The hypothesis that water relations and growth of phreatophytic Tamarix ramosissima Ledeb. and Populus euphratica Oliv. on dunes of varying height in an extremely arid Chinese desert depend on vertical distance to a permanent water table was tested. Shoot diameter growth of P. euphratica was inversely correlated with groundwater depth (GD) of 7 to 23 m (adj. R 2 = = = = 0.69, P = = = = 0.025); growth of T. ramosissima varied independent of GD between 5 and 24 m ( P = = = = 0.385). Pre-dawn (pd) and midday (md) water potentials were lower in T. ramosissima (minimum pd − − − − 1.25 MPa, md − − − − 3.6 MPa at 24 m GD) than in P. euphratica (minimum pd − − − − 0.9 MPa, md − − − − 3.05 MPa at 23 m GD) and did not indicate physiologically significant drought stress for either species. Midday water potentials of P. euphratica closely corresponded to GD throughout the growing season, but those of T. ramosissima did not. In both species, stomatal conductance was significantly correlated with leaf water potential ( P. euphratica : adj. R 2 = = = = 0.84, P < < < < 0.0001; T. ramosissima : adj. R 2 = = = = 0.64, P = = = = 0.011) and with leaf-specific hydraulic conductance ( P. euphratica : adj. R 2 = = = = 0.79, P = = = = 0.001; T. ramosissima : adj. R 2 = = = = 0.56, P = = = = 0.019); the three variables decreased with increasing GD in P. euphratica . Stomatal conductance of P. euphratica was more strongly reduced ( > > > > 50% between − − − − 2 and − − − − 3 MPa) in response to decreasing leaf water potential than that of T. ramosissima (30% between − − − − 2 and − − − − 3 MPa). Tolerance of lower leaf water potentials due to higher concentrations of leaf osmotically active substances partially explains why leaf conductance, and probably leaf carbon gain and growth, of T. ramosissima was less severely affected by GD. Additionally, the complex below-ground structure of large clonal T. ramosissima shrub systems probably introduces variability into the assumed relationship of xylem path length with GD.

SummaryThe terminal branch orders of plant root systems have been proposed as short-lived 'ephemeral' modules specialized for resource absorption. The occurrence of ephemeral root modules has so far only been reported for a temperate tree species and it is unclear if the concept also applies to other woody (shrub, tree) and herb species.Fine roots of 12 perennial dicotyledonous herb, shrub and tree species were monitored for two growing seasons using a branch-order classification, sequential sampling and rhizotrons in the Taklamakan desert.Two root modules existed in all three plant functional groups. Among the first five branch orders, the first two (perennial herbs, shrubs) or three (trees) root orders were ephemeral and had a primary anatomical structure, high nitrogen (N) concentrations, high respiration rates and very short life spans of 1-4 months, whereas the last two branch orders in all functional groups were perennial, with thicker diameters, no or collapsed cortex, distinct secondary growth, low N concentrations, low respiration rates, but much longer life spans.Ephemeral, short-lived root modules and long-lived, persistent root modules seem to be a general feature across many plant functional groups and could represent a basic root system design.

[1] The Japan-Sino joint project, Aeolian Dust Experiment on Climate impact (ADEC), was initiated in April 2000 in order to understand the aeolian dust impact on climate via radiative forcing. As a part of the ADEC project, we have conducted field research in a sand dune and a gobi (i.e., a desert in which the soil surface consists of sand and pebbles with flat surfaces) in the south of the Taklimakan desert, China. The purpose of this study is to understand the wind erosion process and its relation to the meteorological and soil physical parameters. For this purpose, we measured the vertical profiles of wind speed, air temperature, and humidity as well as the other meteorological elements using an automatic weather station. A new sand particle counter (SPC) was newly developed to measure the saltation process. The SPC detects a signal change when a saltation particle passes through the slit between the laser beam transmitter and receiver. From this signal change, we can measure saltation particles from 30 to 667 mm diameter with 32 bin classes and particle numbers of each bin class every second. We have operated this SPC in the field, and it proved to be useful for the saltation process study when data corrections and calibration were properly made. During the observation period (1-21 April 2002), a total of eight dust events occurred; we analyzed two events: 5 April and 14 April cases. The results can be summarized as follows: (1) Total saltation fluxes in the 5 April case from 1223 to 1430 UT were 37.93 kg m À2 at 30 cm height and 43.71 kg m À2 at 20 cm height for the gobi site and 2.61 kg m À2 at 30 cm height for the dune site. (2) In the 14 April case, from 0327 to 0830 UT, the total saltation flux was 8.95 kg m À2 at 30 cm height for the gobi site. (3) Saltation flux at the gobi site in the 5 April case was more than 10 times larger than that of the sand dune, though the distance between the sites is 4 km. This is because the number of the parent soil particles around 80 mm at the gobi site was more than 10 times greater than that of the dune site. (4) Height dependency of saltation particle size and number was found in the gobi site; that is, the particle size distributions at the gobi sites in the 5 April case indicated that the number size distribution of the coarse particles, 117 to 554 mm, at 20 cm height was greater than that at 30 cm height. This size-height dependency was reasonable from a physical point of view. However, present theory cannot explain this well.Citation: Mikami, M., Y. Yamada, M. Ishizuka, T. Ishimaru, W. Gao, and F. Zeng (2005), Measurement of saltation process over gobi and sand dunes in the Taklimakan desert, China, with newly developed sand particle counter,