoinhibition under drought were more marked in EP than in SP. Full sunlight caused in EP a higher non-photochemical quenching, whereas SP showed a better photochemical efficiency. The information here obtained can be important to understand the mechanisms by which olive plants can minimize photoinhibition when subjected to simultaneous abiotic stresses. [Copyright &y& Elsevier]

Water stress is a defining characteristic of Mediterranean ecosystems, and is likely to become more severe in the coming decades. Simulation models are key tools for making predictions, but our current understanding of how soil moisture controls ecosystem functioning is not sufficient to adequately constrain parameterisations. Canopy-scale flux data from four forest ecosystems with Mediterranean-type climates were used in order to analyse the physiological controls on carbon and water flues through the year. Significant non-stomatal limitations on photosynthesis were detected, along with lesser changes in the conductance-assimilation relationship. New model parameterisations were derived and implemented in two contrasting modelling approaches. The effectiveness of two models, one a dynamic global vegetation model ("ORCHIDEE"), and the other a forest growth model particularly developed for Mediterranean simulations ("GOTILWA+"), was assessed and modelled canopy responses to seasonal changes in soil moisture were analysed in comparison with in situ flux measurements. In contrast to commonly held assumptions, we find that changing the ratio of conductance to assimilation under natural, seasonally-developing, soil moisture stress is not sufficient to reproduce forest canopy CO2 and water fluxes. However, accurate predictions of both CO2 and water fluxes under all soil moisture levels encountered in the field are obtained if photosynthetic capacity is assumed to vary with soil moisture. This new parameterisation has important consequences for simulated responses of carbon and water fluxes to seasonal soil moisture stress, and should greatly improve our ability to anticipate future impacts of climate changes on the functioning of ecosystems in Mediterranean-type climates. [ABSTRACT FROM AUTHOR]

We investigated the effects of warming and drought on C and N concentrations, nitrogen use efficiency (NUE), and C and N accumulation in different ecosystem compartments. We conducted a 6-year (1999–2005) field experiment to simulate the climate conditions projected by IPCC models for the coming decades in a Mediterranean shrubland. We studied the two dominant species, Globularia alypum and Erica multiflora, and an N-fixing species, Dorycnium pentaphyllum, also abundant in this shrubland. Warming (1 °C) decreased N leaf concentrations by 25% and increased N stem concentrations by 40% in G. alypum. Although warming changed the available ammonium in soil in some seasons, it did not increase total soil N contents. Drought (19% average reduction in soil moisture) decreased leaf N concentrations in the two dominant shrub species, E. multiflora and G. alypum by 16% and 19%, respectively, and increased stem N concentrations by 56% and 40%, respectively. Neither warming nor drought changed the leaf N concentrations in the N-fixing species D. pentaphyllum, although warming increased stem N concentration by 9%. In G. alypum, the increase of stem N concentrations contributed to the observed increase of N accumulation in stem biomass in drought treatments with respect to control plots (8 kg N ha−1). Neither warming nor drought changed NUE in the period 1999–2005. Warming increased soil organic C relative to drought. The effects of warming and drought on C and N concentrations, on N accumulation and on leaf/stem N distribution were not the result of dilution or concentration effects produced by changes in biomass accumulation. Other factors such as the changes in soil N availability, photosynthetic capacity, and plant internal C and N remobilization must be involved. These changes which differed depending on the species and the plant tissue show that the climate change projected for the coming decades will have significant effects on the C and N cycle and stoichiometry, with probable implications for ecosystem structure and function, such as changes in plant–herbivore relationships, decomposition rates or community species composition. [ABSTRACT FROM AUTHOR]

Abstract: This paper casts light on the events taking place in the root system when seedlings of two deciduous woody Mediterranean species, Quercus pubescens Willd. and Fraxinus ornus L., are stressed by drought or fire or both. Stress treatment mimicked the conditions that often occur in natural Mediterranean ecosystems during dry summers. Allocation of resources to the root system is affected as shown by the fact that taproot biomass decreases under stress conditions only in Fraxinus ornus whereas lateral roots undergo a decrease in length, dry weight, number of apices in both species. An increase in electrolyte leakage from roots of both species during treatments suggests that morphological variations in the root system are associated with damages occurring in root tissue. All the effects observed are reversible if a critical threshold of stress duration is not reached but recovery starts when the aboveground parts are fully replaced. The alteration of a number of root traits suggests that both drought and fire affect root growth and root turnover. Differences in recovery patterns between the two species are observed and attributed to specific tolerances of the root systems. The knowledge of the events taking place at root level might help to understand better the tolerance mechanism to drought or fire characterizing species living in Mediterranean ecosystems. [Copyright &y& Elsevier]

We studied hydraulic traits of young plants of the Mediterranean cedar species Cedrus atlantica (Endl.) G. Manetti ex Carrière (Luberon, France), C. brevifolia (Hook. f.) Henry (Cyprus), C. libani A. Rich (Hadeth El Jebbe, Lebanon) and C. libani (Armut Alani, Turkey). With an optimum water supply, no major differences were observed among species or provenances in either stem hydraulic conductivity (Ks) or leaf specific conductivity (Kl) measured on the main shoot. A moderate soil drought applied for 10 weeks induced marked acclimation through a reduction in Ks, particularly in the Lebanese provenance of C. libani, and a decrease in tracheid lumen size in all species. Cedrus atlantica, which had the smallest tracheids, was the species most vulnerable to embolism: a 50% loss in hydraulic conductivity (ΨPLC50) occurred at a water potential of –4.4 MPa in the well-watered treatment, and at –6.0 MPa in the moderate drought treatment. In the other species, ΨPLC50 was unaffected by moderate soil drought, and only declined sharply at water potentials between –6.4 and –7.5 MPa in both irrigation treatments. During severe drought, Ks of twigs and stomatal conductance (gs) were measured simultaneously as leaf water potential declined. For all species, lower vulnerability to embolism based on loss of Ks was recorded on current-year twigs. The threshold for stomatal closure (10% of maximum gs) was reached at a predawn water potential (Ψpd) of –2.5 MPa in C. atlantica (Luberon) and at –3.1 MPa in C. libani (Lebanon), whereas the other provenance and species had intermediate Ψpd values. Cedrus brevifolia, with a Ψpd (–3.0 MPa) close to that of C. libani (Lebanon), had the highest stomatal conductance of the study species. The importance of a margin of safety between water potential causing stomatal closure and that causing xylem embolism induction is discussed. [ABSTRACT FROM AUTHOR]