Abstract: At the southern fringe of the Gurbantonggut Desert,H.ammodendron and H.persicum are the two dominant woody species.Both species are morphologically similar but H.ammodendron is found primarily at interdunes and H.persicum at the top of dunes in the desert.The contrasting distribution pattern of these two species has long intrigued researchers in plant geography or biogeography.However,we are still far from fully understanding what controls this distribution pattern.In the current study,by a competition experiment,we aimed to determine the competitive abilities under a broad range of soil nutrient supply and to examine the underlying causes of the dune/interdune distribution patterns of the two Haloxylon species in the Gurbantonggut Desert.Our results demonstrate that:(1) in the seedling stage,H.ammodendron was the superior competitor under high nutrient availability and,in contrast,H.persicum was superior under low nutrient availability.The relative dominance index in mixture pots also showed the same pattern.These results resembled the patterns of species distribution observed in the field. (2) At least part of the explanation for the contrasting competitive responses of the two Haloxylon species lies in their differences in nutritional physiology.Its high absorption capacity gave H.ammodendron an advantage only under high nutrient conditions but would be a disadvantage under low-nutrient conditions.Under low-nutrient conditions,diffusion to the root surface is the rate-limiting step in nutrient acquisition from soil,so a high potential to absorb nutrients adds little to the total amount of nutrients acquired.The low absorption capacity of H.persicum suggests high nutrient retention (i.e.low efflux).Thus,H.persicum with high nutrient retention can competitively replace H.ammodendron with higher competitive ability for nutrient uptake under low nutrient availability.In unfertile sites (e.g.desert),the mechanisms of interspecific competition are not always clear.In general,nutrient uptake was less important relative to plants' ability for nutrient retention under unfertile sites.However,our results have showed that both nutrient uptake and nutrient retention can be important under unfertile sites.A key implication of our results is that the inter-specific competition at small scale determines their distribution pattern in unfertile sites.

Key words: inter-specific competition, soil nutrition, N deposition, desert plant, H.ammodendron, H.persicum