Reduced Soil Water Availability Research Articles

Ectomycorrhizal fungal community response to warming and rainfall reduction differs between co-occurring temperate-boreal ecotonal Pinus saplings.

Understanding the responses of ectomycorrhizal (ECM) fungi and their tree hosts to warming and reduced soil water availability under realistic future climate scenarios is essential, yet few studies have investigated how combined global change stressors impact ECM fungal community richness and composition as well as host performance. In this study, we leveraged a long-term factorial warming (ambient, + 1.7 ºC, + 3.2 ºC) and rainfall reduction (ambient, 30% reduced rainfall) experiment in northern Minnesota, USA to investigate the responses of two congeneric hosts with varying drought tolerances and their associated ECM fungal communities to a gradient of soil moisture induced by a combination of warming and rainfall reduction. Soil drying had host-specific effects; the less drought tolerant Pinus strobus had decreased stem growth and lower ECM fungal community richness (fewer ECM fungal Operational Taxonomic Units, OTUs), while the more drought tolerant Pinus banksiana experienced no decline in stem growth but had an altered ECM fungal community composition under drier, warmer soils. Taken together, the results of this study suggest that the combined effects of warming and decreased precipitation will largely be additive in terms of their impact on host performance and ECM fungal community richness, but that drier and warmer soil conditions may also differentially impact specific ECM fungal genera independently of host performance.

Quantifying the Distribution of Evapotranspiration at PV and APV Sites Using Soil Moisture

Solar panels affect the distribution of water and energy reaching the ground causing changes in soil moisture, evapotranspiration and percolation. In the context of Agri-Photovoltaics those changes influence plant growth and yield as well as irrigation demands while large Photovoltaic installations could potentially lead to changes in the water balance of the catchment. In either case, evapotranspiration plays an important role as the installation of panels of any design leads to shading thereby reducing the water loss to the soil through evapotranspiration. As it is difficult to measure evapotranspiration, the authors proceeded using soil moisture observations to quantify evapotranspiration pattern in dry periods. They found on average a 44 % higher evapotranspiration rate over 12 dry periods of varying conditions under the panels compared to a reference area at the research site Pillnitz. However, similar observations at the second site, Weesow show also a reversed behaviour due to reduced soil water availability as a result of the higher evapotranspiration at the reference area.

High sensitivity of hop plants (Humulus lupulus L.) to limited soil water availability: the role of stomata regulation and xylem vulnerability to embolism

Drought poses a serious threat to the productivity of hop, an important perennial crop. However, the precise physiological mechanisms that make it highly susceptible to drought are not yet fully understood. In this study, we investigated stomatal regulation and xylem vulnerability to embolism, which are important traits closely associated with plant drought resistance. In a glasshouse cultivation experiment, we monitored changes in leaf water potential, stem elongation rates, and leaf gas exchange, including net photosynthetic rates, stomatal conductance, and intrinsic water use efficiency, on relatively young hop plants (traditional Saaz - Osvald’s clone 31) exposed to declining soil water availability. The transpiration rate and stem elongation of plants decreased significantly with a small decline in substrate water potential (ΨSUB), indicating a highly sensitive stomata response during early phases of soil dehydration. The stem elongation was completely halted, and the transpiration rate dropped to less than 50% of its maximum at ΨSUB levels below − 0.8 MPa. In well-watered hop plants, xylem in stems operates near the initial point of embolization and is highly vulnerable to embolism, with a water potential corresponding to a 50% loss of xylem conductivity at -1.6 MPa. The sensitive stomatal response to declining ΨSUB likely helps to mitigate the risk of hydraulic failure, albeit at the cost of impaired growth. Scheduled irrigation, particularly during the sensitive stem elongation stage, may be a promising approach to mitigate the detrimental effects of reduced soil water availability on hop growth and yield while also conserving water resources.

Radiation, Air Temperature, and Soil Water Availability Drive Tree Water Deficit Across Temporal Scales in Canada's Western Boreal Forest

AbstractChanges are projected for the boreal biome with complex and variable effects on forest vegetation including drought‐induced tree mortality and forest loss. With soil and atmospheric conditions governing drought intensity, specific drivers of trees water stress can be difficult to disentangle across temporal scales. We used wavelet analysis and causality detection to identify potential environmental controls (evapotranspiration, soil moisture, rainfall, vapor pressure deficit, air temperature and photosynthetically active radiation) on daily tree water deficit and on longer periods of tree dehydration in black spruce and tamarack. Daily tree water deficit was controlled by photosynthetically active radiation, vapor pressure deficit, and air temperature, causing greater stand evapotranspiration. Prolonged periods of tree water deficit (multi‐day) were regulated by photosynthetically active radiation and soil moisture. We provide empirical evidence that continued warming and drying will cause short‐term increases in black spruce and tamarack transpiration, but greater drought stress with reduced soil water availability.

Spatio-temporal dynamics of root water uptake and identification of soil moisture thresholds for precision irrigation in a Mediterranean yellow-fleshed kiwifruit orchard.

Actinidia is highly susceptible to water stress, both excess and shortage, and is therefore a model fruit crop for irrigation management, requiring precise water application. The present study was carried out in a mature kiwifruit orchard in southern Italy to investigate the physiology of a yellow-fleshed kiwifruit cultivar under non-limiting soil water conditions and in response to a progressive decrease in soil water content in a Mediterranean environment, with the aim of defining soil moisture thresholds to guide irrigation management. The progressive lowering in soil moisture was monitored using multi-profile probes, taking into account a 60cm layer. Plant water status and physiological parameters were measured throughout the experiment and were significantly correlated with soil water status, suggesting that the level of soil water deficit affects plant physiological performance. Reference minimum values of stem water potential reached during the day under non-limiting soil water conditions ranged from -0.4 to -0.7 MPa, with a value of -0.8 MPa identifying the threshold below which stomatal conductance began to decrease significantly. Soil moisture thresholds were defined according to the spatio-temporal dynamics of available water (AW) reduction, which decreased by approx. 10% and 1% before the onset of water stress and 16% and 2% at the onset of water stress, considered in the 0-30cm and 30-60cm soil layers, respectively, compared to the AW content of the whole soil profile. Results confirmed that root uptake was mainly concentrated in the first 30cm of soil depth, which should be properly managed by irrigation, as reduced soil water availability could easily lead to plant water stress. An integrated approach, combining plant measurements and soil water content monitoring, together with an assessment of root water uptake dynamics, is essential to identify soil water thresholds and develop precision irrigation, especially for high water-demanding crops and environments.

Sugar Beet Rooting Pattern Mediates Stomatal and Transpiration Responses to Progressive Water Stress

Water stress is the main risk facing sugar beet production in Europe and is expected to worsen with climate change. Therefore, future production essentially depends on the traits that sustain growth during water shortages. In this study, we investigated the interplay of stomatal conductance and transpiration rate as well as the root characteristics of six sugar beet cultivars in a climate chamber experiment under environmental conditions progressing from a non-stressed initial state toward high atmospheric water demand, followed by reduced soil water supply and then by a combination of high demand and low supply. Stomatal conductance quickly responded to changing conditions, dropping from 406.4 to 42.5 mmol m−2 s−1, one order of magnitude, with the transition to reduced soil water availability. The transpiration rate showed a slightly delayed response compared with stomatal conductance, while we observed an inverse influence on the subsequent stomatal behavior exerted by the depletion/conservation of available soil water. The rooting pattern substantially differed among genotypes, predominantly at depths between 60 and 80 cm, where 50.5% of the root length was allocated. Longer roots buffered the effects of the reduction in stomatal conductance at the onset of water supply limitation, with 5.4 mmol m−2 s−1 higher conductance per 100 cm of root length. Therefore, breeding and/or management measures targeting root system vigor are the key to the growth maintenance of sugar beet during dry periods.

Long-Term Tibetan Alpine Vegetation Responses to Elevation-Dependent Changes in Temperature and Precipitation in an Altered Regional Climate: A Case Study for the Three Rivers Headwaters Region, China

Recent studies offer more evidence that the rate of warming is amplified with elevation, indicating thereby that high-elevation ecosystems tend to be exposed to more accelerated changes in temperature than ecosystems at lower elevations. The phenomenon of elevation-dependent warming (EDW), as one of the regional climate-change impacts, has been observed across the Tibetan Plateau. Studies have often found large-scale greening trends, but the drivers of vegetation dynamics are still not fully understood in this region, such that the local implications of vegetation change have been infrequently discussed. This study was designed to quantify and characterize the seasonal changes in vegetation across the Three Rivers Headwaters Region (TRHR), where the land cradles the headwaters of the Yangtze, the Yellow, and the Lancang (Mekong). By mapping the normalized difference vegetation index (NDVI) over the growing season from 1982 to 2015, we were able to evaluate seasonal changes in vegetation cover over time. The results show a slightly increased tendency in green vegetation cover, which could possibly be attributed to sustained warming in this region over the past three decades, whereas a decline in the green-up rate with elevation was found, indicating an inconsistent trend of vegetation greening with EDW. The cause of the green-up rate decline at high elevations could be linked to the reduced soil water availability induced by the fast increase in warming rates associated with EDW. The findings of this study have important implications for devising adaptation strategies for alpine ecosystems in a changing climate.

Irrigation and Crop Load Management Lessen Rain-Induced Cherry Cracking.

The combined effects of deficit irrigation and crop load level on sweet cherry (Prunus avium L.) physiological and agronomic response were evaluated during the 2019 season in a commercial orchard located in southeastern Spain. Two irrigation treatments were imposed: (i) control treatment (CTL) irrigated above crop water requirements at 110% of crop evapotranspiration (ETC) and (ii) a deficit irrigation treatment (DI) irrigated at 70% ETC. Within each irrigation treatment, crop load was adjusted to three levels: 100% (natural crop load-high), 66% (medium crop load), and 33% (low crop load). The water relations results were more affected by the irrigation strategies applied than by the crop load management. The deficit irrigation strategy applied reduced soil water availability for DI trees, which led to a continuous decrease in their gas exchange and stem water potential. At harvest, the fruit water potential and osmotic potential of cherries from the DI treatment resulted in significantly lower values than those measured in cherries from CTL trees. On the other hand, both the irrigation strategies imposed and the crop load management used impacted fruit quality. Trees with the lowest level of crop load had fruits of greater size, regardless of the irrigation treatment assayed, and in the DI treatment, cherries from the trees with the lowest crop load were darker and more acidic than those from the trees with the highest crop load. Our results emphasize the different effects that rainfall before harvest has on mature cherries. Thus, cracked cherries at harvest represented 27.1% of the total yield of CTL trees while they were 8.3% of the total yield in DI trees. Cherries from CTL trees also showed a greater cracking index than those from DI trees. Moreover, a linear relationship between crop load and fruit cracked at harvest was observed, particularly for the CTL treatment; thus, the lower the crop load, the greater the proportion of cracked cherries.

Multi-level physiological and morphological adjustment of Haloxylon ammodendron related to groundwater drawdown in a desert ecosystem

Climate change and anthropogenic exploitation will increase groundwater lowering rates and aggravate groundwater shortage, particularly in desert ecosystems that rely heavily on groundwater. We conducted detailed studies about morphological and ecophysiological traits of two Haloxylon ammodendron communities across different groundwater depths in the Gurbantonggut Desert in China during an extreme drought period. When photosynthetic twig temperatures exceeded 45°C, photosynthesis was suppressed and dropped to zero, while stomatal conductance and transpiration rate increased rapidly. This decoupled behavior may cool photosynthetic tissues to prevent permanent damage. The optimum temperature decreased by ∼3°C in H. ammodendron at deep groundwater sites with less soil-water availability, which contributed to improving water use efficiency (WUE) at lower temperatures. H. ammodendron at the deep groundwater site had lower photosynthetic twig size and plant height while higher proportion of twig and branch biomass, which induced a denser canopy structure, and they had greater leaf dry mass content, photosynthetic pigments, and non-structural carbohydrates, and more negative predawn shoot water potential. Overall, our study indicates that H. ammodendron could alleviate adverse effects of reduced soil water availability and maintain photosynthetic capacity through multiple morphological and physiological adjustments. This establishes the basis for predicting responses of desert shrubs to groundwater drawdown.

Soil nitrogen and water management by winter-killed catch crops

Abstract. Improving N cycling in agroecosystems is one of the key challenges in reducing the environmental footprint of agriculture. Further, uncertainty in precipitation makes crop water management relevant in regions where it has not been necessary thus far. Here, we focus on the potential of winter-killed catch crops (CCs) to reduce N leaching losses from N mineralization over the winter and from soil water management. We compared four single CCs (white mustard, phacelia, Egyptian clover and bristle oat) and two CC mixtures with 4 and 12 plant species (Mix4 and Mix12) with a fallow treatment. High-resolution soil mineral N (Nmin) monitoring in combination with the modelling of spatiotemporal dynamics served to assess N cycling under winter-killed CCs, while soil water was continuously monitored in the rooting zone. Catch crops depleted the residual Nmin pools by between 40 % and 72 % compared to the fallow. The amount of residual N uptake was lowest for clover and not significantly different among the other CCs. Catch crops that produce high N litter materials, such as clover and mustard leaves, showed an early N mineralization flush immediately after their termination and the highest leaching losses from litter mineralization over the winter. Except for clover, all CCs showed Nmin values between 18 % and 92 % higher on the sowing date of the following maize crop. However, only Mix12 was statistically significant. Catch crops depleted the soil water storage in the rooting zone during their growth in autumn and early winter, but preserved water later on when their residues covered the ground. The shallow incorporation of CC residues increased water storage capacity during the cropping season of the main crop even under reduced soil water availability. Hence, catch cropping is not just a simple plant cover for the winter but improves the growth conditions for the following crop with decreased N losses. Mixtures have been shown to compensate for the weaknesses of individual CC species in terms of nutrient capture, mineralization and transfer to the following main crop as well as for soil water management. Detailed knowledge about plant performance during growth and litter mineralization patterns is necessary to make optimal use of their potential.

Grassland restoration on former arable land: Fine‐scale grass accumulation and damaged soil conditions limit species establishment

AbstractQuestionWhich vegetation and soil parameters limit species establishment in restored grasslands? Do these parameters operate predominantly on a fine or a community scale?LocationWhite Carpathian Mountains, SE Czech Republic.MethodsWe compared 16 grasslands restored on former arable land with 9 well‐preserved reference grasslands. We sampled cover of plant species in plots of 2 m × 2 m (community scale) in which four 20 cm × 20 cm subplots (fine scale) were nested. We quantified fine‐scale heterogeneity as the mean Sørensen dissimilarity index among the species composition of the subplots. To characterize site conditions, we analyzed soil properties, above‐ground plant biomass and its nutrient content.ResultsRestored grasslands had a lower number of plant species at both the community and fine scales. Species number at the community scale was positively associated with the species number at the fine scale (irrespective of grassland type), but was not associated with fine‐scale heterogeneity. This indicates the importance of fine‐scale biotic interactions for overall species diversity. In support of this, in both grassland types, the species number was negatively associated with the cover of grasses, but only at the fine scale. Total cover (but not the biomass) was lower in restored grasslands, and was positively related with species number, particularly in restored grasslands. This indicates that the lower species number in the restored grasslands mainly resulted from deteriorated abiotic conditions. Such deterioration of the plant environment can be attributed particularly to the soil of the restored, formerly plowed grasslands. They had a lower soil organic matter content and a higher proportion of fine soil particles (silt and clay), indicating reduced soil water availability. The lower soil N content of restored grasslands was also reflected in the nutrient content of plant biomass, where lower N:P and N:K ratios indicated a limitation of plant growth by N‐supply.ConclusionThe species number was generally driven by fine‐scale biotic interactions, which operated similarly in both grassland types. The reduced number of plant species in restored grasslands is the result of several factors among which altered soil conditions in the former arable land appears to be highly important.

Water Limitation in Forest Soils Regulates the Increase in Weathering Rates under Climate Change

Climate change is generally expected to have a positive effect on weathering rates, due to the strong temperature dependence of the weathering process. Important feedback mechanisms such as changes in soil moisture, tree growth and organic matter decomposition can affect the response of weathering rates to climate change. In this study, the dynamic forest ecosystem model ForSAFE, with mechanistic descriptions of tree growth, organic matter decomposition, weathering, hydrology and ion exchange processes, is used to investigate the effects of future climate scenarios on base cation weathering rates. In total, 544 productive coniferous forest sites from the Swedish National Forest Inventory are modelled, and differences in weathering responses to changes in climate from two Global Climate Models are investigated. The study shows that weathering rates at the simulated sites are likely to increase, but not to the extent predicted by a direct response to elevated air temperatures. Besides the result that increases in soil temperatures are less evident than those in air temperature, the study shows that soil moisture availability has a strong potential to limit the expected response to increased temperature. While changes in annual precipitation may not indicate further risk for more severe water deficits, seasonal differences show a clear difference between winters and summers. Taking into account the seasonal variation, the study shows that reduced soil water availability in the summer seasons will strongly limit the expected gain in weathering associated with higher temperatures.

Seasonal variation of leaf thickness: An overlooked component of functional trait variability.

The dry and wet seasons in the Neotropics have strong effects on soil water and nutrient availability, as well as on forest dynamics. Despite these major effects on forest ecology, little is known on how leaf traits vary throughout the seasons in tropical rainforest trees. Here, we investigated the influence of seasonal variations in climate and soil characteristics on leaf trait variation in two tropical tree species. We measured two leaf traits, thickness and water mass per area, in 401 individuals of two species of Symphonia (Clusiaceae) in the Paracou research station in French Guiana tropical lowland rainforest. We found a significant effect of seasonal variation on these two leaf traits. Soil relative extractable water was a strong environmental predictor of leaf trait variation in response to seasonal variation. Reduced soil water availability during the dry season was associated with increased leaf thickness and water mass per area, possibly as a result of stomatal closure. Our findings advocate the need to account for environmental seasonality when studying leaf traits in seasonal ecosystems such as tropical forests.

Cumulative Deficit Irrigation and Nitrogen Effects on Soil Water Trends, Evapotranspiration, and Dry Matter and Grain Yield of Corn under High Frequency Sprinkler Irrigation

HighlightsEvapotranspiration and corn grain yield under full and deficit water and nitrogen application were measured in a 3-yr study.Soil water trends, evapotranspiration, and dry matter and grain yields were significantly different between irrigation treatments and study year.Nitrogen availability did not significantly affect soil water trends or evapotranspiration within an irrigation level.Grain yield was not significantly different between full irrigation and 75% of full irrigation within a nitrogen level.Results indicate that crop evapotranspiration is independent of crop yield potential when soil water content is similar under high evaporative demand and frequent sprinkler irrigation.Reducing irrigation application to low-yield potential field areas will reduce yield.Abstract.Historically feed corn has been a minor crop in south central Idaho but over the past three decades corn production in southern Idaho has increased fourfold in response to a similar increase in the local dairy industry. Corn seasonal water use and response to water deficits in the region’s climate is lacking. A 3-year field study on corn (Zea mays L.) was conducted in 2017, 2018, and 2019 to evaluate the cumulative effects of continuous water and nitrogen (N) deficits on soil water trends, evapotranspiration, and dry matter and grain yield. Four irrigation rates, fully irrigated (FIT) and three deficit irrigation rates (75% FIT 50% FIT, and 25% FIT) combined with two N supply rates (0 and 246 kg N ha-1) were investigated under lateral-move irrigation. Growing season soil water depletion in 2017 in the 25% FIT and 50% FIT irrigation treatments significantly reduced soil water availability at planting in subsequent years and resulted in reduced yields relative to 2017. Nitrogen treatments had no significant effect on soil water availability, seasonal soil water depletion, or crop evapotranspiration (ETc) for a given irrigation treatment. Crop evapotranspiration was significantly different between irrigation treatments in each study year and decreased as irrigation amount decreased. Dry matter yield was significantly different between irrigation treatments in each study year, but there was no significant difference between the 75% FIT and FIT irrigation treatments for a given N treatment. Differences in dry matter yield decreased between N treatments as irrigation amount decreased. Grain yield was significantly reduced by deficit irrigation in each study year, but there was no significant difference between the 75% FIT and FIT irrigation treatments for a given N treatment in each study year. Grain yield was significantly different between nitrogen treatments for only the FIT irrigation treatment. The lack of significant difference in grain yield between the 75% FIT and FIT irrigation treatments resulted in a curvilinear convex downward water production function regardless of nitrogen treatment. A reduction in applied water resulted in a reduction of grain yield regardless of N availability suggesting that a reduction in irrigation application to less productive areas of a field will cause a yield reduction. The lack of significant difference in crop ETc between N treatments for a given irrigation treatment indicates that crop ETc is unaffected by crop yield potential when soil water contents are similar under high evaporative demand and frequent sprinkler irrigation. Keywords: Corn, Deficit irrigation, Evapotranspiration, Nitrogen, Sprinkler irrigation, Water use, Water production function, Yield.

Ecometabolic mixture design-fingerprints from exploratory multi-block data analysis in Coffea arabica beans from climate changes: Elevated carbon dioxide and reduced soil water availability

Ecometabolic mixture design-fingerprinting in coffee cultivated under climate change was chemically explored using ComDim. Multi-blocks were formed using UV, NIRS, 1H NMR, SWV, and FT-IR data. ComDim investigated all these different fingerprints according to the extractor solvent and in virtue of atmospheric CO2 increase. Ethanol and ethanol-dichloromethane showed the best separations due to CO2 environment. 1H NMR loading indicate increases of fatty acids, caffeine, trigonelline, and glucose in beans under current CO2 levels, whereas quinic acid/chlorogenic acids, malic acid, and kahweol/cafestol increased in beans under elevated CO2 conditions. SWV indicated quercetin and chlorogenic acid as important compounds in coffee beans cultivated under current and elevated CO2, respectively. Based on the ethanol and ethanol-dichloromethane fingerprints, k-NN correctly classified the beans cultivated under different carbon dioxide environments and water availabilities, confirming the existence of metabolic changes due to climate changes. SWV proved to be promising compared with widely used spectrometric methods.

Interspecific and intraspecific phenotypic diversity for drought adaptation in bioenergy Arundo species.

Biomass crops are commonly grown in low‐grade land and selection of drought‐tolerant accessions is of major importance to sustain productivity. In this work, we assess phenotypic variation under different environmental scenarios in a series of accessions of Arundo donax, and contrast it with two closely related species, Arundo donaciformis and Arundo plinii. Gas‐exchange and stomatal anatomy analysis showed an elevated photosynthetic capacity in A. plinii compared to A. donax and A. donaciformis with a significant intraspecific variation in A. donax. The three species showed significantly contrasting behaviour of transpiration under developing water stress and increasing vapour pressure deficit (VPD), with A. donax being the most conservative while A. plinii showed an elevated degree of insensitivity to environmental cues. Under optimal conditions, A. donax had the highest estimated leaf area (projected leaf area) and plant dry weight although a significant reduction under water stress was observed for A. donax and A. donaciformis accessions while no differences were recorded for A. plinii between optimal growing conditions (well‐watered [WW]) and reduced soil water availability (water‐stressed [WS]). A. donax displayed a markedly conservative water use behaviour but elevated sensitivity of biomass accumulation under stress conditions. By contrast, in A. plinii, biomass and transpiration were largely insensitive to WS and increasing VPD, though biomass dry weight under optimal conditions was significantly lower than A. donax. We provide evidence of interspecific phenotypic variation within the Arundo genus while the intraspecific phenotypic plasticity may be exploited for further selection of superior clones under disadvantageous environmental conditions. The extensive trade‐off between water use and biomass accumulation present in the three species under stress conditions provides a series of novel traits to be exploited in the selection of superior clones adapted to different environmental scenarios. Non‐destructive approaches are provided to screen large populations for water‐stress‐tolerant A. donax clones.

Drought Primarily Reduces Canopy Transpiration of Exposed Beech Trees and Decreases the Share of Water Uptake from Deeper Soil Layers

Research Highlights: During drought, reduced soil water availability and increased vapor pressure deficit diminished transpiration in a mature beech stand (Fagus sylvatica L.). Dominant trees were more affected than suppressed trees. The share of soil water uptake from deeper layers decreased. The ability of individual trees in the forest stand to save water during drought was apparently dependent on their social status. This would be relevant for forest management. Objectives: We investigated which basal area classes of trees contribute more or less to total transpiration under wet and dry conditions, and from which soil layers they took up water. We hypothesized that dominant trees have a better adaptability to drought and diminish transpiration more than suppressed trees. Methods: The water budget of the forest stand was continuously monitored throughout the entire observation period. Xylem sap flux measurements using thermal dissipation probes were performed during the vegetation period at different depths in the trunks of ten representative trees. A radial distribution model of the sap flow density pattern was used to compute whole-tree and stand transpiration. Water budget was simulated using a physiology-based model. Results: During drought, the fraction of suppressed trees to whole-canopy transpiration of the forest stand increased and the share of soil water uptake from deeper layers decreased. Conclusions: The behavior of dominant trees under drought conditions could be interpreted as a water-conserving strategy. Thinning by removing suppressed trees should be employed to stabilize forests.

Yield Component Responses of the Brachiaria brizantha Forage Grass to Soil Water Availability in the Brazilian Cerrado

Brazil is one of the world’s largest producers of beef cattle and dairy products, which requires high forage yield to attend grass-fed animals’ demand. Among the grass species adopted in the forage production system in Brazil, the Brachiaria genus stands out. This genus comprises nearly 85% of all planted forage area. In general, forage production systems in Brazil are essentially rainfed, and thus susceptible to seasonal soil water stresses. Selecting the suitable Brachiaria cultivar for lands susceptible to periodic waterlogging and dry spells is crucial to enhance forage yield, and consequently, to reduce the environmental footprint of the livestock sector. In this research, we investigated the performance of three recent commercial Brachiaria brizantha cultivars (Piatã, BRS Paiaguás, and MG13 Braúna) extensively adopted in Brazil’s grazing systems subjected to different ranges of soil water potential. For three cutting periods, yield related-variables (e.g., plant height, leaf area, dry biomass, and water use efficiency) were measured. Our results point to the existence of a low drought-resistant trait among cultivars, indicating the need for releasing better-adapted cultivars to cope with reduced soil water availability. All cultivars achieved higher performance at soil water pressure head between −15 kPa and −25 kPa; and in general, the cultivar. Piatã showed slightly superior results to most of the treatments.

“Responsiveness of mature oak trees (Quercus robur L.) to soil water dynamics and meteorological constraints in urban environments”

Urban trees contribute to the regulation of microclimates of cities through transpirational cooling. The main environmental factors determining transpiration are evaporative demand and soil water availability. However, growth conditions in urban areas are often unfavorable and may promote drought stress. The responses of trees to these conditions may vary between species and age classes, and in addition, can be modified by their environment. For a better understanding of water use patterns of urban tree species, these responses were investigated as a function of different meteorological and soil conditions of their urban environments. To identify physiological responses of mature oaks (Quercus robur L.) to contrasting site conditions, sap flow measurements were carried out in two consecutive growing seasons (2013 and 2014) in urban and suburban areas of Hamburg, Germany. Even considering the varying and sometimes challenging local soil and meteorological conditions, the oaks showed predominantly uniform response patterns. At all study sites, sap flow dynamics were characterized by saturation responses to increasing vapor pressure deficits during both daytime and nighttime conditions, reflecting stomatal responsiveness and hence, a stomatal down-regulation of water loss at demanding atmospheric conditions. Accordingly, the water use of oak trees was characterized by an isohydric strategy, which can help to avoid the risk of cavitation. Moreover, the maximum values of nighttime sap flow exceeded 20% of the daily total at all sites, indicating high rates of nocturnal transpiration and thus a relevant cooling effect during the night. Actual sap flow followed potential sap flow at all sites despite decreasing soil water potentials in the upper soil layers. This suggests actual sap flow during both growing seasons was only limited by atmospheric water demand. Accordingly, the studied mature oak trees maintained high rates of transpiration even in times of reduced soil water availability and thus could provide cooling under challenging site conditions. In contrast to suburban areas, however, oaks in the urban site showed reduced absolute sap flow rates, indicating transpiration sensitivity to different situations of water demand and supply. Due to its water use strategy and its ability to cope with moderate drought, Q. robur can be considered a suitable urban tree species in view of current and future demanding urban growth conditions.

Artificial Top Soil Drought Hardly Affects Water Use of Picea abies and Larix decidua Saplings at the Treeline in the Austrian Alps

This study quantified the effect of shallow soil water availability on sap flow density (Qs) of 4.9 ± 1.5 m tall Picea abies and Larix decidua saplings at treeline in the Central Tyrolean Alps, Austria. We installed a transparent roof construction around three P. abies and three L. decidua saplings to prevent precipitation from reaching the soil surface without notably influencing the above ground microclimate. Three additional saplings from each species served as controls in the absence of any manipulation. Roofing significantly reduced soil water availability at a 5–10 cm soil depth, while soil temperature was not affected. Sap flow density (using Granier-type thermal dissipation probes) and environmental parameters were monitored throughout three growing seasons. In both species investigated, three years of rain exclusion did not considerably reduce Qs. The lack of a significant Qs-soil water content correlation in P. abies and L. decidua saplings indicates sufficient water supply, suggesting that whole plant water loss of saplings at treeline primarily depends on evaporative demand. Future work should test whether the observed drought resistance of saplings at the treeline also holds for adult trees.