Environ Earth Sci (2012) 66:2519–2529 DOI 10.1007/s12665-011-1474-1

ORIGINAL ARTICLE

The threshold of soil moisture and salinity influencing the growth of Populus euphratica and Tamarix ramosissima in the extremely arid region

Aihong Fu • Weihong Li • Yaning Chen

Received: 25 June 2010 / Accepted: 30 November 2011 / Published online: 30 December 2011 Ó Springer-Verlag 2011

Abstract The drought and salt tolerance of two pioneer Introduction species, Populus euphratica and Tamarix ramosissima, were studied by monitoring the stem and leaf water In arid desert zone, soil moisture and salinity are the key potentials under various soil moisture and salinity at depths environmental factors crucial for the survival and growth of of 0–180 cm. The plants are naturally distributed in arid vegetation. It is therefore very important to study the environment in the lower reaches of the Tarim River in influence of soil moisture and salinity on plant growth. Xinjiang, China. The results showed that P. euphratica can Populus euphratica and Tamarix ramosissima are the pri- withhold water by prolonging dewatering and adapt to the mary arbor and shrub trees found in the lower reaches of the dry desert weather by reducing water consumption. The Tarim River in Xinjiang, China. They can adapt to arid lowest soil moisture that would unfavorably influence its desert environment, hold the functions of blocking off wind, growth was found to be 7% soil moisture. T. ramosissima fixing sand and protecting oasis, have economic value and was found to have low water potential and high transpi- maintain ecosystem in the Tarim River Basin. Due to severe ration efficiency. It is capable of absorbing water from the abuse of water resources utilization in the upper and middle soil by keeping a low water potential, so there is no critical reaches, a cut-out of more than 300 km of the riverbed limit of soil moisture for T. ramosissima. In terms of salt occurred in the late 1970s, resulting in withering of forest resistance, P. euphratica was found to secrete salt from its areas and degradation of the environment along both riv- body by discharging salty water through portals in its erbanks (Chen et al. 2004a). The central and local govern- trunks and leaves. A soil salinity of 20% was the minimum ments of China have invested RMB ¥10.7 billion (US concentration at which the salt secretion mechanism of $1 = RMB ¥8.2) for the restoration and reconstruction of P. euphratica was activated. T. ramosissima secreted salt this district. One of the key measures implemented was the by storing the accumulated salt in the vacuoles of its salt ecological engineering of water release from Bosten Lake to secretion glands for separation. Thus, it has no minimum the lower reaches of the Tarim River. In 2007, water soil salinity limit. T. ramosissima was found to have better delivery was carried out nine times (Table 1), which was resistance to drought and salt stress than P. euphratica. shown to improve the efficiency of ecological restoration. Afterward, ecological water delivery was halted for 2 years Keywords Populus euphratica Á Tamarix ramosissima Á and caused the groundwater level to descend and the veg- The soil moisture Á The soil salinity Á The threshold etation to wither. Therefore, it is important to study the stress response of P. euphratica and T. ramosissima to changes in soil moisture and salinity in the ecosystem. By analyzing the characteristics of P. euphratica and A. Fu Á W. Li (&) Á Y. Chen T. ramosissima under different environmental conditions, State Key Laboratory of Desert and Oasis Ecology, we may obtain information valuable for the restoration and Xinjiang Institute of Ecology and Geography, rebuilding of vegetation in arid zone. Chinese Academy of Sciences (CAS), 818 South Beijing Road, Urumqi 830011, China The drought and salt stress were called transpiring stress. e-mail: [email protected] They both could cause the water potential of the soil 123 2520 Environ Earth Sci (2012) 66:2519–2529

Table 1 The nine times of Times of Beginning time End time Duration Volume of water Arrived water delivery in the lower water delivery (year/month/day) (year/month/day) (day) delivery (9104 m3) position reaches of the Tarim River First 2000/5/14 2000/7/13 61 9,883.18 Kardayi Second 2000/11/3 2001/2/14 104 22,000 Alagan Third First phase 2001/4/1 2001/7/6 97 18,400 Alagan Second phase 2001/9/12 2001/11/17 67 19,700 Taitema Fourth 2002/7/20 2002/11/10 110 29,300 Taitema Fifth First phase 2003/3/3 2003/7/11 131 25,000 Taitema Second phase 2003/9/12 2003/11/7 56 9,000 Taitema Sixth First phase 2004/4/22 2004/6/25 62 12,000 Taitema Second phase 2004/8 2004/11 90 23,000 Taitema Seventh First phase 2005/5/7 2005/6/7 30 5,200 Second phase 2005/8/30 2005/10/31 61 22,800 Taitema Eighth 2006/9/25 2006/11/21 56 23,300 Taitema Ninth 2007/10/10 2007/11/18 40 5,000 solution to decrease and consequently the cells to lose water revealed that it had a strong salt resistance capability (Glenn or even die. Thus, they are the most important environ- et al. 1998). It was suggested that most species in the mental factors that influence plant growth (Wang et al. T. ramosissima genus are halophytes and have salt-secreting 2002). Plant’s water potential is an essential physiological capability (Zhang et al. 2003). Their salt-secreting glands indicator that reflects the organic moisture state and the are vital in regulating ion balance and keeping a stable drought-resistant capability of plants. Previous studies on transpiring pressure that functions to increase the salt the drought-resistant characteristics of P. euphratica and resistance capability. The salt resistance limit for T. ramo- T. ramosissima suggested that they had strong drought- sissima was predicted to be about 2.5% (Chen et al. 2005). resistant capability (Wang et al. 1997; Vandersande et al. Thus, T. ramosissima can maintain higher water use effi- 2001). Detailed studies on the physiological and ecological ciency under high salt stress (Vandersande et al. 2001). response of P. euphratica and T. ramosissima under drought The characteristics of drought resistance and salt resis- and salt stress have also been reported (Chen et al. 2004b, tance of P. euphratica and T. ramosissima in extreme arid 2008). The research on salt-resistant characteristics of regions were comparatively analyzed in this paper. Our P. euphratica showed that P. euphratica behaved as non- goal was to study the drought and salt resistance of halophytes. It was found to restrain salt ions from entering P. euphratica and T. ramosissima by investigating the leaf tissues to prevent damage of photosynthetic organs changes in their water potentials under different soil water under salt stress, which indicated that it had a very strong and salt conditions. This may provide further understand- absorb-denying mechanism (Sykes 1992; Greenway and ing of the drought and salt resistance mechanisms, lead to Unns 1980). However, P. euphratica would die under the more effective ecological water delivery and speed up condition of 0.05% NaCl (Fung et al. 1998), and the leaf of restoration of the damaged ecosystem in the lower reaches P. euphratica tree would be damaged under 354 mmol of the Tairm River. NaCl (Chen et al. 2001). Furthermore, some researchers noted that along with the increase in salt concentration, the sprouting percentage of P. euphratica seeds and the Materials and methods expanding percentage of their leaves would reduce corre- spondingly. It was concluded that under salt stress, P. eu- Study area phratica would possess a stable seam structure and synthesize protein (Liu et al. 2004; Wang et al. 1998;Ma Located at the north edge of the Taklimakan Desert, the and Wang 1998). These studies revealed the drought Tarim River is the longest and a well-known inland river in resistance and salt resistance mechanisms of P. euphratica. China. The region investigated is located between Takli- Research on the salt resistance nature of T. ramosissima makan Desert and Kuluke Desert, from the Daxihaizi

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Fig. 1 The map of typical sections and wells in the lower reaches of the Tarim River

Reservoir to Taitema Lake at the lower reaches of the T. ramosissima in the vicinity of each well were measured. Tarim River (Fig. 1). The area is located in the warm Soil samples were collected by digging a 180-cm deep soil temperate zone with continental and extremely arid desert section at one side of every P. euphratica tree, where every climate: little rainfall, much sandy wind, a mean annual 10- or 20-cm interval of soil was collected until a depth of precipitation of 17.4–42.0 mm and a mean annual evapo- 180 cm of soil was dug. ration capacity of 2,500–3,000 mm. The lower reaches of the Tarim River (321 km in length) have been completely Measurement of soil characteristics dried out since 1970; additionally, Luobupo Lake and Taitema Lake at the end of the river have been dry since Soil salinity was measured in the laboratory. The acidity 1970 and 1972, respectively. The groundwater level had and alkalinity of soil samples were measured by the dropped dramatically and the natural vegetation dependent potentiometer method (the ratio of water to soil was 5:1). on groundwater degenerated significantly. Herbs composed The electrical conductance was measured with a conduc- mainly of Phragmites communis, Apocynum venetum and tance meter (the ratio of water to soil was 5:1). After sat- Alhagi sparsifolia died and the arbors or shrubs such as urating the soil with distilled water, the soil salinity was Populus euphratica and Tamarix ramosissima also with- measured by the drying method. Soil moisture was also ered. Wind erosion and desertification accelerated, and the measured by the drying method. desertification of land worsened. Measurement of leaf and stem water potentials Soil collection of P. euphratica and T. ramosissima

In May 2009, based on the groundwater level survey of the The leaf and stem water potentials (wl and ws)ofP. eu- lower reaches of the Tarim River conducted before 2009, phratica and T. ramosissima were measured on location we selected the following wells as the typical study sec- using the HR-33T Dew Point Microvolt-meter (WESCOR tions: B2 well of Yahepu (B), C4 well of Yinsu (C), and E4 company, US). Three normally developed leaves of the and E5 wells of Kardayi (E). The leaf and stem water sample tree were removed from the mid-upper part of the potentials of three mature P. euphratica trees and crown exposed to the sun, and then immediately placed

123 2522 Environ Earth Sci (2012) 66:2519–2529 into an air-proof polyethylene bag. A small part (5 mm water supply was released after 2007, and the groundwater in diameter) of each leaf was cut by avoiding the leaf depth gradually lowered and was deeper in 2009. In 2005, venation and put into the C-52 sample chamber of the the soil moisture at depths from 0 to 180 cm in B2, C4, E4

Microvolt-meter to measure wl. Similarly, the stem water and E5 wells was 10.22, 6.58, 9.53 and 1.83%, respec- potential (ws) was obtained by measuring a piece of tively; in 2009, the soil moisture was 1.17, 12.79, 1.99 and branchlet about 3 mm in length and 1.5–2.0 mm in diam- 2.89%, respectively. In 2005, the soil salinity at depths eter. Sampling was carried out every 2 h from 8:00 to from 0 to 180 cm in B2, C4, E4 and E5 wells was 13.20, 20:00 h; leaf and branchlet measurements of each plant 37.90, 33.63 and 15.04%, respectively; in 2009 the soil were repeated three times and the mean values are repor- salinity was 0.15, 1.11, 0.42 and 0.28%, respectively. Soil ted. Water potential values measured before sunrise (i.e., moisture and salinity in 2009 were lower than in 2005,

8:00 h) is labeled as the predawn water potential (wp), and which showed that soil moisture and salinity gradually w measured at noon (i.e., 14:00 h) is the midday water decreased along with the increase in groundwater depth potential (wm). when water delivery was stopped.

Variations in soil moisture at different groundwater depths Results and analysis Groundwater can provide water to surface soil by going up Soil moisture and salinity at different groundwater along the soil’s capillary tubes. When the groundwater depths level is varied, the amount of water provided to soil is varied accordingly and results in different soil moisture The field monitoring results showed that the 2005 annual (Ye et al. 2009). By analyzing the soil moisture in typical mean groundwater levels in sections B2, C4, E4 and E5 sections at depths from 0 to 180 m (Fig. 2), it was found were 4.28, 5.10, 5.58 and 9.32 m, respectively, and a that as the groundwater depth deepened from 6.07 to combined mean value of 6.07 m. In 2009, they were 5.13, 6.62 m, the soil moisture also decreased. At the B2 well, 5.96, 6.04 and 9.35, respectively, with a mean value of the soil moisture decreased significantly; at the C4 well, the 6.62 m. The groundwater depth was clearly shallower in soil moisture decreased at all the depths except for depths 2005 than in 2009. The reason was because the levels were of 30–40 and 140–160 cm; at the E4 well, the soil moisture measured after the completion of the sixth water release also decreased at all depths except at 40–50 cm; and at the and before the seventh release in May 2005, so that there E5 well, the soil moisture at a depth of 80–180 cm was enough soil water (Table 1). However, no ecological increased as the groundwater depth deepened, but

Fig. 2 Change in groundwater depths in different sections and soil moisture at different soil depths. a B2 section, b C4 section, c E4 section, d E5 section 123 Environ Earth Sci (2012) 66:2519–2529 2523 decreased at all other depths. At different well locations, evaporation (Yin et al. 2007). As the depth of groundwater the groundwater depth yielded different influences on the increases along with the action of leaching, the soil salinity soil moisture. This may be accounted for by the different decreases constantly and leads to variations in the soil extent to which the groundwater depth lowered. At the B2, salinity (Chen et al. 2010). Analysis of salinity in the C4, E4 and E5 wells, their groundwater depths were low- typical sections at depth of 0–180 m (Fig. 3) revealed that ered by 0.85, 0.86, 0.46 and 0.03 m, respectively. The the soil salinity would decrease as the groundwater depth greater the lowering of the groundwater depth, the more increased from 6.07 to 6.62 m. When the groundwater the decrease in soil moisture. At the E5 well location, the depth was 6.07 m, the mean soil salinity at the B2, C4, groundwater depth had lowered the least, so the soil E4 and E5 wells was 13.20, 37.89, 33.63 and 15.04%, moisture did not decrease significantly. respectively. At 6.62 m, the salinity was 12.05, 37.15, At a groundwater depth of 6.07 m, the highest value of 33.29 and 14.97%, with corresponding decreases of 0.15, the soil moisture at the B2 (36.5%), C4 (14.9%), E4 (23.4%) 0.74, 0.34 and 0.27%, respectively. At the location of the and E5 (2.7%) wells were found at depths of 50–80, 30–40, C4 well, the soil salinity decreased most dramatically. This 140–180 and 30–40 cm, respectively. At a groundwater may be due to the fact that the groundwater depth at the C4 depth of 6.62 m, the highest soil moisture at the B2 (2.2%), well had lowered the most. The lower depth of ground- C4 (21.4%), E4 (9.8%) and E5 (12.2%) wells occurred at water and the action of water leaching both contributed to depths of 120–140, 60–80, 40–50 and 120–140 cm, significant soil salt removal and resulted in the decrease in respectively. It can be seen that when the groundwater depth the soil salinity. At the B2 well, the soil salinity decreased increased from 6.07 to 6.62 m, the soil moisture showed a to the least extent, which might be attributed to the com- decreasing trend at the B2, C4 and E5 wells, but an paratively looser sandy soil in the shallow layer of earth increasing trend at the E4 well. This indicated that the soil surface found in this area. With relatively less moisture in moisture at the B2, C4 and E5 wells was dramatically the soil, water was completely evaporated off leaving salt influenced by groundwater. As the groundwater depth accumulating in the soil surface, so the soil salt decreased increased, the soil moisture at greater depths would increase to the least extent. With a groundwater depth of 6.07 m and above the levels found in shallower locations. soil depth of 0–180 cm, the soil salinity clearly yielded a trend of first increasing then decreasing along with an Variations in the soil salinity at different groundwater increase in soil depth. The salinity at B2, C4, E4 and E5 depths peaked at soil depths of 30–40, 30–40, 60–80 and 40–50 cm, with the corresponding peak values of 22.80, The source of soil salt at the earth’s surface in the arid area 66.09, 73.53 and 34.50%, respectively. With a groundwater is mainly from the salt accumulated by groundwater depth of 6.62 m and at a soil depth of 0–180 cm, the

Fig. 3 Change in groundwater depths in different sections and soil salinity at different soil depths. a B2 section, b C4 section, c E4 section, d E5 section 123 2524 Environ Earth Sci (2012) 66:2519–2529

Fig. 4 Relationship between soil moisture at 0–180 cm depth and predawn and midday stem and leaf water potential of Populus euphratica

change in salinity was small and almost linear as the soil need of water. When the soil moisture was above 7%, it depth was increased. This indicated that the increase in began to influence P. euphratica; so, 7% was perhaps the groundwater depth correlated to very small reductions in minimum limit of soil moisture that could act on the soil salinity. P. euphratica.

The relationship between soil moisture and salinity The relationship between soil salinity and the water and the water potential of P. euphratica potential of P. euphratica

The relationship between soil moisture and the water By analyzing the salinity at depths of 0–180 cm and wp and potential of P. euphratica wm of P. euphratica (Fig. 5), the correlation between soil salinity and wms could be given by the following equation: In arid and semiarid regions, water is an important y = 0.0026x2 - 0.1606x - 5.4829, where x is the soil restraining factor that influences the existence and growth salinity, y is the wms of P. euphratica, and the correlation of plants. It was found that at the depth of 0–180 cm, the coefficient was found to be 0.5366. There is no significant correlation between the soil moisture and the predawn fitting curve model correlation between the soil salinity and leaf water potential (wpl)ofP. euphratica could be given wpl and wml of P. euphratica. When the soil salinity 2 by the equation: y = 0.0757x - 0.8864x - 0.973, where increased, wms decreased initially and then became stable. x is the soil moisture, y is the wpl of P. euphratica, and When the soil salinity was below 20%, it imposed a neg- the correlation coefficient is 0.5800 (Fig. 4). However, ative influence on w of P. euphratica; however, there was there is no significant fitting curve model correlation no effect when the salinity was above 20%. This indicated between the soil moisture and the midday leaf and stem that 20% soil salinity was possibly the maximum limit that water potential (wml and wms)ofP. euphratica or the could influence w of P. euphratica. This might be attrib- predawn stem water potential (wps). The water potential uted to the salt-removing capability of P. euphratica,in (wpl) was at the lowest level when the soil moisture which excess salt absorbed by the roots could be dis- reached 7%, followed by a gradual increase as the soil charged via the secreting action of its trunks and leaves. moisture increased. This could be because the soil mois- Thus, 20% is likely the lower limit of soil salinity for ture below 7% has no effect on P. euphratica in extreme P. euphratica to start discharging salt.

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Fig. 5 Relationship between soil salinity at 0–180 cm depth and predawn and midday stem and leaf water potential of Populus euphratica

Fig. 6 Relationship between soil moisture at 0–180 cm depth and predawn and midday stem and leaf water potential of Tamarix ramosissima

The relationship between soil moisture and salinity moisture and wms was found. The wps of T. ramosissima and the water potential of T. ramosissima increased as the soil moisture increased, which indicated that there existed no lowest critical value of soil moisture The relationship between the soil moisture and water for T. ramosissima to utilize the soil water. Even when the potential of T. ramosissima soil moisture was very low, T. ramosissima was still able to make use of it. There was no significant correlation As shown in Fig. 6, the correlation between the soil between the soil moisture and w of T. ramosissima at noon. moisture at a depth of 0–180 cm and wps of T. ramosissima The roots of T. ramosissima extend deeply into the soil could be given by the following equation: y = 0.0049x2 ? where it obtains surface and undergroundwater. It has been 0.1554x - 8.5299, where x is the soil moisture, y is the found capable of transpiring large quantities of water wps, and the correlation coefficient was found to be 0.6261. (Nippert et al. 2010;Salaetal.1996; James et al. 1997; However, no significant correlation between the soil Nagler et al. 2005), because wm of T. ramosissima is

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Fig. 7 Relationship between soil salinity at 0–180 cm depth and predawn and midday stem and leaf water potential of Tamarix ramosissima

mainly influenced by climate factors such as temperature Under different soil water content conditions, we found instead of the soil moisture. For example, the temperature that the soil water content of below 7% had no significant at dawn is much lower than during midday and evaporation influence on the water potential of P. euphratica. Above is also weaker, so the change of the soil moisture could 7% soil water content, the water potential of P. euphratica influence the change of w of T. ramosissima. would gradually increase with increasing soil water con- tent. Therefore, 7% soil water content might be the mini- The relationship between soil salinity and water potential mum concentration that could influence the growth of of T. ramosissima P. euphratica. Similar findings were also reported (Guo and Tian 1992), where eight varieties of coniferous and We found no significant correlation between the soil broad-leaved saplings were found to have a critical soil salinity, wps and wms of T. ramosissima on analyzing the water content that influenced their early morning leaves’ soil salinity at depths of 0–180 cm (Fig. 7), indicating that water potential. When the soil water content was above the soil salinity up to 40% had no influence on its growth. This critical value, the early morning leaves’ water potential might be attributed to the rather strong salt discharge remained linear, but decreased dramatically as the soil capability of T. ramosissima. water content fell below the critical value. For Tamarix spp., zhuang and chen (2006) analyzed the characteristics of the main physiological indexes of T. ra- Discussion mosissima, such as chlorophyll, soluble sugar, proline (Pro), malondialdehyde (MDA), superoxide dismutase

The threshold of soil moisture (SOD), peroxidase (POD), indole-3-acetic acid (IAA), C3- gibberellins (GA3) and abscisic acid (ABA), with different In the lower reaches of the Tarim River, the survival and water depths, and found that the ecological groundwater growth of vegetation mainly depend on the groundwater depth suitable for T. ramosissima was 2–4 m. When the and soil moisture, because precipitation is rare and evap- groundwater depth was over 6 m, it would cause stress to oration is extensive. When the less soil moisture happens, the growth of T. ramosissima, while 10 m was the critical some desert plants may have no way of absorbing water groundwater depth that would threaten its survival, which from the soil to maintain their survival and growth, while provides an important theoretical basis for the ecological others may be able to do so. So the ability of using soil conservation of extremely arid regions. When the moisture for the different desert plants is different, and the groundwater depth reached 2 m, the water use efficiency of lowest threshold of soil moisture affecting different plants T. ramosissima was higher in the irrigating top earth layer needs to be further studied. (Brock 1994), but was reduced dramatically when water In its seedling period, P. euphratica cannot endure depth was above 4 m and caused growth inhibition. drought well, but when fully mature, it acquires certain However, there was no mention of the minimum critical drought resistance capability. The mature P. euphratica soil moisture that would influence the growth of T. ramo- leaves and twigs are coated with wax-like materials and sissima. In this study, the relationship between soil mois- short fine hairs that help reduce water consumption and ture and the water potential of T. ramosissima was adapt to the arid desert climate (Tuerxuntuoheti 2002). analyzed and the results indicated that water potential

123 Environ Earth Sci (2012) 66:2519–2529 2527 would increase with increasing soil moisture, suggesting of each plant after 5 weeks. The results showed that under that there was no critical limit of soil moisture for the a salt stress of 50 mmol NaCl, P. euphratica could have a growth of T. ramosissima. This could be attributed to its comparatively high growth rate, but when the stress deep root system capable of absorbing deep groundwater increased to 150 mmol NaCl, the reduction in the leaf that is inaccessible to other plants (Gay and Fritschen number and accumulated proline in both young and mature 1979). leaves for P. euphratica was observed. The growth would P. euphratica belongs to the drought-resistant species be restrained, suggesting that accumulated proline pro- and is able to withhold water by prolonging dewatering motes osmotic and salt tolerance. However, even under a (Wurigen et al. 2003). It could also adapt to the arid desert high salt stress, the indurated tissue of P. euphratica skin climate by reducing water consumption. In this regard, we could still survive. Gu et al. (2004) characterized the tol- found that P. euphratica would respond to a critical limit of erance of P. euphratica suspended cells to ionic and soil moisture. T. ramosissima belongs to a drought-resistant osmotic stresses, implemented by NaCl, by monitoring cell species that has low water potential and high transpiration growth, morphological features, ion compartmentation and (Zhang et al. 2003), which could increase its ability to polypeptide patterns, and showed that under a stress of absorb water from the soil by keeping a low water potential 137 mmol NaCl, P. euphratica cells could grow normally; and allow for maximum use of limited soil water for sur- under a stress of 222 mmol NaCl, most P. euphratica cells vival and growth. Thus, no critical limit of soil moisture for were still intact; even under a stress of 308 mmol NaCl, the T. ramosissima was found. A previous study using carbon cells still could survive, showing a strong salt resistance. At isotope labeling (Robinson 1965) found that under the 479 mmol NaCl, the multiple-peptide formation of cells same stressing conditions, T. ramosissima’s utilization started to change and cells became gray, which indicated ratio of water was higher than that of P. euphratica, which that 479 mmol NaCl was a critical salt content that indicated that T. ramosissima was more drought resistant inhibited the growth of P. euphratica. The salt tolerance of than P. euphratica. P. euphratica cells might be related to their capacity of adapting to higher osmotic stress by maintaining cell The threshold of soil salinity integrity, sequestrating Cl- into vacuoles and modulating polypeptides that reflect cellular metabolic adaptations. The soil in the desert riverbank forestry area where P. eu- Wang et al. (1996) believed that when the soil salinity was phratica is naturally distributed is mostly alkaline, because below 2%, P. euphratica could grow vigorously. Under salt of dry weather, rare precipitation, extensive evaporation stress, the body fluid of P. euphratica could contain a salt and unique soil moisture and salinity as a result of the great content of 56.2–71.6% with a pH between 9.6 and 11.8. groundwater depth. The effect of too much soil salinity on Our results showed that when the soil salinity was below plant growth includes physiological drought, excluding the 20%, the water potential of P. euphratica gradually absorption of other nutrients and reducing the net photo- decreased with increasing soil salinity, but remained con- synthetic productivity. As pioneer species in the desert stant when the soil salinity was above 20%. This does not vegetation communities, both P. euphratica and T. ramo- suggest that soil salinity above 20% had no influence on the sissima acquired very strong salt resistance capabilities water potential of P. euphratica. This could be explained after a long process of adapting to extreme salt stress. The instead by the fact that P. euphratica has the capability to threshold of soil salinity affecting the growth of P. eu- secrete salt and alkali when the soil salinity is above 20%, phratica and T. ramosissima needs to be further studied. and discharge the excessive salt out of the body in the form The water permeability of the cells of P. euphratica is of white crystals to maintain normal growth. Thus, it was stronger than other plants; the plant can sufficiently excrete found that for P. euphratica, 20% soil salinity was the excess salt and alkalinity by crystallizing them out on its lower limit at which salt secretion would occur. trunks and big branches in the form of white crystals, T. ramosissima is a typical halophyte that can secrete called P. euphratica alkaline. Thus, P. euphratica adapts salt. The salt-secreting cells in the plant’s glands have itself to the alkaline soil. The older the P. euphratica, the many small vacuoles, which can actively store accumu- stronger is its alkaline-resistant capability (Wurigen et al. lated salt to realize subdividing, so as to maintain normal 2003). Different critical salt values affecting the growth of transpiration inside the liquid of the cell. In addition, P. euphratica had been reported by different researchers. T. ramosissima has very strong salt and alkaline-resistant Watanabe et al. (2000) subcultured multiple shoots of capability, and can grow well under a soil salt content of P. euphratica in an in vitro micropropagation system in a 1.4%. The plants could still live even when the soil salt laboratory, in a saturated 10 ml sterile liquid 1/2 MS content reached 2–3% (Bosabalidis and Thomson 1985; medium containing 50, 150 or 250 mM NaCl after 3 weeks Thomson and Kathryn 1985; Lu and Ma 2003).The char- and measured the fresh weights of leaves, stems and roots acteristics (i.e., growth rate, salt resistance and the rate of 123 2528 Environ Earth Sci (2012) 66:2519–2529 utilizing water) of T. ramosissima living in the flood- of soil moisture for T. ramosissima. T. ramosissima washed plain of Krorado River were compared with other secretes salt by storing the accumulated salt in the vacuoles native plants in a greenhouse study (Glenn et al. 1998). The of its salt secretion glands for separation. Thus, it has no comparison study showed that the maximum salt resistance minimum soil salinity limit. T. ramosissima was found to limit for T. ramosissima was 32 g/L, which was much have better resistance to drought and salt stress than higher than the salt resistance limit for other trees. It was P. euphratica. In very dry soil environment, the survival of also found that its rate of water utilization was not influ- T. ramosissima is higher than that of P. euphratica; enced by salt and alkaline content. Furthermore, when therefore, it is suitable to plant T. ramosissima in greening treated with 1,000 mg salt, T. ramosissima could maintain desert riparian forest. The results would provide scientific a water potential of 46 bar and its permeation capability bases in maintaining the survival and growth of P. eu- was higher than that of the soil. The results can aid res- phratica and T. ramosissima in the desert region and the toration efforts by providing quantitative predictors of selection of green trees in different drought and salt stress plant performance as affected by salinity. Our results environments in the desert riparian forest. showed that when the soil salinity was below 40%, it had no negative influence on the water potential of T. ramo- Acknowledgments This study was supported financially by the sissima. By its salt-secreting action, T. ramosissima could National Natural Science Foundation of China (Grant No.91025025), Dr. Funded Projects of West Light Foundation of the Chinese store accumulated salt in the secreting vacuoles of the salt- Academy of Sciences (XBBS201026) and National Basic Research secreting glands. Program of China (973 Program) (2010CB951003). For P. euphratica, when the soil salinity was below 20%, there was a negative influence on its water potential. 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