American Society of Agronomy Crop Science Society of America Soil Science Society of America

ent organs of plants (Comstock and Ehleringer, 1992), and different aged plants (Cavender-Bares and Bazzaz, Water use efficiency (WUE) is an important ecophysiological char2000). In contrast to WUE, the intrinsic water use effiacteristic of plants, especially in semiarid and arid regions. At the scale of community or ecosystem, WUE is difficult to quantify because ciency (IWUE), defined as the ratio of net assimilation the amount of water used per unit dry mass production is a function rate to stomatal conductance, is thought to be less tightly of microclimatic variables and species composition. In this study, we coupled to the instantaneous environmental temperaanalyzed corrected intrinsic water use efficiency (IWUEs) of grass and ture and atmospheric humidity and thus more closely shrub species along the western segment of the Northeast China Tranreflects plant physiological properties (Comstock and sect (NECT) and the relationship between IWUEs and mean annual Ehleringer, 1992; Jones, 1992). rainfall, habitat degradation status, vegetation type, and plant funcModeling the response of plant growth to global clitional type (C3 versus C4) at 22 survey sites. Site intrinsic water use mate change requires the quantification of WUE charefficiency (IWUEv) and its relationship with the aforementioned site acteristics of plant species and their relationships to the variables were analyzed based on species frequencies at each site. First, climatic factors. Studies on water and carbon balances it was concluded that photosynthetic pathway played a very important role in determining species IWUEs . Mean IWUEs for C4 species was in plants in semiarid and arid environments requires the approximately double that of C3 species. Second, mean annual rainfall, quantification of water use efficiency at the community vegetation type, and site degradation status significantly affected IWUEs or ecosystem level, which depends on the WUE of each (p 0.01). Mean IWUEs at degraded sites was twice as high as that species and thus on species composition. Species comat nondegraded sites. The mean IWUEs for meadows was significantly position, a measure of ecosystem structure, is largely higher than those for other vegetation types (p 0.05). Third, the freaffected by the successional status and/or level of disquency of occurrence of C4 plants explained 36% of the variance in turbance of the ecosystem (National Research Council IWUEv across the survey sites. The mean frequency of C4 occurrence Committee on Scholarly Communication with the Peoat degraded sites was more than double that at nondegraded sites. ple’s Republic of China, 1992; Redmann et al., 1995), Consequently, mean IWUEv at degraded sites was more than double and thus the degradation state of the site should be conthat at nondegraded sites. Dominant C4 species in saline–alkaline areas tended to have higher intrinsic WUE than dominant C4 species sidered an important variable when determining WUE in sandy shrub communities. at the community or ecosystem level. The Northeast China Transect (NECT), established by the International Geosphere and Biosphere Program (IGBP), is along a rainfall gradient centered at 43.5 N W loss in terrestrial plants often exceeds carand extending 1400 km from 132 E to 108 E (Interbon gained via photosynthesis by three orders of national Geosphere and Biosphere Program, 1995; Gao magnitude (Farquhar et al., 1989; Marshall and Zhang, and Zhang, 1997; Gao and Yu, 1998). Annual precipita1994). Understanding the water use efficiency of plants tion decreases sharply from around 800 mm in the east in semiarid and arid environments is of particular into less than 100 mm in the west. Vegetation along the tranterest due to the low water availability in these habitats. sect changes from dark conifer forests, conifer–broadWater use efficiency (WUE), the instantaneous ratio of leaved mixed forests, deciduous broadleaved forests, leaf net carbon assimilation rate over transpiration rate, woodlands and shrublands in the east, to meadow steppe has been reported to be different among plants with difand croplands in the middle, and typical steppe and desferent photosynthetic pathways (C3, C4, or CAM) (Ehleert steppe in the west primarily as a result of the rainfall ringer and Osmond, 1989; Marshall and Zhang, 1994), gradient (Gao and Zhang, 1997). Functional types of plants growing in different habitats (Ehleringer and Cooplant species, jointly determined by life form and carbon per, 1988; Garten and Taylor, 1992; Meinzer et al., 1992a, pathway, also vary along the rainfall gradient (Jiang 1992b; Ares and Fownes, 1999), different leaf types et al., 1999). Due to rapid population increase and econ(Marshall and Zhang, 1994; Damesin et al., 1997), differomic development, the transect area has been subjected to increasing influence from intensified human activiM. Yu and X. Zhang, Laboratory of Quantitative Vegetation Ecology, ties, such as cultivation and overgrazing (Yu et al., 2004). Institute of Botany, The Chinese Academy of Sciences, Beijing 100093, Progressive degradation has been reported in the semiP.R. China. M. Yu also at Department of Environmental Sciences, University of Virginia, Charlottesville, VA 22904. Y. Xie, Institute arid and arid grasslands along the transect over the past for Geospatial Research and Education, Eastern Michigan University, 30 yr. Even though water balance has been recognized Ypsilanti, MI 48197. Received 13 Sept. 2004. *Corresponding author as the most important driver of the ecosystem structural ([email protected]). and functional changes along the NECT, quantification Published in J. Environ. Qual. 34: – (2005). Technical Reports: Plant and Environment Interactions doi:10.2134/jeq2004.0352 Abbreviations: IWUE, intrinsic water use efficiency; IWUEs, corrected intrinsic water use efficiency; IWUEv, site intrinsic water use efficiency; © ASA, CSSA, SSSA 677 S. Segoe Rd., Madison, WI 53711 USA NECT, Northeast China Transect; WUE, water use efficiency.