Ecophysiological responses and adaptation of Tamarix ramosissima to changes in groundwater depth in the Heihe river basin

Areas in middle reaches of the Heihe river basin constitute a desert oasis ecotone.The normal growth of vegetation in this area is significantly affected by irrigation practices and seasonal groundwater level fluctuations.This is particularly true for groundwater dependent species.The continued decline of the groundwater level,due high levels of groundwater consumption for agricultural irrigation in the area,threatens the stability and sustainability of this transitional ecosystem.A field experiment was conducted on Tamarix ramosissima,one of the main species of this desert-oasis ecotone,to elucidate the response processes and adaptation mechanisms of this species to changes in groundwater depth.The groundwater depth differences examined included two conditions: spatial differences in absolute groundwater depth and seasonal fluctuations of the groundwater level.Field measurements were carried out at seven sites,where groundwater depth ranged from 0.75m to 7.05m.A number of leaf ecophysiological indicators were recorded,including,midday leaf net photosynthetic rate(Ps),transpiration rate(Tr),stomatal conductance(C),intercellular CO2 concentration(Ci),and midday stem water potential(Ψm).All measurements were made under similar meteorological conditions.The study was conducted from late June to early September.During this period groundwater levels at the different sites fell by differing amounts.The results show that Ps and Tr values among the seven sites were not notably different(the fitted trendline slopes were-0.24 for Ps,and-0.13 for Tr,respectively),indicating that there was little difference in the growth rates of T.ramosissima among sites with different absolute groundwater depth.However,Ps and Tr showed a clear decline with the main seasonal declines in groundwater level(the fitted trendline slopes were-10.10 for Ps,and-6.34 for Tr,respectively).These differing responses to changes in groundwater depth suggest that T.ramosissima may be adapted to the different absolute groundwater depths,but shows a clear response to rapid seasonal fluctuations of groundwater level.C values for T.ramosissima declined with decreasing Ψm when Ψm 3.0MPa.However,C values increased slowly with decreasing Ψm when Ψm 3.0MPa.Nonetheless,both C and Ci/Ca values among the seven sites were similar.These results suggest that T.ramosissima maintains a relatively stable carbon assimilation capacity at different absolute groundwater depths.This is achieved through stomatal regulation which results in stable stomatal conductance and thus stable leaf intercellular carbon dioxide concentration.On the other hand,seasonal declines in groundwater level could have a marked influence on the normal growth of T.ramosissima.The leaf physiological indicators for T.ramosissima showed different responses to seasonal groundwater level fluctuations.At sites where groundwater depths were less than 3m,the leaf gas exchange rates of T.ramosissima increased as the groundwater level declined.On the other hand,at sites where groundwater depths were more than 3m gas exchange rates decreased with a decline in groundwater level.These differences suggest that there is an optimal groundwater depth for T.ramosissima growth in this section of the Heihe river basin.The results indicate that this optimal groundwater depth is about 3m.