Deep Soil Water Research Articles

A data-driven model for Fennoscandian wildfire danger

Abstract. Wildfires are recurrent natural hazards that affect terrestrial ecosystems, the carbon cycle, climate and society. They are typically hard to predict, as their exact location and occurrence are driven by a variety of factors. Identifying a selection of dominant controls can ultimately improve predictions and projections of wildfires in both the current and a future climate. Data-driven models are suitable for identification of dominant factors of complex and partly unknown processes and can both help improve process-based models and work as independent models. In this study, we applied a data-driven machine learning approach to identify dominant hydrometeorological factors determining fire occurrence over Fennoscandia and produced spatiotemporally resolved fire danger probability maps. A random forest learner was applied to predict fire danger probabilities over space and time, using a monthly (2001–2019) satellite-based fire occurrence dataset at a 0.25∘ spatial grid as the target variable. The final data-driven model slightly outperformed the established Canadian Forest Fire Weather Index (FWI) used for comparison. Half of the 30 potential predictors included in the study were automatically selected for the model. Shallow volumetric soil water anomaly stood out as the dominant predictor, followed by predictors related to temperature and deep volumetric soil water. Using a local fire occurrence record for Norway as target data in a separate analysis, the test set performance increased considerably. This demonstrates the potential of developing reliable data-driven models for regions with a high-quality fire occurrence record and the limitation of using satellite-based fire occurrence data in regions subject to small fires not identified by satellites. We conclude that data-driven fire danger probability models are promising, both as a tool to identify the dominant predictors and for fire danger probability mapping. The derived relationships between wildfires and the selected predictors can further be used to assess potential changes in fire danger probability under different (future) climate scenarios.

Infiltration holes enhance the deep soil water replenishment from a level ditch on the Loess Plateau of China

AbstractThe introduction of exotic plants with high‐water consumption and high‐density planting have greatly changed the deep soil water environment on the Loess Plateau of China. Slope measures can intercept runoff to change slope soil water redistribution, however, the existing measures are not enough to improve deep soil water replenishment. Infiltration holes were used as a supplement of the existing slope measures. In this study, the effects of three different measures (the control without any measures, the level ditch, and the combination of level ditch and infiltration hole) on slope runoff and deep soil water redistribution were evaluated under field conditions. The results showed that the combined measure reduced the runoff by 44.1% and the sediments by 78.6% under an intense rainfall event compared with the control. After two rainy seasons, compared with the control and the level ditch, the combined measures significantly increased the soil water storage below 50 cm in the soil layer. Combining infiltration hole with level ditch improved the deep soil water replenishment within 30 cm from infiltration hole in a horizontal plane. Overall, the single‐level ditch was insufficient to improve deep soil water replenishment, and the combined measure can more effectively intercept runoff to increase the water replenishment of deep soils. Our findings provide a new thinking in regulating and utilizing slope runoff to effectively improve deep soil water replenishment in arid and semiarid areas.

The Effect of Woody Encroachment on Evapotranspiration in a Semi-Arid Savanna

Over the past century, increases in indigenous woody plant species, also known as woody encroachment (WE), has occurred in grasslands and savannas across the globe. While the impact on grassland and savanna composition and productivity has been well studied, little is known of the impacts on the hydrological cycle. WE may increase evapotranspiration (ET) losses, leading to reduced infiltration and ultimately reduced freshwater availability, which is of particular concern in arid and semi-arid areas. The aim of this study was to determine the effect of Colophospermum mopane (mopane) encroachment on ET in a semi-arid savanna located in South Africa. Mopane is widely distributed across southern Africa, and is one of the main encroaching species of the region. Following an assessment of the validity of two surface renewal approaches, SR1 and SRDT, against short eddy covariance campaigns for sensible heat flux estimation, the SR1 approach was used to estimate ET at an experimental woody plant clearing trial from November 2019 to July 2022. For the two drier years of the study, the removal of mopane trees had little effect on ET. However, for the wettest year of the study, the removal of mopane trees decreased ET by 12%, supporting the hypothesis that the conversion from grass dominance to woody dominance can increase ET. Annual ET exceeded annual rainfall in all 3 years, indicating that the vegetation supplements its water use with soil water that has accumulated during previous wet seasons, or that tree roots facilitate hydraulic lift of deep soil water, or groundwater, to depths within the rooting depth of both trees and grasses. Further research is needed to confirm the exact mechanism involved, and the consequences of this for groundwater and streamflow at landscape scales.

Partitioned Soil Water Balance and Its Link With Water Uptake Strategy Under Apple Trees in the Loess‐Covered Region

AbstractIt is important, but challenging, to partition soil water balance (SWB) to understand the impacts of afforestation on soil hydrological processes. This study will investigate the partitioning of SWB by combining stable and radioactive water isotopes to quantify the effects of afforestation, and analyze the mechanism by which SWB changes through identifying the water uptake strategies of apple trees of 18 and 26 years old (A18 and A26). Compared to the reference farmland, apple orchards significantly increased evapotranspiration (ET) by 5%–10%, which in turn decreased soil water storage and deep drainage by 5%–14% and 50%–95%, respectively. Further, the partitioning of ET showed that apple tree planting increased transpiration by 15%–28% but decreased evaporation by 17%–30%. The above change in SWB appeared to be closely related to plant water uptake strategies. The apple trees shifted their water source from shallow (0–2 m) to deep soils (below 2 m), utilizing approximately 62% of deep soil water in the late growing season. In particular, 23% of source water may come from soil water older than 50 years. The older apple trees tended to extract more water from deeper soils (45% for A26 vs. 38% for A18). Therefore, the soil water deficit was the cumulative effects of root water uptake. The methods for SWB partitioning provides technical support for similar studies, and the findings are helpful to better understand the hydrological processes in the thick loess deposition.

Quantitative partitioning of temporal origin of transpiration into pre- and post-plantation under deep-rooted vegetation on the Loess Plateau of China

Deep-rooted vegetation transpires a considerable amount of deep soil water with different ages in the unsaturated zone. However, the tradeoffs between new water of transpiration (temporally originating from post-planting precipitation) and old water of transpiration (temporally originating from pre-planting precipitation) across the vegetation lifespan are poorly understood. In this study, we collected soil samples from beyond 28 m soil depth on the Loess Plateau of China to investigate the influence of deep-rooted vegetation on the age of soil water and analyze the proportion of new and old water of transpiration in the unsaturated zone under grassland, 22-year-old apple orchard, and 17-year-old peach orchard. Water isotopes (2H, 18O, and 3H), solutes (chloride, nitrate, sulfate), and soil water content were used to identify the critical water ages in the unsaturated zone (one-year water age, water age corresponding to stand age, and the maximum water age of transpiration), and to determine soil water deficit, soil evaporation loss fraction, and potential groundwater recharge. The results showed that soil water mainly moved as piston flow in these soil profiles, and deep soil water largely came from heavy precipitation. Deep-rooted vegetation restrained new pore water velocity and potential groundwater recharge. New pore water velocity declined from 0.40 m yr−1 to 0.14 m yr−1 and 0.34 m yr−1 for apple and peach, respectively. Deep-rooted vegetation decreased groundwater recharge by 9.46 % for apple and 7.04 % for peach, compared to grassland. Over the vegetation lifespan, annual average transpiration was 500.56 mm yr−1 and 468.89 mm yr−1 with maximum water age of 63 years and 45 years for apple and peach, respectively. The transpiration of deep-rooted vegetation mainly used new water—94.97 % for apple and 97.47 % for peach. The total old water of transpiration was 553 mm for apple and 209 mm for peach. Our results identify the temporal sources of vegetation water use, offering new insights into the transpiration process of deep-rooted vegetation.

Prediction of deep soil water content (0–5 m) with in-situ and remote sensing data

Accurate and timely evaluation of soil water content (SWC) in water-limited arid and semiarid regions will provide an essential reference for scientifically based water management strategies and vegetation restoration practices. Theoretical and empirical models based on remote sensing data have been successfully applied to predict large-scale SWC. However, most existing models are limited to the surface soil layer, while the prediction of SWC in the deep soil layer (1–5 m) has been overlooked because of the shortage of long-term and large-scale filed-observed data. In this study, a south-north transect (ca. 860 km) was selected across the Chinese Loess Plateau (CLP). Based on both long-term remote sensing and in-situ data (2013–2016), pedo-transfer functions (PTFs) for the SWC in the 0–5 m profile were developed using multiple regression, random forest, and artificial neural network. The results showed that evapotranspiration, potential evapotranspiration, soil surface temperature, total shortwave broadband albedo, and normalized difference vegetation index were important remote sensing parameters, whereas soil texture and bulk density were important in-situ parameters for SWC PTFs development. Among the three PTF development methods, machine learning (i.e. random forest and artificial neural network) obtained a higher accuracy than multiple regression. For different combinations of input parameters, the introduction of in-situ factors significantly improved the accuracy of PTFs compared with PTFs based on remote sensing data only. The artificial neural network developed a PTF with only five input variables that predicted SWC with reasonable accuracy (root mean square error = 0.039, R2 = 0.697, mean absolute percentage = 24.8) and is thus useful for many applications on the Loess Plateau of China. In the future, more attention should be given to the role of in-situ parameters when developing PTFs for deep SWC prediction.

In Situ Rainwater Collection and Infiltration System Alleviates the Negative Effects of Drought on Plant-Available Water, Fine Root Distribution and Plant Hydraulic Conductivity

Soil water status and fine-root characteristics are the foundation for implementing forest water-management strategies in semiarid forest plantations, where rainwater is always the sole source of water for plant growth. Rainwater management and utilization are effective strategies to alleviate water scarcity in semiarid areas as ground water is always inaccessible there. Through the implementation of an in situ rainwater collection and infiltration system (IRCIS), we investigated the effects of IRCIS on soil water and fine-root distributions in the 0–5 m soil profile in a wet (2015, 815 mm) and a dry year (2016, 468 mm) in rainfed Robinia pseudoacacia forests in the Loess Plateau region of China. The results showed drought significantly decreased plant water availability and hydraulic conductivity of roots and branches, but strongly increased soil moisture deficits and fine-root (<2 mm diameter) biomass. With the implementation of IRCIS, soil profile available water and plant hydraulic conductivity can be significantly increased, but soil moisture deficits and fine-root (<2 mm diameter) biomass can be significantly decreased. Drought also significantly influenced the root distribution of Robinia pseudoacacia. The maximum depth of Robinia pseudoacacia roots in the dry year was significantly greater than in the wet year. Therefore, Robinia pseudoacacia can absorb shallow (0–1.5 m) soil water in wet years, while utilizing deep (>1.5 m) soil water in dry years to maintain normal growth and resist drought stress. The results of this study will contribute to the formulation of appropriate strategies for planning and managing rainwater resources in forest plantations.

Cover crops and a devigorating rootstock can impart substantial agroecosystem services to high-resource vineyards: A multi-year study

In vineyards, cover crops typically grow concurrently with grapevines and can impart a multitude of agroecosystem services; yet concerns over potential disservices might limit adoption. Reductions in yield and vegetative growth are key disservices of concern, but at sites with high soil resources, reducing excessive vegetative growth is often considered beneficial, if the yield is not strongly penalized. To optimize agroecosystem services provided by cover crops, it is crucial to examine above- and belowground plant responses over multiple years. Documentation of the responses of fruit crop root systems to cover crops may clarify potential benefits for long-term agroecosystem services associated with aboveground growth and soil resources. In 2016, we established a perennial grass under young vines (full floor cover) to examine if agroecosystem services imparted by an under-vine cover crop change in the first five years of vineyard production and depend on rootstock vigor. Overall, cover crop reduced grapevine vegetative growth between 13% and 30% across all years and with a trend of more substantial reductions in the first two years. In contrast, yield was reduced in only three of the five years, between 9% and 25%, depending on the rootstock. Furthermore, neither cover crop nor differences in rootstock vigor resulted in relevant or consistent changes in fruit chemistry during the study. Overall, we found that in a relatively fertile, young vineyard, combining a lower vigor rootstock with a perennial, under-vine cover crop imparts substantial agroecosystem services in comparison to the more common practices of using a medium vigor rootstock and spraying herbicide under the vines. These services included improved crop load (ratio of fruit to vegetative mass), increased soil phosphorus availability, increased deep soil water content, and presumably lower labor and other costs associated with vineyard maintenance. Studies on a decadal timescale are needed to further clarify whether these services persist, accumulate with time, or if competition from the cover crop eventually leads to unacceptable decreases in vine nutrient status, growth, and production.

Effects of stand age and structure on root distribution and root water uptake in fast-growing poplar plantations

Severe soil desiccation in mature forests has been discovered in many water-stressed regions around the world, threatening sustainable forest development. Only by understanding fine root distributions and root water uptake patterns of different forest stands can we timely deal with the severe water stress in tree growth. During the 2019 growing season, we repeated isotopic and soil water content sampling for four Populus tomentosa stands in the North China Plain (two young stands with lower- or higher-competition structure, and two mature stands with lower- or higher-competition structure), and fine root sampling was performed at the end of the growing season. The hydrogen-oxygen stable isotope method and the Bayesian mixture model were used to determine root water uptake patterns. The findings revealed that stand age had no effect on fine root distributions in the 0–2 m profile. However, P. tomentosa became more reliant on the deeper soil water with stand development. The stand structure did not affect fine root distributions of young stands but significantly affected that of mature stands. Regardless of developmental stage, the higher-competition structure would increase trees’ relative water uptake from the middle layers. However, this increase was at the expense of a decrease in the relative water uptake from shallow layers during the young stage and from deep layers during the mature stage. Furthermore, we discovered that the groundwater level in this area may have dropped to an extent that groundwater cannot provide sufficient water supply for P. tomentosa. This study clearly shows that fine root distribution patterns cannot be used to replace root water uptake patterns. In addition, the findings of this article will serve as a theoretical foundation for sustainable forest management in fast-growing plantations in other water-stressed areas around the world.

Comparative Apparent Hydraulic Conductance, Leaf Gas Exchange, and Water Resource Partitioning of Populus euphratica Trees and Saplings

Water acquisition via the root system of woody species is a key factor governing plant physiology. In order to compare the impact of water acquisition on the hydraulic and photosynthetic characteristics of different-sized Populus euphratic, which is a desert riparian tree species, we quantified leaf hydraulic conductance (KL), stomatal conductance (gs), net photosynthetic rate (PN), predawn and midday leaf water potential (Ψ), and the stem δ18O of the saplings and mature trees. The results showed that the saplings had a lower predawn leaf water potential (Ψpd) and soil-to-leaf water potential gradient (ΔΨ) and a higher KL than the mature trees but had a similar gs and PN to the mature trees. In arid zones, probably due to root limitation, the saplings were more likely to use unreliable topsoil water (<80 cm), whereas the mature trees typically uptake reliable deep soil water (>80 cm) and groundwater due to having deeper root systems. The unreliability of the water supply might make saplings hold a higher hydraulic conductance to ensure that the water is transported efficiently to the leaves and to satisfy their transpiration need. In contrast, the mature trees, which uptake the more reliable deeper water resources, had a relatively low leaf-specific hydraulic conductance because of the increased path length versus the saplings. However, adult trees can maintain stomatal conductance by upregulating ΔΨ, thereby facilitating their ability to maintain a carbon assimilation rate similar to that of the saplings. This regulating behavior benefits mature trees’ net carbon gain, but it comes at the expense of an increased risk of hydraulic failure. These results imply that the top priority for saplings should be to maintain hydraulic system functioning, whereas, for mature trees, the priority is to assure stable net carbon gain for their growth.

Using Isotopic Labeling to Investigate Artemisia ordosica Root Water Uptake Depth in the Eastern Margin of Mu Us Sandy Land

The annual precipitation in the eastern Mu Us sandy land is about 400 mm, but the precipitation varies greatly between years and seasons and severe meteorological and seasonal droughts often occur, which makes the ecological environment very fragile. Artemisia ordosica is the most dominant species in the area. We used depth-controlled deuterium labeling technology to study the root water uptake depth of adult Artemisia ordosica to explore how Artemisia ordosica can survive in extreme droughts. In addition, the soil moisture content was analyzed after the rainy season in October 2020 and the dry season in June 2021. We found that under the influence of an extreme seasonal drought in the study area, the soil layer below 180 cm in depth still maintained high water content of more than 2%; the dry sandy soil in the surface layer inhibited the loss of soil water below 180 cm. The maximum water uptake depth of the roots of adult Artemisia ordosica can reach 240–260 cm. In periods of drought, Artemisia ordosica can still maintain life by absorbing deep soil water. In drought-prone environments, Artemisia ordosica evolved a deeper vertical root system to survive dry periods by absorbing soil water from deeper layers, showing a broad water intake capacity and strong adaptability to arid environments. This study can provide a reference for afforestation projects and ecological restoration in Mu Us sandy land and also provide a reference for the ecological restoration of coal mining areas in this area.

Spatio‐temporal variation in deep soil water use patterns of overstorey and understorey layers in subtropical plantations predicts community assembly

Abstract Deep soil water utilization allows plants to cope with drought stress. However, little is known about the roles of the understorey layers in driving spatio‐temporal variations of deep soil water in forests and how the patterns of deep soil water use among life‐forms contribute to community assembly processes. We assessed the spatio‐temporal patterns and determinants of deep water utilization of tree, shrub and herb layers in subtropical coniferous plantations and investigated associations between deep water use parameters and dominance and richness of understorey vegetation. We found that the understorey layer had a higher reliance on deep soil water in the dry season, a larger seasonal plasticity of deep soil water uptake, but lower spatial variability in deep soil water utilization than the tree layer. We showed that greater reliance of the tree layer on deep soil water was associated with decreased shrub layer diversity, whereas greater reliance of the shrub layer on deep water was associated with increased herb layer diversity. Synthesis. Our results highlight the roles of understorey layers in driving the temporal dynamics of deep soil water in forests and improve our understanding of how deep soil water use patterns among life‐forms shape community assembly in forests.

Vapour pressure deficit and solar radiation are the major drivers of transpiration in montane tropical secondary forests in eastern Madagascar

Young secondary tropical forests occupy a larger area than mature forests nowadays but our understanding of their ecohydrological functioning, particularly with respect to tree water uptake, remains poor. Deep soil water uptake may make mature forests resilient to periods of water stress, but little is known in this regard for young forests with possibly less extensive root networks. We, therefore, studied sap flow dynamics for one year in two 50 m x 50 m forest plots: a young secondary forest (YSF, 5–7 years) and a semi-mature forest (SMF; 20 years) in montane eastern Madagascar. Temporal variations in the depth of water uptake were inferred from the stable isotope compositions of soil- and xylem water. Transpiration rates were low for both forest sites (265 and 462 mm y−1 for the YSF and SMF, respectively). Vapour pressure deficit and global radiation explained most of the variation in transpiration rates at both sites. There was little evidence of transpiration limitation by soil water, despite an extended dry season. Trees in the YSF extracted water mostly from the intermediate soil depth (30–70 cm) during the dry season. In the SMF, the depth of uptake increased as the dry season progressed for some species (Abrahamia, Brachylaena and Cryptocaria), but not for others (Ocotea and Eugenia). Although the transpiration rates are low for both forests, they are comparable to results reported for other tropical montane sites after normalising for net energy input and leaf area. Estimated evapotranspiration totals (including interception loss, understorey and litter evaporation) were 679 mm and 1063 mm y−1 for the YSF and SMF, respectively (42% and 61% of precipitation, respectively). These results suggest that the stage of forest regrowth affects water uptake, and thus the water balance during forest succession.

Contrasting water-use patterns of Chinese fir among different plantation types in a subtropical region of China.

Plantation cultivation plays an important role in improving terrestrial ecosystem functions and services. Understanding the water-use patterns of major afforestation species is vital for formulating ecological restoration strategies and predicting the response of plantation to climate change. However, the impacts and drivers of forest types on water-use patterns of key tree species are poorly understood. Here, the combined methods of dual stable isotope of δD and δ18O and Bayesian mixed framework (MixSIAR) were employed to investigate the water-use patterns of Cunninghamia lanceolata (Chinese fir) in a monoculture, mixed forest with Cinnamomum camphora, and mixed forest with Alnus cremastogyne under different rainfall events in subtropical China. Furthermore, the relative contribution of different soil and plant factors to the water-use patterns of Chinese fir was quantified using a random forest model. Our results showed that Chinese fir in the mixed forests (with C. camphora or with A. cremastogyne) utilized less water from shallow soil compared to that in a monoculture but significantly improved the proportion of water absorbed from deep soil with the increase of 55.57%–64.90% and 68.99%–108.83% following moderate and heavy rainfall events, respectively. The most important factors contributing to the differences in water-use patterns of Chinese fir among monoculture and mixed forests were tree attributes (i.e., leaf biomass, eco-physiological regulation, and fine root biomass). These findings reveal that Chinese fir in mixed forests could optimize water-use patterns by adjusting plant properties for interspecific niche complementarity, improving the utilization of deep soil water. Overall, this study suggests that mixed-species plantations could improve water-use efficiency and reduce the sensitivity of tree species to precipitation change, indicating they are better able to cope with expected climate variability.

Seasonal isotopic cycles used to identify transit times and the young water fraction within the critical zone in a subtropical catchment in China

Transit times and the young water fraction (Fyw, the proportion of the water younger than a certain age threshold, τyw) can help characterize the hydrological behaviors within a catchment. Previous studies of water age have concentrated on individual catchment hydrological components, such as river water, groundwater, plant water or soil water, which hinders the dynamic linking of various hydrological components in the critical zone. In this study, the isotopic compositions of precipitation, soil water, xylem water, river water, and groundwater in a subtropical catchment in China were continuously monitored for two years. We found that the seasonal isotopic cycles of shallow soil water (0–60 cm), xylem water and river water had different attenuations and phase shifts relative to those of precipitation. There were no obvious seasonal isotopic cycles of deep soil water (60–130 cm) and groundwater. Additionally, the transit time distributions (TTDs) of soil water at the depth of 0–10 cm, xylem water and river water in the catchment varied between the wet year and normal year. With increasing depth, the mean transit times (MTTs) of soil water increased and the Fyw decreased. For xylem water and soil water, the MTTs and Fyw in the wet year were shorter and larger than those in the normal year, respectively. The river water consisted of a small proportion of fast flow components and a large proportion of slow flow components, with the young water accounting for 16% in the wet year and 21% in the normal year. Our results contribute to an improved understanding of water ages, flow paths and linkages of different hydrological components in the critical zone at the catchment scale.

Identifying multivariate controls of water and nitrate in deep loess deposits under different land use types

Identifying the environmental factors controlling deep soil water and nitrate is important for the sustainability of vegetation and ecosystems. However, the combined effects of multiple factors at varying scales have been poorly understood, especially in those regions with thick vadose zones and deep-rooted plants. The aim of this study is to identify the multivariate controls of water and nitrate in seven > 13 m deep boreholes under different land use types on China’s Loess Plateau. After measuring soil water and nitrate contents, the combined effects of climatic factors (precipitation, temperature, and potential evapotranspiration) and soil properties (soil texture, magnetic susceptibility, pH, EC, and soil organic carbon) were explored by wavelet analysis. Land use is the key factor regulating deep soil water and nitrate reservoirs, with high water deficit and low nitrate accumulation within 0–10 m under non-fertilized forestlands and shrublands. The positive and negative standardized ratios respectively suggested the synergistic and antagonistic relationships of water and nitrate under shallow- and deep-rooted vegetation. As the important agent for nitrate leaching, soil water is influenced by magnetic susceptibility and sand. Furthermore, soil water, pH, EC, and soil organic carbon are individually or simultaneously responsible for nitrate transport and transformation, especially at large scales (> 7.5 m). This study provides novel insights for vegetation and environmental management, and benefits the parameterizing of process-based hydrological and biogeochemical models at regional or global scales, especially in the deep unsaturated zones.

Water use partitioning of native and non‐native tree species in riparian ecosystems under contrasting climatic conditions

Abstract One of the suggested mechanisms behind the success of non‐native plants in recipient ecosystems is competition avoidance with natives by means of different resource‐use strategies, such as deeper water uptake under dry conditions. We aimed at evaluating water source partitioning between native and non‐native tree species coexisting in central Spain floodplains; determining the dependency on drought stress of such water sources use; and assessing if the reliance on deeper water sources relates with physiological and growth performance. We assessed water uptake depth, leaf functional traits related to physiological performance and growth of native (Populus alba) and non‐native trees (Ailanthus altissima, Robinia pseudoacacia) coexisting in riparian forests under different drought conditions (drier, intermediate and wetter). We analysed δ2H and δ18O isotopes in xylem water and in soil water from top, mid and deep soil depths and determined the contribution of each water source to overall plant xylem water. Leaf traits related with resource use and secondary growth were assessed for each species. We found stronger differences between sites than between species, with all species taking more deep water in the driest site (~45% of the xylem water) than in the wettest (~15%). However, under drier conditions, species differences were significant for top‐soil water use, with R. pseudocacia withdrawing more superficial water (~22%) than A. altissima (~8%). These results indicate stronger water partitioning under drier conditions. Non‐native species showed a physiological strategy characterized by greater leaf N, water content, and enriched δ13C and δ15N values independently of the deep‐water uptake. However, a positive relationship between deep soil water use and such strategy was found for P. alba. We highlight that those native and non‐native species differences were more evident regarding physiological performance at leaf level than for deep‐water uptake or growth. Furthermore, our results suggest that differences in water sources used by coexisting species may increase under drier conditions. Read the free Plain Language Summary for this article on the Journal blog.

Divergent roles of deep soil water uptake in seasonal tree growth under natural drought events in North China

Understanding the ecophysiological mechanisms underlying tree adaptation in drought-prone regions is crucial for predicting forest health in the face of future climate changes. However, little is known about how tree hydraulic adaptations modulate the responses of seasonal tree growth to water deficits along a large climate gradient. In this study, the spatially divergent responses of tree growth to natural drought events were assessed through observations of water uptake patterns, leaf water potentials, sap flow rates, and tree stem radial variations in Chinese pine (Pinus tabulaeformis) plantations along a climate gradient in North China. Higher proportions of water were absorbed from the middle and deep soil layers during dry spells compared to the growing season across all study sites. However, at the semiarid and subhumid sites, increased water uptake from deeper soil layers did not completely alleviate drought stress. Instead, the trees adopted strong stomatal control over transpiration at the expense of reduced growth. In contrast, at the humid site, seasonal shifts in water uptake effectively compensated for the negative impacts of seasonal drought on tree growth. Given that intensified climatic droughts are expected under climate warming, the establishment of an early warning drought system is increasingly urgent for plantations in regions prone to drought stress.

Deep soil water deficit causes Populus simonii Carr degradation in the three north shelterbelt region of China

Large-scale afforestation exacerbates soil water scarcity in a semiarid region of northern China, causing the formation of a dried soil layer (DSL) and ecosystem degradation, and overmature Populus simonii Carr (>30 years) degradation in the Three North Shelterbelt Program (TNSP) area has recently been observed. Determining the relationship between degraded Populus simonii Carr and soil water deficit is essential for afforestation in semiarid regions. We monitored soil water content (SWC) to a 160 cm depth in an area with five degradation degrees (healthy, light degradation, moderate degradation, severe degradation, and dead) to investigate soil water dynamics and DSL characteristics from May 2018 to December 2021. The results showed that: compared with that of healthy Populus simonii Carr (94.7 ± 26.2 mm), the shallow layer (0–80 cm) soil water storage of severely degraded (114.1 ± 29.9 mm) and dead Populus simonii Carr (128.0 ± 35.7 mm) was restored by 19.4 mm and 33.3 mm, respectively. However, the deep-layer (80–160 cm) SWC was difficult to recover and a DSL gradually developed, the DSL − SWC was only 9.3–11.95 %. The deep-layer relative extractable soil water (REW) of healthy Populus simonii Carr was significantly higher than that of moderately degraded, severely degraded, and dead Populus simonii Carr (P < 0.05). The deep-layer SWC decreased with increasing degradation due to coarser soils and increased BD. A greater bulk density (BD) and higher sand proportion reduced the soil water holding capacity. Poor site conditions reduced the resistance of shelterbelts, causing the degradation and mortality of Populus simonii Carr during drought. Therefore, our results improve the understanding of the soil water dynamics and degradation mechanism of Populus simonii Carr, providing a theoretical basis for managing and maintaining the TNSP.

Impacts of moss-dominated biocrusts on rainwater infiltration, vertical water flow, and surface soil evaporation in drylands

Globally widespread biocrusts form a “living skin” on most dryland soils, helping govern fundamental ecosystem functions, particularly hydrological processes. However, the underlying mechanisms of biocrust effects on soil water infiltration and evaporation are still controversial. Biocrusts possibly regulate infiltration and evaporation due to their unique physicochemical properties and influence on surface soil moisture (θ), and these assumptions were verified in this study conducted on the Loess Plateau of China. The differences in vertical soil moisture distribution and flow patterns of infiltration water between moss-dominated biocrusts and bare soil were investigated through dye tracing experiments. The evaporation characteristics of biocrusts and bare soil were also obtained by weighing micro-lysimeters in both simulated and in-situ experiments. Results showed that biocrusts retarded deep soil water infiltration below ∼ 15 cm and significantly (F = 48.15, P < 0.001) decreased the stained area ratio (percentage of stained area to total area at the same soil depth) by 16% in contrast to bare soil. Because the infiltrated water was mostly retained in shallow soil (0–15 cm) in biocrust-covered soil, their ratios of low interaction flow (stained path width < 20 mm) and high interaction flow (20 mm < stained path width < 200 mm) were significantly higher (F ≥ 7.82, P ≤ 0.007) than those of bare soil. After the dye tracing experiments, the biocrusts modestly increased θ by 17% throughout the soil profile (0–50 cm), but they remarkably increased θ by 33%−78% in the top 20 cm of soil. Additionally, the biocrusts increased soil daily evaporation rate (E) by 27%−39% (with only the exception of the beginning) and the cumulative evaporation amount by 14% in contrast to bare soil over an 8-day field measurement. More importantly, the biocrust-affected soil properties, such as the contents of fine particles and organic matter, as well as saturated water content and field capacity, were closely correlated with E (|ρ| ≥ 0.52, P ≤ 0.08). Consequently, it was concluded that biocrusts restrict the depth of rainwater infiltration and increase surface θ, which may greatly enhance surface soil evaporation in the Chinese Loess Plateau.