Indigenous vegetation in the foreland of river oases at the extremely arid southern margin of the Taklamakan desert in Xinjiang, NW China, is dominated by a few perennial phreatophytes, primarily Tamarix ramosissima Ledeb., Populus euphratica Oliv., Alhagi sparsifolia Shap. and Phragmites australis (Cav.) Steudel (Bruelheide et al. 2003; Qong, Takamura & Hudaberdi 2002). Due to permanent access to groundwater at depths from 3 to more than 10 m (Thomas et al. 2000a), stands of T. ramosissima and P. euphratica can be nearly as productive as typical vegetation of mesic climates (D. Gries, unpublished), even though mean annual precipitation is less than 50 mm throughout the region (Zhou 1993; Qong et al. 2002). The population of the oases exploits the vegetation by harvest of construction wood, fuel and livestock feed, and by grazing of sheep and goats; it thus plays an important role in the agro-economic system of the oases (Runge et al. 2001).
The productivity of dominant species varies considerably at small spatial scales in the foreland of the oasis (D. Gries, unpublished). Differences in nutrient supply can be ruled out as a reason because plants are uniformly supplied with most nutrients by the groundwater (S.K. Arndt, unpublished). Patterns of stand production in relation to position on dunes or in dune valleys suggest that stand growth is affected by distance to groundwater (D. Gries, unpublished). Greater groundwater depth could also explain why the size of P. euphratica trees generally decreases with increasing elevation on the dunes.
Groundwater depth could affect growth by at least two mechanisms:
- 1Inevitably, more below-ground biomass per unit leaf area is required to reach groundwater from higher dunes, leaving less carbon available for above-ground growth; in this respect the situation may be similar to age-related decline of tree productivity due to a shift of allocation from leaves to transport tissue (Magnani, Mencuccini & Grace 2000).
- 2Lower leaf conductance of T. ramosissima on high dunes than in a dune valley, indicated by higher leaf δ13C during the growing season (S.K. Arndt, unpublished results), suggested that effects of groundwater depth on growth may be mediated by plant water status. This may be anticipated, because as xylem transport distance increases at greater groundwater depth, lower leaf water potentials are required for water uptake and may in turn cause stronger stomatal limitation of photosynthetic carbon gain, similar to mechanisms operative in tall trees (Yoder et al. 1994; Mencuccini & Grace 1996; Ryan & Yoder 1997; Hubbard, Bond & Ryan 1999; Schäfer, Oren & Tenhunen 2000; Hubbard et al. 2001; McDowell et al. 2002).
The effects of groundwater depth on performance of T. ramosissima and related Tamarix species summarized as Tamarix chinensis Lourd., have been widely studied in riparian vegetation in the south-western US (Smith et al. 1998); however, most studies focused on the consequences of temporal depressions of groundwater tables induced by human alteration of riparian hydrology, rather than steady-state effects of different depths to a permanent groundwater table. Some results were unclear with respect to tolerance of deep groundwater: reduced gas exchange and increased canopy dieback was found if groundwater levels decreased below 3 m (T. chinensis, Horton, Kolb & Hart 2001a), whereas occurrence of vigorous T. chinensis stands at 7–8 m groundwater depth was reported by Stromberg (1998). This just indicates that in riparian ecosystems with seasonally variable precipitation, surface flow and groundwater tables, the water sources of facultative phreatophytes such as T. ramosissima are difficult to track; detailed isotope analyses are normally required in order to quantify temporally varying contributions from various sources to plant water supply (Busch, Ingraham & Smith 1992; Smith et al. 1998; Zencich et al. 2002).
The ecological situation at the margin of the Taklamakan desert is unusual and especially well suited for studying growth and water relations of facultative or obligate phreatophytes in relation to groundwater depth because the influence of other water sources, such as precipitation or water stored in the soil, can be ruled out. Annual precipitation is insignificant for plant growth; and typically no water is stored in the soils down to the capillary fringe (Thomas et al. 2000a; F.M. Thomas, unpublished). Moreover, in contrast to typical riparian situations, groundwater levels remain largely stable throughout the year (Thomas et al. 2000a; Bruelheide et al. 2002b; A. Foetzki, unpublished results).
We hypothesize that water relations and productivity of P. euphratica and T. ramosissima growing on dunes at varying elevations above a permanent groundwater table depend on distance to the water table. This distance is likely to affect (1) the whole-plant carbon economy, in particular above-to-below-ground allocation patterns and leaf area ratios, as well as (2) leaf-specific hydraulic conductance, leaf water relations and therefore potentially per-leaf area carbon gain. In order to test our hypothesis, we established elevation transects along depths of groundwater ranging from 7 to 23 m for P. euphratica and from 5 to 24 m for T. ramosissima within two dune complexes in the foreland of Qira oasis. The situation in Qira is typical for oases and riparian dunes along tributaries and ephemeral rivers at the margin of the Taklamakan, the largest desert in China and one of the largest ergs (sand dune deserts) in the world (Zhou 1993). At different elevations along the transects we measured daytime courses of leaf transpiration rate and conductance as well as pre-dawn and midday leaf water potentials on several days during the growing season, and measured the annual basal area increment of stems or shoots. Leaf analyses will be published in a related paper (Arndt et al. in preparation).