Water-use strategy of three central Asian desert shrubs and their responses to rain pulse events

Abstract

Plant water-use strategy is considered to be a function of the complex interactions between species of different functional types and the prevailing environmental conditions. The functional type of a plant's root system is fundamental in determining the water-use strategy of desert shrubs and the physiological responses of the plant to an occasional rainfall event, or rain pulse. In this current study of Tamarix ramosissima Ledeb. Fl.Alt., Haloxylon ammodendron (C.A.Mey.) Bunge and Reaumuria soongorica (Pall.) Maxim., three dominant shrub species in the Gurbantonggut Desert (Central Asia), plant root systems were excavated in their native habitat to investigate their functional types and water-use strategies. We monitored leaf water potential, photosynthesis and transpiration rate during a 39-day interval between successive precipitation events during which time the upper soil water changed markedly. Plant apparent hydraulic conductance and water-use efficiency were calculated for the varying soil water conditions. Our results show that: 1) The three species of shrub belong to two functional groups: phreatophyte and nonphreatophyte; 2) The photosynthetic capacity and leaf-specific apparent hydraulic conductance of the three species was stable during the time that the water condition in the upper soil changed; 3) Transpiration, leaf water potential and water-use efficiency in Tamarix ramosissima Ledeb. Fl.Alt. were stable during the period of observation, but varied significantly for the other two species. Tamarix ramosissima Ledeb. Fl.Alt., as a phreatophyte, relies mostly on groundwater for survival; its physiological activity is not inhibited in any way by the deficiency in upper soil water. Non-phreatophyte Haloxylon ammodendron (C.A.Mey.) Bunge and Reaumuria soongorica (Pall.) Maxim. use precipitation-derived upper soil water for survival, and thus respond clearly to rain pulse events in terms of leaf water potential and transpiration. The observed similarity in leaf-specific photosynthesis capacity among all three species indicates that the two non-phreatophyte species are able to maintain normal photosynthesis within a wide range of plant water status. The observed stability in leaf-specific apparent hydraulic conductance indicates that the two non-phreatophyte species are able to maintain sufficient water supply to their foliage via, mostly likely, effective morphological adjustment at the scale of the individual plant.

Water uptake from different soil depths for halophytic shrubs grown in Northern area of Ningxia plain (China) in contrasted water regimes

“…Presumably, the roots of three-year-old T. ramosissima have not been developed enough to access water in the deeper soil layers during seedling stage, its growth would highly depend on moisture in the upper soil profile through greater capacity in osmotic adjustment (Busch & Smith 1995;Stromberg et al 1996;Horton et al 2003). In contrast, the 20-year-old T. ramosissima mostly used deeper soil water by developing active absorbing roots at depths close to the groundwater table (Xu & Li 2006). Sperry and Hacke (2002) reported that A. Canescens exhibited phreatophytic tendency and had deepest root indices and vulnerable shallow root (Lin et al 1996).…”

“…However, the controversial documents are not scarce. Horton et al (2003) did not find water use from the unsaturated soil by T. ramosissima grown in a serious of sites with a groundwater gradient from 0 to 4.4 m. Xu and Li (2006) reported T. ramosissima relied mostly on groundwater for survival and did not show a significant photosynthetic response to sustained drought or heavy rain pulse event.…”

“…Soil soluble salt content was analyzed by dissolving the soil samples into distilled water to achieve a soil solution, and then drying the solution to obtain the salt residue (Xu & Li 2006). Soil pH was determined by pH-3D pH meter (ZhiGuang Co. Ltd., Shanghai, China).…”

“…Plant functional type of root system is fundamental in determining their water use strategy and physiological responses to a specific water source (Xu & Li 2006). Different rooting patterns among species have resulted from long-period evolution and served as adaptations minimizing competition for water in a particular habitat (Dawson et al 1993;Dawson & Pate 1996).…”

“…A large number of reports concentrating on the water source, water use strategy, and plantÁwater relations of plants using stable isotopic compositions have touched on several ecological systems: riparian areas (Dawson & Ehleringer 1991;Shafroth et al 2000;Horton et al 2003;Chimner & Cooper 2004;Lite & Stromberg 2005), desert regions (Sperry and Hacke 2002;Gries et al 2003;Xu & Li 2006), woodland (Lin et al 1996;Picon-Cochard et al 2001), and grassland (Eggemeyer et al 2009). The information about water use strategies of halophytes, Tamarix ramosissima Ledeb., Atriplex canescens (Pursh) Nutt., and Lycium barbarum L. in salinity-and alkalinity-affected regions is scarce.…”