Decreasing Soil Water Availability Research Articles

Assessing the levels of soil water availability that minimize the damage of heatwaves to potted grapevines

The adverse effects of heatwaves on viticulture could be, at least partly, mitigated by generous soil water availability. As climate change will also likely reduce irrigation water, knowing to which extent soil water availability could be decreased under the heatwaves without compromising plant performances would be important. In this pot experiment, we differentiated soil water availability by compensating 100% (T100), 70% (T70), 40% (T40) and 10% (T10) of vine transpiration. Grapevines (cv. Sauvignon) were then subjected to a 11-day heatwave (Tmax 40 °C) and to a post-heat stress recovery phase. Soil water potential between -0.05 to -0.2 MPa, respectively, in T100 and T70 enabled the plants to maintain high net CO2 assimilation (Pn), stomatal conductance (gs), and leaf transpiration (Eleaf) which kept the leaf temperature (Tleaf) below the ambient temperature and promoted the plant growth. During the heatwave, T100 and T70 vines decreased their photochemical yield and dissipated the excess energy via regulated non-photochemical heat dissipation, and resumed their photochemical activities upon removing the heat stress. Further reduction in soil water potential to -0.4 and and -1.2 MPa in T40 and T10, respectively, decreased Pn and gs and restrained Eleaf, causing a significant increase in Tleaf during the heatwave, and limited the plant recovery after the heat stress. Nonetheless, the maximum efficiency of PSII (Fv/Fm) showed quite high resistance to heat stress in all treatments. These findings suggest that decreasing soil water availability to reach soil water potential levels close to -0.2 MPa implies higher efficiency in using water resources during a heat wave without limiting plant growth.

Interannual variation in gross ecosystem production and evapotranspiration in a temperate semiarid grassland undergoing vegetation recovery

Carbon gains (gross ecosystem production, GEP) and water losses (evapotranspiration, ET) are critical for assessing vegetation recovery potential in drylands. However, large uncertainties remain regarding the controlling factors that regulate GEP and ET in semiarid grasslands, especially during periods of vegetation re-establishment and succession. Using the eddy-covariance technique, we assessed the interannual variability and key biophysical controls of carbon (C) and water vapor (H2O) fluxes in a semiarid grassland ecosystem in northern China from 2012 to 2020. After years of being subjected to heavy grazing and with a recent ban, the shrub-to-herb ratio at the site increased from 0.5 in 2014 to 0.63 in 2020. Associated with decreasing soil water availability (∼4.0% year−1), canopy transpiration and light use efficiency also decreased at rates of about 3.3% and 6.2% year−1, respectively. Multi-year means of GEP and ET were 166.6 ± 20.1 g C m−2 (mean ± SD) and 190.2 ± 21.8 mm, respectively. Interannual variability in GEP was mostly driven by variation in summer GEP, which for the most part was limited by mid-summer air temperature (Ta), particularly when exceeding the plants’ optimal temperature for photosynthesis. Interannual variability in ET was predominantly caused by variation in spring ET, which was mostly affected by spring vapor pressure deficit (VPD) and associated reductions in canopy stomatal conductance. Synchronous changes in GEP and ET resulted in a relatively stable annual water use efficiency (WUE) of 0.97 ± 0.13 g C kg−1 H2O. This stability is indicative of the shrubs’ capabilities to acclimate to arid conditions. Our results suggest that temperature and atmospheric aridity have important roles in regulating C gains and H2O losses in recovering dryland vegetation, respectively. Future increases in climatic variability anticipated with global climate change may augment the regional transition from grass to shrub cover.

Increased wheat yield and soil C stocks after silica fertilization at the field scale

Increased crop production is a main goal to feed the predicted human population in future. The current management practice is, however, not sustainable as it depends on high amounts of fertilizer application and is highly vulnerable to decreased soil water availability. At the same time it becomes more and more crucial to reduce or even mitigate anthropogenic greenhouse gas (GHG) emissions. A possible way to enable this, might be the increase of the soil C sequestration and thus the C sink function of arable lands. A recent and potentially more sustainable idea is the single time fertilization with amorphous silicon (ASi) which is known to increase both nutrient and water availability. Here we show for the first time on the basis of a field plot experiment how a fertilization with ASi is affecting both, crop yield and the C sequestration of the soils in an agricultural system cultivating wheat. We found a strong increase in wheat yield and biomass production after ASi fertilization by increasing soil moisture during the whole growing season. Additionally, despite a relatively short growing season, Si fertilization increased the net C uptake by soils, i.e., C sequestration with both Si fertilized treatments showing a negative net ecosystem C balance (soil C gain) during the measurement period, while the control showed a small positive net ecosystem C balance (soil C loss). To our best knowledge, this is the first time such effect has ever been observed. In summary, our study demonstrates a new management strategy for crop production increasing yield and biomass production as well as soil C uptake on a more sustainable basis, by a single time fertilization with ASi.

Water stable isotope characteristics and water use strategies of co-occurring plants in ecological and economic forests in subtropical monsoon regions

Identifying the spatio-temporal variation in water use by co-occurring species in mixed forests can determine whether there is hydrological niche partitioning or niche overlapping, i.e., whether plant co-occurrence brings about resource complementation or interspecific competition, and provides a reference for plantation management. Water stable isotopic compositions in precipitation, soil, and four plants (xylem and leaf) in ecological and economic forests and hydrometeorological factors were continuously monitored for a year, and the water uptake by plants was determined using a Bayesian MixSIR model. The isotopic compositions of soil water and plant water in the ecological forest (Camphora camphora and Cunninghamia lanceolate) and economic forest (Castanea mollissima and Camellia pitardii) showed similar seasonal variation, and no significant difference between their mean isotopic values was found (p > 0.05). The isotopic compositions in leaf water of different plants in the two forests were controlled by xylem water isotope and meteorological factors, rather than soil moisture conditions. With decreasing soil water availability, the water sources of C. camphora, C. lanceolate, and C. pitardii changed from shallow to deep, while the water use pattern of C. mollissima was insensitive to changing soil moisture conditions. The water source overlapping between C. camphora and C. lanceolate in the ecological forest might generate water resource competition during the dry period, limiting their ecological functions. On the other hand, the water source partitioning between C. mollissima and C. pitardii in the economic forest facilitated the water resource allocation. Our study suggested that the overlapping and partitioning of hydrological niches among species in mixed forests were species-specific and provided a scientific basis for fully exploiting the ecological and economic benefits of silviculture ecosystems.

Extreme drought disrupts plant phenology: Insights from 35 years of cloud forest data in Venezuela.

The potential effects of climate change on plant reproductive phenology include asynchronies with pollinators and reductions in plant fitness, leading to extinction and loss of ecosystem function. In particular, plant phenology is sensitive to extreme weather events, which are occurring with increasing severity and frequency in recent decades and are linked to anthropogenic climate change and shifts in atmospheric circulation. For 15 plant species in a Venezuelan cloud forest, we documented dramatic changes in monthly flower and fruit community composition over a 35-year time series, from 1983 to 2017, and these changes were linked directly to higher temperatures, lower precipitation, and decreased soil water availability. The patterns documented here do not mirror trends in temperate zones but corroborate results from the Asian tropics. More intense droughts are predicted to occur in the region, which will cause dramatic changes in flower and fruit availability.

Physiological strategies for handling summer water stress differ among co-existing species and between juvenile and mature trees

Determining tree response to climate stress is critical for predicting changes in forest carbon dynamics as well as tree mortality. In temperate deciduous forests, describing this response is complicated by the complex diversity of leaf and wood characteristics among co-existing species. Furthermore, because of the inherent logistical limitations of measuring mature forest trees, many carbon models and stress-response studies are informed by physiological data collected from juvenile trees (seedlings or saplings). However, the extent to which juvenile and mature trees differ in their physiological responses to water stress is not well documented under natural conditions. The majority of carbon sequestered in a forest is in mature trees; therefore, direct canopy measurements comparing responses to climate in juvenile and mature trees would allow us to more accurately predict changes in ecosystem carbon uptake. Here, we present data describing the physiological responses to summer water stress in juvenile trees of six temperate deciduous species. Our results indicate that species exhibited variation and plasticity in stress hydraulic parameters yet maintained similar rates of carbon uptake. We demonstrate how integrative photosynthetic parameters, such as photosynthetic capacity and quantum efficiency of photosystem II, are beneficial for wholistically displaying physiological responses at the leaf level. We further compared seasonal patterns of leaf water potential during decreasing soil water availability between the juvenile trees and co-existing mature trees of the same species. Our data reveal that while some species remain static in their hydraulic behavior from the juvenile to adult stage, other species are dynamic between life stages. Models, as well as experimental studies examining tree response to stressors, should plan for plasticity in physiological parameters among co-existing species, and should further allow variability between life stages for particular species. The capacity to effectively inform models from data collected in mature trees will inevitably lead to improved predictions of tree mortality and forest carbon trajectories.

Spatially explicit assessment of genetic variation to inform conservation effort for an endangered Mediterranean conifer, Cedrus atlantica.

Preserving the genetic diversity of forest species is critical for maintaining their adaptive potential and allowing for generation turnover in forest ecosystems. Considering an uncertain future, it is necessary to establish reliable genetic conservation strategies to optimize the genetic variation preserved within populations in a spatially explicit context to assist decision-makers. Hence, we aimed to incorporate genetic information into spatially designed conservation actions. Cedrus atlantica is a large, long-lived conifer native to the mountains of North Africa, threatened by extinction. The relevant genetic units for conservation were selected using Bayesian analysis. The relative contribution of the populations to the genetic pool that maximized the species' genetic diversity was calculated to design an optimal seed bank. Finally, the relationship between the genetic composition and bioclimatic variables was estimated and projected throughout the study area under current and future climatic conditions. Three relevant genetic units were found for C.atlantica conservation that maximizes genetic diversity in a spatial context. Bioclimatic variables with the highest influence on genetic composition were closely related to climate warming and decreased soil water availability. We identified the role of genetic markers in designing a reliable conservation strategy for forest trees considering climate change, increased deforestation, and aridity. Projections of genetic composition due to the climate in the study region of North Africa provide spatially explicit guidance for optimizing the selection and preservation of seed banks.

Branch xylem vascular adjustments in European beech in response to decreasing water availability across a precipitation gradient.

Crucial for the climate adaptation of trees is a xylem anatomical structure capable of adjusting to changing water regimes. Although species comparisons across climate zones have demonstrated anatomical change in response to altered water availability and tree height, less is known about the adaptability of tree vascular systems to increasing water deficits at the intraspecific level. Information on the between-population and within-population variability of xylem traits helps assessing a species' ability to cope with climate change. We investigated the variability of wood anatomical and related hydraulic traits in terminal branches of European beech (Fagus sylvatica L.) trees across a precipitation gradient (520-890mmyear-1) and examined the influence of climatic water balance (CWB), soil water capacity (AWC), neighborhood competition (CI), tree height and branch age on these traits. Furthermore, the relationship between xylem anatomical traits and embolism resistance (P50) was tested. Within-population trait variation was larger than between-population variation. Vessel diameter, lumen-to-sapwood area ratio and potential conductivity of terminal branches decreased with decreasing CWB, but these traits were not affected by AWC, whereas vessel density increased with an AWC decrease. In contrast, none of the studied anatomical traits were influenced by variation in tree height (21-34m) or CI. Branch age was highly variable (2-22years) despite equal diameter and position in the flow path, suggesting different growth trajectories in the past. Vessel diameter decreased, and vessel density increased, with increasing branch age, reflecting negative annual radial growth trends. Although vessel diameter was not related to P50, vessel grouping index and lumen-to-sapwood area ratio showed a weak, though highly significant, positive relationship to P50. We conclude that the xylem anatomy of terminal tree-top branches in European beech is modified in response to increasing climatic aridity and/or decreasing soil water availability, independent of a tree height effect.

Inter-specific transpiration differences between aspen, spruce, and pine in a sky-island ecosystem of the North American Great Basin

Semiarid forests may face future challenges because of climate change-exacerbated soil droughts. Because the risk and rate of tree mortality from soil drying can vary widely between co-occurring species, inter-specific differences in tree response need to be evaluated for designing science-driven best-management strategies specifically tailored to iconic sky-island ecosystems. We analyzed sap flow of quaking aspen (Populus tremuloides), Engelmann spruce (Picea engelmannii), and limber pine (Pinus flexilis) for five consecutive years (2014–2018) at a remote, high-elevation site in the North American Great Basin. Our results revealed species-specific responses of sap flow to declining soil moisture. For quaking aspen, multi-year sap-flow trajectories mimicked growing-season depletion of soil moisture. Ample cool-season precipitation in 2017 diminished the summer drought impact on sap flow of Engelmann spruce and limber pine, indicating that the two conifers could tap a deeper soil water reservoir than the deciduous species. Since species-specific transpiration response to soil droughts was driven by shifting precipitation regimes, our findings suggest niche partitioning in the rhizosphere among coexisting tree species. In addition, spruce and pine rapidly downregulated canopy conductance with decreasing soil water availability, whereas aspen canopy conductance was insensitive to soil drying. This physiological characteristic allows quaking aspen to maximize resource acquisition when growing conditions are favorable, but poses a risk of hydraulic failure and subsequent mortality under soil drying. Overall, we found contrasting hydrological niches and physiological strategies between co-occurring tree species in semiarid, high-elevation ecosystems. We also emphasize the value of long-term, in-situ observations to determine species-specific susceptibility to environmental changes in remote mountain areas.

Temperature Impacts the Response of Coffea canephora to Decreasing Soil Water Availability

Climate change is expected to result in more frequent periods of both low rainfall and above normal temperatures for many coffee growing regions. To understand how coffee reacts to such change, we studied the physiological and gene expression responses of the clonal variety C. canephora FRT07 exposed to water deficits under two different temperature regimes. Variations in the time-dependent impact of water deficits on leaf stomatal conductance and carbon assimilation were significantly different under the 27 °C and 27 °C/42 °C conditions examined. The physiological responses 24 h after re-watering were also different for both conditions. Expression analysis of genes known to respond to water deficits indicated that drought-related signaling occurred at both temperatures. Deeper insights into the response of coffee to water deficits was obtained by RNASeq based whole transcriptome profiling of leaves from early, late, and recovery stages of the 27 °C experiment. This yielded expression data for 13,642 genes and related differential expression analysis uncovered 362 and 474 genes with increased and decreased expression, respectively, under mild water deficits, and 1627 genes and 2197 genes, respectively, under more severe water deficits. The data presented, from a single clonal coffee variety, serves as an important reference point for future comparative physiological/transcriptomic studies with clonal coffee varieties with different sensitivities to water deficits and high temperatures. Such comparative analyses will help predict how different coffee varieties respond to changing climatic conditions, and may facilitate the identification of alleles associated with high and low tolerance to water deficits, enabling faster breeding of more climate-smart coffee trees.

Photosynthetic Characterization and Response to Drought and Temperature in Wild Asparagus (Asparagus acutifolius L.)

The wild asparagus species Asparagus acutifolius L. is widespread in Mediterranean and subtropical environments, where its spears are consumed regularly. The species is known to have ecophysiologic plasticity, however there is no literature on this subject. This work aimed at assessing the photosynthetic characteristics of this wild species of asparagus, grown under full light (FL) and partial (i.e., about 40%) light (PL) conditions, and evaluating its ecophysiologic response to drought and temperature stress. The photosynthetic response to light of spears and of new cladodes (NC; current year) and old cladodes (OC; previous year) was measured using an infrared gas analyzer coupled with a climatized cuvette chamber. Cladodes net photosynthesis at high irradiance was also measured at varying air temperatures and decreasing soil water availability. Results indicate that developing spears were photosynthetically active with no difference between FL and PL treatments. Photosynthetic rates did not differ between NCs and OCs and were greater for FL cladodes, except at low irradiance. Well-watered plants were photosynthetically active from 0 to 45 °C, with a maximum photosynthetic rate of up to 9 µmol·m–2·s–1 at 30 °C and a decrease of about 60% at 45 °C. The species also demonstrated high tolerance to drought, with positive net photosynthesis even at predawn leaf water potential values of –2.4 MPa. Showing great ecophysiologic plasticity, this wild asparagus could be an interesting species in areas were conventional crop species are not profitable economically, or as an intercrop in agroforestry systems.

Soil moisture influences the avoidance behavior of invertebrate species in anthropogenic metal(loid)-contaminated soils

Water availability is paramount in the response of soil invertebrates towards stress situations. This study aimed to evaluate the effects of forecasted soil moisture scenarios on the avoidance behavior of two invertebrate species (the arthropod Folsomia candida and the soft-bodied oligochaete Enchytraeus crypticus) in soils degraded by different types of anthropogenic metal(loid) contamination (mining soil and agricultural soil affected by industrial chemical wastes). Different soil moisture contents (expressed as % of the soil water holding capacity, WHC) were evaluated: 50% (standard soil moisture conditions for soil invertebrates' tests); 75% (to simulate increasing soil water availability after intense rainfalls and/or floods); 40%, 30%, 25% and 20% (to simulate decreasing soil water availability during droughts). Invertebrates’ avoidance behavior and changes in soil porewater major ions and metal(loid)s were assessed after 48 h exposure. Soil incubations induced a general solubilization/mobilization of porewater major ions, while higher soil acidity favored the solubilization/mobilization of porewater metal(loid)s, especially at 75% WHC. Folsomia candida preferred soils moistened at 50% WHC, regardless the soils were contaminated or not and the changing soil porewater characteristics. Enchytraeus crypticus avoided metal(loid) contamination, but this depended on the soil moisture conditions and the corresponding changes in porewater characteristics: enchytraeids lost their capacity to avoid contaminated soils under water stress situations (75% and 20–25% WHC), but also when contaminated soils had greater water availability than control soils. Therefore, forecasted soil moisture scenarios induced by global warming changed soil porewater composition and invertebrates capacity to avoid metal(loid)-contaminated soils.

Drought and heat stress reduce yield and alter carbon rhizodeposition of different wheat genotypes

AbstractDrought and high temperature are major environmental stress factors threatening wheat production during grain filling stage resulting in substantial yield losses. Four wheat genotypes (Suntop, IAW2013, Scout and 249) were planted under two temperature levels (25 and 30°C) and two water levels (15% and 25% soil moisture content). Wheat yield, leaf δ13C, plant rhizodeposition, shoot biomass and root traits were examined. Low moisture (drought stress) and high temperature (heat stress) decreased the grain yield of all wheat genotypes, in particular 249, while combined drought and temperature stresses had the most pronounced negative effect on plant biomass and grain yield. Decreasing soil water availability decreased the allocation of plant‐derived C to soil organic carbon (SOC) and to microbial biomass through rhizodeposition. Leaf δ13C decreased with increased yield, suggesting that higher yielding genotypes were less water stressed and allocated less C to SOC and microbial biomass through rhizodeposition. Wheat genotypes with lower root/shoot ratios and thinner roots were more efficient at assimilating C to the grain, while genotypes with higher root/shoot ratios and thicker roots allocated more C belowground through rhizodeposition at the expense of producing higher yield. Therefore, improving these traits for enhanced C allocation to wheat grain under variable environmental conditions needs to be considered.

Modeling soil evaporation and the response of the crop coefficient to leaf area index in mature Populus tomentosa plantations growing under different soil water availabilities

In order to improve water management in Populus tomentosa plantations, the variation in the FAO-56 parameters of the basal crop coefficient (Kcb) and soil evaporation (Es) in mature P. tomentosa plantations under different soil water availability treatments were investigated over two growing seasons. Empirical models for predicting Es in well-watered drip irrigated stands were constructed and validated. Changes in the relationship between Kcb and leaf area index (LAI) during the growing seasons and differing soil water availabilities, were also studied. The Kcb in all treatments increased rapidly to its maximum in about mid-May, and then decreased and remained relatively constant until late August or September. The Kcb increased sharply with increasing LAI, but plateaued when the LAI reached a critical value. With increasing stand age, the limit of Kcb of all treatments increased, while the critical values of the LAI declined markedly. In contrast, these two values declined with decreasing soil water availability, but the difference in Kcb limit could disappear with increasing stand age. Distinct spatial heterogeneity in Es appeared only from April to June, during which the Es from the wet soil zone accounted for 66% of the total Es on average. Among the five empirical models, the L-ww (constructed model based on LAI data during the “well-watered” period) and LT-ww (constructed model based on data of LAI multiplied by soil temperature at 20 cm depth during the “well-watered” period) models had the lowest Es prediction errors (RMSE of 0.25 and 0.20 mm d−1), and the highest modelling efficiency (0.49 and 0.63) and index of agreement (0.84 and 0.88) for the “whole year” and “well-watered” periods, respectively. In conclusion, for predicting stand transpiration more accurately using a crop coefficient model, the quantitative relationship between LAI and Kcb needs to be adjusted for stand age and soil water availability. Spatial heterogeneity in Es should be considered when estimating Es in drip irrigated plantations of P. tomentosa and other tree species.

Effect of litterfall input on soil respiration and its temperature sensitivity in moso bamboo forest under simulated drought.

Increases in drought frequency and intensity under climate change will have great impacts on the carbon cycle of forest ecosystems. Understanding the responses of soil respiration and its temperature sensitivity to drought is necessary, when we assess whether soil is a carbon sink or source. The effects of litterfall input on soil respiration, temperature sensitivity and its lagging effect were studied in moso bamboo forests under simulated drought by ceiling method in the field with three litterfall treatments, i.e., ambient litterfall (unchanged, LU), litter addition (LA) and litter removal (LR). The results showed that LU decreased annual soil respiration rate in drought treatment (2.34 Μmol·m-2·s-1), compared with that in the control (3.15 Μmol·m-2·s-1) with ambient natural rainfall. LR showed stronger effect on soil respiration than LA. Compared with LU, LR decreased soil respiration rate by 21.0% in ambient condition and by 20.9% in drought treatment, while LA led to 5.3% increase only in drought treatment. Such a result indicated that the effects of LA and LR on soil respiration rate were stronger than LU in the drought condition. Drought decreased the temperature sensitivity of soil respiration by 8.4%, while LA and LR reduced that by 15.4% and 7.6%, respectively. The cumulative CO2 emissions during the whole 18 months were 7.35 and 5.40 kg CO2·m-2 in the control and drought treatment. Compared with LU, LA increased the cumulative CO2 emissions by 1.8% and 10.7%, and LR decreased that by 19.9% and 18.0% in the control and drought treatments. Our results indicated that the relationship between the litterfall amount (addition or removal) and soil respiration rate was nonlinear. The significant lagging effect may be caused by the decrease in root growth and microbial activity due to decreased soil water availability in drought treatment. Litterfall played a more important role in soil CO2 emission under drought, and thus litterfall was a crucial factor in soil carbon emission in the context of climate change.

Physiological Acclimation of Taxodium Hybrid ‘Zhongshanshan 118’ Plants to Short-term Drought Stress and Recovery

The physiological acclimation of Taxodium hybrid ‘zhongshanshan 118’ (T.118) plants to a progressive drought stress and drought-stressed to recovery treatment (DS-R) was investigated in this study. Plants of control (C) treatment were watered daily throughout the experiment. Results indicated that water deficit reduced stomatal conductance (gS) to improve water use efficiency (WUE) and, as a consequence, net photosynthetic rate (Pn), transpiration rate (Tr), and intercellular CO2 concentration (Ci) were also decreased in DS-R T.118 plants compared with C plants. These reductions became more significant with decreasing soil water availability. Correlation analysis showed gS was positively correlated (P < 0.01) with the soil water content as well as leaf relative water content (RWC). There was a tendency to accumulate proline, malondialdehyde (MDA), antioxidases, and membrane electrolyte leakage as stress intensity increased. Moreover, drought stress induced significant (P < 0.05) decline in total chlorophyll contents (Chlt) and increase of nonphotochemical quenching (NPQ) on day 8 as a photo-protective mechanism. Cluster analysis distinguished the adaption of T.118 plants to water deficit in two ways. First, photosynthesis was related to thermal dissipation, and second antioxidation was related to morphology and osmosis. Furthermore, tested parameters showed a reversed tendency and restored equivalently to C levels after 9 days of rewatering. These findings suggest that T.118 plants demonstrated considerable tolerance to short-term drought stress and recovery due to a high degree of plasticity in physiological acclimation.

Drought response transcriptomes are altered in poplar with reduced tonoplast sucrose transporter expression.

Transgenic Populus tremula x alba (717-1B4) plants with reduced expression of a tonoplast sucrose efflux transporter, PtaSUT4, exhibit reduced shoot growth compared to wild type (WT) under sustained mild drought. The present study was undertaken to determine whether SUT4-RNAi directly or indirectly altered poplar predisposition and/or response to changes in soil water availability. While sucrose and hexose levels were constitutively elevated in shoot organs, expression responses to drought were most altered in the root tips of SUT4-RNAi plants. Prior to any drought treatment, constitutively elevated transcript levels of abscisic acid biosynthetic genes and bark/vegetative storage proteins suggested altered metabolism in root tips of RNAi plants. Stronger drought-stimulation of stress-inducible genes encoding late-embryogenesis-abundant proteins in transgenic roots was consistent with increased vulnerability to soil drying. Transcript evidence suggested an RNAi effect on intercellular water trafficking by aquaporins in stem xylem during soil drying and recovery. Co-expression network analysis predicted altered integration of abscisic acid sensing/signaling with ethylene and jasmonate sensing/signaling in RNAi compared to WT roots. The overall conclusion is that steepened shoot-root sugar gradient in RNAi plants increased sensitivity of root tips to decreasing soil water availability.

Climate Change Impairs Nitrogen Cycling in European Beech Forests.

European beech forests growing on marginal calcareous soils have been proposed to be vulnerable to decreased soil water availability. This could result in a large-scale loss of ecological services and economical value in a changing climate. In order to evaluate the potential consequences of this drought-sensitivity, we investigated potential species range shifts for European beech forests on calcareous soil in the 21st century by statistical species range distribution modelling for present day and projected future climate conditions. We found a dramatic decline by 78% until 2080. Still the physiological or biogeochemical mechanisms underlying the drought sensitivity of European beech are largely unknown. Drought sensitivity of beech is commonly attributed to plant physiological constraints. Furthermore, it has also been proposed that reduced soil water availability could promote nitrogen (N) limitation of European beech due to impaired microbial N cycling in soil, but this hypothesis has not yet been tested. Hence we investigated the influence of simulated climate change (increased temperatures, reduced soil water availability) on soil gross microbial N turnover and plant N uptake in the beech-soil interface of a typical mountainous beech forest stocking on calcareous soil in SW Germany. For this purpose, triple 15N isotope labelling of intact beech seedling-soil-microbe systems was combined with a space-for-time climate change experiment. We found that nitrate was the dominant N source for beech natural regeneration. Reduced soil water content caused a persistent decline of ammonia oxidizing bacteria and therefore, a massive attenuation of gross nitrification rates and nitrate availability in the soil. Consequently, nitrate and total N uptake of beech seedlings were strongly reduced so that impaired growth of beech seedlings was observed already after one year of exposure to simulated climatic change. We conclude that the N cycle in this ecosystem and here specifically nitrification is vulnerable to reduced water availability, which can directly lead to nutritional limitations of beech seedlings. This tight link between reduced water availability, drought stress for nitrifiers, decreased gross nitrification rates and nitrate availability and finally nitrate uptake by beech seedlings could represent the Achilles’ heel for beech under climate change stresses.

Above- and below-ground responses of native and invasive prairie grasses to future climate scenarios

More intense and frequent droughts, coupled with elevated temperatures, are projected for grasslands worldwide. Although it has been suggested that alterations in temperature and precipitation will increase the success of biological invasions, studies that combine these climate change scenarios are limited. These changes in climate may increase the success of non-native, invasive plant species directly, as these species often possess traits that are favored by variable climates, or indirectly through negative impacts on native vegetation or alterations in soil microbial communities, such as arbuscular mycorrhizal (AM) fungi. The goal of our research was to assess above- and below-ground production and mycorrhizal fungal abundance of functionally similar native and invasive non-native grass species under projected climate-change scenarios. We assessed plant biomass, intra-radical AM root colonization, and relative abundance of extra-radical fungal biomass of two native (Schizachyrium scoparium (Michx.) Nash, Pascopyrum smithii (Rydb.) Á.Löve) and two invasive (Bothriochloa ischaemum (L.) Keng, Bromus inermis Leyss.) grass species subjected to increased temperature and decreased soil water availability. Regardless of temperature or soil moisture, the invasive grasses in our study displayed greater seedling growth as compared with paired native species. Invasive grasses were also generally characterized by greater intra-radical colonization by AM fungi, compared with native species. Our data suggest that invasive grasses will continue to be problematic and successfully out-compete native grasses following increased temperatures and reduced water availability, as projected by climate-change models.

Simulating transpiration and leaf water relations in response to heterogeneous soil moisture and different stomatal control mechanisms

Aims Stomata can close to avoid cavitation under decreased soil water availability. This closure can be triggered by hydraulic (‘H’) and/or chemical signals (‘C’,‘H + C’). By combining plant hydraulic relations with a model for stomatal conductance, including chemical signalling, our aim was to derive direct relations that link soil water availability, expressed as fraction of roots in dry soil (fdry), to transpiration reduction. Methods We used the mechanistic soil-root water flow model R-SWMS to verify this relation. Virtual split root experiments were simulated, comparing horizontal and vertical splits with varying fdry and different strengths of stomatal regulation by chemical and hydraulic signals. Results Transpiration reduction predicted by the direct relations was in good agreement with numerical simulations. For small enough potential transpiration and large enough root hydraulic conductivity and stomatal sensitivity to chemical signalling isohydric plant behaviour originates from H + C control whereas anisohydric behaviour emerges from C control. For C control the relation between transpiration reduction and fdry becomes independent of transpiration rate whereas H + C control results in stronger reduction for higher transpiration rates. Conclusion Direct relations that link effective soil water potential and leaf water potential can describe different stomatal control resulting in contrasting behaviour.