Spatial and temporal variations in soil and vegetation dynamics on stabilized desert dunes in Ningxia, China

A thesis submitted in partial fulfilment of the requirements of the University of Wolverhampton for the degree of Doctor of PhilosophyThe dynamics of vegetated dune stabilization were investigated at two field stations of the Institute of Desert Research, Academia Sinica, during the summers of 1993 and 1994. The majority of research was conducted at Shapotou Research Station, where an area of dunes, representing a 37 year stabilization chronosequence, allowed the progress of vegetated dune stabilization to be investigated. Investigations were made into dune vegetation dynamics, aeolian deposition, microphytic crust formation and function, dune surface micromorphology and hydrology. Following stabilization by a combination of straw checkerboards and planted shrubs at Shapotou, aeolian deposits formed a finer textured 'grey sand' layer at the dune surface, which thickened at 1.3 - 1.9 mm y-1. Dust gauging and measurement of aeolian enrichment of the stabilized dune surface revealed a complex pattern of deposition, which varied with shrub cover, topography and distance from the margin of the stabilized dunes. An extensive microphytic crust, comprised of cyanobacteria and bryophytes, formed over the surface of the stabilized dunes. The microphytic crusts proved effective stabilizers, protecting deposited dust and promoting the development of the 'grey sand' layer. The accumulation of 'grey sand' and development of the associated microphytic crust appeared to be controlling features of the dune stabilization process at Shapotou. Dune hydrology altered significantly upon stabilization. Dune moisture is transpired by planted shrubs and subsequent recharge is restricted by the moisture retentive 'grey sand' and microphytic crust. This leads to a progressive desiccation of deeper dune sand and an increase in available moisture at the dune surface. Evaporative losses were increased by the development of the 'grey sand' and microphytic crust, which also significantly decreased infiltration rates. The nocturnal distillation and condensation at the dune surface of subsurface moisture became increasingly significant as dune stabilization progressed. Vegetation responded to these changes in surface stability and hydrology. Shrub cover declined with the desiccation of deeper dune sand, whilst the cover afforded by the microphytic crust and annual species increased in response to the increasing availability of near-surface moisture. Initially straw checkerboards and planted shrubs ensure surface stabilization, but as dune stabilization progresses, the surface stability afforded by the microphytic crust becomes of increasing importance