Studies On Selective Utilization Of Water By Plants In Aridland Region

To examine the adaptive strategies of representative desert oasis plants developed underwater stress and to interpret the relationship between plants and environment in both arid andextreme arid area in China, stable isotopes of hydrogen (D), oxygen (¹⁸O), and carbon(¹³C)were used to infer possible water utilization patterns and water utilization efficiency(WUE) of several dominant species in Minqin arid oasis-desert ecosystem, GanSu, China andEjin extremely arid oasis-desert ecosystem, Inner Mongolia, China. The main result of thisstudy are summerized as follows.A. Variation in water utility pattern and WUE among desert plant species:TheδD andδ¹⁸O values for stems water, soil water, and groundwater were compared toidentify water sources for the small arbor Haloxylon ammodendron, the shrub Nitrariatangutorum and Calligonum mongolicum in semi-fixed sand dune, the subshrub Artemisiaarenaria and the perennial grass Thermopsis schischkinii in lowland between sand dunes, andthe shrub Elaeagnus angustifolia near an irrigation well. The stable isotope ratios of xylemwater were significantly different among species (P＜0.05). Strong isotopic enrichment ofhydrogen and oxygen occurred in soil layer shallower than 70 cm. In the dry month of summer,N. tangutorum used shallow soil water (＜1.5 m), H. ammodendron used water in deeper soillayer (＞1.5 m), while C. mongolicum mainly relied on groundwater (or deep layer soil water).E. angustifolia used water in soil layer deeper than 2m, which was recharged mainly by theirrigation water. A. arenaria, T. schischkinii and E.angustifolia used water at seem deep soillayer which was recharged by water stored in wet sand layer or claypan. In autumn, soil waterrecharged by large precipitation was the main water source for three species in semi-fixed sanddune, whereas the growth of these species in spring benefited effectively from water in uppersoil layer. The sequence of depth of soil water layer supporting three dominant species was listfrom deep to shallow as: C. mongolicum. H. ammodendron, N. tangutorum.The utility of groundwater by C. mongolicum based on isotope signal implies that its rootsystem is long enough to uptake groundwater. Nevertheless, soil in deeper layer was notsampled in our study because it was difficult to break through dry sand layer at about 2.0 mdepth.B. Utilization of summer rains by dominant desert plants:The relationship between the size of summer rains and water utilization of desert plantswas evaluated by comparing stable isotope ratios of different waters. The stable isotope data showed that all desert species could use precipitation from large rain event (47.6 mm in thisstudy) for quite a long time up to a few weeks. Both A. arenaria and A. Squarrosum were sosensitive to precipitation that they used rain water on the rain day, but H. ammodendron, C.mongolicum, N. tangutorum began to use rain water one day after the rain day. Comparing withthe large rain event, the small one (3-6 mm in this study) could also favor the developmentof desert plants but only in a relatively brief time up to couple days.C. Physiological Characters of Populus euphratica in different habitat forms:To study possible adaption strategy of P.euphratica to arid environment, stable isotoperatios in conjunction with other ecophysiological characteristics (e.g. photosynthesis character,leaf water potential) were measured. The stable isotope data suggest that P. euphratica uptakemainly groundwater recharged by Heihe river for development. The mature P. euphratica usedgroundwater for development while the saplings tended to use deeper soil water as complementto groundwater. Furthermore, the further the P. euphratica grows to the riverbank, the higherWUE that the individual plant has.The utility of river water by mature P. euphratica near the riverbank increasedsignificantly one week after the water transport event. But no significant increase was foundfor P. euphratica grown in the middle of forest and Gobi, 110 m and 250 m from riverbankrespectively. Water transport in Heihe river can favor the development and restoration of P.euphratica. The water transport with high quantity in relatively long time facilitated the growthofP. euphratica at Gobi far away (18 km) from the riverbank. The water transport with lowquantity in high frequency only facilitated the growth of P. euphratica near the riverbank. Thisfeedback of P. euphratica to water transport is also helpful for designing reasonable mode ofwater transport.D. WUE, water potential, photosynthetic rate of desert plants:Theδ¹³C value of leaves were measured to evaluate the intrinsic water-use efficiency(WUE) of each plant. The predawn water potential (φ_pd) and midday water potential (φ_md) ofeach species were also measured. The ¹³C measurements indicated that all plants have higher¹³C value (high WUE) in arid month (July) of summer season and lower ¹³C value (low WUE)in wet months (August and September). This variation was negatively consistent with the valueof water potential which was higher in August and September than that in July.The shrub C. mongolicum, which has higher evaporation rate and lower water utilizationefficiency than H. ammodendron, mainly use ground water (or deep soil water) freely.Consequently, Planting C. mongolicum in this area might exhaust groundwater. C. mongolicum will compete with H. ammodendron so severely that the restoration of H. ammodendron wasdelayed. P. euphratica and other species which also rely on groundwater for survival. Soreforestation with C. mongolicum might cause irreversible groundwater shortages and is not agood choice.