Water Storage of Plants Cannot Be Ignored During The Process of Water Movement

11 Water migration and use are important processes in trees. However, it is possible to overestimate transpiration 12 by equating the water absorbed by plant roots with that diffused back to the atmosphere via the stomata. The 13 δ 2 H/δ 18 O technique and heat ratio method were used to explore the patterns of water use of coniferous and 14 broad - leaved tree species to determine the proportions of water used for transpiration and water storage. Our 15 results indicate that both species showed strong plasticity in their use of water sources. The species Platycladus 16 orientalis (Oriental arbor - vitae) and Quercus variabilis (Chinese cork oak) primarily absorbed water from 17 groundwater and the 60–100 cm soil layer, and Q. variabilis also absorbed water from the 0–20 cm and 20–40 cm 18 soil layers during the dry season. Only P. orientalis was sensitive to precipitation and used water from the 0–20 cm 19 layers. Q. variabilis did not change its water source but increased its uptake of groundwater during the rainy 20 season. We observed reverse flow and hydraulic redistribution in P. orientalis , which alleviated the spatial 21 heterogeneity of soil water and provided water for neighboring trees. Nocturnal sap flow in P. orientalis and Q. 22 variabilis facilitated the storage of water in the trunk. The water used for storage in both species comprised 6–7% 23 of the total quantity and therefore, should be considered in water balance models.


27
Water is a key factor that affects the circulation of materials and plant growth in forest 28 ecosystems (Nadezhdina et al. 2020). Soil water is absorbed by plants through the roots and stored 29 in the xylem or used for photosynthesis; it also evaporates and reaches the atmosphere through 30 leaf stomata (Weatherley et al. 1982 isotopic fractionation of hydrogen and oxygen isotopes in water during the process of water 35 uptake by plant roots and transportation from xylem to leaves , the 36 hydrogen and oxygen isotopic values of xylem and source water can be compared based on the 37 isotopic mass conservation law. Mixed models (Iso-source or MixSIAR) were used to quantify the 38 ratio of utilization by the plants of each layer of water source (Philips et al. 2005). Dawson and 39 Ehleringer (1991) was the first research group to use stable hydrogen and oxygen isotope 40 technology to explore the water sources of plants. They found that the tree species that grew on 41 the riverside did not use the river water but used the soil water on the riverside. Since then, there 42 have been reports about the water sources of plants in China  Although previous studies state that the stomata are closed at night, and there is no sap flow 47 was recorded in parts per thousand of the "standard average ocean water". The precision of 127 determination was 0.3 and 0.1‰, respectively, and expressed as follows: 128 (3) Ratio of water absorption by trees 130 The isotopic ratios of water in the branches carry the isotopic information of all the water 131 sources as water is absorbed by the tree roots. Owing to this fact, the MixSIAR model can be used 132 to quantify the water absorption ratio of trees (Phillips & Gregg 2003) using the following 133 equations: 134 δX = c1δX1 + c2δX2 + c3δX3 + c4δX4 + c5δX5 + c6δX6 + c7δX7 (Eq. 2) 135 c1 + c2 + c3 + c4 + c5 + c6 + c7 = 1, where δX is the δ 2 H/δ 18 O value of tree branch water (‰); X1, X2, X3, X4, X5, X6, and X7 are the 137 δ 2 H/δ 18 O values of each potential water source, respectively; and c1, c2, c3, c4, c5, c6, and c7 138 represent the absorption ratio of trees to each potential water source. 139

Quantification of the amount of water for migration (Q)
The sap flow meters (SFM1, HRM 30, ICT International PTY, Armidale, Australia), namely 141 flow meters that utilized the heat ratio method (HRM) were installed at the DBH, taproots and 142 used with the fine roots of selected plants to monitor sap flow rate. The SFM1 flowmeter contains 143 three probes, which can simultaneously measure two-way flows. In addition, the SFM1 is more 144 accurate at monitoring the sap flow rate. Three specimens each of P. orientalis and Q. variabilis 145 that met the average tree height and DBH were selected at each observation point, which were the 146 same points for the isotope sample collection in the plot. Three cores were taken from each tree at 147 the breast height (1.3 m) from different sections of the stem using a borer that had 5 mm 148 increments, and the radii of cross-sections and heartwood were measured with a ruler. Three 149 cylindrical probes were inserted into the sapwood of the tree hole at the breast height, taproots, 150 and fine roots of each selected plant. The device was wrapped and sealed with insulated and 151 radiation proof aluminum foil to prevent rainwater from entering and avoid direct solar radiation. 152 The SFM1 flowmeter set the data acquisition interval at 10 min, and the water that migrates 153 through the tree can be calculated using the following equations: 154 where Q is the total migration (mL); Vs is the sap flow velocity (cm/s); As is the sapwood area 158 (cm 2 ); Vot and Vin are the sap flow velocity of the thermocouple inside and outside the temperature 159 probe (cm/s), respectively; A1 and A2 are the areas of the outer ring and the inner ring, 160 respectively (cm 2 ); Vh is the heat pulse rate (cm/s); k is the thermal diffusion coefficient of fresh 161 wood; t1 and t2 are the temperature variations in the upward and downward directions, respectively 162 (ire ρb is the wood density (g/cm 3 ); cw and cs are the specific heat capacity of fresh wood (1,200 163 J·kg -1 ·℃ -1 , 200 and of sap (water, 4,182 J·kg -1 ·℃ -1 , 202, and mc and ρs are the moisture content 164 and density of fresh wood, respectively. 165

Quantification of the water used for migration 166
The water migrated by trees can be divided into transpiration and water storage.
(1) Quantification of the water used for water storage (S) 168 Nocturnal sap flow does not necessarily represent nocturnal transpiration because it is also a 169 very important supplement to diurnal water loss (Phillips et al. 2003). When there is a positive 170 correlation between nocturnal sap flow and the VPD, it is considered that the trees transpire at where Ta and RH are atmospheric temperature ( o ) and relative humidity (%), respectively. 180 (2) Quantification of the water used for transpiration (QTr) 181 Sap flow during the stomatal opening represented plant transpiration. Therefore, the water 182 used for transpiration was quantified by determining the stomatal opening time via the VPD and 183 radiation. 184

Data analysis 185
All the statistical analyses were performed using SPSS 16.0 (SPSS, Inc., Chicago, IL, USA). 186 Descriptive statistics were applied to calculate the means and standard deviations for each set of 187 replicates. First, a one-way analysis of variance (ANOVA) was performed to test the effect of tree 188 species on the migration flux, water quantity, and storage. A two-way ANOVA was then used to 189 analyze the differences in SWC, isotopic composition, season and soil depth as independent 190 factors. In addition, a three-way ANOVA was used to analyze the differences in water source using 191 the soil depth, tree species and season as independent factors. All data met the requirements of 192 normal distribution and homogeneity of variance. 193 100 cm layers during the dry season were 11.6, 12.6, 11.9, 9.8, and 9.2%, respectively. The SWC 201 levels of the five layers during the rainy season were 16.5, 17.0, 17.6, 14.0, and 11.2%, 202 respectively, which were 42.2, 35.0, 39.7, 17.5, and 13.9% higher than those in the dry season. 203

211
The total solar radiation (TR) and VPD varied in a similar manner, with an initial increase 212 and a subsequent decrease; there was one single peak throughout the day (Fig. 2). The peak of 213 VPD appeared 0-3 hours later than that of the TR. The TR was 0 W/m 2 from 19:00 to 06:00 at 214 night, and the VPD values were generally > 0 Kpa owing to the vapor pressure from 19:00 to 215 06:00 at night. In January, the mean and peak values of TR were 87.1 and 174.6 W/m 2 , 216 respectively, and the mean and peak values of VPD were 0.35 and 0.59 Kpa, respectively. The TR 217 and VPD values gradually increased with the seasonal variation from January to July. The mean 218 and peak values of TR reached 176.2 and 491.6 W/m 2 in July, which were 2.0-and 2.8-fold higher 219 than the mean and peak values in January, respectively. The mean and peak values of TR 220 decreased from August to December (96.7 and 216.6 W/m 2 , respectively). The mean and peak 221 values of VPD reached 2.57 and 6.84 Kpa in July, which were 7.3-and 11.6-fold higher than those 222 in January, respectively. The mean and peak values of TR decreased from August to December 223   increased by 6.31% and 9.53%, respectively, during the dry season.

253
The water use ratios of P. orientalis and Q. variabilis to different water sources fluctuated 254 with the seasons. P. orientalis and Q. variabilis primarily absorbed water from the groundwater 255 and 60-100 cm layer during the dry season, and Q. variabilis also took up water from the 0-20 cm 256 (21.7%) and 20-40 cm (19.9%) soil layers. P. orientalis not only used groundwater (30.5%) and 257 water from the 60-100 cm soil layer (21.6%) during the rainy season but also water from the 0-20 258 cm layer (26.6%). The ratio of absorption of water from the 20-40 cm layer used by Q. variabilis 259 during the rainy season was 47.4% lower than that during the dry season, but the ratio of 260 absorption of groundwater uptake by Q. variabilis increased by 68.2%. After the rainy season, P. 261 orientalis transferred its water sources to groundwater (41.2%) and the 60-100 cm soil layer 262 (24.9%). Throughout the experimental period, P. orientalis used less water from the 20-40 cm and 263 the 40-60 cm soil layers, whereas Q. variabilis used less water from the 40-60 cm soil layer.

Water migration of P. orientalis and Q. variabilis
268 The sap flow rates (SFR) of P. orientalis and Q. variabilis exhibited a "single peak" or 269 "double peak," depending on the season (Fig. 6). The variation of SFR of P. orientalis during the 270 dry season showed a "double peak" during the observation period. It gradually increased before 271 11:00, reached its maximum value at 11:00 and 14:30, respectively, and then decreased. In 272 contrast, the variation of the SFR of Q. variabilis showed no peak during the dry season but had a 273 single peak during the wet season. The mean SFR of P. orientalis (0.0008 cm/s) during the dry 274 season was four times higher than that of Q. variabilis (0.0002 cm/s), and the maximum SFR of P. 275 orientalis (0.0022 cm/s) was 4.4 times higher than that of Q. variabilis (0.0002 cm/s). The mean 276 SFR of P. orientalis and Q. variabilis gradually increased during the rainy season with average 277 values of 0.0008 cm/s and 0.0013 cm/s, respectively. The mean SFR of Q. variabilis during the 278 wet season was 6.5 times than that in dry season, while the mean SFR of P. orientalis during the 279 wet season was not significantly different from that in the dry season (P > 0.05). The mean SFR of 280 Q. variabilis (0.0049 cm/s) was 62.5% higher than that of P. orientalis (0.0034 cm/s).

285
Since there was no reverse water migration in Q. variabilis, the relevant data of sap flow in Q. 286 variabilis are not shown here. The "two-way" SFR of taproots, lateral roots, and tree stems of P. 287 orientalis clearly fluctuated throughout the seasons. With the exception of August, the sap flow in 288 the taproot and the lateral root of P. orientalis increased first and then decreased. Typically, the 289 peak value was reached between 12:00 and 14:00, but the peak values depended on the season. 290 The peak values of SFR in taproots and lateral roots were the highest in September, namely 93.6 291 and 60.6 cm 3 /h, respectively. The water in both the taproots and lateral roots migrated in reverse in 292 August, with mean SFR values of 2.73 and -23.6 cm 3 /h, respectively. The sap flow of the stem of 293 P. orientalis migrated upward during the entire experimental period. The SFR of tree stems 294 increased first and then decreased from February to November. The peak value was 127.2 cm 3 /h in 295 September. However, the SFR of the tree stem remained at 6.7 cm 3 /h in January and December 296 and did not fluctuate.  The annual mean upward water fluxes of P. orientalis and Q. variabilis were 374.69 and 303 469.50 mm/a, respectively (Fig. 8). The average sap flux of P. orientalis was 20.19% lower than 304 that of Q. variabilis. The annual mean upward water flux of Q. variabilis was 24.9% higher than 305 that of P. orientalis. The reverse water migration flux of P. orientalis was -8.7 mm/a, comprising 306 2.3% of the forward water flux. The amounts of water used by P. orientalis for transpiration and 307 storage were 350.3 and 24.41 mm/a, respectively, comprising 93.49 and 6.51%, respectively, of 308 the sap flux. The amounts of water used by Q. variabilis for transpiration and storage were 440.85 309 and 28.65 mm/a, respectively, comprising 93.91 and 6.09% of the total sap flow, respectively. This 310 indicates that the plants used most of the water for transpiration. The water in P. orientalis flowed 311 in reverse at a rate of 8.67 mm/a, comprising 2.26%, while Q. variabilis had no reverse flow.