Understanding how olive cultivars coordinate diffusional, biochemical and respiratory processes under drought is essential for identifying traits associated with resilience in Mediterranean agroecosystems. Here, we analysed 204 photosynthetic COQ response curves (A-C-i) from 14 olive cultivars subjected to well-watered conditions, moderate drought (MD) and severe drought (SD). Curves were fitted using a temperature-adjusted Farquhar-von Caemmerer-Berry framework to estimate maximum Rubisco carboxylation capacity (Vcmax), maximum electron transport rate (Jmax), dark respiration (Rd), COQ compensation point (Gamma*), mesophyll conductance (gm), and the ratio between mesophyll and gas-phase COQ conductance (gm/gtc). Moderate drought did not cause a generalized biochemical impairment. Instead, mean Vcmax and Jmax increased from 26.9 to 54.2 & micro;mol m-2 s-1 under control conditions to 35.8 and 63.4 & micro;mol m-2 s-1 under MD, respectively, suggesting partial acclimation of photosynthetic capacity. However, gm declined by nearly half under MD, indicating that internal COQ diffusion was already constrained before major biochemical collapse occurred. This phase was accompanied by increased Rd and elevated Gamma*, consistent with a metabolically costly acclimation response involving higher maintenance and photorespiratory demands. In contrast, SD triggered a strong decline in photosynthetic functionality, reducing Vcmax, Jmax and gm to 18.1 & micro;mol m-2 s-1, 29.3 & micro;mol m-2 s-1 and 0.00335 mol m-2 s-1 bar-1, respectively. The simultaneous increase in gm/gtc under SD did not indicate improved mesophyll performance, but rather a proportionally stronger decline in gas-phase conductance, most likely associated with severe stomatal closure. Multivariate integration of diffusional and biochemical traits separated cultivars into contrasting resilience strategies, with Cornicabra, Koroneiki and Frantoio showing the strongest integrated performance, while Sikitita2 maintained particularly high biochemical capacity under severe stress. Overall, our results support a two-stage drought response in olive, in which moderate drought induces costly acclimation dominated by diffusional constraints, whereas severe drought drives impaired chloroplastic COQ supply, reduced RuBP regeneration, lower carboxylation capacity and loss of physiological coordination.

Water availability and lithology are two key drivers shaping the structure and function of the soil microbiome. While precipitation shapes microbial diversity through soil moisture, oxygen diffusion and substrate transport, lithology determines soil mineral composition, pH, texture, and nutrient availability. Together, these factors define the physico-chemical environment in which soil microbial communities thrive, yet the interactive effects of lithology and water availability on forest soil microbiomes remain poorly understood. Here, we investigated how lithology and precipitation jointly shape soil bacterial and fungal diversity by utilising the unique and complex geomorphology of Pinus pinaster forests across a precipitation gradient in the province of Málaga, southern Spain. Lithology was the dominant factor structuring soil microbial communities, explaining 27% and 16% of the observed variation in bacterial and fungal diversity, respectively, compared to vegetation (1% and 5%) and climatic (5% and 3%) factors. Decreasing precipitation led to an increase in soil microbial diversity across all substrates. At the wettest sites, microbial diversity did not differ among lithologies. However, as precipitation decreased, lithology increasingly modulated microbial diversity, with peridotite soils, the most chemically stressful substrate (characterised by elevated concentrations of magnesium and nickel), supporting the highest bacterial and, to a lesser extent, fungal diversities. These soils also harboured significantly higher relative abundances of ectomycorrhizal fungi and a lower prevalence of plant pathogens and parasites at the driest locations. This study highlights the importance of incorporating lithological substrate as a key modulating factor in assessments of soil microbiome responses to changing water regimes in forest ecosystems at a regional scale, a factor that remains largely unexplored.

Photosynthesis response curves were carried out using different light intensities in 14 olive cultivars subjected to moderate and severe water deficits. For each cultivar, the electronic transfer ratio in response to light was also obtained. The mathematical adjustment of these response curves allowed calculating, with a correlation coefficient above 0.95, parameters such as photosynthetic efficiency, the slope of the curves, the number of electrons required to fix a CO2 molecule, and the number of photons to assimilate a molecule of CO2, among others, as well as their variation with the lack of water. This work represents the first contribution of the response of photosynthesis in olive cultivars to different light intensities in moderate and severe drought conditions for a considerable number of cultivars. The parameters described, and the results provided, pave the road for subsequent work related to plant physiology and other areas of science and technology, and allow the tolerance to water stress to be objectively compared in these fourteen olive cultivars.

Understanding the tolerance of plants to drought and their gradual response to lack of water is a multifaceted challenge that requires a combination of scientific research and technological innovation. Selecting naturally drought-tolerant plants and knowing their response to photosynthesis in a wide range of water availability opens a door to making decisions about the suitability of different cultivars to be implanted in specific geographical areas, based on their tolerance to drought and light absorption capacity. In this work, photosynthesis-light curves were carried out using a LiCor LI-6800 IRGA device, applying increasing light intensities to plants of 14 olive cultivars, either under control conditions (no water stress) or subject to moderate and severe water deficits. The plants were grown in a culture chamber under controlled conditions for photoperiod, air humidity, temperature, and carbon dioxide concentration. For each cultivar, the electronic transference ratio (ETR) in response to light was also obtained. Different equations were used to fit experimental data allowing us to calculate, with a regression coefficient above 0.95, different photosynthetic parameters such as the maximum photosynthetic capacity, the photosynthetic efficiency, the number of electrons or the number of photons to assimilate a molecule of CO2, and the effect of the lack of water on these parameters. This work represents the first contribution of the response to photosynthesis of many olive cultivars subjected to moderate and severe drought conditions. The parameters described, and the results provided, pave the road for subsequent work related to plant physiology and other areas of science and technology, and allow us to objectively compare the tolerance to water stress in these fourteen olive cultivars.

Paratylenchus species are obligate ectoparasitic nematodes on cultivated and wild herbaceous and woody plants occupying numerous soil categories. Several species may cause damage to several crops (viz. P. dianthus, P. enigmaticus, P. microdorus, P. hamatus and P. epacris on carnation, lettuce, rose and walnut, respectively). This investigation proves and emphasizes the relevance of applying integrative taxonomy for the accurate detection of Paratylenchus species in mountainous wild environments in the Malaga province, Southern Spain. This research analyzed 45 soil samples of maritimus pine and one of green heather in southern Spain and identified fourteen Paratylenchus species, two of them are described herein as new species (P. paraaonli sp. nov., P. plesiostraeleni sp. nov.), six of them were first reports for Spain (P. canchicus, P. nainianus, P. neonanus, P. salubris, Paratylenchus sp. 2 SAS, and P. wuae), and six species (P. caravaquenus, P. microdorus, P. nanus, P. neoamblycephalus, P. sheri, and P. variabilis) have been already reported in Spain. Accordingly, these data increase the biodiversity of pin nematodes in Spain comprising a total of 47 species (33.1% out of 142 total species of this genus). Phylogenetic analyses based on ribosomal and mitochondrial markers (D2-D3, ITS, and partial COI) resulted in a consistent position for the newly described Paratylenchus species in this study (P. plesiostraeleni sp. nov., P. paraaonli sp. nov.). Paratylenchus plesiostraeleni sp. nov. grouped in a separated subclade as unequivocal species from the P. straeleni-complex species (including P. straeleni and P. parastraeleni), and P. paraaonli sp. nov. clustered with P. vitecus, but clearly separate from this species. This study indicates that Paratylenchus species diversity in natural environments may be higher than expected, and this study may help in accurate identifications.

Acute and early symptoms of forest dieback linked to climate warming and drought episodes have been reported for relict Abies pinsapo Boiss. fir forests from Southern Spain, particularly at their lower ecotone. Satellite, orthoimages, and field data were used to assess forest decline, tree mortality, and gap formation and recolonization in the lower half of the altitudinal range of A. pinsapo forests (850-1550 m) for the last 36 years (1985-2020). Field surveys were carried out in 2003 and in 2020 to characterize changes in stand canopy structure and mortality rates across the altitudinal range. Time series of the Normalized Difference Vegetation Index (NDVI) at the end of the dry season (derived from Landsat 5 and 7 imagery) were used for a Dynamic Factor Analysis to detect common trends across altitudinal bands and topographic solar incidence gradients (SI). Historical canopy cover changes were analyzed through aerial orthoimages classification. Here we show that extensive decline and mortality contrast to the almost steady alive basal area for 17 years, as well as the rising photosynthetic activity derived from NDVI since the mid-2000s and an increase in the forest canopy cover in the late years at mid and high altitudes. We hypothesized that these results suggest an unexpected resilience in A. pinsapo forests to climate change-induced dieback, that might be promoted by compensation mechanisms such as (i) recruitment of new A. pinsapo individuals; (ii) facilitative effects on such recruitment mediated by revegetation with other species; and (iii) a ‘release effect’ in which surviving trees can thrive with fewer resource competition. Future research is needed to understand these compensation mechanisms and their scope in future climate change scenarios.

Understanding how climate warming and land-use changes determine the vulnerability of forests to drought is critical. However, we still lack: (i) robust quantifications of long-term growth changes during aridification processes, (ii) links between growth decline, changes in forest cover, stand structure and soil conditions, and (iii) forecasts of growth variability to projected climate warming. We investigated tree-ring records over the past 400-700 years, quantified changes in grazing area and forest cover during the 20th century, sampled current stand structure, and analyzed soil organic carbon delta C-13 and total nitrogen delta N-15 of Atlas cedar (Cedrus atlantica (Endl.) Manetti ex Carriere) Moroccan forests to characterize their dieback. Atlas cedar forests experienced massive dieback after the 1970s, particularly in the xeric High Atlas region. Forest cover increased in the less xeric regions (Middle Atlas and Rif) by almost 20%, while it decreased about 18% in the High Atlas, where soil delta C-13 and delta N-15 showed evidences of grazing. Growth declined and became more variable in response to recent droughts. The relative growth reduction (54%) was higher in the Middle Atlas than elsewhere (Rif, 32%; High Atlas, 36%). Growth synchrony between forests located within the Middle and High Atlas regions increased after the 1970s. Simulations based on a worst-case emission scenario and rapid warming forecast a stronger limitation of growth by low soil moisture in all regions, but particularly in the Middle Atlas and after the mid-21st century. Climate warming is expected to strengthen growth synchronization preceding dieback of conifer forests in xeric regions. The likelihood of similar dieback episodes is further exacerbated by historical degradation of these forests. (C) 2020 Elsevier B.V. All rights reserved.

Dagger nematodes of the genus Xiphinema include a remarkable group of invertebrates of the phylum Nematoda comprising ectoparasitic animals of many wild and cultivated plants. Damage is caused by direct feeding on root cells and by vectoring nepoviruses that cause diseases on several crops. Precise identification of Xiphinema species is critical for launching appropriate control measures. We deciphered the cryptic diversity of the Xiphinema hispanum-species complex applying integrative taxonomical approaches that allowed us to verify a paradigmatic example of the morphostatic speciation and the description of a new species, Xiphinema malaka sp. nov. Detailed morphological, morphometrical, multivariate and genetic studies were carried out, and mitochondrial and nuclear haploweb analyses were used for species delimitation of this group. The new species belongs to morphospecies Group 5 from the Xiphinema nonamericanum-group species. D2-D3, ITS1, partial 18S, and partial coxI regions were used for inferring the phylogenetic relationships of X. malaka sp. nov. with other species within the genus Xiphinema. Molecular analyses showed a clear species differentiation not paralleled in morphology and morphometry, reflecting a clear morphostatic speciation. These results support the hypothesis that the biodiversity of dagger nematodes in southern Europe is greater than previously assumed.

Current climate change in the Mediterranean basin is associated to increasing frequency and intensity of droughts. This climate dryness entails a serious impact on drought-sensitive forests, several of them considered as hot spots of biodiversity. Adaptive management, as experimental thinning for stand structural diversity enhancement, may increase tree-level resources availability. However, the long-term stand-level effectiveness of this approach at sustaining forest ecosystem functioning remains uncertain. Here, we attempt to place experimental thinning in a climate change adaptation context, using as experimental system the drought-sensitive fir Abies pinsapo. We conducted a long-term study (2004–2019), focused on tree basal area increment (BAI) and quantified to what extent thinning treatments improved drought resistance, recovery and resilience to drought in the remaining trees. The results support a post-thinning (after 2004) BAI increase in thinned stands (+104% if 30% thinning of basal area was applied; +141% for 60% thinning) compared to controls (+21%). Short-term growth resilience, estimated for an extreme drought occurred after thinning (2005), yielded higher resilience for thinned stands, throughout improved resistance, but similar recovery than control. However, extreme droughts observed thereafter (for instance in 2012) do not support a long-term increase in resilience. Growth resilience, estimated for wetter-than-average years showed similar short-term enhanced resilience in thinned stand, while these differences dispelled afterward.

Stand-level competition and local climate influence tree responses to increased drought at the regional scale. To evaluate stand density and elevation effects on tree carbon and water balances, we monitored seasonal changes in sap-flow density (SFD), gas exchange, xylem water potential, secondary growth, and non-structural carbohydrates (NSCs) in Abies pinsapo. Trees were subjected to experimental thinning within a low-elevation stand (1200 m), and carbon and water balances were compared to control plots at low and high elevation (1700 m). The hydraulic conductivity and the resistance to cavitation were also characterized, showing relatively high values and no significant differences among treatments. Trees growing at higher elevations presented the highest SFD, photosynthetic rates, and secondary growth, mainly because their growing season was extended until summer. Trees growing at low elevation reduced SFD during late spring and summer while SFD and secondary growth were significantly higher in the thinned stands. Declining NSC concentrations in needles, branches, and sapwood suggest drought-induced control of the carbon supply status. Our results might indicate potential altitudinal shifts, as better performance occurs at higher elevations, while thinning may be suitable as adaptive management to mitigate drought effects in endangered Mediterranean trees.

Increasing temperatures are leading to upward migration of plant species in mountains, with elevational shifts of the mountain tree-line being the most commonly documented response ("front-edge" of altitudinal advance). Rapid distributional and compositional shifts in soil microbial communities, especially ectomycorrhizal (ECM) fungi, under climate change are also expected as an indirect effect of the impacts on vegetation. However, and despite the potential consequences on ecosystem functioning, they have seldom been addressed. Here we test the hypothesis that distributional shifts in soil ECM fungal communities will follow climate-driven tree upward migration in a fir forest-high elevation shrubland ecotone. Using Illumina Miseq sequencing, we compare the ECM fungal community associated to roots of adult Abies pinsapo trees at the tree-line with that of individuals involved in the altitudinal advance above it. Our data showed a decreasing trend in ECM fungal species abundance, richness, and proportion of species with epigeous fruiting bodies toward higher altitudes above the tree-line. Agaricales, Boletales and Pezizales were the fungal orders most frequently shared between host-individuals from the tree-line and the upward-migration area. In the latter, the ECM fungal community was mainly affected by the distance from the tree-line. Our results, together with the fact that the tree upward-migration process began several decades ago, suggest that (i) there is a high host-symbiont specificity for the ECM fungal community associated with the relic species Abies pinsapo, (ii) the altitudinal shift in the ECM fungal community lags well behind climate-driven tree upward migration in this forest-high elevation shrubland ecotone, and (iii) ECM dispersal limitation (distance from the forest-edge source) is an important factor controlling the speed of ECM upward migration.

Understanding forest responses to the current climate change requires to investigate the effects of competition, buffering or enhancing process of forests decline. Here we attempt to place intra-specific competition in a climate change context, using as experimental system the drought-sensitive fir Abies pinsapo. We conducted a decade-long (2004-2015) investigation at four permanent plots (two thinned plots and two controls), where all trees were identified, tagged and mapped. Experimental thinning was performed attempting to enhance stand structure diversity by (1) reducing tree competition, (2) increasing the equitability of size classes (i.e. diversity) and (3) randomizing tree spatial pattern. We focus on tree basal area increment and sap flow density, as regards microclimate and competition, to quantify the extent to that thinning alleviates drought in the remaining trees. The results support that lower competition fosters short- to long-term tree-level physiological responses, specifically, improved growth and water uptake in the remaining trees. Density reduction provides a promising strategy for minimizing climate change effects on drought-sensitive tree species by improving resources availability to the remaining trees. Nevertheless, our results also support a higher air temperature and soil water depletion according to density reduction, suggesting that lower competition might also increase soil and understory evapotranspiration. Adaptive management, as the experimental thinning reported here, may increase tree-level resources availability, but the long-term stand-level effectiveness of this approach at sustaining forest ecosystem functioning remains uncertain.

Increasing temperatures and decreasing precipitation in large areas of the planet as a consequence of global warming will affect plant growth and survival. However, the impact of climatic conditions will differ across species depending on their stomatal response to increasing aridity, as this will ultimately affect the balance between carbon assimilation and water loss. In this study, we monitored gas exchange, growth and survival in saplings of three widely distributed European pine species (Pinus halepensis, P. nigra and P. sylvestris) with contrasting distribution and ecological requirements in order to ascertain the relationship between stomatal control and plant performance. The experiment was conducted in a common garden environment resembling rainfall and temperature conditions that two of the three species are expected to encounter in the near future. In addition, gas exchange was monitored both at the leaf and at the whole-plant level using a transient-state closed chamber, which allowed us to model the response of the whole plant to increased air evaporative demand (AED). P. sylvestris was the species with lowest survival and performance. By contrast, P. halepensis showed no mortality, much higher growth (two orders of magnitude), carbon assimilation (ca. 14 fold higher) and stomatal conductance and water transpiration (ca. 4 fold higher) than the other two species. As a consequence, P. halepensis exhibited higher values of water-use efficiency than the rest of the species even at the highest values of AED. Overall, the results strongly support that the weaker stomatal control of P. halepensis, which is linked to lower stem water potential, enabled this species to maximize carbon uptake under drought stress and ultimately outperform the more water conservative P. nigra and P. sylvestris. These results suggest that under a hotter drought scenario P. nigra and P. sylvestris would very likely suffer increased mortality, whereas P. halepensis could maintain gas exchange and avoid water-induced growth limitation. This might ultimately foster an expansion of P. halepensis to higher latitudes and elevations.

Climatic dryness imposes limitations on vascular plant growth by reducing stomatal conductance, thereby decreasing CO2 uptake and transpiration. Given that transpiration-driven water flow is required for nutrient uptake, climatic stress-induced nutrient deficit could be a key mechanism for decreased plant performance under prolonged drought. We propose the existence of an "isohydric trap," a dryness-induced detrimental feedback leading to nutrient deficit and stoichiometry imbalance in strict isohydric species. We tested this framework in a common garden experiment with 840 individuals of four ecologically contrasting European pines (Pinus halepensis, P. nigra, P. sylvestris, and P. uncinata) at a site with high temperature and low soil water availability. We measured growth, survival, photochemical efficiency, stem water potentials, leaf isotopic composition (δ13 C, δ18 O), and nutrient concentrations (C, N, P, K, Zn, Cu). After 2 years, the Mediterranean species Pinus halepensis showed lower δ18 O and higher δ13 C values than the other species, indicating higher time-integrated transpiration and water-use efficiency (WUE), along with lower predawn and midday water potentials, higher photochemical efficiency, higher leaf P, and K concentrations, more balanced N:P and N:K ratios, and much greater dry-biomass (up to 63-fold) and survival (100%). Conversely, the more mesic mountain pine species showed higher leaf δ18 O and lower δ13 C, indicating lower transpiration and WUE, higher water potentials, severe P and K deficiencies and N:P and N:K imbalances, and poorer photochemical efficiency, growth, and survival. These results support our hypothesis that vascular plant species with tight stomatal regulation of transpiration can become trapped in a feedback cycle of nutrient deficit and imbalance that exacerbates the detrimental impacts of climatic dryness on performance. This overlooked feedback mechanism may hamper the ability of isohydric species to respond to ongoing global change, by aggravating the interactive impacts of stoichiometric imbalance and water stress caused by anthropogenic N deposition and hotter droughts, respectively.

•Forest structure modulates Abies pinsapo climate-growth sensitivity.•Thinning may reduce Abies pinsapo decline and mortality by decreasing competition.•Thinning enhances tree-level growth and sap flow.•Density reduction was related to higher air temperature and soil water depletion.•Lower competition might increase soil and understory evapotranspiration.

Summary The growth limitation hypothesis (GLH) is the most accepted explanation for treeline formation, but it has been scarcely examined in Mediterranean regions, where treelines are located at lower elevations than in temperate regions. The GLH states that low temperature is the ultimate environmental driver for treeline formation, constraining C‐sinks (i.e. tissue formation) more than C‐sources. The GLH predicts similar or increasing (but not decreasing) non‐structural carbohydrate (NSC) concentrations with elevation throughout the course of the growing season. We hypothesized that elevational trends in growth and NSC in Mediterranean regions are not determined by low temperature alone. We tested the GLH by examining the mean annual basal area and NSC concentrations in developing (new twigs) and ripened tissues (branch, stem) of trees growing at three elevations in three Mediterranean and three temperate treelines in the Chilean southern Andes (33° S, Kageneckia angustifolia; 36, 40 and 46° S, Nothofagus pumilio) and in Spain (36° N, Pinus sylvestris; 42° N, Pinus uncinata). Samples for NSC were taken at the onset of summer and autumn, which represent periods of contrasting drought intensities in Mediterranean regions. Tree growth decreased significantly with elevation in temperate treelines but not in Mediterranean treelines. In Mediterranean treelines, new twig NSC concentrations increased significantly with elevation in the early summer but not in the early autumn. In temperate treelines, in contrast, no elevational or seasonal variation (or interaction between them) was observed in new twig NSC concentrations. The NSC concentrations of the branches and stems from both climates showed no elevational trends. The soluble sugars' NSC fraction increased over the season in Mediterranean treelines and decreased in temperate treelines. Synthesis. Although we found support for the growth limitation hypothesis (GLH) in temperate and Mediterranean treelines, our study shows that the tree growth and C balance in Mediterranean treelines are not controlled by low temperature alone. We suggest that environmental factors other than temperature explain the lower global elevation of Mediterranean treelines when compared to temperate treelines.

This study aimed to investigate the effects that stand age and forest structure have on microbiological soil properties, enzymatic activities and nutrient content. Thirty forest compartments were randomly selected at the Palancares y Agregados managed forest area (Spain), supporting forest stands of five ages; from 100 to 80years old to compartments with trees that were 19–1years old. Forest area ranging from 80 to 120years old and without forest intervention was selected as the control. We measured different soil enzymatic activities, soil respiration and nutrient content (P, K, Na, Mg, Cr, Mn, Fe, Co, Ni, Cu, Zn, Pb and Ca) in the top cm of 10 mineral soils in each compartment. Results showed that the lowest forest stand age and the forest structure created by management presented lower values of organic matter, soil moisture, water holding capacity and litterfall and higher values of C/N ratio in comparison with the highest forest stand age and the related forest structure, which generated differences in soil respiration and soil enzyme activities. The forest structure created by no forest management (control plot) presented the highest enzymatic activities, soil respiration, NH4+ and NO3−. Results did not show a clear trend in nutrient content comparing all the experimental areas. Finally, the multivariate PCA analysis clearly clustered three differentiated groups: Control plot; from 100 to 40years old and from 39 to 1year old. Our results suggest that the control plot has better soil quality and that extreme forest stand ages (100–80 and 19–1years old) and the associated forest structure generates differences in soil parameters but not in soil nutrient content.