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 non-phreatophyte; 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.