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Restoration of natural secondary forests and afforestation of introduced tree species are major effective measures for revegetation. The semiarid Loess Plateau region, characterised by fragile ecosystems and severe soil erosion, is a key area for ecological restoration and protection in China. To illustrate water use characteristics and adaptation to drought in the main forests in this area, we monitored the xylem sap flow of two typical adjacent forest communities with similar climatic characters, a secondary natural forest dominated by oak (Quercus liaotungensis) and a pure plantation of black locust (Robinia pseudoacacia), during 2011–2019 using Granier‐type thermal dissipation probes. Solar radiation (RS), air temperature, relative humidity (RH), precipitation and soil water content (SWC) were measured simultaneously. Throughout the whole study period, the mean diameter at breast height (DBH) and total sapwood area increased by 4.5 cm and 1.10 m2 ha−1, respectively in the oak forest and by 1.0 cm and 0.22 m2 ha−1, respectively in the black locust plantation. The monthly stand transpiration was jointly determined by phenological and meteorological factors. At the annual timescale, transpiration of the oak stand was significantly correlated with potential reference evapotranspiration (ET0) and rainfall in the previous year, whereas a significant positive relationship was detected between stand transpiration and SWC in the black locust stand. The analyses of differences between dry and wet years showed that the oak forest exhibited significantly different parameters in the regression analysis of stand transpiration to vapour pressure deficit (VPD), whereas only one parameter was clearly distinct in the black locust plantation, suggesting that its transpiration status did not fully recover even in wet years. The management of black locust plantations with weakened growth conditions should be adjusted under prolonged drought conditions. In contrast, oak forest can maintain the water balance and stable growth by efficiently controlling stomatal behaviour. [ABSTRACT FROM AUTHOR]

Headwaters contribute a substantial part of the flow in river networks. However, spatial variations of streamflow generation processes in steep headwaters have not been well studied. In this study, we examined the spatio-temporal variation of streamflow generation processes in a steep 2.98-ha headwater catchment. The time when baseflow of the upstream section exceeded that downstream was coincident with the time when the riparian groundwater switched from downwelling to upwelling. This suggests that upwelling of the riparian groundwater increased considerably in the upstream section during the wet period, producing a shift in the relative size of baseflow between the upstream and downstream sections. The timing of fluctuations among hillslope soil moisture, hillslope groundwater and streamflow reveals that the hillslope contributed to storm flow, but this contribution was limited to the wet period. Overall, these results suggest that streamflow generation has strong spatial variations, even in small, steep headwater catchments. EDITOR A. Castellarin ASSOCIATE EDITOR X. Chen [ABSTRACT FROM AUTHOR]

• Thinning affects ecohydrological processes under different biophysical conditions, silvicultural systems, and time scales. • Throughfall, net precipitation, soil moisture and tree-level water use increase after thinning by 1.19, 1.14 and 1.56 relative to the control, respectively. • Stemflow and stand transpiration decrease after thinning by 0.42 and 0.6 relative to the control, respectively. • Thinning enhances runoff and groundwater recharge and mitigates the effects of drought through increasing Water Use Efficiency. • Thinning intensity of about 50% of the stand density is determined as the threshold at or over which hydrological processes are significantly affected. • The duration of thinning effect can be set between 2.6 and 4.3 (throughfall) and 3.1–8.6 years (soil moisture and transpiration). Forest thinning can significantly affect hydrological processes. However, these effects largely vary with forest types, climate, thinning intensity, and hydrological variables of interest. Understanding these effects and their variations can significantly support thinning treatments' design and selection to ensure desired hydrological benefits. In this global-level review paper, we report the first comprehensive meta-analysis on the effects of thinning on major hydrological processes with an emphasis on rainfall partitioning, soil moisture and evapotranspiration processes. The synthesized and reviewed studies encompass different biophysical conditions (climate and forest ecosystems), silvicultural systems, and time scales (from weeks to decades) across continents. The results showed a significant increase in net precipitation, soil moisture and tree-level water use after thinning (the effect sizes are 1.19, 1.14 and 1.56 relative to the value of the control, respectively), while decreases in stemflow and transpiration (the effect sizes of 0.42 and 0.6 relative to the value of the control, respectively). Thinning intensity of about 50% of the stand density is determined as the threshold at or over which hydrological processes are significantly affected. The duration of thinning effect can be set between 2.6 and 4.3 (throughfall) and 3.1–8.6 years (soil moisture and transpiration), asking for repeated thinning in order to effectively sustain these effects. These global averages can serve as benchmarks for assessment and comparisons, but the effects of thinning depend on local biophysical conditions and thinning treatments. The literature review on the rest of the studied hydrological variables suggests that thinning generally enhance runoff to increase water yield and groundwater recharge. Thinning can also have a positive or limited role in water use efficiency (WUE), but it mitigates the effects of drought through increasing WUE. Moderate adverse effects on water quality can be prevented by adequate forest managements to prevent soil degradation. Nevertheless, more researches at relatively less studied regions are needed to support a more robust analysis of these reviewed hydrological variables. The management implications of the synthesized and reviewed results are suggested and discussed within the context of climate change. [ABSTRACT FROM AUTHOR]

In Japan, forests of Moso bamboo ( Phyllostachys pubescens, an exotic invasive giant bamboo) have naturalized and expanded rapidly, replacing surrounding broadleaved and coniferous forests. To evaluate impacts caused by these forest-type replacements on the hydrological cycle, soil-water content and its spatial variability in a Moso bamboo forest were compared with those in an adjacent evergreen broadleaved forest, in a case study of a stand in western Japan (northern Kyushu). The volumetric soil-water content averaged over depths between 0 and 60 cm was consistently higher in the bamboo stand than that in the broadleaved stand. These results contrast with previous studies comparing the soil-water content in Moso bamboo forests with that in other forest types. The sum of canopy transpiration and soil evaporation ( E) in the bamboo stand tended to be larger than that in the broadleaved stand. Small canopy interception loss was reported in the bamboo forest. Therefore, the large amount of E would counterbalance the small canopy interception loss in the bamboo forest. Differences in soil characteristics between the two stands may be the main factor causing differences in soil-water content. Spatial variation in soil-water content in the bamboo stand was larger than that in the broadleaved stand, confirming findings in a previous series of our study. This could happen because the well-developed root-system in the bamboo forest enhances preferential flow in the soil. To evaluate the effects of aggressive invasion of alien giant bamboo on the ecosystem functions, we recommend further studies measuring various hydrological components in various Moso bamboo forests. [ABSTRACT FROM AUTHOR]

ABSTRACT We investigated the contribution of three evapotranspiration (ET) components including canopy interception ( Ei), tree transpiration ( Et) and forest floor evaporation ( Ef) and identified the sources depth of evaporated water in a Japanese cypress stand. Monitoring primarily focused on the growing season of July-October 2011. In a 12 × 13 m plot with 28 trees with a DBH of 19.1 ± 3.9 cm, Ei was calculated as the difference between precipitation on an open field and precipitation under a canopy as the sum of throughfall and stemflow, and Et was measured using the Granier method. Ef was measured by weighing lysimeters. Total ET during the monitoring period was 446.3 mm, accounting for 47.5% of the total precipitation of 938.8 mm. Ei was the dominant evaporation flux and accounted for 53.6% of ET or 25.5% of rainfall, followed by Et with 33.7% of ET or 16.0% of rainfall. The average Et was 1.5 ± 0.6 mm d−1. It was well correlated with soil moisture at a depth of 5-15 cm, which reflected the forest properties, i.e. tree roots were exposed on the forest floor. Ef accounted for 12.7% of ET or 6.0% of rainfall with a daily mean of 0.55 ± 0.26 mm d−1, and was best correlated with soil moisture in the upper 5 cm of soil. These results can improve the understanding of water budget in forested watershed and also be used to build and validate hydrologic models for water resource managements. Copyright © 2013 John Wiley & Sons, Ltd. [ABSTRACT FROM AUTHOR]

In a deciduous larch forest in eastern Siberia, the mean and standard deviation of the total evapotranspiration ( E) during May to September (day of year (DOY) = 121-274) for 2003-2006 were 181.5 and 26.4 mm, respectively. The interannual variation (IAV) in the total E was caused by the IAV in E for the canopy-foliated period (DOY = 164-253), not by the IAV in the dates of leaf expansion and leaf fall. For the years with higher total E, E in the canopy-foliated period was consistently higher, which corresponded to the higher soil water content in these years. Copyright © 2012 John Wiley & Sons, Ltd. [ABSTRACT FROM AUTHOR]

Black locust ( Robinia pseudoacacia) is a major reforestation species in the semiarid region in the Loess Plateau of China. There has been increasing concern about the sustainability of the plantations because of their possible high water-use. This study was, accordingly, undertaken to quantify the stand-scale water use of a middle-aged black locust plantation in the region. The thermal dissipation probe method was applied to 27 trees to measure sap flux densities in an experimental plot during the growing season of 2008. The monoculture stand has a basal area of 23.3 m2 ha−1 and a maximum plant area index (PAI) of 2.89. Sapwood areas were estimated by use of a regressive relationship with the diameter at breast height (DBH) for scaling up of stand transpiration. The results showed that DBH could be a good predictor of sapwood area of individual trees. The diurnal cycles of average sap flux densities differed among DBH classes. Daily transpiration can be predicted from mean daily daytime vapor pressure deficit (VPDm) using a fitted exponential saturation model. Model variables were different among seasons, probably owing to different soil water conditions and leaf phenology. By using the derived model for each month, stand canopy transpiration over the growing season was estimated to be 73.8 mm, with an average daily value of 0.41 mm day−1 and a maximum of 0.89 mm day−1. The relatively small estimates of stand transpiration might be attributed to low PAI and sap wood area of the middle-aged stand. [ABSTRACT FROM AUTHOR]

The article explores spatial and temporal variations in summer precipitation between 1976 and 2007 in mountain areas in Japan. It notes that positive correlations between the amount of precipitation and the mean depth of precipitation were collected from 28 stations. It mentions that a temporal distribution pattern, based on the data collected, contributed to the determination of hydrological cycles in Japanese mountain areas.

The article looks at a model that estimates monthly forest evapotranspiration (E) from monthly temperature in Japan. It mentions that the model's accuracy was tested using monthly E data derived using micrometeorological methods and short-term water balance for 15 Japan forest sites. It notes that the model overestimated monthly E, and could only be reduced by considering the effect of precipitation of E.

Tree root system development alters forest soil properties, and differences in root diameter frequency and root length per soil volume reflect differences in root system function. In this study, the relationship between vertical distribution of very fine root and soil water content was investigated in intact tree and cut tree areas. The vertical distribution of root density with different diameter classes (very fine <0.5 mm and fine 0.5–2.0 mm) and soil water content were examined along a slope with two coniferous tree species, Cryptomeria japonica (L.f.) D. Don and Chamaecyparis obtusa (Siebold et Zucc.) Endl. The root biomass and length density of very fine roots at soil depth of 0–5 cm were higher in the Ch. obtusa intact tree plot than in the Cr. japonica intact plot. Tree cutting caused a reduction in the biomass and length of very fine roots at 0–5 cm soil depth, and an increment in soil water content at 5–30 cm soil depth of the Ch. obtusa cut tree plot one year after cutting. However, very fine root density of the Cr. japonica intact tree plot was quite low and the soil water content in post-harvest areas did not change. The increase in soil water content at 5–30 cm soil depth of the Ch. obtusa cut tree plot could be caused by the decrease in very fine roots at 0–5 cm soil depth. These results suggest that the distribution of soil water content was changed after tree cutting of Ch. obtusa by the channels generated by the decay of very fine roots. It was also shown that differences in root system characteristics among different tree species affect soil water properties after cutting. [ABSTRACT FROM AUTHOR]

• Manipulated rainfall reduction lowered transpiration and growth of black locust trees. • Simulated drought weakened the response of sap flow to meteorological variables. • It showed different behavior under long or severe drought from short term drought. • The plantations in semiarid area tend to suffer from future precipitation decrease. Black locust (Robinia pseudoacacia) plantations have been widely established in the semiarid and sub-humid areas of central China. Under the condition of global climate change, which is introducing much uncertainty of precipitation patterns in this region, it is of special significance to investigate their responses to precipitation. Here, we investigated sap flow response to reduced throughfall. Stem sap flow was measured from 2011 to 2017 using Granier-type sensors. By placing waterproof panels within tree rows, about 47.5% precipitation was excluded from treated plots since April 2015. Differences in soil water content gradually increased to 4.3% between treated and control plots with continued throughfall exclusion. Decreased precipitation input significantly reduced the average sap flux density in treated plots by 9.1%–45.3%. The extent of this reduction depended on precipitation in the previous and current years. Transpiration and forest growth were negatively affected by the treatment. Furthermore, sap flow response to environmental factors became insensitive, with the discrepancy increasing with increasing drought duration, but was regained by rainfall recharge to soil water in the first two treatment years. However, prolonged drought might damage transpiration resilience capacity, as the saturated sap flux after soil water recharging during the wet period was still lower than that in the control plot in the latter year. Predawn leaf water potential was significantly lower in the treated plot compared to the control plot, whereas midday leaf water potential was similar. Whole tree hydraulic conductance (G P) was also similar between the two plots, except in extreme drought months when G P was much lower in the treated plot. In addition, the specific leaf area and stomatal density decreased in the treated plot. Therefore, decreases in precipitation would cause a transpiration reduction, weakening the tree's response to meteorological variables and lowering growth and productivity, potentially damaging transpiration resilience. These results suggest that black locust is sensitive to water changes, its capacity of drought tolerant is restrained by the drought time scale or drought severity, the use of such species in reforestation in semiarid regions should be implemented with caution. [ABSTRACT FROM AUTHOR]

Soil water repellency (SWR) is a cause of low water infiltration, overland flow and soil erosion in mountainous coniferous plantations in Japan. The factors determining SWR intensity were investigated in two coniferous plantations of Chamaecyparis obtusa (Siebold et Zucc.) Endl. and Cryptomeria japonica (L.f.) D. Don, using intact tree plots and cut tree plots on the same hillslope. The SWR of Ch. obtusa plots was stronger than that of Cr. japonica plots. SWR intensity decreased after tree cutting. There were no significant differences in SWR upslope and downslope of individual trees/stumps for both tree species, though areas downslope of individual Ch. obtusa trees had higher SWR intensity than those upslope. SWR intensity and soil aggregate stability were positively correlated in the Ch. obtusa intact tree plot (r = 0.88, p < 0.01), whereas in the cut tree plot, this correlation was weak with no significance (r = 0.29, p = 0.41). Soil aggregate size had a non-significant influence on SWR intensity. These findings suggest that SWR intensity was not related to the soil aggregate size, but SWR intensity seemed have a role in soil aggregation in the Ch. obtusa intact tree plot. Destruction of soil aggregates could occur after tree cutting because of physical disturbances or increased input of different types of organic matter from other vegetation into soil. The presence of Ch. obtusa introduces a source of SWR, although uncertainty remains about how water repellency is distributed around soil aggregates. The distribution pattern of soil water content and soil hydraulic conductivity around Cr. japonica was related to other factors such as the litter layer and non-water-repellant soil. [ABSTRACT FROM AUTHOR]

• Evapotranspiration features differ in seasonal but are similar in annual scales. • Evapotranspiration can be fitted with single dominant factor in linear form. • Soil water is the main limitation of evapotranspiration in most cases in the region. • Improvement of soil water shifts dominant factors from soil water to energy. • The threshold of dominant factor shift can be determined by ROC curve. Evapotranspiration is an essential process of ecosystem water consumption and is controlled by meteorological factors and soil water conditions. In this study, evapotranspiration based on soil water budget was investigated with respect to major influencing factors during the growing seasons of 2012–2015 for two adjacent forest communities in the semiarid Loess Plateau region: a natural secondary forest dominated by oak (Quercus liaotungensis) and a plantation of exotic black locust (Robinia pseudoacacia). Based on the combination of precipitation, soil water storage, and evapotranspiration dynamics, four distinct periods were differentiated to explore the evapotranspiration response to the main meteorological factors. The correlation matrixes showed that, in most investigated days, evapotranspiration was limited by soil water storage, while an increase in soil water storage shifted dominance from water-related to energy-related factors. The stepwise regression of ≥5-day dry events in each period had a better fitness when paired with the relationship between the summations of evapotranspiration and dominant factors (R2 > 0.7, p < 0.01 for all) than the stepwise results for daily averages. A receiver operating characteristic (ROC) curve was applied to validate the division and determine thresholds. All divisions were thoroughly validated, and the soil water storage thresholds of the black locust (120.2 mm and 150.7 mm) were higher than those of the oak (105.8 mm and 137.9 mm). Our results suggest that the oak had an adaptive water consumption strategy, while the black locust had a higher sensitivity to soil water insufficiency. The method for determining the threshold of soil water storage to separate the dominant factors of evapotranspiration was validated. The approach may be applied to other semiarid forest communities with distinct evapotranspiration response patterns. [ABSTRACT FROM AUTHOR]

Periodic droughts and extreme rainfall are predicted to become more common in the future; therefore, it is important to understand how soil N transformation processes driven by soil microbes respond to rainfall events. Differences in forest type, including those in the predominant mycorrhizal type, can have varying effects on soil N transformation. Therefore, we hypothesised that the response of the N transformation processes to rainfall events would differ between forest types, even those that are adjacently located. In this study, we generated a soil moisture gradient in adjacent dryland black locust (Robinia pseudoacacia , non-ectomycorrhizal [ECM]-type) and oak (Quercus liaotungensis , ECM-type) forests and measured the abundance and community composition of fungi and prokaryotes (including ammonia-oxidisers), extractable N content, and net N transformation rates in the soil. Increasing moisture was found to increase nitrate N content in the black locust forest but not in the oak forest. In the oak forest, the abundance of ECM fungi increased with increasing moisture, subsequently enhancing ECM fungal ammonium N uptake and limiting the N availability for ammonia oxidisers, which may reduce the nitrate N production by ammonia-oxidisers. Increased nitrate N uptake by ECM fungi may also result in low soil nitrate N content. The findings of this study indicate that the responses of the nitrate N dynamics to rainfall events can differ between adjacent forests and that this difference may be attributable to the presence of ECM fungi. Therefore, considering the forest mycorrhizal type is vital for predicating the response of forest N cycling under climate change. • A soil moisture variation was generated by an artificial rainfall experiment. • The response of nitrification to the moisture differed between two forest types. • Soil NO 3 − increased with increasing moisture in the black locust forest. • Soil NO 3 − content was always low in the oak (ectomycorrhizal-type) forest. • The increasing ECM fungi would reduce NO 3 − content in the oak forest. [ABSTRACT FROM AUTHOR]

Black locust (Robinia pseudoacacia) is widely planted throughout semiarid and subhumid regions of the Loess Plateau of China. Determining the changes in transpiration of this species in different climatic areas is important for revealing the acclimation mechanism of black locust and developing suitable forest management practices, particularly in the context of global climate change. Here, sap flow and canopy conductance of black locust plantation trees in semiarid (Yan'an) and subhumid (Yongshou) sites were quantified using Granier-type thermal dissipation probes and concurrent environmental observations from 2012 to 2017. Several physiological parameters were measured throughout the growing season. The results showed that sap flow was correlated with phenological factors across seasons within a year. However, interannual changes in sap flow were affected mainly by the reference evapotranspiration (ET 0) at the Yongshou site, and jointly by precipitation (P), soil water content, and P / ET 0 at the Yan'an site. Sap flow response to meteorological factors showed less discrepancy between pre- and post-rainfall periods at the Yan'an site. Moreover, canopy conductance fluctuated less with a wider range of vapor pressure deficit (VPD) and the slope of canopy resistance as a function of VPD was lower, indicating relatively lower sensitivity of stomatal conductance to environmental factors in Yan'an site. Physiological parameters, except for predawn leaf water potential, were significantly different between the two sites. The results suggested that black locust tended to reduce transpiration, modify leaf morphology, and improve water use efficiency to enhance its adaptability to the dryer site. The species changes stomatal regulation characteristics and general growth rate to acclimatize to distinct water habitats. • Sap flow characteristics of black locust differ between semiarid and subhumid sites. • Dominant environmental factors controlling transpiration varied with habitats. • Adaptability and responses to long-term and short-term drought are different. • Longterm soil water shortage weakened stomatal regulation and growth. • Future intensified drought potentially threatens the plantation stability. [ABSTRACT FROM AUTHOR]

Abstract: In the semiarid Loess Plateau region of China, ecosystems are frequently affected by water shortages. Late spring and early summer are periods when forest communities tend to suffer from soil drought. To clarify the water-use strategies of the main species in this area, the xylem sap flow of trees from three species in the field was monitored for three successive periods in summer using Granier-type thermal dissipation probes. Vapor pressure deficit (VPD), solar radiation (R s) and soil moisture had varying influences on sap flux density (F d) in the species. Normalized F d values could be fitted to VPD using an exponential saturation function, but the fit was better with a derived variable of transpiration (VT), an integrated index calculated from VPD and R s. From differences in model coefficients, the species were roughly divided into two types with contrasting drought sensitivity. The exotic Robinia pseudoacacia was defined as drought-sensitive type. It showed higher sapflow increases in response to rainfall, suggesting a high water demand and high influence of soil water conditions on transpiration. This species showed relatively late stomatal response to increasing VPD. The wide-peak pattern of diurnal sapflow course also suggests relatively low stomatal regulation in this species. The drought-insensitive type consisted of the naturally dominant Quercus liaotungensis and an indigenous concomitant species, Armeniaca sibirica, in the forest. The sap flow of these species was not very sensitive to changes in soil water conditions. The results suggest that typical indigenous species can manage the water consumption conservatively under both drought and wet conditions. Variations in water use strategies within indigenous species are also detected. [ABSTRACT FROM AUTHOR]

Abstract: Plant–soil system patterns and processes along a slope are among the greatest causes of uncertainty in estimating watershed-scale transpiration (E). Tree-to-tree and radial variations in xylem sap flux density (F d), in addition to tree biometrics, were measured over a 2-year period (2005–2006) in two slope stand positions. The areas of interest consisted of an upper slope plot (UP) and a lower slope plot (LP) in a Japanese cedar (Cryptomeria japonica D. Don) forest watershed and the environmental controls of stand E for each plot were compared. Canopy stand E (E C) and canopy stomatal conductance (G C) in the UP were less than those in the LP during the growing season, while those in the UP were greater than those in the LP over winter. In addition, mean stand F d (J S) in the UP was greater than that in the LP over winter, but J S values were similar in the UP and LP except in the winter, which allows us to extrapolate watershed-scale E based on J S estimated from F d measurements of a partial stand in the watershed. However, this relationship contains a bias and differed between 2005 and 2006. Although there were significant differences in soil moisture conditions between the UP and LP in both years, a systematic relationship between the similarity in J S and soil moisture conditions was not found. The bias was due to a tendency for J S in the LP to be greater than that in the UP in 2006. This tendency was amplified because J S in the LP was greater than that in the UP around an atmospheric humidity deficit (D) of 1–1.5kPa and frequencies of this D range were higher in 2006 than in 2005. The greater J S in the LP at D ∼1–1.5kPa could be explained by the difference in the response of G C to D between the UP and LP. Our results suggest this to be the cause of the similarity in J S values for the UP and LP and for the occasional abortion of its similarity. However, even when the bias or the occasional deviation is disregarded, the error in estimating stand E from a partial stand is so small that it is comparable to an F d measurement error. For example, the error when using only the LP was 6.6% for stand E. [Copyright &y& Elsevier]