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• Groundwater depth (GT) controlled transpiration dynamics of Populus tomentosa. • Diurnal (Q day) and nocturnal water use (Q night) had different seasonal trends. • Groundwater recharged Q night and reduced the water consumption from the stem. • Q night helped poplars refill the consumption of water stored in the stem (R e). • When GT < 170 cm of the surface, R e / Q night increased with increasing GT. For theories of forest water utilization over day-night cycles, it is important to have a reliable information on plant transpiration behaviors during daytime and nighttime and their environmental controls. With the help of thermal dissipation probes, our study investigated the seasonal variations in the diurnal (Q day) and nocturnal stand water use (Q night) of a Populus tomentosa plantation and explored their responses to groundwater table (GT) fluctuations and the meteorological factors. The results indicated that Q night occurred on most nights during the growing season in the P. tomentosa plantation, but with a different seasonal dynamic from Q day. A distinct increase in the ratio of nocturnal to daily water use appeared at the beginning and the end of the growing season. During the experimental period, stem refilling accounted for 61% of Q night , indicating that P. tomentosa was able to draw on stored water in the stem to support transpiration. The Q day and Q night were positively and negatively related to GT , respectively. When the GT varied within 170 cm of the surface, the proportion of stem refilling accounting for Q night increased with GT. The positive and negative correlations were found in meteorological factors with Q day and Q night , respectively. Among the single meteorological factors except for ET 0 , vapor pressure deficit (VPD) and photosynthetically active radiation (PAR) explained the highest variation in Q day in 2010 and 2011, respectively. In contrast, Q night was dominantly affected by VPD in 2011. Nocturnal stand water-use could help P. tomentosa to recharge the consumption of water stored in the stem and overcome the seasonal drought. The compensation effect of groundwater on the nocturnal stand water-use of P. tomentosa could reduce its consumption of water stored in the stem. As the stand age of P. tomentosa plantation varied, the key meteorological drivers of both Q day and Q night also changed. Our results enable better understanding of the water-use strategies of poplars in the North China Plain and can lead to refinement of current silvicultural techniques. [ABSTRACT FROM AUTHOR]

Highlights • Four different methods were applied to determine the optimal tensiometer placement. • The optimal tensiometer placement location should not change with stand age. • The tensiometer should best be placed 10 cm deep and 15 cm distant from the dripper. • This will optimize the schedule for drip irrigation in different aged Populus tomentosa forests. Abstract Determining the optimal tensiometer placement is crucial for scheduling irrigation based on soil water potential (SWP) in poplar plantations. However, such work has never been done. The stand-age effect on the tensiometer placement is still unknown, hindering the development of a dynamic irrigation regime. In a surface drip irrigated Populus tomentosa , short-rotation plantation on sandy loam soil, the variation of SWP at different positions [hereafter expressed in spatial coordinates (distance from dripper in cm, depth in cm)] within the soil wetting volume (WV), and the daily tree growth rates were continuously monitored in one growing season. Whereas, the transpiration and fine root distribution were measured in two growing seasons. The HYDRUS model was calibrated using soil water and transpiration data from one growing season and validated using data in another growing season. Then the model was applied to simulate sixty scenarios, which were combinations of different SWP measurement positions (ten scenarios), root distribution patterns (three scenarios; mainly using rooting depth as a proxy for trees at different ages), and irrigation thresholds (two scenarios). Based on the simulation results, we estimated the ratio of actual to potential transpiration (T a / T p) and irrigation efficiency (IE; calculated as (T a / T p)/irrigation amount) for each scenario. The absolute value of the mean relative difference of SWP was smaller at (15, 10) than at any other positions within the WV. The proportion of variation in growth (R2 = 0.294) and transpiration rates (R2 = 0.247) explained by root water-uptake was also highest at (15, 10). Fine roots concentrated on both sides of dripper as the trees grew bigger, with the maximal root density finally occurring at about (15, 10) and (−15, 10). Under the (−15, 10), (0, 10), (15, 10) and (0, 30) position scenarios, the T a / T p was at a high level relative to other scenarios, while the IE were at a middle level. As rooting depth increased, the ranks of different tensiometer positions in T a / T p and IE changed only a little, and T a / T p decreased slightly, implying the optimal placement should not vary with stand age. Therefore, tensiometer was recommended to place at 10 cm depth and about 15 cm distance from the dripper to schedule drip irrigation in different-aged P. tomentosa plantations on sandy loam soil. This will bring benefits to minimizing the use of irrigation water and maximizing production. Besides, the comprehensive approach used in this study can be adopted, when determining the optimal soil water measurement positions for scheduling irrigation for other plant species growing on other soil types. [ABSTRACT FROM AUTHOR]

Populus tomentosa is a dominant poplar species in North China. Clarifying the short-term response of fine roots to soil water gradient in the P. tomentosa plantation is helpful to have a better understanding on the ecological adaption strategy of plant roots to soil moisture. Three irrigation treatments were set in a two-year-old P. tomentosa stand planted on the sandy loam, i.e. full irrigation (DIFI), controlled irrigation (DICI), and control (CK). After 2 months of irrigation, root sampling down to 150 cm depth was conducted in each treatment using the soil core method. Through the measurement of root samples, the root growth, distribution and morphology data in each treatment, soil depth and horizontal distance were obtained. Vertically, in each soil layer, there is no significant difference in fine root biomass density (FRBD) among treatments (p > 0.05). In the whole root zone and the area with a large moisture difference in shallow soil layer among treatments, the distribution depth of fine root showed a characteristic of CK > DICI > DIFI. However, this characteristic was not observed in the area with less difference in soil water content. Horizontally, FRBD in CK decreased gradually with the increase of the distance from tree, but this trend was very weak in DIFI and DICI treatment. There was no significant difference of FRBD among treatments at each horizontal distance (p > 0.05), except for the distance of 30 cm from tree, where FRBD of CK was obviously higher than DIFI and DICI treatment (p < 0.05). On a two-dimensional scale, fine roots distribution tended to be shallower with the increasing distance from tree. Under irrigation, fine roots mainly concentrated in the shallow soil layers at both sides of the dripper, while a majority of fine roots in CK distributed in the relatively deeper soil layers close to the tree. For each soil layer within 0–50 cm, no significant difference in fine root morphology was found among treatments (p > 0.05). Both the variation of the total fine root biomass and length among the treatments followed an order of CK > DIFI > DICI. In conclusion, when a short-term moisture difference occurred in the shallow soil layers, 1) the fine roots of P. tomentosa distributed deeper and tended to concentrate closer to the tree with the aggravation of drought stress, 2) P. tomentosa preferentially adjusted the fine root distribution rather than the fine root morphology in the shallow soil layer to adapt to soil water stress, 3) as to the total amount of fine roots, P. tomentosa adopted a regulation strategy of slight decrease at first and then distinct increase. [ABSTRACT FROM AUTHOR]

Abstract: The form and water uptake characteristics of the fine root system in a 5-year-old triploid Populus tomentosa plantation were investigated to make recommendations related to irrigation water management for P. tomentosa plantation. Fine roots for analysis were collected from 2106 soil cores taken around eight trees. Soil moisture, trunk sap flow and evaporation were measured concurrently for four months in two experimental plots using time-domain reflectometry, thermal dissipation sensors and micro-lysimeters, respectively. Nearly half (44%) of fine roots corresponded to 0.2–0.5mm diameter. Generally, lateral root distribution was even, however, the vertical root profile showed an unusual pattern (nearly an ‘S’ shape). Dense fine roots occurred in surface soil and nearly one third (28%) of total fine roots occurred below 100cm depth, indicating the plantation had developed a dimorphic root system. With increasing distance from the tree, root distribution tended to be shallower. Mean fine root diameter was significantly larger (P <0.05) below 120cm, probably due to soil texture change or/and anoxia. Root water uptake in the 0–20cm layer contributed 58% of that within the 0–90cm soil layer, suggesting surface roots played the major water uptake role in shallow soil (<90cm). On average, P. tomentosa extracted 57% of transpired water from deep soil (>90cm), implying deep roots can contribute significantly to the water relations of mature P. tomentosa plantations. This functional significance of deep roots might be determined by their high length density and relatively large diameter. Based on these results, three irrigation management strategies were recommended: (1) irrigation schedules should be devised based on periodic measurement of the depth to water table; (2) water should be mainly provided to and maintained in the surface 40cm soil; and (3) water should be applied to the zone within 1m from the tree. [Copyright &y& Elsevier]