Interspecific Relationship Between Populus Euphratica And Tamarix Ramosissima And Its Response To Water And Salinity Conditions

Populus euphratica is now faced with hard recruitment and degeneration due to restriction of water and soil salinity in hyperarid area,while the co-dominated shrub Tamarix ramosissima in riparian forests keeps expanding.As the two species are both primary constructive in riparian forest,the relationship between them plays an important role in maintaining desert riparian ecosystem.It has been confirmed that Tamarix ramosissima is more drought tolerant than Populus euphratica for its higher water use efficiency.However,it remains unknown that whether Tamarix ramosissima has become a threat to recruitment of Populus euphratica.And relationship between the two species is controversial.Thus,a replacement series experiment was carried out to clarify relationships between the two species and another controlled greenhouse experiment to investigate response of annual Populus euphratica seedlings to neighboring Tamarix ramosissima seedlings under various water and salinity conditions.Ratios of the two species ranged from 10:0 to 0:10 in the replacement series experiment,including 11 groups in total.The other experiment included 3 regimes of water and salinity respectively.Populus euphratica seedlings were neighbored by intraspecific individuals or Tamarix ramosissima seedlings of the same number.That was,a total of 18 treatments were included in the experiment.By measuring growth indexes of Populus euphratica seedlings in both experiments,we explored how annual Populus euphratica responded to Tamarix ramosissima seedlings under various densities and abiotic factors.Results are as follows.1.Interspecific competition was confirmed by the replacement series experiment.Populus euphratica seedlings suffered from mix cultural with Tamarix ramosissima seedlings.In all mix culture treatments,the total relative yield and relative yield were less than 1 and the attack index and competitive balance index were less than 0.Interspecific competition decreased height,aboveground biomass,belowground biomass,total biomass,main root length and number of leaves of Populus euphratica.As the proportion of Populus euphratica seedlings increased,aboveground biomass,belowground biomass,total biomass of them increased while specific leaf area decreased.However,Tamarix ramosissima seedlings benefited from mix culture.As the proportion of Populus euphratica seedlings increased,Tamarix ramosissima seedlings accumulated more aboveground and belowground biomass and had larger number of branches,indicating intraspecific competition had more impacts on growth of Tamarix ramosissima seedlings instead of interspecific competition.2.Significant interactive effects were observed among water,soil salinity and neighbor conditions.Neighbouring Tamarix ramosissima seedlings inhibited growth and survival of Populus euphratica seedlings under various water and salinity conditions.Survival of Populus euphratica seedlings under drought stress neighbored by Tamarix ramosissima seedlings significantly decreased,but it increased as water condition improved.A series of growth indexes like height,main root length,aboveground and belowground biomass of Populus euphratica seedlings decreased when being neighbored by Tamarix ramosissima under each treatment(p<0.001),among which main root length had the slightest reduction and aboveground and belowground biomass sharply dropped over 75%.On the contrary,root/shoot ratio,specific root length and specific root area of Populus euphratica seedlings increased,indicating a competition induced modification of biomass allocation.As water conditions improved,growth of Populus euphratica seedlings increased significantly.Salinity had no effects on growth of seedlings neighbored by intraspecific individuals,but it influenced total root length,total root surface area and other indexes of seedlings neighboured by Tamarix ramosissima remarkably.Interspecific competition made Populus euphratica seedlings more vulnerable to drought and salinity stress and it had more impacts on root morphology of seedlings than water and salinity conditions did.3.Relative competitive intensity(RCI)for both aboveground and belowground of Populus euphratica seedlings decreased as water availability improved.Importance of competition showed the opposite.By depleting water rapidly,Tamarix ramosissima constrained the growth of Populus euphratica seedlings and even threatened them to death.In summary,interspecific competition is an important cause for failed recruitment for Populus euphratica besides water and salinity conditions.Sufficient water supply is the key point to the two species to co-exist,otherwise it would accelerate the degradation of Populus euphratica and gradual expansion of Tamarix ramosissima to broader range of riparian zone for the latter was more drought tolerant.To reverse the trend,proper water utilization management is also needed for the purpose of maintaining the Populus euphratica forests and their recruitment.