High Soil Water Availability Research Articles

Genetic consequences of landscape features in two rear edge, highly fragmented metapopulations of a mediterranean conifer

ContextHabitat fragmentation is expected to erode genetic diversity, which instead needs to be preserved for promoting species adaptation to a changing climate. As this expectation has found mixed support in forest trees, consistent results on the genetic consequences of fragmentation requires adequately replicated experimental designs, as well as an explicit assessment of which landscape features, if any, could mitigate its detrimental effects.ObjectiveEvaluating the role of several landscape attributes in buffering the detrimental effects of fragmentation in two metapopulations of silver fir.MethodsWe genotyped 904 silver fir (Abies alba Mill.) trees from 18 local populations forming two metapopulations comparable for size and extension in the Apennines, a Mediterranean mountain range. We identified the signatures left by the fragmentation process on the genetic features of silver fir local populations. After removing potentially confounding effects due to different evolutionary histories, we used a multivariate approach for testing the relative effect of demographic, geographic, environmental and topographic factors on genetic features of both metapopulations.ResultsWe found comparable signals of the habitat fragmentation impact on the genetic diversity and structure of both investigated metapopulations. Fragmentation effects were less pronounced in the largest local populations (but not the least isolated), located on gentler slopes with higher soil water availability and lower heat exposure.ConclusionsOur results suggest the existence of a set of demographic and environmental factors that could have coherently buffered the detrimental genetic effects of fragmentation in both metapopulations. These findings could be useful to plan landscape restoration for the evolutionary rescue of mixed forests that once characterized Mediterranean mountain ecosystems.

Water availability is a stronger driver of soil microbial processing of organic nitrogen than tree species composition

AbstractSoil organic nitrogen (N) cycling processes constitute a bottleneck of soil N cycling, yet little is known about how tree species composition may influence these rates, and even less under changes in soil water availability such as those that are being induced by climate change. In this study, we used a 12‐year‐old tree biodiversity experiment in southwestern France to assess the interactive effects of soil water availability (half of the blocks seasonally irrigated to double precipitation) and tree species composition (monocultural vs. mixed plots of coniferous Pinus pinaster, and of broadleaf Betula pendula). We measured gross protein depolymerisation rates using a novel high‐throughput isotope pool dilution method, along with soil microbial biomass carbon and N to calculate microbial biomass‐specific activities of soil organic N processes. Overall, high soil water availability led to a 42% increase in soil protein depolymerisation rates compared to the unirrigated plots, but we found no effect of species composition on these soil organic N cycling processes. When investigating the interactive effect of tree species mixing and soil water availability, the results suggest that mixing tree species had a negative effect on soil organic N cycling processes in the non‐irrigated blocks subject to dry summers, but that this effect tended to become positive at higher soil water availability in irrigated plots. These results put forth that soil water availability could influence potential tree species mixing effects on soil organic N cycling processes in dry conditions.Highlights Tree species (with different litter C:N ratios) had little effect on protein depolymerisation Increasing water availability via irrigation accelerated depolymerisation rates No interactive effect between tree species mixing and water availability, although trends emerged Positive trend of mixing under high water availability and negative trend under low water

Spatial variability of soil organic carbon stock in an olive orchard at catchment scale in Southern Spain

Orchards have a high potential for carbon sequestration. However, little research is available on the spatial variability at catchment scale and on the difference between the tree area and the lanes. We analyzed theik spatial variability of soil organic carbon stock, SOCstock at 90 cm depth in an 8-ha catchment in Southern Spain with olives on a vertic soil. Results showed higher soil organic carbon concentration, SOC, in the tree area as compared to the lane up to 60 cm depth, but its impact on SOCstock was negligible since it was compensated by the higher soil bulk density in the lane. SOC at different depths was correlated with that in the top 0–5 cm. The overall SOCstock of the orchard was 4.14 kg m−2, ranging between 1.8 and 6.0 kg m−2. This SOCstock is in the mid-lower range of values reported for olive orchards, measured at smaller scale, and similar to those other intensive field crops and agroforestry under comparable rainfall conditions. The spatial variability in SOCstock was correlated to several geomorphological variables: elevation, cumulative upstream area, topographic wetness index, sediment transport index, and tillage erosion. Differences in SOC and SOCstock are driven by the sediment redistribution downslope, mainly by tillage erosion, and higher soil water availability in lower areas allowing higher biomass production. These topographic indexes and the correlation between SOC in the topsoil and SOCstock up to 90 cm should be further explored in other typology of olive orchards for facilitating the mapping of SOCstock.

Age-Related Changes in Water and Nitrogen Utilization in Crop Trees and Understory Vegetation in a Hinoki Cypress Plantation Forest in Kochi City, Southern Japan

Age-related changes in water and nitrogen utilization of crop and understory vegetation in a hinoki cypress plantation forest were investigated from the age of 21 to 46 years in Kochi City, southern Japan. Nitrogen concentration in the leaf litter of hinoki cypress showed a decreasing trend with forest age. The leaf δ15N of hinoki cypress was related to a quadratic function and increased from the age of 21 to 26 years and then decreased to the age of 46 years. These results suggest that older hinoki cypress trees utilize soil nitrogen sources with lower δ15N values, and the competition for soil nitrogen with understory vegetation should be stronger. Carbon isotope discrimination (Δ13C) of hinoki cypress decreased from the age of 21 to 30 years and then increased to the age of 46 years. In contrast, the intrinsic water-use efficiency (iWUE) of hinoki cypress increased from the age of 21 to 36 years and then decreased to the age of 46 years. These findings suggest that hinoki cypress trees in the earlier time increased their iWUE by reducing stomatal opening. In the earlier time, the stomatal opening of understory vegetation increased due to higher soil water availability with decreasing stand density of crop trees. In the later time, the iWUE of hinoki cypress decreased due to lower photosynthetic capacity with nitrogen limitation. These results suggest that the increase in the iWUE of hinoki cypress in response to elevated atmospheric carbon dioxide levels should be smaller in the later time because of stronger competition with understory vegetation for soil nitrogen resources.

Long-term drought adaptation of unirrigated grapevines (Vitis vinifera L.)

Resilience to drought stress is an important crop selection and breeding target particularly under an environment of increasing freshwater scarcities resulting from higher evapotranspiration rates and reduced precipitation. Identifying new genetic material and clones with superior drought tolerance would increase available genetic resources and germplasm for both grapevine breeders and propagators, providing genetic material that has greater water use efficiency thereby reducing reliance on supplemental irrigation. Towards this goal, we explored the long-term drought adaptation of field-grown, unirrigated (or dry-grown) and own-rooted grapevines of pre-clonal origin from shallow (SR) and deep (DR) soils representing low and high soil water availability, respectively, in a South Australian vineyard. Despite lower soil moisture available to SR vines, both deep and shallow-rooted vines had similar vine water status, based on measurements of midday stem water potential ({Psi }_{s}), and leaf net photosynthesis (An). Due to the lower stomatal conductance (gs), SR had higher intrinsic water use efficiency (WUEi) than DR, however the carbon isotope ratio ({delta }^{13}C) of the fruit at harvest was similar between the two groups. Our observations suggest a degree of drought adaptation in the SR vines resulting from multi-decadal cyclical droughts. Overall, we demonstrate that pre-clonal Cabernet Sauvignon grapevines dry-grown in shallow soils have an enhanced resilience to drought compared to dry-grown vines in deep soils. This study has implications for selection of crop genetic material in a changing climate.

The effects of periodically stagnant soil water conditions on biomass and methane yields of Silphiumperfoliatum

Resulting from the wide range of usages of bio-methane, anaerobic digestion has become a major issue. Beside biogenic residues, the cultivation of energy crops has significantly been expanded in Europe. Due to the increasing competition for land, sites with a low value for food production are predestined for cultivating feedstocks. Numerous studies have shown that perennial energy crops (PECs) present a valuable alternative to annual energy crops since they combine sound energy yields with substantial ecological advantages. Particularly under unprivileged sites conditions, PECs may fully leverage their strengths. Soils showing long lasting periods of soil moisture above field capacity (Stagnosols) were evaluated to be critical for silage maize cultivation as heavy harvest machinery may cause severe soil damages. Additionally, juvenile development of maize suffers from stagnant soil water.This study aimed to verify the results of a former pot experiment in which cup plant (Silphium perfoliatum) showed early sprouting, fast biomass development and higher yields under a soil water regime typical for Stagnosols. Thus, in the presented field experiment, silage maize and cup plant, were cultivated on well-drained soils (Cambisols) and on Stagnosols, and were compared concerning biomass and energy yields.On Stagnosols, the cup plant benefited likely from the higher soil water availability and produced average biomass yields of 14.3 t ha−1 a−1 being about 40% higher than on well-drained soils. The biochemical methane potential was 305 LN kg−1 VS and not affected by the soil type. Nonetheless, the methane yields per area of cup plant were still 35% lower than those of silage maize.

Soil water availability for crops on landslide deposits in Bompon Sub-Watershed, Magelang

One of the important factors in agricultural practices is the soil water availability for crops. This study was aimed to identify landslide deposits, assess the soil characteristics of each landslide zone, the effect of soil physical characteristics on the soil water availa bility for crops, and determine the availability of water for crops in landslide deposits. The soil sampling was carried out by purposive sampling method at 12 landslide points and 4 zones, i.e. crown, main-scarp, main-body, a nd toe-slope at two soil depths (0-100 cm and 100-200 cm). The data were analyzed with a regression test and standard deviation method. The results showed that the high soil water availability for crops occurred at the toe-slope zone, 0-100 cm depth and at the main-scarp zone, 100-200 cm depth. The soil physical properties clearly correlated with soil moisture content. Based on the soil water a vaila bility level, it might be suitable for annual crops (i.e. cassava, ginger, sweet potatoes, etc.) at the toe-slope zone and perennial crops i.e. Albizia chinensis, etc.) at the main- scarp zone. Therefore, landslides in the Bompon Sub-watershed, Magelang, is potentially utilized for agricultural development.

Response of Gross Primary Productivity to Drought Time‐Scales Across China

AbstractDrought largely affects terrestrial gross primary productivity (GPP) by its duration (i.e., drought time‐scales). However, the large‐scale impact of drought time‐scales on GPP is still not well understood, partly due to limited understanding of ecological processes and difficulties in quantifying drought time‐scales. Here, we examined the responses of GPP to drought in China during the last four decades, using state‐of‐the‐art GPP data estimates and a multiscalar drought metric. Most of the GPP data suggested that the time‐scales at which drought has the largest influence on annual GPP (dominant drought time‐scales) were well related to climate region and vegetation type. The GPP responded to drought more rapidly in arid regions than that in semiarid and subhumid regions, with a mean dominant drought time‐scale of 15 and 20 months, respectively; as plants in arid regions can rapidly adapt to water shortage through physiological strategies, while, in semiarid and subhumid regions plants usually can withstand a degree of water deficits. In a humid region, GPP did not rapidly respond to drought, probably because the high soil water availability buffered the effect of drought. In addition, most of the GPP estimates suggested that grasslands and croplands tended to respond to drought at shorter time‐scales than that of forests, shrublands, and sparse vegetation. The study highlights the importance of improving the effects of water availability on vegetation activity and further consideration of vegetation adaptability to drought in both remote sensing GPP algorithms and parameterizations of current terrestrial ecosystem models.

Characterizing Growing Season Length of Subtropical Coniferous Forests with a Phenological Model

Understanding plant phenological change is of great concern in the context of global climate change. Phenological models can aid in understanding and predicting growing season changes and can be parameterized with gross primary production (GPP) estimated using the eddy covariance (EC) technique. This study used nine years of EC-derived GPP data from three mature subtropical longleaf pine forests in the southeastern United States with differing soil water holding capacity in combination with site-specific micrometeorological data to parameterize a photosynthesis-based phenological model. We evaluated how weather conditions and prescribed fire led to variation in the ecosystem phenological processes. The results suggest that soil water availability had an effect on phenology, and greater soil water availability was associated with a longer growing season (LOS). We also observed that prescribed fire, a common forest management activity in the region, had a limited impact on phenological processes. Dormant season fire had no significant effect on phenological processes by site, but we observed differences in the start of the growing season (SOS) between fire and non-fire years. Fire delayed SOS by 10 d ± 5 d (SE), and this effect was greater with higher soil water availability, extending SOS by 18 d on average. Fire was also associated with increased sensitivity of spring phenology to radiation and air temperature. We found that interannual climate change and periodic weather anomalies (flood, short-term drought, and long-term drought), controlled annual ecosystem phenological processes more than prescribed fire. When water availability increased following short-term summer drought, the growing season was extended. With future climate change, subtropical areas of the Southeastern US are expected to experience more frequent short-term droughts, which could shorten the region’s growing season and lead to a reduction in the longleaf pine ecosystem’s carbon sequestration capacity.

Diagnosis of soil fertility and banana crop nutrition in the state of Santa Catarina

Abstract Soil fertility and nutritional imbalance are among the main limitations in banana production. The present study aimed to diagnose soil fertility and nutrition of bananas trees cultivated in the state of Santa Catarina, southern Brazil. Leaf and soil samples were collected, and banana yield was quantified in 2018 and 2019 in 53 orchards in the regions of ‘Vale do Itajaí’ and northern coast of Santa Catarina. Although orchards had satisfactory yield (42 t /ha/year), limitations related to soil fertility and banana crop nutrition were evident. The observed yield is 59.6 % of its full potential. Liming is still recurrent, mainly before orchard establishment. Fertilizer application is widespread, but in many cases, doses are overestimated. Ca, Zn, Cu, and B contents increased in leaves emitted in periods of higher soil water availability and higher temperatures. The results highlight the influence of weather conditions on the supply of these nutrients to banana plants. Nutritional factors are the main limiting factors in banana production in the state of Santa Catarina, compared to non-nutritional factors.

Anisohydric sugar beet rapidly responds to light to optimize leaf water use efficiency utilizing numerous small stomata.

Under conditions of high transpiration and low soil water availability, the demand for water can exceed supply causing a reduction in water potential and a loss of cell turgor (wilting). Regulation of stomatal aperture mediates the loss of water vapour (gs), which in turn is dependent in part on the anatomical characteristics of stomatal density (SD) and stomatal size (SS). Anisohydric sugar beet (Beta vulgaris) is atypical, exhibiting wilting under high soil water availability. Spinach (Spinacia oleracea) belongs to the same family Chenopodiaceae s.s., but demonstrates a more typical wilting response. To investigate the role of stomatal dynamics in such behaviours, sugar beet and spinach leaves were exposed to step-changes in photosynthetic photon flux density (PPFD) from 250 to 2500 µmol m−2 s−1. Using a four log-logistic function, the maximum rate of stomatal opening was estimated. Concurrent measurements of SD and SS were taken for both species. While sugar beet coupled faster opening with smaller, more numerous stomata, spinach showed the converse. After exposure to drought, maximum gs was reduced in sugar beet but still achieved a similar speed of opening. It is concluded that sugar beet stomata respond rapidly to changes in PPFD with a high rate and magnitude of opening under both non-droughted and droughted conditions. Such a response may contribute to wilting, even under high soil water availability, but enables photosynthesis to be better coupled with increasing PPFD.

Diversity of banana diploid hybrids: An assessment based on a hydroponic system

AbstractBananas are typical to tropical and subtropical climate regions, where they have better conditions due to high air humidity and soil water availability. The production of drought‐tolerant varieties is an alternative to expand the cultivation sites to drier areas. In this work, we present a hydroponic system for the genotype selection of Musa sp. (AA) subjected to water stress. A total of 14 banana genotypes were tested; 12 diploids and two commercial varieties with a known response to drought tolerance. The results show that the improved diploids have different responses with respect to water deficit, with the PMGB043 and PMGB044 genotypes considered more susceptible and the PMGB075 and PMGB099 genotypes more tolerant. Under water deficit conditions, most banana diploids maintain a high relative water content, but there is a reduction in aerial part growth and an increase in root dry mass. The accumulation of alcohol dehydrogenase, especially in control treatment plants, indicate that the glycolytic pathway was used to supply energy demand, showing potential flood‐tolerant genotypes. The results are essential for the preimprovement of banana cultivars.

Understanding effects of genotype × environment × sowing window interactions for durum wheat in the Mediterranean basin

• Advanced sowings compared to traditional may raise durum wheat yield in Mediterranean. • Water stress reduction and longer grain filling were the major factors for yield increase. • Short-cycle cultivars were less affected by hot and dry Mediterranean conditions than medium-long cultivars. Durum wheat is one of the most important crops in the Mediterranean basin. The choice of the cultivar and the sowing time are key management practices that ensure high yield. Crop simulation models could be used to investigate the genotype × environment × sowing window (G × E×SW) interactions in order to optimize farmers’ actions. The aim of this study was to evaluate the performance of the wheat model SiriusQuality in simulating durum wheat yields in Mediterranean environments and its potential to explore the G × E×SW interactions. SiriusQuality was assessed in multiple growing seasons at seven sites located in Italy, Spain and Morocco, where locally adapted cultivars were grown. The model showed good ability in predicting anthesis and maturity date (Pearson r >0.8), as well as above ground biomass and grain yield (6 % < nRMSE < 18 %). The model was then used to find the optimal 30-day sowing window to maximize grain yields at four sites, two were located in Italy (Florence, Foggia), and the other two were in Spain (Santaella) and Morocco (Sidi El Aydi) respectively. Among the cultivars, on the average between all sowing window, Amilcar had the best performance in Foggia (+33 % compared to the traditional cultivar Simeto) and in Sidi El Aydi (+22 % compared to Karim), Karim in Florence (+19 % compared to Creso) and in Santaella (+6 % compared to Amilcar). Instead Creso and Simeto showed the lowest production at all locations. The results showed that an earlier sowing window compared to the traditional one would have a positive effect on wheat yields in all environments tested, because of increased maximum leaf area index, grain number and size, and grain filling duration. Moreover, with earlier sowing, grain filling coincides with higher soil water availability, reducing the water stress and increasing the accumulation of dry mass in grains. In cooler and wetter locations, cultivars characterized by higher leaf area index and radiation use efficiency had the higher number of grains, while in the hottest and driest locations, short-cycle cultivars with high grain dry matter potential (e.g. through enhanced “stay green” capacity) should be preferred.

Vegetable Crops Grown under High Soil Water Availability in Mediterranean Greenhouses

The soil water availability of six vegetable crop cycles, irrigated with water of 0.4 dS m−1 electrical conductivity, was modified by varying the irrigation frequency in typical Mediterranean greenhouses at SE Spain. The soil matric water potential (SMP) in the middle of the loamy soil layer where most roots usually grow was maintained between −10 and −20 kPa (H), −20 and −30 kPa (C), and −30 and −50 kPa (L) for the crops grown under high, conventional and low soil water availability, respectively, while the total irrigation water applied was similar for the three treatments. The high soil water availability (H) did not improve the fresh weight of total, marketable and first class fruits, or the shoot biomass and partitioning. The irrigation frequency did not affect the total root biomass at the end of the autumn–winter cucumber, but the crop under L distributed its root biomass more homogenously throughout the soil profile than the crop under H. Regulating the soil water availability (maintaining the SMP higher than or close to the level at which crop water stress may occur) over the cycle as a function of crop conditions or farmers’ requirements appears to be a useful management practice for controlling soil root distribution or shoot partitioning.

Switching of dominant positions between two sand‐fixing shrub species during the dune revegetation process is underlain by their contrasting xylem hydraulics and water‐use strategies

AbstractDifferences in key physiological characteristics between plant species can result in their contrasting performances in vegetation restoration projects, but ecophysiological comparative studies in close relation to ecological restoration projects are scarce. Hedysarum fruticosum Pall. and Caragana microphylla Lam. are two leguminous shrub species widely used in combination for fixation of active sand dunes in semiarid regions of northern China, but they clearly differ in fitness at different stages of dune vegetation development; that is, H. fruticosum quickly gains a community dominance at the initial stage but yields to C. microphylla in a few years after the fixation of dunes. To reveal the underlying mechanism of such a pattern, key physiological characteristics related to xylem water transport, leaf water use, and drought tolerance were compared between these two species at a typical site of sand dune fixation project in Inner Mongolia. Compared with C. microphylla, H. fruticosum shows an acquisitive and risky water management strategy with substantially higher hydraulic efficiency and photosynthetic capability but reduced resistance to drought‐induced xylem embolism, which endows it an advantage under conditions of relatively high soil water availability at the early stage of dune fixation. In contrast, C. microphylla exhibits more conservative water use and characteristics of greater drought tolerance, which results in greater dominance under the more drought‐stressed conditions of the later stage of dune fixation. Our study for the first time provides a potential mechanistic explanation for the clear switching of dominant positions between the two important sand‐fixing shrub species during the sand dune revegetation process.

Starch‐based biodegradable hydrogel as seed coating for corn to improve early growth under water shortage

ABSTRACTIn this work, the effect of a biodegradable hydrogel as a seed coating on the early growth of corn seed was studied. The hydrogel was a succinate‐modified potato starch, and it had an equilibrium water absorption capacity of 260 g distilled water (DW)/g hydrogel. To coat corn seeds, the seeds were slightly wetted with an aqueous adhesive solution and then, in a closed container, mixed with a dry mixture of the modified starch (MS), bentonite, and talc. The damp coated seeds were air‐dried at 40 °C for 24 h. The seed emergence and postemergence growth of coated and uncoated seeds were assessed with respect to: (1) the amount of MS on coated seeds, and (2) the available water supply. At a water supply of 77% field capacity (FC), the coated seeds showed a significantly higher rate of emergence than uncoated seeds—however, only a minor improvement was observed at higher and lower water supply levels (92, 82, and 65% FC, respectively). This indicates that, while poor water absorption by hydrogel at low water supply (65% FC) has limited benefit to seed water availability, the effect of good water retention by the hydrogel at high water supply (92, 82% FC) appears redundant in the presence high soil water availability. After 14 and 21 days of growth, the dry weights of the corn plants' roots and shoots did not significantly differ between coated and uncoated seeds, which indicate that, after seed emergence and root development, the contribution of the hydrogel coating to the plant's water requirements becomes negligible. © 2019 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2020, 137, 48523.

A New Wetness Index to Evaluate the Soil Water Availability Influence on Gross Primary Production of European Forests

Rising temperature, drought and more-frequent extreme climatic events have been predicted for the next decades in many regions around the globe. In this framework, soil water availability plays a pivotal role in affecting vegetation productivity, especially in arid or semi-arid environments. However, direct measurements of soil moisture are scarce, and modeling estimations are still subject to biases. Further investigation on the effect of soil moisture on plant productivity is required. This study aims at analyzing spatio-temporal variations of a modified temperature vegetation wetness index (mTVWI), a proxy of soil moisture, and evaluating its effect on gross primary production (GPP) in forests. The study was carried out in Europe on 19 representative tree species during the 2000–2010 time period. Results outline a north–south gradient of mTVWI with minimum values (low soil water availability) in Southern Europe and maximum values (high soil water availability) in Northeastern Europe. A low soil water availability negatively affected GPP from 20 to 80%, as a function of site location, tree species, and weather conditions. Such a wetness index improves our understanding of water stress impacts, which is crucial for predicting the response of forest carbon cycling to drought and aridity.

Nocturnal sap flow of Hedysarum scoparium and its response to meteorological factors in semiarid Northwest China

Nocturnal sap flow of shrub may indicate transpiration and water recharge storage at night. Hedysarum scoparium is an important sand fixation shrub in semiarid regions. This paper deals with the water use of Hedysarum scoparium at night in the semiarid Northwest China. Our primary goal was to reveal and understand the nature of nocturnal sap flow and its functional significance. Granier’s stem heat balance method was used to determine the nocturnal sap flow of Hedysarum scoparium (stem of 25, 16, 13 and 9mm). Nocturnal sap flow was substantial and showed seasonal variation similar to the patterns of daytime sap flow. Mean nocturnal sap flow was higher in more precipitation months of year 2011 for more daytime transpiration and high soil water availability which can be the cause of more nocturnal sap flow. And vapour deficit pressure, mean air temperature and wind velocity were significantly correlated with nocturnal sap flow, they could explain a small fraction of the variance in nocturnal sap flow of studied trees. And the meteorological factors have greater impact on nocturnal sap flow rate of Hedysarum scoparium with smaller stem diameter than other diameter classes of Hedysarum scoparium.

Linking photosynthetic performances with the changes in cover degree of three Mediterranean shrubs under climate manipulation

Understanding how different combinations of plant functional traits contribute to species fitness is a question of considerable ecological interest, that can give insights into the mechanisms controlling community assembly, and into the processes by which climate change can modify plant community structure and composition. We investigated the changes in cover degree of three shrubs (Cistus monspeliensis, Dorycnium pentaphyllum and Helichrysum microphyllum) growing in a Mediterranean garrigue subjected for 11 years to a reduced rainfall regime, following a conceptual framework based on the two‐phase resource dynamic model: considering the seasonal drought typical of the Mediterranean climate, the two‐phases were identified based on high (pulse phase) and low (interpulse phase) soil water availability. We developed a parameter proportional to the whole plant photosynthesis (plant photosynthetic index, PPI), scaling up the leaf photosynthesis to canopy level, taking into account the different canopy densities and the fluctuations in leaf biomass due to summer leaf shedding. PPI was used to derive plant performance estimators for both pulse (maximum value reached by PPI, PPImax) and interpulse phase (duration of the exhaustion phase, Durep, when drought constrains PPI below the plant carbon compensation point determining carbon starvation). For each species the ratio between PPImax and Durep (named PPIred) was used as an index of plant performance. The reduced rainfall regime mainly decreased the performances of the dominant species C. monspeliensis, both limiting PPImax and extending Durep. Under both natural and the manipulated rainfall regime, PPIred was proportional to plant success, measured as the cover degree variation rate of the species. This result suggests that a mechanistic approach using functional traits to quantify the different performance of co‐occurring species can be used to investigate 1) the drivers of the medium‐term changes in species abundance and 2) the processes responsible for change in plant community composition under climate change.

The "isohydric trap": A proposed feedback between water shortage, stomatal regulation, and nutrient acquisition drives differential growth and survival of European pines under climatic dryness.

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 2years, 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.