Water Availability Treatments Research Articles

Phenotyping Seedling Root Biometry of Two Contrasting Bread Wheat Cultivars under Nutrient Deficiency and Drought Stress

Roots play a key role in withstanding wheat abiotic stress. In this work, we phenotyped seedling root morphology of two semi-dwarf bread wheat cultivars, the Chinese cv Lankaodali and the Italian cv Rebelde, under the hypothesis that these two genotypes have contrasting root traits and could be used as donors in breeding programs. Root development was compared in a semi-hydroponic screening, where full-strength (FS) vs. half-strength (HS) complete Hoagland’s solution represented high and moderate nutrient availability, and a screening comparing HS solution with tap water corresponding to a condition of nutrient starvation. Genotypes were further compared in soil under full watering (100% of field capacity) vs. drought stress (50% of field capacity). Lankaodali outperformed Rebelde by producing 50% more leaf mass and 70% more root mass in FS solution, 125% more leaf mass and 106% more root mass in HS solution, and 65% more leaf mass and 36% more root mass under nutrient starvation. This cv also showed a positive correlation between leaf mass and root length and mass (between r = 0.82–0.9 and r = 0.83–0.87, respectively, p < 0.05). In the soil screening experiment, Lankaodali produced more biomass than Rebelde regardless of water availability, 48% more leaf mass, 32% more root mass, and 31% more absolute rhizosheath mass (average across water availability treatments). Lankaodali proved to be more responsive than Rebelde to both water and nutrient availability. High values of broad-sense heritability—ranging between 0.80 for root mass and 0.90 for length in a hydroponic screen and 0.85 for rhizosheath size in soil—indicate that these traits could be useful for breeding.

Microbe‐induced plant resistance against insect pests depends on timing of inoculation but is consistent across climatic conditions

Abstract To cope with abiotic and biotic stressors, plants have developed mutualistic associations with beneficial soil microbes, but little is known about how extreme abiotic conditions impact on microbe‐induce resistance to insect herbivores. Extreme temperatures are often accompanied by extremes in plant water availability, which together reduce plant growth and change plant physiology. There are potential consequences for increasing plant susceptibility to biotic stresses, and this poses a real challenge for plant productivity. We evaluated how the effects of beneficial soil bacteria (Acidovorax radicis N35e) on barley plant growth and resultant resistance against aphid infestation (Sitobion avenae) were impacted by a single heatwave event across a plant water availability gradient. We also tested if timing of bacterial inoculation (before or after the temperature treatment) affected bacteria‐plant interactions on aphids. We found that heatwaves affected plant biomass allocation from above‐ground to below‐ground tissues. Inoculation with A. radicis led to reduction of aphid numbers, but depended on timing of inoculation, and led to stronger resistance when inoculations occurred closer to aphid infestation. Remarkably, microbe‐induced resistance against aphids was consistent across heatwave and water availability treatments. This study provides evidence that beneficial plant‐bacteria interactions may represent a potential solution for sustainable agricultural practices to enhance plant growth and response to insect pests under climate change. Future field trials should investigate the consistency of beneficial effects reported here for a better understanding of multispecies interactions in the context of global change. Read the free Plain Language Summary for this article on the Journal blog.

Water stress decreases the biocontrol efficacy of a nucleopolyhedrovirus against the fall armyworm on maize

AbstractDrought events are expected to limit crop productivity in the context of current climate change. Drought is also likely to affect multitrophic interactions such as those involving plants, phytophagous insects and their natural enemies. We designed a two-phase experiment to test the effect of water availability and fertilizer treatment on the interaction between maize, the fall armyworm and its nucleopolyhedrovirus pathogen. Plants grown in soil with high irrigation and mineral fertilization had the highest shoot and root dry weight. Furthermore, plant nitrogen levels were higher in plants with fertilization and a low irrigation scheme compared to other treatments. Low irrigation of maize plants reduced virus-induced mortality of armyworm larvae. Insects did not feed on plants with high irrigation and without fertilization. We conclude that water stress and plant nutrition can affect virus performance and thereby affect the efficiency of biological control and pest management, especially as water and nutrient limitation will likely increase under climate change.

Soil fungal communities contribute to the positive diversity–productivity relationship of tree communities under contrasting water availability

Abstract Plant diversity has often been linked to increased productivity; however, this apparent diversity–productivity relationship may rely on intertrophic interactions such as those between plants and soil microbes. Soil fungi can create complementarity between plant species via altered plant resource partitioning, facilitation via fungal networks or biotic feedbacks, thereby promoting plant diversity–productivity relationships. Furthermore, these relationships are likely to be context dependent in response to resource availability. We used a biodiversity–ecosystem function experiment with trees exposed to high and low water availability treatments to determine the contribution of soil fungal communities to the diversity–productivity relationship in tree communities. We used amplicon sequencing of soil fungi to assess fungal richness, community composition and richness of functional guilds. We then applied structural equation modelling to determine relationships between tree diversity, fungal communities and tree productivity and the role of water availability in these relationships. Tree species richness and functional diversity both increased above‐ground tree productivity and influenced soil fungal community composition. Fungal community composition had a direct impact on tree productivity and enhanced net diversity effects on productivity. Therefore, fungal communities mediated a positive, indirect effect of tree richness on productivity. While total fungal richness was not associated with tree diversity, pathogen richness decreased and mycorrhizal richness increased with tree richness. Pathogen and mycorrhizal richness had either no impact or a weak negative effect on productivity. Tree species traits strongly affected fungal communities and these changes promoted productivity. Finally, water availability greatly influenced fungal communities but did not interact with tree diversity to affect productivity; indicating possible resilience of tree communities to altered precipitation regimes and associated changes in fungal communities. Synthesis: Our study highlights the crucial role that fungal communities play in shaping the relationship between tree diversity, traits and productivity, and resilience to altered water availability.

The effect of pre-drought mepiquat chloride management on cotton sensitivity to drought during peak water demands

Drought can substantially limit yield in cotton, and water loss in any crop is closely associated with plant growth, but studies addressing the effects of growth management on drought sensitivity are limited in cotton. We tested the hypothesis that aggressive plant growth management would decrease cotton sensitivity to drought during flowering and boll development. Cotton was grown at a field site near Tifton, GA during the 2021 and 2022 growing seasons. Treatments included mepiquat chloride (MC) strategies (aggressive and untreated) and two water availability treatments (well-watered and drought-stressed). At approximately 2 weeks after first flower, water was withheld from the drought-stressed treatment for a three-week period, and well-watered plots were kept irrigated using a regionally accepted water balance approach. Measurements throughout the season included heights, nodes, length of the fourth internode below the plant terminal, nodes above white flower (NAWF), and soil moisture. End-of-season measurements included lint yield, fiber quality, and yield component assessments. MC treatment significantly affected plant height, mainstem node number, 4th internode length, and cutout date. Soil moisture was significantly reduced during the drought stress period. Aggressive MC treated plots showed less drought-induced yield reduction than untreated in response to drought, but did not reach the same lint yield, boll density, or uniformity as well-watered, untreated plots. Aggressive MC treatment produced greater seed surface area, seed number boll-1, and boll mass. Aggressive MC management may prevent drought-induced yield reductions but penalize yield under well-watered conditions. Among the potential components driving yield, boll number per unit land area was the most important contributor. Our findings highlight the potential for aggressive MC management to mitigate drought risk, but future efforts should evaluate this possibility in a broader range of cultivars adapted for production in the southeastern United States and with the inclusion of a moderate MC treatment.

Responses of two Acacia species to drought suggest different water-use strategies, reflecting their topographic distribution.

Soil water availability is a key factor in the growth of trees. In arid deserts, tree growth is limited by very dry soil and atmosphere conditions. Acacia tree species are distributed in the most arid deserts of the globe, therefore they are well adapted to heat and long droughts. Understanding why some plants do better than others in some environments is a key question in plant science. Here we conducted a greenhouse experiment to continuously and simultaneously track the whole-plant water-balance of two desert Acacia species, in order to unravel their physiological responses to low water availability. We found that even under volumetric water content (VWC) of 5-9% in the soil, both species maintained 25% of the control plants, with a peak of canopy activity at noon. Moreover, plants exposed to the low water availability treatment continued growing in this period. A. tortilis applied a more opportunistic strategy than A. raddiana, and showed stomatal responses at a lower VWC (9.8% vs. 13.1%, t4= -4.23, p = 0.006), 2.2-fold higher growth, and faster recovery from drought stress. Although the experiment was done in milder VPD (~3 kPa) compared to the natural conditions in the field (~5 kPa), the different physiological responses to drought between the two species might explain their different topographic distributions. A. tortilis is more abundant in elevated locations with larger fluctuations in water availability while A. raddiana is more abundant in the main channels with higher and less fluctuating water availability. This work shows a unique and non-trivial water-spending strategy in two Acacia species adapted to hyper-arid conditions.

Water availability influences both wood anatomy and laticifer density in rubber tree saplings

Given the predicted intensification of droughts and floods in the context of climate change, it is critical to understand the effect of water availability on secondary xylem and laticifers in rubber trees (Hevea brasiliensis (Willd. ex A.Juss.) Müll.Arg, Euphorbiaceae). Thus, we investigated the effects of different substrate water availability treatments on anatomical features of the secondary xylem, and the diameter, density, and proportion of laticifers in saplings of rubber trees. Lower substrate water availability resulted in shorter fibers and higher fiber fraction, lower gelatinous fiber fraction, higher vessel density, and lower laticifer density. Under lower water availability, changes in the secondary xylem favor safety in water transport. In the case of laticifers, which are present in the secondary phloem, their structural characteristics may represent a significant decrease in the latex production capacity, which in large forest plantations may reflect economic losses.

Sensitivity of seed germination to water stress in high-altitude populations of a threatened palm species.

Divergence in seed germination patterns among populations of the same species is important for understanding plant responses to environmental gradients and potential plant sensitivity to climate change. In order to test the responses to flooding and decreasing water potentials we germinated and grew seeds of three habitats of Euterpe edulis Mart. occurring along an altitudinal gradient, over a period of three years. Seed germination and root growth were evaluated under different water availability treatments: control, flood, -0.4MPa, -0.8MPa, for 2012, 2013 and 2014 years, and in the last year of the experiment (2014), -1.0MPa and -1.5MPa. Seeds of the montane habitat did not germinate in the flood treatment. Seed germination of all three habitats decreased in the -1.5MPa treatment. Montane habitat had the lowest seed germination at -1.5MPa. Time required for half of the seeds to germinate increased up to -0.8MPa. Seeds of montane habitats germinated slower in all the treatments. The only difference observed in seed germination synchrony was an increase in submontane population in flooding treatment. However, synchrony decreased at the lowest water potential treatments. Roots of montane population were more vigorous in most treatments, except in -0.8 MPa. The unusual ability of seeds to germinate at low water potentials might be related to early seed germination at the onset of the rainy season, potentially decreasing seed predation pressure. Seeds of the montane population were more sensitive to both types of water stress. A predicted increase in the frequency and intensity of extreme high rainfall or drought events may predispose early stages of this population to adverse factors, which might negatively affect population viability along elevational in future climate change scenarios.

Isolasi dan Karakterisasi Mikroorganisme Endofit Akar Ginseng Jawa (Talinum paniculatum Gaertn.) Yang Diberi Perlakuan Perbedaan Ketersedian Air

A medicinal plant known as Java ginseng (Talinum paniculatum Gaertn.) may thrive in a variety of water availability situations. The quantity of endophytic bacteria a plant contains affects its capacity to survive in stressful environments. The objective of this study was to identify and separate endophytic bacteria from Javanese ginseng roots that had undergone various water availability treatments. A description method was utilized, in which isolated isolates were given morphological descriptions, assessed for similarity indexes, and then categorized. Endophytic microbe abundance and diversity varied according to this study, but plants treated with water scarcity displayed higher abundance and diversity. While endophytes of plant origin treated with 60% field capacity (FC) had the maximum diversity, root samples treated with 80% (FC) had the highest microbial abundance. Ten species groupings were created from the 42 isolated isolates based on a coefficient value of > 70%.

Diaspore dispersal season influences root growth and seed traits, but not germination responses, of sympatric Piper species in a seasonal forest.

Seeds may differ in terms of dormancy, longevity, sensitivity to desiccation and dry mass, according to the timing (dry season/rainy season) of diaspore dispersal. In addition, seasonal variations in temperature and water availability can act as signals of the season during seed development, influencing germination responses and root growth. We evaluated the effects of temperature variations and water availability on germination parameters, root growth and seed traits of four coexisting Piper species in seasonal vegetation that differed in diaspore dispersal timing. Eight temperature treatments (15, 20, 23, 25, 28, 30, 35 °C, and alternate 30 °C-20 °C) and four induced water potentials (0, -0.3, -0.6 and -1.2 MPa) were used. The parameters germination onset, germination percentage (G%), mean germination time (MGT), root elongation, seed longevity during ex situ storage and dry mass of seeds were evaluated. Germination responses observed were independent of the diaspore dispersal timing, such as variations in germination onset, G% and MGT, both in temperature and water availability treatments. In contrast, root elongation, longevity and dry mass of seeds varied according to the time of diaspore dispersal. Our results corroborate the hypothesis that the timing of diaspore dispersal is an important factor in controlling the initial development of seedlings in seasonal vegetation, but not in germination responses. The predominance of negative effects of temperature increases and water deficit on root growth shows that the initial stages of plant development can be strongly impacted by these environmental factors.

Tree species identity drives nutrient use efficiency in young mixed‐species plantations, at both high and low water availability

Abstract Previous studies have demonstrated that tree species diversity increases productivity and may enhance nutrient cycling in forests. The effect of mixing tree species on stand‐level nutrient use efficiency (NutUE) has seldom been studied, and even less so in the context of climate change. Here we present the first study examining how diversity effects on NutUE may be modified by growing season water availability (low vs. high), and importantly, during periodic drought or on water‐limited sites. We tested the interaction of water availability and tree species diversity (i.e. species richness and species identity) on NutUE in two young, experimental plantations located in south‐western France (ORPHEE), and northern Ontario, Canada (IDENT‐SSM). We calculated stand‐level NutUE as above‐ground net primary productivity (ANPP) divided by the product of litterfall mass and macronutrient concentrations, of monocultures and mixed tree communities composed of several temperate tree species, with a focus on birch and pine at both sites. We found significant species richness and water availability effects on NutUE, but they were weakly and inconsistently expressed, detected only for specific nutrients, and differed between the two sites. Species identity had much stronger effects on NutUE when examined using the birch–pine plots at both sites. At ORPHEE, nitrogen use efficiency (NUE) and phosphorus use efficiency (PUE) were significantly greater in the birch–pine mixture than in each monoculture. This was due to an increase in ANPP coupled with a decrease in litter nutrient concentrations in the two‐species plot that was driven by the dominance of pine in the mixture. In the comparatively younger, denser plots at IDENT‐SSM, birch was the dominant species that resulted in positive mixing effects on ANPP and litter nutrient concentrations and a neutral effect of mixing on NUE and PUE in the birch–pine mixture. Overall, the effects of mixing did not differ with water availability treatments, suggesting that species composition of mixtures is more important than water availability for stand‐level NutUE in these young forest communities. Read the free Plain Language Summary for this article on the Journal blog.

Response Analysis of Rice Products on Various Conditions of Water Availability for Development of Optimal Water Sawing Irrigation

Farm systems with stagnant water require adequate water sources, because water is one of the most influential basic factors. Meanwhile, the availability of water decreased, while the level of water consumption increased. Water-saving irrigation solves the problem of water in rice cultivation. This study aims to determine the response of rice yields with the application of water-saving irrigation. The experiment was carried out with 5 treatments of water availability in the field: less than 50% of available water, 50%-60% of available water, 60%-70% of available water, 70%-80% of available water, and saturated). Each treatment was tested three times and the parameter observed was the productivity of rice plants. As a result, statistical analysis stated that there was no difference in rice productivity between all treatments of water availability in the field, with a significance value of 0.600, which is > 0.05. The highest productivity of rice plants is 6.44 tons/ha, namely the availability of water on the land at 70% and 80% available water. There is no difference in the value of rice productivity in saturated conditions with the application of water-saving irrigation, proving that the availability of water in the land in conditions of less than 100% available water has no effect on rice crop production.

Effect of Water Saving Irrigation Method on Physical-Chemical Characteristics of Local Rice

Water-saving irrigation solves the water problem in rice cultivation. Where, the problem of drought due to climatic factors and competition in the use of air between sectors makes the main air requirement for rice plants. The quality of the grain affects the quality of rice as the raw material, while the cultivation technique, climate, and biotic and abiotic inspections affect the quality of the grain. This study aims to determine the physical-chemical characteristics of rice with water-saving irrigation applications. The study used 5 treatments of water availability in the field, namely less than 50% of available water, 50%-60% of available water, 60% -70% of available water, 70% -80% of available water, saturated and ach treatment three replications. Physical properties observed are head rice, broken rice, and groats. Chemical properties analyzed were amylose content, moisture content, ash content, fat content and protein content. Based on the analysis, it is known that the availability of water in the land during the plant growth period affects the chemical properties of the rice of the Bujang Marantau variety and based on the specifications for the quality requirements of rice (SNI 6128: 2015) it is included in the medium quality class 1.

Preconditioning to Water Deficit Helps Aloe vera to Overcome Long-Term Drought during the Driest Season of Atacama Desert.

Throughout evolution, plants have developed different strategies of responses and adaptations that allow them to survive in different conditions of abiotic stress. Aloe vera (L.) Burm.f. is a succulent CAM plant that can grow in warm, semi-arid, and arid regions. Here, we tested the effects of preconditioning treatments of water availability (100, 50, and 25% of soil field capacity, FC) on the response of A. vera to prolonged drought growing in the hyper-arid core of the Atacama Desert. We studied leaf biomass, biochemical traits, and photosynthetic traits to assess, at different intervals of time, the effects of the preconditioning treatments on the response of A. vera to seven months of water deprivation. As expected, prolonged drought has deleterious effects on plant growth (a decrease of 55–65% in leaf thickness) and photosynthesis (a decrease of 54–62% in Emax). There were differences in the morphophysiological responses to drought depending on the preconditioning treatment, the 50% FC pretreatment being the threshold to better withstand prolonged drought. A diurnal increase in the concentration of malic acid (20–30 mg mg−1) in the points where the dark respiration increased was observed, from which it can be inferred that A. vera switches its C3-CAM metabolism to a CAM idling mode. Strikingly, all A. vera plants stayed alive after seven months without irrigation. Possible mechanisms under an environmental context are discussed. Overall, because of a combination of morphophysiological traits, A. vera has the remarkable capacity to survive under severe and long-term drought, and further holistic research on this plant may serve to produce biotechnological solutions for crop production under the current scenario of climatic emergency.

Moderate shading did not affect barley yield in temperate silvoarable agroforestry systems

With climate change and an increasing global human population, the concept of agroforestry is gaining economic and environmental interest. The practice of growing trees and crops on the same land is mainly applied in (sub)tropical climate and rarer in temperate areas where farmers fear decreased understorey crop yields due to competition with trees. However, whether competition is stronger below- (soil moisture, nutrients) or aboveground (light) in a temperate silvoarable agroforestry system (AFS) is not clear. The effects of different treatments of light, water and nutrient availability on crop production in two temperate AFS in Central Switzerland were investigated, where summer barley (Hordeum vulgare L.) was grown as understorey crop under 90%, 40% and 0% shade nets, with and without irrigation and/or fertilisation in a fully factorial design. Yield was reduced by 26% under heavy shade; yield reductions under moderate shade were not significant. Fertilisation and irrigation increased crop yield by 13% and 6–9%, respectively, independent from shade. Individual seed mass was significantly increased by fertilisation from an average of 0.041 g (± 0.008 SD) in unfertilised treatments to an average of 0.048 g (± 0.010) in fertilised treatments. Fertilisation had the biggest impact on total seed number (p < 0.001) with on average 36 (± 26) seeds per individual in unfertilised plots and 61 (± 33) in fertilised plots. This study demonstrates that moderate shade (as can be expected in modern AFS) was not a major limiting factor for barley yield in these two AFS in Switzerland, indicating that AFS with appropriate management combined with suitable selection of understorey crops are an option for agricultural production in temperate regions without significant yield losses.

Irrigation and fertilization of Chrysothamnus nauseosus (Asteraceae) affect the attack and gall growth of Rhopalomyia chrysothamni (Cecidomyiidae)

The quality of the environment and the host plant influence the development of the galls as well as the performance of gall inducing insects. The effect of nutrients and water availability on the development of galls induced by Rhopalomyia chrysothamni (Diptera: Cecidomyiidae) on Chrysothamnus nauseosus subsp. hololeucus (Asteraceae) was tested. The study was conducted in a xeric area with no signs of disturbances in Coconino National Forest, Arizona, USA. Individuals of C. nauseosus randomly selected underwent three treatments of water and nutrients availability. At the end of the growing season, the number of attacked plants, abundance and the development of galls in each treatment were evaluated. The differences in habitat quality simulated by the fertility and humidity conditions influenced both the number of attacked plants and the growth of galls. Plants with lower hydric-nutritional quality were more attacked by the galling insect. Otherwise, when both water and nutritional stresses were relieved plants developed galls with higher dry biomass and density. Therefore, this field experimental study indicates that while galling females attacked plants more in xeric environments, the galls grew larger in relatively more mesic conditions. The results showed a trade-off between the quantity and quality of galls of R. chrysothamni depending on the nutritional quality of the host plants.

The effect of water-saving irrigation on the growth of local rice plants

The decline in the function of the watershed causes the increasing scarcity of water, while the competition for water use is increasing. A conventional method of rice cultivation (continuous inundation) is very wasteful in the use of irrigation water. Water-saving irrigation by regulating the water availability in rice fields is an effort to reduce water loss due to percolation, seepage, and runoff. The study’s purpose was to examine the effect of water availability in the field on the growth of local rice varieties. The study used 5 treatments of water availability in the field (GW < 50% AW, 50% AW < GW ≥ 60% AW, 60% AW< GW ≥ 70% AW, 70% AW < GW ≥ 80% AW, and saturated) with 3 replicates and the parameters observed were the height of plants and number of tillers. The results showed that the availability of water in the field did not affect the plant height but did affect the number of tillers. From the vegetative phase at the age of ±28 days after planting (DAP) to the beginning of the generative phase ±55 DAP, there was an addition of tillers. When the generative phase period from ± 55 DAP, the formation of the tillers stopped, and some of the tillers dried up or died.

The contribution of phenotypic traits, their plasticity, and rapid evolution to invasion success: insights from an extraordinary natural experiment

The context‐depency of biological invasions makes it difficult to understand why some species become succesfull invaders and others not. Such understanding requires studying closely‐related invasive and non‐invasive alien taxa sharing the same introduction history in the same environment. We identified this unusual situation in Kenya where the individuals that founded invasive Prosopis juliflora and non‐invasive P. pallida populations are still present in original plantations. We evaluated field‐measured traits, conducted glasshouse experiments simulating different nitrogen and water availability treatments, and did reciprocal transplants to compare functional traits and plasticity between the founders of both species (i.e. ‘invasive–non‐invasive congeners' comparison), and between P. juliflora individuals from plantations and invaded sites (i.e. testing for rapid evolution during invasion). We found that planted individuals of P. julifora and P. pallida differed in a number of key traits related to performance and spread (root:shoot ratio, number of stems and susceptibility to seed damage) as well as in levels of phenotypic plasticity in growth responses to resource availability, which may explain their differential invasiveness. Offspring of invasive P. juliflora individuals had higher seed mass and production, germination, survival, produced more stems, matured earlier and had higher plasticity compared with those of founder individuals, indicative of rapid post‐introduction evolution. By using this exceptional study system, we show that differences in values of only a few key traits, increased phenotypic plasticity and post‐introduction evolution have all contributed to the success of P. juliflora as an invasive species in Kenya.

Plant Size Plays an Important Role in Plant Responses to Low Water Availability and Defoliation in Two Woody Leguminosae Species.

Plant size influences plant responses to combined environmental factors under climate change. However, their roles in plant ecophysiological responses are not fully understood. Two rapidly growing Leguminosae species (Robinia pseudoacacia and Amorpha fruticosa) were used to examine plant responses to combined drought and defoliation treatments (two levels of both treatments). Both 1.5 month-old seedlings and 3 month-old seedlings were grown in a greenhouse, and seedling growth, leaf gas exchanges, stem hydraulics, and concentrations of non-structural carbohydrates were determined after 60 days of treatment. Our results indicated defoliation had no significant effect on plant height, basal diameter, and total biomass whatever plant sizes and species. Under the low water availability treatment, the defoliated seedlings significantly increased by 24% in stem water potential compared with non-defoliated seedlings in large R. pseudoacacia. Compared with the high water availability in large non-defoliated R. pseudoacacia seedlings, the low water availability significantly reduced by 26% in stem starch concentration to maintain the stem soluble sugar concentration stable, but not in small R. pseudoacacia seedlings. We also found a negative correlation between leaf and root soluble sugar concentration under low water availability in A. fruticosa. The results demonstrate defoliation could relieve the effect of low water availability in large seedlings. Large seedlings had more compensatory mechanisms in response to defoliation and drought treatments than small seedlings, thus species with large carbon reserves are more recommended for vegetation restoration under combined drought and defoliation conditions. Future studies with more species are crucial for obtaining more rigorous conclusions.

Tree species richness and water availability interact to affect soil microbial processes

The objective of this study was to examine the interactive influence of tree diversity and water availability on microbial functioning in surface soil (0–10 cm). The study was conducted at two field experiments (ages 4.2 and 9.0 years, respectively) of the TreeDivNet platform that established plantations with a gradient of tree species richness exposed to high and low water availability treatments. Soil microbial biomass, activity, functional diversity and functional identity were estimated using the MicroResp™ method. Soil organic carbon and several other variables potentially associated with soil processes (soil moisture, soil and air temperature, soil pH, soil C:N ratio, tree biomass) were also measured. Our results indicate that tree species richness significantly increased soil microbial biomass and, to a lesser extent, soil microbial activity in the high water availability treatment, at both trial sites. However, tree species richness effects on microbial processes were not expressed in the low water availability treatment, contrary to the stress-gradient hypothesis. Tree species richness and water treatment interaction impacted microbial biomass and activity through their effects on microbial functional diversity and identity at one site and air temperature at the other site. In conclusion, the mechanisms through which tree diversity creates bottom-up effects on biological soil processes depend on environmental conditions and time following the establishment of tree communities. Future research relating diversity and water availability effects on soil microbial processes should consider functional traits of tree species and a larger range of trophic levels including herbivores and soil fauna to better understand biodiversity-soil ecosystem function relationships in forest ecosystems.